Wear characteristics of normal compressive crushing rings Master’s thesis in Materials Engineering Ahmadreza Babaahmadi DEPARTMENT OF INDUSTRIAL AND MATERIALS SCIENCE CHALMERS UNIVERSITY OF TECHNOLOGY Gothenburg, Sweden 2023 www.chalmers.se www.chalmers.se Master’s thesis 2023 Wear characteristics of normal compressive crushing rings Ahmadreza Babaahmadi Department of Industrial and Materials Science Division of Engineering Materials Chalmers University of Technology Gothenburg, Sweden 2023 Wear characteristics of normal compressive crushing rings AHMADREZA BABAAHMADI © AHMADREZA BABAAHMADI, 2023. Supervisor: Emmy Yu Cao, Industrial and Materials Science, Magnus Evertsson, Comminution Reimagined Sweden AB Examiner: Emmy Yu Cao, Industrial and Materials Science Master’s Thesis 2023 Department of Industrial and Materials Science Chalmers University of Technology SE-412 96 Gothenburg Telephone +46 31 772 1000 Cover: Typeset in LATEX, template by Kyriaki Antoniadou-Plytaria Printed by Chalmers Reproservice Gothenburg, Sweden 2023 iv Wear characteristics of normal compressive crushing rings AHMADREZA BABAAHMADI Department of Industrial and Materials Science Chalmers University of Technology Abstract This thesis examines the wear characteristics of a promising new rock-crushing tech- nology. This novel method employs a normal compressive force to comminute rocks between two rotating rings. To further understand the wear mechanism of these rings and the ring materials response to this type of wear, a variety of ring mate- rials, including Hardox600, two types of sintered Tungsten Carbide (WC)/Cobalt (Co) matrix, identified as X1 and X2 - where X1 has less WC compared to X2 was used. This range of materials enabled a comparative study of wear patterns under the same operating conditions. The thesis was carried out through a set of surface studies using Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS), and Backscatter Electron (BE) imaging. These tools were used to closely examine the worn features on the surfaces of the tested materials. These detailed methods provided a clear view of wear patterns, enabling a better understanding of how the ring materials behave during rock crushing. After a comprehensive analysis of the crushing principle and surface wear, the high stress three-body abrasive wear was identified as the primary wear mode for this application. In response to this type of wear, the tested materials displayed contrasting material removal mechanisms. Hardox600, being the softest of the materials, primarily expe- rienced cutting (Ploughing) and repeated plastic deformations, evidenced by slivers on the worn surface. Conversely, the sintered WC materials exhibited completely different post-wear sur- face topography. For these materials, most material removal was due to WC particle breakage and subsequent dislodging from the matrix. Of the two sintered materials tested, the one with a higher WC content percentage demonstrated superior wear resistance, underscoring the significance of hardness in relation to the hardness of the abrasives. The superior wear can also be explained by the smaller and more uniform WC used in the Sintered X2 compared to X1. Based on the wear resistance and hardness of the evaluated materials, we conclude that for optimal wear resistance in this wear application, materials should resist plastic deformation, as it leads to severe wear. To control wear, materials should have a hardness that is on par with that of the abrasives to minimize the severity of wear. Keywords: Crushing- Wear mechanisms- three body abrasive wear- Hardox600- Sin- tered WC/Co- Surface analysis v Acknowledgements I would like to express my deepest gratitude to my supervisors, Emmy Yu Cao and Magnus Evertsson, for their continuous guidance, support, and encouragement throughout my thesis journey. Their valuable insights, thoughtful feedback, and unwavering support were absolutely essential for the successful completion of this work, and I am incredibly grateful for the opportunity to learn and grow under their mentorship. Additionally, I must extend my sincere thanks to Comminution Reimagined Sweden (CRS) for their trust, support, and collaboration in this project. Their constructive feedback, insightful comments, and ongoing support had a significant and positive impact on the quality and depth of this thesis. I am truly thankful for the oppor- tunity to work with such a supportive and innovative team. Lastly, I cannot express enough thanks to my family for their constant love, support, and encouragement. Their unwavering belief in my abilities, even during the most challenging times, has always been a great source of strength and motivation for me to strive for excellence and continually improve myself. Ahmadreza Babaahmadi, Gothenburg, Jun 2023 vii List of Acronyms Below is the list of acronyms that have been used throughout this thesis listed in alphabetical order: BE Backscatter Electron Imaging CRS Comminution Reimagined Sweden AB EDM Electro-discharged Machining EDS Energy Dispersive X-ray Spectroscopy HC Hertzian Cone Crack HPGR High Pressure Grinding Rolls HVOF High-Velocity Oxygen Fuel L Lateral crack MP Median Half-penny Crack MMC Metal Matrix Composites PSD Particle Size Distribution SEM Scanning Electron Microscopy ix Contents List of Acronyms ix Nomenclature xi List of Figures xiii List of Tables xv 1 Introduction 1 1.1 Thesis aim and Deliverable . . . . . . . . . . . . . . . . . . . . . . . . 1 1.2 Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2 Theory and Background 5 2.1 Comminution Industry . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.2 Crushing Principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.2.1 Particle Grip Condition . . . . . . . . . . . . . . . . . . . . . 6 2.3 Wear Principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.3.1 Wear Classification . . . . . . . . . . . . . . . . . . . . . . . . 9 2.4 Abrasive Wear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2.4.1 Abrasive wear classification . . . . . . . . . . . . . . . . . . . 11 2.4.2 Different contact types . . . . . . . . . . . . . . . . . . . . . . 13 2.4.2.1 Elastic contact . . . . . . . . . . . . . . . . . . . . . 13 2.4.2.2 Elastic-plastic and fully plastic contact . . . . . . . . 15 2.4.2.3 Brittle contact . . . . . . . . . . . . . . . . . . . . . 16 2.4.2.4 Viscoelastic contact . . . . . . . . . . . . . . . . . . 17 2.4.3 Material removal mechanisms . . . . . . . . . . . . . . . . . . 18 2.4.3.1 Cutting . . . . . . . . . . . . . . . . . . . . . . . . . 19 2.4.3.2 Fracture and grain pull out . . . . . . . . . . . . . . 19 2.4.4 Abrasive characteristics . . . . . . . . . . . . . . . . . . . . . 19 2.4.4.1 Hardness . . . . . . . . . . . . . . . . . . . . . . . . 20 2.4.4.2 Shape and Size . . . . . . . . . . . . . . . . . . . . . 20 3 Materials Background 23 3.1 Hardox600 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 3.2 Sintered WC/Co . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 3.2.1 Manufacturing process . . . . . . . . . . . . . . . . . . . . . . 24 3.2.1.1 Powder manufacturing . . . . . . . . . . . . . . . . 25 xi Contents 3.2.1.2 Consolidation and sintering: . . . . . . . . . . . . . . 26 4 Test Procedure and Methods 29 4.1 Test Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 4.1.1 Wear Experiment . . . . . . . . . . . . . . . . . . . . . . . . . 30 4.1.2 Sample Preparation . . . . . . . . . . . . . . . . . . . . . . . . 32 4.1.3 Surface Analysis . . . . . . . . . . . . . . . . . . . . . . . . . 32 4.1.4 Micro-hardness . . . . . . . . . . . . . . . . . . . . . . . . . . 33 4.2 Test Plan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 5 Results 35 5.1 Wear experimental results . . . . . . . . . . . . . . . . . . . . . . . . 35 5.2 Characterization of Hardox 600 . . . . . . . . . . . . . . . . . . . . . 36 5.2.0.1 Microstructure . . . . . . . . . . . . . . . . . . . . . 36 5.2.1 Micro-hardness . . . . . . . . . . . . . . . . . . . . . . . . . . 38 5.2.2 Stereo Microscope observation of the worn surface . . . . . . . 40 5.2.3 SEM and EDX analysis . . . . . . . . . . . . . . . . . . . . . 41 5.3 Characterization of sintered X1 and X2 . . . . . . . . . . . . . . . . . 44 5.3.1 Microstructure . . . . . . . . . . . . . . . . . . . . . . . . . . 44 5.3.2 Micro-hardness . . . . . . . . . . . . . . . . . . . . . . . . . . 47 5.3.3 SEM and EDX analysis . . . . . . . . . . . . . . . . . . . . . 49 6 Discussion 53 6.1 Hardox600 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 6.2 Sintered WC/Co . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 7 Conclusion 57 Bibliography 59 A Appendix A I B Appendix B III C Appendix C V xii List of Figures 2.1 Force visualization from the front side of the machine . . . . . . . . . 7 2.2 An example of a tribosystem with it’s important elements . . . . . . 10 2.3 Different wear classification with focus on contact type . . . . . . . . 11 2.4 An example of a contact point for two spherical bodies . . . . . . . . 15 2.5 Indentation-crack evolution in loading and unloading . . . . . . . . . 18 2.6 Probable material removal mechanisms for abrasive wear . . . . . . . 19 3.1 Consolidation flowchart of WC/Co . . . . . . . . . . . . . . . . . . . 28 4.1 Overview of the rings . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 4.2 Wear experiment procedure flowchart . . . . . . . . . . . . . . . . . . 31 4.3 Schematic of two different samples taken from rings . . . . . . . . . . 33 5.1 Comparison of wear between different tested materials . . . . . . . . 35 5.2 Martensite structure with presence of bands observed by optical mi- croscope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 5.3 Optical microstructure difference in the cross-section between surface regions and distant from the surface . . . . . . . . . . . . . . . . . . . 38 5.4 SEM comparison of surface and bottom region on cross-section . . . . 39 5.5 Micro-hardness profile through thickness of Hardox600 . . . . . . . . 39 5.6 Stereo microscopy pictures of Hardox600 rings after third run . . . . 40 5.7 Stereo microscopy pictures of Hardox600 rings after sixth run . . . . 41 5.8 SEM image of Hardox600 surface with presence of embedded rocks . . 42 5.9 Chemical mapping of Hardox 600 surface . . . . . . . . . . . . . . . . 42 5.10 SEM material removal . . . . . . . . . . . . . . . . . . . . . . . . . . 44 5.11 Microstructural analysis of sintered WC with Co matrix . . . . . . . 45 5.12 Comparison of the microstructure near-surface region on cross-section 47 5.13 Hardness profile of sintered rings through thickness . . . . . . . . . . 48 5.14 SEM image of crack appeared after indentation on sintered X2 . . . . 49 5.15 Chemical mapping of Sintered X2 surface . . . . . . . . . . . . . . . . 50 5.16 SEM comparison of sintered surfaces at different magnifications . . . 52 6.1 Schematic of wear mechanisms on Hardox600 . . . . . . . . . . . . . 54 6.2 Schematic of wear mechanisms on sintered rings . . . . . . . . . . . . 55 C.1 PSD of Hardox600 . . . . . . . . . . . . . . . . . . . . . . . . . . . . V C.2 PSD of Sintered WC/Co X1 . . . . . . . . . . . . . . . . . . . . . . . V C.3 PSD of Sintered WC/Co X2 . . . . . . . . . . . . . . . . . . . . . . . VI xiii List of Figures xiv List of Tables 2.1 General abrasive wear classification . . . . . . . . . . . . . . . . . . . 12 3.1 Hardox600 Chemical composition . . . . . . . . . . . . . . . . . . . . 23 5.1 Comparison of WC particle size between sintered X1 and X2 . . . . . 46 5.2 Elemental difference between sintered X1 and sintered X2 . . . . . . . 46 xv List of Tables xvi 1 Introduction The process of comminution, a critical component of the rock milling industry, sig- nificantly impacts the global environment. It’s estimated to consume as much as 4% of the world’s electricity, accounting for half of the energy used at mining sites. Moreover, it produces ultrafine waste material that necessitates specific disposal methods in slimes dams [1]. Traditional milling techniques, such as tumbling mills, have been widely used but are known for their high energy inefficiency. High Pressure Grinding Rolls (HPGR) have been introduced as a more energy- efficient alternative [2], primarily due to their normal crushing component that re- sults in a wear mechanism involving normal compression, as opposed to the other extensively wear mechanisms. The rock milling industry has been evolving in response to growing demands for more efficient and sustainable processes. As concerns for the environment and re- source conservation continue to gain traction, it has become increasingly important for the industry to explore new materials and technologies that can improve energy efficiency and reduce waste generation. HPGR technology offers a promising avenue for achieving these goals, but further research is needed to optimize its performance and material selection. Although the compressive fracture wear mechanism significantly affects equipment procurement costs, operational expenses, and machine availability, it remains an under-researched field, lacking a standardized testing method. Previous investiga- tions into High-Pressure Grinding Rollers (HPGR) and their wear characteristics provide useful perspectives for this project. However, given the intricate nature of wear phenomena, it is essential to analyze each wear application individually. Comminution Reimagined Sweden AB (CRS), a company working to improve crush- ing technology, has developed its own test equipment and procedures for wear in- vestigations. 1.1 Thesis aim and Deliverable This Master’s thesis aims to assist the company in understanding the wear mech- anism behind the crushing machine. Three materials were analyzed in the current study: tempered steel, and two variations of sintered tungsten carbide (WC) with a cobalt (Co) binder, each having different WC contents. 1 1. Introduction The primary goal of this thesis is to better understand the wear mode/modes when subjected to this specific wear during compressive crushing. Utilizing the scientific method to establish hypotheses and test them to understand the materials resistant to normal wear loading, will form the basis of the experimental design. The development of a reliable and accurate test method for evaluating compres- sive fracture wear is a crucial step toward understanding the complex interactions between material properties and wear mechanisms. The proposed testing method and equipment will provide a valuable foundation for future investigations in this area, contributing to a more comprehensive understanding of the wear process and guiding the development of improved materials and technologies for rock milling applications. A thorough investigation of surface topography alterations and microstructure changes after wear testing was conducted using various techniques such as Scanning Electron Microscopy (SEM), Energy Dispersive X-ray spectroscopy (EDX), Backscattered Electrons (BE), and other microscopy methods. It is anticipated that these studies will shed light on the wear behavior of these materials under specific conditions. This analysis will provide valuable insights into the development of new wear resis- tant materials and help to inform decision-making in material selection and manu- facturing for rock milling equipment. The deliverables of this Master’s thesis include: • Obtain a wear graph after the experimental wear tests to rank the tested ma- terials • Explain the wear mode of the machine based on the test work performed for each of the tested materials • Determine the material removal mechanism(s) for each material • Identify crucial parameters related to the tribosystem that will affect the wear 2 1. Introduction 1.2 Limitations The execution of this thesis encountered several limitations, which are listed as follows: • Wear, by nature, is a complex and unpredictable phenomenon. Even identi- cal wear applications may yield different results under varying environmental and working conditions. Therefore, finding equivalent wear cases for a more in-depth understanding was challenging due to the absence of directly compa- rable situations, such as the crushing mechanism and crush machine design. • No reference material was available to benchmark the original states of the materials against their post-wear conditions. This constraint was particularly noticeable in the analysis of micro-hardness. • The shape and size of the ring made it difficult to monitor wear progression after each run, posing a challenge in tracking the gradual changes on the sur- face and within the microstructure. • As the materials tested varied, and each had a different manufacturer, there might be an unseen influence from manufacturing constraints that were not accounted for in the overall analysis. • Some tests, particularly those involving smaller gaps, required up to 12 hours of testing. This extended duration meant the thesis was restricted to one test(consisting of 6 individual runs) for each material, potentially compromis- ing the validity of the wear values. 3 1. Introduction 4 2 Theory and Background Based on the conducted literature review, this chapter aims to explain the basic concepts of how the machines cause abrasive(rock) breakage and the wear principles that are relevant to this application. Understanding these elements is crucial before analyzing the materials after the conducted wear experiment tests. 2.1 Comminution Industry Comminution is a sub-sector that falls under the broader mineral processing indus- try. Mineral processing involves the physical and chemical transformation of mineral resources from their raw state into usable products, such as metals, chemicals, or construction materials. Comminution is a crucial step in the mineral processing industry, which involves the crushing, grinding, and milling of ores to reduce their size and prepare them for fur- ther processing. Comminution is critical to the recovery of valuable minerals from ores and is responsible for a significant portion of the energy consumed in mineral processing operations. Comminution is the biggest individual energy consumer in hard rock mining op- erations, accounting for a quarter of the total energy used in mining. Thus, even minor enhancements in these circuits can result in substantial energy savings and reductions in greenhouse gas emissions [3]. In the pursuit of reducing energy consumption and carbon emissions within the industrial sector, enhancing comminution equipment efficiency represents a promis- ing strategy. Accomplishing this objective may entail the use of more energy-efficient equipment, optimizing operational conditions, and reducing the size of the particles to be processed. To further advance this aim, a comprehensive and systematic exploration of alter- native comminution methods that offer more environmentally friendly, sustainable, and energy-efficient solutions is required, as opposed to conventional approaches commonly employed in the industry. 2.2 Crushing Principle There is a limited amount of research available that investigates the mechanisms and behavior of the crushing machine used in this project. Based on the conducted lit- erature review [4, 5, 6, 7], two techniques share high similarities in terms of machine 5 2. Theory and Background design, crushing behavior, and principles. HPGR and smooth roll crushers(Can be identified in a more general group of double roll crushers). In both machines, almost similar to our case, rocks are crushed using mostly com- pression force and some degree of shear force [6]: • Compression: When the material is fed between the rolls, it experiences com- pressive forces, which cause the particles to break. • Shear forces: The rolls, rotating in opposite directions, create shear forces as they pull the material through the gap. These forces act on the particles, causing them to break along their natural weak points. As previously noted, the CRS crushing machine exhibits similar crushing mecha- nisms to both HPGRs and smooth double-roll crushers. In these machines, the comminution of ores takes place through the application of normal forces as abrasive ores are fed into the gap between two rotating rings. The normal compression force acting within this gap causes the ores to fracture. To ensure the effectiveness of this fracture process, a set of parameters must be metic- ulously taken into account. It’s important to note that despite some similarities, the crushing mechanisms and machine designs of HPGR and the machine examined in this thesis are significantly different. In HPGR, rocks are subjected to a high degree of pressure between rolls, which is usually facilitated by hydraulic systems. However, in the case of the CRS machine, rocks are compressed solely due to the difference between their original size and the gap distance of the rotating roll, without any application of external pressure. 2.2.1 Particle Grip Condition Figure 2.1 provides a schematic representation of an HPGR or double-roll crusher, illustrating the corresponding component forces involved. To effectively trap a par- ticle between the rings, the drawing force (Fd) must surpass the ejecting force (Fe). Since the horizontal components of the forces from both rings counteract each other, we only need to consider the vertical components. These vertical forces are mainly derived from compression and friction actions. The ejecting force (Fe) stems from the vertical aspect of the normal force (Fn), which acts as the counterforce at the contact point between the particle and the rings. On the other hand, the drawing force (Fd) originates from the vertical component of the friction force (Fr) [5]. The relationship among these forces is as follows: Fe = Fd.sin(α/2) (2.1) Fd = Fr.cos(α/2) (2.2) In above equations α is determined as the gripping angle [5],[8]. As can be under- 6 2. Theory and Background stood the particle will be captured by the rings when: Fd ≥ Fe (2.3) With having equations 2.1,2.2,2.3 we will have: µ ≥ tan(α/2) (2.4) Figure 2.1: Force visualization from the front side of the machine[5],[8],[7] Generally, friction can manifest in two primary forms: static friction, denoted by the coefficient µH , and kinetic friction, represented by µG. Static friction arises when a particle touches the roll surface moving at the same speed, facilitating immediate engagement. This situation arises when the circumferential speed vectors (Vr) and the particle speed (Vp) align, which happens exactly along the line connecting the roll rotation centers within the crushing gap. Yet, the crushing process for the particle starts well before this point and is usually finalized by the time the smallest crushing gap is reached. In other scenarios where the particle strikes the roll surface at a differing speed, it slides until its speed aligns with the rolls. Here, the kinetic friction coefficient µG becomes essential for accurate computations [5]. 2.3 Wear Principle Wear is a common phenomenon in various industrial and day-to-day situations. The concept of wear involves diverse interpretations when analyzed and anticipated 7 2. Theory and Background within various contexts. In most definitions, wear signifies the "loss" or "damage" of material due to the relative motion of a counterpart surface against the material [9, 10, 11]. This intricate process leads to degradation and, ultimately, the gradual removal of material as the contacting surface displaces the subjected material[12]. Addressing a particular wear problem relies on determining the characteristics of the specific wear system involved. Depending on a tribosystem’s parameters, di- verse wear mechanisms might be present. The task of predicting and modeling wear presents significant challenges due to several underlying factors. Three primary reasons contribute to this complexity [12]: • Transformation of Near-Surface Layer:During wear, the properties and composition of the near-surface region experience alteration, warranting sepa- rate and distinct investigation as an independent material layer. The historical progression and evolution of this layer necessitate separate investigation apart from the material undergoing wear. • Dynamic Topographical Changes: As the material is worn from the sur- face, it initiates alterations in topography, perpetually influencing the behavior of the tribosystem through continuous modifications. • Intricate Wear Mechanisms: Wear frequently occurs through intricate mechanisms, encompassing a range of mechanical material removal approaches and potential chemical interactions between the surface and external agents, such as lubricants. Consequently, due to the intricate nature of wear, a definitive empirical correlation that quantitatively links wear quantity with operational factors like load, speed, and material properties has not yet been established. Pursuing a singular relation of this kind is, in fact, somewhat futile due to the diverse and distinct phenomena encompassed by wear. The absence of a unified empirical wear law has significantly restricted engineers from effectively addressing wear-related challenges during machinery design. Unlike established quantitative relationships like Hook’s law for elastic bodies, there isn’t an equivalent law that explains wear. This deficiency makes it challenging to apply dimensional analysis for predicting wear in actual machinery based on results from small-scale laboratory tests [13]. Understanding wear rates, and various types of wear mechanisms, are crucial for designing tribosystems and choosing suitable materials according to the material’s wear map. The predominant type of wear in a system can change from one form to another, influenced by surface material characteristics and dynamic surface reactions from friction between the elements in contact. Wear mechanisms provide insights into the intricate alterations that occur during friction. Typically, wear does not result from a single wear mode, thus, it becomes important to comprehend each individual wear mechanism within a particular wear 8 2. Theory and Background application [14]. A wide range of techniques are used for wear testing, and while there is no universal standard specification for such tests, some methods have gained broad acceptance in laboratories globally. The reason for this assortment of testing techniques lies in the multitude of wear systems found in practical applications. Even when employing similar methodologies, test results from different researchers and laboratories can generally only be qualitatively compared due to variations in testing procedures, like test piece dimensions, geometry, environment, and so on. As a result, it is es- sential to be aware of the exact testing conditions when evaluating test outcomes. The successful application of laboratory test results to real-world industrial scenar- ios can be expected only when the key parameters of tribosystems in both lab and practical settings are closely aligned [15]. In this section, we will investigate the wear mechanisms relevant to the CRS crushing machine by comparing it with similar crushing machines or crushers used in wear tests. We aim to highlight the similarities and differences in the wear mechanisms to provide a better understanding of the wear behavior in the CRS crushing machine. 2.3.1 Wear Classification It is crucial to identify the tribosystem when investigating a wear case, as emphasized repeatedly. For conducting this we must identify four important elements listed below [15]: • The material undergoing wear (Body) • The second material (Counter body) • The interfacial region or elements contacting the two bodies • Environmental conditions An example of this can be seen in Figure 2.2. As previously stated, a crucial factor in the identification of a tribosystem in- volves analyzing the interfacial region between the interacting bodies, along with the subsequent determination of their contact type and configuration. This crite- rion serves as a valuable tool for categorizing distinct processes, a method that has been employed by Kato and Adachi [17]. With this graph(Figure 2.3), any type of wear application can be categorized into four major modes as Burwell has suggested as well [13]: • Adhesive wear or galling wear • Abrasive wear and cutting wear • Fatigue wear • Corrosive wear By categorizing the nature of body interactions and the method of material removal according to the state of deformation, it becomes easier to discern the potential processes leading to material removal and eventual material failure. This organized 9 2. Theory and Background Figure 2.2: An example of a tribosystem with it’s important elements [16, 15] approach facilitates clearer identification for subsequent investigative purposes. By identifying contact types as wear modes we can better understand and differen- tiate obscure wear mechanisms such as abrasive or fatigue wear. As explained in the previous section, a significant component of the CRS machine is the presence of ores, which serve as the abrasives to be crushed within the gap between two rotating rings. This particular setup, along with the cyclic interaction between the ring materials and the abrasives during the crushing process, highlights the significance of investigating abrasive wear processes. In the coming sections abrasive wear as the probable wear mechanism for the CRS machine will be described. 2.4 Abrasive Wear Given the operational principle of the CRS crushing machine and the characteris- tics of its tribosystem, it seems intuitive to primarily focus on abrasive wear when exploring the wear mechanism. Abrasive is a type of wear that occurs due to the contact between a particle and a solid material. It refers to the removal of material from a surface as hard particles pass over it. Abrasive wear involves material loss as hard particles move across the surface. This type of wear occurs when a solid object comes into contact with particles that have equal or greater hardness than the subjected material [15][18]. 10 2. Theory and Background Figure 2.3: Different wear classification with focus on contact type Modified from [17, 15] 2.4.1 Abrasive wear classification The primary abrasive wear mechanism, which significantly impacts mining and min- eral processing equipment, can be categorized into four major sub-mechanisms [19]: • Gouging abrasion • High stress (Grinding) abrasion • Low stress (Scratching) abrasion • Erosion-corrosion This categorization is primarily based on how the abrasives interact with the worn material’s contact surface. It revolves around understanding the nature of contacts, the abrasives’ behavior during wear, and, most importantly, the worn material’s response when in contact with abrasives. Gouging abrasion: Gouging happens when large segments of hard abrasive par- ticles press into worn surfaces and slide across them with strong force. This type of wear includes both cutting and tearing actions. Sharp points of rock under pressure cause small pieces of metal to be pulled away from the surface. Gouging abrasion results in the surface becoming plastically changed and harder due to the abrasive impact. The degree of surface deformation that warrants the use of "gouging" term is not clearly demarcated [19],[18]. High stress(Grinding) abrasion: This type of abrasion happens when the abra- sive particles fracture during the wear process. How the abrasives during the wear remove surface material is not well known yet. It has been seen that material re- moval in this type occurs by a combination of different modes like cutting, plastic deformation, fracture, tearing, fatigue, and delamination. 11 2. Theory and Background A significant consideration in this classification is the relative hardness between the worn material and the abrasives. When the material and abrasives have similar hardness levels, it prevents the material from undergoing extensive plastic deforma- tion and substantial wear before the abrasives break.[19],[20]. Low stress(Scratching) abrasion: Despite the previous class, this type won’t cause breakage of abrasives due to considerably lower stresses during wear. When abrasive particles make lightly loaded contact with the worn surface, low-stress scratching abrasion occurs, which results in microscopic cutting and ploughing. Corrosion can also affect the general wear rate in circumstances involving water or other liquids. The active wear mechanism is known as erosion-corrosion in this case. Low-stress abrasion is the main wear mode in both situations. The wear rates for low-stress abrasion are comparatively low [19]. Table 2.1 is the summary of this classification based on the configuration of contact and environmental conditions [19],[21]. Table 2.1: General abrasive wear classification [19, 21] The wear rate for the mentioned mechanisms is influenced by how deeply abrasive particles indent the worn material. The depth of this indentation under specific wear conditions relies on the hardness of the worn material and the attributes of the abrasive particles, including their hardness, shape, and size. Additional information about these abrasive characteristics will be discussed in the upcoming section [19]. There is another way to see the interaction between hard abrasives and the ma- terial’s surface. Based on that wear can be categorized into two distinct types of wear [22]: • Two-body abrasive wear • Three-body abrasive wear Two-body abrasion takes place when wear is induced by rigid protrusions on the counterface or hard particles adhering to it. On the other hand, three-body abra- sion occurs when hard particles move freely, rolling and sliding between distinct sliding surfaces. 12 2. Theory and Background From the explanations given for high-stress and low-stress abrasion, it can be un- derstood that three-body abrasion is predominantly associated with high-stress sit- uations, while two-body abrasion is more closely linked to low-stress scenarios. It’s important to note, however, that there are instances where two-body abrasion can occur under high-stress conditions as well [19]. The wear process cannot be fully described by simply categorizing it into "two-body" and "three-body" based on whether the particles are fixed or loose. Free particles can either slide or roll in the space between surfaces and in certain situations, they might simultaneously slide in one area and roll in another. As such, it is often more accurate to label abrasion as either sliding or rolling abrasion, in accordance with the motion of the particles. Consider the instance of a drill cutting through rock, which typically experiences two-body sliding abrasion. On the other hand, grit particles trapped between slid- ing surfaces, perhaps as impurities in lubricating oil, could either slide or roll - both circumstances are categorized as three-body abrasions. Generally, the wear rates associated with rolling abrasion are lower than those due to sliding abrasion. However, the methods of material removal in both scenarios might primarily differ in their relative significance [18]. Before exploring the detailed material removal mechanisms linked to abrasive wear, it is beneficial to develop a fundamental understanding of the diverse contact types. This foundational knowledge helps in interpreting how materials react to specific wear mechanisms, such as abrasive wear. 2.4.2 Different contact types Generally, contacts can be categorized into [23]: • Elastic contact • Elastic-plastic contact • Fully plastic contact • Brittle contact • Viscoelastic contact 2.4.2.1 Elastic contact The contact between two bodies can be classified as either conformal or non-conformal, depending on their geometric arrangement. Conformal contact takes place when the surfaces of two bodies come into complete contact with each other, as seen in sce- narios like bearings and shafts. On the other hand, if the contact doesn’t encompass this full contact, non-conformal contact happens at a specific point or along a line. For instance, point contact occurs between balls and races in a bearing, while line contact occurs between the teeth of gears [23]. The example below (shown in figure 2.4 a) is from an ideal elastic point contact 13 2. Theory and Background for two different bodies. Based on this setting we can have these relations between the corresponding parameters [23], as given in Eq 2.6: R, E, and ν with their subscripts are the radiuses, Young’s modulus, and Pois- son’s ratios of the two contacted bodies respectively. With a as the radius of the circular contact region, the apparent contact area of two bodies will be as given in Eq 2.5: An = π.a2 (2.5) a = (3.FN .R′ 2E ′ )1/3 (2.6) FN is the normal force, R′ is the reduced radius of curvature and E ′ is the effective Young’s modules, given by the following Eq 2.7 and 2.8, respectively: 1 R′ = 1 R1 + 1 R2 (2.7) 1 E ′ = 1 2 .(1 − ν1 2 E1 + 1 − ν2 2 E1 ) (2.8) Based on the Hertz theory the contact pressure distribution, as shown in Figure 2.4 b), will have its maximum value at r=o, which is the center of contact(Eq 2.9) : p = −σz(z = 0) = pmax. √ 1 − r a 2 (2.9) The Hertzian pressure (maximum pressure) which occurs at the center of contact is given by the following Eq 2.10: pmax = 3FN 2πa2 (2.10) At the edge of the nominal area of contact, a tensile radial stress occurs. Its maxi- mum value( at r=a) can be determined by following Eq 2.11: σr = pmax 1 − 2ν 3 (2.11) At 45◦ with respect to the contact surface, maximum shear stress(τmax) will develop. Maximum shear stress distribution can be found by Eq 2.12: τmax = 1 2 |σz − σr| (2.12) Another important parameter is the occurrence of the shear stress(τyz ), which is normal to the z and y axis and happens due to lateral displaces beneath the contact area (Figure 2.4c). All the equations mentioned are significant not only for the calculation of stress and pressure within tribosystems but also for analyzing the nature of contact. In 14 2. Theory and Background Figure 2.4: An example of a contact point for two spherical bodies Taken from [23] practical situations, purely elastic deformations are limited; instead, there is usually some degree of plasticity involved. Hence, it’s essential to initially analyze a sys- tem in its simplest configuration, characterized by elastic behavior. Subsequently, considering other conditions and criteria, the type of contact can be determined. 2.4.2.2 Elastic-plastic and fully plastic contact Depending on the material’s ductility, the contact area can experience localized plas- tic deformation. This occurs when stresses reach or surpass the yield strength at a critical point, leading to a shift from purely elastic behavior to a combination of elastic and plastic responses. In the case of conformal contacts, yielding typically starts at the surface and may extend to the edges. Conversely, non-conformal contacts see yielding initiation at a certain depth, denoted as zm. According to the Tresca criterion, when τmax = τY (Shear yield stress) = σY 2 happens, the material begins to yield. This yielding results in localized plastic zones beneath the surface. With increasing the load, the plastic zone can expand, if the load is removed and the pressure is below the critical value, the remaining load will cause elastic deformation only. This phenomenon is known as elastic shakedown. If the load is high enough that expand the plastic zone to the surface, then the contact transfers to the fully plastic. This happens when the nominal pressure (p0) equals to the critical value( yield pressure pY ). Yield pressure is greater than the yield stress and it is defined as pY = bσY . b value can be as difficult to spread the plasticity. As can be understood, the b value depends on the geometry, loading conditions, and the characteristics of the material [23]. 15 2. Theory and Background 2.4.2.3 Brittle contact When the material possesses high yield strength but relatively low fracture tough- ness, the likelihood of encountering a brittle contact increases. In scenarios char- acterized by brittleness, the contacting bodies could exhibit micro-cracks on either one or both surfaces. These micro-cracks become subjected to critical stress that leads to their opening or propagation. When the radial stress(σr), as introduced in Eq 2.11, exceeds the critical value(σF ) defined by the following Eq 2.13[23]: σF = K1C 1.12 √ πc (2.13) In this equation c and k1C are the length of the crack and the fracture toughness of the material, respectively. During this type of contact, there are some alterations in the material which lead to material loss, as described in the following [24, 25]: Crack initiation: Cracks in a material often originate from its existing imperfec- tions. This means that the initial cracks, or "crack nuclei," typically emerge from the material’s inherent defects(Figure 2.5 a). However, there’s another way that cracks can form: during the process when the material is indented or pressed(when deforma- tion is induced). The type of indenter used—whether it’s blunt or sharp—determines how these cracks will appear: • With a blunt indenter, cracks tend to begin at the pre-existing imperfections on the material’s surface. • On the other hand, when using a sharp indenter, cracks are more likely to start from new flaws that develop below the surface due to the deformations during indentation. Crack formation: When a solid material is subjected to pressure, it can start to develop cracks. These cracks initiate and expand from the most significant imper- fection within the material as the force from the indenter is increased. The growth of this crack, particularly in its early stages, is steady and doesn’t happen sponta- neously (Figure 2.5 b). This is because there exists an energy threshold that the crack must overcome to grow further. This energy barrier can be attributed to cer- tain mechanisms within the material that limit the stress, termed here as "stress cutoffs." Crack propagation: When the energy needed to start a crack exceeds a cer- tain threshold, the crack rapidly spreads. However, it stabilizes once it reaches a depth beyond its initial point of contact or pressure. At this stabilized stage, the crack is termed "well developed." This means that with increasing the load as the driving force, the deformation-induced zone stably increases. There are two primary types of such mature cracks: • The Hertzian cone crack starts from a surface defect and is caused by the pres- sure of a sphere pressing against the material. When you look at its pattern 16 2. Theory and Background of spread (visualized in a referenced figure), it looks like it grows outward in a circular manner, centered around where the initial pressure was applied. This crack grows in a direction perpendicular to the material’s surface. • The median half-penny crack is initiated by the sharp tip of a cone or pyramid- shaped indenter. This type of crack starts from a deformation caused by the pointed pressure. Its growth pattern is also radial and circular but spreads parallel to the surface of the material. Interestingly, several of these mature cracks can grow either sequentially or even simultaneously, influenced by other physical or chemical forces. This behavior is especially evident when the material is subjected to varied pressures or conditions (Figure 2.5 c). Unloading crack: When the pressure from the indenter is released, the cone and median cracks tend to close, but not completely (Figure 2.5 d). This unloading process leads to the emergence of residual stresses. These stresses result from the mismatch between areas that were permanently altered due to the indenter’s plastic deformation and the adjacent, unchanged elastic regions. This stress discrepancy sets the stage for a new type of crack formation (Figure 2.5 f). These new cracks, known as ’lateral’ cracks, originate from the previously deformed zone and grow both sideways and upward towards the material’s surface. The exact mechanics of these lateral cracks remain a bit elusive, primarily because the under- lying residual stresses are intricate and not entirely understood. Yet, it’s evident that these lateral cracks play a significant role in detaching material from the surface. The formation and growth of these lateral cracks are closely tied to how intensely the material was deformed. Sharp indenters, which concentrate stress more in- tensely, are especially influential in this process, accentuating the deformation and, consequently, the development of these lateral cracks. 2.4.2.4 Viscoelastic contact This form of contact is primarily observed in polymeric materials. In this context, when a load is applied, the material exhibits both elasticity and viscosity. One noteworthy distinction from other contact types is its time-dependent nature. Unlike other materials, viscoelastic substances rely on time to respond to stress and strain. Meaning, when subjected to an abrupt load or stress, they initially behave like an elastic material. However, over time, they can also exhibit the characteristics of a viscous fluid. This implies that similar to elastic deformation, viscoelastic deformation can indeed recover, but this recovery happens gradually over a duration of time following the release of the load, rather than instantly[23]. 17 2. Theory and Background Figure 2.5: Indentation-crack evolution in loading and unloading. (HC) denotes Hertzian cone crack, (MP) denotes Median half-penny crack and (L) lateral crack. Taken from [24] 2.4.3 Material removal mechanisms Abrasive wear can be more extensive than it appears initially. One of the chal- lenges in preventing and controlling abrasive wear is that the term ’abrasive wear’ doesn’t accurately capture all the wear mechanisms involved. In reality, many wear modes, or to put it another way, material removal mechanisms frequently coexist [22]. Figure 2.6 illustrates some of the probable mechanisms that are associated with abrasive wear. All of the four: cutting(ploughing), fracture, fatigue, and grain pull-out can be present in an abrasive wear system to some extent based on their material char- acteristic. An important thing is regardless of the material’s hardness or ductility, the material can experience ductile or brittle, and both during the abrasive wear [22],[18]. 18 2. Theory and Background These different mechanisms can be better understood by considering the contact type which was discussed in the previous section. 2.4.3.1 Cutting Figure 2.6a, illustrates one of the most well-known mechanisms associated with abrasive wear, where a sharp, hard particle cuts a softer surface. Two primary mechanisms occur in this cutting process: cutting the surface and forming a wedge buildup after the cut, also known as "ploughing" [22]. Depending on the specific wear modes, the formation of wear particles will vary. When the cutting mode is active, the resulting wear patterns often have an elongated and curled appearance, resembling ribbons. This particular type of wear is influenced by minimal friction. These particles form at the tip of the grooving asperity as demonstrated in 2.6a, where they remain and act like built-up wedges, facilitating the continuation of the grooving process [14]. The ploughing mechanism is mostly involved in the plastic contact and deforma- tion of the worn material. Thus, the hardness and toughness of the worn material would be an important factor. Figure 2.6: Probable material removal mechanisms for abrasive wear [22] 2.4.3.2 Fracture and grain pull out If the material exhibits brittle behavior upon contact, it will either develop fractures or experience grain pullouts, depending on how it responds to the cracks. The continuous stress from indentations can cause localized fractures, which can escalate into significant fractures and grain pullouts. 2.4.4 Abrasive characteristics One crucial aspect to examine in a three-body abrasive wear tribosystem is the third body, which is the abrasive particles between two contact surfaces. The character- 19 2. Theory and Background istics of these particles, including their hardness, shape, and size, play a significant role in determining the type and rate of wear. This section will explore the impact of these particle attributes on wear. 2.4.4.1 Hardness One can realize that as the hardness of the abrasive particle increases the overall wear on the corresponding surface will increase as well. Considering the hardness of the surface and abrasive particle as Hs and Ha respectively, It has been found that wear is most sensitive to these hardness values when Ha/Hs ≲ 1. This implies that when abrasive particles are significantly harder than the contact surface, the precise values of these hardness levels are not crucial. This means for situations where the abrasive particles are considerably harder than the contact surface the exact values of this hardness is less important [18]. This behavior can be explained by looking at the interaction between a single grit particle and a flat surface. When Ha ≳ 1, 2Hs the surface material experiences plastic flow upon surpassing its yield point(Y), substantial plastic deformation oc- curs on the surface as the average contact pressure reaches roughly 3Y (three times its uniaxial yield stress). The surface’s indentation hardness dictates this contact pressure, which is not significantly influenced by the particle’s specific shape. If the particle maintains this contact pressure without breakage(deforming), the surface will undergo plastic indentation as the normal load on the particle increases. How- ever, when Ha ≲ 1.2Hs the particle undergoes flow or breakage before the pressure on the surface reaches 3Y, the surface will experience only a small degree of plastic deformation [18]. This indicates that as long as the material being tested has not undergone plastic deformation, the wear can be managed. Generally, if the mechanism of material removal is more brittle in nature, the severity of abrasive wear will be lower. 2.4.4.2 Shape and Size The rate of wear is significantly affected by the particle shapes. Particles having sharper edges lead to increased wear compared to those with round edges. Generally with increasing the angularity of the abrasives, the wear will increase. This differ- ence can get up to 10 times higher for angular abrasives compared to round ones [18]. Abrasive wear’s intensity is governed by both the shape and size of the abrasives. A notable decrease in wear is evident when the abrasive’s size is under 100 microm- eters. This correlation between abrasive size and wear isn’t specific to a singular process; it’s prevalent across various wear methods, including two- and three-body abrasion, erosion, grinding, and metal cutting. While multiple mechanisms may describe this behavior across diverse types of wear, such as erosive or abrasive, one consistent factor emerges: the amplified residual stress beneath the worn surface. This pattern implies that the abrasive size effect might be intricately tied to the material’s properties, especially at its surface. Kramer and Demer have observed that the surface layer undergoes greater work-hardening compared to the material’s 20 2. Theory and Background deeper layers during wear [26]. A compelling explanation for this phenomenon related to size is the "hard (de- bris) layer" model, as proposed by Kramer and Demer in 1961 [26, 27]. This model assumes that a layer on the surface, roughly 50-100 micrometers thick, experiences more hardening than the underlying material. Hence, smaller abrasive particles, when interacting with the surface, primarily engage with this hardened layer, facing a tougher resistance than the larger particles. These larger abrasives impact and de- form the layers beneath the surface-hardened layer, inducing more wear. However, there’s a threshold abrasive size, beyond which the impact of the hardened layer becomes insignificant, leading to a plateau in wear rate despite increasing abrasive size. It should be mentioned that this is not an unexceptional case. Some materials ex- hibit regions that are softer in the deformed layer than in the bulk material, as observed by Kramer and Fourie in the case of a single crystal of copper[28, 27]. The study on tool steel used for the hot stamping process also revealed that the material near the worn surface exhibited signs of softening [29]. Thus, it’s crucial to examine this phenomenon on a case-by-case basis rather than making broad generalizations for all wear scenarios. 21 2. Theory and Background 22 3 Materials Background In this section, an overview of the properties and characteristics of Hardox600 and WC materials embedded within a Cobalt matrix is presented. Given the lack of comprehensive data regarding the manufacturing process of sintered WC rings, the focus in this section is primarily on studying the general characteristics manufac- turing procedure of these Metal Matrix Composites (MMC). 3.1 Hardox600 Hardox 600 is a good choice for tasks demanding superior abrasive resistance, due to its impressive hardness level of approximately 600 Brinell [30]. Classified as a medium carbon steel, it contains roughly 0.47% carbon, as illustrated in Table 3.1. The steel undergoes a quenching and tempering process, which transforms it into martensitic steel, tailoring it specifically for extreme abrasive and impact wear situa- tions. While its unique manufacturing process contributes to its distinctive mechan- ical traits, its chemical composition is equally vital. The table below underscores that Hardox 600 comprises various elements that significantly enhance its harden- ability and toughness. Besides C, this steel also contains elements like chromium (Cr), nickel (Ni), and molybdenum (Mo), each of which contributes specific benefits. For instance, hard carbides can be formed by Cr and Mo. Mn is a austenite stabilizer. Mo retard the austenite-to-ferrite and pearlite transformation. Boron also plays a critical role in enhancing steel properties. It significantly enhances steel hardness by hindering the onset of phase transformations through its segregation on austenite grain boundaries [31, 32]. Based on the unique properties in hardness, strength, and toughness Hardox600 can be used in versatile applications of the mining, transport, and construction industries. Table 3.1: Hardox600 Chemical composition[30] 23 3. Materials Background 3.2 Sintered WC/Co Cemented carbides, commonly known as hard metals or cermets, are composite ma- terials designed to combine the hardness of ceramics with enhanced fracture tough- ness. These materials are crafted by sintering a blend of metal carbides. Tungsten carbide, with a hardness ranging from 1900 to 2100 HV, is the most prevalent carbide used, typically set within a cobalt matrix. However, variations with other carbides, like TiC and TaC, known as ternary carbides, are also available. During the sin- tering process, the metal powder(3–30% of the mix) melts and forms a liquid film around the carbide particles, facilitating their bonding (sintering). This results in a composite characterized by remarkable hardness, stiffness, and a fracture toughness significantly greater than most advanced ceramics. The metallic binder, or the softer phase, plays a crucial role in this, as it hinders the initiation and growth of cracks [33] [18]. Cemented carbides are recognized for their remarkable hardness, boasting an elastic modulus three times greater than steel and exhibiting high density. Their hardness values range between 800 and 2000 HV [33] [18]. Notably, when the metallic binder content increases, the fracture toughness gets better, but it diminishes the hardness. The carbide grain size, which typically ranges from 1 to 6 m, is essential in deter- mining their attributes. There are also finer grain sizes in use, such as submicron (0.5–0.8µm), ultrafine (0.2–0.5µm), and nanopowders (<0.2 µm). Generally, a smaller grain size enhances hardness without negatively affecting frac- ture toughness [33]. Carbides are highly resistant to sliding and abrasive wear, even at high temper- atures, making them ideal for machining tools, sliding bearings, seals, and other wear-prone components. However, they come with drawbacks like limited forma- bility and increased cost. To overcome these, powder metallurgy is used to create near-perfect shaped components. Machining techniques, such as electro-discharged machining (EDM) and grinding, are also utilized. Furthermore, the High-velocity oxygen fuel(HVOF) method allows cemented carbides to be used as surface coatings [33]. 3.2.1 Manufacturing process The production of sintered WC/Co involves two main steps: first, creating the el- emental powders, and then sintering these powders to form the solid material. In this section, key parameters involved in the manufacturing process are highlighted. The primary reference utilized is the esteemed book, "Cemented Tungsten Carbides: Production, Properties, and Testing" [34]. 24 3. Materials Background 3.2.1.1 Powder manufacturing To produce WC/Co, it’s essential to understand the creation process of each indi- vidual powder component. In the following section, we’ll provide a concise overview of this process. Tungsten Powder: WC-Co hard metals are mainly composed of Tungsten mono- carbide (WC), obtained from carburizing tungsten, which is produced by reducing WO with hydrogen. Tungsten is primarily sourced from minerals like Scheelite (CaWO) and Wolframite ((Fe,Mn)WO). Due to its high melting point, tungsten is extracted using hydro processes. The extraction process, starting from ore concen- trate, aims to produce an intermediate compound, often tungstic acid or ammonium paratungstate. Before refining, the ore is pretreated to remove impurities. It’s then decomposed to isolate tungsten, forming a compound that undergoes further purifi- cation. The final step, turning this compound into metallic tungsten, requires little purification compared to other metals [35]. Cobalt powder: Cobalt, a crucial metal binder in WC-based cemented carbides, is typically found alongside other metals in the earth’s crust. The primary sources for cobalt extraction include sulphides, arsenides, oxides, and hydroxides. Histori- cally, cobalt powder was produced by reducing cobalt oxide with hydrogen. Modern cemented carbide industries, however, rely on ultra-fine cobalt powders, which are created through the pyrolysis of cobalt salts, like cobalt oxalate. This salt is derived from the reaction between oxalic acid and cobalt chloride. These powders, initially around 2 µm in size, become even finer during milling with carbide, partly due to a phase change in the cobalt’s crystal structure. Milling further reduces the size, en- suring thorough mixing and coating of tungsten carbide particles. A novel method, termed ’Polyol Cobalt,’ has been introduced, utilizing the reduction of cobaltous hydroxide with specific glycols. This process yields micron-sized cobalt particles with a narrow size distribution. Notably, while nickel is reduced alongside cobalt, other impurities like Ca, Na, and S are excluded in polyol cobalt [35]. Tungsten carbide: Once the tungsten powder is ready, it’s mixed with carbon black and heated to temperatures between 1400-1800°C. The resulting WC grain size usually falls between 0.8-7.0µm, optimized for the final sintered hard metal product. Each mixture batch is standardized for consistency, with both grain size and carbon content being key parameters. While the carburizing temperature can influence grain size, it’s vital to maintain exact carbon levels. Insufficient carbon can lead to the formation of a brittle ’eta’ phase, while a high amount of carbon produces weakening graphite flakes. Therefore, for optimal hard metal properties, the carbon content must be strictly controlled. High-quality carbon black used in hard metal production should have low ash (typ- ically 0.1%, indicating impurity levels) and sulphur content, with the latter ideally being below 0.01% due to its significant impact on tungsten carbide grain size. The mixing method of tungsten and carbon black can influence the carbide’s grain size. 25 3. Materials Background While the mixing method is less crucial for individual or mechanically aggregated tungsten particles, it becomes vital if the metal contains a firmly bonded poly- crystalline structure of the original primary tungsten compound used in refining processes. Effective mixing can produce uniform carbide grains, but if the original structures aren’t adequately broken down, it results in larger tungsten carbide par- ticles during carburization, affecting the desired grain size [35]. Carburization Tungsten monocarbide (WC) is primarily produced by carburiz- ing tungsten powder with carbon black. This process requires a specific carbon gas environment, especially in a graphite tube furnace where carbide formation is opti- mized using carbon under hydrogen. An alternative method uses a high-frequency induction furnace, where the carbon-tungsten mixture is rapidly heated. During carburization, the grain size of the resulting carbide is influenced by factors like temperature and the initial grain size of the tungsten. Notably, finer tungsten pow- ders transform differently into carbide compared to coarser ones. The carburizing temperature plays a pivotal role in determining the properties of the final tungsten carbide. Impurities introduced during the process, especially from carbon sources like lamp black, can affect the grain growth and overall quality of the carbide. The purity of the initial tungsten metal also impacts grain growth, with purer tungsten leading to more significant WC grain growth. This is because the formation of monoatomic layers of impurities at the interfaces and grain boundaries alters the rate of diffusion. Some foreign elements can either reduce or enhance this grain growth, depending on their nature. Carburizing fine tungsten powder at lower temperatures (around 1300°) leads to a loose agglomeration of WC grains, reducing the risk of local grain growth during WC-Co sintering. The resulting tungsten carbide’s grain size mirrors that of the original tungsten, and these disturbed WC crystals enhance reactivity during sin- tering. In contrast, carburizing at higher temperatures (around 1600°) causes strong agglomeration, increasing the risk of local grain growth and producing larger WC grains with reduced sintering reactivity. For tungsten powder larger than 2.5µm, carburization transforms the original single crystal into a polycrystalline tungsten carbide particle. During production, various factors can affect the size of single crystal domains in WC particles, with temperature being the most crucial. Higher temperatures en- hance carbon diffusion, leading to larger single crystal domains. These domains, rather than WC particle size, determine the microstructure of sintered WC-Co hardmetals[35]. 3.2.1.2 Consolidation and sintering: To produce a fully sintered WC/Co, several steps are required after the initial powder creation. These steps include milling, pressing, dewaxing (or presintering), sinter- ing, and post-sintering operations. A brief overview of each step is provided [36]: 26 3. Materials Background • Milling: The carbides and metal binder powders are mixed together. The fi- nal product’s mechanical properties are significantly influenced by the uniform distribution of cobalt, which is achieved during this milling process. Moreover, milling produces new active surfaces and enhances the defect structures in both carbides and the metal binder. These freshly formed surfaces are highly reac- tive to gases in the surroundings. • Granulation: After milling, the carbide slurry dries into a very fine powder that doesn’t flow easily and has a low density. This makes pressing challenging because of the friction between the fine particles. To overcome this, the pow- der is granulated, forming loosely-bound clusters. These granulated clusters are generally coarser and rounder, improving flow and making the pressing step more efficient and quicker. Various granulation techniques include com- pression, rotation, spray drying, and vacuum drying, each offering its own advantages and disadvantages. • Green consolidation: Green compacts are formed by pressing powder with an external force, usually in the range of 21-42 kg/mm2, to achieve aspecific shape and maintain dimensions. These compacts typically reach around 60% of their potential density. While hard metal components can be almost fully densified during liquid phase sintering, starting with a denser green compact minimizes shrinkage and post-sintering grinding. Double-acting presses, which offer more consistent density distribution, are preferred over single-acting ones. For the compaction process, both hydraulic (ideal for larger parts) and me- chanical presses are used. Occasionally, isostatic pressing is chosen for its even pressure application, especially beneficial for bigger components or when aim- ing to minimize pressure loss. For components like drills and reamers, which have a significant length-to-diameter ratio, extrusion is a favored method. • Dewaxing and presintering: During hardmetal manufacturing, paraffin wax is mixed with the powder to ease the pressing process and protect the material from damage during handling. Once the pressing is done, the wax, now redundant, is removed by heating, either in a hydrogen atmosphere or in a vacuum. It’s vital that this removal process doesn’t compromise the integrity of the compacted material. The industry predominantly uses fully refined waxes, comprised of compounds like normal paraffins and isoparaffins. These waxes melt between 40-50°C. The process of wax elimination starts at about 150°C, but to thoroughly remove it, temperatures between 250-300°C are necessary. Notably, in environments like hydrogen, the wax remains stable and doesn’t break down until temperatures exceed 400°C. During the production of hardmetals, components are first shaped and then undergo a final sintering process. These components are initially pressed into basic forms, such as rectangles or circles, and are later refined using techniques like turning, drilling, and grinding. It’s important to account for an antici- pated shrinkage of approximately 20-25% during the final sintering. 27 3. Materials Background To prevent breakage during the shaping process, the components are presin- tered at temperatures ranging from 750-1000°C, enhancing their strength. This presintering step can be intricate due to the fine quality of the pow- der, potentially causing changes in its composition. • Sintering: In the WC-Co composition, a unique liquid known as eutectic forms at 1320°C. The common sintering temperature for the popular hard- metal variant, WC-10 Co, is set at 1400°C. At this temperature level, around 15% of the mixture turns into a liquid phase, where WC blends into cobalt. As the temperature drops, WC particles start to solidify, causing the grains to grow larger. Interestingly, no eutectic structure emerges during the sintering process. When the liquid phase becomes solid, it retains 20-25% of WC, but this percentage decreases as the temperature goes down. Sintering WC-Co hard metals involves a series of transformations. Even at sub- dued temperatures, cobalt either envelops or seeps into the carbide particles. As the heat intensifies, cobalt starts dissolving the carbide surfaces nearby. This action paves the way for WC particles to rearrange more closely, leading to a noticeable contraction in size, especially between 800°C and 1250°C. Several key variables can significantly affect the sintering process, including the sin- tering atmosphere, the temperature and duration of sintering, the rates of heating and cooling, and the presence of impurities. To provide a summarized overview of the consolidation phase in the manufacturing of WC/Co, we can refer to the flowchart 3.1, which highlights the key aspects of each step. Figure 3.1: Consolidation flowchart of WC/Co Modified from [36] 28 4 Test Procedure and Methods For measuring and investigating the wear, a laboratory-scale crusher machine has been employed. This crushing apparatus operates by a unique technique that exclu- sively uses normal force for crushing the ores. The figure below 4.1 illustrates the front view of this machine. Two test rings are mounted to a shaft by means of a clamping system. Each of the rings slides onto a split inner ring, which is bolted to a conical shaft. When tightening the screws, the inner ring expands and grips the outer test ring. The test rings have an outer diameter of 125 mm and 30mm width. The gap between the shafts is adjustable. By means of this, the gap between the rolls can be changed for different tests. (a) Front view of the rings (b) Clamping system of the rings Figure 4.1: Overview of the rings Feed material will be directed to the rings using a feed system. The feed system consists of a hopper, an adjustable discharge gate, and a controllable belt feeder. Using adjustment screws more control on the feed has been achieved which is cru- cial for attaining an optimal falling angle and grip between the rings and the feed material. Materials pass through an adjustable gate and with a feed belt pores down to the spinning rings. Depending on the test gate height, belt speed and gap between the rolls will change. Further details on these factors will be provided in subsequent sections. 29 4. Test Procedure and Methods 4.1 Test Procedure The test procedure will be explained in two different sections of wear experiments when wear is introduced to the material and then post-wear tests that have been conducted to understand the characteristics of wear by exploring the material’s surface and microstructure. 4.1.1 Wear Experiment As mentioned in the previous sections the mechanism behind breaking the ores is based on compression and using a normal force to all the ores passing the gap be- tween the rolls. This compression force is accommodated by having gap distances smaller than the feed particle size. The primary factor in the present wear process is the loss reduction of rings after processing a predetermined quantity of ore in the testing apparatus. It’s important to clarify that while measuring the reduction in volume of the rings might be a more accurate way to measure wear for this application, the lack of specific density in- formation for each material made it necessary to use mass loss as the measurement instead of volume loss. Wear assessment will be conducted through a sequence of tests, involving the alter- ation of the gap between the rings. In each test, the particle size of the feed material will exceed the gap between the rings. This discrepancy in reduction causes the par- ticles to undergo compression and eventually break down. Wear evaluation is carried out by performing a series of progressively smaller gaps, using the product generated from the prior test as the feed material for the subsequent test, which features a reduced gap. This method ensures a thorough examination of wear under various conditions and contributes to a comprehensive understanding of the wear mecha- nism. Additionally, considering the various materials being evaluated for wear, differences in wear properties necessitate an appropriate volume of feed material to effectively discern the distinctions among them. This ensures a comprehensive assessment of the wear performance of each material under the test. The ring gap is precisely set using feeler gauges. However, difficulties arise due to the rings being somewhat out of round and the mounting surface having similar irregularities. This can result in a deviation as the rings rotate. Furthermore, a skewness between the rings can create a difference in the gap at one end of the ring compared to the other. During wear evaluations, the feed rate is maintained to ensure that particles en- ter the rolls in a single layer such that particles are crushed independently. The rings’ rotation speed is set to coincide with the falling particles’ speed. Previous ex- periments determined this to be approximately 200 rpm, which is used for all tests. 30 4. Test Procedure and Methods Fines accumulate in a cyclone, but the wear apparatus’s air system is designed solely to minimize dust escape, not to extract the final product. The feed hopper is filled with the entire test sample, and the feed rate is set to provide the appropriate feed rate to the rings for the specific gap. The data logger is activated, and the test commences, with samples collected randomly throughout the test. A remote control and test stop system allows the equipment to be stopped remotely, as tests can take up to 12 hours for the finest feed. The flowchart of the test procedure can be seen in the figure below 4.2. Figure 4.2: Wear experiment procedure flowchart 31 4. Test Procedure and Methods Following the test, the rings are removed, cleaned in an ultrasonic bath, dried, and then weighed. The feed and product samples are sized to evaluate the reduc- tion extent. To perform a representative and reproducible wear analysis on various materials, a consistent test plan is implemented to measure wear for tested materials. Each test consists of six individual tests. For having consistent and similar condi- tions in all tests, a compression of 0.8 has been chosen for the tests. Compression 0.8 means that first the feed material(ores), with a particle size of 1mm, will be discharged and crushed with a gap distance between rings that is set to 0.8 mm. This is considered as the first run of a whole complete test. The rings after this test will be weighed. 4.1.2 Sample Preparation To study the wear behavior, each test was followed by a thorough examination of the samples. After the wear tests were completed, the rings were cut to gather two types of samples. The first type of sample was used to investigate the charac- teristics and topography of the worn surface, providing insights into how the wear process affected the material surface. The second type of sample was consisted of cross-sectional cuts from the rings, which were used to examine the microstructure of the tested materials and understand how the wear process may influence( or be influenced) by the internal structure. A schematic illustration of the samples taken for analysis can be seen in Figure4.3. The samples for microstructure analysis on the cross-section were grinded, polished, and then etched. The etchant used was, Nital, and Murakami’s reagent for the sintered WC/Co, as recommended in the literature [37]. 4.1.3 Surface Analysis Due to the size and shape of the rings, observation of the wear surface after each test was limited. After completion of each test, the surface and cross-sectional fea- tures were examined for the tested rings. SEM, EDX, and BE were used. Optical and stereo microscopy were also performed for a more detailed examination of the samples. To monitor the topographic changes of the surface during the wear tests, a stereo microscope was used before the start of each test, in the third run, and in the last run. This approach helped to better understand the surface changes throughout the testing process, which allowed for a comprehensive analysis of the wear behavior of the materials. To ensure a more reliable analysis, it was tried to examine the same area after each test. This was particularly important for the Stereo Microscope tests, which were designed to monitor the progression of wear for each material throughout the tests This information is essential for the development of improved wear-resistant mate- 32 4. Test Procedure and Methods Figure 4.3: Schematic of two different samples taken from rings: a)from the worn top surface of the ring b)from a cross-section of the ring rials and coatings, and the selection of suitable materials considering manufacturing and application constraints. 4.1.4 Micro-hardness The micro-hardness test was carried out on all the materials tested in this study. The purpose of this test was two-fold: firstly, to compare the hardness values of the tested materials and secondly, to create a hardness profile across the thickness of the tested rings on the cross-section(refer to Figure 4.3). The hardness profile was obtained by making indentations across the width of the cross-section samples. The indentations were positioned depending on the thickness of the rings, with each indentation being 0.7mm apart from the next. All tests used the Vickers hardness testing method with a load of 5kg (HV5). To ensure the reliability of the tests, several rows of indentations were applied. The average values derived from these multiple indentations were then used to generate the overall hardness profiles. 4.2 Test Plan To achieve a representative and reproducible wear analysis across various materials, a systematic test plan has been implemented to measure wear for the materials being tested. This plan consists of six distinct tests for each material, ensuring compre- hensive investigation and analysis. A consistent compression ratio of 0.8 is employed for all tests to maintain unifor- mity in test conditions. This compression ratio facilitates six individual runs within 33 4. Test Procedure and Methods a complete test, an optimal number for thorough material examination. With a 0.8 compression ratio, the feed material with a particle size of 1 mm is initially subjected to a gap distance of 0.8 mm between rings, signifying the first run of a complete test. In subsequent tests, the gap is reduced by a fraction of 0.8, resulting in tests conducted at 0.8, 0.64, 0.51, 0.41, 0.33, and 0.26 mm gap distances between rings. The tests commence with approximately 60 kg of feed material, which is consid- ered an appropriate weight for several reasons. It allows for a reasonable timeframe for each test to be completed and enables the wear differences among the various materials to be clearly observed. The tests consistently used "Quarry rock" as the feed material. While it’s chal- lenging to specify the exact composition for each test, it is known that over a third of the material is Quartz, a notably hard mineral. References indicate that the equivalent Vickers hardness of quartz ranges between 750-1200 HV [38, 18]. During each test, random samples were collected from the crushed rocks to examine their particle size distribution (PSD). Analyzing the PSD not only provided insights into the specific patterns of particle sizes for each run, but also helped us gain a better understanding of the machine’s crushing accuracy and consistency between each trial. Additionally, this analysis allowed us to identify any potential areas for improvement or optimization, ensuring that the machine operates at its highest level of efficiency when processing rocks. All of the PSD graphs can be find in Appendix C. As discussed earlier three different types of materials are used for the wear in- vestigation. Hardened Steel: • Hardox 600 Sintered WC/Co with different WC percentages • Sintered X1, with 60% WC • Sintered X2, with 90% WC 34 5 Results The results from the experimental tests and post-wear analysis of each material will be introduced in this chapter. 5.1 Wear experimental results After carrying out the experiments according to the plan detailed in prior sections, the wear findings for the rings that were tested are provided and illustrated in the figure 5.1. Figure 5.1: Comparison of wear between different tested materials Examining the wear results, we observe that wear increases for all materials as test- ing progresses with reducing the gaps. This rise is expected since more number of individual rocks need to be crushed, leading to greater wear on the rings being tested after each run. The wear growth pattern predominantly follows an exponen- tial trend rather than a linear one. However, an exception to this observation is the 35 5. Results Sintered X2, which exhibits an almost linear trend, but with a significantly lower rate of wear compared to the other materials tested. This observation highlights the importance of understanding wear behavior through- out testing, particularly during the final stages, for accurate crushing machine eval- uation. The graph reveals that the wear rate does not consistently increase between two con- secutive individual runs; instead, there are instances where the wear rate decreases before increasing again. This pattern is particularly noticeable in the sintered rings, and it would be intriguing to investigate the cause of this behavior. It’s important to consider that by reducing the gap in each run, more rock par- ticles must be crushed by the rotating rings, even though the total surface area of the rings involved in crushing remains similar in each test. In simpler terms, as the gap between the rolls decreases, the rock size decreases as well, meaning more particles come into contact with the ring’s surface. This could be one reason for the exponential growth of wear during a test. As we explore more details about the wear mechanism in the following sections, this trend will become clearer. According to the findings, the wear resistance ranks from highest to lowest as follows: sintered X2, sintered X1, and Hardox 600. Sintered X2 demonstrates the highest wear resistance among the tested materials. Various factors could explain the differences in wear resistance among the materials. These factors may include the composition of the materials, their microstructure, the methods used to create them, and their hardness. Another key aspect to con- sider is the wear mechanism and how each material behaves (the type of wear and the process behind material removal) under this specific wear condition. In this chapter, each material will be analyzed in terms of the factors mentioned, and then compared with one another to have a better understanding of this specific wear. 5.2 Characterization of Hardox 600 Hardox600 was the first material tested for surface examination. Using SEM, EDX, and Stereo microscopy helped track wear progress during the tests, giving a clearer understanding of how material was removed. These findings will be introduced in the following sections. 5.2.0.1 Microstructure After completion of the wear test, the ring was cut and a cross-section was prepared for the microstructural analysis. For the etching Nital was used. Being a quenched and tempered steel, the expected microstructure of Hardox 600 is martensite, which is what was found. The presence of longitudinal tempered martensite laths is visible in the microstructure. The rolling direction of the original sheet plate is visible with the presence of wide longitudinal martensite bands. 36 5. Results As mentioned the Hardox600 is a sheet plate which is made by rolling. The figure 5.2 shows noticeable longitudinal martensite bands(wide light lines in the microstruc- ture). These bands could potentially indicate the direction in which the original sheet plate was rolled. It’s worth noting that the crushing direction for the rings aligns with this as well. Given that it was not possible to examine the microstruc- ture of the rings before the wear test, drawing any concrete conclusions regarding this aspect is challenging at this point. Figure 5.2: Martensite structure with presence of bands observed by optical microscope The microstructure reveals a distinct contrast between the areas near the worn sur- face and the more distant regions, or middle parts, as shown in figure 5.3. The etching effect is more pronounced at the locations close to the surface. This no- ticeable contrast might come from various factors. For example, one possibility is that the etchant might have become trapped in the space between the mounting and the sample due to the unevenness of the worn surface, which caused prolonged contact of the etchant leading to more overetched-like microstructure. However, other hypothesis should also be considered: • Heat during crushing: During the crushing process, the significant heat generated can alter the microstructure near the surface. Therefore, the ob- served inconsistencies in the microstructure and etching response might be attributable to variations in the heat process. • Plastic deformation: The process of crushing abrasives could introduce in- ternal stresses close to the rings’ surface. This stress field could trigger plastic deformation, which in turn might lead to strain hardening of the surface ar- eas. Consequently, this work-hardened regions could contribute to observable differences in the microstructure after etching. In figure 5.4, a comparison of the microstructure from the surface and bottom on the cross-section of the samples is presented using SEM imaging. It’s noticeable that the surface region has undergone significant grooving, a feature that will be further 37 5. Results (a) Near surface (b) In distance from the surface (c) Overall view of the microstructure Figure 5.3: Optical microstructure difference in the cross-section between surface regions and distant from the surface explored in the surface analysis section. It becomes apparent that martensite bands near the surface have been deformed, aligned along the direction of the applied stress. However, this contrasts with the bottom regions of the ring, where the formation of martensite blocks does not appear to be altered. Based on these observations, we can infer that there might have been some degree of plastic deformation near the surface, possibly leading to a degree of work hardening. Further investigation, including a micro-hardness test detailed in the upcoming section, will provide additional insights to support or challenge this hypothesis. 5.2.1 Micro-hardness As noted in the earlier section, it is plausible that a degree of plastic deformation may have occurred, especially in regions near the surface. To explore this hypothesis, a micro-hardness profile test was performed. Prior to delving into any results, it should be highlighted that Hardox 600 has undergone a quenching and tempering process, which would likely impact the hardness results. The sample that was used for this test is the cross-section sample (refer to figure 4.3). Three rows of indentations on the width of the sample, one in the right side,middle and left part of the sample, has 38 5. Results (a) Near surface of the ring (b) Near bottom of the ring Figure 5.4: SEM comparison of surface and bottom region on cross-section been applied. Detailed results of each row can be seen in appendixB The average of these has been considered as the overall values for the hardness profile. In figure 5.5, you can see the micro-hardness profile along the thickness of the Hardox 600: Figure 5.5: Micro-hardness profile through thickness of Hardox600 As can be seen, the hardness profile shows a steady increase from the bottom to the surface region. Considering the impact of heat treatments, the observed hardness increase, particularly close to the worn surface regions, corroborates the hypothesis of work hardening in these areas. A notable observation is the significantly lower hardness in the initial indentations close to the sample’s bottom edge. Even though they are 0.3mm away from the edge, this pronounced difference could be attributed to the edge effect in the micro- hardness test. 39 5. Results 5.2.2 Stereo Microscope observation of the worn surface As mentioned in the 4.1.3 the only way to monitor the wear during the test was the stereo microscope. In the figure 5.6. After the third run, the ring surfaces were noticeably roughened, as illustrated by the transformation in topography. Some dark spots, which appear as holes in the rings, are visible in figure 5.6a. Upon closer examination using higher magnification, it was found that these dark spots are caused by significant variations in surface levels across different areas. (a) 7.5x magnification (b) 15x magnification (c) 25x magnification Figure 5.6: Stereo microscopy pictures of Hardox600 rings after third run After six run some small changes in the surface can be seen, as shown in figure 5.10. But, these changes aren’t very substantial. Looking at the pictures, it is not clear if the surface has gotten rougher or not. It seems like the surface is still not even, just like it was before(Run 3). The most noticeable alteration was the emergence of significant disparities in surface level, almost resembling material peeling off. This change was observed in various regions of the rings after the sixth run, a feature that wasn’t apparent after the third run. (See figure 5.7c) 40 5. Results (a) 7.5x magnification (b) 15x magnification (c) 25x magnification Figure 5.7: Stereo microscopy pictures of Hardox600 rings after sixth run 5.2.3 SEM and EDX analysis To study the worn surface, sample a was used(refer to figure 4.3). The analysis consists of SEM, EDX, and BE to gain a comprehensive understanding of the wear occurring on the Hardox surface. A broad view of the surface can be found in figure 5.8, where the surface roughness is quite apparent. Given the challenges of distinguishing between Hardox material and abrasives, BE was used to clarify the features. The areas appearing darker in the image signify the embedded rocks, while the brighter areas show the worn surface of the Hardox600 material. As displayed, about half of the area (in the context of this image) is embedded with abrasive particles(rocks). The combination of the rings’ relative movement and the contact pressure exerted between the rings and rocks during rock grinding could account for this embedding phenomenon. As will be discussed subsequently, these embedded rocks play a crucial role in understanding the wear mechanism and how the material is removed during the wear process. 41 5. Results (a) SE image of surface (b) BE image of surface Figure 5.8: SEM image of Hardox600 surface with presence of embedded rocks To gain a clearer understanding of the surface’s characteristics, EDX was performed for a chemical mapping of the surface. The comparison of the BE image and chem- ical map can be seen in Figure 5.9 These analyses complement each other as the embedded rocks, represented as dark regions in the BE image, also appear as Si elements in the chemical map. Interestingly, varying shapes of these embedded rocks can be seen on the surface. Two distinct forms are present on the surface: one displays a flake-like shape, while the other exhibits a striation-like pattern. Figure 5.9: Chemical mapping of Hardox 600 surface: a)BE image b)Chemical map C)Element image Apart from the embedded rocks, the SEM images offer insights into the Hardox material itself. It appears that the surface predominantly suffers from a cutting mechanism, as discussed previously. Evidence of material displacement, or ’plough- ing,’ can be seen in figure 5.10d with the direction of this ploughing mostly aligned 42 5. Results with the direction of rock crushing. An understanding of the wear process can be drawn from the study of both the embedded rocks and the material displacement. It is hypothesized that following the initial contact and impression of the sharp abrasive into the material, rotation is induced. This causes a sliding across the surface, leading to observed ploughing. These ploughs often appear to be trapped between various embedded rock regions. (Refer to figure 5.10d) Further examination suggests a hypothesis. It is suggested that once these embed- ded rock regions are formed, any subsequent ploughing continues until encountering these embedded rocks. These rocks, therefore, may act as a barrier, inhibiting the ploughs from further cutting into the surface. An additional noticeable feature on the surface is the presence of material slivers dispersed across it. These slivers exhibit a flake-like appearance, almost suggesting a separation or detachment from the surrounding material. This particular charac- teristic can be attributed to the impact of the abrasives on the surface. As described, the crushing mechanism involves a cyclic rotation of the rings, thus continuously crushing the rocks. The compression of the rock against the surface, which indents the sharp abrasives into it, is likely to induce a certain degree of deformation, as previously proposed. Each cycle of this repeated deformation, ac- companied by the indentation of the abrasives, results in a tearing effect on the material. This is likely to contribute to the observed delamination and the forma- tion of slivers. 43 5. Results (a) Slivers SE (b) Slivers BE (c) Ploughing SE (d) Ploughing BE Figure 5.10: SEM material removal 5.3 Characterization of sintered X1 and X2 In this study, two different variants of WC with Co matrix are employed. The primary difference between them lies in the proportions of WC and Co - the sintered X1 material contains a lower proportion of WC and a higher proportion of Co compared to the sintered X2. As these materials were crafted specifically for this wear application rather than being commercially available products, there is limited information about the sintering process and its parameters. First, a detailed examination of the materials’ microstructure will be conducted. This will be followed by an analysis of the surface features, just as was carried out for the Hardox600 material. 5.3.1 Microstructure After the completion of the wear test, cross-section samples were extracted from the Sintered X1 and X2 rings. These samples were subjected to a sequence of grinding, polishing, and etching. The microstructures of the sintered materials, X1 and X2, are depicted in Figure 5.11. Here, one can observe the dispersion of WC particles within the Co matrix. The WC particles are irregularly shaped, with various sizes. 44 5. Results (a) Sintered X1 (b) Sintered X2 Figure 5.11: Microstructural analysis of sintered WC with Co matrix: a)SEM image b)Chemical mapping c)Elemental mapping of the cross-section sample Upon comparing the microstructures of X1 and X2, it’s evident that the WC parti- cles in X1 are considerably larger than those in X2. In addition, the dispersion of WC particles in X2 is more uniform, demonstrating less size variation between the largest and smallest WC particles than X1. To provide a more quantitative analysis, the ImageJ software was utilized to deter- 45 5. Results mine the sizes of the WC particles and calculate the respective ’Area’ values. The data pertaining to the measured particles can be found in Table 5.1. Table 5.1: Comparison of WC particle size between sintered X1 and X2 WC Place WC X1 Area [µm2] WC X2 Area [µm2] 1 27,296 4,362 2 32,182 2,557 3 21,711 1,689 4 28,612 2,915 5 25,669 4,531 6 40,589 4,312 7 9,866 1,596 8 15,735 6,541 9 5,704 4,1947 10 6,059 1,996 11 1,304 1,773 Corresponding to the measured WC particles from Figure 5.11 and Table 5.1, we can see that sintered X1 has WC particles from largest 40 µm2 to smallest 1 µm2. This is in contrast with the sintered X2 as the largest WC is 6.5 µm2 and smallest 1 µm2. Figure 5.11 presents the EDX analysis results, offering insights into the chemical mapping and elemental breakdown of the observed surface. This analysis reveals the distribution and presence of various elements such as Tungsten, Cobalt, and Carbon, among others. The comparison of the overall elemental compositions for the sintered materials X1 and X2 is provided in Table 5.2. Table 5.2: Elemental difference between sintered X1 and sintered X2 Elements Sintered X1 (Wt%) Sintered X2 (Wt%) Tungsten (W) 68,2 77 Cobalt (Co) 8,7 10,6 Ni(Ni) 9,4 1,7 Titanium (Ti) 0,4 0,3 Given the superior wear performance of the sintered X2 material compared to the other materials tested, we can propose the following hypotheses about the wear resistance of sintered rings: • Tungsten Carbide Content: Increasing the amount of tungsten in the ma- terial appears to enhance its wear resistance. • Size and Dispersion: Utilizing smaller WC particles and achieving a ho- mogeneous distribution of these particles within the matrix seem to boost the wear performance. We should note that limitations inherent to the manufacturing process of these materials have not been factored into this comparison. Despite the two materials 46 5. Results having been produced by the same manufacturer, numerous manufacturing param- eters could differ, such as the conditions under which sintering occurred, the type of powder and matrix employed, and more. Each of these factors could considerably influence the overall performance of the material. (a) SEM image for Sintered X1 (b) SEM image for Sintered X2 Figure 5.12: Comparison of the microstructure near-surface region on cross-section Figure 5.12 shows the microstructure from the surface regions of the sintered X1 and X2 rings on the cross-section. On first inspection, we see noticeable differences in the extent of wear between the sintered rings. The surface of the sintered X1 ring is visibly rougher than that of the X2 ring. This observation is supported by the unevenness across the top layer of the X1 cross-section, indicative of its worn surface. Upon closer examination, we notice that wear has caused extensive damage to the WC particles in the X1 sample, resulting in significant material removal from the surface. This level of wear is not as prominent in the sintered X2 sample, which aligns with our experimental wear results. A more detailed look at the SEM images reveals additional differences. For the sintered X1, we see micro-cracks and fractures within the WC particles near the surface. In contrast, the sintered X2 ring shows a different worn surface: WC particles near the surface have merely detached from the matrix binder due to the stresses incurred during crushing, with far fewer observed fractures within the WC particles. 5.3.2 Micro-hardness Similar to the procedure employed for the Hardox600, we also evaluated the hardness through the thickness of the sintered rings. The cross-section samples were utilized for this examination. We performed hardness tests in three rows, each indentation separated by a distance of 0.7mm along the thickness. Detailed hardness values for 47 5. Results each of these rows can be found in Appendix B. Figure 5.13 illustrates a comparison of the hardness profile through the thickness of both sintered X1 and X2 rings. It can be observed that there are variations in the hardness throughout the thickness. These fluctuations can be attributed to the inherent non-uniformity of the sintered material, with possible variations in the dis- tribution of matrix and hard phases. This suggests that regions of lower hardness might have a higher binder content and a lower pr