Functional Coatings through Axial thermal spraying of powder-suspension feedstocks
Typ
Examensarbete för masterexamen
Program
Materials chemistry (MPMCN), MSc
Publicerad
2020
Författare
Örnfeldt, Louise
Modellbyggare
Tidskriftstitel
ISSN
Volymtitel
Utgivare
Sammanfattning
Thermal spraying is a widespread method for applying coatings on top of a material to enhance, protect or add certain properties. As the standard has been to use one feedstock, often in powder form, an interesting area to explore is the effect of adding a supplementary feed during the coating process. Recent studies have also shown several advantages when axial loading has been used. Thus, this thesis has studied the influence of a simultaneously sprayed hybrid powder-suspension as well as premixing two different suspension feeds before spraying. Additionally, the influence of the spray distance on the coating and the effect of adding different amounts of hBN wt% in suspension feed have also been examined. HVAF and APS in an axial setup were the applied coating techniques with NiCr-Cr3C2, Cr2O3, water and hBN used as the feedstocks.
Results show that when spraying NiCr-Cr3C2 powder together with either a water or hBN suspension feed in a HVAF setting, more delaminations occur. There are also more particles with sharper edges embedded into the coating when a liquid feed has been added to the spraying process. Different Ni-Cr and Cr-C phases are produced in all of the NiCr-Cr3C2 containing coatings, a Ni-rich phase and a Cr-rich phase. All NiCr-Cr3C2 sprayed coatings show a homogeneous distribution of the phases with both fine and globular pores in the coating. Cracks in the different phases can also be seen, especially in the Cr-rich phase.
SEM imaging of Cr2O3 coatings sprayed together with a 5% hBN suspension in an APS setup reveals that varying the spray distance between 100 – 130 mm influences topography and microstructure of the coating. When a shorter distance is used, bulges on top of the coating are formed with a higher pore content underneath them as well as larger vertical or diagonal cracks in the coating. All Cr2O3 sprayed coatings show a homogeneous distribution with a laminar microstructure containing a mixture of fine and globular pores embedded into the coating. Moreover, all Cr2O3 coatings showed splats that they were prone to cracking. For most of the hBN sprayed coatings, only small amounts of hBN could be detected by SEM-EDS.
Beskrivning
Ämne/nyckelord
Thermal spraying , coating , hybrid powder-suspension , HVAF , APS , SPS , NiCr- Cr3C2 , Cr2O3 , hBN