Detailed Microstructure and the Influence of Post-Treatment on CVD TiAlN Wear Resistant Coatings

Examensarbete för masterexamen
Master's Thesis
Materials engineering (MPAEM), MSc
Mead, Monica Audrey
The deposition of TiAlN by chemical vapour deposition (CVD) for the application as wear-resistant coatings for cutting tools has been the subject of research interest for numerous years, yet a comprehensive understanding of its growth mechanism and intricate microstructural characteristics remains incomplete. Furthermore, although the positive effect of blast-treatment on the stress state of wear-resistant coatings has received attention in scientific studies, there remains a relative sparsity of research investigating its influence on the microstructure. In this work, the detailed microstructure of nano-lamellar low-pressure CVD TiAlN coatings on cemented carbide substrates and the influence of post-treatment on the microstructure was investigated by scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM) and transmission Kikuchi diffraction (TKD). SEM imaging revealed two distinct types of surface morphologies corresponding to specific grain orientations unveiled by TKD on thin foil cross-sections. As previously shown [1], pyramidal surface morphology is connected to growth in a <111> direction, additionally, a ridge-like surface morphology was connected to growth along <110> directions. Both growth directions enable fast growth with low-energy {100} facets. Furthermore, truncation of pyramidal and ridge-like surface morphologies was observed. A growth mechanism was proposed aiming to explain the characteristics of the truncated grain morphology. Here, strongly facetted surfaces emerging on the truncated grains increase the surface reaction kinetics, leading to an increased Al/Ti-ratio in the core region of the grain compared to its sides with large {100} facets. Blast-treatment of the TiAlN coatings with corundum particles led to plastic deformation up to a few hundred nanometres in depth. Impact jet wear introduced a high defect density and crack or void formation beneath the surface. Plastic deformation of the near-surface region of the coating led to the bending or disappearance of the Ti- and Al-rich lamellae typical in TiAlN coatings prepared by CVD and a continuous lattice rotation in three dimensions. The available data indicates that grains which have grown along one of their <110> directions exhibit a more pronounced lattice rotation when compared to grains that have grown along a <111> direction.
TiAlN , LPCVD , lamella , SEM , STEM , TKD , film growth , texture , blasting , plastic deformation
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