Electron Microscopy Investigation of Detailed Microstructures of CVD TiAlN and TiN Coatings - Effects of Gas Flow and Substrate on Coating Microstructure

Licentiate thesis, 2020

This thesis contains studies of topics of relevance to increase the understanding of TiN and TiAlN coatings synthesized by chemical vapour deposition (CVD) and low pressure CVD (LPCVD), respectively. This work contains two parts: (i) nanolamella LPCVD TiAlN coatings that were found to show a spatial variation of their chemical composition and microstructure, which was influenced by the gas flow speed of reactants; (ii) microstructure of CVD TiN coating deposited onto a CoCrFeNi multi-principal element alloy (MPEA)substrate.
Depositing homogeneous TiAlN coatings on cutting tool inserts to achieve extraordinary mechanical performance and good chemical stability remains a challenging task. In this work, the detailed microstructure of high-Al cubic Ti1-xAlxN coatings (average x = 0.8) made by LPCVD was studied by state-of-the-art electron microscopy. A periodic structure with Ti-rich and Al-rich nanolamellae was found. Ti enrichment appears at high gas flow rate, which is suggested to be due to the increase of mass transport of certain reactants. However, the higher mass transport does not influence the Al-deposition much, as it is mainly limited by surface kinetics. A co-growth of h-AlN and TiAlN was investigated by electron microscopy, which showed that the h-AlN prefers to form on the Ti-rich cubic phase following a specific crystallographic orientation relationship: [1 1 0]cub // [2 -1 -1 0]hex, and (0 0 1)cub // (0 0 0 1)hex. This can be explained by a low interface energy between the h-AlN and Ti-rich cubic phase according to density functional theory (DFT) modelling. The microstructural inhomogeneity of TiAlN coatings can therefore be explained by the co-growth of the h-AlN phase with the Ti-rich cubic phase, which is caused by a fast gas flow.
An electron microscopy study was done on the microstructure of TiN coatings grown on a CoCrFeNi MPEA substrate. The MPEA shows a good capability of serving as a CVD substrate. No severe etching of the substrate was found, and the growth of TiN was not disturbed by the substrate material. However, grain boundary diffusion of Cr into the TiN coating was revealed by analytical electron microscopy.