Microstructural investigation of CVD TiAlN, TiN and WN coatings

Doktorsavhandling, 2021

High speed machining of workpiece materials puts extreme thermal and pressure loads onto the cutting tool inserts, which thus must possess both high hardness and good toughness. Nowadays, most cutting tools are made of cemented carbide substrates that are coated with wear-resistant coatings. Chemical vapour deposition (CVD) is a widely used industrial method for producing the wear-resistant coatings, and has advantages like conformal coverage of irregular shapes and high purity of the deposited materials. However, CVD is a complex process and the coating growth is therefore not fully understood.

This thesis focuses on examining topics of relevance to increase the understanding of hard nitride coatings synthesized by CVD. The main research methods are analytical transmission and scanning electron microscopy (TEM and SEM), with complementary X-ray diffraction (XRD), atom probe tomography (APT) and simulations. Three types of CVD coatings were studied in this work: TiAlN coatings, TiN coatings and WN coatings.

The work on TiAlN coatings: (i) First, the growth facets and texture were revealed. (ii) Second, an effect of precursor gas flow on the growth of the coating was studied, including the correlation between the formation of a nanolamella structure and the rotating precursor gas supply, and a microstructural inhomogeneity relevant to the varying gas environment. (iii) In addition, the full chemical composition of the TiAlN coating was studied via APT and electron microscopy. (iv) Finally, an intra-grain misorientation that forms in TiAlN during the CVD growth, and the formation of relevant dislocations, was studied. The work on TiN coatings: (i) The microstructure of the CVD TiN coatings deposited on a CoCrFeNi multi-principal elemental alloy (MPEA) substrate was studied, and (ii) the etching effect of the corrosive gas environment on the MPEA substrate was evaluated. The work on WN coatings: The microstructure and grain morphology of WN deposited on a (0001) sapphire substrate, especially the influence of deposition temperature on the microstructures, were studied.

In conclusion, the results presented in this thesis provide insights into the detailed microstructures of TiAlN, TiN and WN coatings, which will increase the understanding of the growth mechanisms for these CVD coatings.