Effect of microstructure and phase constitution on mechanical properties of Ti1-x Al x N coatings

Monolithic Ti-Al-N coatings with varying Al content in the range 0-65at.% were deposited by cathodic arc evaporation. The variation in mechanical properties was studied by nanoindentation and scratch testing, and correlated with the phase constitution, grain size and residual stress. The hardness was found to be nearly stable up to Al content of 53% followed by a large drop at 65%. Depending on the stoichiometry, phase constitution and microstructure of the Ti1-x Al x N coatings, the mechanical property measurements were observed to reveal distinct trends at particular Al contents-ranging from a large scatter to clustering of data around specific values. Focused Ion Beam milling and Transmission Electron Microscopy studies showed a gradual change in microstructure, from large columnar grains in TiN to finer columns at intermediate Al content and near equiaxed, ultrafine grains with a nanocomposite structure in case of Ti0.35Al0.65N. Scratch studies revealed the deformation modes to vary with Al content, with the ductile failure modes at low Al content giving way to brittle failure at the highest Al content. Toughness studies showed a gradual increase in toughness with Al%, with the maximum seen at 53% and a moderate drop seen at 65%. The toughness shows a close dependence on the mechanical properties, phase constitution and microstructure. The study outlines the role of Al content on the microstructure of PVD TiAlN coatings and highlights the advantage of a cubic, nanocomposite structure for enhancing the toughness of these coatings.