Effect of Ti to Al ratio on the crystalline structure and hardening of a Ti1 − xAlxN/CrN nanoscale multilayered coating

The crystalline structure and hardening of Ti1 − xAlxN/CrN nanoscale multilayered coatings with various Ti to Al ratios have been investigated. The Ti1 − xAlxN/CrN coatings with different Al contents were deposited on M2 HSS (high speed steel) by alternating the deposition of Ti1 − xAlxN and CrN, using reactive D.C. magnetron sputtering. The Al content (x) in the Ti1 − xAlxN coatings was controlled to be 0.263, 0.526 and 0.702 by using Ti1 − xAlx alloy targets with varying Ti to Al ratios. For the Ti1 − xAlxN/CrN nanoscale multilayered coatings, bilayer periods were controlled between 2.0 nm and 9.9 nm by changing the rotation speed of the substrate holder. The crystalline structure of Ti0.737Al0.263N, Ti0.474Al0.526N and CrN coatings was B1 cubic, whereas Ti0.298Al0.702N coating showed a mixed phase of würtzite and B1 cubic structure. In contrast, the crystalline structure of the Ti1 − xAlxN/CrN nanoscale multilayered coatings was B1 cubic, irrespective of the Al content (x). Although the Ti1 − xAlxN coatings showed a maximum hardness value at x = 0.526, the hardness of the Ti1 − xAlxN/CrN nanoscale multilayered coating was maximal for x = 0.702. For the Ti1 − xAlxN/CrN nanoscale multilayered coating, hardening was pronounced at lower Ti to Al ratios. The hardening phenomena observed for the Ti1 − xAlxN/CrN coatings using nanoscale multilayering structurization is believed to be due to the disparity in elastic moduli, rather than the coherency strain caused by the difference in lattice parameters between Ti1 − xAlxN and CrN layers.