Effect of Ti to Al ratio on the crystalline structure and hardening of a Ti 1 − xAl xN/CrN nanoscale multilayered coating

Abstract

The crystalline structure and hardening of Ti 1 − x Al x N/CrN nanoscale multilayered coatings with various Ti to Al ratios have been investigated. The Ti 1 − x Al x N/CrN coatings with different Al contents were deposited on M2 HSS (high speed steel) by alternating the deposition of Ti 1 − x Al x N and CrN, using reactive D.C. magnetron sputtering. The Al content ( x) in the Ti 1 − x Al x N coatings was controlled to be 0.263, 0.526 and 0.702 by using Ti 1 − x Al x alloy targets with varying Ti to Al ratios. For the Ti 1 − x Al x N/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 Ti 0.737Al 0.263N, Ti 0.474Al 0.526N and CrN coatings was B1 cubic, whereas Ti 0.298Al 0.702N coating showed a mixed phase of würtzite and B1 cubic structure. In contrast, the crystalline structure of the Ti 1 − x Al x N/CrN nanoscale multilayered coatings was B1 cubic, irrespective of the Al content ( x). Although the Ti 1 − x Al x N coatings showed a maximum hardness value at x = 0.526, the hardness of the Ti 1 − x Al x N/CrN nanoscale multilayered coating was maximal for x = 0.702. For the Ti 1 − x Al x N/CrN nanoscale multilayered coating, hardening was pronounced at lower Ti to Al ratios. The hardening phenomena observed for the Ti 1 − x Al x N/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 Ti 1 − x Al x N and CrN layers.