A study on the characteristics of (Ti1−xAlx)N coatings deposited by plasma-enhanced chemical vapor deposition after heat treatment

(Ti1−xAlx)N films were deposited on M2 high-speed steel (HSS) and α-Al2O3 substrates by a plasma-enhanced chemical vapor deposition (PECVD) process, and the change in hardness and the phase transformation of the coatings after heat treatment were examined. The hardness of the (Ti1−xAlx)N coatings deposited on HSS increased with increasing heat treatment time at 1100 °C, reaching a maximum hardness. Further increases in heat treatment time led to a decrease in hardness. However, the hardness of the (Ti1−xAlx)N coatings deposited on alumina substrates decreased slightly after heat treatment. XRD and TEM analysis indicated that the hardness change of the (Ti1−xAlx)N coatings after heat treatment is related to the precipitation of a stable würzite AlN phase from the metastable NaCl-(Ti1−xAlx) phases. Moreover, this precipitation effect depends on the Al content and the grain size of the as-deposited coating.