Effect of laser heat treatment on AlxTi1-xN-based PVD coatings, deposited on carbon and tool steel substrates

Abstract

This paper highlights the effect of laser heat treatment on the adhesion and sliding wear of physical vapour deposited (PVD) multilayer AlxTi1-xN and nanocomposite AlxTi1-xN/α-Si3N4 coatings on carbon (C45E, preliminarily bulk hardened) and tool steel (Vanadis® 6, preliminarily bulk hardened and tempered), as well as substrate steel microstructure and hardness. The hardened zone in carbon steel generally comprises martensite and retained austenite. The hardened zone in tool steel contains martensite and retained austenite, as well as M7C3 (M = Fe, Cr) and MC (M = Fe, V) carbides. Laser heat treatment increased the average surface hardness of the carbon steel by 1.5–3.1 times and that of the tool steel by 1.1–1.2 times. The critical adhesion loads Lc1 and Lc2 enlarged by 1.22.6 times and by 1.2–1.3 times, respectively, in the case of the coatings deposited on the carbon steel. However, only slightly positive or no changes in the critical adhesion loads were observed for the tool steel case. The scratch crack propagation resistance (CPRs) of the coatings increased by 1.1–4.8 times, being more pronounced for the carbon steel substrate. The improvement of adhesion was assumed to be the result of the increased hardness (H) to Young's modulus (E) H/E and H3/E2 ratios of the substrate steel. Wear resistance of the coatings improved by 1.3–1.7 times. Scuffing and surface fatigue wear were the principle wear mechanisms in all the cases. However, the first mechanism was more remarkable for laser heat treated samples, and the second for the samples that remained untreated by laser. Apart from the above-mentioned increment of H/E and H3/E2 ratios, the improvement of wear resistance was explained by the increased CPRs values of the coatings and by the presumed precipitation of the AlN phase within them.