Effects of high temperature treatment on microstructure and mechanical properties of laser-clad NiCrBSi/WC coatings on titanium alloy substrate

Abstract

Laser-clad composite coatings on the Ti6Al4V substrate were heat-treated at 700, 800, and 900°C for 1h. The effects of post-heat treatment on the microstructure, microhardness, and fracture toughness of the coatings were investigated by scanning electron microscopy, X-ray diffractometry, energy dispersive spectroscopy, and optical microscopy. The wear resistance of the coatings was evaluated under dry reciprocating sliding friction at room temperature. The coatings mainly comprised some coarse gray blocky (W,Ti)C particles accompanied by the fine white WC particles, a large number of black TiC cellular/dendrites, and the matrix composed of NiTi and Ni3Ti; some unknown rich Ni- and Ti-rich particles with sizes ranging from 10nm to 50nm were precipitated and uniformly distributed in the Ni3Ti phase to form a thin granular layer after heat treatment at 700°C. The granular layer spread from the edge toward the center of the Ni3Ti phase with increasing temperature. A large number of fine equiaxed Cr23C6 particles with 0.2–0.5μm sizes were observed around the edges of the NiTi supersaturated solid solution when the temperature was further increased to 900°C. The microhardness and fracture toughness of the coatings were improved with increased temperature due to the dispersion-strengthening effect of the precipitates. Dominant wear mechanisms for all the coatings included abrasive and delamination wear. The post-heat treatment not only reduced wear volume and friction coefficient, but also decreased cracking susceptibility during sliding friction. Comparatively speaking, the heat-treated coating at 900°C presented the most excellent wear resistance.