Effects Of Post Heat Treatment On Microstructure And Properties Of Laser Clad Self-lubricating Anti-wear Composite Coatings

Laser cladding belongs to a rapid melting and rapid non-equilibrium solidification process, which leads to high residual stress and microstructure instability in the coating. When high residual stress and even cracks exist, industrial applications of the coating will be critically restricted. Post heat treatment, as an efficient method, can adjust microstructure and relieve residual stress of laser clad self-lubricating anti-wear coating to acquire the desired properties. Furthermore, post heat treatment will be beneficial to the self-lubricating anti-wear composite coatings during the long-term service process.In this paper, the laser clad Ni60/h-BN self-lubricating anti-wear composite coating was prepared on 304 stainless steel substrate, and the laser clad in situ synthesized Ti₂SC/CrS self-lubricating anti-wear composite coating was prepared on Ti6Al4 V substrate, and coatings were post heat treated. The phase composition, microstructure, hardness, friction and wear properties of coatings were investigated by X-ray diffraction（XRD）, scanning electron microscope（SEM）, energy dispersive spectroscopy（EDS）, microhardness tester, nano-indentation tester and HT1000 ball-on-disc wear tester. Effects of post heat treatment on tribological properties of self-lubricating anti-wear composite coatings were investigated especially.Results indicated that the Ni60/h-BN self-lubricating anti-wear composite coating was metallurgically bonded with the substrate and free from holes and cracks. The as-clad coating and the coating with post heat treatment（1 h and 2 h） mainly consist of γ-（Ni, Fe） solid solution, CrB ceramic phase and h-BN self-lubricating phase. The microhardness of the coating was first increased from 641.1 HV_0.5 to 701.2 HV_0.5 after post heat treatment for 1 h, and then decreased to 652.4 HV_0.5 after post heat treatment for 2 h. Compared with the coating without post heat treatment, the friction coefficients of the coatings after post heat treatment were decreased obviously. Effects of the post heat treatment time on friction coefficient were negligible, but were significant on wear volume loss. Comparatively speaking, the laser clad self-lubricating after post heat treatment for 1 h presented the best anti-wear and friction reduction properties, the worn surface was almost smooth, the wear debris were mainly powder-like, and the wear mechanism was mild abrasive wear.The laser clad in-situ synthesized Ti₂SC/CrS self-lubricating anti-wear composite coating on Ti6Al4 V substrate mainly consists of NiTi₂, Cr₃C₂, Cr₇C₃, TiC, Ti₂ SC and CrS. Effects of the post heat treatment for 1 h on the phase composition of the coating were negligible, but the post heat treatment decreased microhardness of the coating. The microhardness of the coating was decreased from 1168 HV_0.5 to 1076 HV_0.5 after post heat treatment. Compared with the anti-wear coating without adding WS2, the self-lubricating anti-wear coating shows better anti-wear and friction-reducing properties owing to the existence of in-situ synthesized self-lubricating phases Ti₂ SC and CrS. Furthermore, post heat treatment could slightly increase the friction-reducing property and significantly enhance the anti-wear property of the self-lubricating anti-wear coating. The worn surface of the coating after post heat treatment was almost smooth, and the counter-body presented good running-in surface without plastic deformation obviously. The wear mechanism is mainly abrasive wear, oxidative wear and the formation of oxide films and lubricating films.