Effect of heat treatment on microstructure and tribology behaviour of electroless Ni-P/BN(h) composite coating

Abstract

ABSTRACTSub-micron (0.1–0.3 µm) BN(h) particles were incorporated into Ni-P plating coatings to enhance their mechanical and tribological properties. The morphology, phases and elemental analysis of Ni-P/BN(h) coatings were characterised by Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), Energy Dispersive Spectroscopy (EDS) and 3D surface profiler, respectively. The tribological properties of Ni-P/BN(h) coatings annealed at different temperatures (200–600°C) were investigated to determine the optimal annealing temperature to enhance their wear resistance as well as to reveal the underlying mechanisms. The results illustrated that co-deposited BN(h) particles can lower friction coefficient and enhance wear resistance. Increasing the annealing temperature, the micro-hardness of the annealed coatings gradually decreased from 1008 HV0.1 (400°C) to 848 HV0.1 (600°C) as a result of the Hall-Petch effect, nevertheless these values were higher than that of the as-deposited coating (692 HV0.1). With regard to the tribological property, the Ni-P coating presented a poorer wear resistance (6.7 × 10−5 mm3 N−1 m−1) in comparison with the Ni-P/BN(h) coating (3.1 × 10−5 mm3 N−1 m−1). In addition, coatings annealed at different temperatures ranging between 200∼600°C exhibited improved wear resistance (2.7 × 10−5–7.2 × 10−6 mm3 N−1 m−1) attributable to their higher hardness, the self-lubricating nature of BN(h) and the lubrication effect of oxides arising from the tribochemical reaction between Ni3P and the environment. Overall, the Ni-P/BN(h) coating annealed at 400°C for 1 h lowers the coating friction coefficient by about 58% and the wear resistance of the Ni-P composite is about 10 times higher than Ni-P coating.