Comparison of microstructure and residual stress characteristics of electroless Ni–W–P coatings annealed with a laser and a furnace

This paper presents an investigation of microstructures in mixstructural electroless Ni–W–P coatings annealed by a laser and a furnace. Effects of the annealing temperature and the laser scanning velocity on microstructural characteristics, in terms of degree of crystallisation, grain size, microstrain and residual stress, were investigated using SEM/EDX and XRD techniques. Microhardness of the coatings before and after the heat treatments was evaluated by Vickers measurements. The results showed that the laser treatment at 8 to 14 mm/s and the furnace annealing at 200 to 700 °C resulted in incomplete crystallisation and the size of Ni3P phase was larger than that of the Ni phase while the Ni3P crystallisation reaction occurred at 14 mm/s for laser annealing or at 400 °C for furnace annealing. The laser annealing was useful for precipitation of Ni3P phase and produced higher microhardness as compared to the furnace annealing. The phenomena were discussed considering the effect of heating and cooling rates in laser and furnace annealing on the degree of crystallisation, grain sizes of both nickel and Ni3P phases, microstrain and residual stress.