Influence of pH value on microstructure and thermal stability of Ni–P electroless coating prepared in acidic condition

Three kinds of Ni–P electroless coatings were prepared in nickel sulphate solution at different pH values of 4.5, 5.5 and 6.5 with the purpose of ascertaining the influence of pH value on microstructure, internal stress statue and thermal stability of the coatings. Laser curvature (LC) method was used to measure the residual stress level in the coatings. Scanning electronic microscopy (SEM) and transmission electronic microscopy with energy dispersive spectrum (TEM/EDS) were used to examine the surface morphology and internal phase structure of the coatings, respectively. Differential scanning calorimeter (DSC) was used to analyze the phase transformation and thermal stability of the coatings at high temperature. Results showed the Ni–P coating prepared at pH 5.5 with nano-crystal mixed in amorphous structure had the worst thermal stability. The relatively higher stability of Ni–P coatings prepared at pH 4.5 and 6.5 was ascribed to the lower tensile stress level and much finer grain size, respectively. Besides, inverse Hall–Petch effect of annealing strengthening might also contribute to the integrity of Ni–P coating prepared at pH 6.5.