Phase structure and microstructure,and thermal stability of electroless Ni-P coatings deposited at different pH values

Ni-P electroless coatings were prepared in nickel sulfate solution at different pH values of 4.5,5.5 and 6.5 with the purpose of determining the influence of pH value on microstructure,internal stress statue and thermal stability of the coatings.The laser curvature(LC)method was used to measure the residual stress level in the coatings.Inductively coupled plasma massive spectrometer(ICPM)was used to analyze the P content in the as-deposited Ni-P 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 a high temperature.The results showed that the Ni-P coating prepared at pH 4.5 had well-developed amorphous structure,the Ni-P prepared at pH 6.5 had thorough nano-crystal structure,and the Ni-P prepared at pH 5.5 had a mixed structure with middle level of P content and tensile level.The Ni-P prepared at pH 5.5 experienced obvious cracking even after 250℃ intentional heat treatment,and TEM result revealed the micro-cracking originated from the Ni/Ni-P interface.On the contrary,the relatively higher stability of Ni-P coatings prepared at pH 4.5 and 6.5 was ascribed to the former’s lower tensile stress level and latter’s much finer grain size.Besides,the diminished local stress concentration in the nano-structure and the inverse Hall-Petch effect might also contribute to the integrity of Ni-P coating prepared at pH 6.5.