Structure and phase transformation behavior of electroless Ni–P alloys containing tin and tungsten

Autocatalytic ternary Ni–Sn–P, Ni–W–P and quaternary Ni–W–Sn–P films were prepared using alkaline citrate-based baths and compared with binary Ni–P coatings. Energy dispersive analysis of X-ray (EDAX) showed that binary Ni–P deposit contained 11.3 wt.% of phosphorus. Codeposition of tungsten in Ni–P matrix resulted in ternary Ni–W–P with 5 wt.% P and 7.8 wt.% of tungsten. Incorporation of tin led to ternary Ni–Sn–P deposit containing 0.4 wt.% Sn and 10.3 wt.% P. Presence of both sodium tungstate and sodium stannate in the basic bath had resulted in quaternary coating with 6.9 wt.% W, traces of Sn and 6.4 wt.% P. X-ray diffraction patterns of all the deposits revealed a single, broad peak which showed the nanocrystalline nature of the deposits. For the first time in related literature, the presence of a metastable phase Ni12P5 in ternary deposits is reported in the present study. Metallographic cross-sections of all the deposits revealed the banded/lamellar structure. Scanning electron microscopy (SEM) studies of the deposits showed smooth nodules for ternary deposits, but coarse and well-defined nodules for quaternary deposits. DSC studies of phase transformation behavior of the ternary Ni–Sn–P deposit revealed a single sharp exothermic peak at 365 °C. However, ternary Ni–W–P and quaternary Ni–W–Sn–P deposits exhibited a low temperature peak at 300 °C, a split type high temperature peak at 405 and 440 °C and a very high temperature peak at 550 °C. Higher activation energy values were obtained for W-based alloy deposits. Presence of W and Sn has helped to retain high microhardness values even at higher temperatures indicating an improved thermal stability.