Solid-state reactions in low-phosphorus autocatalytic NiP–SiC coatings

Composite NiP–SiC coatings with a nanocrystalline nickel matrix produced by autocatalytic deposition were subjected to phase transformations by isochronal and isothermal heating. Isochronal heating to 700°C (heating rate 10°C min−1) revealed three exothermic effects (reactions) occurring in the coatings. X-Ray diffraction showed that only the second and third thermal effects were associated with phase transformations. The first peak was associated with chemical and structural relaxation and a slight grain growth of the matrix. The second reaction was attributed to the nucleation and growth of Ni3P precipitates, while the third was related to the complete dissolution of SiC particles in the matrix with the formation of Ni3Si and carbon. A similar trend in the phase formation sequence was observed by isothermal heating. However, the formation of nickel silicides at the SiC/matrix interface occurred at lower temperatures (i.e. 500°C for 1 h). The formation of silicides appeared to be governed by the diffusion of nickel atoms into the SiC lattice, as indicated by transmission electron microscopy.