Characterization of phase transformation behaviour and microstructural development of electroless Ni–B coating

Phase transformation behaviour of amorphous electroless Ni–B coating with a targeted composition of Ni–6 wt% B is characterized in conjunction with microstructural development and hardness. Microscopic observations of the as-deposited coating display a novel microstructure which is already phase separated at multiple length scales. Spherical colonies of ∼5 μm consist of 2–3 μm nodular regions which are surrounded by ∼2–3 μm region that contains fine bands ranging from 10 to 70 nm in width. The appearance of three crystalline phases in this binary system at different stages of heat treatment and the concomitant variation in hardness are shown to arise from nanoscale fluctuations in the as-deposited boron content from 4 to 8 wt%. High temperature annealing reveals continuous crystallization up to 430 °C, overlapping with the domain of B loss due to diffusion into the substrate. The implications of such a microstructure for optimal heat treatment procedures are discussed.