Phase transformation and hardness of the Ni–P–Al ternary coatings under thermal annealing

Ternary Ni–P–Al coatings were fabricated by the dual-gun rf magnetron sputtering technique. The as-deposited Ni–P–Al coatings exhibited a major Ni nanocrystalline phase (with a (111) texture) with Al and P co-deposited. After 400 °C heat treatment, various NixPy compounds, including Ni12P5, Ni5P2, and Ni3P, formed within the recrystallized Ni matrix. Accordingly, the hardness of the coating increased to 10 GPa due to the NixPy precipitation. For heat treatment temperatures higher than 450 °C, NipAlq hard phases were observed in the ternary Ni–P–Al coating. A further increase in coating hardness from 10 to 12 GPa was revealed. The hardening of the annealed Ni–P–Al coatings was attributed to precipitation of Ni–P and Ni–Al compounds formed around 400 and 500 °C, respectively. A two-stage hardening in Ni–P–Al coating by NixPy and NipAlq precipitation through heat treatment was then demonstrated. Through surface analysis, the increase in surface roughness for the Ni–P–Al coating due to the formation of NixPy and NipAlq compounds was revealed.