Mechanical properties of sputtered nickel–phosphorus-based hard coatings under thermal annealing

Ternary Ni–P–W and Ni–P–Cr alloy molding coatings were fabricated by radiofrequency (RF) magnetron sputtering onto steel substrate, AISI 420 tool steel. Cycling annealing up to 500 °C was performed on various Ni–P-based coatings to simulate the cycling injection or press-molding process in practical use. The mechanical properties, including hardness and Youngʹs modulus of the alloy coatings, were evaluated by continuous stiffness measurement. It was observed that the ternary Ni–P–Cr and Ni–P–W coatings exhibited a maximum hardness up to 14 and 15.5 GPa under 8 and 12 h of annealing test at 500 °C, respectively. With long-term annealing up to 20 h at elevated temperature, the Ni–P-based coatings retained a steady hardness of 12 GPa. The influence of the third element introduction on the mechanical properties of the Ni–P-based coating was probed and discussed. The incorporation of W and Cr into the NiP-based coating exhibited superior mechanical behavior under elevated temperature environments as compared to Ni–P binary coating. In addition, the correlation between indentation penetration depth and mechanical behavior was also evaluated. Reliable hardness values of the coatings were extracted from the indentation range of 200–400 nm, while the gradual descent in hardness with increasing indentation depth was influenced by the substrate effect.