Fabrication and crystallization behaviors of sputtered Ni–Cu–P films on tool steel

Electroless nickel is widely used as a hard coating for many industrial applications due to its extreme hardness, uniform thickness, corrosion and wear resistance. For advanced industrial applications, it is essential to promote the crystallization temperature of Ni–P deposits. In this study, Cu is introduced in Ni–P to improve its thermal stability. An alternative coating technique by r.f. magnetron sputtering is applied to deposit Ni–Cu–P coatings to solve the problem of composition control in the conventional chemical solution method. The Ni–Cu–P coatings were deposited by r.f. magnetron sputtering on 420 tool steel substrates with Cu+Ni–P compound targets. A novel design of compound targets with Cu and Ni–P by consideration of the surface ratio of constituents exhibits a controllable composition in the deposited film. The compositions of the Ni–Cu–P coating can be modified by the original Ni–P deposits of the compound target along with the Cu area ratio in the compound target. The planar uniformity of the compositions of the as-deposited Ni-Cu–P films is also achieved. All the as-sputtered (70.18–82.26 at.%)Ni–(17.17–1.39 at.%)Cu–(12.65–16.35 at.%)P deposits reveal an amorphous structure. The hardness of as-deposited Ni–Cu–P coatings decreases with increasing Cu content. Nevertheless, the hardness of the Ni–Cu–P films is still greater than that of the substrate. After annealing, the structure of the amorphous Ni–Cu–P deposits will transform directly into the Ni–Cu alloy and the Ni3P phase. The as-deposited Ni–Cu–P films can be hardened by precipitation of the Ni3P phase and crystallization of Ni–Cu. The introduction of Cu into the ternary Ni–Cu–P deposits increases the crystallization temperature as compared with the binary Ni–P film. The annealing temperature associated with the occurrence of the peak hardness in the (82.26 at.%)Ni–(1.39 at.%)Cu–(16.35 at.%)P and (74.27 at.%)Ni–(12.65 at.%)Cu–(13.08 at.%)P films is approximately 365 and 380°C, respectively. It is shown that the thermal stability of the Ni–Cu–P films is enhanced by increasing the Cu content in the sputtered deposits.