Microstructure and mechanical properties of magnetron co-sputtered Ni–Al coatings

Binary Ni–Al alloy coatings were fabricated by magnetron co-sputtering with dual targets of pure Ni and Al metals. The chemical composition variation, microstructure evolution and related properties of the coatings caused by sputtering power modulation were investigated. The Ni–Al coatings with Al contents ranging from 2.7 to 24.9 at.% were manipulated through dual-gun sputtering. The as-deposited Ni–Al films revealed crystalline and nano-crystalline microstructures with respect to sputtering powers. Microstructure consisting of a significant crystallization feature of a metastable (Ni,Al) matrix and Ni3Al precipitations was observed for the coatings under high sputtering powers, while coatings deposited at lower sputtering powers exhibited a (Ni,Al) solid solution polycrystalline structure with (111) preferred orientation. The coatings with (Ni,Al) matrix and Ni3Al precipitations exhibited a higher hardness than those with a single (Ni,Al) phase. The formation of (Ni,Al) nano-crystallites and Ni3Al phases was the strengthening mechanism for Ni–Al coatings deposited under high energy input during sputtering. With the increase in Al sputtering power, a slight decrease in grain size due to a higher Al concentration in the Ni–Al coatings was observed. The variation in hardness of the magnetron co-sputtered Ni–Al coatings could be attributed to the grain size and microstructure evolution.