Microstructure and mechanical properties of Ni-Al composite coatings for tribological applications

Abstract

Composite coatings consisting of alternating layers of Ni and Al were fabricated using a radio frequencymagnetron sputtering system. The composite coatings, which were 15 μm thick and consisted of 30 layers with an average thickness· of 900 nm for the Ni and 200 nm for the Al layers, were fabricated and then annealed at temperatures from 200 to 750°C. In as sputtered coatings a metastable η phase (Al₉ Ni₂) was found at the interface between Ni and Al layers. In the coatings annealed at 550°C, there were the stable intermetallic phases, Al₃ Ni and Al₃ Ni₂, and the metastable 1] phase. The as sputtered coatings had a hardness of 7·30 GPa and elastic modulus of 123·4 GPa; hardness was found to decrease with increasing annealing temperature. The strength decrease arose mainly from an increase in the grain size of the nickel and aluminium. The coefficients of friction of the as sputtered and annealed (at 550°C) coatings were 0·36 and 0·27, respectively, lower than that for an as sputtered Ni film (0·54). The improvement in tribological behaviour is mainly attributable to the production of the intermetallic phases.