Surface mechanical properties of aluminum implanted nickel and co-evaporated Ni–Al on nickel

Implantation of Al into nickel and co-evaporation of Ni–Al films onto nickel substrates followed by ion irradiation was conducted in order to investigate the mechanical properties of ion beam surface modifications and their relation to the composition and microstructure of the surface. Implantations were made using 400 keV Al⁺ to doses from 1 × 10¹⁵ to 6 × 10¹⁷ cm⁻² at room temperature. Nickel films with 0–25% Al were co-evaporated onto nickel substrates and mixed using the same irradiation conditions but to a lower maximum dose. Hardness was measured using ultra-low load indentation, and residual stress was measured by the bending beam method. Results indicate that the primary contributor to the hardness increase of as-implanted surfaces is the irradiation-induced defects. The effect of Al in solution (γ phase) or Al in γ′ (Ni₃Al) in either implanted or co-evaporated and mixed surfaces is evident only upon thermal treating to remove radiation damage. The high inherent hardness of the co-evaporated films is due to the small grain size of the film. The magnitude of the hardness in Al implanted nickel is very sensitive to the surface condition of the substrate. The observed hardness effects are all directly relatable to microstructure or phases present. Residual stresses change from tensile to compressive as a result of damage from low dose irradiation and heat treatment.