Surface alloying of stainless steel 316 with copper using pulsed electron-beam melting of film–substrate system

The surface morphology, chemical composition, microstructure, nanohardness, and tribological properties of a film (Cu)–substrate (stainless steel 316) system subjected to pulsed melting with a low-energy (20–30 keV), high-current electron beam (2–3 μs, 2.8–8.4 J/cm2) have been investigated. The film was deposited by sputtering a Cu target in the Ar plasma of a microwave discharge. To prevent the local delamination of the film due to the cratering, the substrates were repeatedly pre-irradiated with 8–10 J/cm2. Single pulsed melting of this system resulted in the formation of a diffusion layer of thickness 120–170 nm near the interface, irrespective of the energy density. The layer has the subgrain structure consisting of the γ-Fe-solid solution and submicrometer or nanocrystalline Cu particles. The nanohardness and the wear resistance of the surface layer of thickness 0.5–1 μm, including the molten film and the diffusion layer, non-monotonically vary with energy density, reaching a maximum in the range of 4.3–6.3 J/cm2. As the pulse number is increased to five in the same range of energy density, the film dissolves in the substrate, and a ∼2-μm-thick surface layer is formed which contains ∼20 at.% Cu. Under these conditions, the segregation of Cu during resolidification leads to the formation of two-phase nanocrystalline layers separating γ-phase grains.