Surface alloying of metals by nanosecond laser pulses under transparent overlays

Abstract

A thorough study was made of the processes determining the formation of metastable surface alloys in a film–substrate system [using Au(110 nm)–Ni and Sn(40 nm)–Cr systems as an example] irradiated by laser pulses of nanosecond duration. The irradiation was performed in conventional conditions (in air) and, following the applied method, through a transparent overlay. Scanning/transmission electron microscopy and Rutherford backscattering spectroscopy investigations of alloys were complemented by the measurements of acoustic signals initiated by laser pulses in the substrate. It has been ascertained that the alloy formation in air is accompanied by two competitive processes: laser sputtering of the film substance and its diffusion into the molten substrate layer. As the irradiation intensity goes up, the hydrodynamical sputtering mechanism is replaced by the atomic vaporization. As a result, sufficiently high vapor pressure (up to 102 bar) is generated above the film surface, the film is pressed to the substrate, and the heat and mass transport through the film–substrate interface is realized. However, no more than 20% of the atoms (of their total amount in the film) penetrate into the substrate, and the depth of alloying is within 100 nm. The experiments on the irradiation of the same systems through transparent overlays (water or “liquid glass”) have demonstrated a marked change in the pattern of processes: alloying atoms efficiently penetrate from the film into the substrate, the depth of alloying increases to 400 nm, and defects are formed in deep layers of the substrate.