The influence of hydrogen on ion beam mixing of multilayer films

Abstract

Previous qualitative studies of ion beam mixing of Ni–Ti and Fe–Ti multilayers at room temperature have shown the Ni–Ti samples to mix considerably faster than the Fe–Ti, in apparent contrast with theory. Furthermore, the Fe–Ti mixing was strongly inhibited by previous charging of the sample with hydrogen, whereas only a small effect was seen for Ni–Ti. We have quantified the mixing and extended the study to four more systems (Al–Ti. Co–Ti, Cu–Ti, and Pd–Ti) and lower temperatures. This allows some important conclusions to be drawn. Predictions based on a thermal spike model underestimate the larger room temperature mixing rates (Cu–Ti, Ni–Ti, and Pd–Ti), apparently because of contributions from a temperature dependent mechanism such as radiation enhanced diffusion. The lower mixing rates (Fe–Ti, Co–Ti, and Ni–Ti at ∼80 K) are overestimated by a factor of 2–3.5, possibly because of hydrogen contamination of the as-deposited samples. For the Al–Ti sample, the experimental mixing rate was in good agreement with predictions. Except for the Cu–Ti sample, results were seen to vary with heat of solution, rather than heat of mixing, suggesting significant contributions from the lower temperature after-spike regime. Hydrogen charging was found to reduce the Fe–Ti mixing rate by a factor of 7 at room temperature, whereas the Co–Ti and Ni–Ti rates were only reduced by a factor of 2, and the mixing of the Pd–Ti was influenced very little. Near liquid nitrogen temperature the Ni–Ti mixing rate was more strongly reduced (by a factor of 3–4). Our results suggest that the original hydrogen contamination in as-deposited samples may also cause significant reduction of mixing rates in some materials.