Investigation of plasma immersion ion implanted surfaces by instrumented indentation

Plasma immersion ion implantation (PI3) involves the energetic implantation of nitrogen at energies up to 50 keV combined with the thermochemical absorption of nitrogen from a low pressure r.f plasma at temperatures up to 550 degrees C. At low temperatures the treated surfaces are similar to those obtained by conventional ion beam implantation, but at temperatures where the mobility of nitrogen is significant, diffusion becomes important. In this case, thicker treated layers are obtained which give greater improvements in mechanical properties to depths of several micrometres. The development of instruments which measure continuously the force and displacement as an indentation is made in a material has led to great advances in probing the mechanical properties of materials on the submicrometre scale. We have used such an instrumented indenter at loads up to 500 g, in order to characterize the surface layers produced by PI3 in a range of alloys. Results are given for standard test blocks, demonstrating the comparability of hardness values derived from force vs. displacement data with those measured by the conventional Vickers technique. It is also shown that the instrumented indenter can be used to investigate the variability of the hardness profile across an engineered surface. Specific examples are given for an austenitic stainless steel treated at elevated temperatures.