Expanded austenite has been established as a hard and wear resistant layer obtained after surface modification while generally maintaining the excellent corrosion behavior of the base material. Using in-situ X-ray diffraction during low energy ion nitriding has allowed us to elucidate fundamental processes as phase formation, diffusion and decay. Nevertheless, the existing system is limited in two ways: only low energy ion nitriding as a method can be investigated and dynamic processes during the implantation result in a final layer which may not be representative of intermediate stages due to additional relaxation processes.
Here, it is shown that low energy ion beam etching coupled with in-situ XRD measurements can be used to obtain additional information on the nitriding processes. With argon ions accelerated to 1 keV, only minor modifications of material to be sputtered will occur, restricted to the immediate surface zone of only up to 10 nm, thus avoiding potential plastic or elastic deformation during mechanical removal. In this presentation, three examples pertaining to open questions on expanded austenite will be presented with new information accessible for theoretical considerations.
When comparing plasma immersion and low energy ion implantation, the plasma immersion samples always present much broader XRD peaks. A rough comparison of the nitrogen depth profiles shows that the nitrogen content alone cannot explain this effect. In-situ XRD measurements during sputter depth profiling indicate that this behavior is present throughout the layer. Thus, a discussion about local stress distributions should be necessary.
An anisotropic layer growth during nitriding has been observed using some methods with the (111) oriented grains growing slower than (200) oriented grains. When processing with low energy nitrogen ions, no such effect was observed whereas Ar sputtering of the samples indicates a much slower erosion of the (200) oriented grains. Thus, sputtering may be involved in the appearance or non-appearance of the anisotropic growth.
For low current ion nitriding, a very fast transition from a low lattice expansion to a high lattice expansion phase is observed. However, it is impossible to ascertain whether the complete layer is transformed simultaneously as these nitriding experiments indicate. Nitriding until this transformation is occurring, followed by subsequent cooling and sputtering has helped to solve this question.