In situ stress measurements during low-energy nitriding of stainless steel

Nitriding of austenitic stainless steel at moderate temperatures leads to the formation of a hard and wear-resistant surface layer of expanded austenite, characterised by an expansion of the lattice constant of up to 12%. This large expansion can be assumed to be accompanied by a corresponding high compressive stress parallel to the substrate at the interface between the implanted layer and the bulk substrate. In situ stress measurements using the beam bending method were performed to capacitively determine the curvature of thin samples, prepared from X5CrNi18.10 (DIN 1.4301, AISI 321), during low-energy nitrogen implantation. The thickness of the expanded austenite layer was derived afterwards from glow-discharge optical spectroscopy. Assuming an inversely parabolic growth rate for diffusion-limited growth, a rather high compressive stress of 1.4–1.5 GPa was reached after 90 s and stayed constant for 1 h. This value is near the plastic flow limit of stainless steel. Furthermore, while keeping the sample at 400 °C and switching off the nitrogen beam, the curvature of the sample almost instantaneously started to decrease exponentially by approximately 50% on a time scale of 15–30 min, indicating ongoing stress relaxation.