Improving the mechanical properties of steels using low energy, high temperature nitrogen ion implantation

The use of nitrogen ion implantation to increase the surface hardness of structural steels is well documented. Traditionally this involves the use of high energy nitrogen ion beams (approximately 100 keV), with a relatively low beam current density because high energy beams are necessary to produce the required penetration into the material to achieve a significant depth of hardened material. Hardening needs to occur in a region whose size is comparable with the scale of the deformation associated with the tribocontact. 100 keV nitrogen ions typically penetrate into steels only about 0.1 mu m and the range of possible tribological applications is thus restricted by this shallow treatment depth. In plasma nitriding processes the nitrogen ions approach the substrate with much lower energies but the ion currents are sufficiently high to cause considerable substrate heating. In this study an ion beam process has been used which more closely approximates plasma nitriding conditions. This involves the use of comparatively low energy nitrogen ion beams (1-2 keV), with higher beam current densities and, where necessary, additional heating to increase the sample temperature to approximately 400 degrees C at the surface. Secondary ion mass spectrometry has been used to assess the extent of nitrogen diffusion into a range of steels. The nitrogen penetration depth (and profile shape) depends both on the ion beam parameters and the structure and composition of the steel. Additional experiments conducted with a supplementary supply of nitrogen gas in the form of a vacuum chamber backfill demonstrate that increasing the nitrogen supply can increase nitrogen take-up for some materials. For a 30 min implantation at 263 mu A cm(-2) the penetration depth was a substantial fraction of a micrometre and is greater than the profile produced by high energy implantation in some cases. The hardness of the implanted steels can be correlated with the extent of nitrogen take-up, the amount of diffusion during implantation and softening induced by annealing of the steel substrates. Considerable hardening is achieved for some steels (e.g. M2 high speed steel) whereas for other steels the improvements are less obvious (e.g. 304 stainless). These observations are discussed in the light of the nitridability of the steels.