Changing the tribological performance of steels using low energy, high temperature nitrogen ion implantation

Traditional nitrogen ion implantation can lead to considerable improvements in the wear resistance of steel components. For instance, implantation of 100 keV nitrogen to doses of approximately 2 x 10(17) ions cm(-2) can lead to a fourfold increase in the life of injection moulding dies or cemented carbide cutting tools. Typically, high energy nitrogen ion beams are required with a low beam current, and heating of the steel component is kept to a minimum during the process. The high energy is needed to achieve a reasonable ion penetration depth into the material, but even so this is only a fraction of a micrometre. Conventional ion implantation is thus most suitable for tribological applications where the depth of wear is low. Recently, it has been shown that comparatively low energy nitrogen ion beams (1-2 keV) with very much higher beam current densities can lead to significant increases in the hardness of some steels, particularly when some extra nitrogen is introduced into the system using a backfill gas. In this process, the temperature of the sample is allowed to rise to around 400 degrees C during implantation and may be boosted by additional heating. The elevated temperature allows diffusion to occur and hardening to be achieved at a depth considerably greater than that obtained by high energy implantation at lower temperatures. The extent of hardening depends on the ion beam parameters and the structure and composition of the steel substrate. In this study, the tribological performance of such implanted layers has been assessed by pin-on-disc wear testing. The wear behaviour is compared with that of the same steels subjected to conventional ion implantation, and the results are discussed in the light of the hardness and compositional changes induced by the treatment.