High current density, low energy, ion implantation of AISI-M2 tool steel for tribological applications

Abstract

The effects of the temperatures of fully hardened and mill-annealed AISI M2 steels during N implantation at low energy and high current density on their hardnesses, sliding wear behaviors, microstructures and N concentration profiles were investigated. It is shown that N implantation at 1 keV and 2 mA cm −2 can increase the hardness of the steel in either condition, but does not improve the wear resistance of the fully hardened material. Implantation of the mill-annealed material, however, yields a surface that is as wear resistant as the fully hardened surface. Strengthening and hardening are induced through enrichment of the matrix that bonds hard (Fe,Cr) 3(W,Mo) 3C and V 4C 3 particles with an iron nitride phase that appears to be predominantly ε-Fe 2+ x N. The implantation temperature that yields the greatest surface hardness increase in either state is near 450 °C. The exponential prefactor and activation energy associated with N diffusion into heat-treated M2 steel are approximately 1.4 × 10 −5 cm 2 s −1 and approximately 0.70 eV respectively. In order for ions to be implanted, it is pointed out that their energy must be great enough to carry them through any barriers that may exist on a surface. It is shown that atomic B from an ion source can condense on a surface and produce a surface barrier that inhibits subsequent B + implantation at the nominal N implantation energy of 1 keV.