Surface Modification of Austenitic Stainless Steels by High-Flux Elevated-Temperature Nitrogen-Ion Implantation.

Nitrogen diffusivity is found to be enhanced under unusual N ion beam conditions used for modification of fcc AISI 304 stainless steel surfaces. The unusual conditions also lead to the development of various near-surface microstructures and enhanced mechanical properties. The relative importance of ion energy and current density on N penetration was studied in order to help understand the enhanced N diffusivity. The role of residual stresses in the N implanted layers was also investigated. The N beam conditions included: (1) ion beam energies from 0.4 to 60 keV; (2) beam current densities from 0.1 to 5 mA/cm^2; (3) an elevated substrate temperature of 400^ circC; (4) implantation times of 10 to 30 minutes. Mossbauer spectroscopy and x-ray diffraction (XRD) were used to characterize the near-surface N ion implanted microstructures. Supplemental data were obtained by Auger electron spectroscopy, scanning electron microscopy (SEM), magneto-optic Kerr effect (MOKE) and electron probe micro-analysis (EPMA) on selected samples. A metastable, fcc, high-N phase (gamma _{N}) is found to be generally produced in fcc 304 SS for all ion energies and current densities at 400^circC. The gamma_{N} was found to be either paramagnetic or magnetic in nature depending on the N content. With a low-energy, high-flux N beam, magnetic gamma_{N} was found to be ferromagnetic at room temperature. The N contents and depths were found to depend on the grain orientation relative to the ion beam direction for low -energy, high-flux conditions. The N was found to diffuse deeper in the (200) oriented grains compared to the (111) oriented grains and the N contents were significantly higher in the (200) planes relative to the (111) planes. Post-implantation annealing experiments showed that the magnetic gamma_{N} phase was destabilized as a result of annealing it at 400^circC, thereby resulting in thicker and predominantly paramagnetic gamma _{N} layers with less N in solution and less lattice expansion. Based on XRD data, the N diffusivity in the absence of the N ion beam was found to be 2 times 10^{-13} cm^2/s at 400^circ C, two to three orders of magnitude higher than that in fcc-SS with low N content, but too small by about 2 orders of magnitude to explain the N diffusivity during the low-energy, high-flux N ion implantation into 304 SS. The gamma_{N} layer thickness and the load-bearing capacity was maximum near 1 keV implantation conditions. Further lowering of ion energies while increasing the flux was found to reduce the gamma_{N} thickness. The enhanced strength demonstrated by pin-on-disk wear tests was attributed to high N contents and sufficiently large N layer thicknesses. Based on the hardness data of the low energy, high flux samples, a 10-fold increase in surface layer yield strength was observed. (Abstract shortened by UMI.).