Low-pressure rf plasma as a medium for nitriding iron and steel

This study compares the nitriding efficiency of a low-pressure pure-nitrogen rf plasma for the materials AISI-316 stainless steel, 99.8% iron and the low-alloy steels AISI 4140 and En41B. The process temperature was 400 °C; a range of gas pressures and biasing conditions was explored. The treated samples were examined using instrumented microhardness measurements, proton backscattering, X-ray diffraction and other diagnostics. The degree of nitriding is enhanced by increased bombardment by excited neutral molecules. Ion bombardment assists in the nitriding of iron and the low-alloy steels studied, apparently by aiding the production of Fe2-3N. This phase does not form in the nitriding of 316 stainless steel, and therefore treatment of this material is not assisted by ion bombardment. In comparing low-pressure rf plasmas to procedures involving higher pressures and temperatures, the process described here is more appropriate for hardening austenitic stainless steel, but perhaps less appropriate for the other materials.