Microstructure and wear properties of AISI M2 tool steel on RF plasma nitriding at different N2–H2 gas compositions

Abstract Wear behavior of quenched-tempered AISI M2 tool steel samples has been studied after plasma nitriding at different N 2 –H 2 plasma gas flows containing 25, 50 and 75 sccm N 2 . Plasma nitriding was performed at 450 °C for 8 h under floating potential using a plasma reactor equipped with a radio frequency power generator. Microstructure, phase composition, nitrided layer thickness, hardness and surface roughness of the samples were studied using optical microscopy, X-ray diffraction, microhardness and surface profilometry measurements. Dry sliding wear resistance of samples was determined by performing ball-on-disc wear testes. The results revealed formation of mainly a diffusion zone at the 25 sccm N 2 –75 sccm H 2 gas flow and mono-phase e-Fe 2–3 N compound layer at higher N 2 concentrations. Plasma nitriding increases near surface hardness up to 50% (about 1600HV 0.025 ) irrespective of the N 2 :H 2 ratio, where nitrided layer depth and surface roughness increase with increasing the N 2 flow rate in the plasma gas. Depending on the nitrogen content, sliding wear resistance may be improved between 20 and 90% with respect to the un-nitrided substrate. Among the nitrided samples the maximum and minimum wear resistance was obtained at plasma gases containing higher and lower H 2 fractions, respectively. Decreasing wear resistance with increasing N 2 flow rate in the plasma gas attributed to formation of the hard and brittle compound (white) layer on the sample surface and development of residual stress profiles.