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

• Published in: Surface & coatings technology 2014-11, Vol.258, p.566-573
• Main Authors: Mohammadzadeh, R., Akbari, A., Drouet, M.
• Format: Article
• Language: English
• Subjects: Applied sciences; Contact of materials. Friction. Wear; Cross-disciplinary physics: materials science; rheology; Exact sciences and technology; Flow rate; Gas composition; Gas flow; Heat treatment; High speed tool steels; Ion nitriding; M2 tool steel; Materials science; Mechanical properties and methods of testing. Rheology. Fracture mechanics. Tribology; Metals. Metallurgy; Microhardness; Microstructure; Nitrided layer depth; Physics; Plasma nitriding; Production techniques; Radio frequencies; Sliding friction; Surface treatments; Thermochemical treatment and diffusion treatment; Wear; Wear resistance
• ISSN: 0257-8972; 1879-3347
• DOI: 10.1016/j.surfcoat.2014.08.036

Wear behavior of quenched-tempered AISI M2 tool steel samples has been studied after plasma nitriding at different N2–H2 plasma gas flows containing 25, 50 and 75sccm N2. Plasma nitriding was performed at 450°C for 8h 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 25sccm N2–75sccm H2 gas flow and mono-phase ε-Fe2–3N compound layer at higher N2 concentrations. Plasma nitriding increases near surface hardness up to 50% (about 1600HV0.025) irrespective of the N2:H2 ratio, where nitrided layer depth and surface roughness increase with increasing the N2 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 H2 fractions, respectively. Decreasing wear resistance with increasing N2 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. •The enhancement of the wear resistance of the M2 tool steel by plasma nitriding•Reporting the lowest wear rate for samples plasma nitrided without compound layer•The absence of a compound layer by plasma nitriding at 25N2–75H2 gas mixture•The presence of a mono phase compound layer at 50N2–50H2 and 75N2–25H2 gas mixtures•Increasing surface roughness and nitrided case depth by N2 flow in the plasma gas