Effect of methane concentration on surface properties of cathodic cage plasma nitrocarburized AISI-304

Plasma nitrocarburizing is a thermochemical diffusion process with simultaneous diffusion of nitrogen and carbon on the surface of metals at elevated temperatures. In this work, the influence of methane (carbon-containing precursor) concentration on the surface properties of the nitrocarburized AISI-304 in the presence of a cathodic cage is studied. The samples are treated at fixed conditions (400 °C, 3 h, 150 Pa), while 0–10% methane is admixed in nitrogen–hydrogen mixture discharge. The addition of methane results in a gradual decrease in the surface hardness with indentation depth of sample compared to the plasma nitriding, which indicates an improved load-bearing capacity. The XRD pattern shows the abrupt phase transformation from austenite (fcc) to the martensite (bcc) phase for a small methane concentration and simultaneous carbon and nitrogen expanded austenite phases for higher methane fraction. The thickness of both N-enriched and C-enriched layers increases up to 4% of methane addition, and then the thickness of the N-enriched layer decreases. The corrosion rate also showed a non-monotonic response to the methane fraction.