AEGD-Assisted Plasma Nitriding of AISI M2 Steel: Influence of Treatment Time on Structure and Scratch Resistance

The effect of treatment time on arc-enhanced glow discharge plasma-assisted nitriding (AEGD-PAN) of AISI M2 high-speed steel was investigated for non-heat-treated and heat-treated substrates. Nitriding treatments were carried out at 350 °C for 1.5 and 3.5 h, producing diffusion layers with thicknesses ranging from approximately 38 to 75 µm without formation of a continuous brittle compound layer. X-ray diffraction combined with Rietveld refinement revealed the progressive formation of γ′-Fe4N and ε-Fe23N nitrides together with lattice expansion of the α-Fe matrix, indicating nitrogen supersaturation and precipitation strengthening within the diffusion zone. Heat-treated specimens exhibited higher surface hardness, reaching ~1350 HV0.1, while non-heat-treated substrates developed pronounced hardness gradients associated with diffusion-controlled layer growth. Scratch testing showed improved resistance to contact-induced damage with increasing nitriding time, particularly for the 3.5 h treatment, where lateral cracking was significantly reduced and load-bearing capacity increased. Multi-pass scratch wear tests revealed a reduction in the Archard wear coefficient by up to four orders of magnitude compared with untreated M2 steel. These results demonstrate that AEGD-PAN at moderate temperature enables efficient diffusion layer formation and significant improvement in the tribological performance of high-alloy tool steels.