Tribological Performance of LPBF-Fabricated 316L Stainless Steel by Hybrid Thermochemical Treatment

This study presents a comprehensive investigation into the microstructural evolution and tribological performance of 316L stainless steel (SS316L) components fabricated by Laser Powder Bed Fusion (LPBF), enhanced through nitriding and hybrid thermochemical surface treatment. The hybrid process which involves simultaneous diffusion of nitrogen and carbon, resulted in the formation of a single, homogeneous expanded austenite (S-phase) layer with a thickness of 12.2 μm, over six times thicker than the 1.98 μm layer formed through nitriding alone. This study is among the first to evaluate hybrid thermochemical treatment on LPBF 316L, addressing a gap in surface engineering strategies for additive manufacturing. X-ray diffraction confirmed significant lattice expansion indicative of dual interstitial incorporation. Microhardness measurements confirmed the enhancement, with the hybrid-LPBF specimen achieving a peak surface hardness of ~1209 HV compared to ~345 HV for nitride-LPBF specimens and ~265–297 HV for untreated conditions, representing a nearly fivefold enhancement. Then, tribological testing under dry sliding conditions revealed that the hybrid-LPBF specimens achieved the lowest coefficient of friction (COF = 0.45) and negligible wear depth, outperforming both nitride-LPBF (COF ~ 0.60, wear depth ~ 18 μ μm) and untreated-LPBF specimens (COF = 0.77, wear depth ~ 40 μm). Counter face analysis further showed reduced material transfer and smoother wear scars in treated specimens especially hybrid-LPBF. These findings demonstrate that hybrid thermochemical treatment significantly mitigates the surface limitations of as-built LPBF components, offering a significant potential to enhance the wear resistance and operational durability of stainless steel parts for high-demand applications. Future research will explore anisotropic behaviour and simulation-based optimization of treatment parameters.