S-phase layer by hybrid low temperature thermochemical treatment process on 316 LVM stainless steel: corrosion and biocompatibility test / Siti Umrah Zainal

The 316 LVM stainless steel has been used in biomedical applications. The present study focuses on the improvement of surface properties of medical grade austenitic stainless steel (ASTM F138). The aim is to develop a homogenous supersaturated hard layer of expanded austenite (S-phase) at the surface of the AISI 316LVM using a low temperature hybrid thermochemical heat treatment process in a customised mixing chamber of a tube furnace. Hybrid means treating both nitriding and carburizing at the same time. The S-phase layer would improve the surface integrity at the surface by increasing the hardness and improving the wear resistance without impairing the corrosion resistance of the steel. During the thermochemical process, ammonia (NH3) and methane (CH4) gases were introduced into the furnace for 6 and 12 hours at 425 °C, 450 °C, 475 °C, and 500 °C with a gas composition of 75% NH3, 5% CH4, and 20% nitrogen (N2) and 75% NH3, 10% CH4, and 15% N2.The S-phase characterization was analysed by XRD. The thick bright layer of the S-phase formation was measured at up to 15.9 μm as analysed by SEM. Besides, the surface hardness also significantly increased from 210.9 HV to 1121.0 HV. These results have significant effectiveness in corrosion resistance at 475°C. The E(corr) increased from a value of 364 mV (untreated) to -217 mV. Biocompatibility of treated samples shows enhancement of osteoblast cell proliferation on the S-phase surface, implying the low temperature hybrid thermochemical treatment could improve up to 8 % of cell viability after 24 hours. Finally, the low-temperature hybrid heat treatment process can improve the surface integrity of 316 LVM for use in biomedical implants.