Development of AISI 316LVM austenitic stainless steel hybrid S phase layer for medical application / Mohammad Firdaus Mohammed Azmi

Attempts to improve the surface hardness and wear resistance of austenitic stainless steels using surface treatments in the past have resulted in corrosion resistance degradation due to chromium precipitation in the hardened layer. In this study, systematic gas diffusion thermochemical treatments and characterisation were performed on medical grade austenitic stainless steel AISI 316LVM (Sandvik Bioline) in order to establish the optimised treatment conditions (temperature, time and gas composition) which can maximise the austenitic stainless steel performance without sacrificing corrosion resistance. The hybrid S phase layer was systematically characterized by microscopy investigation, microhardness, phase analysis, potentiodynamics, pin on disk test, nanoindentation and nanoscratch according to each testing standards. According to the DOE optimisation results, the ideal treatment parameters for the low temperature hybrid heat treatment were when the temperature was set at 475 °C for 12 hours of holding time. The optimum gas composition was when methane, ammonia and nitrogen simultaneously introduced at 10 %, 80 % and 10 % respectively. The nitrogen and carbon element dissolved in the austenitic lattice forming an interstitial supersaturated solid solution called the hybrid S phase layer. From the microscopy result, up to 13.3 μm hybrid S phase layer thickness was developed. The characterization results found the hybrid S phase layer able to significantly increase the bulk material surface hardness up to 1461 HV0.025, the wear resistance where about three times improvement and the corrosion resistance compared to the untreated material. The nano-tribological behaviour of the hybrid S phase layer shows improved the wear resistance coefficient and decreased the coefficient of friction. In addition, it has good cohesion where no evidence of delamination when the samples are nanoscratched vertically up to 50 mN and it is a noteworthy discovery for the biological field. The biocompatibility studies on the hybrid S phase found that they are biocompatible under the cytotoxicity (ISO 10993-5) and cell adhesion (ASTM F813-20) tests conducted. Therefore, the use of hybrid S phase as surface modification process on medical grade austenitic stainless steel might be suitable in biomedical applications.