Plasma surface engineering and characterisation of biomedical stainless steels

Abstract

Low temperature surface alloying with nitrogen (nitriding), carbon (carburising) and both (carbonitriding) has been successfully employed in hardening engineering grade AISI 316 by the formation of a modified layer better known as S-phase or expanded austenite. However little or no research has been directed towards the surface modification of medical grade austenitic stainless steels, such as ASTM F138, ASTM F1586 and ASTM F2581. In this study, systematic plasma surface alloying treatments and characterisation were performed on Fe-Cr-Ni medical grade ASTM F138 and ASTM F1586 as well as engineering grade AISI 316 austenitic stainless steel for comparison in order to establish the optimised treatment conditions (especially temperature) which can maximise the hardened case depth without any detriment in corrosion resistance. Based on the first phase process condition optimisation results, the optimum treatment temperature for nitriding and carbonitriding is 430°C whilst for carburising it is 500°C The established optimised treatment temperatures that formed precipitate free Sphase layers on the nickel-containing medical grade austenitic stainless steels were also performed on the nickel-free ASTM F2581 medical grade alloy. For the first time S-phase was created in the surface of nickel-free austenitic stainless steel by low temperature plasma surface alloying but only carburising yielded a precipitate free Sphase layer. This implies that nickel is not essential in the formation of S-phase and that manganese together with chromium plays an important part in precipitation kinetics of Ni-Free (Fe-Cr-Mn) alloys. The surface of a biomaterial must not adversely affect its biological environment and return the material surface must not be adversely affected by the surrounding host tissue and fluids. Experimental results have shown that this duality of concern can be addressed by creating S-phase in the surface of medical grade austenitic stainless steel since biocompatibility, corrosion and wear studies have manifested positive results. It has been shown that low-temperature nitriding, carburising and carbonitriding can improve the localised corrosion resistance of medical grade stainless steel as long as the threshold sensitisation temperature is not reached. The improvement seen after treating medical grade austenitic stainless steel was not limited only to electrochemical but also to mechanical-electrochemical properties. In fact all plasma surface alloyed medical grade austenitic stainless steels have shown a general improvement in wear-corrosion and fretting-wear resistance over the untreated materials. Also since biocompatibility studies on N, C and hybrid C/N S-phase have proved, for the first time, that they are biocompatible under the realms of the tests conducted in this study therefore the use of hardened medical grade austenitic stainless steel might be suitable in implant applications. The renaissance in metal-tometal implants and the high performance of the S-phase have raised the possibility of utilising stainless steel as an alternative joint bearing.