Current Status of Supersaturated Surface Engineered S-phase Materials

Thomas Bell^1,2,*

¹ Hanson Professor of Metallurgy University of Birmingham, UK

² Director International Surface Engineering Research & Development Centre, Xi’an Jiaotong University, China

Abstract. S-Phase as it is now know was mentioned for the first time by Zhang and Bell at the Third International Congress on Heat Treatment of Materials, November 1983, Shanghai¹. In 1985 it was reported that S-Phase was created in AISI 316 stainless steel by means of low temperature plasma nitriding carried out at 400 °C. Zhang and Bell described the S-Phase layer as being hard and corrosion resistant with anticorrosion properties equivalent to the original material².

In a parallel investigation by Ichii et al. the S-Phase was again created using low temperature nitriding at 400 °C. The authors presented XRD data which showed peaks shifting to lower angles. These shifted peaks were not listed in the ASTM index and were denoted as S1-S5. Ichii et al. used the term ‘S-Phase’ to describe this layer and thus the term was created³.

The present paper reviews the scientific development of our understanding of S-Phase since those early times. It is now known that S-Phase formation is an example of Para equilibrium phenomena. A necessary but not sufficient condition for S-Phase formation is the presence of an fcc structure in the starting alloy^4-6. An essential requirement is for a nitride forming element⁷, usually chromium, to be present in the starting material which may be iron or nickel^4,7-9 or cobalt based alloys⁵.

After surface engineering with carbon, nitrogen or carbon and nitrogen to generate supersaturated solid solutions tribological, corrosion, mechanical and microstructural studies were conducted for the various alloy systems. These data have been used to generate S-Phase stability maps¹⁰. Finally the industrial status of S-Phase technology on an international basis is described.