H<SUB>2</SUB>-Ar系プラズマ処理によるオーステナイト系304ステンレス鋼の表面形態の変化

Abstract

The change in the surface layer of SUS304 steel exposed to d.c. glow discharge plasma in a H2-Ar mixed gas was investigated, in order to elucidate the influence of H2 plasma concerning the austenitic stainless steels.Generally, grain boundaries are ion-etched by Ar 100% glow discharge plasma. Some surface reliefs like slip lines except for grain boundaries were formed on the plasma treated specimens in a mixed gas (80 vol%H2+20 vol%Ar) under a reduced pressure of 665 Pa at 623 K, as observed in the previously reported H2-N2 plasma treatment. By detailed observation of the microstructure, it was found that these surface reliefs were due to the growth of plate-like precipitates along {111} planes of the matrix, being similar to the morphology of ε martensite. Furthermore, we found that this surface layer contained newly formed two f.c.c. phases (a=0.367 nm, 0.363 nm) and a small amount of α phase and showed a high carbon content and a high hardness of about 500 Hv.From these results, the following mechanism for the formation of surface reliefs was proposed: by the absorption of carbon being mixed in the plasma, new f.c.c. phases precipitated in the plate-like form along {111} planes of the matrix and surface reliefs were generated combining with the effect of ion-etching.In austenitic stainless steels treated in the plasma using H2-mixed gas, the formation of surface reliefs is considered as a characteristic phenomenon.