Characterisation of dual S phase layer on plasma carbonitrided biomedical austenitic stainless steels

Dual S phase layers without precipitates were formed on the surface of two biomedical austenitic stainless steels, ASTM F138 and ASTM F1586, by plasma surface alloying at 430 degrees C with both nitrogen and carbon (i.e. carbonitriding). Systematic characterisation of the dual surface layers were conducted using cross-sectional TEM (XTEM), depth-profiling X-ray diffraction, glow discharge optical emission spectroscopy and electrochemical corrosion tests. The cross-sectional TEM studies have revealed that this dual S phase layer consists of a nitrogen rich S phase and a carbon rich S phase sublayer with crystal constant of a=50.384 and 0.373 nm respectively. Quantitative calculation based on glow discharge optical emission spectroscopy composition depth profiling indicated that more alloying species were absorbed during nitrocarburising than during nitriding or carburising. Based on the experimental results, the response of these two medical grade stainless steels to plasma carbonitriding is compared.