Air plasma sprayed metallic coatings are accompanied by oxides formed at different stages of spraying. It is widely believed that the formed oxide in metallic deposits influence their properties. Oxides formed at every stage during plasma spraying of 316L austenitic stainless steel powder on 1040 plain carbon steel were characterized.
Two different types of austenitic stainless steel particles were sprayed using a dc plasma gun (PTF4 type) and were collected in an argon atmosphere. Oxide on substrates was studied by preheating them at 300°C at 400°C and 450°C in air. Coatings were sprayed under "cold condition" (temperature kept to its minimum value during spraying), at 300°C and 450°C. The formed oxides were characterized using scanning electron microscopy, energy dispersion spectroscopy, x-ray diffraction, microprobe, Fourier transform infrared spectroscopy and Mossbauer spectroscopy. Mixed oxide layer of hematite and magnetite forms on 1040 substrate surface while plasma heating them between 300 and 400°C whereas above 400°C only hematite was identified. Oxide islands form in the interior of the sprayed particles due to convective moment of liquid particle in plasma jet which are likely composed of CrO and FeO. FeCr2O4 was detected as major oxide phase in stainless steel coatings.