Effect of load on the behaviour of tribofilms formed at the interface of austenitic steel and ductile iron – a Raman spectroscopic study

This paper presents Raman spectroscopic characterization of the tribofilms formed during dry sliding at the interface of an exhaust valve material, austenitic steel 21–4 N and valve-seat material, ductile cast iron GGG-40, at loads of 20–50 N at a temperature of 500°C. The behaviour of the tribofilms formed at the contact interface with increasing loads is highlighted. Pin-on-disc experiments were conducted on a high-temperature tribometer and low coefficient of friction (COF), in the range of 0.12–0.15, was observed at loads up to 40 N which increased by 35% to 0.19 at a load of 50 N. Scanning electron micrographs indicate the presence of well-developed tribofilms at lower loads while the formation, subsequent deterioration and finally the reformation process of tribofilms is observed at the higher loads. Energy dispersive spectroscopy analysis primarily indicates the presence of oxides of Fe in the tribofilm region. Ex-situ Raman spectroscopy confirms the presence of haematite (α-Fe2O3) on the wear tracks, especially at loads of 20–40 N. However, the spectra indicate a strong presence of magnetite at a load of 50 N. It is observed that haematite is a major constituent of the wear protective tribofilms.