Amelioration of the tribological behaviour of carbon/Ti–6Al–4V and graphite/Ti–6Al–4V couples by plasma nitriding of the titanium alloy

Abstract This article reports on the influence of nitriding treatment of titanium alloy on the tribological behaviour of amorphous carbon or graphite/Ti–6Al–4V couples. This contact was investigated in pin-on-disc geometry in an ambient atmosphere. The pins were made of amorphous carbon or graphite and the discs were made of Ti–6Al–4V, nitrided or not. The treatment of nitriding for this study was an N2–H2 plasma nitriding at a low temperature (700 °C) for 12 h. In these conditions, the top surface hardness was improved by a factor of 3. In this study, the mechanism of transfer and wear of carbon against Ti–6Al–4V, nitrided or not, has been studied with a scanning electron microscope, an optical microscope, etc. The analyses show that the nitriding of Ti–6Al–4V improves the tribological behaviour of the couples investigated. In fact, with the disc made of Ti–6Al–4V, the wear of carbon is very high and the disc is worn down at the end of the test. In contrast, after nitriding of the disc, the wear of carbon decreases, and there is a carbon film transfer on the disc. Moreover, with the nitriding of Ti–6Al–4V, the friction coefficient is stabilized, whereas without the nitriding of Ti–6Al–4V, the curve of the friction coefficient is marked by a very important increase during a short time. With the pin in graphite, the nitriding decreases the friction and increases the adhesion on the disc. Furthermore, the tests have shown that the contact temperature increases with the nitriding of Ti–6Al–4V.