Study on growth behaviour of oxide scale and its effects on tribological property of nano-TiO2 additive oil-in-water lubricant
Oil-in-water (O/W) emulsion is widely applied as a lubricant during hot rolling process. For reducing environmental impact caused by O/W emulsion, a novel nano-TiO2 additive O/W lubricant with improved stability and reduced coefficient of friction (COF) is developed. Little research, however, has been focused on the growth behaviour of oxide scale formed on the workpiece surface and its effects on the tribological property of nano-TiO2 additive O/W lubricant. In the current work, oxidation tests of high strength steel were conducted to investigate the growth behaviour of oxide scale and obtain different surface morphologies of the oxide scale. Ball-on-disk tribological tests under a temperature of 80 °C were carried out to investigate the effects of surface morphology of oxide scale on the tribological behaviour under nano-TiO2 additive O/W lubrication. The performance of the new developed lubricant was evaluated by hot rolling process. The growth mechanism of oxide scale and the modified oxide scale growth model were proposed. Tribological results indicate that oxide scale plays an important role in the tribological behaviour of nano-TiO2 additive O/W lubricant. A reduction in surface roughness can result in a decrease of COF, while the COF will increase if the surface roughness reduces too much because some of the nano-TiO2 particles cannot go through the contact area to play a positive role in lubrication.