Discussion on "Scale and contact geometry effects on friction in thermal EHL: Twin-disc versus ball-on-disc" by Liu, Zhang, Bader, Venner, Poll, Tribology International 154, 106694, 2021

Liu et al. recently published a study on friction in heavily loaded lubricated contacts. They compare measurements obtained from tests carried out on two tribometers, at constant entrainment speed, temperature and contact pressure. They observe deviations they attribute to the influence of scale and contact geometry effects. We show, by means of experiments conducted under the same conditions and with the same lubricant, that three alternative effects, somehow dependent on each other, can explain their results: very different elastohydrodynamic conditions, the pervasive presence of significant thermal effects and the occurrence of a mixed lubrication regime in some tests. This discussion provides an opportunity to reconsider how to conduct friction tests with in mind to focus on the lubricant response only. Proposals are presented in terms of operating conditions and lubricant selection, with the subsequent objective to better understand the mechanisms behind friction in highly loaded EHD contacts.