Tribological behavior and machining performance of non-hydrogenated diamond-like carbon coating tested against Ti–6Al–4V: Effect of surface passivation by ethanol

The role of an ethanol environment on the tribological behavior of non-hydrogenated DLC (NH-DLC) coatings sliding against Ti–6Al–4V was investigated. Pin-on-disk tests were performed under different test atmospheres consisting of dry argon (0% RH) and ambient air (40% RH) and also while the samples were submerged in water (H2O) and ethanol (C2H5OH). The running-in friction of the NH-DLC coatings under each condition was studied and the results were used to rationalize the drilling behavior of Ti–6Al–4V by NH-DLC coated tools. The highest running-in coefficient of friction (μR = 0.55) was observed under a dry argon atmosphere. In ambient air, the μR was 0.42, while a reduction in μR to 0.21 was recorded when the tests were carried out in H2O. The observed reductions in μR were attributed to OH passivation of the NH-DLC surfaces. Ethanol was effective in providing OH passivation, as evident from the lowest μR of 0.15. Consistent with the friction measurements, a 45% reduction in drilling torque was obtained when 10% ethanol was blended with the metal removal fluid (MRF) consisting of water–oil (1:3) mixture compared to an ethanol free MRF.