Tribochemical induced wear and ultra-low friction of superhard ta-C coatings

Employing a broad parameter study, friction and wear properties of amorphous carbon coatings (a-C), tetrahedral amorphous carbon coatings (ta-C) and single crystalline diamond were investigated in boundary lubrication regime, using an oscillating ball-on-flat test setup with engine oil, glycerol, and glycerol monooleate (GMO) lubrication. Ultra-low friction (µ < 0.04) was confirmed with glycerol and GMO. At the same time, very high wear of the superhard ta-C coating was found for GMO solely. A systematic study of lubricant chemistry, surface impurities, testing temperature, counter body material, and the carbon coating regarding sp3-fraction and crystallinity was conducted, indicating a strong difference between low abrasion wear and high chemical wear. Based on the measured data, the chemical nature and wear promoting factors are considered, suggesting detrimental interaction of amorphous sp3-carbon in the coating and unsaturated carbon bonds in the lubricant. Furthermore, a relation for low friction of GMO and contact pressure was established, leading to lowest friction for contact pressure below 600 MPa. High wear on either component of the ball-on-plate-geometry of the test setup led to a change of the contact area, thus strongly influencing frictional behavior of the tribometer model test.