Effect of C₂H₂/Ar Flow Rate on Microstructure and Tribological Properties of a-C:H Coatings

a-C:H coatings have widely used as the surface protection material in different applications due to their high hardness, low friction coefficient and good chemical inertness, while the H content and sp²C/sp³C ratio in the a-C:H coatings are the key factors to affect the mechanical and tribological properties of the coatings. In this study, the a-C:H coatings were prepared on the surface of 9Cr18 steel substate via unbalanced magnetron sputtering technology, and the influences of precursor composition on the H content, microstructure, mechanical and tribological properties of the coatings were systematically studied. The results showed that the growth rate and H content of the a-C:H coatings monotonically increased as the increase of C₂H₂/Ar flow rate ratio, while the compactness decreased. The hardness and elastic modulus gradually decreased as a function of the C₂H₂/Ar flow rate ratio owing to the variation of the C-H bonds contents and compactness of the a-C:H coatings, but the adhesion strength between the substrates and coatings displayed a trend of first increased and then decreased. The coatings exhibited good friction and wear performances when the C₂H₂/Ar flow rate ratio was below 4:3, but the friction coefficient and wear rate of the coating significantly increased when the ratio was higher than the value. Overall, with the increase of the C₂H₂/Ar flow rate ratio, the friction coefficient and wear rate of the a-C:H coatings always decreased at first and then increased. Moreover, the main wear mechanisms of the a-C:H coatings were changed from the abrasive wear and adhesive wear to single abrasive wear as a result of the passivation effect of H atoms and changes of mechanical properties. In the case of C₂H₂/Ar flow rate ratio of 1:1, the a-C:H coatings had the lowest average friction coefficient and wear rate, and the values were 0.1 and 8.0×10^-8 mm³/(N·m), respectively. This property superiority of the a-C:H coating was closely related to the H content and sp²C/sp³C ratio in the coating.