The present study discusses the influence of an a-C:H coating on the tribological parameters of sliding pairs under mixed friction. Using the PVD method, the a-C:H coating was formed on specimens made from AISI 4337 steel. The sliding friction and wear process was carried out on the pairs of AISI 4337 steel and SAE- 48 bearing alloy, which were lubricated with 10W40 biodegradable engine oil. The investigation showed significant differences in the friction coefficient and temperature in the tested pairs with the steel surface layer and the a-C:H coating. In the friction pairs with the a-C:H coating, the tested parameters of friction were higher than in pairs with the steel surface layer. The pairs with the a-C:H coating showed more intensive wear of the SAE-48 bearing alloy than those with a steel surface. The surface layer used in a friction pair leads to the deterioration of the lubricating properties of engine oil and reduces its resistance to scuffing.