Tribological Compatibility of Diamond-like Carbon Coatings and Molybdenum Friction Modifiers in Engine Oil

The synergistic integration of solid lubricants (e.g., diamond-like carbon, DLC) and oil-based friction modifiers (FMs) holds promise for enhancing engine fuel efficiency, yet their compatibility remains underexplored. This study investigates the tribological interactions between three DLC variants (a-C, a-C:H, ta-C) and molybdenum friction modifiers (MoFMs) in a Group III base oil and an SAE 0W-20 engine oil, along with the antagonistic/synergistic effects of zinc dialkyldithiophosphate (ZDDP)-MoFM blends. Friction and wear behaviors under boundary lubrication were evaluated using a ball-on-disc tribometer. Results reveal that both MoDTC and moly trimer decompose to form MoS ₂ -rich tribofilms on steel and DLC surfaces, achieving significant friction reduction (up to 35%). However, MoDTC induces higher wear on amorphous DLCs (a-C and a-C:H) compared to moly trimer, attributed to accelerated graphitization. ZDDP forms protective phosphate films on DLCs, mitigating MoFM-induced wear, but its anti-wear performance is compromised by MoFM-derived sulfides. The combined use of ZDDP and MoFMs leads to mutual inhibition, diminishing their individual friction-reduction and anti-wear benefits. These findings highlight the necessity for tailored additive formulations when pairing DLC-coated components with MoFM containing lubricants, balancing friction optimization and durability in advanced engine systems.