Synergism of TiO2 and Ni on the Growth of a Double-Layered Tribofilm: New Strategy for ZDDP Replacement

Phenolic resin (PF) coatings were formulated and deposited onto Al alloy surfaces, and their tribo-performance was evaluated. It is demonstrated that even when being lubricated with poly(alpha olefin) 4 (PAO 4) base oil, the PF-based coatings reinforced with titanium dioxide (TiO2) or/and nickel (Ni) exhibit excellent tribo-performance under harsh boundary lubrication conditions. Interestingly, synergetic antiwear roles of TiO2 and Ni were identified. In comparison to the conventional PF coating filled with molybdenum disulfide (MoS2), friction and wear of the hybrid coating containing TiO2 and Ni are reduced by up to 80.7% and 83.2%, respectively. Furthermore, the hybrid coating lubricated with PAO 4 base oil exhibit a much lower friction and wear than the uncoated Al alloy lubricated with zinc dialkyldithiophosphat (ZDDP)-additivated oil. Our work provides direct evidence that the addition of TiO2 or Ni leads to the growth of tribofilms of distinctly different nanostructures. It is interesting that the addition of a TiO2/Ni combination gives rise to the formation of a double-layered tribofilm, i.e., a TiO2/NiO bottom layer and a carbon-based top layer. This work offers an avenue for minimizing dependency on hazardous lubrication additives.