Positive Antiwear Interaction Between ZDDP and CNTs, GNPs and FLGs Under Boundary Lubrication

Industrial gear contacts operate under mixed-to-boundary lubrication where reliable antiwear protection is essential. This study assesses whether carbon nanomaterials can enhance the performance of zinc dialkyldithiophosphate (ZDDP) under severe conditions. A crossed-cylinder Reichert configuration (2 GPa, 75 °C, 1 m/s) with PAO6 was used to test ZDDP (1 wt%) and its blends with carbon nanotubes (CNT, 0.05 wt%), graphene nanoplatelets (GNP, 0.05 wt%), and few-layer graphene (FLG, 0.05 wt%) at 1, 10 and 60 min. The lubrication regime was boundary. Friction, specific wear rate (k), and tribofilm coverage were quantified. Oils containing only carbon nanoparticles could not sustain the test (seizure within minutes), confirming the necessity of ZDDP. After 60 min, average CoF remained similar across formulations and largely governed by ZDDP. By contrast, wear showed marked differences: relative to ZDDP alone (A), ZDDP + CNT (F) and ZDDP + GNP (G) reduced k by 52% and 48%, respectively, and exhibited higher tribofilm coverage (F = 68%, G = 72% vs. A = 57%). Time-resolved tests revealed that long-duration degradation was mitigated in F and G: from 10 to 60 min, k rose by 72% (F) and 58% (G) versus 159% for A; coverage decreased by only 8% (F) and 3% (G) versus 22% for A. SEM–EDS indicated no major differences in average elemental chemistry among formulations, suggesting an improvement on tribofilm coverage/stability rather than compositional change.