Characterization of ZDDP and ionic liquid tribofilms on non-metallic coatings providing insights of tribofilm formation mechanisms

Lubricant anti-wear additives are known to chemically interact with metallic surfaces to form a self-healing, wear-protection tribofilm. Their interactions with non-metallic surfaces are however less understood. Here we report recent findings on whether and how a zinc dialkyldithiophosphate (ZDDP) and a phosphonium-organophosphate ionic liquid (IL) form tribofilms on three hard coatings, AlMgB₁₄-TiB₂, TiB₂, and diamond like carbon (a-C:H DLC), when sliding against a steel ball. Systematic characterization was conducted on the coating wear scars including top surface morphology imaging and elemental mapping, layer-by-layer chemical analysis, and cross section nanostructural examination. The ZDDP and IL tribofilms on the boride coatings are up to 50-70nm thick with 75-80% surface coverage while the tribofilms on DLC were <25nm thick and only covered 20-30% of the contact area. The presence of iron compounds in the tribofilms suggests a critical role for wear debris in tribofilm formation. Oxidation products of TiB₂ were detected in the tribofilms, while no involvement of the DLC surface in tribofilm formation was observed. Results suggest that wear debris digestion and contact surface reaction both are critical in tribofilm formation: the former process is responsible in forming the bulk of the tribofilm and the latter provides strong bonding of the tribofilm to the contact surface.