Nanoscale Generation of Robust Solid Films from Liquid-Dispersed Nanoparticles via in Situ Atomic Force Microscopy: Growth Kinetics and Nanomechanical Properties

Inorganic metal-oxide nanoparticles have been identified as candidate materials for replacing zinc dialkyldithiophosphate (ZDDP) antiwear additives in automotive lubricants due to their ability to form protective antiwear tribofilms. However, the nanoscale mechanisms that control the growth and properties of these tribofilms remain unknown. Here, we present the results of an in situ study of the kinetics of nanoparticle tribofilm growth using colloidal-probe atomic force microscopy (AFM). We report on the nucleation and growth of antiwear tribofilms formed with a dispersion of a novel zirconia (ZrO₂) nanoparticle additive in low-viscosity oil. Tribofilms in this study are generated in situ at the lubricated nanoscale contact of the colloidal-probe AFM, which helps elucidate the effects of localized contact parameters on tribofilm nucleation and growth. The results strongly support that ZrO₂ nanoparticle tribofilms grow through a process of stress-activated tribosintering, after removal of organic ligands that attached to the nanoparticles. In contrast to ZDDP-derived tribofilms, ZrO₂ tribofilm growth occurs across temperatures from -25 to 100 °C. Nanomechanical properties of the resulting tribofilms are found to approach values for single-crystal and conventionally sintered macroscale structures, suggesting the potential for robust wear protection across a broader range of conditions than those where ZDDP is effective.