Tribological sintering of antiwear films as a function of applied load has been investigated using a novel zirconia nanoparticle antiwear additive. Spherical five nanometer diameter zirconium oxide (ZrO2) nanoparticles are dispersed in polyalphaolefin (PAO) synthetic base oil and tested between AISI 52100 steel counterfaces in a ball-on-disk tribometer with a slide-to-roll ratio of 50%. The apparatus allows tribofilm thickness data to be tracked in-situ at set intervals, and tribofilms reaching a maximum thickness of 160 nm have been measured. A tribofilm formation dependence on load has been found, as higher loads assist initial growth and final thickness. Contrary to the chemical tribofilm formation processes of traditional antiwear additives such as zinc dialkyldithiophosphate (ZDDP), testing suggests the primary mechanism of tribofilm growth for zirconium oxide is nanoparticle adsorption followed by particle accumulation and sintering. A strong correlation between friction force, tribofilm width and Hertzian contact theory has been found. Initial tribofilm growth rate grew exponentially with applied load, whereas isolation of the linear growth phase yielded a linear relationship. Mechanical stylus, scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS) are employed to investigate the roughness, morphology and chemical nature of the sintered tribofilms, respectively.
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