Wear mechanisms of a sintered tribofilm in boundary lubrication regime

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A research effort was conducted to investigate wear mechanisms of a sintered tribofilm formed by a zirconia nanoparticle antiwear additive. Spherical five nanometer diameter zirconium oxide (ZrO2) nanoparticles were dispersed in polyalphaolefin (PAO) synthetic base oil and used to generate approximately 120 nm thick tribofilms on AISI 52100 steel counterfaces in a ball-on-disk tribometer under boundary lubrication conditions. The sintered tribofilms were subsequently worn using the same conditions without the nanoparticle additive. Semi-in situ measurements of tribofilm thickness were conducted with an optical interference-measuring device to observe tribofilm growth and subsequent tribofilm wear. A rapid initial wear process was observed, however, a thinner tribofilm provided enduring protection of the substrate steel for the 120 min test duration. Without nanoparticles to replenish the tribofilm, the wear was deconvoluted from growth. Comparatively, lack of a tribofilm would result in scuffing within 10 min when testing the unadditized PAO base oil. Scanning electron microscopy and profilometry suggest the tribofilms were smoothened by the wear process. Evidence of crack formation between pores of the tribofilm illustrate a wear mechanism for the tribofilms. The cracks were concentrated where the contact pressure was the largest. Mild abrasive wear appeared as minor furrows in the sliding direction throughout the full width of the tribofilm.