MEAM Ph.D. Thesis Defense: “Tribosintering of Metal Oxide Nanochrystals”

Machinery relies on lubrication to regulate friction and wear at contacting interfaces. As lubricants become less viscous to save energy and cost, and as new technologies like electric vehicles operate in harsher conditions, the risk of surface-initiated failure grows. We show that metal oxide nanocrystals (NCs) dispersed in lubricants form protective coatings, or tribofilms, in situ at contacting interfaces via tribosintering. Compared to state-of-the-art antiwear additives and surface coatings, metal oxide tribofilms have several advantages, but the lack of fundamental knowledge about the growth and wear processes of these tribofilms is a key factor hindering adoption of NCs in lubricants. We reveal mechanisms behind metal oxide tribofilm formation through macro-scale, application-relevant experiments.

We use a benchtop tribometer capable of commercially relevant conditions — the mini-traction machine (MTM) with spacer layer imaging (SLIM) — to refine experimental study of metal oxide tribofilm formation. First, we consider the nanocrystals as anti-wear additives, demonstrating that the SLIM technique captures and quantifies local wear events during tribofilm growth. The competition wear and growth can be tuned by, e.g., the addition of S- and P-based co-additives, which increase the initial rate of tribofilm growth while contributing to a more polished steady-state tribofilm morphology. We then use the nanocrystals to form in situ metal oxide surface coatings at low homologous temperatures in a variety of realistic conditions. We show that ZrO2, TiO2, and BaTiO3 all form durable, anti-scuffing coatings near room temperature, offering robust practical benefits. We then generalize an under-appreciated feature of SLIM, disaggregating tribofilm thickness measurements to correlate pixels of data with local contact stress and contact time in contact. Local tribofilm thickness variations at a given time are best explained by differences in exposure to interfacial sliding, not differing contact stresses — an unacknowledged driver of tribosintering. We then show that a property-dependent critical length scale explains the differing wear behavior of ZrO2 and TiO2 tribofilms, providing a physically motivated design criterion for application. Finally, we discuss data-driven efforts to model tribofilm formation, demonstration unique benefits including correction of corrupted data, and a process to iteratively test analytical models.