Improving energy efficiency in the transportation sector is a major avenue towards both economic growth and the reduction of greenhouse gas emissions. Low viscosity (LV) lubricants that reduce viscous losses can boost automotive efficiency, but the resulting thinner lubricating films increase the risk of boundary contact, requiring antiwear (AW) additives for surface protection. We have recently shown that zirconium oxide (ZrO2) nanoparticle (NP) additives in lubricant base oils produce stable, optically transparent dispersions that form AW tribofilms in sliding contacts. In these microscale sliding contacts, ZrO2 NP additives develop tribofilms through a tribosintering mechanism, with a linear dependence on contact pressure. However, their performance in a fully formulated LV gear oil remains unknown, since co-additives in such oils can impact tribofilm growth rate and mechanisms through synergistic or antagonistic chemical interactions with the NPs. To understand interactions with such additives, particularly sulfur- and phosphorous-containing AW and anti-scuffing compounds, we evaluate tribofilm growth across different length scales comparing ZrO2 NP behavior in pure base oil to a fully formulated LV gear oil with and without ZrO2 NPs. We use a mini traction machine (MTM) to understand the effect of slide-to-roll ratio (SRR) on tribofilm growth rate in macroscale contacts. Additionally, we use in situ atomic force microscopy (AFM) tribofilm generation experiments to help assess the impact of NP/co-additive interactions on contact-stress dependent film growth in microscale contacts. We further characterize the structure of ZrO2 tribofilms with cross-sectional transmission electron microscopy (TEM). We find that surface-active co-additives do not prevent ZrO2 tribofilm nucleation and growth. Moreover, we observe NP/co-additive interactions synergistic to tribofilm development. We will discuss the proposed underlying mechanisms for these differences in ZrO2 behavior. Overall, understanding ZrO2 NP behavior in a LV gear oil aids in improving vehicle efficiency while maintaining component durability. DISTRIBUTION A. Approved for public release; distribution unlimited. OPSEC #: 3359

Interactions between nanoparticles and extreme pressure additives: Toward high performance low viscosity lubricants

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.