Robust Interfacial Tribofilms by Borate- and Polymer-Coated ZnO Nanoparticles Leading to Improved Wear Protection under a Boundary Lubrication Regime.

This work reports on the development of borate- and methacrylate-polymer-coated zinc oxide nanoparticles (ZnOBM) via a plasma polymerization technique to replace the harmful conventional antiwear additive zinc dialkyl dithiophosphate (ZDDP) in automotive lubricants. Here, the tribochemistry across the interfaces formed between sliding ferrous surfaces and coated and uncoated ZnO nanoparticles is thoroughly studied from the perspective of elucidating the tribofilm formation, wear, and friction performance of a novel ZnOBM-based nanolubricant. Tribological tests conducted under a boundary lubrication regime revealed that oil formulations containing only ZnOBM nanoadditives and a mixture of ZnOBM with a low amount of ZDDP (350 ppm of P) significantly improve wear performance (up to 95%) compared to the base oil. Electrical contact resistance results acquired in situ during tribological tests demonstrated that lubricants containing ZnOBM nanoparticles at sliding interfaces undergo tribochemical reactions to form stable tribofilms that reduce friction and wear. Atomic force microscopy (AFM), X-ray absorption near-edge spectroscopy (XANES), and X-ray photoelectron spectroscopy (XPS) analysis revealed that ZnOBM nanoparticles, by themselves, form patchy interfacial tribofilms containing iron borate, boron oxide, and zinc oxide and lead to superior tribological performance. Interestingly, ZnOBM nanoparticles interact synergistically with ZDDP to form a hierarchical interface of boron-doped tribofilms, with zinc-iron polyphosphates at the surface and iron oxide, zinc and iron sulfides in the bulk. These encouraging results suggest the potential effective use of the ZnOBM nanoparticles to significantly reduce harmful levels of ZDDP (350 ppm) in the engine oil without compromising the antifriction and antiwear performance and to develop eco-friendly high-performance lubricant additives.