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In this work, the effect of the addition of reduced graphene oxide (rGO) nanoflllers to UHMWPE was investigated. The UHMWPE/rGO nanocomposites were prepared at rGO contents of 0.010, 0.10, 0.25, and 0.50 wt%. The morphological, mechanical, and surface properties of the nanocomposites and the neat UHMWPE were evaluated and related with their tribological behavior. For that, DSC, SEM, contact angle, MO, and profilometry techniques, besides shore D hardness tests, instrumented indentation tests and pin-on-disc sliding wear tests were performed. The results showed that nanocomposites with rGO contents of 0.10 and 0.25 wt% exhibited higher crystallinity degree than the neat UHMWPE, which improved significantly their mechanical and tribological behavior through increases of around 40% in the instrumented hardness and the elastic modulus, and a significant reduction of up to 48% in the coefficient of friction when compared to the neat UHMWPE. Additionally, the presence of rGO decreased the work of adhesion of the nanocomposites/ counterface pairs, resulting in friction and wear mechanisms changes in comparison with the neat polymer. KEYWORDS friction, nanocomposites, reduced graphene oxide, sliding wear, UHMWPE, work of adhesion, 1 | INTRODUCTION Ultra-high molecular weight polyethylene (UHMWPE) has been extensively used in engineering applications that involve wear and friction due to their self-lubricity, lightweight, corrosion resistance, and low coefficient [...]

Moving mechanical interfaces are commonly lubricated and separated by a combination of fluid films and solid 'tribofilms', which together ensure easy slippage and long wear life (1). The efficacy of the fluid film is governed by the viscosity of the base oil in the lubricant; the efficacy of the solid tribofilm, which is produced as a result of sliding contact between moving parts, relies upon the effectiveness of the lubricant's anti-wear additive (typically zinc dialkyldithiophosphate) (2). Minimizing friction and wear continues to be a challenge, and recent efforts have focused on enhancing the anti-friction and anti-wear properties of lubricants by incorporating inorganic nanoparticles and ionic liquids (3,4). Here, we describe the in operando formation of carbon-based tribofilms via dissociative extraction from base-oil molecules on catalytically active, sliding nanometre-scale crystalline surfaces, enabling base oils to provide not only the fluid but also the solid tribofilm. We study nanocrystalline catalytic coatings composed of nitrides of either molybdenum or vanadium, containing either copper or nickel catalysts, respectively. Structurally, the resulting tribofilms are similar to diamond-like carbon (5). Ball-on-disk tests at contact pressures of 1.3 gigapascals reveal that these tribofilms nearly eliminate wear, and provide lower friction than tribofilms formed with zinc dialkyldithiophosphate. Reactive and ab initio molecular-dynamics simulations show that the catalytic action of the coatings facilitates dehydrogenation of linear olefins in the lubricating oil and random scission of their carbon-carbon backbones; the products recombine to nucleate and grow a compact, amorphous lubricating tribofilm., Transportation vehicles account for about 19% of the world's energy consumption and some 23% of total greenhouse-gas emissions every year (6). With ever-increasing mobility, these numbers will undoubtedly surge and [...]