Tribochemical and tribomechanical effect of TiO₂ nanoparticle: Different influence on MoDTC and ZDDP additives

This study investigates the formulation-dependent tribochemical regulation of TiO₂ nanoparticles in molybdenum dithiocarbamate (MoDTC), zinc dialkyldithiophosphate (ZDDP), and their blended lubricant systems. Reciprocating ball-on-disk tribological tests were conducted under a normal load of 20 N, corresponding to Hertzian contact pressure of 1.33 GPa. TiO₂ nanoparticles promoted MoS₂-rich tribofilm formation in the lubricant with single MoDTC, which led to reduction of both the coefficient of friction and wear of counterparts. Conversely, in the MoDTC-ZDDP blended system, TiO₂ drove interfacial reactions toward high-valence Mo oxides and sulfate species, leading to increased wear. Atomic force microscope measurements confirmed that protruded MoS₂-rich tribofilm formed by lubricant containing MoDTC and TiO₂ exhibited low shear strength, whereas the protruded oxide/sulfate-rich tribofilm formed by lubricant containing MoDTC-ZDDP and TiO₂ generated higher local friction. These findings reveal that TiO₂ nanoparticles function as selective tribochemical regulators whose effects are governed by interfacial reaction competition and additive compatibility, rather than universally enhancing lubrication performance. This work provides critical mechanistic insights for rational design of nanoparticle-modified lubricant formulations.