Investigation of the electrical contact resistance between tribo-pairs lubricated by nanoparticle additive

A UMT-2 tribometer was performed to measured the Electrical contact resistance (ECR) between tribo-pairs lubricated by base oil and base oil containing nano-LaF₃ (NLa), nano-Ag (NA), nano-SiO₂ ( NS) and nano-A1₂O₃ (NAl) particles. ECR measuring technique was used to verify the formation of tribo-film generated by nanoparticulate oil additive. The test was done in a reciprocating ball-on-disk mode under boundary lubricated condition at frequency of 15 Hz and normal load of 5 N. Variation of electrical contact resistance between tribo-pairs was observed during the first 250 s when it was lubricated by base oil, then a drop of base oil containing 2% NLa, NA, NS or NA1 was added. ECR was recorded continuously, thus a curve of the ECR versus time was obtained. Because the interface film generated by nanoparticulate oil additive acted as either an insulating barrier or electrical layer for electrical current, ECR was used to visualize the formation of interface film. The formation of tribo-films was further proved by surface analysis on wear scar using XPS. ECR measuring technique provided real-time information about the formation of surface films. The chemical state change of nanoparticles did not happen during rubbing, but its capability of forming a film were different. The deposit velocity and the protective film thickness of NLa and NA were larger than that of NS and NAl. Friction test performed in the presence of NLa or NA was characterised with the formation of an interface film which was strongly bonded to worn surface.