Caracterização tribológica da lubrificação sólida

Abstract

This work aims to study the tribological behavior of two solid lubricant materials used in different conditions: nanoparticles as additives for lubrication fluids and self-lubricating sintered steels. The synthetic base oil with different concentrations and kinds of nanoparticles (MoS2 particles, MoS2 nanoflowers, carbon nanotubes+MoS2, silver nanoparticles) was extensively evaluated through tribological tests: ball on disk scuffing; ball on disk constant load (250 N, 20min. duration) and constant load (2N, 20min. duration) reciprocating linear sliding tests assessing friction coefficient. None of the nanoparticles affected the friction coefficient in a significant way. The nanoparticles were found to be an aggregated state. On the other hand, the effect of sintering temperatures (1100, 1150 and 1200 °C) and precursor content (0-5%) on the tribological behavior of self-lubricating sintered steels, produced by metal injection molding (MIM), was analyzed by using linear reciprocating sliding tests (constant load 7N, 60 min. duration). Solid lubrication effect was produced by in situ formation of graphite nodules due to the dissociation of precursor (SiC particles) during sintering. It was shown that presence of the graphite nodules significantly improved the friction coefficient and the wear rate of the alloys and that the sintering temperature little affected these parameters. Chemical analyses of the wear scars by scanning electron microscopy (SEM-EDX) and auger electron spectroscopy (AES) showed the presence of a tribolayer composed predominantly by carbon and oxygen. The graphite on the samples contributed to the formation of the tribolayer on the contact interface. It is suggested that during sliding graphite foils are removed from the in situ generated graphite nodules and remain at the interface thus contributing to the formation of the protective tribolayer. This tribolayer breaks up and forms during sliding and it is continuously replenished by graphite. Analyses of the wear scars showed the presence of plastic deformation traces on the samples and counter-bodies and the predominance of the wear mechanism by abrasion. Study of the influence of roughness surface showed that polishing the surfaces closed the graphite sources due to plastic deformation. This fact has resulted in a friction coefficient higher than 0.2, which means that, in this condition, this material does not act anymore as solid lubricant.