Silicon Nitride Boundary Lubrication: Effect of Sulfonate, Phenate, and Salicylate Compounds
Richard S. Gates, Stephen M. Hsu
The boundary lubrication characteristics of sulfonates, phenates, and salicylates in a silicon nitride on silicon nitride contact were investigated. The additives were blended at one percent by weight solution in a purified paraffinic base oil and tested on a ball-on-three flat geometry in the Four Ball Tester. These compounds all exhibited effective lubricating properties for silicon nitride, including both low base and high base types. Lower friction and wear were observed as compared to the base paraffin base oil case. Load capacity tests indicated that these compounds extended load range for silicon nitride. Boundary films were observed in the wear scar suggesting the occurrence of tribochemical reactions.Surface analysis of the worn surface revealed the presence of the metal salt portion of the compound but without the calcium carbonate which was used for overbasing. This suggests that these compounds probably oxidize and then adsorb onto the surface to form lubricating films containing the metal salts. It might be hypothesized that upon decomposition, the metal portion of the detergent molecule reacts with the silicon oxide surface layer of the Si 3N4, producing a complex silicate amorphous glassstructure which protects the Si3N4 from fracture and damage.