Silicon nitride boundary lubrication: Effect of sulfonate, phenate and salicylate compounds

The boundary lubrication characteristics of sulfonates phenates, and salicy lates in a silicon nitride on silicon nitride contact were investigated. Th e additives were blended at one percent by weight solution in a purified pa raffinic base oil and tested on a ball-on-three flat geometry in the Four-B all Tester. These compounds all exhibited effective lubricating properties for silicon nitride, including both loew base and high base types. Lower fr iction and wear were observed as compared to the base paraffin base-oil cas e. Load capacity tests indicated that these compounds had extended load ran ges for silicon nitride. Boundary films were observed in the wear scar sugg esting the occurrence of tribo-chemical reactions. Surface analysis of the worn surface revealed the presence of the metal sal t portion of the compound but without the calcium carbonate which was used for overbasing. This suggests that these compounds probably oxidize and the n adsorb onto the surface to form lubricating films containing the metal sa lts. It is hypothesized that upon decomposition, the metal portion of the d etergent molecule reacts with the silicon oxide surface layer of the Si3N4, producing a complex silicate amorphous "glass" structure which protects th e Si3N4 from fracture and damage.