Effect of Selected Chemical Compounds on the Lubrication of Silicon Nitride

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Abstract
Successful use of advanced ceramics in many tribological applications requires an understanding of the physical, chemical, and mechanical properties of the material. Physical and mechanical data are relatively abundant for most ceramics. However, information on the chemical interactions of ceramics is scarce. This is especially true for chemical interactions with regard to lubrication of these materials. This paper investigates the influence of selected chemical compounds on the friction and wear of silicon nitride under boundary lubrication conditions. A ball-on-three-flat modification of the four-ball wear tester was utilized to evaluate the tribological characteristics of a hot pressed silicon nitride lubricated with a paraffinic base oil containing 1 weight percent additives. Friction, wear, and. film formation tendencies were observed for a range of oil soluble chemical compounds containing oxygen, sulfur, nitrogen, chlorine, and phosphorous. A wide range of additive response was observed. Friction coefficient varied from increases of 30 percent to decreases of 47 percent below that of the base case of paraffin oil without additive. Wear was measured as the wear scar diameter increase above the Hertz diameter. Relative to the base case, wear additive response ranged from an increase in diameter of 61 percent to a decrease of 96 percent. Successful wear reduction was obtained by all phosphorus containing compounds, several glycol compounds, oleic acid, sulfonates, a salicylate, an imidazoline, and a phenate, and was generally associated with the formation of a film in the contact region.