Silicon Nitride Boundary Lubrication: Effect of Oxygenates

A ball-on-three-flat (BTF) wear tester was used to investigate the boundary lubricating characteristics of oxygenates on a commercial silicon nitride. A wide variety of oxygen-containing compounds were tested neat and/or at 1% by weight in a paraffin oil. Compounds containing hydroxyl functional groups were more effective compared to a base case of neat paraffin oil. Decreases of up to 58% in friction coefficient, and 95% in wear were obtained. In most cases, films were observed in and around the wear scar, suggesting chemical reactions had taken place in the contact.

Additional wear tests, conducted using neat shorter-chain linear primary alcohols, i.e., 6–10 carbons, demonstrated boundary lubrication protection, with longer chain length providing better antiwear performance. A study of several C₈ compounds with specific oxygen-containing functional groups (primary alcohol, secondary alcohols, acid, aldehyde, and ketone) demonstrated that the primary alcohol had the strongest boundary lubricating effect. Varying the amount of water in the alcohols had little effect on friction and wear, suggesting that the boundary lubrication effects observed were not merely due to dissolved water in these fluids, but some characteristic chemical interaction with the hydroxyl functional group of the alcohols and acids.