Borate esters possess friction‐reducing, antiwear, and anti‐oxidant characteristics when blended in lubricating oils. However, borate esters are susceptible to hydrolysis. The formation of a stable five‐member ring structure in the ester molecules, involving coordination of nitrogen with boron, contributes substantially to the resistance to hydrolysis of borate esters. The susceptibility of borates to hydrolysis can be reduced by introducing N,N‐dialkylaminoethyl groups with alkyl radicals containing more than three carbon atoms. X‐ray photo‐electron spectroscopy and X‐ray diffraction reveal that the borate ester can be adsorbed on the rubbing surface, and some of the adsorbed borate film degrades and forms boron nitride. Four‐ball wear tests indicate that the combination of oil‐soluble copper, tin, and cadmium compounds with organoborates gives better antiwear properties than the components separately. An antiwear synergistic mechanism is postulated in which borates with electron‐deficient boron p orbitals catalyse the tribo‐reduction of the metal compounds on the rubbing surfaces, producing elemental metals.
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