Effect of Surface Oxide Layer of Steel on the Tribological Characteristics of Load-bearing Additives for Multiply-Alkylated Cyclopentane Oil under High Vacuum

The tribological performance of two types of additives—alkylated phenyl phosphate and lead naphthenate—dissolved in multiply-alkylated cyclopentane was evaluated under a high vacuum using two types of ball-on-disk tribometers: a reciprocating motion tribometer under mild loading conditions and a unidirectional rotating motion tribometer under heavy loading conditions. A ball and a flat disk made of SUS440C stainless steel were used as specimens for both tribometers. The surface of the as-received flat disk specimen was covered with a thick (>40 nm) oxide layer. For the examination of the effect of the surface oxide layer on the tribological performance of the liquid lubricants under a high vacuum, another specimen with an oxide layer (approx. 4 nm thick) was prepared. The alkylated phenyl phosphate additive showed better lubrication performance with the specimen with the thicker oxide layer, but the lead naphthenate additive showed superior performance with the thinner oxide layer specimen. It is also shown that these opposite tribological characteristics are explained by the hard and soft acids and bases principle.