| The oxidation and polyesterification in bulk lubricant and lubricant-steel interface were studied in this work. The oxidation products of bulk base oil oxidized at 180{dollar}spcirc{dollar}C were identified by FTIR to have five functional groups: alcohol, aliphatic acid/ketone, aromatic acid/ketone, ester/polyester, and lactone; and that of paraffinic mineral oil and tetradecane were also identified to have five functional groups: alcohol, aliphatic acid/ketone, ester/polyester, lactone, and cyclic anhydride. The order of oxidation rate was found to be: tetradecane {dollar}>{dollar} paraffinic mineral oil {dollar}>{dollar} base oil. The oxidation of base oil increased with the surface area of stainless steel powder linearly. The route to form polyester in bulk lubricant may be dominated by alternative oxidation and esterification. Ring-forming reactions to form lactone and cyclic anhydride compete with polyesterification.; Polyesterification in sliding under boundary lubrication conditions was found to take place mainly on steel surface rather than in the bulk. Polycondensation of hydroxy carboxylic acids is the most probable route to form polyester. Without antiwear additive ZDTP (zinc dialkyldithiophosphate), the surface films were found to be composed of iron oxide and polyester with the polyester on the top. With ZDTP in the lubricant, the films are composed of polyester and organometallic compounds of sulfur, phosphorus, oxygen, carbon, and iron. More polyester is easier to form on the top of organometallic compounds than on iron oxide. |
