Influence of Cycloalkane Carboxylic Acid Ring Size on Tribological Properties of TiN Coatings Under Lubricated Conditions

This study aimed to investigate the tribological properties and lubrication mechanisms of three cycloalkane carboxylic acids (cyclopropane, cyclobutane, and cyclopentane) as additives in PAO4 base oil. The testing was conducted in a TiN/steel tribosystem under ball-on-flat reciprocating sliding motion with a maximum contact pressure of 0.9 GPa. Notably, cyclopropane carboxylic acid exhibited excellent anti-friction and anti-wear properties, achieving a microscale coefficient of friction of 0.008 and a wear rate of only 7.0 × 10−8 mm3/N·m. XPS and Raman analyses of the tribofilms revealed that all three cycloalkane carboxylic acids underwent tribochemical reactions during frictional motion to form carbon-based tribofilms with varying contents. Moreover, cyclopropane carboxylic acid exhibited significant tribochemical reactions, forming carbon-based tribofilm with superior tribological properties. This behavior can be attributed to the stability and size of the carbon ring in cycloalkane, which influences its dissociation and the formation of carbon-based tribofilm under high-temperature and contact stress conditions.