In this study, the structural characteristics of TiN coatings deposited by DC magnetron sputtering on the surface of AISI 304 stainless steel were modulated by performing deposition at four different nitrogen flux ratios. The XRD results indicated that the coatings mainly contained face-centered cubic TiN phase. The experimental results showed that the nitrogen flux ratio played a major role in determining the microstructure and the mechanical and tribological properties of the TiN coatings. SEM images revealed that the thickness of the TiN coatings decreased linearly as the nitrogen flux ratio increased from 0.25 to 0.55. However, the grain size had a nonlinear relationship with the nitrogen flux ratio. When the nitrogen flux ratio was 0.45, the grain size was only 5.3 nm. Theoretical and experimental analysis showed that the TiN coating deposited at a nitrogen flux ratio of 0.45 had the best mechanical properties, which due to its minimum grain size and (111) orientation, and the best tribological performance under unlubricated conditions, may have been due to its higher fracture toughness and plastic deformation resistance among the four TiN coatings.

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.