Kara, Levent; Küçükömeroğlu, Tevfik; Efeoğlu, İhsan

Many engineering components are required to operate under both atmospheric and vacuum environments, where friction and wear mechanisms differ significantly. Therefore, solid lubricant coatings designed for such applications must maintain stable tribological performance across multiple environments rather than being optimized for a single condition. In this study, Ti/MoS₂–Ti composite coatings were deposited on AISI 52100 bearing steel using pulsed-DC closed-field unbalanced magnetron sputtering. A Taguchi L9 experimental design was employed to systematically investigate the influence of key deposition parameters, including working pressure, substrate bias voltage, and target currents, on coating composition, microstructure, mechanical properties, and tribological behavior. The coatings were characterized using SEM/EDS, XRD, and nanoindentation analyses, while friction and wear behavior were evaluated by ball-on-disc tests under atmospheric, low-vacuum (5 ×10⁻³ mbar), and high-vacuum (8 ×10⁻⁶ mbar) conditions. The coatings exhibited sulfur-to-metal ratios S/(Mo+Ti) ranging from 0.28 to 0.48, with hardness and elastic modulus values between 9.9–32.4 GPa and 150–398 GPa, respectively. Tribological performance strongly depended on both environmental conditions and coating composition. Under vacuum conditions, coatings with higher sulfur retention showed significantly improved wear resistance due to the formation of a stable MoS₂-based tribofilm, whereas sulfur-deficient coatings exhibited severe wear and tribofilm instability. The results demonstrate that high hardness alone does not guarantee improved wear resistance in vacuum environments, and coatings combining sufficient sulfur retention with balanced mechanical properties provide superior tribological stability across different environments.

Çelik, A.; Bozkurt, Y.B.

Making Mg and its alloys suitable for use as biodegradable implant material was the subject of innovative studies in this field. In this study, it was aimed to improve the surface properties of the AZ31 alloy and to increase the wear performance as the first step to improve its biodegradable character. In this context, AlTiN and Ag doped TiN films were grown on the surface by PVD method. In order to correlate wear performance with biological effects, three different solutions and two wear loads were used for liquid media experiments in terms of in vitro simulation of body fluid. An extensive post-wear performance data was collected for in vitro analysis of biodegradation.

Kovacı, H.; Bozkurt, Y.B.; Yetim, A.F.; Baran, Ö.; Çelik, A.

In this study, a comprehensive analysis was performed to investigate the corrosion and tribocorrosion behaviour of untreated and duplex ceramic coated AISI 4140 steel. Therefore, samples were plasma nitrided in a glow discharge environment and then, they were coated with diamond-like carbon (DLC) film. Electrochemical impedance spectroscopy, potentiodynamic polarization and tribocorrosion tests were performed in fourteen different parameters. Nitriding treatment increased the surface roughness compared to the base material. The negative effect of roughness on corrosion and tribocorrosion performance has disappeared with the ceramic film thickness increasing with temperature and time. Harder surfaces compared to the base material contributed to the wear resistance. Compound, diffusion layers and DLC film have a barrier effect, delaying the arrival of the corrosive liquid to the base material.