Role of sp3 fraction and surface morphology on the wear mechanisms of ta-C coatings under current-carrying sliding

This study examines how bonding structure and surface quality govern the tribological behaviour of tetrahedral amorphous carbon (ta-C) coatings under applied electrical currents, addressing their suitability for current-carrying sliding applications. Two ta-C coatings—designated as soft and hard— were deposited on high-speed steel (HSS) substrates via filtered cathodic vacuum arc (FCVA) and tested against SAE 52100 steel under currents from 100 to 1500 mA. The soft coating, deposited at 150 °C without substrate bias, had a hardness of 51 GPa, while the hard coating, grown at room temperature with a −100 V pulsed bias, reached 69 GPa. Both coatings improved the wear resistance of the substrate. Compared to pulsed vacuum arc (PVA) ta-C, the FCVA coatings—being smoother, harder, and less defective—were less prone to arc-induced damage. The soft ta-C consistently exhibited low coefficient of friction (COF) about 0.2 (rising to about 0.3 at 1500 mA) and minimal wear, while the hard ta-C degraded above 500 mA due to iron oxide-filled pores and localized arcing. With its lower pore and defect density, the soft coating resisted oxidation and showed limited current-induced damage, while Raman spectra confirmed the formation of few-layer graphene on its wear track, facilitated by its moderate sp³ content and aromatic sp² structures. These results indicate that bonding structure and the ability to undergo structural transformation play key roles in governing tribological behaviour under electrical current. Coatings with smooth, low-defect surfaces and moderate sp³ content demonstrated stable performance and low wear, supporting their suitability for current-carrying sliding applications.