Achieving low friction and wear in superhard TiCₓ coatings on GCr15 steel via Ti-induced graphitization and TiO₂ stabilization

Ti-based carbon coatings are promising for low-friction and wear-resistant applications, but their performance on steel substrates is often limited by poor adhesion, high residual stress, and unstable tribological behavior under ambient conditions. In this work, titanium carbide (TiC_x) coatings were deposited on GCr15 bearing steel using a hybrid strategy combining magnetic-filtered cathodic arc ion sputtering and magnetron sputtering (MS). Dense Cr/CrN/CrTiN/TiCN graded interlayers ensured strong adhesion and a smooth mechanical transition to the substrate, while the TiC_x top layer was tuned via the carbon-target direct current (DC). The TiC_x-1 coating, deposited at 4 A, exhibits an exceptional hardness of 43 GPa and a low coefficient of friction (COF) of ~0.18, demonstrating a superior combination of elastic–plastic properties and wear resistance. Structural analyses reveal that increasing the DC promotes sp² carbon formation and graphitization, while Ti incorporation facilitates TiC reinforcement and the in situ generation of TiO₂ during sliding. Tribological tests, Raman spectroscopy, and focused ion beam high-resolution transmission electron microscopy (FIB-HRTEM) observations confirm that friction and wear are dominated by the formation of a stable graphitized transfer film supported by TiO₂, which reduces shear stress and stabilizes the sliding interface. However, excessive carbon enrichment at higher currents destabilizes the transfer film and degrades mechanical integrity, leading to higher friction and wear. These results highlight the synergistic roles of interfacial architecture, carbon-bonding evolution, and Ti-assisted tribochemistry in governing the tribological performance of TiC_x coatings. This study provides a novel design strategy and fundamental insights for developing superhard, low-friction coatings for steel components under ambient conditions.