From structural collapse to nanophase reinforcement: The dual-edged role of Cr in modulating the tribological performance of GLC coatings

To mitigate the serious adhesive wear of cemented carbide (WC–Co) tools, Cr-doped graphite-like carbon (GLC-Cr) coatings with varying Cr contents were prepared by using plasma-enhanced magnetron sputtering (PEMS). The evolution of their microstructural, mechanical, and tribological properties was systematically investigated. The GLC (G1) coating possessed a dense, sp³-rich amorphous structure, exhibiting the highest hardness (13.08 GPa) and superior load-bearing capacity. Cr doping at 1.87 at.% (G2 coating) induced the transition from sp³ to sp² hybridization and led to a porous, defective network, resulting in a sharp decline in mechanical resistance and catastrophic adhesive failure. Conversely, further increasing Cr incorporation at 4.05 at.% (G3 coating) triggered a microstructure recovery, where intensified ion bombardment densified the matrix and facilitated the precipitation of dispersed CrC nanoclusters. This structural optimization of G3 coating improved the coefficient of friction (COF) stability during the running-in stage and restricted its wear rate to 1.89 × 10⁻⁶ mm³ N⁻¹ m⁻¹, lower than that of the G2 coating (4.61 × 10⁻⁶ mm³ N⁻¹ m⁻¹) but higher than that of G1 coating (7.72 × 10⁻⁷ mm³ N⁻¹ m⁻¹). The wear mechanism transitioned from mild abrasive wear and tribofilm protection in G1 coating to catastrophic delamination in G2, and finally to a combination of fatigue spallation and Cr interlayer protection in G3. This work suggests that the overall performance is governed by the synergistic competition between Cr-induced graphitization, structural densification, and nanophase strengthening, providing a basis for the rational design of durable protective coatings for cemented carbide cutting tools.