The friction and wear performances of aluminum extrusion die surfaces are important factors affecting the quality of aluminum or aluminum alloy products and the life of extrusion dies. The deposition of wear-resistant films on the surface of extraction dies is one of the most effective strategies, and the friction and wear performances of different coatings against aluminum have been previously investigated. However, studies regarding the influence of the coating microstructure on the tribological behavior of the same coating material when applied against aluminum remain limited. Plasma-enhanced magnetron sputtering introduces an extra electron-emitting source into conventional magnetron sputtering equipment to obtain a densified and controllable plasma around the substrates, producing wear-resistant coatings with similar compositions but significantly different microstructures. The TiN coating, which is a widely used wear-resistant coating, is selected as an example and deposited by plasma-enhanced magnetron sputtering at various substrate bias currents to obtain coatings with different microstructures. The effect of the substrate bias current on the microstructure, mechanical properties, and tribological behavior of the TiN coatings against aluminum is systematically investigated to further optimize the deposition process of the wear-resistant TiN coating applied on aluminum extrusion dies. The TiN coating is prepared using plasma-enhanced magnetron sputtering under varied substrate bias currents of 0.1 A, 1.5 A, 3.0 A, and 4.5 A. The chemical compositions of the TiN coatings are analyzed using X-ray photoelectron spectroscopy (XPS). The surface and cross-sectional morphologies of the coatings are observed using scanning electron microscopy (SEM). The 3D surface microstructure and surface roughness of the coatings are studied using atomic force microscopy (AFM). The phase structures of the coatings are determined using X-ray diffraction (XRD). The comprehensive mechanical properties and tribological behavior of the TiN-coated samples against aluminum are investigated using a nano-indenter and a rotary ball-on-disk friction and wear tester, respectively. The surface morphologies and chemical compositions of the wear tracks are analyzed using laser confocal microscopy, SEM, and EDS. The results show that the variation in the substrate bias current has little impact on the chemical composition of the TiN coatings deposited by plasma-enhanced magnetron sputtering, and all the coatings have a nearly stoichiometric composition. The cross-sectional microstructure of the TiN coating is gradually refined with an increasing substrate bias current, and the surface microstructure of the coating is consisting of island-like microprojections. When the substrate bias current increases from 0.1 A to 4.5 A, the size and amount of the microprojection are gradually decreased along with the surface roughness (from 77.67 nm to 15.67 nm). The preferred growth along the TiN(111) direction dominates in all the coatings, and it is further enhanced when the substrate bias current reaches 3.0 A. The grain size of the TiN coating is pronounced decreased from 44 nm to 11 nm as the substrate bias current increases from 0.1 A to 1.5 A, and the comprehensive mechanical properties of the TiN coating are significantly improved. When the substrate bias current is further increased, the effect of the substrate bias current on the grain size and the mechanical properties of the coatings becomes unobvious. Adhesive and abrasive wear are dominated in the wear process against aluminum of TiN-coated samples, and the friction-reduction and wear-resistance performance of the TiN-coated samples is negatively correlated with the aluminum adhesion area. In conclusion, the substrate bias current plays an important role in controlling the surface cross-sectional microstructure, grain size, mechanical properties and tribological behavior of the TiN coatings deposited by plasma-enhanced magnetron sputtering. When deposited at a substrate bias current of 1.5 A, the TiN coating with excellent mechanical properties and a rough surface microstructure has the lowest friction factor and wear rate of 0.41x10(-15) and 3.03x10(-15) m(3) / (N center dot m), respectively. This study is theoretically significant and practically valuable for the research and development of high-performance and long-life protective coatings on the surfaces of aluminum-forming dies.
