Coating Life and Failure Mechanism of TiN Coating under Sliding Wear Condition with Different Normal Loads

This work aimed to explore the characteristics of wear process and failure form of TiN coating under different normal loads, and to explore the coating life under sliding wear condition of coatings on two kinds of substrates (high-speed steel (HSS) and cemented carbide (WC-Co)), and then to guide the matching between the substrate and coating and the choice of coating for cutting tools based on the service load condition. Titanium nitride (TiN) coating was prepared on HSS and WC-Co substrates using the Cathodic Arc Plasma Deposition technology under the same deposition process and parameters. The chemical component of TiN coating, HSS substrate and WC-Co substrate were detected via the energy dispersive spectrometer. The surface morphologies and surface roughness of TiN coatings were observed using scanning electron microscopy and profile-meter. The friction coefficient during the wear process of two substrates and TiN coatings were measured using a reciprocating friction wear test machine. After reciprocating friction and wear tests, wear profiles and wear morphologies of coating samples were analyzed via profile-meter, scanning electron microscope and energy dispersive spectrometer. Furthermore, based on the comprehensive analysis of friction coefficient, wear profiles and wear surface morphologies, maps of TiN coating life under sliding wear condition on different substrates were constructed. The stable friction coefficient of the HSS substrate was between 0.7 and 0.8, while that of the WC-Co substrate was between 0.2 and 0.3. For the friction coefficient of coated samples, with the increase in the number of cycles, the friction coefficient of the TiN coating on HSS were comparable to the stable friction coefficients of the substrates. The friction coefficient of TiN coating on WC-Co was about 0.5. It was found from the depth of the wear profile increased with the increase in the normal loads and the number of cycles. Based on observation and analysis of wear morphologies of the wear scars. The main failure forms of TiN coatings on HSS were abrasive wear, mild adhesive wear and oxidation wear under 30 N. Under the normal load of 60~120 N, the main failure forms were fracture, delamination and wear. During the wear process, the HSS substrate material underwent plastic deformation, and the broken coating would be embedded in the substrate for protection of the substrate. After that, as the number of cycles increased, the coating embedded in the substrate would be completely removed. The failure form of cemented carbide TiN coating was abrasive wear under 30~50 N. Under the normal load of 60~120 N, the main failure form was severe delamination. The map of TiN coating life under sliding wear condition was divided into two parts: coating working area and coating failed area. The life of the TiN coating on both substrates decreased with the increase in the normal load. TiN coating on HSS had the best ability to resist normal loads among the tested samples. The life of TiN coating was affected by the contact pressure and the ability to resist deformation of substrate material. The larger contact pressure led to the shorter coating life. Under the similar contact pressure conditions, the stronger ability of substrate to resist the deformation, the longer the coating life was.