Novel Ti-Enhanced Plasma Nitriding (PNTi)/Diamond-like Carbon (DLC) Composite Coating with Strong Adhesion Strength and Excellent Wear Resistance

To improve the adhesion and tribological performance of diamond-like carbon (DLC) coatings on steel substrate, a Ti-enhanced plasma nitriding (PNTi) layer was formed on the surface of 38CrMoAl steel, followed by deposition of a Cr-based interlayer (mainly CrN) and then a W interlayer. Finally, a DLC coating was deposited, resulting in a novel PNTi/DLC coating. For comparison, a conventional PN/DLC coating was prepared under the same processing conditions. Optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy, hardness tests, and tribological experiments were performed to systematically investigate the effect of TiN-enriched PNTi supporting layer on the performances of the PNTi/DLC composite coating. The results show that comparing with PN/DLC composite coating, the critical load (Lc2) of the PNTi/DLC coating was increased from 28.89 N to 43.25 N—about a 50% enhancement. The microhardness was increased from 2650 HV0.05 to 4400 HV0.05 (corresponding to 28.2 GPa to 44.1 GPa). The friction coefficient was decreased from 0.28 to 0.11, about a 60% reduction, and the wear rate declined more than 40%, from 4.81 × 10−6 to 2.90 × 10−6 mm3·N−1·m−1. The introduction of Ti promoted the in situ formation of TiN phase in the nitrided layer, which significantly improved the compactness of the nitrided layer and the adhesion at the film–substrate interface. Consequently, the PNTi/DLC composite coating exhibited excellent wear resistance and friction stability under high-load and severe tribological conditions. This study provides a promising perspective for engineering applications of steel-based DLC coatings in harsh service environments.