Design and Performance Evaluation of a WC-10Co4Cr/ta-C Duplex Coating System on Carbon Steel for Valve Sealing Applications

Valve sealing surfaces demand coatings with high load-bearing capacity, low friction, and corrosion resistance. This study presents a duplex coating system, comprising an AISI 4130 steel substrate, a WC-10Co4Cr interlayer, and a tetrahedral amorphous carbon (ta-C) top film—a hydrogen-free member of the diamond-like carbon (DLC) family—designed for such applications. A thick HVOF-sprayed WC-10Co4Cr interlayer (~300 µm, ~1100–1200 HV) provides mechanical support for a thin PVD-ta-C top layer (1.8 µm), preventing the “eggshell effect” on the soft steel substrate. A NiCr/Ti sublayer (650 nm) is added to improve compliance. Four systems—AISI 4130 steel substrate (S), single WC-10Co4Cr (WC), single ta-C, and duplex (WC-ta-C)—were tested under dry sliding (Al2O3, 20 N, 300 min) and in 3.5 wt.% NaCl solution. The duplex WC-ta-C coating achieves a mean CoF of 0.084 under dry sliding against Al2O3, representing a reduction of approximately 74% relative to the uncoated steel substrate. The corresponding specific wear rate is 2.08 × 10−8 mm3/(N·m), which is substantially lower than those of the single WC-10Co4Cr coating (by ~42%) and the single ta-C film (by ~90%), and more than three orders of magnitude lower than that of the bare steel. Electrochemically, WC-ta-C exhibits Ecorr = −0.181 V vs. SCE, Icorr = 1.19 × 10−8 A/cm2, and a total charge-transfer resistance Rtotal = 3.32 × 107 Ω·cm2, orders of magnitude higher than single coatings. These results demonstrate that the WC-10Co4Cr interlayer enables the ta-C layer to fully express its tribological and anti-corrosion functions, offering an effective surface strategy for valve seals in aggressive environments.