To address the simultaneous and critical demand of marine moving parts for advanced protective coatings that synergistically combine high hardness, low friction coefficient, and exceptional corrosion resistance, an innovative diamond/DLC duplex coating architecture is strategically designed and fabricated through a hybrid deposition approach integrating hot-filament chemical vapour deposition (HFCVD) and magnetron-sputter-assisted ion-beam deposition. This sophisticated fabrication methodology is specifically conceived to engineer a composite coating system that overcomes the inherent limitations of monolithic coatings. The deposition sequence commences with the synthesis of foundational diamond layers onto silicon carbide (SiC) substrates by HFCVD. Two distinct diamond morphologies are engineered: a micro-crystalline diamond (MCD) layer, characterized by its faceted, large-grain structure providing supreme load-bearing capacity, and an ultra-nanocrystalline diamond (UNCD) layer, featuring exceptionally fine, nanometer-scale grains that yield a naturally smoother surface finish. These diamond layers serve as the primary, rigid mechanical support, and are engineered to resist extreme contact pressures and abrasive wear in harsh marine environments. Subsequently, a uniform, hydrogenated diamond-like carbon (DLC) lubricating topcoat is deposited onto these diamond underlayers via the magnetron-sputter-assisted ion-beam deposition technique, renowned for producing dense, adherent amorphous carbon films with controlled sp3/sp2 bonding ratios. This precise two-stage synthesis successfully yields a functionally graded "rigid underlayer-lubricating top layer" architectural design. The fundamental operational principle of this architecture lies in its ability to synergistically harness the ultrahigh hardness and outstanding chemical inertness intrinsic to the diamond phases, while simultaneously capitalizing on the excellent solid-lubricating properties and low shear strength characteristic of the DLC topcoat, thereby effectively overcoming the performance compromises typically encountered in single-layer coating systems, such as the high initial roughness of MCD or the limited mechanical support of standalone DLC films. Comprehensive tribological characterization is systematically performed in a simulated seawater environment to evaluate the coating performance under conditions relevant to marine applications. Surface profilometry measurements quantitatively demonstrate the critical role of the DLC overlayer in surface engineering, showing that it significantly reduces the arithmetic mean surface roughness (Ra) of the underlying MCD base layer from an initial 155.33 nm to a final 123.77 nm, thereby mitigating the initial abrasive interactions and potential stress concentration. A corresponding, though less pronounced, smoothing effect is observed on the inherently finer UNCD base layer, where the Ra parameter is further diminished from 92.43 nm to 81.90 nm following the conformal DLC deposition. Ball-on-disk tribometer testing under controlled conditions provides quantitative evidence of substantial performance enhancement for the duplex coatings relative to their uncoated diamond counterparts. Specifically, the MCD/DLC hybrid configuration exhibits a remarkable 32.08% reduction in its steady-state friction coefficient and a 12.22% decrease in its calculated specific wear rate. Meanwhile, the UNCD/DLC architecture demonstrates even more impressive friction performance with a 26.67% reduction in the steady-state friction coefficient, coupled with a more substantial 20.92% improvement in wear resistance, highlighting the particular efficacy of combining the DLC topcoat with the naturally smoother UNCD underlayer. Post-test characterization of the worn surfaces utilizing scanning electron microscopy (SEM), micro-Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) provides mechanistic insights into the tribological behavior. These analytical techniques collectively reveal that the DLC film performs multiple critical functions during sliding contact: it effectively alleviates interfacial shear stresses by acting as a compliant, low-shear-strength layer, thereby enhancing boundary lubrication regimes. Furthermore, the metastable nature of the DLC's amorphous carbon network accelerates the kinetics of friction-induced graphitization, facilitating the formation of a sp2-carbon-rich tribofilm that provides easy shear planes at the contact interface. The synergistic interaction between this graphitized tribolayer and the aqueous environment of the simulated seawater, where water molecules act as both cooling agents and passive lubricants, contributes significantly to the observed friction and wear reduction. This research, therefore, provides both a fundamental scientific basis for understanding the tribochemical mechanisms in hierarchical carbon-based coatings in aqueous environments and a practical, scalable engineering route for developing next-generation, long-life protective coatings for critical moving parts in high-end marine equipment, such as propeller shaft bearings, hydraulic piston pumps, and underwater robotic joints, operating under simultaneous mechanical and corrosive challenges.
Key words
diamond /
diamond-like carbon (DLC) /
composite coating /
seawater environment /
tribological mechanism
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Funding
National Natural Science Foundation of China (52475231); the Science and Technology Innovation 2035 Major Projects, Ningbo, China (2024Z097); the Science and Technology Innovation 2025 Major Projects, Ningbo, China (2023Z009)
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