Key Takeaways & Executive Findings
- •• • DLC topcoat reduces MCD surface roughness from 155.33 nm to 123.77 nm and UNCD roughness from 92.43 nm to 81.90 nm, directly lowering asperity contact and interfacial shear stress in seawater—critical for minimizing seal leakage and wear in rotary equipment. • • MCD/DLC composite achieves a 32.08% reduction in steady-state friction coefficient and a 12.22% reduction in specific wear rate compared to monolithic MCD, extending seal ring service intervals and reducing maintenance costs in marine propulsion systems. • • UNCD/DLC composite delivers a 26.67% friction coefficient reduction and a 20.92% wear rate reduction versus monolithic UNCD, with the finer grain structure providing a smoother foundation that enhances DLC lubrication efficiency under boundary conditions. • • DLC incorporation accelerates the sp2 phase transformation and graphitization at the sliding interface, as confirmed by Raman and XPS, reducing debris generation and counterface ball damage—mitigating abrasive wear and prolonging the lifespan of mating components in seawater environments.
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Abstract
Silicon carbide mechanical seal rings in marine rotary equipment suffer severe wear, demanding coatings that simultaneously deliver high hardness, low friction, and corrosion resistance. This study fabricates diamond/diamond-like carbon (DLC) duplex coatings via hot-filament chemical vapor deposition (HFCVD) followed by magnetron-sputter-assisted ion-beam deposition. Microcrystalline diamond (MCD) and ultra-nanocrystalline diamond (UNCD) underlayers are grown on SiC, then capped with a hydrogenated DLC lubricating topcoat, forming a rigid-underlayer/lubricating-top-layer architecture. Tribological tests in simulated seawater reveal that DLC reduces MCD surface roughness from 155.33 nm to 123.77 nm and UNCD roughness from 92.43 nm to 81.90 nm. The MCD/DLC coating lowers steady-state friction coefficient and specific wear rate by 32.08% and 12.22%, respectively; UNCD/DLC achieves 26.67% and 20.92% improvements. SEM, Raman, and XPS analyses of worn surfaces indicate that the DLC top layer mitigates interfacial shear stress, enhances boundary lubrication, and accelerates friction-induced graphitization. The composite coating also reduces counterface ball damage and debris accumulation, extending the service life of mating components. These findings demonstrate that the duplex architecture overcomes the inherent limitations of monolithic diamond coatings, offering a viable route for durable marine seal applications.
1. Introduction
Silicon carbide mechanical seal rings in marine rotary equipment face relentless degradation from seawater corrosion and abrasive wear. Monolithic diamond coatings, despite exceptional hardness, exhibit high surface roughness and intrinsic brittleness, leading to elevated friction coefficients and premature failure under boundary lubrication. The hard, faceted grains of microcrystalline diamond (MCD) and the finer but still rough ultra-nanocrystalline diamond (UNCD) generate severe interfacial shear stresses, accelerating counterface wear and debris accumulation. These limitations stall the direct application of diamond coatings in critical marine sealing systems where both low friction and high wear resistance are mandatory.
This study engineers a duplex architecture that decouples mechanical load-bearing from tribological function. HFCVD-grown MCD and UNCD underlayers provide a rigid, wear-resistant foundation, while a magnetron-sputter-assisted ion-beam-deposited DLC topcoat delivers a low-friction, graphitizable lubricating surface. The DLC layer reduces roughness and promotes sp2 phase transformation during sliding, effectively lowering interfacial shear stress and enhancing boundary lubrication. By systematically quantifying friction and wear reductions in simulated seawater, this work establishes a scalable surface modification strategy that addresses the high-wear bottleneck of SiC seal rings in marine environments.
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YANG Chengye, SONG Hui, LI He, MU Yuanyuan, YANG Shihao, GUO Peng, JIANG Nan, MAO Xinbiao, Kazuhito Nishimura (2026). Tribological Performance of Diamond/Diamond-like Composite Coatings in Simulated Seawater Environment. Surface Technology (表面技术). https://doi.org/10.16490/j.cnki.issn.1001-3660.2026.11.002
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Frequently Asked Questions
What is the primary failure mechanism of monolithic diamond coatings in seawater that this duplex architecture addresses?
Monolithic diamond coatings fail due to high surface roughness (MCD: 155.33 nm; UNCD: 92.43 nm) that generates severe interfacial shear stress and abrasive wear. The hard, faceted grains cause counterface ball damage and debris accumulation, leading to elevated friction coefficients and accelerated wear. The DLC topcoat reduces roughness to 123.77 nm (MCD/DLC) and 81.90 nm (UNCD/DLC), lowering shear stress and promoting graphitization, which mitigates abrasive wear and extends mating component life.
How does the DLC layer affect the friction and wear performance of MCD versus UNCD underlayers?
DLC reduces the steady-state friction coefficient of MCD by 32.08% and specific wear rate by 12.22%. For UNCD, the reductions are 26.67% and 20.92%, respectively. The UNCD/DLC system benefits from a finer grain structure that provides a smoother foundation, enhancing DLC lubrication efficiency. Both composites outperform their monolithic counterparts, but the relative improvement in wear rate is more pronounced for UNCD/DLC due to its inherently lower roughness.
What evidence confirms that DLC accelerates graphitization at the sliding interface?
Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) analyses of worn surfaces reveal an increased sp2/sp3 ratio and graphitic carbon formation. The DLC top layer promotes friction-induced sp2 phase transformation, creating a graphitic tribofilm that reduces interfacial shear stress and enhances boundary lubrication. This graphitization process is more pronounced in composite coatings than in monolithic diamond, as indicated by the Raman D and G band intensities.
Can this duplex coating be scaled for industrial marine seal applications, and what are the cost implications?
The hybrid HFCVD and magnetron-sputter-assisted ion-beam deposition process is compatible with batch production and has been demonstrated on SiC substrates. While the two-stage deposition adds cost compared to single-layer coatings, the 12.22–20.92% wear rate reduction and 26.67–32.08% friction reduction translate to extended seal life and reduced maintenance downtime. For high-value marine rotary equipment, the cost parity is favorable when factoring in avoided replacement expenses and improved operational reliability.
How does the composite coating perform in terms of counterface damage and debris generation?
The DLC top layer reduces counterface ball damage by lowering interfacial shear stress and minimizing hard debris accumulation. The smoother surface (roughness reduced by 20.3% for MCD/DLC and 11.4% for UNCD/DLC) decreases abrasive wear and the generation of wear debris. This effect is critical for maintaining a stable tribofilm and preventing third-body abrasion, thereby increasing the service life of the mating pair in seawater environments.
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