Microstructure and Cryogenic Wear Resistance of Spherical Tungsten Carbide Reinforced Cobalt-Based Overlay Coating

Literature Overview

Published in Surface Technology in 2022 by Shi Liang, Chen Kun, Chang Xuetin, Wang Dongsheng, and Yin Yansheng, this paper investigates the use of plasma transfer arc (PTA) overlay welding to deposit cobalt-based coatings reinforced with spherical tungsten carbide (WC) particles on E32 low-temperature steel. The research was supported by the National Key R&D Program of China (2016YFB0300704), the National Natural Science Foundation (52071091), and the Pudong New Area Science and Technology Project (PKJ2019-C03). The work addresses a critical need for improving the wear resistance of cryogenic marine steels used in polar vessel hulls, which operate in extremely cold environments and are subject to severe abrasive wear from ice and snow.

Experimental Design and Coating Development

The authors prepared three groups of cobalt-based coatings with different spherical WC contents on E32 steel substrates using the PTA overlay welding process. The coatings were then subjected to dry sliding wear tests at -20°C under a load of 50 N and a sliding speed of 20 mm/s for 2 hours. The wear performance was evaluated by measuring the friction coefficient, volume wear rate, and wear scar geometry (width and depth).

The results demonstrate a clear trend: increasing the WC content improves the cryogenic wear resistance of the coating. The unmodified cobalt-based coating (without WC) already provides better wear resistance than the bare E32 steel, but the addition of WC particles produces a significant additional improvement. The wear mechanism transitions from a combination of abrasive and adhesive wear in the unmodified coating to predominantly three-body abrasive wear in the WC-reinforced coatings.

Microstructural Analysis and Wear Mechanism

The authors used X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and 3D optical profilometry to characterize the coating microstructure and wear behavior. The WC-reinforced coatings contain a rich array of hard carbide phases, including WC, W₂C, Cr₂₃C₆, Cr₇C₃, Co₆W₆C, and Fe₆W₆C. These carbide phases are distributed throughout the cobalt-based matrix, providing multiple mechanisms for wear resistance enhancement.

The presence of multiple carbide phases is significant because each phase contributes differently to the coating's wear resistance. WC and W₂C provide high hardness and abrasion resistance. Cr₂₃C₆ and Cr₇C₃ contribute to both hardness and corrosion resistance. The Co₆W₆C and Fe₆W₆C phases, which form during the welding process from the interaction of WC with the molten pool, provide additional hardening and may influence the coating's thermal stability.

The transition from two-body to three-body abrasive wear with increasing WC content is an important finding. Three-body abrasive wear occurs when hard particles become embedded in the softer coating surface and act as abrasives against the counterface. In the WC-reinforced coatings, the spherical WC particles can become dislodged from the matrix during sliding and act as grinding agents, but the overall wear rate still decreases because the matrix itself is harder and more resistant to material removal.

Engineering Practice Implications

The application of PTA overlay welding for cryogenic marine steel repair and protection is a significant engineering advancement. E32 steel is a nickel-aluminum low-temperature steel used in polar vessel construction, and its wear resistance at cryogenic temperatures is a critical concern for hull durability. The PTA process offers several advantages for this application: it produces dense, well-bonded coatings with low dilution; it can be applied to large structures in the field; and it allows for the incorporation of hard particles such as WC to enhance wear resistance.

The spherical morphology of the WC particles is worth noting. Spherical WC particles are expected to distribute more uniformly in the molten pool and produce a more homogeneous coating microstructure compared to irregularly shaped WC particles. The spherical shape also reduces stress concentration at the particle-matrix interface, which is beneficial for coating toughness and crack resistance.

Key Questions and Reflections

Several important questions arise from this work. First, the test temperature of -20°C is relatively mild compared to actual polar service conditions, where temperatures can reach -40°C to -50°C. The wear behavior of the coating at more extreme cryogenic temperatures may differ from what is observed at -20°C, and additional testing at lower temperatures would be valuable. Second, the wear testing was conducted under dry sliding conditions, while in actual polar service, the coating is exposed to ice, snow, salt water, and potentially abrasive sediments. The interaction between corrosion, abrasion, and thermal cycling in a real marine environment is likely to be more complex than what is captured in laboratory tests.

The paper also does not address the long-term stability of the coating under repeated thermal cycling, which is a critical concern for polar vessel hulls that experience temperature variations between the cold air and warmer seawater. Thermal fatigue can degrade the coating-substrate bond and initiate delamination, which would compromise the coating's protective function.

Study Insights and Implications

This research demonstrates that the PTA overlay welding process is an effective method for depositing WC-reinforced cobalt-based coatings on cryogenic marine steels, with clear improvements in wear resistance as WC content increases. The identification of multiple hard carbide phases in the coating microstructure provides a scientific understanding of the wear resistance mechanism. For engineers working on polar vessel hull protection, the key takeaway is that PTA overlay welding with WC-reinforced cobalt-based coatings offers a practical and effective solution for extending the service life of cryogenic steel components. The work also highlights the importance of matching the coating's wear mechanism to the service environment: the transition to three-body abrasive wear with increasing WC content suggests that the coating's wear behavior is dominated by particle-matrix interactions, which can be optimized through careful control of WC size, distribution, and bonding with the matrix. Future work should focus on testing at more extreme cryogenic temperatures, evaluating the coating's performance under combined corrosion and abrasion conditions, and assessing the long-term stability under thermal cycling to fully qualify the coating for polar vessel applications.