Microstructure and Tribological Properties of Spherical WC-Reinforced Iron-Based Composite Plasma Overlay Coatings

Literature Overview

This paper, authored by Fan Li and colleagues from Shanghai Maritime University and published in Tribology International in 2018 (Vol. 38, Issue 1, pp. 17-27), investigates the microstructure and dry sliding friction-wear behavior of WC-reinforced iron-based composite coatings deposited on 304 stainless steel by plasma arc overlay welding. Funded by the Marine Public Welfare Industry Research Special Fund (201405013-3), the National Natural Science Foundation of China (51609133), and Shanghai Maritime University Research Fund (20130448), the study addresses the challenge of improving the tribological performance of 304 stainless steel, which is widely used in marine and offshore applications but suffers from relatively low hardness and poor wear resistance. Two WC volume fractions—30% and 60%—were investigated to determine the optimal reinforcement level.

Core Technical Findings

The plasma arc overlay process was used to deposit iron-based composite coatings containing spherical WC particles onto 304 stainless steel substrates. The choice of spherical WC is significant, as the particle shape affects both the microstructure of the coating and the tribological behavior during sliding contact. Spherical particles distribute stress more uniformly than angular or irregular particles, reducing the likelihood of stress concentration at the particle-matrix interface.

The microstructural analysis reveals that the coatings consist of a Cr-rich austenitic solid solution matrix, high-hardness Cr7C3 carbides, and WC reinforcement particles. The presence of both Cr7C3 and WC provides a dual hardening mechanism: Cr7C3 forms in situ during solidification from the iron-based matrix, while WC particles are retained from the composite powder feedstock. The spherical WC particles remain largely intact after the plasma arc overlay process, indicating that the thermal exposure was sufficient to melt the iron-based matrix but not to dissolve or decompose the WC particles completely.

Microhardness and Tribological Performance Comparison

The 30% WC coating exhibits a microhardness of HV0.2 665, representing a 21.1% improvement over the iron-based coating without WC, while the 60% WC coating reaches HV0.2 724, a 31.9% improvement. Both coatings are 3.7 and 4 times harder than the 304 stainless steel substrate, respectively. The tribological testing, conducted under a constant load of 50 N and a sliding velocity of 20 mm/s in dry conditions, reveals that the 60% WC coating outperforms the 30% WC coating in both friction coefficient (0.42 vs. 0.59) and wear rate (1.111 × 10⁻⁶ vs. 2.639 × 10⁻⁶ mm³·N⁻¹·m⁻¹).

The wear mechanism analysis reveals an important distinction between the two WC contents. The 30% WC coating exhibits a mixed wear mechanism of adhesive wear and two-body abrasive wear, while the 60% WC coating transitions to a predominantly three-body abrasive wear mechanism. This transition is attributed to the higher hardness and greater volume fraction of WC particles in the 60% coating, which reduces the tendency for adhesive transfer and promotes the generation of loose abrasive debris that mediates three-body abrasion.

Engineering Practice Implications

For marine and offshore applications where 304 stainless steel is commonly used for corrosion resistance but requires enhanced wear performance, this study provides a practical solution. Plasma arc overlay welding is a well-established industrial process with relatively low equipment costs compared to advanced thermal spray methods, making it accessible for retrofitting existing components. The 60% WC coating offers superior tribological performance, but engineers must consider the trade-off between performance and cost, as higher WC content increases material cost and may require more careful process control to prevent particle cracking or matrix discontinuity.

In practice, the choice between 30% and 60% WC should be guided by the specific application requirements. For components experiencing moderate wear with some adhesive contact—such as pump impellers, valve seats, and marine hardware—the 30% WC coating may provide adequate performance at lower cost. For severe abrasive wear conditions—such as propeller shafts, thruster nozzles, and anchor chain components—the 60% WC coating's superior wear resistance justifies the additional cost. The dry sliding conditions tested in this study represent a conservative case; in actual marine environments, the presence of water and marine debris may alter the wear mechanisms, and this should be considered when extrapolating laboratory results to field conditions.

Study Insights and Reflections

This study is notable for its systematic approach to WC content optimization and its detailed tribological characterization. The distinction between two-body and three-body abrasive wear mechanisms is a valuable insight for engineers, as it explains why simply increasing hard particle content does not always produce proportional improvements in all tribological parameters. The transition from adhesive-plus-two-body to three-body wear at 60% WC is a critical threshold that should be considered in coating design.

One limitation of the study is the use of a single WC particle size. In practice, WC particles are available in various sizes, and the particle size distribution can significantly affect coating properties. Smaller WC particles provide better dispersion and more uniform hardness but may be more susceptible to dissolution during the plasma arc process. Larger particles provide higher local hardness but may create stress concentrations. A multi-size WC approach could potentially optimize both dispersion and hardness, and this represents an interesting direction for future research. Additionally, the study does not address the effect of WC content on the coating's corrosion resistance, which is a critical parameter for marine applications. The presence of WC particles could potentially create galvanic coupling with the iron-based matrix, and this interaction should be evaluated for long-term service in seawater.