Microstructure and Tribological Properties of WC-Reinforced Iron-Based Composite Plasma Surfacing Layers

Literature Overview

This paper, published in the Tribology journal (2018, Vol. 38, No. 1, pp. 17-27) by Fan Li et al. from Shanghai Maritime University, investigates the microstructure and dry sliding friction-wear behavior of WC-reinforced iron-based composite coatings produced by plasma surfacing on 304 stainless steel substrates. The study compares coatings with 30% and 60% WC content against an unreinforced iron-based coating. Funded by the Marine Public Welfare Industry Research Special Project (201405013-3) and the National Natural Science Foundation (51609133), the research addresses the tribological enhancement of marine-grade stainless steel components.

Core Technical Points

The plasma surfacing process produces coatings with a microstructure consisting of Cr-rich solid solution strengthened austenite, high-hardness Cr7C3 carbides, and WC reinforcement particles. The WC addition significantly enhances both hardness and wear resistance, with the 60% WC coating outperforming the 30% WC coating in all tribological metrics.

Hardness and Microstructural Comparison

Coating Type	Microhardness (HV0.2)	Relative to Iron-Based	Relative to 304 Substrate
Iron-based (baseline)	~536	1.00	1.00
30% WC	665	1.211	3.7
60% WC	724	1.319	4.0

The hardness improvement of 21.1% and 31.9% for 30% and 60% WC coatings, respectively, is attributed to the combined effects of WC particle reinforcement, Cr7C3 carbide precipitation, and solid solution strengthening of the austenitic matrix by chromium. The WC particles, with an intrinsic hardness of approximately 1500-2000 HV, act as load-bearing elements that resist plastic deformation under sliding contact.

Tribological Performance

Coating Type	Friction Coefficient	Wear Rate (×10⁻⁶ mm³·N⁻¹·m⁻¹)	Dominant Wear Mechanism
30% WC	0.59	2.639	Adhesive + two-body abrasive
60% WC	0.42	1.111	Three-body abrasive

The 60% WC coating exhibits both a lower friction coefficient and a significantly lower wear rate compared to the 30% WC coating. The shift from a mixed adhesive/two-body abrasive mechanism to a three-body abrasive mechanism indicates that the higher WC content promotes the formation of a protective transfer film composed of detached WC particles and debris that act as a lubricating layer between the sliding surfaces.

Plasma Surfacing Process Parameters

Plasma surfacing is a thermal spray-based process that uses a plasma torch to simultaneously melt the substrate surface and the feed powder, creating a metallurgically bonded coating. The process offers excellent dilution control, high deposition efficiency, and the ability to produce coatings with minimal porosity. For WC-reinforced coatings, the plasma temperature must be carefully controlled to avoid excessive WC decomposition. WC has a melting point of approximately 2870°C, but it decomposes at temperatures above approximately 1500°C in the presence of oxygen and carbon. Excessive decomposition leads to the formation of W2C and Fe3W3C, which are softer than WC and reduce the coating's wear resistance.

Engineering Practice Integration

For marine applications—such as shafts, propeller hubs, and pump components exposed to seawater and sand-laden flows—this study provides a quantitative basis for selecting WC content in composite plasma surfacing coatings. The 60% WC coating's superior tribological performance suggests that higher WC content is beneficial for severe abrasive environments, provided that the coating remains intact and does not spall.

However, engineers must consider the brittleness associated with high WC content. WC particles are inherently brittle, and a coating with 60% WC may exhibit reduced fracture toughness. In applications subject to impact loading or thermal cycling—such as marine engine components or offshore platform structures—this brittleness could lead to coating delamination or spalling. A practical approach would be to use a graded coating design, with a higher WC content at the surface transitioning to a lower WC content near the substrate, balancing wear resistance with toughness.

Key Questions and Reflections

The transition from two-body to three-body abrasive wear with increasing WC content raises an important question about the stability of the protective debris layer. In real-world marine environments, the debris layer may be continuously removed by water flow, undermining the three-body abrasive mechanism and potentially exposing the coating to more severe wear. Long-term tribological testing under conditions that simulate actual service environments—such as immersion in seawater with periodic sand abrasion—would provide more reliable performance data.

The decomposition behavior of WC during plasma surfacing is another critical consideration. The extent of WC decomposition depends on the plasma power density, powder feed rate, and travel speed. Engineers must optimize these parameters to maximize WC retention while ensuring adequate melting and bonding. In-situ monitoring of the plasma process, such as optical emission spectroscopy or thermal imaging, could provide real-time feedback on WC decomposition levels and enable adaptive process control.

Study Insights and Implications

This study demonstrates that WC reinforcement of iron-based plasma surfacing coatings significantly enhances both hardness and tribological performance, with the 60% WC coating achieving a fourfold improvement in hardness over the 304 stainless steel substrate. The shift to three-body abrasive wear at higher WC content suggests a self-lubricating mechanism that could be valuable in marine and offshore applications. For engineers designing protective coatings for marine equipment, this work provides a clear performance benchmark and a practical guideline for WC content selection, while also highlighting the need to address brittleness concerns through coating design optimization.