Zhuojin Pipe Fitting Co., LtdMicrostructure and Tribological Properties of Spherical WC-Reinforced Iron-Based Composite Plasma Overlay Layer
Literature Overview
This paper by Fan Li, Chen Haiyan, Liu Shanshan, Dong Yaohua, Dong Lihua, and Yin Yansheng, published in the Journal of Tribology (2018, Vol. 38, No. 1, pp. 17-27), investigates the effect of spherical tungsten carbide (WC) reinforcement on the microstructure and tribological performance of iron-based composite plasma overlay coatings deposited on 304 stainless steel. Funded by multiple sources including the Marine Public Welfare Industry Research Special Fund (201405013-3) and the National Natural Science Foundation (51609133), this work addresses the challenge of enhancing the tribological performance of marine-grade stainless steel components.
Core Technical Findings
The study employs plasma transfer arc (PTA) welding to deposit iron-based composite coatings reinforced with 30% and 60% spherical WC particles by mass fraction onto 304 stainless steel substrates. The characterization includes microstructural analysis, phase identification, microhardness measurement, and dry sliding friction-wear testing at 50 N load and 20 mm/s sliding velocity.
Performance Comparison
| Parameter | 304 SS Substrate | Iron-Based Coating | 30% WC Coating | 60% WC Coating |
|---|---|---|---|---|
| Microhardness (HV0.2) | ~180 | ~550 | 665 | 724 |
| Hardness improvement over iron-based | — | — | +21.1% | +31.9% |
| Hardness ratio to 304 SS | 1.0 | ~3.1× | 3.7× | 4.0× |
| Friction coefficient | — | — | 0.59 | 0.42 |
| Wear rate (×10⁻⁶ mm³·N⁻¹·m⁻¹) | — | — | 2.639 | 1.111 |
Microstructural Analysis
The microstructure of the WC-reinforced iron-based composite overlay coatings consists of three primary constituents:
- Chromium-rich solid solution strengthened austenite: The base matrix phase, providing ductility and corrosion resistance. The Cr-rich austenite is solid solution strengthened by dissolved alloying elements (Cr, Ni, Mo, etc.).
- Cr7C3 carbides: High-hardness carbide particles formed in situ during solidification, contributing to wear resistance.
- WC reinforcement particles: The added spherical WC particles that survive the PTA process and act as hard reinforcement phases.
The spherical morphology of the WC particles is significant. Compared to irregularly shaped WC particles, spherical particles have a more uniform stress distribution, reducing the likelihood of stress concentration at particle boundaries. This is particularly important in composite coatings where the interface between the reinforcement and the matrix is a critical factor in mechanical performance.
WC Particle Survival and Interface Quality
A critical consideration in PTA deposition of WC-reinforced composites is the survival of WC particles through the high-temperature molten pool. WC has a melting point of approximately 2,870°C, which is well above the PTA process temperature (typically 1,500-2,000°C in the molten pool). Therefore, WC particles are expected to survive the thermal cycle. However, the interface between WC and the iron-based matrix is a potential weakness.
The spherical morphology of the WC particles is advantageous for interface quality. During solidification, the spherical shape minimizes the contact area between WC and the matrix, reducing the interfacial energy and the likelihood of interface cracking. This is in contrast to angular WC particles, which can create stress concentration points at sharp corners.
Tribological Analysis
The dry sliding friction-wear test results demonstrate that both WC content levels significantly improve wear resistance compared to the base iron-based coating. The 60% WC coating exhibits superior performance with a lower friction coefficient (0.42 vs. 0.59) and a substantially lower wear rate (1.111 vs. 2.639 ×10⁻⁶ mm³·N⁻¹·m⁻¹).
Wear Mechanism Analysis
The wear mechanisms differ between the two WC content levels:
- 30% WC coating: Mixed mechanism of adhesive wear and two-body abrasive wear. The relatively lower WC volume fraction allows some matrix material to be exposed to the counterface, leading to adhesive transfer. The exposed WC particles act as abrasive agents in two-body abrasion.
- 60% WC coating: Three-body abrasive wear mechanism. The high WC volume fraction ensures that the surface is dominated by WC particles, and wear debris generated during sliding is trapped between the coating and counterface, creating a third-body abrasion regime.
The lower friction coefficient of the 60% WC coating is attributed to the higher hardness of the surface, which reduces the real contact area and the adhesive component of friction. The three-body abrasive wear mechanism, while not ideal, results in lower overall material loss because the hard WC particles resist penetration by the abrasive debris.
Engineering Considerations
PTA Process Parameters
| Parameter | Typical Range | Influence |
|---|---|---|
| Arc current | 200-400 A | Affects heat input and dilution |
| Travel speed | 50-200 mm/min | Controls cooling rate and grain size |
| Powder feed rate | 50-200 g/min | Controls WC incorporation efficiency |
| Shielding gas flow | 15-25 L/min | Prevents oxidation of molten pool |
| Wire-powder ratio | Variable | Controls WC content in deposit |
The PTA process is well-suited for composite coating deposition because the wire serves as both the heat source conductor and a base alloy source, while the powder delivers the WC reinforcement. This dual-feed approach allows independent control of matrix composition and reinforcement content.
Interface Quality and Coating Integrity
A critical concern in WC-reinforced composite coatings is the bond strength between the coating and the substrate, as well as the interface between WC particles and the matrix. Insufficient bonding can lead to spalling under cyclic loading. The PTA process provides good metallurgical bonding due to the deep penetration of the arc, but the interface quality must be verified by sectioning and metallographic examination.
Key Questions and Reflections
The significant improvement in wear resistance with WC addition is encouraging, but several practical considerations must be addressed. First, the cost of WC powder is substantially higher than iron-based powder, and the 60% WC content represents a significant material cost increase. Engineers must evaluate whether the performance improvement justifies the cost premium for each specific application.
Second, the three-body abrasive wear mechanism observed at 60% WC content suggests that the coating may generate significant wear debris during service. In closed systems, this debris can accumulate and cause accelerated wear of mating surfaces. This is a critical consideration for marine applications where components operate in confined spaces.
Third, the study does not address the effect of WC particle size distribution on coating performance. In practice, WC powder is available in various particle size ranges, and the optimal size for PTA deposition may differ from the size used in this study. Smaller particles may provide better dispersion but may also be more susceptible to melting or decomposition during the PTA process.
Practical Implications for Marine Component Protection
For marine engineers seeking to enhance the tribological performance of 304 stainless steel components, the WC-reinforced PTA coating offers a proven solution. The 60% WC content provides the best wear performance, but the 30% WC content may offer a more cost-effective solution with acceptable performance. The choice between the two should be based on the specific operating conditions, including load magnitude, sliding velocity, and service life requirements.