CLADDING TECHNOLOGY SHANXI CO., LTDStudy Note on Microstructure and Low-Temperature Wear Resistance of Spherical Tungsten Carbide Reinforced Cobalt-Based Overlay Coatings
Literature Overview
This 2022 study published in Surface Technology, funded by the National Key R&D Program (2016YFB0300704), the National Natural Science Foundation (52071091), and the Pudong New Area Science and Technology Project (PKJ2019-C03), investigates the microstructure and low-temperature wear resistance of spherical tungsten carbide (WC) reinforced cobalt-based overlay coatings. Conducted by researchers at Shanghai Maritime University and Guangzhou Maritime College, the work addresses the specific challenges of marine equipment operating in harsh ocean environments where low-temperature wear and corrosion-wear synergistic degradation are dominant failure modes. The spherical morphology of WC particles represents a deliberate design choice to improve particle dispersion and bonding within the cobalt matrix.
Spherical WC Particle Characteristics and Composite Design
The use of spherical WC particles, as opposed to conventional angular WC particles, offers several advantages: improved flowability during powder feeding, more uniform dispersion in the matrix, reduced stress concentration at particle-matrix interfaces, and enhanced coating integrity under cyclic loading. The study employed spherical WC particles with mean diameter of 15-30 μm and carbon content of 6.0-6.5 wt%, obtained through a specialized gas atomization process.
| Composite Parameter | Specification | Rationale |
|---|---|---|
| WC particle size | 15-30 μm (spherical) | Uniform dispersion, reduced stress concentration |
| WC volume fraction | 20-40 vol% | Balance hardness and toughness |
| Cobalt matrix composition | Co-15Cr-5W-3Mo (wt%) | High-temperature strength, corrosion resistance |
| Overlay thickness | 2-3 mm | Adequate protection depth |
| Deposition method | SAW overlay with powder feeding | Industrial scalability |
The cobalt-based matrix alloy was specifically formulated to provide high-temperature strength (retained strength at 400-500°C), excellent corrosion resistance in marine environments, and good bonding with WC particles through the formation of a Co-C interfacial layer.
Low-Temperature Wear Testing and Mechanism Analysis
Wear testing was conducted at temperatures of -40°C, -20°C, 0°C, and 25°C using a reciprocating pin-on-disk tribometer with Al2O3 counterfaces under loads of 10 N and 20 N. The low-temperature conditions simulate polar ocean environments where marine equipment such as propeller shafts, rudder stocks, and hull coatings experience severe wear degradation.
| Temperature | Baseline Co Matrix Wear Rate (mg/km) | WC-Reinforced Wear Rate (mg/km) | Improvement Factor |
|---|---|---|---|
| 25°C | 8.5 | 3.2 | 2.7× |
| 0°C | 12.3 | 4.1 | 3.0× |
| -20°C | 18.7 | 5.8 | 3.2× |
| -40°C | 25.4 | 8.2 | 3.1× |
The results demonstrate that WC reinforcement provides a consistent 2.7-3.2× improvement in wear resistance across the entire temperature range, with the relative benefit slightly increasing at lower temperatures. This is attributed to the fact that the cobalt matrix undergoes increased embrittlement at low temperatures (transition from ductile to brittle behavior), while the spherical WC particles maintain their hardness and provide mechanical support to the embrittled matrix.
Microstructural Evolution and Wear Mechanism
X-ray diffraction and scanning electron microscopy revealed that the as-deposited coating microstructure consisted of a dendritic Co-Cr solid solution matrix with spherical WC particles uniformly distributed at interdendritic regions. After low-temperature wear testing, the wear surface exhibited a polished appearance with embedded WC particles protruding above the matrix surface, indicating that the matrix material was preferentially removed by adhesive wear while the WC particles remained anchored in the coating.
At -40°C, the cobalt matrix showed evidence of microcracking along grain boundaries due to thermal contraction and stress concentration, but the spherical WC particles prevented crack propagation through their stress-relieving morphology. This is a critical advantage over angular WC particles, which would create sharp stress concentration points at their corners and edges, accelerating crack initiation and propagation at low temperatures.
Corrosion-Wear Synergistic Testing and Engineering Implications
The study also conducted corrosion-wear synergistic testing in simulated seawater (3.5% NaCl solution) at 0°C and -20°C, where the combined effects of electrochemical corrosion and mechanical wear were evaluated. The WC-reinforced coating exhibited a synergistic corrosion-wear rate 4.5× higher than the sum of individual corrosion and wear rates, confirming the severe degradation mechanism in marine environments. The spherical WC particles provided a degree of protection by creating a more uniform wear surface that reduced the active corrosion area exposed at any given time.
For engineering practice in marine applications, the study recommends overlay thickness of 2.5-3.0 mm with WC volume fraction of 30-35 vol% for optimal performance in low-temperature marine environments. The spherical WC particle design is strongly preferred over conventional angular particles for applications involving thermal cycling or low-temperature service. The research demonstrates that particle morphology is not merely a processing consideration but a fundamental design parameter that significantly influences coating performance under extreme environmental conditions. The findings provide a scientifically grounded basis for the selection and design of overlay coatings for marine equipment operating in polar and sub-polar regions, where conventional coatings often fail prematurely due to the combined effects of low temperature, corrosion, and wear.