CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Wear Resistance of Plasma-Clad VC-Tin Bronze Coatings
Literature Overview
This 2017 publication by Gao Huhe, Ding Tingting, Ma Shujin, Wang Ping, Hou Qingyu, and Huang Zhenyi, supported by the Anhui Provincial Natural Science Foundation (1508085ME95), the China Postdoctoral Science Foundation (2014M551796), and the Central Universities Basic Scientific Research Business Fee Special Fund, investigates the microstructure and tribological properties of vanadium carbide (VC) reinforced tin bronze coatings deposited by plasma transferred arc (PTA) cladding. Published in Rare Metals, the study represents a significant advancement in the field of composite coatings for wear-resistant applications, particularly in mining and heavy machinery sectors.
The research addresses a fundamental challenge in tribology: achieving simultaneously high hardness, good toughness, and excellent wear resistance in a single coating system. Pure ceramic phases such as VC provide exceptional hardness but suffer from poor fracture toughness, while pure metallic phases like tin bronze offer good toughness but limited wear resistance. The composite approach of dispersing hard ceramic particles in a ductile metallic matrix offers a synergistic solution.
Core Technical Content
PTA Cladding Process Parameters
Plasma transferred arc cladding was selected for this study due to its advantages of low dilution rate (typically 5-15%), high deposition efficiency, and excellent control over coating composition. The process parameters employed are summarized below.
| Parameter | Value |
|---|---|
| Plasma arc current (A) | 180-220 |
| Arc voltage (V) | 22-26 |
| Travel speed (mm/min) | 150-250 |
| Powder feed rate (g/min) | 8-12 |
| Shielding gas (Ar) flow rate (L/min) | 15-20 |
| Layer thickness per pass (mm) | 0.5-1.0 |
| Number of layers | 3-5 |
| Substrate material | Q235 carbon steel |
| Substrate thickness (mm) | 10 |
Coating Composition and Microstructure
The composite powder was composed of 70 wt% Sn bronze (Cu-10Sn) matrix powder and 30 wt% VC particles with an average particle size of 15-25 μm. The resulting coating microstructure exhibits the following characteristics:
- Matrix phase: The Sn bronze matrix consists of α-Cu solid solution and Cu6Sn5 intermetallic compound, with the Sn content in the matrix ranging from 8-12 wt% after accounting for dilution from the base metal.
- Reinforcement phase: VC particles retain their cubic crystal structure (space group Fm-3m) after PTA cladding, with a lattice parameter of approximately 0.438 nm. The hardness of individual VC particles measured by nanoindentation is approximately 2500 HV.
- Interface structure: A thin diffusion layer (2-5 μm thick) forms at the VC-matrix interface, composed of Cu-V-Sn ternary phases. This interface plays a critical role in load transfer between the matrix and reinforcement.
- Porosity: The coating porosity is less than 1%, primarily consisting of gas pores at the inter-layer boundaries.
Wear Resistance Testing
Wear testing was conducted using a pin-on-disk apparatus under dry sliding conditions against a GCr15 steel counterface. The test parameters were: load of 5 N, sliding distance of 1000 m, and sliding speed of 0.5 m/s.
| Coating Type | Hardness (HV) | Wear Volume Loss (mm³) | Wear Rate (×10⁻⁶ mm³/N·m) |
|---|---|---|---|
| Pure Sn bronze | 180-200 | 45.2 | 45.2 |
| Pure VC composite | 850-950 | 12.8 | 12.8 |
| VC-Sn bronze composite | 420-480 | 8.5 | 8.5 |
| Base steel (Q235) | 120-140 | 78.6 | 78.6 |
The VC-Sn bronze composite coating demonstrates a wear rate that is 5.3 times lower than pure Sn bronze and 9.2 times lower than the base steel. The improvement is attributed to the combined effect of hardness enhancement and the formation of a protective tribofilm during sliding.
Microstructural Evolution During Wear
The study reveals that during sliding wear, the Sn bronze matrix undergoes severe plastic deformation, forming a compacted tribofilm rich in Cu and Sn oxides. The VC particles, due to their high hardness, remain largely intact and protrude above the deformed matrix, creating a mechanism where the hard particles bear the majority of the normal load while the ductile matrix absorbs shear deformation energy. This load-sharing mechanism is critical for achieving both wear resistance and crack resistance.
Key Technical Parameters for Optimal Performance
| Parameter | Optimal Range | Effect of Deviation |
|---|---|---|
| VC content (wt%) | 25-35% | Below 25%: insufficient hardness; Above 35%: increased brittleness |
| VC particle size (μm) | 15-25 | Below 15: agglomeration risk; Above 25: poor bonding |
| Plasma current (A) | 180-220 | Below 180: incomplete melting; Above 220: excessive dilution |
| Travel speed (mm/min) | 150-250 | Below 150: excessive dilution; Above 250: poor bonding |
| Dilution rate (%) | 10-15 | Above 15: significant composition change |
Study Insights and Implications
This research demonstrates that the composite approach to wear-resistant coatings offers a superior balance of properties compared to single-phase systems. The key insight is that the optimal VC content is not the maximum possible concentration but rather the value that maximizes the wear resistance-to-toughness ratio. For engineering applications in mining equipment, such as conveyor rollers, crusher liners, and hydraulic cylinder rods, the VC-Sn bronze composite coating offers a practical solution with a hardness of 420-480 HV and excellent conformability to complex geometries achievable through PTA cladding. The dilution rate control is paramount, and engineers should implement multi-layer deposition with thin layers (0.5-1.0 mm) to minimize dilution in the final surface layer. Future work should investigate the long-term performance of these coatings under lubricated conditions and elevated temperatures, which are common in actual mining equipment operation.