Part 2: Preparation Process of Cemented Carbide
Chapter 6: Coating and Composite Technology
Hard alloy ( WCCo ) plays an important role in aerospace, mining , energy and deep-sea engineering fields due to its excellent hardness (HV1500-2500±30), good toughness ( K₁c 8-20 MPa·m¹ / ² ± 0.5 ) and excellent compressive strength (>4000 MPa±100 MPa) . However, under extreme working conditions, high temperature (>1000°C±10°C), strong corrosion (pH<4±0.1) and high impact (>10³Hz ± 100Hz ) put forward higher requirements on surface performance, and single hard alloy is difficult to fully meet the requirements.
Coating and composite technology significantly enhances wear resistance (wear rate <0.06 mm³/N·m ± 0.01 mm³/N·m), corrosion resistance (corrosion rate <0.01 mm/year ± 0.002 mm / year ) and thermal fatigue resistance (lifespan > 10 ⁵ times ± 10 ⁴ times) through surface modification (coating thickness 10-200μm±1μm) and structural optimization (gradient layer, nano WC <100 nm±5 nm ) . These technologies not only extend the service life of cemented carbide, but also expand its application range, such as aviation turbine blades (lifespan > 5000 hours ± 500 hours), mining drill bits (> 1500 m ± 100 m) and deep-sea valves (> 5 years ± 0.5 years).
This chapter discusses the key processes and technical principles from four aspects: cemented carbide coating preparation , coating materials, gradient and nanostructured cemented carbide and coating performance testing . Coating preparation focuses on thermal spraying technology (such as HVOF, APS, detonation spraying), and achieves high hardness coating (HV 1200-1500±30) by optimizing spraying parameters (speed 600-4000 m/s±10 m/s, temperature 2000-15000°C±100°C); material optimization covers WCCo , WCNiCr and multiphase coatings (such as WCTiCNi), balancing hardness and toughness ( K ₁ c 10-15 MPa·m ¹ / ² ± 0.5); gradient and nanostructure improve comprehensive performance (strength>4500 MPa±100 MPa) through interface engineering and nanocrystal strengthening ; performance testing verifies coating reliability according to standards (such as ASTM G65, ISO 6508). Each section combines process details, scientific mechanisms, optimization strategies and engineering practices to reveal the core value of coating and composite technology.
For example, HVOF spraying WC12Co coating (speed 700 m/s±10 m/s, thickness 100μm±1μm) enables aviation turbine blades to maintain low wear (<0.05 mm³/N·m±0.01 mm³/N·m) in high-temperature airflow (1000°C±10°C) , with a service life of more than 5000 hours ± 500 hours; gradient WCCo (Co content 5%-15%±1%) improves the impact resistance of mining drill bits, with a drilling depth of 1800 m±100 m; nano WC coating (grain <100 nm±5 nm) is used for deep-sea valves, with corrosion resistance of more than 5 years±0.5 years. This chapter seamlessly connects with Chapter 5 (molding and sintering, WC particle size 0.1-10μm±0.01μm, density>99.5%±0.1%) through process parameters and performance data, laying the foundation for subsequent chapters (application and optimization).
6.1 Preparation of cemented carbide coating
Cemented carbide coatings are prepared by thermal spraying, physical/chemical vapor deposition (PVD/CVD) or laser cladding to deposit functional coatings (thickness 10-200μm±1μm, hardness HV 1200-1500±30) on high-performance substrates (hardness HV 1500-2500±30, surface roughness Ra<0.05μm±0.01μm). These coatings significantly improve wear resistance (wear rate <0.06 mm³ / N · m ± 0.01 mm³ / N · m ) , corrosion resistance (corrosion rate <0.01 mm/year±0.002 mm/year) and high temperature oxidation resistance (oxidation weight gain <0.1 mg/cm² ± 0.02 mg/cm² ) , meeting the requirements of demanding working conditions. Thermal spraying technology is the preferred choice due to its high efficiency (deposition rate > 90% ± 2%), flexibility (applicable substrate size > 100 mm ± 1 mm) and economy (cost < $500/m² ± $ 50), and is widely used in the aviation, mining and energy fields.
This section discusses in detail the three mainstream technologies of high velocity oxygen fuel spraying (HVOF), plasma spraying (APS) and detonation spraying, and analyzes their process principles, parameter optimization and application scenarios. The coating quality depends on the spraying parameters (speed, temperature, spray distance), powder characteristics (particle size 10-50μm±1μm, fluidity 12-15 seconds/50g±0.5 seconds) and substrate pretreatment (roughness Ra 2-5μm±0.1μm). Through thermal fluid mechanics (jet velocity 600-4000 m/s±10 m/s) and interface bonding mechanism (bonding strength 50-80 MPa±5 MPa), this section reveals the core technology.
For example, HVOF sprayed WC12Co coating (porosity <1%±0.2%) is used for aviation turbine blades, with a wear life of more than 5000 hours±500 hours; APS sprayed WCNiCr coating (thickness 150μm±1μm) increases the life of mining drill bits to 1500 m±100 m. The following is a comprehensive guide for the preparation of high-performance coatings from the perspective of process details, influencing factors and engineering practice.
6.1.1 High velocity oxygen fuel spraying (HVOF, coating hardness HV 1200-1500)
Process Principle and Technology Overview
High velocity oxygen fuel spraying (HVOF) is a highly efficient thermal spraying technology that generates a high-temperature and high-speed jet through the combustion of oxygen and fuel to deposit powder materials on the cemented carbide substrate to form a high-hardness, wear-resistant coating. The core of HVOF is the supersonic jet (speed 600-800 m/s±10 m/s), which partially melts the powder particles ( WCCo , particle size 10-45μm±1μm) and impacts the substrate at high speed to form a dense coating (porosity <1%±0.2%).
Compared with traditional spraying, HVOF has a lower temperature (2000-3000°C±50°C), effectively avoiding WC decomposition (<0.5%±0.1%), and is suitable for the preparation of high-performance cemented carbide coatings. HVOF equipment includes a spray gun (power>100 kW±10 kW), a combustion chamber (pressure 5-10 bar±0.5 bar) and a Laval nozzle ( throat diameter 8-12 mm±0.1 mm). Oxygen (purity>99.5%±0.1%, flow rate 800-1200 L/min±10 L/min) reacts with fuel (such as kerosene, flow rate 0.3-0.5 L/min±0.01 L/min) in the combustion chamber , releasing high enthalpy (>10 MJ/kg±0.5 MJ/kg).
The jet is accelerated to supersonic speed through the Laval nozzle, driving the powder to hit the substrate (roughness Ra 2-5μm±0.1μm), forming flat splash particles (diameter 50-100μm±5μm), ensuring a bonding strength of 50-80 MPa±5 MPa. This process makes the coating hardness reach HV 1200-1500±30, which is widely used in aviation, mining and other fields.
Process parameters and deposition mechanism
HVOF coating formation involves four stages: combustion, particle acceleration, melting and deposition:
Combustion stage
Oxygen reacts with fuel to generate a high temperature jet (3000°C±50°C), the temperature is lower than the decomposition point of WC (~3500°C±50°C), reducing carbide loss (<0.5%±0.1%). The thermal enthalpy (>10 MJ/kg±0.5 MJ/kg) ensures that the particles are fully heated.
Particle Acceleration
The powder is accelerated in the jet (speed 700 m/s±10 m/s, residence time <1 ms±0.1 ms ), the surface melts (melting rate 70%-90%±2%), the core remains solid (<50%±5% melted), and WC grains (0.5-2μm±0.01μm) are retained.
Particle melting and impact
The semi-molten particles impact the substrate to form flat spatter, and mechanical interlocking and trace diffusion (depth <1μm±0.1μm) form high bonding strength (>60 MPa±5 MPa).
Coating solidification
Rapid cooling (rate >10 ⁶ K/s±10 ⁵ K/s) results in a dense coating (porosity <1%±0.2%) with a hardness of HV 1400±30.
The jet dynamics follows the Bernoulli principle (velocity ~√(2ΔP/ρ), ρ~1 kg/m ³ ± 0.1 kg/m ³ ), and the thermal conductivity is 10 ⁴ W/m ² · K±10 ³ W/m ² · K. Optimizing the spray distance (250 mm±5 mm) and oxygen flow rate (1000 L/min±10 L/min) can reduce the porosity to <0.8%±0.1%. For example, the HVOF sprayed WC12Co (velocity 700 m/s±10 m/s, thickness 100μm±1μm) coating has a hardness of HV 1400±30 and a porosity of <0.8%±0.1%, which meets the high wear resistance requirements of aviation turbine blades.
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