What are the factors that affect the spraying performance of tungsten carbide?

What are the factors that affect the spraying performance of tungsten carbide?

The factors that affect the spraying performance of tungsten carbide, such as hardness, wear resistance, bonding strength, porosity, etc., mainly involve material systems, equipment parameters, construction processes, and post-treatment. The following is a specific analysis:

1、 Material system

Powder composition and particle size

Types of bonding phases: Tungsten carbide powder usually requires the addition of metal bonding phases (such as Co, Ni, Cr), and the content of bonding phases directly affects the toughness and bonding strength of the coating. For example, the bonding strength of WC-12Co coating (70-100MPa) is higher than that of pure WC coating, and its impact resistance is better.

Powder particle size distribution:

Particle size too small (such as<15 μ m): prone to excessive oxidation or evaporation in the flame flow, resulting in the coating containing WC ₁₋ₓ (carbon deficient phase) and a decrease in hardness.

Particle size too large (e.g.>45 μ m): Insufficient melting, increased coating porosity, and decreased bonding strength.

Ideal particle size: Supersonic flame spraying (HVOF) is suitable for 15-45 μ m, while plasma spraying is suitable for 20-60 μ m.

Workpiece material and pre-treatment quality

Surface roughness: Insufficient surface roughness (Ra 12.5~50 μ m) after sandblasting can lead to weakened mechanical bite of the coating and decreased bonding strength.

Surface cleanliness: Residual grease and oxides can hinder the metallurgical bonding between molten particles and substrates, which may cause coating peeling.

2、 Equipment and process parameters

Type of heat source and energy density

Supersonic flame spraying (HVOF): The flame velocity is greater than 1000m/s, the particle kinetic energy is high, the coating density is high (porosity<1%), but the temperature is low (about 3000 ℃), and it is necessary to ensure that the WC Co powder is fully melted.

Plasma spraying: With a temperature of up to 16000 ℃, it is suitable for high melting point materials, but the particle velocity is low (300-500m/s) and the coating porosity is high (3%~5%). It is necessary to balance the melting degree and oxidation degree by adjusting the power (such as 30-50kW).

Spray parameter settings

Fuel to gas ratio:

The imbalance of propane/oxygen ratio in HVOF (such as excessive oxygen) can lead to the oxidation of WC to form WO3, reducing the hardness of the coating (WC hardness is about 2300HV, WO3 is only 700HV).

The ratio of argon/hydrogen in plasma spraying affects the ionization degree of the flame flow. Insufficient hydrogen can lead to a decrease in temperature and insufficient powder melting.

Spray distance and speed:

Distance too close: HVOF can easily cause coating overheating and oxidation; Plasma spraying is prone to burning the substrate.

Distance too far: The particle cooling rate is fast, and the insufficiently melted particles do not deform sufficiently when they collide with the substrate, resulting in a decrease in bonding strength.

Powder delivery rate: If the rate is too high, it will cause the powder to accumulate and not melt, resulting in a "dry spray" phenomenon and a significant increase in porosity; If the speed is too low, the efficiency will be low and the coating thickness will be insufficient.

environmental control

Atmosphere oxygen content: When spraying in air, WC is prone to react with oxygen at high temperatures (especially plasma spraying). Low pressure plasma spraying (LPPS) or inert gas protection can be used to reduce oxidation.

Workpiece temperature: During the spraying process, if the workpiece temperature is too high (such as>200 ℃), it will cause thermal stress accumulation and the coating is prone to cracking; If the temperature is too low, it will affect the spreading of particles and decrease the bonding strength.

3、 Construction technology and operation

Coating structure design

Single layer vs multi-layer: If the thickness of a single layer is too large (such as>0.3mm), it is prone to cracking due to internal stress concentration. Thin coating of multiple layers (each layer ≤ 0.2mm) and controlling the interlayer temperature (≤ 100 ℃) can reduce stress.

Transition layer use: Spray NiCr or NiAl transition layer (50-100 μ m) on stainless steel and other substrates to alleviate the difference in thermal expansion coefficient (WC Co thermal expansion coefficient ≈ 6 × 10 ⁻⁶/℃, steel ≈ 12 × 10 ⁻⁶/℃) and reduce interface stress.

Stability of spray gun operation

Uneven movement speed or angular deviation of the spray gun (angle with the substrate ＜ 60 °) can lead to uneven coating thickness and increased local porosity.

When manually spraying, avoid the "sweeping" operation and maintain a straight and uniform movement to ensure the uniformity of the coating.