What are the process parameters related to the bonding strength of plasma sprayed coatings? How to improve the density of coatings? ​

What are the process parameters related to the bonding strength of plasma sprayed coatings? How to improve the density of coatings?

Influence parameters on bonding strength of plasma sprayed coatings and methods for improving density

1、 Process parameters affecting the bonding strength of coatings

Plasma arc parameters

Arc power: Low power (<30kW) can cause insufficient melting of the sprayed material, resulting in semi melted particles and a significant decrease in coating bonding strength (usually<30MPa); If the power is too high (>80kW), it is easy for the substrate to overheat and oxidize, forming a brittle oxide layer, which in turn reduces the bonding strength. It is necessary to match the power according to the melting point of the material. For example, when spraying ceramic materials (melting point 1500-2500 ℃), the power should be controlled at 40-60kW, and for metal materials (melting point 600-1500 ℃), it should be controlled at 30-50kW to ensure that the particles are completely melted and the substrate is not excessively heated.

Plasma gas type and flow rate: commonly used is a mixture of argon (Ar) and hydrogen (H ₂) gas. Argon provides a stable arc, while hydrogen increases the arc temperature (for every 5% increase in hydrogen proportion, the arc temperature can be increased by about 300 ℃). A high hydrogen ratio (>20%) will exacerbate particle oxidation, while a low ratio will result in insufficient melting efficiency. The total gas flow rate needs to be matched with the nozzle diameter (e.g. 8mm nozzle corresponds to a flow rate of 50-80L/min). Unstable flow rate can cause fluctuations in flame energy, resulting in a deviation of bonding strength exceeding ± 5MPa.

Spray distance and angle

Spray distance: If the distance is too close (<80mm), it will cause the substrate surface to overheat, undergo phase transformation or oxidation, and the coating is prone to cracking; If the distance is too far (>200mm), the cooling of the molten particles during flight will be too fast, and the kinetic energy will be insufficient when impacting the substrate, resulting in a decrease in bonding strength (for every 50mm increase in distance, the bonding strength can decrease by 10% -15%). The optimal distance needs to be adjusted according to the material, with metal coatings controlled at 100-150mm and ceramic coatings at 120-180mm, ensuring that the temperature of the particles reaching the substrate is ≥ 0.8Tm (Tm is the melting point of the material).

Spray angle: Deviation from the vertical direction (<85 °) can cause particles to splash along the tangent direction of the substrate surface, forming layered overlapping defects, and the bonding strength decreases with decreasing angle (the bonding strength is only 70% of that when the angle is 60 °). For complex surface spraying, it is necessary to adjust the posture of the spray gun through a robot to ensure that the spraying angle at any position is ≥ 80 °. Special parts (such as deep hole inner walls) can be matched with rotating fixtures with an angle of not less than 75 °.

Powder parameters

Powder particle size and morphology: A wide powder particle size distribution (such as 10-100 μ m) can lead to uneven melting, excessive melting of fine powder can cause splashing, and incomplete melting of coarse powder can affect bonding. The particle size range needs to be controlled (metal powder 20-60 μ m, ceramic powder 30-80 μ m), and the sphericity should be ≥ 80%. Irregular shaped powders can easily cause turbulence in the airflow, leading to increased fluctuations in bonding strength.

Powder feed rate: If the rate is too low, it will cause the single coating to be too thin and the substrate to be heated unevenly; If it is too high, the powder will not melt sufficiently and the number of unmelted particles will increase. The feed rate should be matched with the power (such as 20-40g/min for metal powder and 15-30g/min for ceramic powder at a power of 50kW), and accurately controlled by a weight loss powder feeder with a fluctuation of ≤± 2g/min to ensure coating uniformity.

Substrate pretreatment status

Surface roughness: The substrate surface is sandblasted (such as using 80 mesh white corundum sand), and the roughness Ra is controlled at 3.2-6.3 μ m to form a mechanical anchoring effect. Insufficient roughness (Ra<1.6 μ m) can lead to a decrease in mechanical bonding force; If it is too coarse (Ra>10 μ m), it is easy to leave sand particles and form coating defects.

Surface cleanliness: Oil stains and oxide layers can hinder bonding and need to be removed through degreasing (ultrasonic cleaning), acid washing, or laser cleaning. After cleaning, the surface water film should be uniform and continuous (water film rupture time>30s) to avoid residual impurities from forming interface pollution.

2、 Methods to improve coating density

process parameters optimization

Improving plasma arc energy density: Using a compression nozzle (throat diameter of 3-5mm) to enhance the compression effect of the arc column, the flame velocity is increased to Mach 1.5-2.0, the particle flight velocity is greater than 300m/s, and the deformation is sufficient when high-speed impacts the substrate, reducing pores. Simultaneously control the arc voltage fluctuation to ≤± 2V and the current fluctuation to ≤± 5A to ensure energy stability.

Optimize spraying distance and environment: shorten the spraying distance to the critical value (such as metal coating 100-120mm), and cooperate with inert gas protection (such as nitrogen atmosphere) to reduce oxidation and cooling during particle flight, so as to maintain high temperature and kinetic energy of particles. For oxygen sensitive materials (such as titanium alloy coatings), vacuum plasma spraying (vacuum degree ≤ 10Pa) can be used, and the porosity can be reduced to below 1%.

Material and powder modification

Powder pretreatment: Coating modification of ceramic powder (such as coating Ni with Al ₂ O3 powder) to improve its wetting and melting properties; Metal powder undergoes atomization and spheroidization treatment to improve fluidity and melting uniformity. Powder drying treatment (drying at 120 ℃ for 2 hours) can remove moisture and avoid gas pores during spraying.

Adding alloying elements: Adding a small amount of low melting point elements such as silicon and boron to metal powder (such as NiCrAlY powder with 0.5% Si) to form a low melting point eutectic phase, promoting melting and bonding between particles; Introducing nanoparticles (such as nano TiO ₂ - doped Al ₂ O3) into ceramic coatings, utilizing the nano effect to refine grains and reduce pore channels.

Enhancement of post-processing technology

Remelting treatment: Laser remelting or induction remelting is performed on the sprayed coating to completely melt the surface layer within the range of 50-100 μ m, eliminate pores, and form metallurgical bonds. The density can be increased to over 98%, but the remelting rate (5-10mm/s) needs to be controlled to avoid coating cracking.

Hot isostatic pressing (HIP) treatment: The coating is treated at high temperature (0.6-0.8Tm) and high pressure (100-200MPa) to eliminate internal pores through diffusion welding, especially suitable for thick coatings (>200 μ m). The density can be increased from 90% to 99% of the sprayed state, but attention should be paid to the matching of thermal expansion between the substrate and the coating to prevent cracking.

Rolling and shot peening strengthening: Mechanical rolling (pressure 50-100MPa) or shot peening (shot diameter 0.2-0.5mm) is performed on the surface of the coating to close the surface pores through plastic deformation, while introducing residual compressive stress to improve the density and fatigue strength of the coating.

Equipment and Environmental Control

Adopting high-energy plasma spraying system, such as supersonic plasma spraying (flame velocity>1000m/s), the particle kinetic energy is significantly increased, and the coating density is increased by 10% -15% compared to conventional plasma spraying; Vacuum or low-pressure plasma spraying can avoid oxidation in the atmosphere, reduce porosity and inclusions, especially suitable for coatings of active metals such as titanium and zirconium.

Environmental atmosphere control: The relative humidity in the spraying area should be ≤ 60%. Excessive humidity can easily cause powder to absorb moisture and create gas pores during spraying; For highly active materials, they need to be sprayed in an inert gas protection chamber with an oxygen content controlled below 50ppm to prevent the formation of pore cores from oxidation products.