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Selection of Plasma Spraying Parameters
Release time:
2019-12-12 16:58
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Selection of Plasma Spraying Parameters
The main parameters of plasma spraying include the composition and flow rate of the working gas, electrical parameters, powder feed rate, spraying distance and spraying angle, as well as the relative moving speed between the spray gun and the workpiece.
1. Gas parameter (flow rate)
The flow rate of the primary gas is one of the critical process parameters, directly affecting the enthalpy and velocity of the plasma jet, which in turn influences spray efficiency and coating porosity. When the spraying power remains constant, both excessively high and excessively low primary gas flow rates will lead to reduced spray efficiency and increased coating porosity. If the gas flow rate is too high, the ion concentration decreases, and the excess gas cools the plasma jet, hindering proper heating of the powder. As a result, the powder fails to melt adequately, reducing spray efficiency and leading to a loose coating microstructure with higher porosity. Conversely, if the primary gas flow rate is too low, the plasma jet becomes weak and ineffective, increasing the relative proportion of secondary gas in the working gas. This leads to a rise in the jet’s enthalpy and temperature, causing the sprayed powder to overmelt.
The variation in the flow rate of secondary gas is primarily reflected in changes in the spraying voltage.
The pressure and flow rate of the powder-supply gas also significantly affect coating quality. For external powder-feed torches, the impact of the powder-supply gas on coating quality is particularly severe. As shown in the figure, if the pressure and flow rate of the powder-supply gas are too low, the powder will have difficulty reaching the center of the flame; if they are too high, the powder will pass through the center of the jet, resulting in a severe “boundary effect” that leads to a loose coating and reduced bonding strength. For internal powder-feed torches, excessively high pressure and flow rate of the powder-supply gas will likewise prevent the powder from being delivered into the flame core. Conversely, if the pressure and flow rate are too low, the nozzle is prone to clogging, and in severe cases, the nozzle may even be burned out. If very high pressure and flow rate of the powder-supply gas are required to deliver the powder into the flame core, it is essential to check the air tightness of the powder supply system for any leaks.
Therefore, the pressure and flow rate of the powder-feeding gas should be selected based on the amount of powder being fed, the specific gravity of the powder, the powder’s flowability, the performance of the powder-supply system, as well as the jet’s power and rigidity.
2. Electrical Parameters
(1) Power
The magnitude of the input power must first ensure that the powder can be melted effectively. The thermal power required for the powder to form a coating should be:
In the formula: Gf —— the powder feeding rate per unit time
T0, Tm, Tr—initial temperature of the powder, melting point of the powder, and temperature of powder superheating;
Cs, Cm — specific heat capacities of the powder in solid and molten states;
Hr——The enthalpy increment of the molten powder material at Tr.
Based on the plasma flame energy utilization coefficient ηf, the thermal power qp of the plasma at the nozzle exit can be estimated as follows:
Finally, the required input power P can be estimated based on the spray gun efficiency η:
In the formula: 0.24 —— the coefficient for converting electrical energy into thermal energy
In general, it’s better to use a higher power rating. The commonly used power range for conventional plasma spraying is 20–35 kW, whereas the power range typically employed for HEPJet high-efficiency supersonic plasma spraying is 45–65 kW.
(2) Voltage and Current
The plasma arc voltage is determined by the torch design and the working gas. The arc voltage can be adjusted by modifying the distance between the cathode and the nozzle and by varying the composition of the working gas. With the torch design already selected and the main gas flow rate held constant, the adjustment of voltage and current can be achieved by changing the power supply regulator and the H2 flow rate. It should be noted that when the voltage or current is adjusted, the main gas flow rate will also change accordingly. Therefore, to ensure stable spraying parameters, it is essential to adjust and maintain the main gas flow rate constant whenever the voltage and current are modified.
Once the power has been determined, you should, whenever possible, select a higher voltage and lower current, as this will help improve the thermal efficiency of the spray gun.
3. Spray distance
The spray distance refers to the straight-line distance from the nozzle’s end face to the surface of the substrate. Since it takes time for the powder to be heated and accelerated within the plasma jet, there should be an appropriate spray distance. If the spray distance is too close, the powder will not have sufficient heating time, resulting in inadequate deformation upon impact and thereby compromising coating quality. Moreover, the part will rapidly heat up under the influence of the plasma jet, leading to severe oxidation and eventual coating delamination. On the other hand, if the spray distance is too far, the powder—already heated to a molten state—will cool down by the time it comes into contact with the part, causing its flight velocity to decrease as well. This also adversely affects coating quality and significantly reduces spraying efficiency. Typically, the spray distance for plasma spraying ranges from 70 to 150 mm.
4. Powder feeding rate
The powder feed rate refers to the amount of powder delivered per unit time, and it directly affects both the spraying efficiency and the coating quality. The powder feed rate should be matched with the parameters of the heat source. For powders of the same grade and particle size, different input powers must be applied at different powder feed rates. When the powder feed rate remains constant, if the heat source power parameter is too low, the powder will melt poorly, resulting in a higher proportion of unmelted particles trapped within the coating. Additionally, the powder will not deform sufficiently upon impact with the workpiece, leading to increased powder rebound losses and lower deposition efficiency, thereby degrading the coating quality. Conversely, if the heat source power parameter is excessively high, although the powder will melt and deform adequately upon impact, the powder will suffer severe thermal oxidation and ablation. This will result in a greater inclusion of soot and smoke in the coating, as well as significant spattering of molten particles, again reducing deposition efficiency and degrading coating quality. Therefore, for a given powder grade and particle size, the magnitude of the powder feed rate must be appropriately matched with the heat source parameters.
5. Spray angle
The spray angle refers to the angle between the axis of the spray jet and the tangent to the substrate surface. The typical spray angle ranges from 60° to 80°; when the spray angle is no less than 45°, it will not significantly affect the coating’s microstructure or deposition efficiency. It is generally accepted that spray angles below 30° are not permissible for spraying operations.
When the spray angle is too small, fine powder particles adhere to the sprayed surface, obstructing the subsequent particles from being deposited. As a result, a “shield” forms behind these particles, leading to the creation of a porous coating characterized by numerous irregular voids. These voids not only weaken the coating’s strength but also allow fine particles rich in oxides to accumulate from the spray jet, thereby altering the coating’s chemical composition.
When the spray angle is less than 45°, a “shadowing effect” occurs during spraying, which affects the interlayer adhesion of the coating and significantly reduces the bonding strength between the coating and the substrate.
6. Preheating and Temperature Control of Sprayed Workpieces
Before spraying on parts with complex structures or internal cavities during winter, preheating is required. The typical preheating temperature ranges from 80 to 150 ℃. The purpose of preheating is to remove moisture from the substrate surface, improve the activation state of the substrate surface, reduce the cooling rate of spray particles upon impact with the substrate, and mitigate thermal stresses generated during the cooling of sprayed particles.
During the spraying process, the temperature rise of the workpiece must be carefully controlled. It is essential not only to maintain the overall temperature of the workpiece below a maximum of 200 ℃ but also to prevent localized overheating in specific spray areas. Compared to overall overheating, localized overheating has a more significant impact on the coating—especially when preparing ceramic coatings, where the coating is highly susceptible to cracking. Typically, auxiliary air blowing is employed to cool and control the workpiece's temperature.
7. The moving speed of the spray gun
The moving speed of the spray gun is generally determined based on the diameter of the spray-beam spot. Since different spraying processes result in varying diameters of the spray-beam spot, the typical range is 30% to 50% of the capillary-spot diameter—never less than 30%. After the spray-gun moving speed has been set, it must also be matched with the rotational linear speed of the workpiece to ensure that the coating thickness achieved per pass meets the specified requirements. Under a given powder-feed rate, a slower or faster moving speed of the spray gun—or, equivalently, a lower or higher relative speed between the spray gun and the workpiece—directly affects the area of the workpiece surface swept by the spray gun per unit time and the thickness of each applied coating layer. Therefore, adjusting the spray-gun moving speed essentially means controlling the thickness of each coating layer. The thickness of each spray application should not be too great. In general, for thin coatings with a thickness of 0.15 mm or less, the thickness of each spray application should not exceed 0.02 mm. Moreover, the spray-gun moving speed also influences the temperature rise of the workpiece. To prevent excessive local temperature increases in the substrate that could lead to thermal deformation or excessive thermal stress, one can slightly increase the workpiece’s linear speed to accelerate the spray-gun moving speed.
8. Spray Atmosphere Control
During the spraying process, flying particles come into contact with gases or the atmosphere and undergo chemical reactions, leading to the incorporation of oxide inclusions in the coating. The formation of these oxides is influenced by both the atmosphere surrounding the heat source and the ambient atmospheric conditions. Employing certain low-pressure atmosphere spraying techniques can help reduce the degree of particle oxidation—for example, low-pressure plasma spraying, which is particularly suitable for preparing coatings from metals and their alloys that are prone to oxidation. Additionally, inert gases can be used to shield the particle beam.
In the entire spray-coating environment, dust is unavoidable. It is essential to ensure thorough ventilation and dust removal throughout the entire environment to minimize the impact of impurities in the coating as much as possible.
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Core spraying equipment: SX-80 plasma spray main unit, with a 15,000℃ flame suitable for multiple materials including zirconia and silicon carbide; powder feeding accuracy of ±1%, and coating thickness tolerance ≤ ±5 μm. Environmental protection support system: Customized soundproof booth + pulse dust collector—effectively addressing spray noise pollution while achieving highly efficient dust recovery, compliant with Thai environmental standards. Automated equipment: 6-axis industrial robot + intelligent spray cart, enabling precise spraying of complex workpieces and automated loading/unloading, boosting production efficiency by 40%.
On-site engineers provide hands-on training, focusing on the core operations of plasma spraying equipment such as the SX-80. Sanxin has customized a complete solution for its customers, including an automated spraying robotic arm, a soundproof booth, and a dust removal system—ensuring both high spraying efficiency and compliance with environmental standards.
Guangzhou Sanxin Thermal Spraying Turntable Shipment
Guangzhou Sanxin Company manufactures single-station and multi-station thermal spray turntables, which can be paired with plasma equipment, supersonic equipment, and arc equipment. We offer rapid, customized production tailored to customer requirements and product dimensions. A thermal spray turntable is a high-precision rotary worktable specifically designed for thermal spraying processes—including flame spraying, plasma spraying, and arc spraying. By providing controllable rotational speed and exceptional stability, it ensures uniform coating thickness on the workpiece surface, thereby enhancing spraying efficiency and quality.
SX-80 plasma equipment is being used at a customer site in Russia.
SX-80 plasma equipment is being used at a customer site in Russia.
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