Plasma spraying equipment and process
Plasma spraying equipment is the core guarantee for realizing plasma spraying technology, and its performance directly affects the quality and stability of the coating. A complete set of plasma spraying equipment mainly consists of a plasma spray gun, a power supply system, a powder feeding device, a working gas supply system, a cooling system, a control system and auxiliary equipment (such as a spraying robot and a turntable). As a core component, the plasma spray gun plays an important role in generating plasma arc, melting and accelerating the spraying material. Its structural design (such as nozzle shape, electrode material, and powder feeding channel position) has a significant impact on the temperature and speed of the plasma and the melting state of the spraying material. Commonly used spray guns mostly use copper anodes and tungsten cathodes, and maintain the working temperature through water cooling to avoid high temperature damage.
The power supply system provides stable DC power for plasma spraying, typically using a thyristor rectifier or inverter. The output voltage ranges from 30-80V, and the current can be adjusted from 200-1000A depending on the spraying requirements. The current directly determines the power and temperature of the plasma arc. For example, at high current, the plasma arc temperature can reach 30,000°C, effectively melting high-melting-point ceramic materials. The powder feeder is responsible for evenly feeding the spray powder into the plasma arc of the spray gun at a set rate. The powder feed rate is generally 5-50g/min, and can be precisely controlled by a screw or pneumatic powder feed mechanism to ensure uniform coating thickness. The working gas supply system provides working gases such as argon, nitrogen, and hydrogen. The gas flow is regulated by a flowmeter. Different gas combinations affect the thermal enthalpy and velocity of the plasma. For example, an argon-hydrogen mixture can increase the temperature and flow rate of the plasma arc.
The cooling system is an integral part of plasma spray equipment. Because the plasma spray gun operates at high temperatures, heat must be removed through a continuous water cooling cycle. Deionized water is typically used as the cooling medium, with the inlet temperature controlled between 20-30°C and the outlet temperature below 50°C to ensure stable operation and a long service life. The control system integrates various subsystems through a PLC or industrial computer, enabling precise control of parameters such as current, voltage, gas flow, powder feed rate, and spray gun movement speed. Some high-end equipment also features touchscreens and data logging capabilities for easy storage and traceability of process parameters. Auxiliary equipment such as multi-axis robots or turntables can automate the spraying of complex workpieces, improving coating uniformity and production efficiency.
The plasma spraying process is a systematic process that requires the optimization of coating performance by reasonably setting process parameters. The pretreatment process is crucial to the bonding strength of the coating, and includes surface cleaning, roughening, and preheating. Surface cleaning uses solvent cleaning or sandblasting to remove oil and scale; roughening is usually performed by sandblasting with aluminum oxide or silicon carbide sand to achieve a surface roughness of Ra3.2-6.3μm on the substrate, thereby enhancing the mechanical bond between the coating and the substrate; preheating heats the substrate to 100-200°C to reduce thermal stress during cooling of the coating. The core parameters of the spraying stage include gun power, spraying distance, spraying speed, and powder particle size. The power of the spray gun determines the degree of material melting. Insufficient power will result in insufficient melting of the powder, forming a loose coating. The spraying distance is generally 80-200mm. Too close will cause the substrate to overheat, while too far will cause the molten droplets to cool too quickly and the bonding strength will decrease. The spraying speed must match the powder feeding rate to ensure uniform coating thickness. The powder particle size is usually 10-100μm. Fine powder is conducive to forming a dense coating, while coarse powder is suitable for preparing thick coatings.
Post-processing processes are used to further enhance coating performance. Commonly used methods include sealing, heat treatment, and machining. Sealing uses resin, metal, or ceramic slurry to fill the coating’s pores, improving corrosion resistance and making it suitable for corrosive environments such as chemical equipment. Heat treatment eliminates internal stress in the coating through low-temperature annealing (200-400°C), reducing the risk of cracking. Machining, such as grinding or polishing, can achieve the required dimensional accuracy and surface finish required for precision parts. In actual production, a personalized process plan must be developed based on the substrate material, spraying material, and application environment. For example, when spraying a ceramic coating on an aluminum alloy substrate, the preheating temperature and spraying power must be strictly controlled to avoid substrate deformation. When spraying a nickel-based alloy coating on a high-temperature alloy, the gas flow rate must be optimized to increase the coating’s density. With the application of intelligent technology, plasma spraying processes are gradually achieving adaptive parameter adjustment and online quality monitoring, providing strong support for the stable production of high-performance coatings.
