Plasma spraying technology
As an advanced surface modification technology, plasma spraying technology has become an important means of preparing high-performance coatings after continuous development and improvement since its advent in the 1950s. This technology uses high-temperature plasma as a heat source to melt the spray material and spray it at high speed onto the surface of the substrate to form a coating. It can significantly improve the wear resistance, corrosion resistance, high temperature resistance, thermal insulation and other properties of the substrate, and is widely used in aerospace, energy and electricity, machinery manufacturing, automotive industry and other fields. For example, spraying zirconium oxide thermal insulation coating on the surface of aircraft engine blades can reduce the operating temperature of the blades and increase their service life; spraying nickel-based alloy coating on the surface of power station boiler pipes can resist corrosion and wear from high-temperature flue gas.
The core equipment of plasma spraying technology includes the plasma spray gun, power supply, powder feed system, control system, and cooling system. The plasma spray gun is a key component, consisting of electrodes (cathode and anode), nozzle, and powder feed channel. The cathode is typically a tungsten electrode, and the anode is a copper nozzle. Working gas is introduced at the rear end of the spray gun, generating an arc between the electrodes and ionizing them into plasma. The power supply provides direct current (DC), typically 30-80V and 200-1000A. Adjusting the current controls the power and temperature of the plasma arc. The powder feed system feeds the spray powder into the plasma arc at a constant rate. The powder feed rate directly affects the thickness and uniformity of the coating. The cooling system uses water to cool the anode and nozzle of the spray gun to prevent damage from high temperatures. The control system regulates parameters such as current, voltage, working gas flow, and powder feed rate to ensure a stable spraying process.
The plasma spraying process primarily consists of three stages: substrate pretreatment, spraying, and post-treatment. Substrate pretreatment aims to enhance the bond between the coating and the substrate and primarily involves surface cleaning, roughening, and preheating. Surface cleaning removes oil, rust, and scale, and can be accomplished by solvent cleaning, sandblasting, or pickling. Roughening involves sandblasting (e.g., with aluminum oxide grit) to increase the substrate’s surface roughness, creating a rugged surface that enhances the mechanical bond between the coating and the substrate. Preheating heats the substrate to 100-200°C to remove surface moisture and reduce stress during cooling. During the spraying phase, the pretreated substrate is placed in front of the spray gun. The distance between the gun and the substrate (usually 80-200 mm) and the relative speed are adjusted. The machine is then activated to melt the spray material and spray it onto the substrate surface, forming a coating. Post-treatment includes pore sealing (e.g., applying resin or metal to fill pores in the coating), heat treatment (to relieve coating stress), and machining (e.g., grinding to achieve dimensional accuracy).
Plasma spraying technology can be categorized into atmospheric plasma spraying, low-pressure plasma spraying (vacuum plasma spraying), and water-stabilized plasma spraying, depending on the material form and process characteristics. Atmospheric plasma spraying, performed in air, features simple equipment and low cost, making it the most widely used. However, the coating is susceptible to oxidation contamination and is therefore suitable for applications requiring less demanding coating performance. Low-pressure plasma spraying, performed in an environment below atmospheric pressure, reduces oxidation of the sprayed material, improving the density and bonding strength of the coating. It is suitable for high-end applications such as aerospace, but the equipment is complex and the cost is high. Water-stabilized plasma spraying uses water as the working medium. The plasma arc temperature is higher, capable of melting high-melting-point materials (such as refractory metals and ceramics), and boasts high spraying efficiency, but equipment maintenance is challenging.
The development trends of plasma spraying technology are toward high performance, intelligent technology, and environmental friendliness. To achieve high performance, this technology is developing new spray materials (such as nanopowders and composite powders) and optimizing process parameters to improve coating density, bonding strength, and performance stability. To achieve intelligent technology, this technology utilizes computer simulation to optimize the spraying process and uses sensors to monitor coating parameters such as temperature and thickness in real time, enabling automated control and online quality monitoring. To achieve environmental friendliness, this technology is developing low-pollution working gases and spray materials, and improving dust removal and exhaust gas treatment equipment to minimize environmental impact. In the future, plasma spraying technology will be applied to more high-end fields (such as new energy and biomedicine), providing more advanced solutions for improving material surface properties.
