Principles and characteristics of plasma spraying
Plasma spraying is a surface enhancement technology that uses the high temperature of a plasma arc to melt the spray material and spray it at high speed onto the substrate surface, forming a coating. Its core principle is to generate high-temperature plasma through a plasma generator, heating the solid spray material (powder, wire, or rod) to a molten or semi-molten state. This material is then atomized into tiny particles by a high-speed plasma gas flow and sprayed at high speed (typically 50-500 m/s) onto the pretreated substrate surface. After impacting the substrate, the particles rapidly cool and solidify, accumulating to form a coating with sufficient bonding strength and performance. This technology can produce functional coatings such as wear-resistant, corrosion-resistant, high-temperature-resistant, and thermal-insulating coatings on a variety of substrates, including metals, ceramics, and plastics, making it a key technology in the field of surface engineering.
The high temperatures in plasma spraying arise from the energy released by the plasma arc. Powered by a DC power supply, the plasma arc ionizes a working gas (typically argon, nitrogen, or an argon-hydrogen mixture) between electrodes, creating a high-temperature ionized gas with temperatures reaching 10,000-30,000°C, far exceeding the melting point of most materials. During the spraying process, the working gas is ionized into a plasma as it passes through the nozzle of the plasma spray gun. The spray material is fed into the plasma arc through the gun’s feed port, where it is instantly heated to a molten state. Simultaneously, it is accelerated by the high-velocity plasma flow, forming a high-speed stream of molten droplets. Upon impacting the substrate surface, these droplets undergo plastic deformation and form a tight bond. As the droplets accumulate, a coating gradually forms. The bond between the coating and the substrate is primarily mechanical, with some micrometallurgical bonds also present. The bond strength is typically 10-100 MPa.
Plasma spraying technology has remarkable characteristics. The first is that it is applicable to a wide range of materials. Almost all solid materials (metals, alloys, ceramics, cermets, plastics, etc.) can be used as spraying materials. It can prepare single material coatings or composite coatings to meet different performance requirements. For example, ceramic powders (such as alumina and zirconium oxide) can be used to prepare high-temperature resistant and wear-resistant coatings; metal powders (such as nickel-based alloys and cobalt-based alloys) can be used to prepare corrosion-resistant coatings. Secondly, the coating has excellent performance. The high temperature of plasma spraying can ensure that the material is fully melted. The formed coating has high density and good uniformity, and the coating thickness is controllable (usually 0.1-5mm) and can be adjusted according to needs.
Another key feature of plasma spraying is its high process flexibility. It can process substrates of various shapes and sizes, from small parts to large components (such as boiler pipes and rollers), while minimizing thermal impact on the substrate. Due to the rapid ejection velocity and rapid cooling of the droplets, the substrate temperature is typically kept below 200°C, preventing deformation, phase change, or performance degradation caused by high temperatures. Plasma spraying is particularly suitable for precision parts and workpieces undergoing heat treatment. Furthermore, plasma spraying offers high production efficiency, rapid coating deposition, and the ability to achieve mass production. It is also relatively simple to operate and easily automated.
However, plasma spraying technology also has some limitations. For example, the coating has a certain porosity (usually 1%-10%). Although some pores can be filled through sealing treatment, this will still affect the density and corrosion resistance of the coating. The bonding strength between the coating and the substrate is relatively low, and it may peel under high impact loads. The equipment investment is large, and the operating costs (such as working gas consumption and electricity consumption) are high. The spraying process generates dust, noise, and harmful gases, requiring effective environmental protection measures. Despite this, plasma spraying technology, with its unique advantages, still has an irreplaceable position in the fields of aerospace, electricity, metallurgy, machinery, etc., and with the continuous advancement of technology, its limitations are gradually being improved.
