Principle of Plasma Coating

　　What is the principle of plasma spraying? Plasma spraying is a thermal spray surface‑processing technique that uses a rigid, non-transferred plasma arc as the heat source to heat the powder material to a molten or semi‑molten state. The molten particles are then atomized and accelerated by a high‑velocity flame jet, which propels them onto the pre‑treated substrate surface to form a sprayed coating.

　　The principle of plasma spraying relies on generating a plasma jet using a plasma spray gun—also known as a plasma arc generator. The gun’s tungsten electrode (cathode) and nozzle (anode) are connected to the negative and positive terminals of the power supply, respectively (with the workpiece remaining electrically neutral). A high‑frequency spark ignites an arc, causing the process gas supplied to the gun—typically argon or nitrogen—to ionize into a plasma under the influence of the arc. Under the combined effects of mechanical compression, self‑magnetic compression, and thermal compression, the arc is constricted, forming a non‑transferred plasma arc. A powder feed stream carries the coating material into the plasma arc, where it is rapidly heated to a molten or semi‑molten state. The molten particles are then propelled at high velocity by the plasma jet, impacting the pre‑treated substrate surface and forming a robust coating. As a result, the coated surface of the component acquires a range of specialized physicochemical properties—such as enhanced hardness, wear resistance, heat resistance, corrosion resistance, electrical insulation, thermal insulation, and lubricity—thus meeting the diverse performance requirements of the part under various operating conditions.

　　During the coating formation process, a single molten particle serves as the fundamental building block of the coating, and its behavior largely reflects the characteristics of coating development. The behavior of an individual particle comprises three essential stages: first, the delivery of the spray material into the heat source; second, the interaction between the material and the heat source, during which the material is heated, melted, and accelerated under thermal exposure, while also undergoing reactions with the surrounding atmosphere—particularly in the case of metallic materials, where entrainment of atmospheric air can lead to oxidation and other surface‑atmosphere interactions; and third, the collision of these high‑temperature, high‑velocity molten particles with the substrate, followed by lateral spreading and flattening, and rapid cooling and solidification. This entire sequence typically unfolds within an extremely brief timescale of tens of microseconds.

　　The bonding mechanisms of a coating encompass both the adhesion between the coating and the substrate surface and the cohesive bonding within the coating itself. The strength of this bonding is referred to as the adhesive strength, while the strength of the bond between the coating and the substrate surface is termed the cohesive strength.

　　Research and empirical evidence indicate that the “macroscopic” interfacial bonding between the coating and the substrate is predominantly mechanical, with physical and chemical interactions playing a secondary role; meanwhile, the “microscopic” interfacial bonding among the layered particles within the coating likely involves a greater contribution from physicochemical interactions.