Principles and characteristics of metal thermal spraying
Metal thermal spraying is a surface enhancement technology that uses a heat source to heat a metal material to a molten or semi-molten state, atomizes it into tiny particles with the help of a high-speed airflow, and sprays it onto the surface of a substrate to form a coating. Its core principle is to use thermal energy to make the sprayed material plastic or molten, and then transfer it to the surface of the substrate through kinetic energy. After the particles impact the substrate, they undergo plastic deformation and rapidly cool and solidify, accumulating to form a coating with certain bonding strength and performance. This technology can be used to prepare coatings on the surfaces of almost all solid materials such as metals, ceramics, and plastics, thereby endowing the substrate with special properties such as wear resistance, corrosion resistance, high temperature resistance, and heat insulation. It is an important technical means in the field of surface engineering.
Thermal metal spraying uses a variety of heat sources, including flame, arc, and plasma arc. These sources vary significantly in temperature and energy density, making them suitable for different spray materials. Flame spraying (including powder flame spraying and wire flame spraying) operates at lower temperatures (2000-3000°C) and is suitable for low-melting-point metals (such as zinc, aluminum, and copper) and some alloys. Arc spraying operates at a moderate temperature (3000-6000°C) and is suitable for spraying wire. Plasma spraying operates at the highest temperatures (10,000-30,000°C) and can melt high-melting-point materials (such as ceramics and refractory metals). The choice of heat source must be determined based on the melting point and performance requirements of the spray material to ensure sufficient melting.
Metal thermal spraying technology has remarkable characteristics. The first is that it is applicable to a wide range of materials. Almost all metals, alloys, metal ceramics, ceramics, etc. can be used as spraying materials. It can prepare single material coatings or composite coatings to meet different performance requirements. For example, nickel-based alloys can be used to prepare corrosion-resistant coatings, alumina ceramics can be used to prepare wear-resistant coatings, and zirconium oxide can be used to prepare thermal insulation coatings. Secondly, the coating has diverse functions. By selecting different spraying materials and process parameters, the coating can have multiple functions such as wear resistance, corrosion resistance, high temperature resistance, conductivity, insulation, and heat insulation, which can solve various surface failure problems.
Another key feature of metal thermal spraying is its high process flexibility, allowing it to process substrates of various shapes and sizes, from small precision parts to large components (such as bridges, storage tanks, and rollers), with minimal thermal impact on the substrate. Since the substrate temperature is typically kept below 200°C during the spraying process, deformation, phase change, or performance degradation caused by high temperatures is avoided, making it particularly suitable for precision parts and workpieces undergoing heat treatment. Furthermore, metal thermal spraying offers a wide controllable coating thickness range (0.1-10mm), enabling the production of thin coatings for surface modification or thicker coatings for dimensional repair, as needed.
Metal thermal spraying technology also has some limitations. For example, the coating and substrate rely primarily on mechanical bonding, resulting in relatively low bonding strength (typically 10-100 MPa), which can cause flaking under high impact loads. The coating also has a certain porosity (1%-15%), requiring sealing to improve corrosion resistance. Equipment investment and operating costs are high, especially for plasma spraying and high-velocity flame spraying. Despite this, metal thermal spraying, with its unique advantages, continues to play an irreplaceable role in aerospace, energy and power, machinery manufacturing, automotive and other industries. With continuous technological advancements, its performance is gradually improving and its application range is expanding.
