Classification of thermal spray materials
Thermal spray materials can be divided into four categories based on their physical form: powder, wire, rod, and paste. Different forms of material are suitable for different thermal spray processes, each with its own characteristics and application range. Powder is the most widely used material form in thermal spraying. Its particle size typically ranges from 10-150μm and can be applied via various processes, including plasma spraying, flame spraying, and supersonic flame spraying. The advantage of powder materials lies in their flexible composition adjustment, allowing them to produce coatings of various alloys, cermets, and ceramics, such as tungsten carbide-cobalt powder and alumina-zirconia composite powder. Wire is primarily used in arc spraying and flame spraying processes. Its diameter typically ranges from 1.2-3.0mm. It features stable feeding and high deposition efficiency. Commonly used wires include aluminum wire, zinc wire, and nickel-aluminum composite wire, and are widely used in the preparation of anti-corrosion coatings. Rods are typically made of ceramic or cermet materials, with diameters ranging from 3 to 13 mm. They are primarily used in flame spraying processes, where the ends of the rods are melted and atomized into particles. Zirconia rods, for example, are often used to produce high-temperature-resistant coatings. Pastes, a new type of thermal spray material, are a mixture of powder and binder. They are suitable for certain specialized processes and can improve coating density and uniformity, but their application range is relatively narrow.
Based on their chemical composition, thermal spray materials can be categorized into four major categories: metals, alloys, ceramics, and composites. Each type of material has unique properties and applications. Metals, primarily pure aluminum, pure zinc, pure copper, and pure nickel, possess excellent electrical and thermal conductivity, as well as corrosion resistance, and are commonly used in the preparation of anti-corrosion and conductive coatings. For example, pure zinc powder spray coatings form sacrificial anodes on steel surfaces, effectively preventing corrosion. Pure copper coatings, due to their excellent electrical conductivity, are widely used in conductive connections within the electronics industry. Alloy materials, such as nickel-based alloys, cobalt-based alloys, and iron-based alloys, are composed of two or more metallic elements. By adjusting their composition, they can achieve superior properties compared to pure metals. Nickel-based alloys offer excellent corrosion resistance and high-temperature oxidation resistance, making them suitable for surface protection of components such as chemical equipment and turbine blades. Cobalt-based alloys, with their exceptional wear and high-temperature resistance, are commonly used in coatings for high-speed wear components such as aircraft engine valves and bearings. Ceramic materials primarily include oxide ceramics (such as alumina and zirconia), carbide ceramics (such as tungsten carbide and chromium carbide), and nitride ceramics (such as silicon nitride and boron nitride). These materials exhibit high hardness, high-temperature resistance, and corrosion resistance, making them suitable for the preparation of wear-resistant and high-temperature resistant coatings. For example, alumina ceramic coatings are used for wear protection of pump shafts, and zirconia ceramic coatings are used as thermal insulation layers for high-temperature furnaces. Composite materials are composed of two or more different materials, combining the excellent properties of each component. Examples include metal-ceramic composites (such as nickel-coated alumina and cobalt-coated tungsten carbide) and metal-metal composites (such as aluminum-magnesium alloys). These composite materials can meet the comprehensive coating performance requirements of various operating conditions. For example, nickel-coated alumina composite coatings combine the toughness of metal with the wear resistance of ceramics, making them suitable for complex wear environments.
Thermal spray materials can be categorized by coating function into wear-resistant, corrosion-resistant, high-temperature-resistant, conductive, and insulating materials. Each functional material is designed for specific application requirements. Wear-resistant materials are primarily used to resist mechanical wear and extend component life. Commonly used materials include tungsten carbide-cobalt, chromium carbide-nickel-chromium, and high-chromium cast iron. Tungsten carbide-cobalt powder spray coatings can achieve a hardness exceeding HRC60 and offer wear resistance 10-20 times that of ordinary steel. They are widely used in heavily worn components such as mining machinery and oil drilling equipment. Corrosion-resistant materials protect metal substrates from corrosive media. These primarily include zinc, aluminum, and their alloys, as well as nickel-based corrosion-resistant alloys. Zinc-aluminum pseudo-alloy coatings offer excellent salt spray resistance in marine environments, with a salt spray resistance of over 5,000 hours, making them ideal for corrosion protection of steel structures. High-temperature-resistant materials maintain stable performance even in high-temperature environments. Examples include ceramics such as zirconium oxide, magnesium oxide, and lanthanum chromate, as well as metals such as nickel-chromium alloys and cobalt-based superalloys. Zirconia coatings maintain excellent stability at temperatures exceeding 1000°C and are commonly used for thermal insulation in aircraft engine combustion chambers and high-temperature furnace linings. Conductive materials such as copper, silver, and nickel are used to create conductive coatings to ensure efficient current transmission. For example, copper coatings are used for surface treatment of motor commutators to improve their conductivity and wear resistance. Insulating materials such as ceramics like aluminum oxide and magnesium oxide offer excellent electrical insulation properties, with breakdown voltages reaching several thousand volts, making them suitable for insulating coatings in electronic equipment.
Thermal spray materials can be categorized by their suitability for the spraying process, including those suitable for flame spraying, arc spraying, and plasma spraying. Different processes have varying requirements for material form and properties. Flame spray materials include powders, wires, and rods. Powders must possess good fluidity and a moderate melting point to facilitate thorough melting in the flame. Wires must possess a uniform composition and good plasticity to ensure stable feeding. For example, nickel-coated aluminum powder, commonly used in flame spraying, forms a strong bond after flame heating, enhancing the bond between the coating and the substrate. Arc spraying primarily consists of wires, typically two metal wires of the same or different compositions. These wires must possess excellent electrical conductivity and melting properties, such as aluminum wire, zinc wire, or nickel-aluminum composite wire. Arc spraying aluminum wire can achieve deposition efficiencies exceeding 80%, making it suitable for corrosion protection coatings on large steel structures. Plasma spraying materials are primarily powders, making them particularly suitable for high-melting-point materials such as ceramic powders and refractory metal powders. The high temperatures of the plasma arc (reaching over 10,000°C) fully melt these materials, forming a dense coating. For example, when plasma spraying zirconium oxide powder, the coating density can reach over 95%, demonstrating excellent high-temperature resistance and thermal insulation properties.
New thermal spray materials have been a research hotspot in recent years, primarily including nanostructured materials, amorphous materials, and functionally gradient materials. These materials offer new directions for the development of thermal spray technology. Nanostructured thermal spray materials are composed of nanoscale particles, and the coating retains its nanostructure after spraying, resulting in higher hardness, toughness, and wear resistance. For example, coatings sprayed with nano-alumina-zirconia composite powders have a hardness increase of over 30% and toughness increase of over 50% compared to traditional micron-scale coatings, making them suitable for surface enhancement of precision molds. Amorphous materials have a long-range disordered atomic structure and are free of defects such as grain boundaries and dislocations. Consequently, they possess excellent corrosion resistance, wear resistance, and magnetic properties. For example, coatings sprayed with iron-based amorphous alloy powders have corrosion resistance over 10 times that of stainless steel and are widely used for corrosion protection in chemical equipment. Functionally graded materials (FGMs), whose composition and properties vary continuously across their thickness, can effectively mitigate thermal and interfacial stresses between the coating and the substrate. For example, FGMs that transition from a metal substrate to a ceramic coating combine the toughness of metal with the high-temperature resistance of ceramics, making them suitable for high-temperature components in the aerospace industry, such as surface coatings for rocket engine nozzles. The emergence of these new materials not only expands the application scope of thermal spray technology but also provides new solutions to the problems inherent in traditional coatings.
