Selection of thermal spraying materials and processes
The selection of thermal spray materials and processes is crucial to ensuring that coating performance meets application requirements. This requires comprehensive consideration of multiple factors, including the workpiece’s operating environment, performance requirements, substrate characteristics, and economic viability. The primary consideration is the operating environment, which includes factors such as temperature, corrosive media, and wear type. For example, in high-temperature oxidizing environments (such as gas turbine blades), nickel- or cobalt-based high-temperature alloys should be selected, combined with a plasma spray process to achieve a high-temperature, oxidation-resistant coating. In seawater-corrosive environments (such as offshore platform steel structures), zinc, aluminum, and their alloys should be selected, using an arc spray process and utilizing the sacrificial anode protection principle to prevent substrate corrosion. Furthermore, the stress state of the workpiece is crucial. For components subject to impact loads (such as crusher hammers), a metal-ceramic composite with good toughness should be selected, combined with a supersonic flame spray process to balance the hardness and toughness of the coating and prevent it from flaking under impact.
Substrate characteristics significantly influence the selection of thermal spray materials and processes. Different substrates vary significantly in thermal conductivity, thermal expansion coefficient, hardness, and other properties, necessitating a suitable matching of materials and processes to reduce stress between the coating and the substrate. For common structural materials like low-carbon steel, most thermal spray materials and processes are applicable, such as flame-spraying aluminum wire to produce anti-corrosion coatings. However, for non-ferrous metals like aluminum alloys and titanium alloys, due to their high thermal conductivity and susceptibility to oxidation, heat input during the spraying process must be controlled to prevent overheating and deformation of the substrate. Low-temperature flame spraying or cold spraying processes can be used, with aluminum- or titanium-based alloys being the primary materials. For brittle substrates like ceramics and glass, due to their poor thermal shock resistance, materials with a thermal expansion coefficient close to that of the substrate must be selected (e.g., matching a zirconia coating to a ceramic substrate). A step-by-step spraying process is also employed to gradually increase the coating thickness and reduce cracking caused by thermal stress.
Coating performance requirements are the core considerations for selecting materials and processes. Different performance indicators correspond to different material and process combinations. For applications requiring high hardness and wear resistance (such as machine tool guide rail surfaces), tungsten carbide-cobalt powder and HVOF spraying are ideal. This process can achieve a coating hardness exceeding HRC65 and a wear resistance over 20 times that of ordinary steel. For applications requiring excellent corrosion resistance (such as the inner walls of chemical reactors), nickel-chromium-molybdenum alloys should be selected. Plasma spraying creates a dense coating with acid and alkali corrosion resistance comparable to stainless steel. For conductive coatings (such as motor commutators), pure copper or silver combined with arc spraying ensures excellent electrical conductivity, reaching over 90% IACS. For insulating coatings (such as transformer cores), ceramic materials such as aluminum oxide and magnesium oxide should be selected. Plasma spraying can achieve a breakdown voltage exceeding 5000V.
Economic efficiency is a crucial factor in large-scale industrial applications, requiring the selection of low-cost materials and processes while meeting performance requirements. Arc spraying and flame spraying processes require low equipment investment and operating costs, making them suitable for mass production applications such as corrosion protection and repair of large components. For example, zinc-aluminum coatings for bridge steel structures can be applied at a cost of only one-third that of plasma spraying. Powder materials are generally more expensive than wire materials. Therefore, where performance permits, wire materials are preferred. For example, using aluminum wire arc spraying instead of aluminum powder flame spraying can reduce material costs by approximately 40%. Furthermore, the deposition efficiency of the process also influences economic efficiency. Arc spraying can achieve a deposition rate of 5-10 kg/h, far exceeding the 1-3 kg/h of plasma spraying. For workpieces requiring thick coatings (such as roll repair), arc spraying can shorten production cycles and improve economic efficiency.
Process maturity and equipment availability are also practical considerations when making a selection. Equipment resources and technical reserves vary between regions and companies, necessitating the selection of materials and processes that are easy to implement and control. In small and medium-sized enterprises, flame spraying equipment is widely used due to its simple structure and ease of operation, making it suitable for repairing and protecting small and medium-sized workpieces. Large enterprises or specialized spray processing centers can be equipped with high-end equipment such as plasma spraying and supersonic flame spraying to handle demanding precision components. Furthermore, the stability of the material supply is crucial. While commonly used materials such as zinc wire, aluminum wire, and nickel-based alloy powders are in sufficient supply in the market, some new nanomaterials or specialty ceramic materials may have long supply cycles and significant price fluctuations. Therefore, supply chain risks must be assessed during selection to ensure production continuity. By comprehensively weighing these factors, the optimal selection of thermal spray materials and processes can be achieved, ensuring both coating performance and affordability and operability.
