Performance and Design of Thermal Spray Coatings
The performance of thermal spray coatings is the core indicator of their quality, mainly including mechanical properties, protective properties, and functional properties. These properties are interrelated and have different focuses, and together they determine the coating’s effectiveness. Among mechanical properties, bond strength is a key indicator, referring to the ability of the coating to bond to the substrate or between coatings. It is usually measured through tensile testing. High-quality coatings can achieve bond strengths of 30-100 MPa. For example, the bond strength of plasma-sprayed nickel-based alloy coatings to steel substrates is generally above 50 MPa. Hardness reflects the coating’s resistance to deformation. Tungsten carbide-cobalt coatings can reach hardnesses as high as HRC65, while aluminum coatings are only HRC15-20. The hardness should be selected based on the application requirements. Protective properties include corrosion resistance and high-temperature resistance. Corrosion resistance can be assessed through salt spray and immersion tests. Zinc-aluminum coatings have a neutral salt spray lifespan of over 5,000 hours, while nickel-chromium alloy coatings exhibit a corrosion rate of less than 0.01 mm/ year after 1,000 hours of immersion in a 5% sulfuric acid solution . High-temperature resistance focuses on the coating’s stability at high temperatures. Zirconia coatings can withstand long-term use above 1,200 °C, while copper coatings soften above 300 °C. Functional properties such as electrical and thermal conductivity are also evaluated. Pure copper coatings can achieve an electrical conductivity of up to 90% IACS , making them suitable for conductive applications. Zirconia coatings, with a thermal conductivity of only 0.1-0.3 W/(m · K) , are excellent thermal insulators.
The design of thermal spray coatings needs to be guided by performance requirements and targeted in combination with the parameters of the use environment to ensure that the coating can function stably during service. First, the working environment of the workpiece needs to be clarified, including the temperature range, type and concentration of corrosive media, wear form and load, etc. For example, when designing wear-resistant coatings for mining machinery, it is necessary to select high-hardness, high-toughness metal ceramic materials for working conditions dominated by abrasive wear; when designing anti-corrosion coatings for chemical equipment, it is necessary to select alloy materials resistant to corresponding corrosion according to the pH value and temperature of the medium. Secondly, the thickness design of the coating must be considered. A coating that is too thin cannot provide effective protection, and a coating that is too thick can easily generate internal stress and cause cracking. Usually, the thickness of the wear-resistant coating is 0.1-1mm, the anti-corrosion coating is 50-200μm, and the thermal insulation coating can reach 0.5-2mm. In the coating design of a rolling mill’s working rolls, a 0.5mm thick high-chromium cast iron coating was selected based on the rolling force (50-100kN) and rolling temperature (800-1000℃). This coating not only meets the wear resistance requirements but also avoids the problem of poor heat dissipation from the roll body caused by excessive thickness.
Coating structural design is a key means of improving performance. Layered and gradient structures can effectively alleviate stress, increasing coating reliability and service life. A layered design typically consists of a base layer, a transition layer, and a working layer. The base layer (e.g., nickel-aluminum alloy) is 5-10 μm thick and primarily serves to enhance the bonding strength between the coating and the substrate, forming a strong metallurgical bond. The transition layer (e.g., nickel-chromium alloy) is 10-20 μm thick and serves to balance the difference in thermal expansion coefficients between the coating and the substrate, reducing thermal stress. The working layer, selected based on functional requirements, provides key properties such as wear resistance and corrosion resistance. A more advanced approach is gradient structural design, where the material composition changes continuously from the substrate to the surface. For example, a gradient coating from metal to ceramic can completely eliminate interfacial stress and is highly effective in extreme environments such as rocket engine nozzles. A certain aircraft engine combustion chamber coating employs a gradient design, with a gradual change in composition from a nickel-based alloy base layer to a zirconium oxide surface layer. The coating exhibits no cracking or flaking when used at temperatures of 1100°C, and its service life is more than doubled compared to traditional layered coatings.
The material matching design of thermal spray coatings must consider the physical and chemical compatibility between the coating and the substrate to avoid coating failure due to significant performance differences. Thermal expansion coefficient matching is crucial. If the difference in thermal expansion coefficients between the coating and substrate exceeds 10×10⁻⁶/ °C, significant thermal stresses can easily be generated during temperature fluctuations, leading to cracking or flaking of the coating. For example, when spraying an alumina coating ( 8×10⁻⁶/ °C) onto an aluminum alloy substrate ( 23×10⁻⁶/ °C), an aluminum – silicon transition layer ( 18×10⁻⁶/ °C) is required to relieve stress. Chemical compatibility is also crucial. Chemical reactions between the coating and the substrate must be avoided at high temperatures. For example, when spraying a nickel-based alloy coating onto a titanium alloy substrate, the formation of brittle titanium-nickel intermetallic compounds must be prevented at high temperatures. Therefore, controlling the spraying temperature or adding an isolation layer is necessary. In addition, hardness matching is also very important. When the hardness of the coating is much higher than that of the substrate, the coating is easily peeled off due to deformation of the substrate under impact load. Therefore, when spraying a high-hardness coating on a soft substrate (such as aluminum and copper), a toughness transition layer needs to be added. For example, a pure aluminum layer is first sprayed on the aluminum substrate, and then a tungsten carbide coating is sprayed to improve the impact resistance.
The design of thermal spray coatings must balance process feasibility and cost-effectiveness. While meeting performance requirements, a method that is easy to prepare and cost-effective should be selected. Regarding process feasibility, the material’s sprayability must be considered. For example, ceramics, with their high melting points, are well-suited to plasma spraying, while metals with lower melting points (such as zinc and aluminum) are more suitable for arc spraying or flame spraying. For workpieces with complex shapes (such as cavities or deep holes), a more flexible spraying process, such as flame spraying, should be selected over the less adaptable plasma spraying. Economical design requires a comprehensive consideration of material cost, processing cost, and service life. For example, in the corrosion protection of steel bridge structures, zinc-aluminum arc spray coatings have higher initial costs than paint, but their service life is over five times that of paint, resulting in lower lifecycle costs. When designing an engine valve coating, an automotive parts manufacturer compared plasma spraying of a nickel-based alloy with high-velocity flame spraying of tungsten carbide. The plasma spraying method offered a 30% lower cost and met the application requirements. The manufacturer ultimately chose the plasma spraying method, achieving both performance and cost savings. Only through scientific performance design and comprehensive considerations can thermal spray coatings achieve optimal results in practical applications.
