Design, selection, function and application of thermal spray coatings
The design of thermal spray coatings is a systematic process. Starting with the workpiece’s operating conditions, the coating’s performance specifications must be clearly defined. Then, a layered design is implemented, taking into account the substrate’s characteristics to ensure an integrated, integrated coating. This operating condition analysis includes detailed parameters such as temperature range, corrosive media concentration, wear load, and impact frequency. For example, when designing a coating for an oil drilling bit, multiple factors must be considered, including downhole temperatures (150-200°C), drilling fluid corrosion (pH 3-11), and rock impact wear (impact loads of 50-100N). Therefore, the coating must possess comprehensive resistance to high temperatures, acids and alkalis, and impact wear. A layered design is key to improving coating reliability and typically comprises a base layer, a transition layer, and a working layer. The base layer (such as a nickel-aluminum alloy) strengthens the coating-substrate bond, the transition layer (such as a nickel-chromium alloy) mitigates thermal stress, and the working layer (such as tungsten carbide-cobalt) provides primary functional performance. In the coating design of a certain aircraft engine blade, the service life of the blade at a high temperature of 1100°C was increased by 2 times by plasma spraying a nickel-aluminum base layer (thickness 5-10μm), a nickel-chromium transition layer (thickness 10-20μm) and a zirconium oxide working layer (thickness 50-100μm).
The selection of thermal spray coatings must adhere to the principles of performance matching, process feasibility, and economic rationality. A multi-dimensional evaluation system should be established to determine the optimal solution. Performance matching requires that the coating’s key performance indicators (such as hardness, temperature resistance, and corrosion rate) exceed the workpiece’s operational requirements. For example, when selecting a coating for the inner wall of a chemical reactor, the corrosion resistance must meet a corrosion rate of less than 0.01 mm/year after immersion in a 5% sulfuric acid solution for 1000 hours. In this case, a nickel-chromium-molybdenum alloy coating is a suitable choice. Process feasibility assessments include equipment compatibility, operational difficulty, and quality stability. For complex workpieces (such as engine block waterways), the highly flexible flame spraying process should be selected over the less adaptable plasma spraying process. For standardized, mass-produced components, automated arc spraying lines can be used to improve efficiency and consistency. Economic rationality comprehensively considers material cost, processing cost, and service life. For example, when selecting an anti-corrosion coating for steel bridge structures, zinc-aluminum arc spraying, while initially more expensive than paint, offers a service life over five times that of paint and lower overall lifecycle costs, making it a more suitable option.
The functional design of thermal spray coatings must achieve synergistic performance across multiple functions to meet the comprehensive demands of complex operating conditions. This approach overcomes the limitations of single performance through material composites and structural optimization. Material composites can combine the strengths of different materials. For example, metal-ceramic composites retain the toughness of metals while possessing the wear resistance of ceramics. Nickel-coated tungsten carbide coatings used on crusher liners have an impact toughness (≥15J/cm²) three times that of pure ceramic coatings, while also possessing a hardness (HRC60) approaching that of pure ceramic. Structural optimization achieves functional control by adjusting microstructural parameters such as coating porosity and density . For example, when preparing heat dissipation coatings, copper coatings with a porosity of 20%-30% are designed to utilize the insulating effect of air trapped in the pores, minimizing heat loss while maintaining thermal conductivity. When preparing filtration coatings, the pore size distribution (5-20μm) is controlled to achieve specific particle filtration. In the coating design of a new energy battery heat sink, a porous copper coating is sprayed with supersonic flame. Its heat dissipation efficiency is 15% higher than that of a solid copper coating, while the weight is reduced by 10%, meeting the battery’s lightweight and efficient heat dissipation requirements.
The application of thermal spray coatings requires customized design based on the specific characteristics of each industry, ensuring that coating performance precisely matches industry requirements and resolving technical challenges encountered in actual production. In the automotive industry, plasma-sprayed iron-based alloy coatings are applied to engine valve tappets, achieving a hardness of HRC 55-60 and increasing wear resistance by more than five times, addressing the wear resistance limitations of traditional quenching treatments. Molybdenum coatings are sprayed onto transmission synchronizer rings, leveraging their low coefficient of friction (0.1-0.2) to achieve smooth shifting and reduce shifting noise. In the power industry, tungsten carbide coatings are applied to turbine blade tips using supersonic flame spraying to resist steam erosion wear, extending blade maintenance intervals from one year to three years. Insulating ceramic coatings are sprayed onto transformer cores, reducing iron loss by 10%-15% and improving transformer energy efficiency. In the medical device industry, plasma-sprayed hydroxyapatite coatings are applied to artificial joints, leveraging their biocompatibility to promote osseointegration, shortening postoperative healing time by 30%, and reducing the risk of prosthetic loosening.
The design, selection, and application of thermal spray coatings must keep pace with technological trends, continuously incorporating new materials, processes, and concepts to drive performance upgrades and expand their applications. Regarding new materials, nanostructured coatings (such as nano-alumina-zirconia) achieve 30% higher hardness and 50% higher toughness through grain refinement. Application to precision mold surfaces can extend the service life by up to twice that of traditional coatings. Amorphous alloy coatings (such as iron-based amorphous) exhibit significant potential in chemical equipment due to their lack of grain boundary defects, offering corrosion resistance over 10 times that of stainless steel. Regarding new processes, cold spraying can produce oxidation-free coatings, achieving tensile strengths exceeding 800 MPa for titanium alloys, making them suitable for precision aerospace components. Laser remelting can reduce coating porosity to below 1%, significantly improving corrosion resistance. Regarding innovative concepts, functionally graded coatings eliminate interfacial stresses through continuous compositional changes. Application to rocket engine nozzles can withstand temperatures exceeding 2000°C and severe thermal shock. These technological advances not only raise the performance ceiling of thermal spray coatings, but also enable their application in strategic emerging fields such as high-end equipment manufacturing, new energy, and biomedicine, providing continuous impetus for industrial development.
