The Development Trend of Aerospace Emerging Surface Treatment Technology

According to the commercial aircraft market outlook published by Boeing in 2018, it is predicted that 42700 new aircraft will be delivered globally in the next 20 years, with an output value of 6.3 trillion U.S. dollars, driving huge business opportunities in aviation systems and components markets. The rapidly developing aerospace industry has increasingly higher requirements for oxidation resistance, corrosion resistance, wear resistance, heat resistance, fatigue resistance, and special functions of products and components.

Overview of the aerospace market

According to the commercial aircraft market outlook published by Boeing in 2018, it is predicted that 42,700 new aircraft will be delivered globally in the next 20 years, with an output value of 6.3 trillion U.S. dollars, driving huge business opportunities in aviation system and component markets. The rapidly developing aerospace industry has increasingly higher requirements for oxidation resistance, corrosion resistance, wear resistance, heat resistance, fatigue resistance, and special functions of products and components. A single material often cannot meet the requirements of high-performance aviation equipment, and surface treatment technology can very effectively make up for the lack of material performance and adapt it to harsh working conditions. The mastery and innovation of coating materials and key technologies for coating preparation are of great significance to prolong the service life of products, reduce energy consumption, and improve product reliability. At the same time, the Taiwanese government is actively coaching manufacturers to invest in the development of key aerospace technologies and energy, establish aerospace quality systems and special process certifications. By promoting the aerospace industry clusters, they will jointly build a high-quality supply chain for Taiwan’s aviation industry and integrate with global aviation manufacturers. An important partner of the aviation industry in the Asia-Pacific region.

The development trend of emerging coating technology in aerospace

Currently, the rapidly developing aviation surface treatment technologies include Thermal barrier coatings (TBCs), Environmental barrier coatings (EBC), high-temperature abradable sealing coatings, wear-resistant coatings such as WC-Co and aluminum oxide titanium, absorbing and infrared stealth coatings, etc. The application of coating greatly improves the performance, reliability, economy, service life, and survivability of aerospace products.

1. New high-performance thermal barrier coating
   Thermal barrier coatings are usually applied to the surface of high-pressure turbine blades of aero-engines, which play a role in thermal insulation protection and longevity of the blade matrix. It is mainly composed of a ceramic surface layer with excellent thermal insulation performance and a metal bottom layer with adhesion, which can avoid high-temperature combustion. The gas is in direct contact with the metal substrate, which effectively protects the substrate, prolongs the working life of the engine, and improves the combustion efficiency. It has become one of the three key manufacturing technologies for the high-pressure turbine blades of modern aero engines. At present, the most widely used thermal barrier coating preparation technology is Air plasma spray (APS) and Electron beam physical vapor deposition (EB-PVD) technology.
   Plasma spray-physical vapor deposition (PS-PVD) is a recently developed functional film preparation technology. PS-PVD technology combines the characteristics of plasma spraying and physical vapor deposition technology, which represents the future development direction of high-performance thermal barrier coating preparation technology. Compared with the former two, PS-PVD uses rapid thermal spraying to achieve large-area, uniform physical vapor deposition. By changing the plasma jet state, PS-PVD can also realize the deposition of multi-phase composite coatings and expand the design and preparation of thermal barrier coatings with different tissue structures. More importantly, the plasma jet of PS-PVD has good coating performance, which can realize non-direct-view thermal barrier coating deposition on the surface of workpieces with complex shapes. In general, the PS-PVD method is expected to play a key role in the development of a new generation of ultra-high temperature, high heat insulation, and long-life thermal barrier coatings.
2. Environmental barrier coating on the surface of high-temperature composite materials
   The environmental barrier coating is the key to ensuring the long-term service of composite materials in aero engines. It is mainly used to resist environmental corrosion, and it also has the function of blocking and healing cracks and gaps. Therefore, the choice of environmental barrier coating materials must meet the following four basic conditions.
   • Must have good resistance to oxidation and corrosion, but also need to have a low oxygen diffusivity
   • The thermal expansion coefficient must match the base material to avoid cracks, delamination, or even peeling caused by thermal stress during the thermal cycle
   • It must have good thermal stability within the operating temperature range, which means that no phase change occurs
   • There is no harmful chemical reaction with the base material, which means that it has good chemical compatibility with the base.
   The environmental barrier coating material system has roughly gone through four stages: the first generation-Mullite/YSZ system; the second generation-Barium strontium aluminosilicate, BSAS, Ba1-xSrxAl2Si2O8, 0≤x≤1 system; the third generation-Rare-earth (RE) silicates, RE: rare earth elements system; the fourth generation -Thermal/Environmental barrier coatings (T/EBC) system.
   The third-generation environmental barrier coating has basically met the long-term use in the 1,400℃ gas environment, but focusing on the future development and application of composite materials, NASA combined with the thermal barrier coating used on the superalloy and proposed the design of the thermal/environmental barrier coating. The concept is to meet the future use requirements of silicon carbide/silicon carbide lining and blades in a 1,650℃ water-oxygen coupled environment. The main structure consists of four floors. The first layer is a high-temperature-resistant thermal barrier coating on the surface, which is mainly composed of ceramic materials with low thermal conductivity, including La2Zr2O7 and Gd2Zr2O7, etc., which provide thermal insulation protection for the underlying film and substrate and can be used as the first-level radiation shielding layer. In order to reduce the infrared heat radiation from the high-temperature combustion chamber environment and the high-temperature surface of the film; the second layer is an energy consumption layer and a chemical barrier layer; the third layer is an environmental barrier layer, and the fourth layer is a nanocomposite bonding layer.
3. High-temperature abradable sealing coating
   As the energy crisis intensifies, improving engine efficiency and reducing fuel consumption have become one of the current researches and development focuses of aero engines. In order to improve the efficiency of the engine and protect the blades and casing from scratches and damage, an abradable sealing coating is introduced in the design and development of aero-engine air path seals to maintain a minimum air path gap and improve engine performance. The high-temperature abradable sealing coating is used to seal the gas path of the engine turbine, which can reduce the gap between the tip of the turbine blade and the outer ring of the turbine, thereby reducing gas leakage and improving the engine efficiency. The general design requirement is that when the turbine blade and the sealing coating are in contact and scraped, the coating is scraped and the blade wear is very small, and the friction coefficient should be small, to avoid the high temperature generated by the scraping and the coating or blade ablation and cracking, so the high temperature can wear the seal. The coating must have a certain anti-friction function. The metal-based abradable seal coating has excellent airflow erosion resistance, while the oxide ceramic-based abradable seal coating has relatively poor airflow erosion resistance. Therefore, great attention must be paid to the material composition and the parameter control of the coating preparation method to ensure the service life of the coating.
   In recent years, plasma spraying MCrAlY high-temperature alloy type (such as NiCrAlY, CoCrAlY, NiCrAlYSi, etc.) abradable sealing coating and ceramic-based (such as rare earth oxide stabilized ZrO2, A12O3, etc.) abradable sealing coating has made significant progress, and the film has abradable performance. And the erosion resistance is obviously improved. MCrAlY has high-temperature oxidation resistance and thermal corrosion resistance. Generally, polyphenyl ester is added as a pore-forming agent. After the polyphenyl ester is heated and removed, many fine and uniformly distributed pores are left in the film, which can reduce the hardness of the film and increase the abrasion of the film. Performance reduces the wear of the film on the turbine blades. Adding hexagonal BN or fluoride as a friction-reducing self-lubricating material reduces the coefficient of friction. The thickness of the high-temperature abradable seal coating generally exceeds 1.5mm. Robot automatic spraying technology, computer closed-loop control of spraying parameters, and online coating thickness monitoring can be used to ensure the uniformity and reproducibility of the coating structure and thickness.
4. Thermal spraying ceramic film instead of hard chrome plating
   Because hard chromium electroplating creates a long-lasting hazard to the environment, and the hexavalent chromium in the electroplating waste liquid causes serious harm to human health, it is of great significance to reduce or even cancel the hard chromium electroplating method. In recent years, High-velocity oxygen fuel (HVOF) and High-velocity air fuel (HVAF), WC-Co, WC-Co-Cr, Cr3C2-NiCr cermet coatings, plasma-sprayed Cr2O3, and A12O3-TiO2 oxide ceramic coatings have been used in the industry. It has been widely used, and it will inevitably completely replace the hard chromium plating method. High-speed flame spraying WC-Co-Cr coating has been successfully applied to advanced military/commercial aircraft (including Airbus A380, Boeing 787, F-35, etc.) produced by Airbus, Boeing, Lockheed Martin, etc., and the results show high-speed flame the sprayed WC-Co-Cr2 coating is significantly better than the traditional hard chromium electroplating layer in key performance indicators such as wear resistance, corrosion resistance, and fatigue resistance. Cr3C2-NiCr coating is widely used in high-temperature friction and wear environments. Turbine guide vane seals adopt high-speed flame spraying Cr3C2-NiCr hard coating or plasma spraying A12O3-TiO2 ceramic hard coating, which can resist corrosion, high-temperature oxidation, and wears resistance. Plasma sprayed Cr2O3 ceramic hard coating is widely used in engine sealing and abrasion protection of aircraft operating components. Its wear resistance and corrosion resistance are several times higher than traditional hard chromium plating.
5. Nano coating
   Nanomaterial technology is a new technology that was born in the 1980s and is still developing rapidly and is highly valued by countries all over the world. Physical vapor deposition (PVD), chemical vapor deposition (CVD), thermal spraying, molecular beam epitaxy (MBE), chemical deposition, electrodeposition, and other methods are typical methods for obtaining nano-coatings or thin films. In the past 10 years, researchers have used PVD (including magnetron sputtering, ion beam sputtering, radiofrequency discharge ion plating, plasma ion plating, EB-PVD, etc.) to obtain many results in the preparation of nano monolayer and nano multilayer films. Nano Ti (N, C, CN), (V, Al, Ti, Nb, Cr) N, SiC, β-C3N4, α-Si3N4, TiN/CrN, TiN/A1N, WC-Co coating can be used for aircraft shaft parts and are wear-resistant and anti-corrosion. Plasma-sprayed nano A1203-TiO2 coating has been used for air sealing of aero engines. Nano Y2O3-ZrO2 coating has been used for heat insulation protection of turbine blades. Adding graphene and carbon nanotube composite gives the coating a radar stealth function.
   Thermal spraying is one of the most competitive methods for producing nano-coating. Compared with other technologies, it has many advantages, such as simple construction method, a wide selection of coating materials and substrates, thick film preparation, high deposition rate, and layer composition that is easy to control, easy to form a composite functional coating, and suitable for large-scale components. Using nano-agglomerated powder as the thermal spraying material, by strictly controlling the parameters of the process, shortening the residence time of the nano-material in the flame, limiting the diffusion of atoms and the growth of crystal grains, the nano-coating can be prepared.

Future prospects of the aerospace market

According to a survey conducted by the Japan Aircraft Association, the annual growth rate of the global commercial aircraft market is 5% due to strong customer demand in emerging countries and increased cargo volume. At the same time, the demand for aircraft has also increased year by year and is expected to approximately double in the next 20 years. Each aircraft is assembled from about 3 million components. In order to meet this mass production demand, the application demand of surface treatment in the aviation industry cannot be underestimated. In addition, according to statistics from the European Commission, aircraft emit greenhouse gases, accounting for about 4% of global greenhouse gases, and they are still growing. The EU’s goal is to reduce aircraft carbon dioxide emissions per kilometer by 60%, nitrogen oxide pollution by 90%, and noise by 75% by 2050. In order to achieve this goal, aircraft manufacturers are bound to be more active in introducing new materials and key components, which is bound to further increase the demand for corresponding emerging surface treatment technologies.