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The increasing cost of fuel and the push for cleaner skies is compelling aerospace manufacturers to search for more efficient turbine engines. Combustion temperatures and efficiencies increase concurrently, leading to a consequent decrease in emissions of carbon dioxide and nitrogen oxides, as engines burn fuel more thoroughly.
The development of materials that can work at these extreme temperatures has been a key objective of R&D in recent years.
In a commercial jet engine, the fuel burns in the combustion chamber at temperatures approaching 2000 °C [1]. The turbine, which extracts energy from the hot gases and supplies the engine with power, is subject to temperatures ranging from 850 to 1700 °C.
However, any increase to engine efficiency means raising the temperature in the turbine inlet to even more extreme levels. The turbine blades are required to operate and withstand prolonged periods at temperatures extending beyond their melting point.
Thermal Barrier Coatings
The best approach to protecting turbine blades from this intense heat is to utilize thermal barrier coatings (TBC). TBC systems are constituted by an insulating ceramic top coating administered over a metallic bond coating. A thermally grown oxide is formed between these two coatings [2].
Ceramics are employed in the top coating because they typically embody low thermal conductivity and higher melting points than metal. As ceramics do not conduct the heat of the gas, the blade alloy temperature remains stable. Therefore, the turbine can operate at higher temperatures, and efficiency increases as a result [3].
Zirconia is the most typically employed ceramic. In most instances, zirconium oxide (ZrO2) is stabilized with yttrium oxide (Y2O3) [4]. This compound has an extremely low thermal conductivity. The ceramic is administered in layers of 250 to 375 µm thickness, and an intricate alloy plating (the bond coating) is utilized to adhere it to the turbine blade alloy. This oxidation-resistant metallic bond coating also allows a reduction in heat transfer to the base material.
To summarize, TBCs lower the temperature of the blade alloy and ensure protection against oxidation and hot corrosion from high-temperature gas [2]. This leads to significant improvements to turbine performance, life expectancy and efficiency as a result.
Coating Solutions for the Aerospace Industry
Saint-Gobain Coating Solutions produces a broad array of technically accomplished thermal barrier coatings in the form of thermal spray powders and EB-PVD ingots. Implementing its experience and proficiency in process engineering and materials technology, Saint-Gobain can provide the foremost solutions to fortify equipment, such as turbine blades, against high-temperature erosion, abrasion, oxidation, and wear.
Most significantly, its products guarantee lengthier use life and enhanced performance in extreme environments.
With its advanced process technology, Saint-Gobain can supply uniform chemistry and particle size control for absolutely homogeneous powders. For each particular application, its team can customize powder chemistry and particle size in accordance with the equipment requirements and the properties of the desired coatings.
The capacity to adjust the morphology, purity and material homogeneity to the needs of the customer enables maximal deposit efficiency and optimum coating performance.
Thermal Spray Powders
Saint-Gobain produces an assortment of zirconia-based thermal spray powders for TBC, each of which embody the key properties of low thermal conductivity and long coating lifespan. They possess outstanding wear-resistant features for high-temperature applications, including gas turbines and combustion engines.
The company’s yttria zirconia powders comprise remarkably few impurities, and are characterized by a tightly controlled particle morphology and size. These properties enable high deposit efficiency during spraying and prolonged coating life throughout their use life. With their distinctively low thermal conductivity, they are particularly resistant to thermal shock, erosion, and corrosion in high-temperature applications.
Saint-Gobain presents an array of configurations and particle sizes, such as those powders which constitute the 204 Series, featuring 8% stabilized yttria, and exhibiting superior mechanical robustness. These embody a very homogeneous chemistry and are available in multiple particle sizes to meet individual porosity needs.
The spherical shape of the 204 powder enables more constant powder delivery without plugging or pulsing of feed lines. The hollow particles offer more complete melting, which further enhances deposit efficiency. The 204 PR series is tailored to attain higher levels of porosity, while the 204F series is recommended for dense coatings.
The 202 powder series is an exceptionally homogeneous, hollow sphere powder comprising 20% yttria. Its powders yield outstanding thermal barrier protection in air engine applications and embody lower thermal conductivity than the 8% partially stabilized yttria zirconia.
The 1190 series comprises further hollow-sphere, high-purity powders, which generate particularly reliable coatings. The powders offer the lowest thermal conductivity for a highly durable TBC and embody a superior stress/strain response. They possess no monoclinic crystal phase and silica levels are typically lower than 0.1%. The aforementioned morphology, shape, and purity properties can be provided in bespoke configurations in accordance with customer needs.
Environmental Barrier Coating Powders (EBC's)
EBC's fortify engine parts against corrosion and erosion from water vapor and gases. Saint-Gobain supplies high-purity rare earth silicates and barium strontium aluminum silicate (BSAS) thermal spray powders [6].
Aerospace manufacturers require high-purity environmental barrier protection. Rare earth powders can be utilized to fortify silicon carbide parts against gas erosion, and Saint-Gobain can customize their chemistry and size to satisfy particular criteria.
Yttrium monosilicate and yttrium disilicate are two examples of sophisticated EBC powders. BSAS is an important coating utilized in sealing the base material from high-temperature water vapor and other corrosive gases located in gas turbines. Rare earth silicates are implemented with BSAS to decelerate gas penetration.
Ingots for Thermal Barrier Coatings
Saint-Gobain’s EB-PVD ingots are manufactured from the legacy yttria-stabilized zirconia [7], or any customized advanced TBC. Stringent quality control standards during the production stage are vital to an unparalleled reliability. Exclusive production instruments ensure that every ingot manufactured embodies consistent density and morphology from lot to lot.
To guarantee that every ingot can be traced from raw material to end product, the company executes broad testing of materials, systems, and processes. The ingots are characterized by very high stress compliance and superior surface finish.
References
- https://cs.stanford.edu/people/eroberts/courses/ww2/projects/jet-airplanes/how.html
- https://www.hindawi.com/journals/mpe/2017/2147830/
- https://awamdhlwmaterialsengineers.wordpress.com/2014/12/01/thermal-barrier-coatings-turbine-blades/
- http://iieng.org/images/proceedings_pdf/IAE0716412.pdf
- https://www.coatingsolutions.saint-gobain.com/
- https://www.coatingsolutions.saint-gobain.com/materials/thermal-spray-powders/ebc-powders
- https://www.coatingsolutions.saint-gobain.com/
This information has been sourced, reviewed and adapted from materials provided by Saint-Gobain Specialty Grains and Powders
For more information on this source, please visit Saint-Gobain Specialty Grains and Powders.