Engineered coating kindness!

Turbine engines in the modern aerospace industry are subjected to some of the most extreme and demanding environments, and must be manufactured to stringent specifications to deal with such strains. Notably, the turbine blades of aircraft engines are under constant mechanical stress and intense temperature variations, with maximum temperatures reaching up to 1,400°C. Thermal barrier coatings (TBCs) have been employed for several decades to form protective layers over the alloy parts. These coatings provide extensive thermal insulation and shielding from wear-and-tear, increasing the engine's tolerance to raised temperatures for efficient fuel combustion and consumption and enhancing the engine's lifespan. Such advantages lead to more sustainable and environmentally friendly aircraft promoting decreased carbon footprints. Since the introduction of TBCs, advances in the development and physical application of the coatings have been fuelled by technological, economic and societal pressures to increase turbine efficiency. Recent advances have led to the development of nanostructured, partially stabilised zirconia (PSZ), such as Innovnano's 4 mol% yttria stabilized zirconia (4YSZ), which displays excellent mechanical and physical properties, and is ideal for the production of finely structured layers. Compared to the traditional use of micromaterials, nanostructured ceramic powders can be used to create thinner coatings, minimising component weight without any compromise in performance. Unsurprisingly, nanostructured feedstock is finding an increasingly prominent place in the production of aircraft TBCs thanks to a major advancement in TBC application - the new suspension plasma spray technique - it can now offer an enhanced and even more economic protection method for turbine engines. Advancements in TBC technologies For turbine engine TBCs, a columnar structure is best suited (Figure1), offering high strain tolerance and shock resistance. As such, techniques to develop successful columnar coatings are highly sought-after by the aerospace industry and have been the subject of many studies. Until recently, electron beam physical-vapour-deposition (EB-PVD) was the most popular technique for applying columnar TBCs. However, since the advancements of suspension plasma spray (SPS) using nanostructured PSZ in suspension, it is possible to generate the desirable columnar structure at a substantially lower cost. Not only are there significant capital savings to be made using SPS, but operational costs are reduced due to the increased efficiency of TBC deposition enabled by the technique. Undeniably, properties of the suspension, procedural requirements and spraying conditions play a significant role in the formation of the coating, and once optimal conditions are set, SPS could become the preferred technique over EB-PVD for TBC application. To develop a suitable rheological suspension, the characteristics of the suspension need to be considered carefully. For example, the morphology of the TBC is strongly dependent on the solvent used for suspension, so the choice between a water-based or organic-based suspension is critical. Water is a more stable solvent, with minimal associated environmental problems, although its higher enthalpy of vaporisation means that more energy is required. Conversely, while ethanol-based suspensions are considered to produce a better quality coating, they have a high content of hydroxide and carbon groups which can pollute the coating if the process is not controlled completely. Essentially, it has become a balancing act when choosing between solvents, weighing up the mechanical benefits of the resulting coating and any environmental implications. The use of suspension sprays for generating TBCs has been proven to improve the overall performance of the coating, producing a thin but dense nanostructured ceramic coating with much finer grain and pore sizes. The resulting structure ensures significant thermal insulation, chemical inertness and superior hardness for maximum protection against the harsh environment of the engine. As such, the use of nanostructured feedstocks with SPS will increase the durability of TBCs, reducing the need for service intervention and the overall cost of maintenance, to enhance efficiency within the aerospace industry. SPS process and spraying conditions For optimal SPS processing, the method used to inject the suspension into the plasma spray for TBC application must be carefully considered. Currently, axial injection is more popular, however clogging problems can exist. Radial injection is therefore an alternative, although due to an increased sensitivity to fluctuations in the plasma jet velocity, the liquid injection conditions have to be meticulously controlled to overcome any unwanted interactions which would directly affect the end properties of the coating. The ideal coating requires consideration of the type of injector used, in order to avoid any possible issues in the application process, such as sedimentation of the suspension or deposition in the combustion chamber/nozzle. For successful injection, the liquid stream (4YSZ suspension) has to have a higher momentum density than the plasma gas flow. When using this novel technique, the velocity of the plasma flow and density of the suspension can be increased in order to overcome any tendency of the powder to agglomerate. Usually, the use of dispersants can also help avoid particle agglomeration during SPS. However, advantageously, as a result of its unique manufacturing process, Innovnano's 4YSZ nano-powder has properties which negate any need for dispersants. This means the ceramic powder is less expensive, has a lower contamination risk and has fewer controllable variables that can affect the suspensions' characteristics. Furthermore, despite long shipping distances, the powder can be developed easily into a stable, high performance suspension, by simple addition of the solvent. Improving aero engine performance Thermal barrier coatings are a necessity in the manufacture of aircraft turbine engines, protecting them from physical wear, chemical corrosion and thermal degradation. With SPS, the desirable columnar-based coating can be produced at a much lower cost, saving valuable resources and improving accessibility to high quality coatings in the turbine industry. Furthermore, the use of nanostructured feedstocks ensures the resulting mechanical properties of the TBCs are further enhanced, with low thermal conductivity, high melting points and excellent adhesion to the underlying metallic substrates, to name a few. Importantly, the high thermal insulation properties of the coatings produced by this technique will also allow aircraft engines to be operated at higher temperatures, improving the engine's burn efficiency for lower fuel consumption. This not only contributes to important cost savings, but also a reduced carbon footprint, favouring a more environmentally-friendly engine lifecycle. www.innovnano-materials.com