It has been couple of decades since India embarked on a project to build a modern low-bypass turbofan jet engine compatible with the Light Combat Aircraft (Tejas) at the Gas Turbine Establishment (GTRE) of the Defence Research & Development Organization (DRDO). The engine under development, named Kaveri, has faced multiple hurdles both technological and bureaucratic over the years. Nonetheless, the Defence Metallurgical Research Lab (DMRL), a lab under DRDO, has produced and proven a new thermal barrier coating (TBC) material that has a maximum surface temperature capability of 1200°C. This development, as we shall see, provides a useful baseline for the progression of jet engine technology in India.
A Brief on the Significance of Thermal Barrier Coatings
A jet engine produces thrust not unlike an internal combustion engine. Air is drawn in and compressed by the compressor section. Fuel is then mixed with this compressed air and ignited producing a great amount of gases which are then used to rotate a turbine to extract work. The turbine and the compressor are connected via a common shaft such that fuel ignition is utilized to drive the compressor and continue engine operation. Military as well as modern civilian use place a great premium on weight, specific fuel consumption, maximum thrust as well as low maintenance requirements and long life of components.
One of the primary limiting factors that limit thrust and fuel consumption in a jet engine is the temperature of exhaust gases. Titanium, a metal widely used in aerospace applications, readily undergoes oxidation below temperatures of exhaust gases. Hence, Nickel alloys are used for ‘hot section’ components of the jet engine. Today, even the high pressure compressor turbine blades of the Kaveri engine utilise indigenous Ni alloy SUPERNI718A due to the high temperatures. Ni-alloys by themselves however are not robust enough to prevent heat and aero-mechanical stress related deterioration of turbine blades. Extended exposure to high temperature and mechanical stress results in creep-fatigue that causes failure of turbine blades. Reducing thermal exposure of the turbines therefore is vital to extend their life. Layers of oxidised material called TBCs are applied to high temperature sections of the engine such as turbine blades and vanes for this purpose. Air cooling vents are also used to provide a barrier on top of the blades and vanes to lower heat transfer.
Picture Courtesy : ARCI, Hyderabad.
Yttria Stabilised Zirconia (YSZ) has been the gold standard of thermal barrier coatings (TBC) for many years. The need for increasing exhaust gas temperature, Turbine Entry Temperature (TeT), is one of the ways to extract greater work from fuel. A stoichiometric air-fuel ratio would produce the maximum temperature. However such a temperature would also produce the greatest thermal stresses on the turbines and vanes in the engine. TeT is therefore limited to the extent of a designer’s target of component life. Therefore any TBC that can enable higher gas temperature operation would be greatly valued.
Rare Earth pyrochlores (Re2Zr2O7, Re = La, Gd) are very stable materials. La and Gd zirconates remain structurally stable under reducing atmosphere of Ar(g)/3%H2(g) at 1400 °C. The zirconates with pyrochlore structure, are predominantly cubic and ionic. They also allow a variety of atomic substitutions at the A, B and O sites when the ionic radius and charge neutrality conditions are met. Since Gadolinium is heavier and earmarked for strategic applications, Lanthanum was chosen for TBC purpose. Also, Lanthanum has a higher ionic radius which helps mixing it with Zirconium to produce Lanthanum Zirconate.
Among the pyrochlores, La2Zr2O7 (LZ) seems to have great potential as a TBC material due to its excellent bulk properties vis a vis YSZ. But it has a lower coefficient of thermal expansion and slightly higher specific gravity compared to YSZ. Though nano LZ can mitigate the problem to a certain extent, it can not be applied directly on the MCrAlY bond coat. A double-layer coating with La2Zr2O7 as top coat was adopted since it is reported that such a bi-layer coating enhanced the temperature capability of the coating by >100K. Therefore LZ is applied as a top coat material over YSZ for enhanced coating life. A nano-structured bi-layer is also expected to reflect certain amount of radiations thus providing a more effective TBC. LZ has good chemical compatibility with YSZ at least up to 1250C. LZ remains stable in a large La/ Zr molar ratio range, from 0.87 to 1.15 and remains so even when the La2O3 composition changes ±10 per cent from the stoichiometry. This is significant since in the absence of cooling or heat losses the substrate temperature equals that of the environment eventually. The synthesis and impurities in TBC materials also influence the stability and life of the coating.
IR radiation which has wavelength in the 700 nm –1100 nm range results in heating of the surface if absorbed. To achieve the highest Near-IR reflectivity, particle size needs to be more than half the heat wavelength that is to be reflected. A comparison of the NIR reflectance of nano and their micro crystalline forms show that the nano-crystalline metal oxides are about 15 per cent – 20 per cent more reflective. A decrease in mean particle size usually increases the reflectance. Particle size also dictates diffuse reflectance. The reduction in particle size increases the inter particle boundaries and therefore the number of reflections at the boundaries increases.
Nano- and mesostructured zirconia ceramics combine many desirable properties like low thermal conductivity (k), high refractive index, high chemical and thermal stability. The wavelength of the reflected light is directly proportional to the particle diameter. Hence TBCs require micro-particles of the order of 1-3 μm to reflect heat in the near IR spectrum. In the medium and far IR bands, larger particles are relevant. Particle size reduction also stabilizes the high-temperature phases at room temperature. The nano-material always has a larger thermal expansion coefficient and higher toughness than its micro counterpart. The thermal cycling life of nano LZ is reported to be six times that of micro LZ coatings. The indentation toughness of the nanocrystalline SPPS 7YSZ TBC was found to be five times that of corresponding APS TBC in the most critical in-plane orientation. Due to the lower in-plane tensile stress and higher fracture toughness of the nano-composite TBCs, they have higher thermal shock resistance than the conventional TBCs. It is reported that the yield stress (τ) and micro-hardness (Hv) of nano-crystalline materials are 2–10 times that of the coarse-grained counterparts of the same composition. Particle size reduction also reduces the flaw sizes in the coating. Therefore the fracture resistance in nano-ceramics is higher compared to conventional micron-sized materials. Grain boundary scattering, an extrinsic mechanism limiting the thermal conductivity decreases in nanocrystalline materials. Mean free path of the point defects in YSZ is significantly smaller than even the smallest grain size attainable in nano-crystalline YSZ. Nano-structured TBCs often exhibit excellent performance compared with conventional TBCs such as adhesive strength, thermal shock resistance, thermal insulation, corrosion resistance and so on. Since Phonons play a major role in heat transport in ceramics, spatial confinement of Phonons in nanostructures can strongly affect the phonon spectrum and thus the thermal properties at nanoscale. The superior thermal shock resistance and thermal cycling capability than conventional TBCs is attributed to the formation of a large number of micro-cracks, uniformly distributed tiny pores and a large area of nanostructured region with high stress relief capability.
Crystallite size of approximately 10 nm, could be synthesized employing the stearic acid and co-precipitation routes. For uniform particle size, care was taken to produce fine particles with lower dispersity. LZ powders with excellent phase and compositional control via co-precipitation method was obtained and the absence of other precursor ions was validated by ICP-MS. Lower the impurity content (namely oxides), higher the stability of YSZ powder. Silica, even at <1 wt. % in YSZ coating, adversely affects the thermal cycling life of the coating. It was found that silica gets segregated especially at grain boundaries triple points where it may lead to local instability. For preparation of LZ, zirconium oxy-chloride is the key starting material produced from Zircon, a natural combination of Zirconia (ZrO2) and Silica (SiO2). It is also found concentrated with other heavy minerals e.g., Ilmenite, Rutile, Garnet, Sillimanite, Monazite and Xenotime in beach sand. Newly synthesized YSZ powder showed maximum amount of ‘tetragonal’- t’ phase compared to the other two powder variants. This is due to complete intermixing of species in the atomic scale and the fact that particle size reduction stabilizes the high-temperature modifications. It is this t’ structure in YSZ, that is responsible for thermal stability and endurance of the coating.
The TBC material was then applied to a component by first by grit-blasting with synthetic alumina of 120-140 grit sizes. Calcined and screened TBC powder was air plasma sprayed on to cast Ni-base super alloy substrates and components preliminarily coated with MCrAlY bond coat (Cr 15-19%, Al 5-7%, Y 0.2-0.7%) and less than 50 μm size) and YSZ top coat (thickness 100 μm ). LZ was applied (thickness 50 μm – 60 μm) over and above the YSZ to produce a bi-layer TBC. The interruption between YSZ and LZ coatings was kept as low as practicable for better adhesion. The total maximum thickness was kept well below 250 μm. To check the adhesion and quality of the coating on the specimens (70 mm x 20 mm x 2.5 mm), production bend tests on 10 mm diameter mandrels were done. No visible spalling/ delamination was observed up to 60°. The bend test was also used to optimise the process parameters. On more ductile substrates, no delamination/spalling was observed up to 90° bending.
After coating, the stoichiometric ratio of ZrO2/La2O3 differed from initial composition of the powder due to the evaporation of La2O3 in high temperature reducing environment of plasma. This was expected and monitored. Higher current plasma spray was employed to prepare a coating with significant resistance to high velocity gas erosion. The bi-layer YSZ-LZ coated flaps were assembled and tested in an aero-engine which was under accelerated mission testing for long endurance. The coating withstood rapid thermal transients, supersonic flow of combustion products along with vibratory loads of about 4 ‘g’. The coating sustained 1000 h equivalent of engine operation and more than 30000 nozzle actuations. No chipping off or spallation of the coating was observed. It was also evident that the two flap with indigenous nanostructured bilayer coating were hardly tarnished compared to the other flaps which were coated with commercial YSZ grade. Also the two flap had lesser warpage compared to others which could be due to higher insulation effectiveness of bi-layer coating. This could be realized right first time due to the application of multiple strategies like nanostructures, bi-layer thermal barrier, high purity materials and a thermally stable sinter resistant coating as the top coat among others.
This process can be scaled up for bulk production as LZ has shown good long term physico-chemical compatibility with YSZ. Higher thermal insulation and durability make this bi-layer TBC an excellent candidate for widespread use. Further research is required to obtain cleaner powders for TBC preparation.
With due regards to R.K Satpathy, Defence Metallurgical Research Laboratory (DMRL).
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