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|Thick Thermal Barrier Coatings – State-of-the-Art|
|Samppa Ahmaniemi, Petri Vuoristo, Tapio Mäntylä
Surface Engineering Laboratory, Institute of Materials Science, Tampere University of Technology.
Background of Thermal Barrier Coatings
Thermal Barrier Coatings (TBCs) are widely used in gas turbine hot section components such as burners, transition ducts, shrouds, blades and vanes. The most common TBC material is yttria stabilised zirconia (8Y2O3-ZrO2) because of its high temperature stability, low thermal diffusivity and high coefficient of thermal expansion (CTE) for a ceramic material.
Figure 2: TBC coated gas turbine vane (static turbine component) and diesel engine piston head.
From the early 1980s, there have been many investigations to apply TBCs also in diesel engines, but their use is still quite limited. The conditions in diesel engine combustion chamber differ considerably from those in gas turbine hot sections. Temperatures are lower, but thermal and mechanical loads and hot corrosion conditions still set very demanding requirements for TBCs. Combustion section components of the gas turbine and diesel engine are typically coated by the atmospheric plasma spray (APS) process using coating thicknesses of 200 – 500µm. However, electron beam physical vapour deposition (EBPVD) is the state-of-the-art coating process in manufacturing relatively thin (150 – 300µm) strain tolerant TBCs for gas turbine components. The vertically oriented columnar structure of the EB-PVD TBCs offers better strain tolerance for the component under thermal cycling, compared to the lamellar microstructure of the APS coatings. In addition, the surface finish of the EB-PVD coatings is aerodynamically advantageous in rotating turbine aerofoils. In figure 2 a TBC coated gas turbine vane and diesel engine piston head are presented.
Demand for Thicker TBCs
Increasing the turbine hot gas inlet temperature (TIT) is a potential way to improve the efficiency of the gas turbine driven combined cycle process. Currently, in land based gas turbines, the maximum TIT is around 1500°C and in aero engines even higher. Since structural materials such as nickel and cobalt based superalloys cannot face temperatures higher than 950°C, thermal barrier coatings (TBC) with better insulation properties are needed. Surface temperatures of the gas turbine hot section components are mainly controlled by different cooling techniques and TBCs. Although air cooling is necessary, the cooling air is taken straight from the compressor and this lowers the output of the compressor. Calculations have shown that the thermal gradient through a coating is 150°C for a TBC of standard 500 µm thickness and 320°C in the case of a 1800 µm thick TBC (coating surface temperature approx. 1250°C).
With thicker TBCs the mean combustion temperature of the diesel process can also be increased. This increased temperature does not directly affect the efficiency of the diesel process, but the extra heat can be recovered in a turbocharger or in a flue gas boiler in combined cycle. Some studies have shown that TBCs can increase the coefficient of thermal efficiency of the diesel combustion process and lower fuel consumption. However, in the literature, there are also some results indicating that there are no advantages in using TBCs in diesel engines. Without question the diesel process has to be adjusted correctly to utilise the benefits of thermal barrier coatings. Thick TBCs up to 3.5 mm have been studied for diesel engine applications.
Lowering the thermal conductance (thermal conductivity of the coating/coating thickness) of TBCs can be approached in three ways: (1) lowering the thermal conductivity of the coating material, (2) lowering the thermal conductivity by increasing the porosity of the coating, (3) increasing the thickness of the coating. When tailoring low thermal conductance TBCs, all three methods should be considered. In this paper we focus mainly on the thick TBCs.
Drawbacks with Thick TBC (TTBC) Coatings
During operation TBCs are exposed to various thermal and mechanical loads such as thermal cycling, high and low cycle fatigue, hot corrosion and high temperature erosion. Currently, because of reliability problems, the thickness of TBCs is limited, in most applications, to 500µm. Increasing coating thickness increases the risk of coating failure and leads to a reduced coating lifetime. The failure mechanisms that cause TTBC coating spallation differ in some degree from that of the traditional thinner coatings.
A major reason for traditional TBC failure and coating spallation in gas turbines is typically bond coat oxidation. When the thickness of the thermally grown oxide (TGO) exceeds a certain limit, it induces the critical stress for coating failure.
Thicker coatings have higher temperature gradients through the coating and thus have higher internal stresses. Although the coefficient of thermal expansion (CTE) of 8Y2O3-ZrO2 is close to that of the substrate material, the CTE difference between the substrate and coating induces stresses at high temperatures at the coating interface. The strain tolerance of TTBC has to be managed by controlling the coating microstructure. Use of thicker coatings generally leads to higher coating surface temperatures that can be detrimental if certain limits are exceeded. In the long run, the phase structure of yttria stabilised zirconia (8Y2O3-ZrO2) is not stable above 1250°C. Also the strain tolerance of the coating can be lost rapidly by sintering if too high surface temperatures are allowed.
Development and Future Trends of TTBCs
In order to overcome the problems with TTBCs much research has been carried out. In figure 3 a problem-reason-solution flowchart for solving the problems and for developing more durable TTBCs is presented.
Figure 3: Problems and solutions concerning spraying of thick thermal barrier coatings .
Most solutions in figure 3 are related to reducing Young’s modulus and residual stresses in the coatings. In practice this can be achieved by controlling the spray parameters, but also the substrate and coating temperature during the coating deposition. If the system heats too much in spraying, compressive stresses will be developed in the coating structure. Thus, active substrate and surface cooling is normally used during spraying. Spray parameters can also be fixed to obtain desired levels of porosity and microcracks. Vertical segmentation cracks, passing through the whole coating, can be produced by producing rather thick spray passes during coating deposition. In addition to strain tolerance, pores and especially horizontal cracks are naturally advantageous in lowering the thermal conductivity of the coating. Extremely high porosity values (up to 25 vol%) in TBCs have been obtained by spraying polymers together with zirconia. However, when spraying very porous and thick coatings the deposition efficiency (DE) decreases.
Various gradient and layered structures have been studied in order to lower the critical stresses caused by the CTE difference between the coating and substrate materials. Several attempts have been made to modify the properties of the TBCs by post treatment processes. The coating surface can be modified by liquid metal impregnation, laser-glazing, hot isostatic pressing (HIP), sol-gel processing, and phosphate impregnation. When modifying the TBC structures, one should remember that the primary functions of the coating, thermal insulation and strain tolerance, should not be deteriorated. Microstructures of porous, modified 8Y2O3-ZrO2 TTBCs are presented in figure 4.
Figure 4: Optical micrographs of the 8Y2O3-ZrO2 TTBC coatings, (a) reference, (b) aluminum phosphate impregnated, (c) laser-glazed coating.
Sintering of TBCs can be caused by impurities (mainly SiO2) in feedstock spray powder. At high temperatures sintering increases the Young’s modulus of the coating as a function of time and therefore is an important factor that should be considered when choosing and developing powders for TBC applications.
TTBC Research Activity at TUT/IMS
The Surface Engineering Laboratory of TUT/IMS is involved in TTBC research in national and European research programmes. Structure modifications of TTBC are studied in a national project « Extreme Values of the Piston Engine », which belongs to the TEKEVA research programme, funded by the Finnish Academy. Mechanical and thermal properties, as well as high temperature testing, of modified TTBC are studied in European Gas Turbine Materials Program, COST522. Recent results of these studies can be found in references 2 to 5.
1. H.-D. Steffens, Z. Babiak, M. Gramlich, Some Aspects of Thick Thermal Barrier Coating Lifetime Prolongation, Journal of Thermal Spray Technology, 8(4), 1999, p. 517-522.
2. S. Ahmaniemi, P. Vuoristo and T. Mäntylä: Improved Sealing Treatments for Thick Thermal Barrier Coatings, Surface & Coatings Technology, 151-152, 2002, p. 412-417.
3. S. Ahmaniemi, P. Vuoristo, T. Mäntylä: « Comparative Study of Different Sealing Methods for Thick Thermal Barrier Coatings », Proceedings of the International Thermal Spray Conference, 2001, p. 157-166.
4. C. Gualco, E. Cordano, F. Fignino, C. Gambaro, S. Ahmaniemi, S. Tuurna, T. Mäntylä, P. Vuoristo: « An improved deposition process for very thick porous thermal barrier coatings », Proceedings of the International Thermal Spray Conference, 2002, p. 195-201.
5. S. Ahmaniemi, P. Vuoristo, T. Mäntylä « Effect of Aluminum Phosphate Sealing Treatment on Properties of Thick Thermal Barrier Coating » Proceedings of the International Thermal Spray Conference, 2000, p. 1087.
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