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Leonardo Times MARCH 2013BACKGROUND
Gas turbine engines generate thrust by expanding the hot gases (coming out of the combustion chamber) in the turbine. With the fact that thermodynamic ef-fi ciency of any heat engine depends on the maximum operating temperature, eff orts are continuously made in order to increase the temperature of the hot gases coming out of the combustion chamber in gas turbine engines. However, the max-imum attainable temperature is limited by the material resistance to higher tempera-tures. To withstand higher temperatures, turbine blades are made of nickel-based superalloys which possess high strength even at elevated temperatures. In order to further increase the turbine inlet tem-perature without damaging the blades, a coating of ceramic material is depos-ited onto the blades, which is commonly called a thermal barrier coating (TBC). The blades and vanes in the hot section of turbine engines and the walls of the com-bustion chambers are coated to increase the energy effi ciency of the engine, by allowing for higher operation tempera-tures, and to enhance the structural
integ-rity of the blades, by protecting the core from the aggressive environment during service [Nichols, 2003].
THERMAL BARRIER COATINGS
A modern high-temperature coating sys-tem comprises a thermal barrier coating (TBC) layer on top of a bond coating (BC) layer. A thin thermally-grown oxide (TGO) layer is formed during operation between the TBC and BC layers as a result of oxida-tion of metallic constituents of the bond coat. The structure of a typical TBC system is shown in Figure 1. The TBC is made of yttria-partially stabilised zirconia (YPSZ) to allow higher operation temperatures [Levi, 2004]. The BC is usually composed of a MCrAlY alloy (where M denotes nickel and/or cobalt), which protects the under-lying substrate material.
TBC systems experience thermal cycles due to starts and stops of a gas turbine en-gine, as shown in Figure 2. Especially dur-ing cooldur-ing from the operation tempera-ture to room temperatempera-ture, high stresses develop due to a mismatch between the coeffi cients of thermal expansion of the substrate and the diff erent layers in the
coating system [Hille et al., 2009]. These stresses result in the development of crack patterns in the TBC that coalesce and ultimately lead to failure. Cracks that run through the TBC perpendicular to its surface are not detrimental per se, but in conjunction with cracks that develop par-allel to the interface lead to spallation, i.e., fragmentation of the TBC.
NEED FOR SELF-HEALING
The lifetime of TBC systems currently lies between 2000 and 4000 thermal cycles (or fl ights) [Stolle, 2009]. Correspond-ingly, TBC systems on average need to be replaced about four times during the lifetime of an aircraft and these are cost-intensive maintenance operations. Hence, life extension of such systems is always desirable in order to reduce maintenance costs. Several eff orts were made in gas turbine industry to enhance the life time of the TBC system, for example, by vary-ing the deposition method, coatvary-ing com-position, etc. One of the innovative ideas to improve the lifetime is to incorporate a self-healing mechanism into the system. This, in turn, means that the cracks formed
With application to gas turbine engines
Thermal Barrier Coating (TBC) systems have been applied in turbine engines for
aerospace and power plants since the beginning of the 1980s to increase the energy
effi ciency of the engine, by allowing for higher operation temperatures. TBC systems
on average need to be replaced about four times during the lifetime of an aircraft.
Hence, life extension of such systems are always desirable in order to minimise
cost-intensive maintenance operations. This research focuses on developing self-healing
TBC systems to enhance their lifetime.
TEXT Sathiskumar A. Ponnusami, PhD Researcher, Aerospace Structures
SELF-HEALING THERMAL BARRIER COATINGS
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during operation should heal by itself. Inother words, the system automatically regains its strength and toughness even after the formation of microcracks.
MECHANISM OF SELF-HEALING:
The principle of the self-healing mecha-nism is demonstrated in Figure 3. A self-healing agent is encapsulated and embedded within the TBC topcoat layer during the coating process. When the crack reaches the microcapsule, the cap-sule breaks and the self-healing agent dif-fuses into the crack, where it can oxidise and heal the crack. In an explorative re-search [Sloof, 2007], it was demonstrated that the addition of Mo-Si based particles leads to the fi lling of cracks in the TBC layer. As shown in Figure 4, it was success-fully demonstrated that (i) Mo-Si (molyb-denum alloyed with silicon) based heal-ing particles can be deposited together with the yttria-partially-stabilised zirconia (YPSZ) using plasma spraying to produce the thermal barrier coating, and (ii) cracks developing in the TBC layer are healed by oxidation of the Mo-Si based particles. The principle of the crack healing with particles containing Mo-Si is based on the formation of SiO2 by oxidation when
such a particle is exposed to the ambient gas at high temperatures through a crack in the TBC. The Mo forms a volatile oxide (MoO3) and will leave the coating via the crack path, thereby compensating for the volume increase upon oxidation. The SiO2 fi lls the crack and closes it, thus postpon-ing failure of the TBC system.
Nonetheless, this self-healing mechanism is not fully understood and therefore needs to be thoroughly analysed in or-der to signifi cantly improve its effi ciency. Hence, the research is aimed at optimiz-ing the self-healoptimiz-ing capacity of thermal barrier coatings with Mo-Si based dis-persed particles for application in gas
turbine engines. This will be achieved through a combined experimental-mod-elling analysis of a modifi ed self-healing approach that relies on the encapsulation of the healing particles. The purpose of the encapsulation is to prevent premature oxidation of the healing agent. A shell of alumina (Al2O3) will be created around the healing particles by selective oxidation of a limited amount of Al that is added to the particles. With this new approach, the healing mechanism will become ac-tive only when required, i.e., when a crack breaks up the alumina shell.
RESEARCH PLANS
The project is divided into two concurrent parts that will be executed at Delft Uni-versity of Technology. The fi rst part will be carried out by Zeynep Derelioglu and supervised by Dr Wim Sloof at the Depart-ment of Materials Science and Engineer-ing (Faculty 3mE). It comprises the experi-mental understanding of the mechanisms of damage development and crack heal-ing in a self-healheal-ing TBC upon thermal cy-cling and the practical realization of such a system. The second part will be con-ducted by Sathiskumar A. Ponnusami and supervised by Dr Sergio Turteltaub at the Aerospace Structures and Computational Mechanics group of the Faculty of Aero-space Engineering. This part concerns the modelling of damage and healing processes and the development of design strategies for self-healing TBCs.
The extent of the self-healing eff ect of the modifi ed TBC can be determined ex-perimentally in a thermal cycling test and theoretically from numerical simulations. In the experiments, the evolution of dam-age (cracking and delamination) is moni-tored quantitatively as a function of the number of thermal cycles with acoustic emission and microscopic techniques. In addition, detailed microstructure analyses
will reveal the transformation of the heal-ing particles into crack-fi llheal-ing oxides. The modelling approach allows for the explicit simulation of complex damage patterns, generation of healing oxides and subse-quent repair processes. Virtual prototyp-ing through parametric studies is meant to guide the design process of new self-healing coatings and thus improve the ef-fi ciency during development. The manu-facturing of the self-healing TBC system will be done by KLM and Sulzer and the testing of the system under thermal cy-cling conditions will be carried out by the National Aerospace Laboratory (NLR). This research project is part of the IOP Self-Healing Materials Program chaired by Prof Sybrand van der Zwaag of the Faculty of Aerospace Engineering and is funded by the Dutch government.
CHALLENGES INVOLVED
Many challenges are involved in the devel-opment of self-healing TBC systems. First of all, the manufacturing of encapsulated healing particles itself is a big challenge. Then, when the crack emanates during operation, triggering of the mechanism that repairs cracks is crucial. First, the crack must run into the healing particles and not be defl ected. Next, the crack must break-up the alumina shell encapsulat-ing the particles. Then, high temperature oxidation must result in the formation of SiO2 to fi ll the cracks. Optimal size and dis-tribution of the healing capsules should be determined with guidance from the modelling and design process. Healing effi ciency has to be studied in order to en-sure proper healing has occurred in order to regain the toughness. These challenges will be addressed in this combined ex-perimental-modelling research project in order to arrive at an optimal self-healing TBC system.
RESULTS
The research will focus on developing a novel ceramic thermal barrier with self-healing capability. Therefore, under-standing of the mechanisms of damage development and crack healing is es-sential. Modelling of these mechanisms will enable optimization and design of new TBC systems. Routes will be devised for controlled manufacturing of both the healing particles and the modifi ed TBCs. If successful, this project will lead to a new generation of aff ordable TBCs with improved lifetime in gas turbine engines. Consequently, a signifi cant economic benefi t can be obtained by reducing the number of TBC replacements in critical turbine engine components.
QUALITY AND INNOVATION
Self-healing TBCs with extended lifetime do not exist for commercial applications yet. The development and practical
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Figure 1. Structure of TBC system
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Leonardo Times MARCH 2013 plementation of a novel self-healing TBC is very attractive, since the maintenance frequency of jet and gas turbines criti-cally depends on the lifetime of the TBC. The proposed research involves (i) the development of the TBC material, (ii) the advancement of coating manufacturing technology and (iii) the implementation of self-healing coatings into industrial ap-plications (jet and gas turbine engines). In this project, the scientifi c research groups will collaborate with industry which en-sures knowledge transfer from academia to industry and users, and vice versa.ECONOMIC PERSPECTIVE
If a TBC system with suffi cient self-healing capacity can be realised, then the eco-nomic benefi ts will be signifi cant. First, the manufacturer of such a coating system can off er a unique product and thereby acquire a stronger position in the market. Secondly, the users of a TBC system with self-healing capacity, e.g. airline compa-nies and producers of electricity, will ben-efi t from the longer lifetime of the coating system itself and hence the critical com-ponents of the gas turbine, which brings about less engine revisions and thus a
re-duced engine downtime. Both users and manufacturers of the self-healing TBC sys-tem within the Dutch industry participate in this project to defi ne requirements for successful application.
SUSTAINABILITY
A TBC system itself already contributes considerably to sustainable technol-ogy, because it enhances the engine ef-fi ciency by allowing higher operation temperatures, which saves fuel and thus reduces CO2 emissions. Furthermore, it protects the high-tech structural compo-nents (made of single-crystal superalloys) against severe high temperature corro-sion, thereby contributing to durable use of resources. The materials and their amounts used to produce the TBC system are abundant and not environmentally hazardous.
CONCLUSION
It has already been demonstrated that a TBC with a healing agent can be manu-factured with existing technology (i.e., plasma spraying) used in industry [Sloof, 2007]). Thus, once an optimal design is realised and eff ective healing particles are developed, it is anticipated that there are no obstacles for introduction of self-healing TBCs into industrial practice. The involvement of industrial participants (i.e., KLM and Sulzer) in this research project, and in particular with the manufacturing of the self-healing TBC system, ensures a successful transition into applications.
Figure 3. Schematic of crack-healing mechanism in a TBC system with encapsulated Mo-Si based particles.
Figure 4. Crack healing in a thermal barrier coating (TBC) system by Mo-Si based healing particles. (a) TBC layer composed of yttria-partially-stabilised zirconia and Mo-Si particles co-deposited by plasma spraying on a MCrAlY bond coating (BC). (b) Micrograph of self-healed TBC by SiO2 (dark gray) formed due to high-temperature oxidation of Mo-Si particles (arrows indicate remnants of Mo-Si particles). (c) Si distribution map of a selected area showing that the crack is fi lled with SiO2.
References
1. J.R. Nichols, Advances in coating design for high-performance gas tur-bines, MRS Bull 28 (2003) 659-670. 2. T.S. Hille, T.J. Nijdam, A.S.J. Suiker, S. Turteltaub, W.G. Sloof, Damage growth triggered by interface irregu-larities in thermal barrier coatings, Acta Materialia, 57 (2009) 2624-2630. 3. C. Levi, Emerging Materials and processes for thermal barrier systems, Current opinion in solid state and materials science, 8 (2004) 77. 4. R. Stolle, Conventional and ad-vanced coatings for turbine airfoils. Available from: http://www.mtu.de/ en/technologies/engineering_news/, retrieved in January 2009. MTU Aero Engines, 80995 München, Germany. 5. W.G. Sloof, Self Healing in Coatings at High Temperatures in: Self Healing Materials – an Alternative Approach to 20 Centuries of Materials Science, S. van der Zwaag Editor, Springer, Dordrecht, The Netherlands, 2007, pp. 309-321.
Figure 2. Typical thermal cycle of a gas turbine engine
