Directions of the development of thermal barrier coatings in energy applications

https://doi.org/10.1016/S0924-0136(99)00244-7Get rights and content

Abstract

Increased efficiencies in energy conversion systems are the driving forces for the development and introduction of new improved materials as part of the operating component. Due to the large amounts of energy produced, even the smallest changes in inlet temperatures result in a considerably smaller demand of fuel to produce the same amount of energy. Thermal energy conversion systems according to Joule's Process (the Brayton Process) therefore need steadily enhanced inlet temperatures and temperature differences to approach this goal (η = 1  Tout/Tin). Higher temperatures to be applied make necessary the use of enhanced temperature-resistant structural ceramic, or as a temporary solution the application of ceramic thermal barrier coatings. To achieve sufficiently low metallic substrate temperatures in the latter case, the material system has to be operated under a temperature gradient by cooling. The well-established material for thermal barrier coatings is partially (Y2O3) stabilized zirconia combined with oxidation-resistant bond coats of special chemical composition (nickel/cobalt base alloy as alumina former). Coatings processing via plasma spraying as well as by physical vapor deposition is considered. For use at extremely high temperatures completely new materials are sought with potentially enhanced temperature durability and reduced failure probability. New developments have taken the task of identifying the life-time limiting influencing parameters, if possible by separating the different mechanisms acting during model experiments. The main parameters are: residual stresses, thermo-mechanical loading, cyclic strain loads, creep and sintering, as well as interface oxidation. The main unsolved problem, hitherto, is the existence of a reliable model for long-term life-time prediction under the complex loads of operation.

Introduction

Low thermal conductivity, high melting point and good resistance against oxidative and a corrosive environment are the required advantages of ceramic coatings applied in energy applications. However, compared to metals, ceramics are not reliable with respect to mechanical properties. Despite intensive research on structural ceramics, this non-reliability hinders the use of bulk ceramic parts in turbines and diesel engines. Instead, the advantages of ceramics and metals are combined in utilizing ceramic thermal barrier coated (TBC) metallic substrates. Its extremely low thermal conductivity and good phase stability makes Yttrium-stabilized zirconia the most successful ceramic top-layer, when combined with a metallic interlayer. This interlayer acts on the one hand as a bond coat and on the other hand as an oxidation and corrosion protection barrier. The alloy normally consists of a base of M = Ni, Co and/or Fe, Cr, Al, Y and additional active elements such as Si, Ti and Re. Both layers can be applied by thermal spraying and/or by electron beam physical vapor deposition (EB-PVD).

The limited life-time of the TBC system forms the boundary of this 40-year-old concept [1]. Until today the use of TBCs on aircraft turbines blades is not design-integrated: they are used frequently to lower the metal temperature, and therefore elongate the life-time of a blade itself. If the coating spalls off, metal temperature will increase, but not above a critical point. For design-integrated TBCs with improvement of efficiency, fuel consumption and exhaust pollution, 100% reliability is necessary.

One driving force in the development of gas turbines is increased efficiency resulting in less fuel consumption and less environmental pollution like such as CO2 and NOx exhaust gases. Fig. 1 shows the gas turbine process that is called the Joule or Brayton process. The thermal efficiency of this process is determined by temperature and pressure ratios [2], see Eq. (1).ηth,ideal=1+qinqout=1−T4T3=1−p4p3κ−1κwhere κ is an adiabatic exponent.

One possibility for increasing efficiency is increasing the turbine inlet temperature, T3, whilst keeping a constant cooling level [3]. The highest gas temperature in a turbine engine occurs in the combustion chamber, followed by the inlet temperature at the first blades. This inlet temperature may reach 1400°C in advanced turbine engines [4], but is limited by the materials used in the hot sections. TBC coating of the hot parts (combustion chamber tiles and rotating blades) increases their potential in and resistance against hot corrosive and oxidative environments. Fig. 2 shows the rotor and heat shield tiles in a ring combustion chamber with the outer ring part removed.

The workhorse of transportation, industrial or utility and marine power, has long been the diesel engine. Plasma spray-applied TBC consisting of MCrAlY alloy and zirconia ceramics protect pistons, valves and piston fire decks from heat, high temperature oxidation and corrosion and thermal shock [5]. The heat from a coated piston reflected back to the chamber leads to higher temperatures in a shorter time, and therefore, reduced ignition delay on start-up, with lower peak combustion temperatures. All of this improves the `overall burn' of the heavy fraction of diesel fuels, and therefore, can improve fuel efficiency or allows the use of heavier grade fuel oils. Nevertheless the reported data about increase in fuel efficiency vary from negative (i.e. more consumption) to positive values [5] and are expected to be not relevant. The decisive improvement of TBCs in diesel engines is lower peak temperatures and decrease in NOx emission rate, together with the steep fall of CO and particulate emission. At the same time TBCs protect the metallic substrate against the corrosive attack of fuel contaminants (Na, V, S).

The mean difference in loading TBC in turbines or diesel engines is the thermal cycle characteristic: the cycle of a diesel engine lasts only a fraction of a second whilst the maximum temperature is about 800°C. Thermo-mechanical fatigue is the most important life-time limiting factor. Aircraft turbines exhibit thermal cycles of several hours at much higher temperatures. Bond coat oxidation becomes more important and determines, together with thermo-mechanical fatigue, the life-time of airplane TBC coatings. Thermal cycles of stationary gas turbines last from several hours for peak-load operation up to 1 year at base-load operation. Here, for the life-time of turbine TBCs oxidation and other time-on-temperature parameters become increasingly more important.

For all of these differently used TBCs, a life-time model which predicts 100% reliability of the coating has to be developed or, based on the NASA model 6, 8, deepened. Research will focus on understanding of all failure mechanisms under a large variety of applied conditions. Further development in the improvement of materials (ceramic, bond coat and superalloy substrate), material properties (oxidation resistance, thermal expansion, thermal conductivity, thermo-mechanical fatigue, sinter- and phase-stability, chemical inertness etc.) and deposition processes, are the future goal.

The following text deals with selected topics of this TBC research, especially for gas turbine applications.

Section snippets

The status of TBCs for gas turbines with increased turbines inlet temperatures

The improvement of the chemical composition of nickel-base superalloys has led to extreme mechanical strength of materials, even at high-temperature, mostly correlated with good corrosion resistance [9]. The introduction of new processing techniques such as directional solidification (DC) and single crystal techniques (SC) has reduced microstructure correlated problems such as grain boundary corrosion and crack initiation at grain boundaries. Today, superalloys are used at up to homologous

Life-time limiting factors

During operation in high or changing temperatures the joining of three different materials (substrate, bond-coat and ceramic) causes problems that finally limit the life-time by the spallation of the TBCs. The following paragraph describes the interaction between the different layers, and with their environment. Fig. 8 presents a schematic view of TBC-components, properties and processes during operation.

Life time prediction

For integrating TBC in the design of gas turbines, their desired effect has to be 100% reliable. In the past decade a lot of work has been done by NASA 6, 7, 8. The described life-time prediction models are based on a general form of power law:Nf=ΔεiΔεfbwhere Nf is the number of cycles to failure, Δεi is the total cyclic inelastic strain range, Δεf is the failure strain and b is the empirical power-law coefficient. The cyclic failure strain Δεf was found to depend on the grown oxide thickness:Δε

Conclusions

The applications and purposes of ceramic thermal barrier coatings for operation in gas turbines or diesel engines have been reviewed. The status quo in knowledge about the properties of traditional Yttrium stabilized zirconia TBC and their degeneration during operation in turbines have been described. An overview about future research for life-time elongation of TBCs has been given. Furthermore, the existing NASA model for life-time estimation has been described, together with recent directions

Acknowledgements

The authors would like to thank Dr. W.J. Quadakkers for helpful discussions concerning bond-coat oxidation and interdiffusion.

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