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Ceramic materials for thermal barrier coatings

Time:2012-06-29 09:06:45  From:Journal of the European Ceramic Society  Author:X.Q. Cao, R. Vassen, D. Stoeve

Ceramic materials for thermal barrier coatings
X.Q. Cao, R. Vassen, D. Stoever
Journal of the European Ceramic Society
Abstract
This paper summarizes the basic properties of ceramic materials for thermal barrier coatings. Ceramics, in contrast to metals, are often more resistant to oxidation, corrosion and wear, as well as being better thermal insulators. Except yttria stabilized zirconia,other materials such as lanthanum zirconate and rare earth oxides are also promising materials for thermal barrier coatings.

Keywords:Coatings; Mechanical properties; Thermal barrier coatings; Thermal properties

1. Introduction
    During the past decade, research efforts were devoted
to the development and manufacturing of ceramic thermal barrier coatings (TBCs) on turbine parts because
the traditional turbine material have reached the limits of their temperature capabilities. TBCs are deposited on
transition pieces, combustion lines, first-stage blades and vanes and other hot-path components of gas turbines either to increase the inlet temperature with a consequent improvement of the efficiency or to reduce
the requirements for the cooling system.The earliest ceramic coatings for aerospace applications were frit enamels developed by the National Advisory Committee for Aeronautics (NACA) and the coating of calcia stabilized zirconia on the exhaust nozzle of the X-15 manned rocket plane in 1960s is believed to be the first use of TBCs in manned flight.The working parts of aircraft jet engines are subjected to serve mechanical, chemical and thermal stresses. Several ceramic coatings such as Al2O3 , TiO2, mullite, CaO/MgO+ZrO2 , YSZ, CeO2+YSZ, zircon and La2Zr2O7,etc. have been evaluated as TBC materials.The selection of TBC materials is restricted by some basic requirements: (1) high melting point, (2) no phase transformation between room temperature and operation temperature, (3) low thermal conductivity, (4) chemicalinertness, (5) thermal expansion match with the metallic substrate, (6) good adherence to the metallic substrate and (7) low sintering rate of the porous microstructure.The number of materials that can be used as TBCs is very limited. So far, only a few materials have been found to basically satisfy these requirements. This paper is believed to be the first review about the ceramic TBC materials and is helpful to the selection of TBC materials. In Ref. 6 the development of new TBC systems is described. The following are TBC materials under investigation.Properties of some ceramics that can be used in TBC system are summarized in Table 1. Among those properties, thermal expansion coefficient and thermal conductivity seem to be the most important. These data are
collected from different references and hence may not be complete. Metal substrate and bond coats are also
included for comparison. The number before yttria stabilized zirconia (YSZ) represents the weight percentage
of Y2O3 in ZrO2. The advantages and disadvantages of other TBC materials are compared with YSZ and listed
in Table 2. The improvement techniques of YSZ coatings are also summarized in this table.

2. Materials for TBCs

2.1. YSZ
7-8YSZ is the most widely studied and used TBC material because it provides the best performance in high-temperature applications such as diesel engines and gas turbines,and reports about this material are numerous. YSZ coating has been proved to be more resistant against the corrosion of Na2SO4 and V2O5 than the ZrO2 coating stabilized by CaO or
MgO.18-20YSZ coatings has also been studied.A major disadvantage of YSZ is the limited operation temperature ( < 1473 K) for long-term application. At higher temperatures, phase transformations from the t0-tetragonal to tetragonal and cubic (t +c ) and then to monoclinic (m ) occur, giving rise to the formation of cracks in the coating.A practical upper-use temperature of 1223 K in gas turbine for the ZrO2 coating stabilized by CaO and MgO was reported.On the other hand, these coatings, possess a high concentration of oxygen ion vacancies, which at high temperature assist oxygen transport and the oxidation of the bond coat at the ceramic–bond coat interface, namely the formation of thermally grown oxide (TGO) on the bond–coat sur-face. This leads to spallation of the ceramic and such a mode of failure of the TBC is predominant when the coatings are thin as in gas turbines. This problem
has been overcome to a large extent by providing oxidation resistant bond coats such as alumina and
mullite.A model of life prediction of TBCs has been developed, in which the coating failure was attributed to stresses arising from the formation of TGO,and in Fig. 1 is shown the relationship between the thermal cycling life of TBCs and the substrate temperature.The silica impurity (even as low as 1 wt.%) in YSZ coating has a strong detrimental effect on the thermal cycling life.In bulk zirconia-based ceramics, silica
segregates to grain boundaries with excessive amounts collecting at triple points. Silica at the grain bound-aries leads to changes in the size and shape of grains and it may dissolve Y2O3 from the YSZ grain boundary regions leading to localized destabilization.Silica can also cause ZrO2 polycrystal super-plasticity, dramatic increases in sintering rates, and decreases in electrical conductivity. It may also lead to increased creep rates, as has been observed with silicon-base ceramics.However, silicates have much lower oxygen conductivity than YSZ, and a thin layer of sili-cates on the top of the bond coat as oxygen barrier might improve the oxidation resistance of the bond
coat.

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