Microstructure and Mechanical Properties of Yttria-Stabilized Zirconia Coatings Produced by Eletrophoretic Deposition an
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INTRODUCTION
THE Ni-base superalloy is widely used in the severest operating conditions in the combustor and gas turbine sections because of their creep, toughness, and low cycle fatigue properties. However, combustor and turbine gas temperatures may exceed the melting point of the Ni superalloy, leading to structural failure by melting. To increase the temperature capability of gas turbines, a coating of heat-insulating zirconia ceramics is applied on the surface of the turbines as a thermal barrier coatings (TBCs).[1–3] At present, there are two principle technologies for fabrication of TBCs. One is plasma spraying[4] and the other is electron beam–physical vapor deposition.[5] While these techniques are applied with great success, they are cost and time intensive, and coating of complex shapes may be difficult or even impossible. The eletrophoretic deposition (EPD) technique, with range of novel applications in the processing of advanced ceramic materials and coatings, has recently gained increasing interest not only because of the high versatility of its use with different materials but also because of its costeffectiveness requiring simple apparatus.[6–8] The use of EPD for obtaining such coatings will be investigated in this article. The loose powder coating needs to be densified after EPD, yet the high temperatures necessary for sintering with conventional furnaces may be detrimental for the W. WANG, Associate Professor, S.Q. QIAN, Professor, and H. SHEN, Graduate Student, are with the School of Materials Engineering, Shanghai University of Science Engineering, Shanghai 201620, People’s Republic of China. Contact e-mail: [email protected] Manuscript submitted December 14, 2009. Article published online August 10, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
metal. Microwave sintering is a relatively new technique employed for sintering of ceramics. It is a faster method and high densities can be achieved over a shorter period of sintering time compared with conventional sintering techniques.[9,10] Microwave sintering also has the potential to produce ceramics with better microstructures due to more uniform heat distribution leading to lower thermomechanical stresses. In this work, Y2O3stabilized zirconia has been selected for the TBCS; the effect of microwave heating on final microstructure and grain size of these materials has been discussed. Also, mechanical properties of the Y2O3-stabilized zirconia coatings, densified by microwave and conventional sintering, have been investigated.
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EXPERIMENTAL PROCEDURE
In this work, EPD was carried out with electric field applied between a graphite counter electrode (anode) and a substrate (cathode). The distance between the electrodes was 20 mm, and they were kept in a liquid solvent, which contained a suspension of powders of the materials to be deposited. EPD was carried out at room temperature using applied voltages in the range of 60 to 120 V and deposition times of 1 to 5 minutes. The substrate was K17 superalloy. The substrate sample was of dimensions 20
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