Optimization of High Porosity Thermal Barrier Coatings Generated with a Porosity Former
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Jan Medrˇicky´, Nicholas Curry, Zdenek Pala, Monika Vilemova, Tomas Chraska, Jimmy Johansson, and Nicolaie Markocsan (Submitted June 5, 2014; in revised form October 26, 2014) Yttria-stabilized zirconia thermal barrier coatings are extensively used in turbine industry; however, increasing performance requirements have begun to make conventional air plasma sprayed coatings insufficient for future needs. Since the thermal conductivity of bulk material cannot be lowered easily; the design of highly porous coatings may be the most efficient way to achieve coatings with low thermal conductivity. Thus the approach of fabrication of coatings with a high porosity level based on plasma spraying of ceramic particles of dysprosia-stabilized zirconia mixed with polymer particles, has been tested. Both polymer and ceramic particles melt in plasma and after impact onto a substrate they form a coating. When the coating is subjected to heat treatment, polymer burns out and a complex structure of pores and cracks is formed. In order to obtain desired porosity level and microstructural features in coatings; a design of experiments, based on changes in spray distance, powder feeding rate, and plasmaforming atmosphere, was performed. Acquired coatings were evaluated for thermal conductivity and thermo-cyclic fatigue, and their morphology was assessed using scanning electron microscopy. It was shown that porosity level can be controlled by appropriate changes in spraying parameters.
Keywords
gas turbines, high temperature application, porosity of coatings, stabilized zirconia, thermal barrier coatings (TBCs)
1. Introduction The need for increasing of power and efficiency of gas turbines for land and aviation applications is inevitably connected with combustion of fuel at high temperatures. Superalloys that make up the turbine components are not able to withstand these high temperatures therefore thermal barrier coatings (TBC) have been used for several decades in order to provide thermal protection (Ref 1). However, conventional air plasma sprayed yttria-stabilized zirconia coatings are reaching the limit of their capabilities. Development of new generation coatings is required to push the technology to higher efficiency levels (Ref 2). This article is an invited paper selected from presentations at the 2014 International Thermal Spray Conference, held May 21-23, 2014, in Barcelona, Spain, and has been expanded from the original presentation. Jan Medrˇicky´, Czech Technical University in Prague, Prague Czech Republic; Nicholas Curry and Nicolaie Markocsan, University West, Trollha´ttan, Sweden; Zdenek Pala, Monika Vilemova, and Tomas Chraska, Institute of Plasma Physics AS CR, Prague, Czech Republic; and Jimmy Johansson, GKN Aerospace Engine Systems, Trollha´ttan, Darrasa, Sweden. Contact e-mail: [email protected].
Journal of Thermal Spray Technology
Crucial factor for new TBCs is lowering of thermal conductivity of the coatings. It has been demonstrated that thermal conductivity of zirconia can be lowered by substitution of a st
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