Fabrication and Evaluation of Plasma-Sprayed Microlaminated Thermal Barrier Coatings
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FABRICATION AND EVALUATION OF PLASMA-SPRAYED MICROLAMINATED THERMAL BARRIER COATINGS A.M. RITTER*, J.R. RAIRDEN* AND R.L.MEHAN* * GE Corporate Research & Development, P.O. Box 8, Schenectady, NY 12301 ABSTRACT Microlaminated metal matrix (M3 ) composite coatings of MCrA1Y/A10 3 were fabricated using coordinated two-gun low pressure plasma deposition rocessing. These coatings are shown to hold promise as durable thermal arrier coatirngs (TBC's) for aircraft engine applications. The properties of analogous MJ coatings of MCrA1Y/Zr)2.Y 2 0 3 also were evaluated and found to be significantly less durable when thermpl1y cycled. Microstructural analysis of the MJ coatings revealed that no interaction occurred between the alumina and metal lamellae in the MCrAlY/AI 2 0 3 TBC's. However, alumina and chromia layers formed at the metal/ceramic interfaces in the ZrO2-containing TBC's, and grew into the metal layers of the ZrO2-containing TBC's during air-cycling. This growth may contribute to TBC cracking and spallation by lowering the overall coating ductility. INTRODUCTION Thermal barrier coatings are designed to reduce heat transfer to the airfoil in the hot section of aircraft engines, but the spallation tendency of monolithic partially-stabilized ZrO 2 .YO 3 coatings has severely limited their introduction into the high pressure turbine section of these engines. The coatings are commonly fabricated by plasma-spraying a metal bond coat onto the part, and then spraying an insulating layer onto the bond coat. In this study, microlaminated metal matrix composite coatings were evaluated as potential TBC's. Coordinated two-gun low pressure plasma deposition processing was developed [1] to achieve deposit size and shape control. Similarly, metal powder can be plasma-sprayed from one gun, and ceramic powder sprayed from a second gun, with both guns aimed at the same area of the substrate, to form a finely-dispersed, swirly microlaminated metal-matrix composite. Application of this process to the fabrication of microlaminated TBC's was given impetus by an approach [2] which suggested that the numerous interfaces in alternating layers of metals and oxides would lead to a high thermal resisitivity, and that the composite microstructure would resist failure by cracking. Microstructure, properties and cracking in conventional, or monolithic, TBC's have been studied in some detail [3-8]. The cracks are generally found in the oxide, near the bond coat/oxide interface [8-11], and appear to form due to stresses generated by thermal expansion mismatch on cooling [8]. TBC failure has been associated with oxidation of the bond coat [7,8], although the contribution of this to coating degradation and the type of oxide formed are not clearly established. This study examines these effects in MJ TBC's. EXPERIMENTAL PROCEDURE Deposits were made using two 80-kW EPI Model 03CA guns that were mounted in a water-cooled low-pressure chamber and positioned 43 cm from the substrate. Metal powder was sprayed from one gun and ceramic powder from the other
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