Improved Microstructures for Thermal Barrier Coatings Produced by Glancing Angle Deposition
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ABSTRACT A new approach to deposition of thin films for thermal barrier applications is described. During electron beam evaporation, the extreme shadowing effect that is present at highly oblique incidence is employed to introduce porosity into thin films of zirconia. Using controlled substrate motion a solid capping layer may be applied to these porous films. By depositing layers of porous material and capping in an alternating fashion a new structure is produced which warrants evaluation as an improved thermal barrier coating.
INTRODUCTION Thermal barrier coatings (TBCs) are thin films of low thermal conductivity materials applied to any surface that is to be protected from intense heat. TBCs are in common usage in
turbine engine applications where critical components are required to operate above their normal melting temperatures.I Two major types of thermal barriers have been described in the literature. The first method applies a film of zirconia (ZrO2) that has been stabilized by inclusion of yttria (Y20 3 ) onto the critical surface by a means of a plasma spray.2 The resulting structure is primarily dense zirconia, however, interspersed throughout the material are voided regions. For high temperature operation, the thermal conductivity of air (2.4x 10 2W/mK at 0,C) 3 is much lower than that of zirconia (-1.7W/mK at 0,C) 4, thus, the ability of heat to propagate across the structure is reduced. One unfortunate difficulty inherent to this structure is that zirconia and its substrate may have vastly different thermal expansion coefficients. After multiple cycles of heating and cooling, induced stress tends to cause cracking and spallation in the structure rendering it less 5 effective. A second deposition technique, utilizing electron beam evaporation, attempts to solve this problem by introducing a vertical columnar microstructure having grain boundaries oriented perpendicular to the substrate. 6 Expansion stress relief is provided, but any pores that exist may form paths for heat flow between the ambient environment and the substrate. The thermal conductivity of such an electron beam evaporated coating is greater than that of a plasma sprayed coating.
97 Mat. Res. Soc. Symp. Proc. Vol. 555 ©1999 Materials Research Society
EXPERIMENT In order to allow for the provision of stress relief while minimizing thermal conductivity we have used a technique employing substrate motion known as GLancing Angle Deposition (GLAD). 7 As is depicted in Figure 1 a test substrate may be rotated about two axes: the first, denoted as 0 in the figure, varies the angle at which the arriving flux is incident upon the surface. At highly oblique incidence (>800) extreme shadowing effects allow for the introduction of well defined microstructure and porosity into thin films of yttria stabilized zirconia.8 The second axis of revolution passes directly through the substrate center. It is found that, at oblique incidence, growth will occur in a direction favoring the vapour source. 9 Since it is possible to vary the rotational positio
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