A high-temperature displacement-sensitive indenter for studying mechanical properties of thermal barrier coatings
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Electron beam physical-vapor-deposited Y2O3-stabilized ZrO2 thermal barrier coating (TBC) samples were indented from room temperature to 900 °C using an instrumented high-temperature vacuum displacement-sensitive indenter. Hardness and elastic modulus were determined from the load–displacement curves recorded during indentation. Both the hardness and the elastic modulus of the TBCs were much lower than those of dense ceramics of a similar composition; this is attributed to the increased compliance that results from the porous columnar microstructure of the TBCs. In addition, the TBCs showed an unusual absence of elastic recovery at the residual indents compared to the dense ceramics.
I. INTRODUCTION
Recently, much attention has been paid to the displacement sensitive indentation (DSI) technique for determining material properties at the nanoscale because conventional methods are not easily applicable to the study of hardness (and other mechanical properties) at such fine length scales.1–4 In conventional hardness testing, the hardness is determined by the size of residual indents on the sample surface. This becomes difficult when the indent is small or the indents have irregular shapes because of the porous microstructure of the sample.5,6 In the DSI technique, the load and displacement experienced by the indenter are monitored during the indentation process; both hardness and elastic modulus can be determined from the load–displacement curve. In the current study, we have investigated the mechanical properties of ZrO2-based thermal barrier coatings (TBCs), which are highly engineered thermal insulation systems applied to superalloy components of gas-turbine engines experiencing high-temperature environments.7,8 An important current issue in enhancing the reliability of state-of-the-art TBCs is understanding of the failure mechanisms of TBCs, particularly issues related to spallation damage or erosion damage. The hardness and elastic modulus of TBCs have previously been investigated.9–14 It is known that TBCs are softer and more compliant than dense oxide ceramics or oxide single crystals of the same composition15,16 due to their porous columnar microstructure.
Address all correspondence to Arthur H. Heuer. e-mail: [email protected] J. Mater. Res., Vol. 19, No. 1, Jan 2004
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For establishing the mechanical properties of these coatings at service temperatures, a high-temperature vacuum displacement-sensitive indentation apparatus was constructed17 and used to indent, at temperatures up to 900 °C, an as-deposited Y2O3-stabilized ZrO2 (YSZ) top coat on a coupon sample, a similar sample that had been “aged” at 1204 °C for 1 h, and a Y2O3-stabilized tetragonal ZrO2 polycrystal (Y-TZP). The hardness and elastic modulus were determined from the load– displacement curves, and the extent of elastic recovery was estimated from scanning electron microscopy (SEM) images of residual indents.
II. EXPERIMENTAL A. Sample preparation
The as-deposited and aged TBC samples were s
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