Determination of the stress-dependent stiffness of plasma-sprayed thermal barrier coatings using depth-sensitive indenta
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The elastic response of atmospheric plasma-sprayed coatings was investigated using Vickers and spherical indenter geometries. In both cases a strong dependency of the stiffness on the applied load (indentation depth) was observed. The stiffness of the coatings decreased with increasing load for a Vickers indenter, whereas it increased for a spherical indenter. This contrary behavior was related to the relative crack density in the deformed volume and to the stress dependence of the stiffness due to crack closure. The effect of annealing on the stiffness was quantified for both tip geometries. The heat treatment yielded additional information on the relationship between the indentation data and the microstructural defects. From the results it was concluded that the stiffness measured using a sharp indenter and small load reflected the elastic behavior of single spraying splats. With the relatively large spherical indenter, the average global stiffness of the thermal barrier coating was measured even at small loads. From the data obtained using the spherical indenter, a compressive stress–strain curve was suggested. Furthermore, values of the apparent crack density and yield strength were determined from the indentation tests.
I. INTRODUCTION
Ceramic thermal barrier coatings (TBCs) are increasingly utilized on high-temperature exposed components of advanced gas turbines to extend the operational temperature or to decrease internal cooling, which in both cases increases the efficiency of the turbine.1,2 Typically the components consist of a three-layer composite. The single-crystal Ni-superalloy substrate is corrosion protected by a NiCoCrAlY layer that also provides the bond coat (BC) for the insulating porous ceramic top coat. The standard TBCs are either atmospheric plasma sprayed (APS) or physically vapor deposited (PVD) yttria stabilized zirconias. Depending on the coating process, different microstructures develop; that is, spraying lamellae in the case of APS or columnar grains in the case of electron beam-physical vapor deposition (EB-PVD). The mismatch in thermal expansion coefficients between substrate, BC, and TBC results in residual stresses, which, along with the brittleness of ceramic coatings and the formation of a thermally grown oxide (TGO) between BC and TBC, often lead to delamination or segmentation cracks, which ultimately cause spallation.3–6 The stiffness has to be known as a key parameter to quantify the stress situation and the brittle failure behavior of the TBC. J. Mater. Res., Vol. 18, No. 8, Aug 2003
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Frequently a relatively low macroscopic stiffness is reported for TBCs and attributed to pores and a large number of microcracks,7 the latter being a result of the differences in thermal expansion coefficients and residual stresses that arise during cooling from deposition temperature or created in service. The low stiffness leads to low residual stresses and in consequence to relatively high thermomechanical stability. The microstructur
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