Bulk Property Evaluation of a Thick Thermal Barrier Coating

Abstract

Thick thermal barrier coatings (TTBCs) for diesel engine applications are being developed to improve engine performance through increased operating temperatures and lower emissions. To more completely assess the bulk properties of coating materials, a miniature test stand for the mechanical testing of coating materials independent of the substrate was developed. Using a piezoelectric translator as an actuator and a miniature load cell, it was possible to conduct uniaxial testing in both compression and tension of very small samples. In this study, room temperature deformation experiments were conducted on an air plasma-sprayed 24% CeO2-ZrO2 coating material. Mechanical properties in both the in-plane and transverse coating directions were evaluated in both compression and tension. From simple monotonic tests, the anisotropy of the material could be quantified. A key finding was that both the loading modulus and tensile strength were about two to three times higher in the in-plane direction. This anisotropy is believed to be due to the directionality of microcracking in the material. Cyclic loading experiments showed that the coating material also exhibits considerable irreversible strain behavior in both the transverse and in-plane directions. A model describing the irreversible strain behavior based on the combined sliding and closing of pre-existing microcracks is proposed and compared with experimental results. It is shown that the model describes the qualitative and quantitative aspects of the material behavior quite well, especially in compression.

Fracture of plasma‐sprayed thick thermal barrier coatings under multiaxial stress states

“…2. Lower out‐of‐plane strengths have been observed for tension loading 6 . The figure shows that the difference between the two is small for compression shear failure.…”

“…Lower out-of-plane strengths have been observed for tension loading. 6 The figure shows that the difference between the two is small for compression shear failure. It appears that the maximum principal stress theory provides an adequate representation of the test data given the typical variability associated with this material.…”

“…Although crack sliding has been used in many models of deformation and fracture of brittle materials, 6,7,9,13,14 there is rarely any direct evidence of crack sliding (to our knowledge), except Nemat‐Nasser's experiments with intentionally introduced artificial cracks. Figure 10 is a micrograph showing crack sliding between two crack surfaces in an in situ SEM shear test.…”

“…Various models and failure criteria have been proposed in attempts to understand the mechanisms of fracture of brittle materials with randomly distributed cracks. [6][7][8][9][10] Most of this work was carried out on rock, soil and concrete materials under compression with or without confining pressure.…”

“…The state of stress in a coating is a complex combination of both normal and shear stresses from the combined thermal and mechanical loading. Various models and failure criteria have been proposed in attempts to understand the mechanisms of fracture of brittle materials with randomly distributed cracks 6–10 . Most of this work was carried out on rock, soil and concrete materials under compression with or without confining pressure.…”