Title

Failure modes of plasma-sprayed thermal barrier coatings

Date of Completion

January 2000

Keywords

Engineering, Materials Science

Degree

Ph.D.

Abstract

Conventional plasma-sprayed thermal barrier coatings (TBCs) are known to fail by spallation of the yttria-stabilized zirconia (YSZ) topcoat exposing the underlying metal to high temperatures. Failure takes place by crack propagation in the YSZ just above the YSZ/thermally grown oxide (TGO) interface. Compressive stress in the TGO due to thermal expansion coefficient mismatch and oxidation is believed to play a key role in the failure. However, non-destructive measurement of the compressive stress in the TGO has been challenging due to the overlying ceramic top layer. In this study, TBC samples coated to current industrial specifications were thermally cycled to various fractions of their life to determine the failure mechanisms. The technique of Cr3+ piezospectroscopy was successfully applied to the plasma-sprayed samples for the first time in an effort to measure compressive stress in the TGO through the ceramic top layer. In addition, a new nano-grained plasma-sprayed TBC was studied in order to develop a next generation TBC with enhanced properties. ^ Results from observations on cross-sections and spalled surfaces have identified two competing failure mechanisms for TBCs: (1) cracking along asperity tips at the TGO/bond coat interface, and (2) cracking in the ceramic between the asperity tips. TGO residual compressive stress was found to increase in the first 1 to 10 cycles and then decrease with increasing number of cycles. The standard deviation of the stress measurement, which is a measure of damage accumulation in the TGO layer, was found to increase at higher numbers of cycles. Measurement of compressive stress in the TGO using Cr3+ piezo-spectroscopy was limited to YSZ thicknesses of <50 μm due to an impurity present in the YSZ layer. When no impurity was present the limiting thickness was <170 μm due to scattering by microstructural defects such as solute, porosity, and most importantly splat boundaries. ^ A new nano-grained TBC was fabricated with a resulting microstructure that contained no splat boundaries or microcracking. The coating had a high porosity, 22 vol%, and strain relieving vertical microcracks. When compared to conventional plasma-sprayed TBCs, the nano-grained TBCs have a similar cyclic lifetime and failure mechanism but a lower compressive stress in the TGO. ^ Since thermal conductivity is a key physical property of interest in TBCs, a fundamental study was performed to understand the effects of grain boundaries and porosity on the thermal conductivity of YSZ. ^ To that end, monolithic YSZ samples were manufactured using the fugitive sphere method to create tailored porosity utilizing polymer spheres. The grain boundaries were found to have little effect on the thermal conductivity while the porosity was found to have a small effect in the size range studied (5μm–15μm). ^