This work has addressed the gas transport rate and mechanism through plasma sprayed coatings. This is of particular interest for thermal barrier coatings (TBCs) and solid oxide fuel cell electrodes. Deposits of ZrO<SUB>2</SUB> - 8 wt% Y<SUB>2</SUB>O<SUB>3</SUB>, ZrO<SUB>2</SUB> - 14 wt% Y<SUB>2</SUB>O<SUB>3</SUB> and A1<SUB>2</SUB>O<SUB>3</SUB> have been plasma sprayed under varying conditions. Microstructural studies and density measurements have been carried out to characterise the porosity content and microcrack distribution. Crystallographic phase analysis and Young's Modulus measurements have also carried out. The gas permeability of each specimen has been measured at temperatures up to 600°C. These measurements involve a thin disc of the coating being sealed over a ceramic tube. A mass flow controller was used to set a constant gas flow rate, and the resulting pressure difference across the coating was measured once steady state had been reached. D'Arcy's Law was then used to determine the specific permeability of each specimen. Measurements were carried out on the coatings using hydrogen, oxygen and nitrogen as the permeating gases. Values of the specific permeability were of the order of 10<SUP>-16</SUP>m<SUP>2</SUP> for zirconia coatings and 10<SUP>-17</SUP> m<SUP>2</SUP> for alumina coatings. These results were correlated with microstructural observations via a simple analytical model for gas permeation, based on Percolation Theory. In this model, large pores were treated as isolated cavities with connecting microcracks, predicting a high sensitivity to the density and connectivity of microcracks. Good agreement was obtained between theory and experiment in terms of the magnitude of the permeability. The oxygen flux through the top coat of a TBC has been calculated from the permeability of the coating. Published values of the diffusion coefficient have been used to calculate the oxygen flux by diffusion. These 2 transport mechanisms have been compared and gas permeation has been found to dominate over diffusion at the operating temperatures of a TBC. Oxide growth at the bond coat / top coat interface of a TBC has been shown to be controlled by diffusion through the oxide layer.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:599155 |
| Date | January 1999 |
| Creators | Fox, A. C. |
| Publisher | University of Cambridge |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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