Effect of gas pressure on thermal conductivity of zirconia thermal barrier coatings

The thermal diffusivity of thermal barrier coatings (zirconia stabilized with 7-8 wt% yttria) deposited by air plasma spraying has been measured by the laser flash method. Free-standing samples (0.5-0.6-mm thick) polished on both sides were coated with graphite. Deduced values of thermal conductivity (e.g., 1.5 Wm-1K-1 at 200 C in 1 atm. of nitrogen) are in the range measured in other studies for this type of thermal barrier coating. The % increase in thermal conductivity in 1 atm. of gas compared to its value in vacuum was found to be about 10% in nitrogen at 500 C, close to a calculated value of 7% using a model in the literature. A simple extension of the model to higher temperatures and gas pressures predicts a significant rise, so that by 10 atm. of gas pressure the % increase would be 30% (with little sensitivity to temperature). (orig.)