Sensitivity to experimental errors in evaluating the thermal expansion coefficient of a thermal barrier coating by using coating system specimens

The coefficients of thermal expansion (CTEs) of coatings were necessary to evaluate the thermal stresses of the coated component. In general, the CTEs were measured from the free-standing coatings. It is useful if the CTEs of the coatings are able to be measured from the coating system specimens. The resulting CTE of a coating by the coating system is sensitive to experimental errors when compared with that for the free-standing coating. In this study, the sensitivity to experimental errors in evaluating the CTEs from the coating system specimens was investigated. The experimental study was also carried out on six kinds of specimens with different sensitivities. The surface strain during thermal cycling from 850 °C to 300 °C measured by a laser speckle method was used to evaluate the CTE. The CTEs of Yttria-stabilized-zirconia thermal barrier coating and CoNiCrAlY bond coatings were measured by a composite cylinder model. It was found from the sensitivity calculation that the errors of the CTE of the substrate and the measuring slope between strain and temperature caused a large error in the resulting CTE of the coating. The sensitivity calculation also showed that the increase of the thickness of the coating was effective in decreasing the sensitivities. A reasonable estimation was experimentally obtained if the specimens had low sensitivities.