A new approach for quantifying thermal barrier effect. Case study: RE_20 enamel

Abstract

Thermal barrier coatings (TBC) are designed to reduce the temperatures at the surface of metallic hot working pieces. A new refractory enamel, denoted RE_20, was developed at INCAS SA. The RE_20 is designed to protect pieces made of EI 468 superalloys. Thermal barrier effect (TBE) is frequently used in the literature to refer to the temperature decreasing at the heat exposed surface, but it is not defined as a quantitative measurand. The paper aims to mitigate this shortcoming by introducing two measurands for a proper assessment of the TBE i.e. relative temperature decreasing (RTD) and relative heat flow decreasing (RHD). The TBE depends on the working temperature, therefore it has to be estimated at elevate temperatures in the 900-1100oC range. This is possible only through thermal diffusivity flash method (ThDM) which facilitates measurements up to 1000oC. Two mathematical models were derived for RTD and RHD. The models were applied to the RE_20/ EI868 systems for two cases: as obtained and as cyclic thermal shock tested at 900oC. The paper addresses the following novelties: two new measurands (RTD, RHD), the TBE behavior depending on the working temperature and the TBE dependance on upper temperature of the thermal shock.