Studies of the sintering kinetics of thick thermal barrier coatings by thermal diffusivity measurements

Better thermal insulation of the hot path components is needed in state-of-the-art gas turbines and diesel engines, because of the increasing demands of the higher process temperatures. In these processes, thermal barrier coatings (TBCs) and various cooling techniques mainly control the component surface temperatures. For this reason low thermal conductivity of the TBCs are extensively studied. One of the main factors determining the TBC thermal conductivity is the coating microstructure, with specific attention to the porosity content, as well as to its morphology and orientation. One important feature of TBCs is the stability of their thermal properties as a function of time at service conditions. In fact the prolonged exposure to high temperature can promote shrinkage phenomena within the TBC, which make the coating less strain tolerant and more heat conductive. This leads to a drastic reduction of the functional effectiveness of this ceramic protective top layer. In order to study the evolution of thermal properties of TBC, as a function of time and temperature, thermal diffusivity evaluation by laser flash method has been performed. The measurements have been performed on freestanding yttria-stabilized zirconium oxide (YPSZ) TBCs. In particular, measurements have been carried out at five different temperatures in the range 900-1300 degreesC, for different ageing times (from I up to 150 h). The data show a significant increase of the thermal diffusivity also after exposures of few hours, especially at the highest testing temperatures. Microstructural analysis carried out by optical and electron microscopy clearly showed that the observed thermal diffusivity variations can be ascribed to sub-micrometric crack healing and sintering neck formation. Mechanical testing confirmed the microhardness increase of TBC as well. Finally the data have been summarised in order to experimentally define a functional life curve of the TBC, as a function of ageing time and temperature. (C) 2004 Published by Elsevier Ltd.