Correlation between spraying conditions and micro crack density and their influence on thermal cycling life of thermal barrier coatings

It is generally known that the porosity of thermal barrier coatings is essential to guarantee a sufficiently high strain tolerance of the coating during thermal cycling. However, much less is known about the influence of the specific morphology of porosity, such as microcracks and typically larger pores, on the performance of the coatings. Both features are usually formed during plasma spraying of yttria-stabilized zirconia (VSZ) thermal barrier coatings (TBCs). In this investigation, the influence of microcracks on the thermal cycling behavior was studied. The amount of microcracks within VSZ thermal barrier coatings was changed by changing the powder-feeding rate. Only small changes of the total porosity were observed. Mercury porosimetry served as a tool to investigate both the amount of microcracks and pores in the coating. Additionally, microcrack densities were determined from metallographical investigations. A linear dependence between the amount of fine pores determined by Hg porosimetry and the crack density was obtained for one set of coatings. Thermal cycling TBC specimens with different microcrack densities were produced and tested in a gas burner test facility. At high surface temperatures (above 1300 degreesC), failure occurred in the ceramic close to the surface. Under these conditions, the samples with increased horizontal microcrack densities showed a significant increase of thermal cycling life

A life time model for ceramic thermal barrier coatings

The life time Of Y2O3 stabilized ZrO2 thermal barrier coatings (TBC) has been modelled using a rather simple fracture mechanical approach. The basis of the model, is a finite element analysis of the thermal stresses and approximate assumptions of crack growth along the bond coat (BC)-ceramics interface. The FE calculations show the influence of several microstructural features of the TBC system, as profile of the BC TBC interface, thickness of thermally grown oxide formed during thermal cycling and others, on the stress state. From these results, a specific way of crack growth is predicted and included into the model. The modelling results are compared to life times obtained from thermal cycling experiments. An analysis of the location of failure within the samples, as well as the influence of a variation of the roughness of the BC-ceramics interface on life time are presented. Both are in reasonable agreement with the modelling results. Finally, the shorter life times, which are predicted for samples exposed to an additional compressive mechanical strain, are discussed. (C) 2003 Elsevier Science B.V. All rights reserved

New double-ceramic-layer thermal barrier coatings based on zirconia-rare earth composite oxides

A series of La2O3-ZrO2-CeO2 composite oxides were synthesized by solid-state reaction. The final product keeps fluorite structure when the molar ratio Ce/Zr >= 0.7/0.3, and below this ratio only mixtures of La2Zr2O7 (pyrochlore) and La2O3-CeO2 (fluorite) exist. Averagely speaking, the increase of CeO2 content gives rise to the increase of thermal expansion coefficient and the reduction of thermal conductivity, but La-2(Zr0.7Ce0.3)(2)O-7 has the lowest sintering ability and the lowest thermal conductivity which could be explained by the theory of phonon scattering. Based on the large thermal expansion coefficient of La2Ce3.25O9.5, the low thermal conductivities and low sintering abilities of La2Zr2O7 and La-2(Zr0.7Ce0.3)(2)O-7, double-ceramic-layer thermal barrier coatings were prepared. The thermal cycling tests indicate that such a design can largely improve the thermal cycling lives of the coatings. Since no single material that has been studied so far satisfies all the requirements for high temperature thermal barrier coatings, double-ceramic-layer coating may be an important development direction of thermal barrier coatings. (c) 2004 Elsevier Ltd. All rights reserved

Thermal cycling setup for testing thermal barrier coatings

Different mechanisms of failure can be induced and studied with the experimental setup for thermal-cycling tests developed by the authors and shown in the Figure. It offers a realistic test rig for a broad range of applications, being suitable for a large surface temperature range (1050-1400degreesC) and thermal gradients of 0.4-1.0 K/mum through the ceramic layer. Cycle lengths are variable; typically, 5 min heating and 2 min cooling are chosen, since a steady state is normally reached after about 1 min of heating or cooling