Advanced materials for high temperature coatings
Thermal barriers are multilayered materials with dual chemical and thermal functionality. Due to their ability to inhibit degradation of the underlying structural superalloy, TBS can significantly limit thermo-mechanical fatigue and oxidation. This feature makes TBS suitable for further exploitation in gas turbine technology. Project partners focused on novel TBCs that are based on rare earth zirconates and co-doped zirconias as alternative materials to yttria-stabilised zirconia (YSZ). Unlike standard material based on USZ, rare earth zirconates can suitably combine low thermal conductivity with improved morphological stability. Until the beginning of the HIPERCOAT project, rare earth zirconate compositions were considered to be thermochemically incompatible with alumina in all cases. Experimental studies showed that interphases are formed when rare earth zirconates come in contact with alumina at high temperature. Most interestingly, an aluminate layer was found to be formed resulting to pore development at the interface of gadolinium zirconate with alumina. This may cause degradation of adherence putting at risk the integrity of the coating. However, the effect was found to be minimised at lower temperatures (1100 °C), thus interphase formation may be avoided over the required life of the coating system. Another finding offered indications that the actual gadolinium content of the zirconate has an impact on kinetics. Since thermal conductivity is almost constant for the same composition range, it seems that the use of sub-stoichiometric zirconate phases would be preferable for reducing the risk of interface interactions. For more information click at: http://www.materials.ucsb.edu/~nsf/
