| Abstract: |
It has been considered that the current effective high temperature capability limit of APS YSZ TBCs is at ~1200oC. This is partially related to the formation of the so called non-transformable tetragonal t-phase during the rapid quenching of the YSZ particles during spraying. Although in fact metastable, the t-phase transforms slowly into the equilibrium tetragonal and cubic phases at temperatures higher than 1200oC. These phase transformations are not desirable in ceramic materials due to their lack of plastic deformation, i.e., they tend to crack due to the stresses generated during the phase changes, thereby leading to TBC failure. The NRC optimized an APS TBC to allow it to operate at ~1500oC temperature levels. The bond coat MCrAlY and top coat YSZ powders were commercial off-the-shelf products and both sprayed via the APS torch Metco 3MB. The spray parameters for the MCrAlY and YSZ were developed to engineer a (i) low-oxidized bond coat and a (ii) low thermal conductivity (at high temperatures) YSZ top coat. The TBCs were sprayed onto metallic super-alloy substrate pucks (2.5 cm diameter x 3.2 mm thick). By using the laser gradient rig, a temperature gradient was generated across the TBC/substrate system. The YSZ Tfront temperature and substrate backside Tback temperature levels were ~1500oC and ~1000oC, respectively. Two types of cycles have been performed: (i) 5 min hot and 2 min cooling for 1000 cycles and (ii) 1 h hot and 2 min to cooling for 10 cycles. In both cycle conditions, no TBC failure has been observed. Although preliminary, these results are promising regarding the potential to increase the effective high temperature capability limits on optimized APS YSZ TBCs. In addition to the thermal cycle results, this presentation will include (i) the microstructural characteristics of the TBC (via SEM), (ii) TBC elastic modulus and hardness values measured via instrumented indentation, (iii) YSZ thermal diffusivity and conductivity values of the YSZ up to 1500oC, (iv) phase evolution of the YSZ analyzed via XRD.
|
|
