Low conductivity thermal barrier coatings

The dissertation begins by exploring the growth of 7YSZ coatings at different rotation rates. The experiments show that as the rotation rate was increased, the texture changes from <111> to <100> and the total pore fraction slowly decreased. The intercolumnar pores perpendicular to the coating surface are very effective at strain accommodation during thermal cycling. The intra-columnar pores appear the most effective for the reduction of thermal conductivity of the coatings. The minimum thermal conductivity occurs at a low rotation rate and is 0.8 W/mK.;The failure modes and mechanisms of 7YSZ coatings during thermal cycling have been investigated. The primary mode of failure on rough bond coat surfaces involves delamination within the ceramic coating, just above the thermally-grown oxide (TGO). It was initiated by a bond coat rumpling mechanism. The delaminations were initiated preferentially at "corn kernel" growth defects in the coating. Ceramic coatings applied to polished bond coat surfaces had much longer spallation lifetimes and the delamination fracture shifted to the interface of TGO/bond coat. These delaminations were extended by a mechanism involving the formation and coalescence of interfacial voids. The enhanced coating life is shown to be a consequence of their lower density and hence, lower elastic modulus.;Rare earth zirconates appear to be a promising candidate due to their reported low intrinsic thermal conductivity, good phase stability and greater resistance to sintering and CMAS attack compared to 7YSZ. The SZO coatings had as-deposited conductivities of 0.5+/-0.1 W/mK. When these SZO coatings were subjected to thermal cycling, it was found to have a much shorter lifetime (on both rough and smooth bond coats) than similarly deposited 7YSZ material. It was also found that samaria tended to react with alumina to form a SmAlO 3 interphase of the TBC/TGO interface which appears to significantly lower the interface toughness.;To improve the durability of SZO coatings, 7YSZ/SZO bilayer coatings were grown by the DVD method so that a thin 7YSZ layer separated the SZO from the TGO layer. The SZO layer was found to thermochemically compatible with YSZ without the degradation during the cycling exposure. Sm was detectable near the TGO when the bilayer with 10 mum of 7YSZ was subjected to thermal cycling for more than 1000 hours.