Session BB8.3

2:00 PM BB8.3
THE INFLUENCE OF TEMPERATURE AND STRESS-STATE ON THE YOUNG'S MODULUS OF PLASMA-SPRAYED YTTRIA STABILIZED ZIRCONIA THERMAL BARRIER COATINGS. Frigyes Szuecs, Hans-Jörg Fecht, Technical University of Berlin, Dept of Materials Science and Engineering, GERMANY; Thomas Cosack, Joachim Bamberg, Christian Schwarminger, Daimler-Benz Aerospace, MTU Munich, GERMANY.

Thermal barrier coatings (TBC) become an important structural element in the design of modern flight and land-based gas turbine engines. Yttria stabilized zirconia is a common material for thermal barriers, because this material combines a low thermal conductivity with a relative high thermal expansion and fracture toughness. TBC's have to be highly strain tolerant in order to avoid delamination. It is reported in literature that thermal barrier coatings with superior lifetimes have a low apparent Young's modulus. The thermoelastic behavior of plasma-sprayed 8 wt$\%$ yttria stabilized zirconia thermal barrier coatings with an underlying low pressure plasma-sprayed M-CrAlY bond coat on superalloy substrates is analysed between room temperature and 1000$^{\circ}$C by dynamic three-point-bending experiments performed on freestanding and composite beams. The measurements on the composite beams are analysed concerning the apparent Young's modulus of the TBC using a composite beam deflection model, which also allows the calculation of inner stresses. The as-sprayed TBC has an apparent Young's modulus between 20 and 24 GPa. This strongly reduced stiffness compared to sintered zirconia is discussed in relation to the TBC defect structure. Above 700$^{\circ}$C an irreversible increase in apparent Young's modulus can be observed. Isothermal measurements at 1000$^{\circ}$C show a parabolic time law dependency. We explain this effect with a change in the defect structure due to surface diffusion. After heat treatments the apparent Young's modulus of freestanding zirconia coatings is nearly temperature independent. Zirconia coatings attached to a substrate show a different behavior: The apparent Young's modulus is now strongly temperature dependent, which can be explained by the influence of the stress-state on the elastic behavior. Compressive stresses arise on cooling as a result of relaxation of tensile stresses above 650$^{\circ}$C, when the bond coat becomes ductile. The observed stiffening can be explained by a reversible closure of microcracks under compressive stresses. The irreversible change in the defect structure of plasma-sprayed zirconia coatings has to be interpreted as a degradation phenomena and a life limiting factor. Whenever simulating the thermomechnical behavior of TBC composite systems time-dependent behavior as well as the observed eigenstress-dependency of the elastic properties has to be considered.