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Yttria stabilized zirconia has become the preferred TBC layer material for gas turbine
engine applications because of its low thermal conductivity, κ, and its relatively high
(compared to many other ceramics) thermal expansion coefficient, Fig. 2.2[27]. This
reduces the thermal expansion mismatch with the high thermal expansion coefficient metals
to which it is applied. It also has good erosion resistance which is important because of
the entrainment of high velocity particles in the engine gases[1]. The low thermal conductivity
of bulk YSZ results from the low intrinsic thermal conductivity of zirconia (reported
to be between 2.5 and 4.0 depending on the phase, porosity and temperature[28]) and
phonon scattering defects introduced by the addition of yttria[29]. These defects are introduced
because yttria additions require the creation of O2- vacancies to maintain the electrical
neutrality of the ionic lattice. Since both the yttrium solutes and the O2- vacancies are
effective phonon scattering sites the thermal conductivity is decreased as the yttria content
is increased. In practice, a yttria concentration in the range of 6 to 8 wt.% is generally used
since this composition maximizes spallation life due to the formation of the metastable t’
phase[30],