Plasma-sprayed coatings have exhibited transversely
isotropic symmetry. Although properties of coating materials are
different in through thickness and in-plane directions, the material
behaves linearly in each direction. Elasticity modulus of the thermal
spray ceramic coating in the in-plane directions is approximately
Ex ¼ Ez ¼ 1.6 Ey [22]. The most important problem with the coated
systemis the thermal stresses which occur during operation because
of the considerable mismatch between the thermal expansions coefficients
of the metal substrate and the ceramic coating. TBCs are
preferred because of their low conductivity and their relatively high
coefficients of thermal expansion.