Non-destructive characterization of plasma-sprayed ZrO2 coatings by ultrasonic surface waves

A surface wave method has been used to determine the elastic modulus of ZrO2 coatings plasma-sprayed onto a nickel-base superalloy and aluminium. The phase velocity depending upon the frequency of laser induced surface waves has been measured. The inverse problem of the surface wave dispersion relation has been solved to calculate the elastic modulus of the coatings from the measuring data. Modulus values in the range 11.2-46 GPa were obtained for the coatings applied on a nickel-base superalloy by atmospheric plasma spraying. These moduli are lower by up to one order of magnitude than those measured on compact ZrO2 samples (240 GPa) owing to a network of long streched- out pores which intersperse the coating material. An elastic modulus of 90 GPa has been determined for the ZrO2 coating vacuum-plasma-sprayed on aluminium. The micrograph of this coating showed a structure with more isolated pores having almost equiaxial form. The nature of the pore structure can be predicted from the m easured elastic modulus using the theory of elastic modulus of heterogeneous solids.