Effect of Porosity on the Infrared Radiative Properties of Plasma-Sprayed Yttria-Stabilized Zirconia Ceramic Thermal Barrier Coatings

Far- and mid-infrared spectroscopy is presented as a powerful and nondestructive tool for estimation of the porosity of oxide ceramic films, which are typically employed as part of thermal barrier coatings. A radiative model that takes porosity into account and includes the optical response of thin films has been used. Provided that pore size is small enough as compared to the infrared wavelength, the porosity level can be included as an adjustable parameter of effective medium theories (EMT) such as Maxwell-Garnet or Bruggeman. Periodic interferential oscillations are found on radiative properties in spectral ranges where the coatings are semitransparent and do not scatter significantly infrared radiation. This information has been used to retrieve the porosity level of several semitransparent samples. Analysis of plasma-sprayed yttria-stabilized zirconia (YSZ) ceramic coatings shows that the radiative model is relevant, and the comparison between experimental and theoretical porosity values illustrates the level of accuracy of the technique.