Stress Distributions in Plasma-Sprayed Thermal Barrier Coatings Under Thermal Cycling in a Temperature Gradient

Abstract

The residual stress distribution in plasma-sprayed zirconia thermal barrier coatings subjected to cyclic thermal gradient testing was evaluated using Raman piezospectroscopy and finite element computation. The thermal gradient testing (approximately 440°C/mm at temperature), consisted of repeated front-side heating with a flame and constant cooling of the back-side of the substrate either with front-side radiative cooling only or with additional forced air cooling between the heating cycles. The coatings exhibited characteristic “mud-cracking” with the average crack spacing dependent on the cooling treatment. This is consistent with finite element calculations and Raman spectroscopy measurements in which the sudden drop in coating surface temperature on initial cooling leads to a large biaxial tension at the surface. The key to proper interpretation of the Raman shifts is that the stress-free Raman peaks need to be corrected for shifts associated with the evolution of the metastable tetragonal phase with aging.

The use of Larson–Miller parameters to monitor the evolution of Raman lines of tetragonal zirconia with high temperature aging

“…As shown in this study, the Raman peak positions shift with aging, hence they have to be calibrated in order to perform either temperature or strain measurement. It was shown previously that this type of correction is necessary in the residual strain measurement using Raman spectroscopy on plasma-sprayed tetragonal zirconia subjected to a thermal gradient [22].…”

The role of beam dispersion in Raman and photo-stimulated luminescence piezo-spectroscopy of yttria-stabilized zirconia in multi-layered coatings

“…There are six peaks for the Raman spectrum of YSZ. Each spectrum was fitted using mixed Gaussian and Lorentzian equations [39]. The peaks centred at 608 and 640 cm −1 were chosen for investigation because of their large signal-to-noise ratio, and the intensity recorded at each YSZ thickness.…”

Role of internal oxidation on the failure of air plasma sprayed thermal barrier coatings with a double-layered bond coat

“…The CTE of the outer bond coat decreases with the increase of the internal oxides fraction. With more than 20 vol.% internal oxides, it is very close to that of the APS YSZ, 9.7×10 -6 /K ~ 11×10 -6 /K [26,29,30]. The Young's modulus of the outer bond coat increases with the increase of internal oxides, which is significantly higher than that of the substrate (Fig.…”

“…Hence, the stress in the YSZ layer was measured by Raman technique using the approach employed by Limarga et al [26]. Since the top coat is much thinner than the substrate, the coating stress can be approximated to a biaxial stress, , which can be calculated from the Raman peak shift, , as follows [26]:…”

“…GPa and 210 GPa [26], respectively; is the piezospectroscopic (PS) coefficient for the dense 7YSZ under uniaxial stress, which is 2.01 cm -1 /GPa for 465 cm -1 [18]. It should be noted that the measured stresses represent the stress state near the top coat surface, since the…”