Strain measurements with GHz and THz radiation

Abstract

With the aim of moving towards quantitativeanalysis of thermal barrier coating (TBC) lifetime at manufacture, observations of stress-induced birefringence in yttria partially stabilised zirconia (YTZP) as well as air plasma sprayed (APS) yttria stabilised zirco nia (YSZ) TBCs are reported with gigahertz (GHz) illumination as well as time-domain spectroscopy (TDS). For the YTZP, values of 1.3×10−1 ± 2×10−2 GPa−1 and 1.3×10−1 ± 1.6×10−2 GPa−1 were observed with a GHz polariscope in transmission and reflection respectively. TDS yielded values of 9.8×10−2 ± 4.3×10−2 GPa−1 and 1.4×10−1 ± 1.3×10−2 GPa−1 for the transmission and reflection cases respectively. There is good agreement between the reflection values and the differences in the transmission between the two techniques can be explained by the approach that needed to be taken to the TDS measurements. For the APS coatings, strain-optic coefficients of (-2.4 ± 0.7)× 10−5 µ s−1 and (-2.5 ± 1.0) × 10−5 µ s−1 for samples with a 1 mm and 2 mm thick mild steel sub strates respectively were measured with the GHz system. The TDS measurements lead to 5 times lower coefficients which was explained through plastic deformations and over a year’s worth of ageing of the coatings. First measurements of electron beam physi cal vapour depsoition (EB-PVD) YSZ coatings using TDS lead to the conclusion that the EB-PVD structure was relieving strain throughout the coating and required further understanding of how these coatings reacted to applied stresses to be able to perform measurements which yielded meaningful results. Finally, a confocal imaging set-up with a GHz source is presented for stress map ping in 2D. The 3rd dimension showed an axial resolution in excess of 30 mm. The lateral resolution was imrpoved to 0.315 lp· mm−1, in other words, elements of 1.58 mm in size could be distinguished. While the imaging capabilities were very reasonable for a 330 GHz input signal, the strain measurements were hampered by unsufficiently con trolled samples, though the system was be deemed suitable with either samples with better controlled thicknesses or a higher frequency input signal which would also improve the spatial resolution of the system further.