The cold sintering process (CSP) is a low temperature processing technique that utilizes a transient phase to synthesize dense ceramics. However, some CSP parts contain microflaws that arise due to pressure gradients and inhomogeneities in temperature and distribution of the transient phase. This work uses high frequency ultrasound (20 MHz) to verify the presence of defects larger than 15 μm in effective radius in CSP ZnO samples of varying densities (84–97%). The acoustic data were compared to X-ray Computed Tomography (XCT) images to validate the findings. Acoustic metrics used in this work include wave speed, which is affected by differences in the effective elastic properties of the medium, and attenuation, which is wave energy loss due to scattering from defects, including those smaller than the wavelength. Wave speed maps were inhomogeneous suggesting density gradients which were verified with SEM. Areas of high attenuation (> 300 Np/m) are present in all samples independent of relative density and correspond to defects identified in XCT ranging from 15 μm to 50 μm in effective radius. This suggests the presence of microflaws possibly due to the inhomogeneous removal of the transient phase. However, some high attenuation spots do not correspond to visible defects in XCT which suggests the presence of features such as texturing which are undetectable with XCT. These results show the viability of high frequency ultrasound for defect detection in cold sintered ZnO.

The cold sintering process (CSP) is a low temperature processing technique that utilizes a transient phase to synthesize dense ceramics. However, some CSP parts contain microflaws that arise due to inhomogeneities in pressure, temperature, and transient phase. This work uses 20 MHz ultrasound to verify the presence of defects in CSP ZnO samples of varying densities (84%–97%). Acoustic metrics used in this work include wave speed, which is affected by differences in the effective elastic properties of the medium, and attenuation, which quantifies wave energy loss due to scattering from defects. Wave speed maps were inhomogeneous, suggesting density gradients which were verified with scanning electron microscopy. In addition, it was demonstrated that the pores produced by cold sintering are anisometric, which increases the anisotropy in the elastic properties. High attenuation regions (>300 Np/m) are present in all samples independent of relative density and correspond to defects identified in X‐ray computed tomography (XCT) which were as small as 38 µm in effective diameter. However, some high attenuation spots do not correspond to visible defects in XCT, which suggests the presence of features undetectable with XCT such as residual secondary phases at the grain boundaries.

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