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The cold compression behavior of alumina samples, with average grain sizes of 350 nm, 7.9 μm, and 18.2 μm, has been investigated under non-hydrostatic conditions by using a diamond anvil cell. After treatment with various pressures, the in situ axial X-ray diffraction patterns, transparency variations, and microstructures after decompression of the samples were also analyzed. The results showed that the submicron grains became easily translucent. The larger the grain size, the higher the pressure needed to compress the sample to transparency. On the other hand, for a similar grain size, the various geometrical shapes of the grains did not affect the pressure dependency on transparency. The elastoplastic characteristics of the alumina grains of different sizes under cold compression were found to be consistent with the transparency variation. As a conclusion, alumina particles under cold compression were found to be applicable in the pressure-assisted preparation of alumina transparent ceramics. [ABSTRACT FROM AUTHOR]

The in situ axial X-ray diffraction patterns of four ceramic powder samples (MgO, Al 2 O 3 , AlN, and cBN) that were compressed in a diamond anvil cell under uniaxial non-hydrostatic conditions were recorded. The microscopic deviatoric stress as a function of the pressure was determined from the X-ray diffraction peak broadening analysis: the curves increased approximately linearly with the pressure at the initial compression stage and then levelled off under further compression. Pressure-induced transparency was observed in all of the samples under compression, and the pressure at the turning point on the curves of the microscopic deviatoric stress versus pressure corresponded to the pressure at which the samples became transparent. Analysis of the microstructural features of the pressure-induced transparent samples indicated that the compression caused the grains to fracture, and the broken grains bonded with each other. We demonstrated that the ceramics' pressure-induced transparency was a process during which the grains were squeezed and broken, the pores were close between the grains, and the broken grains were re-bonded under compression. [ABSTRACT FROM AUTHOR]

High pressure obviously decreased the sintering temperature of high melting point ceramics and produced bulks with excellent mechanical and thermal properties. In this study, the effect of pressure alone on the densification process of alumina ceramics was demonstrated. The translucent alumina bulks with unexpected hardness were obtained by compressing α-Al 2 O 3 powder under ultra-high pressure at room temperature. Through analyzing microstructure and hardness of cold compressed samples, the results reveal that ultra-high pressure could crush alumina grains under appropriate pressure, and initiate plastic deformation of grains when the applied pressure exceeds the yield strength of alumina. Thus to enhance the hardness of the samples by grain boundary strengthening and plastic deformation at higher pressure. [ABSTRACT FROM AUTHOR]