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A silicon carbide disk was sintered from 2090" to 2190°C in 25°C steps. After each sintering step, the disk was examined using a precision acoustic scanning system to determine acoustic attenuation and velocity. The bulk density was found to vary nonmonotonically with sintering temperature. The density varied as much as 10% from its value at 2090°C during the sintering process. Local density fluctuations occurred in an organized and history-dependent way. These local density fluctuations varied up to ?7% of the bulk density and were made visible by acoustic attenuation and velocity mapping. [Key words: silicon carbide, sintering, density, porosity, ultrasonics imaging.] ERAMIC processing is being investiCgated extensively by an international effort. A major goal of current processing research is to control the amount, size, uniformity, and distribution of porosity in ceramics. Considerable effort is being made to control porosity variations by modifying the ceramic powder processing that is done to form green or unsintered ceramics. Milling techniques, powder-size distributions, binders, pressing pressures, etc., all have an effect on the final poresize distribution of a sintered ceramic. There are relationships between the type of powder processing and the final microstructure of the sintered ceramic. '** We will show that there exist large local density fluctuations that occur within the bulk ceramic during sintering. These local density variations are historydependent and can be tracked or followed during the sintering process.

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SUMMARYThe reliability of microfocus x-radiography for detecting internal voids in structural ceramic test specimens was statistically evaluated. The microfocus system was operated tn the projection mode using low x-ray p'hoton energies (~20 keV) and a 10 lim focal spot. The statistics were developedJor implanted internal voids in green and sintered silicon carbide and silicon nitride test specimens. These statistics were compared with previouslyobtained statistics for implanted surface voids in similar specimens. Problems associated with void implantation and characterization are discussed. Statistical results are given as probability-of-detection curves at a 95 percent confidence level for voids ranging in size from 20 to 528 lim in diameter ..