The relation between a change in grain-boundary composition and a change in color for silicon nitride (Si
3N
4) ceramics, during capsule-free hot isostatic pressing (HIP), were investigated by EPMA color mapping and microanalysis using transmission electron microscopy (TEM) with energy dispersive X-ray spectroscopy (EDS). It was concluded that the color change in the sintered body of Si
3N
4, during HIP using AlN powder bed, was not attributed to the change in the density or the crystalline phase, but to the change in a composition of Si, Al, and Y in the glassy grain- boundary phase. In the case of HIP in a Si
3N
4 crucible without any powder bed, the color change was very small.
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