Silicon nitride ceramic images

Because Si,N ceramics are partially translucent to light and the strength limiting flaws are normally within a shallow depth under fte sur ce, optical methods are effective to detect and characterize these types of subsurface flaws. Argonne National Laboratory (ANL) has developed and utilized several optical methods for nondestructive evaluation (NDE) of subsurface flaws in silicon nitride ceramics. In this study, three optical methods are presented and evaluated (1) laser backscatter, (2) optical coherence tomography (OCT) and, (3) confocal microscopy. The la.ser backscatter is a two-dimensional method while both (XT and confocal are three-dimensional methods. It is demonstrated in the following that subsurface flaws of various types, sizes, and depths can be identified and imaged by these NDE methods. 

Fig. 46 provides an example of a high depth of focus combined with high resolution. This is an image of the very fine-grained microstructure of a silicon nitride ceramic after etching in molten NaOH. 

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