It is known that the strength and lifetime of silicon nitrides are strongly affected by subsurface flaws that are either inherent to the material (voids, porosity, etc.) or induced by component processing such as machining damage (e.g., cracks). Because ceramics are translucent, optical methods are effective to detect and characterize these types of subsurface flaws. In this study, three optical methods were developed/utilized for nondestructive evaluation (NDE) of subsurface flaws in silicon nitride ceramics: (1) laser backscatter, (2) optical coherence tomography (OCT) and, (3) confocal microscopy. The laser backscatter is a two-dimensional method while both OCT and confocal are three-dimensional methods. Subsurface flaws of various types, sizes, and depths can be identified and imaged by these NDE methods. In particular, subsurface Hertzian cracks, induced by surface indentations with various loads, were clearly imaged for the first time by the confocal method. This paper describes these methods and presents NDE data and their correlation with surface photomicrography results.
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