Characterization of Subsurface Defects in Ceramic Rods by Laser Scattering and Fractography

Evaluation of Silicon‐Nitride Ceramic Valves

“…Flaws in this region may be intrinsic to material defects such as porosity or voids, but also extrinsic (e.g., machining damage). For high‐strength Si 3 N 4 (e.g., SN235P), failure (fracture) initiation flaws can be <30 μm in size 8,27,28 . Traditional NDE methods are not effective for detecting subsurface flaws with such small dimensions.…”

“…The resolution can be further improved to the submicrometer level when using a high numerical‐aperture focusing lens with a confocal setup 40 . This system has been used to detect fracture initiation flaws in machined SN235P cylindrical specimens 28 . Subsurface flaws ranging from 25 to 35 μm in size were identified to be fracture origins, which were verified by detailed fractographic examinations.…”

“…For valve‐train applications, the most critical portions of a ceramic component, that is, those under the highest stress during operation, are the surface or near‐surface regions (usually to depths of <200 μm). For high‐strength Si 3 N 4 , fracture‐initiation flaws may be smaller than 50 μm in size and are located on or beneath the surface 7,8,28 . The fracture origins normally consist of microstructural irregularities that act as a stress concentration from the applied load.…”