As-fired strength of sintered silicon nitride ceramics

An increased machined, bulk strength is seen to lead to an improved as-fired strength. Silicon nitride with an as-fired strength as high as 750 MPa can be obtained, sufficient for most structural applications. Microstructural examinations reveal that the reduction of surface and subsurface pores (or equivalent, may be treated as cracks) both in size and occurrence frequency is the primary reason for the strength increase observed. A linear elastic fracture mechanics model correlates the as-fired and bulk strength properties, and predicts that subsurface flaws located in a reasonably deep site underneath the surface would dominate the flexural strength if their length exceeds 2.3–2.8 times of the length of surface defects. The subsurface flaws very close to the surface are the most damaging and should be avoided by process optimizations.