Raman microprobe mapping of residual and bridging stress fields in AlN ceramics

Residual and bridging microstress fields developed from frictional/interlocking mechanisms along the crack wake have been evaluated by Raman microprobe spectroscopy in various AlN materials. AlN materials with different grain size were prepared with or without addition of (oxide) densification additives. Their fracture behavior was evaluated and related to the respective microstructural characteristics, which were assessed by image analysis techniques on scanning electron micrographs. The results showed a general tendency for coarse grained materials to undergo inter-granular fracture. Such an intergranular fracture mode activated bridging mechanisms along the crack wake, leading to a rising R-curve behavior. However, residual microstress fields, developed upon cooling from densification temperature, provided the strongest influence on fracture mode of the AlN materials. Mapping of residual stresses by Raman microprobe spectroscopy revealed that intergranular fracture occurred only when compressive stresses generated in the AlN grains (i.e. tensile stresses stored in the grain-boundary phase) due to thermal expansion mismatch between AlN matrix and oxide secondary phases. Maps of microscopic stress fields were also collected in the neighborhood of the crack path under a constant external load. Bridging stresses arising from frictional and grain-interlocking effects were extracted from the maps under the assumption of a three-dimensional stress field and used to interpret the rising R-curve behavior of the material.