Abstract
In situ Raman microprobe spectroscopy has for the first time provided direct experimental evidence that Si3N4 is toughened by virtue of local microscopic bridging stresses operated by unbroken grains, which resist crack opening upon external load. The bridging stresses may arise from elastic tractions of unbroken Si3N4 grains which bridge the crack in the very neighborhood behind its tip, or be of frictional nature at the grain interfaces in regions more far behind the tip. Particularly remarkable is the magnitude of the elastic bridging tractions which achieve the GPa order prior to microscopic fracture of the bridging site. Frictional tractions in correspondence of elongated Si3N4 grains may also locally achieve the order of several hundreds MPa, but they are shown to contribute toughening in a less effective way as compared to elastic bridges. Conventional fracture mechanics characterization and theoretical considerations are also provided which prove the rising R-curve effect in Si3N4 ceramics being explainable simply in terms of the microscopic crack bridging mechanism.
