PROBING FRACTURE OF SILICON-NITRIDE BY IN-SITU MICROSCOPIC RAMAN-SPECTROSCOPY

In situ Raman microprobe spectroscopy has for the first time provided direct experimental evidence that Si3N4 is toughened by virtue of loca l microscopic bridging stresses operated by unbroken grains, which res ist 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 a t the grain interfaces in regions more far behind the tip. Particularl y remarkable is the magnitude of the elastic bridging tractions which achieve the GPa order prior to microscopic fracture of the bridging si te. Frictional tractions in correspondence of elongated Si3N4 grains m ay also locally achieve the order of several hundreds MPa, but they ar e shown to contribute toughening in a less effective way as compared t o elastic bridges. Conventional fracture mechanics characterization an d 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.