DIFFERENTIAL INTERFERENCE MICROSCOPY OF MICROSCOPIC FRACTURE IN SI3N4 CERAMICS

In order to make clear the origin of the break-down of the linear elastic fracture mechanics for small cracks of ceramics, microscopic observation, using a differential interference microscope, was made for the stable crack growth developed at the fracture origin in hot-pressed Si3N4 samples under the stress field. Three-point bending tests were carried out to investigate the relationship between intrinsic flaw size and strength of the Si3N4 samples. It was found that the linear elastic fracture mechanics holds only within 50 mu m in the equivalent crack length. Knoop flaws were introduced, before the three-point bending test, on the specimen surface to check the microscopic fracture machanism: For the flaws larger than the critical crack size (>50 mu m), the fracture took place after stable crack growth, while for the smaller flaws than the critical one (<50 mu m), the fracture occurred with no stable crack growth. It is concluded from the above experimental results that the microscopic fracture originates in the combination of the main crack and microcracks and the break-down of the linear elastic fracture mechanics in small cracks occurs due to the microcracks generated in the proxmity of the crack-tip of the main crack, which causes the stress relaxation corresponding to the mechanism of plastic deformation in metals.