A Fractographic Study on Glass and Silicon Nitride Fractured under Monotonic Tensile and Cyclic Tension-Compression Tests

A fractographic analysis was made on two kinds of silicon nitride, glass ceramics and borosilicate glass fractured under monotonic tensile loading and tension-compression cyclic loading. Emphasis in the fractographic investigation was placed on the fracture initiation area. The relation between mirror size and fracture stress was investigated under various stress conditions. The mirror sizes observed on the fracture surfaces of tensile fractured specimens were approximately 60% of those for bending tests when the comparison was made at the same fracture stress. The fractographic observation on the area of fracture initiation revealed that the fracture in silicon nitride-B always occurred from the defects formed during sintering, while in silicon nitride-A, in which fracture always occurred from the surface, no defect was observed in the fracture initiation area. This result means that the fracture in silicon nitride-B occurs by the crack growth from these defects (formed during sintering), while in silicon nitride-A, fracture initiation occurs by the cracking formed along the grain boundaries by SCC in the surface layer, and then these cracks grow by SCG to final failure. The results of EDX analysis on the fracture initiation area of silicon nitride-A showed that the relative content of Y, which is the main element of sintering aid additions, was higher in the region inside of the critical crack size than in the region outside this region. This will probably show that the crack growth occurred by SCG in this area and by rapid growth in the area outside of the critical crack size. This observation seems to support the fracture intiation process discussed above on the fracture of silicon nitride-A.