Fracture strength of silicon carbide microspecimens

Polycrystalline silicon carbide tensile microspecimens 3.1 mm long were produced by deep reactive ion etching of wafers on the order of 150 mu m thick. The gage sections, which were nominally 200 mu m wide, were either straight, slightly curved, or contained double notches in order to vary the size of the highly stressed region. The fracture stresses of 190 specimens from three process runs were measured in a novel test setup. The average local fracture strengths for the last run were: straight 0.38 +/- 0.13 GPa, curved 0.47 +/- 0.15 GPa, notched 0.78 +/- 0.28 GPa. The corresponding Weibull characteristic strengths were, 0.42 GPa, 0.53 GPa, and 0.88 GPa with respective moduli 3.3, 3.4, and 3.1. These results show a clear increase in the strength of the material as the size of the highly stressed region decreases. Fractographic analyzes showed failures initiating from the bottoms of side grooves left by the etching process. The grains of the material were quite heterogeneous, varying from a few microns in size to columnar grains through the entire specimen thickness. The curved specimens were used as the base for predicting the probability of failure of the other two shapes. While the Weibull approach was quite accurate for the straight shape, it over-predicted the strengths of the notched specimens. Given the microstructure of the material relative to the size of the specimen, a continuum analysis is questionable.