Size-Strength Effects in Sapphire and Silicon Nitride Whiskers at 20 degrees C

Tensile tests at 20 degrees C have been carried out on seventy-three sapphire whiskers and on seventeen silicon nitride whiskers. The sapphire whiskers were of < 0001 >, < 1<(1)over bar>20 >, < 10<(1)over bar>0 >, and < 10<(1)over bar>1 > orientations, while the silicon nitride whiskers were < 0001 >, < 11<(2)over bar>0 >, and < 10<(1)over bar>3 >. Tensile strengths were in the range 45 to 1500 kg/mm(2), and deformation was found to be purely elastic. The tensile strength data have been analysed and fitted to empirical equations describing the effect of size on strength for different orientations. These empirical equations have been used to deduce possible fracture nucleation mechanisms. It is concluded that, in the case of < 0001 > sapphire whiskers, fracture nucleation may be due to dislocation pile-ups or interactions, while in the other cases a Griffith flaw mechanism is probably applicable.