The fracture toughness (KIC
) was measured for quartz and sapphire single crystals as a function of crystal orientation at room temperature. The controlled surface flaw method popularized by J. J. Petrovic et al
. was employed in this experiment. Knoop indentor was used to introduce a semicircular microflaw on the surface of the specimen. Then the 3-point bending rupture strength was measured and KIC
was calculated from the flaw size and strength.
Five kinds of crystal orientations, namely (0001), (0110), (0111), (1120) and (1121) planes as fracture surface, were used for quartz, and four crystal orientations, namely (0001), (1100), (1102) and (1120) planes as fracture surface, were used for sapphire. The relation between KIC
and the Young's elastic modulus (E
) perpendicular to the fracture surface was investigated.
It is expected from the theoretical consideration that KIC
follows a linear dependence of E
. The linear relation between KIC
was actually observed for both quartz and sapphire single crystals. This kind of relation was also reported for MgAl2
single crystal. The KIC vs. E
plot shows different lines for different crystals, namely quartz, sapphire and MgAl2
. As a consequence, it is concluded that the proportional constant of this relation is dependent on the crystal structure.