Room- and high-temperature tensile fracture of directionally solidified chromia-doped sapphire fibers

Delayed failure of single crystal ruby fibers along <0001> direction (containing 300 ppm and 10% of a nominal amount of Cr2O3) has been studied in tension both at room temperature (RT) and at 1400degreesC. Commercial sapphire c-axis fibers have been also studied to establish a comparison between the mechanical properties of doped and undoped Al2O3. True tensile stressing rates in the range 0.37 to 1353 MPa.s(-1) were applied. Average rupture stress is higher at RT, 1.67 - 3.19 GPa, than at high temperature (HT), 397 - 675 MPa, and increase in each range with the stressing rate. Both at RT and at HT 300 ppm. Cr3+-doped fibers are stronger than sapphire fibers, and both are stronger than 10% Cr2O3-doped fibers. Solution hardening is considered to be responsible for the observed increase of tensile strength in the 300 ppm fibers, and an increased density of internal defects is considered responsible of the reduction of tensile strength in the 10% fibers. At RT, typical slow-crack growth (SCG) exponents are higher than 40. At 1400degreesC, SCG exponents are smaller than 20, i.e., N=12-19.