Stress rate dependence of fracture strength in pre-cracked silicon nitride ceramics doped with ytterbium oxide

High-temperature dynamic fatigue behavior has been investigated in 6 wt% ytterbium oxide and 2 wt% alumina-doped silicon nitride ceramics by nitrogen gas pressure sintering. The specimens were pre-cracked by Vickers indentation to prevent creep damage and to ensure dynamic fatigue dominating. The tests were performed in four-point flexure in air at temperatures of 1000°, 1200°, 1300°, and 1400°C and by varying the loading rate from 1, 0.5, 0.1-0.01 mm/min at each temperature. The analyses were conducted by plotting fatigue stress against loading rate at each testing temperature in double logarithm coordinates. The material was found to be the least susceptible (the highest slow crack exponent number N) to slow crack growth at 1200°C, as reflected by the comparison of the plot slopes for the four testing temperatures. The explanation and analyses take into consideration the grain-boundary phase crystallization, crack healing, and oxidation during testing evidenced by X-ray diffraction and transmission electron microscopy. The fracture surfaces were characterized by three well-defined zones, namely zone I, II, and III, referring to the pre-cracked area, slow crack growth area, and fast fracture area, respectively.