High-temperature fatigue behavior in pre-cracked sil icon nitridecomposites doped with ytterbium oxide

Silicon nit ride materials doped with 6 wt % ytterbium oxide and 2 wt % alumina were prepared by nit rogen gas pressure sintering. The high temperature dynamic fatigue behavior of the Si₃N₄ composites was investigatedin terms of the loading rate dependence of fatigue st rength at high temperatures. The specimens were pre-cracked byVickers indentation to avoid creep and to ensure slow crack growth dominating in fatigue damage. The tests wereperformed in the four-point bending mode in air at the temperature of 1000 ℃, 1200 ℃, 1300 ℃and 1400 ℃respectively. The cross head speeds were chosen of 1 , 0. 5 , 011 and 0. 01 mm/ min at each temperature. By plotting fatiguest rength against cross-head speed in double logarithm coordinates , the highest slow crack exponent number N wasfound in the test at 1200 ℃, indicating that the critical crack growth was the slowest at this temperature. X-raydiff raction and t ransmission elect ron microscopy show that the grain boundary crystallization and crack healinginhibit the crack growth thus increasing the slow crack growth exponent number. Energy dispersive spect rumreveals that the crack surface oxidation may benefit the crystallization at 1200 ℃, while slow crack growth may bepromoted at higher temperatures because of over-oxidation.