Yb2O3 掺杂氮化硅复合材料的高温动态疲劳行为

研究了N2 气氛加压烧结制备的6 wt %Yb₂O₃ 和2 wt %Al₂O₃ 掺杂氮化硅复合材料在高温下的动态疲劳行为。用维氏压痕法在试样上获得规范的预制裂纹。分别在1000 ℃、1200 ℃、1300 ℃、1400 ℃下, 以1 、0. 5 、0. 1 、0. 01 mm/ min 的不同压头速率对试样进行四点弯曲试验。不同温度下疲劳应力2加载速率函数曲线的对比表明, 这种材料在1200 ℃时, 对亚临界裂纹生长具有最高的抵抗力(即具有最大的亚临界裂纹生长指数N) 。XRD、TEM 分析表面晶界相的晶化、裂纹愈合可以提高亚临界裂纹生长指数。EDS 分析表明, 1200 ℃下氮化硅陶瓷断裂面的氧化有助于晶界相的晶化, 但是在更高的温度下, 则会产生不利影响。 

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.