Study on the High Temperature Strength of Ceramics and Ceramics Coating Materials Using Laser Speckle Strain/Displacement Gage

在本研究中，对氧化铝陶瓷和氮化硅陶瓷进行了室温和高温疲劳裂纹扩展试验。试验中还测量了裂纹张开位移。然后利用这些实验数据讨论了陶瓷的裂纹扩展特性。有关裂纹扩展试验的结果如下：1。随着裂纹进一步扩展，裂纹扩展速率逐渐减小，裂纹扩展后，两种陶瓷材料的裂纹都暂时停止扩展。由COD与应力强度因子K的关系可知，裂纹张开水平Kop随着循环次数的增加而增加。在这些曲线中也观察到稍宽的迟滞。这些迟滞被认为与粒子桥接对裂纹张开行为的影响相对应。随着循环次数的增加，试样的柔度呈不连续变化。裂纹尖端附近颗粒桥接效应的存在对裂纹的柔度产生了影响。由于这种桥接效应，裂纹尖端的应力强度因子通过减去摩擦应力而减小。其次，对陶瓷涂层材料进行了室温和高温疲劳试验。涂层材料的实验结果如下：室温下喷砂试样的疲劳极限高于相同温度下退火试样，而氧化铝包覆试样的疲劳极限低于喷砂试样。在较高的外加应力水平下，陶瓷包覆试样在疲劳寿命初期表现为弹性变形，而在疲劳寿命中期，塑性变形随循环次数的增加而增加。这种行为符合基体的循环软化行为。在疲劳寿命的最后阶段，试样的表面应变呈不连续变化。这意味着由于涂层材料和衬底之间的分层而没有应变的区域出现了。在较低外加应力水平下，试样早期处于弹性状态，随后逐渐表现出塑性变形行为。在疲劳寿命的最后阶段，试样的塑性应变范围随着循环次数的增加而减小。这意味着单个主裂缝只能在这种条件下扩展。少

In this study, fatigue crack growth tests for alumina ceramics and silicon nitride ceramics were performed at both room and elevated temperature. The crack opening displacement was also measured during the tests. Then the crack growth characteristics of ceramics were discussed using these experimental data. The results concerned with the crack growth tests are as follows.1.The crack growth rate was found to decrease gradually with a further crack growth, and after some crack extension a crack stopped growing temporary for both ceramics materials.2.It was found from the relationship between COD and stress intensity factor, K,that a crack opening level, Kop, increased with increasing the number of cycles. A little bit wide hysteresis was also observed in these curves. These hysteresis were thought to correspond with the effect of particle bridging on the crack opening behavior. Furthermore, the compliance of sample varied discontinuously with increasing the number of cycles.3.Both the cr … More ack arrest mentioned the above and the variation on the compliance related to the existence of the particle bridging effect near the crack tip. The stress intensity factor at crack tip was reduced by subtracting the friction stress due to this bridging effect.Next, fatigue tests for ceramics coating materials were conducted at both room and elevated temperature. The results for coating materials are as follows.1.While the blasted sample at room temperature has a higher fatigue limit than the annealed sample at the same temperature, the alumina coated sample has a lower fatigue limit that the blasted sample.2.Though the ceramics coated sample shows the elastic deformation at an early stage of fatigue life under a higher applied stress level, the plastic deformation increased with increasing the number of cycles at a middle stage of fatigue life. This behavior corresponded with the cyclic softening behavior of substrate. At a final stage of fatigue life the surface strain of the sample varied discontinuously. This means that the region with no strain due to the delamination among a coat material and a substrate appeared.3.Under a lower applied stress level, the sample was under an elastic state at an early stage, and gradually showed the plastic deformation behavior. At a final stage of fatigue life, however, the plastic strain range of sample decreased with increasing the number of cycles. This means that a single main crack can only propagate under this condition. Less