Investigation on Fatigue Strength of Nano/Micromatereials Subjected to Very High Cycle Fatigue

硅基MEMS/NEMS的应用已经大大改善了我们的生活。微机械部件经常受到千兆赫和兆赫频率的循环应力，积累了大量的应力循环。为了确保这些微小系统的性能和可靠性，必须阐明疲劳机制，以解释材料在感兴趣的尺寸尺度上的及时和循环依赖的退化。在本研究中，利用一种新型应变控制、压电驱动的可安装在扫描电子显微镜（SEM）内外的独立拉伸疲劳系统，研究了LP_CVD多晶硅薄膜在10^7-10^9循环域内的疲劳强度和断裂机制。用CCD摄像机观察并记录了疲劳小裂纹行为。得到了以下结果，并讨论了可能的机理。A)本研究开发的压电驱动拉伸疲劳系统具有位置传感器，该传感器采用MEMS技术制造，可用于监测试件中心在循环加载过程中的运动。B)利用商业有限元软件ANSYS对独立拉伸试样的几何设计进行了优化，确定了试样中心梁处的最大应力和最小面外位移。c)发现LP-CVD多晶硅薄膜即使在室温（~20℃）和大气环境（~26-35%的湿度）下也会产生疲劳。但S-N图的分布非常分散，说明LP-CVD多晶硅薄膜的疲劳强度对微加工引起的试样形貌差异和湿度偏差非常敏感。D)试样失败后，几乎没有明显的损伤演变，可以通过光学显微镜或电子显微镜识别。此外，有限断口分析认为疲劳形核和早期生长是局部的，其他断口形态以脆性为主。换句话说，疲劳寿命的大部分消耗在微小裂纹的形核上。少

The application of silicon-based MEMS/NEMS has already resulted in significant improvements in our lives. Micromechanical components are routinely subjected to cyclic stresses at kilo- and megahertz frequencies, accumulating large numbers of stress cycles. To ensure performance and reliability of these tiny systems, fatigue mechanism must be elucidated to account for the timely and cyclically dependent degradation of the material at the size-scales of interest.In the present study, fatigue strength and fracture mechanism of LP_CVD polycrystalline silicon thin films were investigated in the domain of 10^7-10^9 cycles by using a novel strain controlled, piezoelectric actuated free standing tensile fatigue system that can installed either within or outside a scanning electron microscope (SEM). Fatigue small crack behavior were observed and taped by CCD video camera. The following results were obtained and the possible mechanism was discussed.A) The piezoelectric actuated tensile fatigue s … More ystem developed in the present study possesses a position sensor, which was fabricated by MEMS technology and can be used to monitor the movement of specimen center during cyclic loading.B) The geometry design of the free standing tensile specimen was optimized with the help of commercial FEM software, ANSYS, in terms of the maximum stress available at the center beam of the specimen and the minimum out of plane displacement occurred accordingly.C) It was fund that LP-CVD polycrystalline silicon thin films fatigued even at room temperature (~20℃) and at atmospheric environment (~26-35% of humidity). However, S-N diagram scattered tremendously, implying that the fatigue strength of the LP-CVD polycrystalline silicon thin films was quite sensitive to the difference in specimen morphology caused by micromachining as well as the deviation of humidity.D) Specimens failed with little remarkable damage evolution that can be recognized by either an optical or an electron microscope. Furthermore, limited fractographic analysis assumes that fatigue nucleation and early growth are localized, and the other fracture morphologies are most brittle featured. In other words, the most of fatigue life was consumed in nucleating tiny cracks. Less