Nano Scale Fatigue Crack Propagation Behavior in Micromaterials

近年来，微机电系统（MEMS或微机械）在汽车、航空、人造卫星以及生物医学工业中的应用越来越多，其中机器或部件的机械可靠性必须在10年的典型寿命期间得到保证。然而，关于MEMS材料的强度可靠性以及断裂机制的信息是极其重要的。另一方面，当材料的尺寸减小到微米/纳米尺度时，由于缺陷或孔隙率的减少以及位错的加速发射，材料通常表现出比相同块体材料更强的物理和机械性能。与此同时，MEMS材料由于表面积越来越多地暴露于周围环境而受到更多的环境影响。据报告，近90%的机械失效是由疲劳引起或与疲劳相关。明显 ...更多信息 实际上，如果MEMS器件疲劳并且初始裂纹以大于10 -9 mm/周期的速度扩展，则MEMS器件不能经受超过几千个周期的时间，该速度被定义为体材料的非扩展裂纹的文件阈值增长率。因此，为了确保MEMS器件的安全性，研究疲劳裂纹在其非常缓慢的扩展范围内的扩展行为是至关重要的。然而，这是不现实的，通过使用传统的疲劳试验机在非常缓慢的增长区域的疲劳试验是非常耗时和昂贵的，在本研究中，一种新的超声疲劳方法被用于研究小裂纹的疲劳行为在纳米级的扩展区域，特别是在典型的合金已被用于MEMS器件中的裂纹扩展速率的阈值的存在。应变速率的影响，以及环境因素的影响，如水分和高温的疲劳强度，裂纹萌生和扩展，和断裂机制方面进行了研究。少

In recent years, microelectromechanical systems (MEMS or micro-machines) have found increasing applications in automobiles, aeronautic, man-made satellites as well as biomedical industries, where the mechanical reliability of machines or components has to be secured during a typical life span of〜10 years. However, information regarding the strength reliability as well as the fracture mechanism of MEMS materials is extremely important. On the other hand, when the dimensions of a material are reduced down to micron/nanometer scale, the material usually demonstrates stronger physical and mechanical properties than the same bulk material due to the decrease of defects or porosities as well as the accelerated emission of dislocations. Meanwhile, MEMS materials are more environmentally affected because of the increasing exposure of surface area to the surrounding environment. It was reported that nearly 90 % out of the total mechanical failures were caused by or related with fatigue. Obvious … More ly, MEMS devices cannot stand longer than a few thousands of cycles if the device is fatigued and the initiated cracks propagate at a speed fester than 〜10^-9 mm/cycle, which is defined as file threshold growth rate for non-propagating cracks for bulk materials. Therefore, it is critically important to investigate into the growth behavior of a fatigue crack in its very slow extension range in order to secure the safety of MEMS devices. This is, however, not realistic by utilizing traditional fatigue machines because fatigue tests at very slow growth region are tremendously time consumed and expensive.In the present study, a novel ultrasonic fatigue methodology was employed to investigate fatigue behavior of small cracks in the region of nanometer order propagation, especially the exist of a threshold crack growth rate in typical alloys that have been using in MEMS devices. The effects of strain rate as well as the influences of environmental factors such as moisture and elevated temperature were examined in terms of fatigue strength, crack initiation and propagation, and fracture mechanism. Less