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行业的应用越来越多，在这些行业中，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材料的强度可靠性和断裂机制的信息是非常重要的。另一方面，当材料的尺寸缩小到微米/纳米尺度时，由于缺陷或孔隙的减少以及位错的加速发射，材料通常表现出比相同块状材料更强的物理力学性能。同时，MEMS材料受到的环境影响更大，因为其表面积暴露于周围环境的面积越来越大。据报道，近90%的机械故障是由疲劳引起的或与疲劳有关。更重要的是，如果MEMS器件疲劳并且初始裂纹以超过~ 10^-9 mm/周期的速度扩展，则MEMS器件不能承受超过几千次循环，这被定义为块体材料非扩展裂纹的文件阈值增长率。因此，研究疲劳裂纹在其极慢扩展范围内的扩展行为对于保证MEMS器件的安全至关重要。然而，使用传统的疲劳试验机是不现实的，因为在非常慢的生长区域进行疲劳测试是非常耗时和昂贵的。本文采用一种新颖的超声疲劳方法，研究了纳米级扩展区域小裂纹的疲劳行为，特别是在MEMS器件中使用的典型合金中存在阈值裂纹扩展速率。从疲劳强度、裂纹萌生与扩展、断裂机理等方面考察了应变速率的影响以及湿度、高温等环境因素的影响。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