Nano Scale Fatigue Crack Propagation Behavior in Micromaterials

近年来，微电子机械系统(MEMS或微型机械)在汽车、航空、人造卫星以及生物医学行业得到了越来越多的应用，在这些行业中，机器或部件的机械可靠性必须在大约10年的典型寿命内得到保证。然而，有关MEMS材料的强度可靠性以及断裂机理的信息是极其重要的。另一方面，当一种材料的尺寸减小到微米/纳米级时，由于缺陷或孔洞的减少以及位错的加速发射，该材料通常表现出比相同的块体材料更强的物理和力学性能。同时，由于MEMS材料的表面积越来越大，对周围环境的影响也越来越大。据报道，近90%的机械故障是由疲劳引起的或与疲劳有关。显而易见的…更确切地说，如果MEMS器件疲劳，并且起始裂纹的扩展速度超过~10^-9 mm/周期，则该器件不能持续超过几千个周期，该速率被定义为块体材料的非扩展裂纹的文件阈值增长率。因此，研究疲劳裂纹在其非常缓慢的扩展范围内的扩展行为对于确保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，因为在非常慢的扩展区进行疲劳试验是非常耗时和昂贵的。在本研究中，采用一种新的超声疲劳方法来研究纳米级扩展区域的小裂纹的疲劳行为，特别是在已经用于MEMS器件的典型合金中存在阈值裂纹扩展速率的情况。从疲劳强度、裂纹萌生和扩展以及断裂机制等方面考察了应变率的影响以及湿度和高温等环境因素的影响。较少

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