Fracture and Fatigue of Microelements for Micro/Medical Applications

Abstract

Microelectromechanical systems (MEMS) are expected to be applied to micro-photonics and bio-medical devices such as optical switches for electro-optical communications and micro-catheters for brain surgery. The size of the components used in such devices is considered to be in the order of microns. Therefore, the evaluation of mechanical properties of micro-sized materials are essential for practical applications of such MEMS devices. In particular, the fracture and fatigue properties of micro-sized components are of crucial importance to the structural integrity and long-term reliability of actual MEMS devices. In order to evaluate fracture and fatigue properties of such micro-sized specimens, we have developed a new type mechanical testing machine for micro-sized specimens, which can apply small amount of static and cyclic loads to the specimens. In this investigation, fracture and fatigue tests were carried out for micro-sized specimens prepared from an electroless deposited Ni-P amorphous alloy thin film using the mechanical testing machine which we have developed and the size effects on the fracture and fatigue crack growth behavior have been discussed. Cantilever beam type specimens (10×12×50(μm)^3) with notches were prepared from a Ni-P amorphous thin film by focused ion beam machining. This specimen size is approximately 1/1000 of ordinary sized specimen. Fatigue crack growth tests were carried out in air at room temperature under constant load amplitude using the testing machine for micro-sized specimens. Fracture tests were performed for the specimens with fatigue pre-cracks ahead of the notches. Fatigue crack growth resistance curves were obtained from the measurement of striation spacing on the fatigue surface and closure effects were observed even for micro-sized specimens. Once fatigue crack growth occurs, the specimens were failed after several thousand cycles. This indicates that the fatigue life of micro-sized specimens is dominated by a crack initiation. This also suggests that enen micro-sized surface flaw may be an initiation site of fatigue crack and this will shorten the fatigue life of micro-sized specimens. As the results of fracture toughness tests, plane strain fracture toughness, K_<IC>, values were not obtained since the criteria of plane strain were not satisfied for this size of specimens. As the plane strain requirements are determined by stress intensity, K, and yield stress of the material, it is rather difficult for micro-sized specimens to satisfy these requirements. Plane stress and plane strain dominated regions were clearly observed on the fracture surfaces and their sizes were consistent with those estimated by fracture mechanics calculations. This indicates that fracture mechanics is still valid for such micro-sized specimens. It is required to consider the results obtained in this investigation when designing actual MEMS devices.