Anisotropic Fracture Behavior of Electroless deposited Ni-P Amorphous Alloy Thin Films
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Anisotropic Fracture Behavior of Electroless deposited Ni-P Amorphous Alloy Thin Films Kazuki Takashima, Akio Ogura, Yusuke Ichikawa and Yakichi Higo Precision and Intelligence Laboratory, Tokyo Institute of Technology, 4259, Nagatsuta, Midori-ku, Yokohama, 226-8503, JAPAN ABSTRACT Fracture tests have been carried out for an electroless deposited Ni-P amorphous alloy thin film with different crack growth directions. Cantilever beam type specimens with dimensions of 10 x 10 x 50 µm3 were prepared from a Ni-P amorphous thin film and notches with different directions, which are perpendicular and parallel to the deposition growth direction, were introduced by focused ion beam machining. Fatigue pre-cracks were introduced ahead of the notches. Fracture tests were performed using a mechanical testing machine for micro-sized specimens. Fracture behavior is different between the two types of specimens. As KIC values were not obtained because the criteria of plane strain were not satisfied for this size of the specimen, the provisional fracture toughness KQ values were determined. The KQ value of the specimen with crack propagation direction being perpendicular to the deposition growth direction was 4.2 MPam1/2, while that with crack propagation direction being parallel to the deposition growth direction was 7.3 MPam1/2. This result suggests that the electroless deposited Ni-P amorphous alloy thin film has anisotropic fracture properties. INTRODUCTION Microelectromechanical systems (MEMS) are expected to be applied in many scientific and technological fields such as information and biomedical technologies. These MEMS devices are usually fabricated from a thin film deposited on a substrate by suitable surface micromachining techniques, and the micro-sized elements prepared from a thin film layer are used as mechanical components. The evaluation of fracture toughness of thin films is then extremely important to ensure the reliability of MEMS devices. The fracture toughness measurements of such thin films have been attempted by a nanoindentation technique [1] and an on-chip type test which includes both a specimen and a comb type electrostatic actuator on one Si chip [2]. In the nanoindentation technique, fracture toughness of a thin film on a substrate can be measured, but it is rather difficult to measure fracture toughness of free standing micro-components. In the on-chip type testing, only fracture toughness for “in-plane” crack propagation of films can be determined, and it is almost impossible to measure fracture toughness for “out-of-plane” crack propagation of films. Micro-elements on MEMS devices are considered to be subjected to load in both the direction of “in-plane” and “out-of plane” of the thin film. The fracture toughness values for both in-plane and out-of-plane are thus required to be evaluated for actual design of MEMS devices, as the fracture toughness of thin films prepared by sputtering or other deposition techniques has been considered to have anisotropy [3]. In this study, micro-sized cantilever type spe
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