Fracture behavior of micro-sized specimens prepared from an amorphous alloy thin film at ambient and elevated temperatur
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Fracture behavior of micro-sized specimens prepared from an amorphous alloy thin film at ambient and elevated temperatures K. Takashima, R. Tarumi and Y. Higo Precision and Intelligence Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, 226-8503, Japan ABSTRACT Fracture behavior of micro-sized cantilever beam type specimens prepared from an electroless deposited Ni-P amorphous alloy thin film has been investigated at ambient and elevated temperatures. Cantilever beam type specimens with dimensions of 10 x 12 x 50 µm3 were prepared from an electroless deposited Ni-P amorphous alloy thin film and notches were introduced by focused ion beam machining. Fatigue pre-cracks were introduced ahead of the notches. The introduction of fatigue pre-crack and fracture toughness tests were carried out using a mechanical testing machine for micro-sized specimens. The temperature of the specimen was controlled from room temperature to 473 K using a newly developed heating system. Compared with room temperature, fracture toughness increased approximately 40 % at 373 K but decreased 19 % at 473 K. The increase of fracture toughness at 373 K is considered to be related with the formation of nano-sized crystals and the decrease of fracture toughness at 473 K is considered to be due to the growth of crystals. It is required to consider the fracture behavior obtained in this investigation when designing actual MEMS devices using electroless deposited amorphous films.
INTRODUCTION Amorphous alloy thin films prepared by deposition or sputtering are expected to be potential candidate materials for micro elements in MEMS devices because of their isotropic and excellent mechanical properties and high corrosion resistance. Therefore, the evaluation of mechanical properties of micro-sized amorphous thin films is important for the practical application of MEMS devices. Several mechanical properties such as fracture toughness [1], fatigue life and crack growth properties [2, 3] and corrosion fatigue properties [4] of a micro-sized amorphous alloy have already measured. In particularly, fracture toughness is one of the most important properties to design actual micro-sized machine or MEMS devices. In addition, the temperature of substrate is considered to increase up to approximately 473 K during service in MEMS devices. Amorphous alloys are in a thermally non-equilibrium state at room temperature so that structural changes such as medium range ordering or formation of nano-sized crystals may occur at elevated temperature even if the temperature is lower than its crystallization temperature [5-7]. These structural changes may affect the fracture properties of amorphous alloy. It is therefore important to investigate fracture properties of amorphous alloy thin film at elevated temperature. However, mechanical testing machine for micro-sized materials equipped with temperature controlling system has not been developed yet. The aim of this study is to develop a fracture toughness testing apparatus for micro-sized specimen
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