7C-6-38L

Fatigue tests of NI-P amorphous alloy microcantilever beams

S.Maekawa*, K.Takashima*, M.Shimojo*, Y.Higo*, S.Sugiura t , B.Pfister * and M.V.Swain * * Precision and Intelligence Laboratory, Tokyo Institute of Technology 4259, Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japan t Nissei Sangyo Co., Ltd. 1-24-14, NishiShinbashi, Minato-ku, Tokyo 105-8717, Japan * Telecomunicatins & Industrial Physics, CSIRO Bradfield Road, West Lindfield, NSW 2070, Australia.

ABSTRACT

Development of micromachines and MEMS (microelectromechanical system) devices are making steady progress at present. The size of components used in these micromachines and MEMS devices is expected to be in the order of p m, and the mechanical properties of such materials may be different from those of bulk materials. Therefore, the evaluation of mechanical properties such as Young’s modulus, tensile strength and fatigue properties is essential for the design of micromachines and MEMS devices. Young’s modulus, fracture stress and hardness of thin films and small elements of silicon single crystals have been measured [la]. In addition to modulus and strength of such mimsized materials, the fatigue properties are extremely important to design micromachines and MEMS devices. However, there have been few fatigue data for micresized materials, and the fatigue properties of such materials are still unclear. In the present study, fatigue life and fatigue crack propagation tests have been performed on Ni-P amorphous alloy microantilever beams by using a newly developed fatigue testing machine [5].

The material used in this investigation was a Ni-Il.Swt%P amorphous thin film, which had been electroless plated on an AI42wtOhMg alloy substrate. The thicknesses of the thin film and the AI- 4.2WhMg alloy substrate were 12 p m and 0.79mmI respectively. A disk with a diameter of 3mm was cut from the Ni-P/AI-Mg sheet by electro discharge machining. The amorphous layer was separated from the AI-Mg alloy substrate by dissolving the substrate with NaOH aqueous solution. Cantilever beam type specimens with dimensions of 30 x 12 x 5 0 p m3 were prepared by focused ion beam machining as shown in Fig.1. A Notch (depth of 3p m) was introduced in some specimens. Cyclic load was applied to the specimens at a constant load ratio (the ratio of minimum and maximum load applied over the fatigue cycle) of 0.1 under load control. The frequency of cyclic load was set at IOHz, and the wave form of the cyclic load was a sinusoidal.

To confirm that cyclic loading caused fatigue fracture of the specimens, fatigue crack propagation tests were performed on 2 specimens having a notch. Figure 2-a shows a FE-SEM (field emissionsun type scanning electron microscope) image of the sp&imen failed under the cyclic loading. Fine equispaced markings are observed on the fatigue surface. These markings are aligned perpendicular to the crack propagation direction (Fig.2-b). Fig.3 shows a fracture surface of a specimen fractured under monotoic loading, and no striation-like markings are found on the fracture surface. Therefore, the markings seen in Fig.2-b are considered to be striations, and the crack is deduced to be propagated by the cyclic loading. Fatigue lie tests were performed on 4 specimens, and Fig.4 shows an S-N curve obtained in these fatigue tests. U in FigA indicates the maximum tensile stress at the circles indicated in Fig.?, and the value of U was calculated by FEM (Finite Element Method). The fatigue strength of Ni-P amorphous alloy micro-sized specimen is determined to be 1.89 GPa from FigA. In this investigation, the fatigue mechanisms and the size effect on the fatigue properties are not clear. However, the newly developed machine and the experimental method used in thii investigation are considered to be effective for the evaluation of fatigue properties of micro-sized specimens.

132

REFERENCES

I . W.N.Sharpe, Jr., Bin Yuan, and R.L.Edwards, J.Microelectromechanical System, Vo1.6., 1997, pp.193- 199 2. J.Mencik, E.Quandt, D.Munz, Thin solid films, V01.287.~ 1996, pp.208-213 3. Ashok VKulkami, B.Bhushan, Thin solid films, Vo1.290-291., 1996, pp.206-210 4. K.Sato, TYoshioka, TAndo, M.Shikda, and TKwabata, Sensors and Actuators A:physical, Vo1.70., 1998, pp.148-152 5. K.Takashima, S.Maekawa, M.Shimojo, YHigo, S.Sugium, B.Pfister and M.VSwain, Proc. Fatigue 99, in press

Fig.1 FE-SEM image of a micro cantilever beam specimen prepared by focused ion beam machining.

Fig.2-b FE-SEM image of markings aligned

fig.2-b

Fig.2-a FE-SEM image of the specimen failed under the cyclic load.

4

3. 5

a 3 LL a \

............................ : ............................. ; ........................... perpendicular to the crack propagation direction. b

t 0 1

Fig.3 FE-SEM image of a fracture surface of a specimen fractured under monotonic loading.

133