Fracture Behavior of Micro-Sized Ni-P Amorphous Alloy Specimens
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Mat. Res. Soc. Symp. Proc. Vol. 605 © 2000 Materials Research Society
EXPERIMENT
The material used in this study was a Ni- 11.5 wtP amorphous thin film that was electroless-plated on an Al-Mg alloy. As amorphous alloys have isotropic mechanical properties and high corrosion resistance, it is one of the candidate materials for micromachines. The material in this study has been used for hard disk substrates. This material is mass-produced with uniform quality, the roughness of the surface is in the order of nano-meters, and it has been confirmed that there are no surface defects in this material. From these features, this material is considered to be one of the suitable materials for the evaluation of the methodology of micro-sized testing. Specimen Preparation A disk with a diameter of 3 mm was cut from the Ni-P/Al-Mg plate by electric discharge machining. The amorphous layer was separated from the Al-Mg alloy substrate by dissolving the substrate with a NaOH aqueous solution. After these processes, a cantilever beam type specimen was produced on the amorphous layer by focused ion beam machining as shown in Fig. 1.
Fig. 1 Micro-cantilever beam specimen. The thickness of the specimen, B, was 10 ,im. The distance from the loading point to the notch position, L, was 30 •.m. The width of the specimen, W, was 12 j.tm. Schematic diagrams around the notch are shown in Fig.2. A notch was introduced in some specimens with a depth of about 6 p~m. In the other specimens, a fatigue pre-crack of about 3 pm was introduced after the introduction of a 3 iim notch (The total crack length was thus about 6 j.im.). The notch position was 10 p~m from the root of the specimen. The width of the notch was 0.5 j•m, and the notch radius was thus deduced to be 0.25 jim. The fatigue precracks were introduced at a stress ratio of 0.5, a maximum load of 8 inN, and a cyclic frequency of 10Hz using the mechanical testing machine for micro-sized material, which was developed in our previous investigations [5-8].
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LtNotch: 64.mTALIA a(n) - 6um
I___
Notch: 3m A •
I_____
Fatigue
pre-crack:3
I
,
a(nf) n
6m 6
(p=0.25jim) W=12um
W=12um with a notch only
with a fatigue pre-crack
Fig2. Schematic diagrams around the notch. Mechanical Testing Machine for Micro-sized Specimens The mechanical testing machine can apply static and cyclic loading. The specimen can be positioned with an accuracy of 0.1 jim using a precise X - Y stage. The load resolution is 10 jiN, and the displacement resolution is 0.005 jim. The details of the testing machine are described in our previous papers [5-6]. Experimental Procedure Fracture toughness tests were performed on micro-cantilever beam specimens with a fatigue pre-crack, and these with a notch only using the same mechanical testing machine as was used to introduce the fatigue pre-cracks. All tests were performed in air and at room temperature (-20'C) in laboratory environment. Fracture surfaces were observed after the tests using a field emission gun scanning electron microscope (FE-SEM). RESULTS AND
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