| Summary: | 碩士 === 逢甲大學 === 機械工程學所 === 100 === Most materials utilized in micro-electro-mechanical devices are classified as brittle ceramic materials. The strength of a brittle material is related to the distribution of defects, which randomly changes. Thin film multilayers are commonly adopted in advanced micro-electro-mechanical devices such as micro cantilevers. One of the main reasons that cause the failure of the multilayer is the difference between coefficients of thermal expansion and the interface delamination resulted from mechanical stresses. However, there is limited knowledge about the degradation of these microscopic materials and the failure mechanism of multilayer structure under stresses.
This study includes the setup of mixed mode fracture toughness experiments and measurement of the critical strain energy release rate. Four kinds of multilayer coatings are manufactured on 4-inch silicon wafer by micro-electro-mechanical processing with sample dimension of 40mm × 5mm, which are (a)Si3N4/SiO2/Si, (b)Si3N4/Si, (c)PZT/Pt/Ti/Si, and (d)PZT/Al/Si. The goal is to develop a novel mixed mode double cantilever beam (MMDCB), which is capable of measuring the strain energy release rate, namely fracture toughness, in both tensile and in-plane shearing modes and the single-mode fracture toughness (opening or in-plane shearing). Three parameters are discussed: (i) stress buffer layer, (ii) grain growth layer for piezoelectric film, and (iii) thickness of previous two layers.
Results show that the average maximum force is proportional to the increase of phase angle for the four samples and the average force reaches its maximum at ψ = 60°, which are 366.50 N, 361.87 N, 335.10 N, and 287.70 N for samples (a), (b), (c), and (d) respectively. Phase angle is related to the extent of mixed mode. For fracture toughness, (a), (b), (c), and (d) samples have the values of 185.04, 62.66, 626.34, and 139.10 J/m2 at ψ = 0°(pure tensile rupture). A comparison between samples (a) and (b) shows that the stress buffer layer in (a) helps eliminate the residual stress and increase the fracture toughness. In comparison of samples (c) and (d), the adoption of Pt/Ti as grain growth layer in (c) leads to better fracture toughness. Comparing (a) and (b) to (c) and (d), we find that the latter group possess better fracture toughness at small phase angles due to the grain growth layer for piezoelectric film. At large phase angles, ψ=45°for example, the fracture toughness of samples (a), (b), (c), and (d) is 1927.90, 808.60, 2944.98, and 2137.04 J/m2, respectively, implying that the composition of grain growth layer for piezoelectric film can better resist the shear stress.
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