| Summary: | 碩士 === 國立清華大學 === 材料科學工程學系 === 104 === Abalone shells are composed mainly of minerals (i.e. calcite), yet it is much tougher as compared to that of calcite. The excellent mechanical property of nacre is attributed to its organized layered-like structure consisting of aragonite platelets and organic materials. Consequently, a hybrid multilayered configuration is inspired from nacre, and a hybrid system combining radio frequency (RF) sputtering and pulsed laser deposition (PLD) is established to synthesize the bio-inspired organic/inorganic layers sequentially in present work. The bio-inspired multilayered thin films are composed of an inorganic layer of 100 nm- and 150 nm-thick TiO2 as well as an organic layer of polyimide (PI) ranging from 5 to 30 nm. Thin films are characterized by an atomic force microscopy (AFM), scanning electron microscopy (SEM) and X-ray diffraction (XRD). Mechanical properties of thin films and nacres are evaluated using nanoindenter and scratch tester. The electrochemical and ball-on-disk wear testers are used to conduct the corrosion and tribological properties of thin films, respectively. The fracture toughness of thin films is characterized via substrate indentation and nano-indentation methods, and the fracture mechanisms and interface phenomenon have also been discussed.
The hardness of deproteined shell decreases to 4.2 from 6.9 GPa after deproteinized process, yet the values are still in range of literature (4~9 GPa) and close to those of calcite around 7 GPa. It means that the protein does not affect the mechanical properties of nacre. The interesting result of multilayers for hardness measurement also shows similar trend to shell while the polymer thickness is less than 5 % organic layer. Then, the hardness and elastic modulus significantly decrease with increasing PI thickness. Based on the results of adhesion test, it can be confirmed the thinner polymer in multilayer, the stronger interface strength. The wear test illustrates that a minimum coefficient of friction around 0.4 is found in 20 nm-thick PI in 100 nm-thick TiO2 series. The tribological enhancement is resulted from the shear zone provided by polymer layer under loads. Meanwhile, thicker PI in multilayered coating blocks the path for corrosion solution so that the 20 nm-thick PI exhibits better corrosion resistance. For fracture toughness, 10 nm-thick PI layer in all multilayers shows better fracture toughness. 100TiO2/10PI multilayer even reveals the maximum fracture toughness of 3.2 MPa∙m1/2, suggesting that more layers in coatings also provide positive contribution on fracture toughness.
The bilayer thickness of hybrid multilayered thin films developed in this study with adequate hardness, good anti-wear ability, corrosion resistance, and better fracture toughness can be used as a guideline for designing bio-inspired thin films.
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