Elastic Constants and Graphitic Grain Boundaries of Nanocrystalline CVD-Diamond Thin Films: Resonant Ultrasound Spectros
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Elastic Constants and Graphitic Grain Boundaries of Nanocrystalline CVD-Diamond Thin Films: Resonant Ultrasound Spectroscopy and Micromechanics Calculation Hirotsugu Ogi1, Nobutomo Nakamura1, Hiroshi Tanei1, Ryuji Ikeda2,3, Masahiko Hirao1, and Mikio Takemoto3 1 Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan 2 Asahi Diamond Ind Co Ltd, Res & Dev, Chiba 290-0515, Japan 3 Faculty of Science and Engineering, Aoyama Gakuin University, Kanagawa 229-8558, Japan ABSTRACT Using resonant-ultrasound spectroscopy coupled with laser-Doppler interferometry, we determine the independent elastic constants of nanocrystalline CVD-diamond thin films with thickness between 2-12 µm. They are deposited on oriented monocrystal silicon substrates by the hot-filament methane/nitrogen CVD method. The diagonal components of the elastic constants are smaller than those of microcrystalline CVD diamond films and bulk diamond. However, the off-diagonal component is larger. We attribute these observations to the presence of sp2-bonded graphitic phase at grain boundaries. A micromechanics model assuming inclusions of thin graphitic plates consistently explains the observations. INTRODUCTION Nanocrystalline diamond (NCD) originates an emerging materials-science field because of their remarkable properties such as high stiffness, high hardness, enhanced electrical conductivity, and flat surface, which greatly improve surface-acoustic-wave (SAW) devices, field-emission transistor, and machine tools for high-precision processing. Recently their microstructure and bond configuration are studied by transmission-electron microscopy [1-4], Raman-spectroscopy [1-5], and electron energy-loss spectroscopy [3]. These efforts reveal that NCD films consist of sp3-bonded diamond grains and sp2-bonded grain boundaries. Therefore, it is expected that strongly-bonded diamond grains in NCD films are less strongly connected via sp2 bonds. Such a specific microstructure can show anomalous elastic properties. However, few reports appear concerning the elastic constants of NCD films because of the difficulty of the measurement. The purpose of this study is to measure the independent components of the elastic-constant matrix of NCD thin films and analyze the results with micromechanics modeling. For this, we use resonant-ultrasound spectroscopy with laser-Doppler interferometry (RUS/Laser method) [6-8], which determines independent elastic constants of anisotropic thin films. Several methods are reported for evaluating the elastic constants of thin films, including microtensile tests, microbending tests, and nanoindentation method. However, these previous methods involve difficulties for determining the thin-film elastic constants. First, they are severely affected by dimension errors. Second, some of them involve ambiguous gripping conditions. Third, they cannot determine independent elastic-constant component simultaneously: Polycrystalline thin films usually show elastic anisotropy between the film-growth direction an
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