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Title: Mechanical properties of amorphous tetrahedral carbon
Other Titles: Fei jing si mian ti tan de li xue xing neng
Authors: Lam, Chun Wing (林俊榮)
Department: Dept. of Physics and Materials Science
Degree: Master of Philosophy
Issue Date: 2005
Publisher: City University of Hong Kong
Subjects: Amorphous substances -- Mechanical properties
Carbon -- Mechanical properties
Notes: CityU Call Number: TA455.C3 L36 2005
Includes bibliographical references (leaves 87-93)
Thesis (M.Phil.)--City University of Hong Kong, 2005
xii, 98 leaves : ill. (some col.) ; 30 cm.
Type: Thesis
Abstract: Amorphous tetrahedral carbon (ta-C) films were deposited on silicon substrates using a Filtered Cathodic Vacuum Arc (FCVA) technique. The amorphous ta-C films were prepared with various thicknesses ranging from 37 to 120 nm. Their structural properties were tailored by the ion kinetic energies induced by the variable bias voltage within the range of –30 to –150 V. Studying the prepared films shows that the kinetic energy of ions is closely associated with the material structures and mechanical properties of the films evolved. The deposition rate of amorphous ta-C varies with altering the bias voltage. The magnitude of deposition rate is first increased and after passing a maximum at the –800 V bias is then reduced due to self-sputtering process. The scratch resistance, hardness, and tribological properties of amorphous ta-C films deposited on silicon substrates were investigated by nano-scratch, nano-indentation and ball-on disk sliding tests. Since hardness of super-hard thin films is particularly difficult to determined, two different, i.e., conventional and extrapolation approaches were undertaken. Both approaches give the highest hardness for the films prepared at the –90 V bias, which correlates with the highest density of sp3 bonding. Hardness of amorphous ta-C determined by conventional methods is 47 GPa while the method established on further calculation and extrapolation of data yields hardness of 60 GPa. This discrepancy results from underestimation of the substrate effect in conventional method though the measurement is referenced to fused silica standard. Deeper insight on hardness measurement provides investigation of the substrate effect on hardness employing ta-C films with different thickness but with equal structures as presumed from equivalent deposition conditions. Tribological performance of amorphous ta-C films was also examined. The lowest friction coefficient and rate were 0.101 and 0.0285 mm3/cycle, respectively. However, these parameters were also affected with surface roughness and film thickness. The surface roughness increases with the increase in film thickness over whole investigated range of thicknesses (37 – 120 nm). The minimal root-mean-square (RMS) roughness of the films concerned was 7 Å. Phase structures of the ta-C films were characterized by Raman spectroscopy. The Raman spectra collected from amorphous ta-C films were fitted with a single skewed Lorentzian peak described by the Breit-Wigner-Fano lineshape. The magnitude of coupling coefficient (Q –value) of the films deposited at a substrate bias of –90 V is the minimum, which implies that these films are characteristic with highest sp3 and lowest sp2 contents. In addition, the films prepared at the – 90 V bias shows the Raman G peak centered at the lowest wavenumber, which corresponds to the structures with the most abundant sp3 bonding when compared to other films. Thus the correlation among spectral features, structural properties and mechanical properties is evident.
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