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Title: The study of cubic boron nitride/diamond composite films for sensing and mechanical applications
Other Titles: Li fang dan hua peng/jin gang shi fu he mo de chuan gan yu li xue ying yong yan jiu
Authors: Chong, Yat Ming (莊一鳴)
Department: Department of Physics and Materials Science
Degree: Doctor of Philosophy
Issue Date: 2009
Publisher: City University of Hong Kong
Subjects: Boron nitride.
Diamond thin films.
Notes: CityU Call Number: TA455.N5 C48 2009
xiii, 166 leaves : ill. (some col.) 30 cm.
Thesis (Ph.D.)--City University of Hong Kong, 2009.
Includes bibliographical references (leaves 127-136)
Type: thesis
Abstract: Cubic boron nitride (cBN) is isostructural and isoelectronic to diamond. It has excellent properties being the second best in materials. Cubic BN is the second hardest material and the second most thermally conductive material. However it surpasses diamond in thermal stability, chemical inertness and radiation resistance, which make cBN very interesting for mechanical, optical and electronic applications. Commercial products of cBN is limited to cBN powders produced by high pressure high temperature (HPHT) methods, assisted with various metal catalysts and their residues being left in cBN products. Tool inserts are then fabricated by molding and sintering cBN powders with aid of metal binders. Therefore, novel synthesis method of cBN are required for production of cBN with high phase purity, complex surface shapes, combination of cBN into superlattice structures, cBN patterning for specific mechanical, optical or electronic functions including cost effective fabrication. Cubic BN films have been synthesized by low pressure methods. However, cBN deposition involves energetic ion bombardment which gives rise to high residual film stress. The deposition on most of substrates is accompanied with the evolution interfacial precursor aBN/tBN layers which are soft in nature, and have poor adhesion to the substrates. The stress and presence of the precursor layers thus restrains the ultimate thickness of cBN to ~200 nm. Very recently, thick and adherent cBN films with high phase purity were deposited on polycrystalline diamond in our group using electron cyclotron resonance microwave plasma chemical vapor deposition (ECR MPCVD) employing fluorine chemistry. This achievement offers opportunities to investigate the superior physical and chemical properties of pure cBN films, and apply them for sensing, electronic optical and optoelectronic as well as mechanical applications. In this work, the chemically inert surfaces of cBN are demonstrated to serve as biological and chemical sensors via photochemical surface functionalization. To serve as a biological sensor, the cBN surfaces are functionalized with allylamine photochemically. The amine-terminated cBN films are further connected to glutaraldehyde and DNA probes. The amine-modified cBN surface shows selectively to its complementary DNA which demonstrates the surface modified cBN as DNA sensors. The distinctive properties, especially their chemical stability and inertness upon the exposure to various body fluids, make cBN to be highly desired in biological sensing applications. In addition to biological sensing, cBN film-based sensors for detecting Hg2+ ions were developed by functionalizing cBN surfaces with dansyl chloride. The dansyl chloride-functionalized cBN films show fluorescence quenching upon the addition of metal ions in an ethanol solution, and a high selectivity and characteristics being dependent on the concentration of Hg2+ ions. More importantly the cBN film chemical sensors are reusable for multiple sensing tests. In this study, cBN films are also demonstrated to be an effective solar-blind deep UV photodetector. A metal semiconductor metal (MSM) photodetector based on cBN has been fabricated. The device is insensitive in visible irradiation while its photoconductance is changed by four orders of magnitude in a deep UV region. This deep ultraviolet cBN detector shows a sharp cut off wavelength at 193 nm which is the shortest value of semiconductors photodetectors reported so far. By depositing a diamond transition layer, the thickness and adhesion of cBN are significantly enhanced and reach the level which meets the requirement for tooling and tribological applications. The diamond layer serves as a mediating layer that provides strong adhesion to the substrate and allows the direct growth of cBN. The usually observed aBN/tBN precursor layers are absent. Therefore this work also focuses on preparing this diamond/cBN bilayer structures on commercial cutting inserts. Thus cBN has been synthesized on diamond precoated WC:Co cutting inserts by using ECR MPCVD and fluorine chemistry. The effect of the growth parameters on the structure and phase composition of cBN films is studied systematically herein. Films with thickness of 2.8 μm and high phase purity have been prepared via optimizing the growth parameters. The cBN and diamond show hardness of 70 and 90 GPa, respectively as determined on the polished surfaces. Finally this work investigates tribological behaviors of different BN films. The cBN films in friction with counterparts of hard silicon nitride balls show high wear resistance. The tribological behavior and wear mechanism of cBN films are discussed.
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