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Title: Deposition and characterization of cubic boron nitride diamond composite films
Other Titles: Li fang dan hua peng jin gang shi mo de chen ji yu jian ce
Authors: Chong, Yat Ming (莊一鳴)
Department: Dept. of Physics and Materials Science
Degree: Master of Philosophy
Issue Date: 2006
Publisher: City University of Hong Kong
Subjects: Boron nitride
Diamond thin films
Notes: CityU Call Number: TA455.N5 C47 2006
Includes bibliographical references (leaves 86-94)
Thesis (M.Phil.)--City University of Hong Kong, 2006
xii, 101 leaves : ill. (some col.) ; 30 cm.
Type: Thesis
Abstract: This work presents the latest and the most advanced technology for synthesis of cubic boron nitride (cBN) films, and their characterization. Cubic BN is a synthetic material which was made in 1950-s for the first time using a high pressure high temperature (HPHT) method. Couple years later, synthesis of thin films was also demonstrated. These films were too thin, very unstable and suffered from poor quality and delamination problems. Since then the technology of cBN films has stagnated. The quality of cBN films has not improved for nearly a half century and many researchers therefore deserted the field. Nevertheless the technology of cBN thin films has progressed by introducing several innovative technological approaches but only in recent several years. The major breakthroughs account the introduction of fluorine assisted growth process, lowering the kinetic energies of particles closer to thermal energies, passivation of boron dangling bonds at substrate interface and employing carbon substrates or buffer layers. In all these technological improvements, City University of Hong Kong has played pioneering roles and a part of these efforts are also results presented in this work. Incentives behind the development of cubic boron nitride films are cBN properties and many potential applications. HPHT cBN was commercialized, however, it can be only prepared in powder forms which are used as abrasive materials. Powders are also molded and cemented with metal binders to form machining tools. The cemented cBN products are thus not composed of pure cBN. The larger cBN crystals than ~1 mm are unavailable and cBN films has not been commercialized because problems from which they suffer. Cubic BN is extremely attractive materials because of it extreme properties. Its many properties are very similar to those of diamond. Cubic BN is the second hardest and the second most thermally conductive material just next to diamond. It is far more superior to diamond in thermal and chemical stability particularly in contact with molten ferrous material and oxidation. In contrast to diamond it is piezoelectric, and as a wide band gap semiconductor, it can be doped for both p- and n-type conductivities. The detailed analysis of cBN properties suggests a wide range of applications including mechanical, chemical, electronic and photonic use. The extreme thermal and chemical properties suit it for space, fusion reactor applications which are related to exploring the space and generation energy, for example, by nuclear synthesis of light elements. Therefore cBN should be treated as a multi-functional material. The implementation of cBN films into practice has however been hindered by soft and mechanically weak aBN/tBN interlayers forming prior to the cBN growth and large internal stress which is magnifies by energetic ions which otherwise promote the formation of cBN bonding. The same energetic ions also induce defects associated with stress level, secondary nucleation centers that prevent the development of larger cBN crystallites. The overall film thickness of cBN films has then been limited to about 200 nm. The drawbacks above made practical use of cBN films very challenging. This work however shows that some practical applications with the developed film quality are already feasible. The fluorine technology established in our laboratories and within this work enables to produce thick BN films (>1μm) with high content of cubic phase. The cBN films are deposited from complex plasmas induced by electron cyclotron resonance microwave plasma (ECR MP) CVD. The ECR plasma is formed in an environment composed of five precursor gases. Nitrogen and boron trifluoride are gas precursors for growing boron nitride films. Fluorine and hydrogen components mediate the deposition. The former one selectively etches non-cubic phases and its reactivity is compensated by hydrogen. Hydrogen on the other hand also stimulates the film growth. Inert argon ions contribute to the overall momentum transferred to surface atoms of growing films to open surface bonds for incorporation of arriving activated radicals and promote the formation of sp3 bonds. Helium enhances plasma density, which in return allows reduction of the bias voltage and ion kinetic energy of impinging particles. The low threshold bias voltage of –30 V enhances the quality of cBN crystallites which obviously is evident from visible Raman spectra collected form the cBN structures. At cBN growth on silicon, however humid sensitive aBN/tBN interlayers are inevitably formed due to the discrepancies in physical parameters particularly cBN–silicon lattice mismatch. The deposited cBN films delaminate after a period of 1 to 2 months. On the other hand, the cBN deposition on carbon substrates led to understanding of the role of carbon atoms at passivation of boron dangling bonds in addition to the compatibility of cBN and sp3 bonded carbon structures. Therefore polycrystalline diamond (polyD) films were introduced herein as buffer layers on silicon and other substrates prior to cBN deposition. At tuned conditions, the interfacial polyD films constrain the formation of amorphous and hexagonal BN (aBN/hBN) phases and moderate discrepancies between the substrate and cBN film in their physical parameters. The aBN and hBN phases are virtually undetectable by conventional techniques, Fourier transform infrared (FTIR) and visible-Raman spectroscpies. The FTIR analyses thus indicate 100 % pure cBN phase in BN films. The analysis on nano- and atomic-scales by high-resolution transmission electron microscopy (HRTEM) is consistent with spectroscopic data and show epitaxial relationship between the polyD and cBN. These conditions led to the synthesis of thick cBN films (~3.6 μm) which are stable at ambient. The films also yield visible-Raman peaks which are usually not observed at analysis of cBN films prepared by other technologies except DC jet using analogous CVD process. The disclosed local heteroepitaxial growth suggests viability for growing large thin film crystals without interfacial constrains, i.e., formation aBN/hBN precursor layers. The developed method of synthesis then allows engineering diamond/cBN superlattices for enhancement of mechanical and electronic properties and further development of novel electronic devices operating in harsh environments. The work also proves that conclusion on unimportance temperature are incorrect. Systematic study the temperature parameters in a range of 200 -1100 oC in this work shows that hasty conclusions of many previous works on unimportance of the temperature parameter are not entitled. The nucleation and growth of cBN at a variety of temperatures, for example, at cBN at 130 oC and subsequent grow at room temperature or using temperature in a neighborhood of 600 oC may lead to such statements. These syntheses were abstracted from the concepts of their effects on the quality films, crystallinity, stress, formation of interfacial precursor layers, the presence of other BN phases on top of cBN, overall film thickness and others. This work demonstrates that the thick films (on μm range) prepared at temperature of 1100 oC are characteristic with fused and hardly notable interfaces, whereas the film prepared at 600 oC confined interfacial voids and the film synthesized at 400 oC tended to delaminate. These and following analysis disclose that the cBN films synthesis require the substrate temperature at least 800 oC to sustain thick cBN film on substrates and provide films with highly performing properties. In this context the role of substrate is also vital. The surface roughness is another parameter which governs the cBN nucleation particularly at physical vapor deposition processes. It was reported that polyD with rough and faceted surface promotes the formation of non-cubic BN and delays the cBN nucleation time. Amorphous BN phase is evolved in the valleys between the polycrystalline grains particularly at PVD process where higher biases and kinetic energies of particles are used. The formation of interfacial aBN then can be restricted if the substrate surface is smooth. Therefore this work also examines the cBN synthesis on nanocrystalline diamond (nanoD), which is characteristic with inherent smoothness, and the same time preserves similar properties as diamond. NanoD is also very analogue in nanocrystalline structure. Alike cBN/polyD composite films, thick and adherent cBN films were formed on nanoD while the overall deposition rate is twice faster than that found at the cBN deposition on polyD. The smoother surface is certainly favorable for mechanical applications including tools and tribological films since these films will not require further polishing. The hardness and elastic modulus of the films were 70 GPa and 800 GPa, respectively. A critical load of 420 mN was found corresponding to the film adhesion strength by a nanoschratch test method. The value can be used as reference for other measurements that use similar testing instruments and methodology for determination of adhesion strength. The cBN films with diamond substrates or buffer layer can easily be prepared with thickness of 3–4 μm. Since the films thickness is not restricted and the non-cubic phases do not emerge the films in polyD/cBN layer configuration were also used prepared on tungsten carbide (WC) cutting inserts by the methodology which is described above briefly and in detail in following chapters. The cutting test performed on coated and referenced uncoated cutting inserts revealed that the adhesion and properties of the films are already suitable for some industrial applications through the cutting tools failed. The failure is however in the WC region whereas the separation as well as any cracks at cBN-diamond and diamond–substrate interfaces were not observed. The quality of cBN coating approaches the level suitable for implementation of cBN films into practice.
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