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Synthesis and characterization of cubic boron nitride

City University of Hong Kong
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  • Boron Nitride
  • Chemistry
  • Design
  • Physics


Cubic boron nitride (cBN) is an extraordinary material, which possesses similar physical and chemical properties like diamond and even it is superior to diamond in several aspects of applications. This work focuses on the synthesis of cubic boron nitride (cBN) films on silicon substrates by a radio-frequency magnetron sputtering and electron cyclotron resonance microwave plasma assisted chemical vapor deposition (ECR-MPCVD). The former technique belongs to the methods designated as physical vapor deposition (PVD) while the later technique is classified as chemical vapor deposition (CVD). In both cases the variable deposition conditions were optimized for the maximum content of cBN phase. The structure and physical properties of cBN films prepared were investigated in terms of deposition parameters including substrate bias, gas flow rate, gas composition, substrate temperature and duration of deposition. For example, introducing He into H2 plasma in deposition reduced the bias voltage required for cBN nucleation and growth via Penning effect. Cubic BN films prepared on silicon substrates using magnetron sputtering grew with soft interfacial layers which thickness and incubation time needed for the cBN formation varied with the deposition parameters employed. The soft interfacial layer was amorphous BN (aBN) normally followed by turbostratic BN (tBN) on top which cBN nucleated as proved by Fourier transform infrared (FT-IR) spectroscopic analysis. The surface structure and properties of these BN films were also studied with surface sensitive methods including x-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and electron energy loss spectroscopy (EELS). These three independent surface analytical techniques indicate very thin hBN layers on top of the cBN films suggesting the continuous conversion of hBN to cBN structure through structural densification. The phenomenon of the structural densification supports the subplantation model of cBN growth. The thicker cBN films (>200 nm) prepared by PVD magnetron sputtering were sensitive to ambient and readily delaminated due to the high internal stress induced by the ion bombardment and hydroscopic properties of the transition (incubation) aBN/tBN layers. On the contrary the cBN films prepared by ECR CVD yield distinct Raman spectra characteristic to cBN phase. The Raman signature unavailable in the case of the first technique (magnetron sputtering) demonstrates that cBN films are thicker and quality of cBN crystallites (referring to the size of crystallites and defect density) is much higher when compared to the films prepared by magnetron sputtering. The characteristic thickness of cBN films was 1 – 2 micrometers. Except substrate temperature, the effect of bias voltage on the cBN properties was thoroughly investigated as well. Cubic BN is the best material for machining hard steels and all ferrous materials. However, due to the poor adhesion of cBN films to the substrates they have never been used in any practical application. Therefore the important step for the first ever implementation of cBN film into practice is this work which pioneers deposition of cBN films on diamond interfacial buffer layers employing fluorine chemistry using ECR-MPCVD process. The choice of the diamond films was guided by the striking similarity between the cBN and cubic diamond lattices, which have an identical structure but a slightly different lattice constant (that of cBN larger by 1.34% than that of diamond). Diamond films were prepared on silicon with variable thickness of 1 to 10 μm. On top of the diamond films cBN films were grown (with thickness ranging from 200 nm to 2 μm) in ECR plasma made of He-Ar-N2-BF3-H2 gas mixtures. The growth was performed at 950 oC and a low substrate bias of –20 V. This work shows that cBN directly grows on diamond substrates without the conventional nucleation stage however using extremely low bias voltage (–20 V). Hence the resulting energy is small and rather consumed on inducing phones than subplantation and densification of BN structure. The soft interfacial aBN/tBN layers are absent and the interface between the diamond and cBN is seamless. The cBN deposition is characteristic with columnar growth in which each cBN column extends the diamond column from its base to the film surface. The seamless epitaxial growth of cBN on diamond occurs because of the similarity in lattice parameters and the condition of reducing the surface energy when diamond is continuously covered by cBN structure. This explains the importance of the substrate selection. The high quality of the CVD cBN films is confirmed by the distinct Raman peak, which has not been possible to observe for nanosized cBN films grown by PVD ion assisted techniques. The hardness and the elastic modulus of these cBN films on diamond are the highest ever measured (70 and 830 GPa, respectively) and are only to second to that of diamond. The coating combination of the cBN/diamond provides extraordinary film properties because beneath the extremely chemically resistive and thermally stable cBN is the hardest material, diamond. Diamond imparts extreme mechanical supporting capacity which is important in all mechanical applications, including tooling and tribological use.

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