|Title:||A study on various fabrication routes for preparing multilayered cubic boron nitride films and sp3-like boron nitride films|
|Subject:||Hong Kong Polytechnic University -- Dissertations.|
|Department:||Department of Applied Physics|
|Pages:||1 v. (various pagings) : ill. ; 30 cm.|
|Abstract:||Cubic boron nitride (cBN) has a sp3-bonded structure which leads to excellent mechanical properties. Though cBN-rich films have been successfully fabricated by using many ion assist techniques, the adhesion of the films on most substrate materials is still unsatisfactory. The maximum sustainable thickness of cBN-rich films with good adhesion is widely reported to be around 200 nm, so many practical applications of cBN coatings are hindered. The poor adhesion arises from (i) the appearance of a weak sp2-bonded BN layer prior to the cBN nucleation; (ii) high compressive stresses evolved in the deposits; (iii) possible existence of excessive B causing reaction of the film materials with the moisture in the atmosphere. In this study, we designed a series of deposition schemes in a logical sequence, in order to explore whether some of the above problems can be solved, or other structural forms of BN with potential applications can be made, and to gain more fundamental understanding on the mechanisms controlling the growth of various phases observed in the films prepared. Various fabrication processes were employed according to the following sequence: 1. A single-step process was used to find out the maximum tolerable thickness of the cBN-rich films prepared by our own system, compare with the widely reported result, i.e. 200 nm. 2. A multilayered deposition process was used to add a thick gradient sp2-bonded BN buffer layer in order to support a thicker cBN-rich layer on top. 3. An advanced multilayer process with subsequent annealing process was used to remove the sp2-bonded buffer layer through solid state reaction with a pre-deposited zirconium (Zr) metal layer. 4. Ion assist deposition at unheated condition was used to create composite BN films containing sp3 nanoclusters in a sp2-bonded matrix. Experimentally, a dual ion beam deposition (DIED) technique was used to prepare samples. In this technique, one ion beam was generated to sputter a B target, while another ion beam was produced to bombard the growing film surface. To characterize the film structure, we employed infrared absorption (IR) to analyse the phase composition; X-ray photoelectron spectroscopy (XPS) to determine the elemental composition and chemical state; and transmission electron microscopy (TEM) to directly observe the microstructure. Electron energy loss spectroscopy (EELS) was conducted by both XPS and TEM techniques. For the EELS(XPS) analysis, a new method was developed to quantitatively determine the volume fractions of different BN phases in a film sample. It was assumed that a BN film normally consisted of a cBN phase, a hexagonal BN (hBN) phase, and a highly disordered amorphous BN (aBN) phase. We found that the EELS(XPS) method was more effective in detecting the nano-sized sp3 structure in a film, compared to the IR absorption technique. The EELS method conducted by TEM system can identify whether the BN material in a tiny region (~ 1 nm) is mainly sp2-bonded or sp3-bonded. Results obtained from Process 1 showed that the maximum thickness of cBN-rich film obtainable with our DIED system (183 nm) was consistent with that widely reported in the literature, suggesting that new processes were required to produce thicker cBN-rich films. The application of Process 2 verified our idea that the addition of a thick gradient sp2-bonded BN buffer was relatively deformable, and hence some stresses would be released during the growth of a cBN-rich top layer so as to allow a thicker cBN-rich layer with acceptable adhesion to grow on top. The optimum assist beam energy was identified to be 450 eV (at 680oC), and a simple model taking account of the equilibrium between the generation and annihilation of defects was applied to explain the results. Process 3 showed the effectiveness of removing the sp2-bonded layer, and resulted in a boride / cBN-rich layer film on tungsten carbide substrate. Process 4 produced composite BN films containing sp3 nanoclusters embedded in a sp2-bonded matrix. The IR technique was not sensitive enought to detect sp3 nanoclusters, but their presence was verified by the results of high resolution TEM analysis, EELS(XPS) analysis, EELS(TEM) analysis and hardness measurements. In particular, the sp3 content can be over 30 vol.%, with a hardness of 20 GPa compared to 0.9 GPa of a pure sp2-bonded BN. The influences of the assist beam energy and substrate temperature on the generation of the sp3 nanoclusters were investigated in detail. Interestingly, by observing the annealing effects on the structure and mechanical properties of the films, we concluded that the volume fraction of the sp3 phase dropped with increasing annealing temperature, verifying that some of sp3 nanoclusters were metastable. Two theoretical models were applied to describe the formation mechanisms of these nanoclusters.|
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