| Cubic boron nitride (cBN) is a kind of new material which has excellent properties in mechanical, thermal, optical, chemical, and other fields. Its hardness and thermal conductivity is close to those of the diamond’s. Due to the high oxidation resistance, good chemical stability and inertia, cBN shows special superiorities while processing ferrous metal materials. Therefore, cBN cutting tool not only can process all kinds of hard and wear-resistant materials, but also be used for titanium alloys, pure nickel, tungsten, and other materials processing. Meanwhile, with being applied to automotive, aerospace, shipbuilding, steel, machine tools, mold, and all kinds of mechanical processing field, cBN abrasive is becoming the most preferred in grinding machining.According to experimental results, it is found cBN single crystal surface is always covered with a layer of molten material in the synthesis.It is considered that using this catalyst layer contains catalyst and hexagonal boron nitride (hBN) under high temperature and high pressure. Due to its direct contact with cBN single crystal, the fine structure of catalyst layer can reflect relevant information of cBN transformation and growing environment. As a result, by the characterization of catalyst layer, important reference basis can be obtained to explore cBN single crystal transition mechanism. In this paper, using scanning electron microscope (SEM), X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM), the microstructure and phase structure of the catalyst layer are hierarchical characterized and analyzed to study the transformation and growth process of cBN single crystal.Meanwhile, using auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS), the fine structure of cBN single crystal catalyst layer is characterized, and distribution regularity of sp2 and sp3 hybridization of B and N are emphatically analyzed. Moreover, in order to provide evidence for cBN single crystal transition mechanism, electron energy loss spectroscopy (EELS) are used for the further stratified characterization of catalyst layer to study the change of the electronic structure of B and N atoms and the distribution of sp2, sp3 hybridization and to calculate the relative content of B elements in different layers.By means of XRD and HRTEM for the characterization of cBN single crystal catalyst layer, it is found each layer exists cBN, hBN, Li3BN2, the content of cBN reduce from the inner layer near the cBN single crystal to outer, but the original catalyst Li3N is not found. It follows that in the process of the synthetic Li3N reacts with hBN to generate Li3BN2. However, the highest content of Li3BN2 is the middle layer, the inner is lower, and the outer is the least. Based on the phase distribution, it is thought that with the catalysis of Li3BN2, molten hBN in catalyst layer is turned into short-range ordered cBN under high pressure and high temperature (HPHT), and the crystal nucleus which gradually grows into large cBN single crystal is formed in the inner layer with the highest content of cBN growth primitives. In addition, because Li3BN2 is direct contact with cBN growth primitives, it is concluded hBN is transformed to cBN under the catalysis of Li3BN2.Using XPS and AES for the characterization of B and N elements in the catalyst layer, the results show that the spectra of the inner layer is largely consistent with pure cBN, which explains the phase composition is the same with pure cBN single crystal. However, the phase of outer layer is quite different from cBN. It can be thought content of sp3 hybridization increases and content of sp2 hybridization gradually reduces while closing up to cBN single crystal, namely hBN content gradually reduces and cBN increases. Through EELS the catalyst layer is further stratified characterized, by the method of double window the contents of B-sp3 of each layer are calculated. The results show that the content of inner, middle and outer layer of B-sp3 are 79.53% ,67.24% and 63.47% respectively. Through comparing the spectra of samples, it is found B-π* peak has high strength in the outer structure, and intensity of B-π* peak reduces in the inner structure, which shows that cBN content in the inner and hBN content in the outer layer are the most, namely, from the outer to the inner, the electronic structure of BN has gradually transformed from sp2 hybridization into sp3 hybridization.Combining with the experimental results, it is thought that firstly the original catalyst Li3N reacts with the raw material hBN to generate Li3BN2 under HPHT, then hBN is catalyzed in the melt by Li3BN2 and transformed into cBN. As the synthesis process going on, in the area... |
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