Effect of granulated sugar as pore former on the microstructure and mechanical properties of the vitrified bond cubic boron nitride grinding wheels

For vitrified bond cubic boron nitride (CBN) grinding wheel, introduced pores play a very important role for its mechanical properties and performance. In this paper, granulated sugar was used as pore former of the vitrified bond in CBN grinding wheel. The effects of content and particle size of the granulated sugar on the porosity and the flexural strength of the sintered vitrified bond CBN wheel samples have been investigated. It was found that the porosity of the vitrified bond CBN wheel is positively correlated with the content of the granulated sugar. The smaller and more irregular shaped pores are uniformly distributed in the bond when the content of granulated sugar is between 1 and 3 wt.%. Larger and more non-uniform pores and pore channels appear as the content of granulated sugar is increased from 5 to 7 wt.%. The flexural strength of the vitrified bond CBN wheel specimens decreases with an increase in pore former’s content and the porosity. With the increase of pore former’s particle size at the content of 3 wt.%, the flexural strength reaches to a peak value of 49 MPa with average particle size of granulated sugar is 250 μm. When the average size of granulated sugar is from 100 to 125 μm, the pores’ size is similar with the size of pore former and distributed homogeneously. The larger granulated sugar with the size from 160 to 500 μm can introduce different size of pores which could be smaller or larger than the size of pore former.     

High-pressure sintering of cubic boron nitride

Cubic boron nitride (cBN) powder was compacted at high pressure and high temperature using a solid solution of titanium carbide and titanium nitride (TiC _x N_1–x ) as binding material in the presence of a small amount of aluminium. Different compositions of (TiC _x N_1–x ), 0<x<1 were used as binders. The weight percentage of cBN, TiC _x N_1–x and aluminium were optimized and found to be critical; any marked deviation from these optimized values deteriorated the quality of compacts. Various high-pressure sintering parameters such as pressure, temperature and sintering time, etc, were optimized for this binder. The compacts were also characterized using X-ray diffraction, scanning electron microscopy and microhardness measurements. The microstructural and X-ray diffraction observations indicated no marked changes in the compacts as the value ofx in TiC _x N_1–x was varied, but the microhardness was found to depend on the value ofx.     

The study of dry grinding of steel cams using cubic boron nirtride tools

Experimental studies that simulate grinding of hardened alloyed steel cams have demonstrated that these workpieces can be machined with a minimum work surface heating by using highly porous cubic boron nitride wheels with the oil mist cooling or without cooling. It has been found out that for minimizing the thermodynamic stress level in the dry grinding mode it is advisable to preset machining conditions with maximum possible depth of cut and minimum workpiece rotational speed.     

Synthesis of cubic boron nitride with boron nitride powder formed from triammoniadecaborane

Cubic BN was synthesized under high temperature and pressure conditions from BN powder formed by reaction of triammoniadecaborane (TAD) with ammonia. The BN powder formed from TAD and ammonia had a low degree of ordering. The crystal lattice of the BN powder increased in regularity with increasing synthesis temperature and time for the reaction of TAD with ammonia. The conversion yield of cubic BN at 1300 °C and 6.5 GPa in the presence of AIN increased with decreasing of reaction temperature of TAD and ammonia from 1000–700 °C. Cubic BN decreased in yield with increasing reaction time of TAD and ammonia at 800 °C. BN powder pre-heat treated at 1550 °C had a crystallite size,L _c, of 22 nm, and was converted to cubic BN in a 43% yield at 1300 °C and 6.5 GPa for 10 min. The activation energy for cubic BN synthesis from BN powder-20 mol% AIN was 97 kJ mol^–1, when the starting BN was synthesized at 800 °C. The conversion yield of cubic BN from the disordered BN-20 mol% AIN was 100% after heat treatment at 1300 °C and 6.5 GPa for 20 min.     

Shock compaction of cubic boron nitride powders

C-BN powders with different grain sizes were dynamically compacted by explosive shock loading using approximate peak pressures from 33 to 77G Pa. The density and the microhardness of the resulting c-BN compacts were strongly dependent upon the grain size of the c-BN powders used as the starting materials. The best c-BN compacts, with 98% of the theoretical density and microhardness of 51.3G Pa, were obtained from the coarse c-BN powder (40 to 60m). In the compacted fine c-BN powder (2 to 4m) conversion of the c-BN to low density forms of BN at a residual temperature degraded the interparticle bonding significantly. X-ray line-broadening analysis of the compacted c-BN powders indicated that the residual lattice strain increased with the increase in grain size of the starting powder, while the crystallite size was independent of the grain size.     

立方氮化硼薄膜表面的XPS研究

研究立方氮化硼薄膜表面的性质对于研究立方氮化硼薄膜的成核机理和应用,具有重要的价值.本文用XPS对立方氮化硼薄膜表面进行研究,并对有关问题进行了讨论.XPS分析表明,立方氮化硼薄膜表面除了B、N外,还含有C和O.从XPS谱图计算得到含有立方相的氮化硼薄膜的N/B为0.90,较接近于氮化硼的理想化学配比11;不含立方相的氮化硼薄膜的N/B为0.86,离氮化硼的理想化学配比11较远.计算表明立方氮化硼薄膜的顶层六角相的厚度约为0.8nm.     

Intrinsic metallic and semiconducting cubic boron nitride nanofilms

We show by density functional theory calculations with both hybrid and semilocal functionals that cubic boron nitride (111) nanofilms are intrinsically metallic and even turn into semiconductors once the thickness is less than 0.69 nm, which is in sharp contrast to the known insulating nature of boron nitride materials. The exceptional metallic or semiconducting band gap is due to a combined effect of thickness-dependent inbuilt electric polarization and labile near-gap states unique in the polar nanofilms. The band gap and dipole moment of the nanofilms can be further significantly tuned by applying an in-plane strain. These distinguished features of the boron nitride nanofilms are robust to surface passivation and can be enhanced by hybridizing with diamond films, thereby opening an exciting prospect of using the versatile cubic nanofilms in future electronic and piezoelectric devices.     

Increasing workpiece peripheral speed as a means for improving ergonomics of cylindrical up grinding with cubic boron nitride tools

The author pioneered in experimentally finding out that an increase in the workpiece peripheral speed proper in a wide range invariantly leads to a definitely better surface roughness, i.e., smaller thickness of cut, suggesting an improvement in the process ergonomics, in cylindrical centered up grinding under elastic attrition and linear wear conditions, with cubic boron nitride tools of various specifications, on the grinders differing in static stiffness (8.3 and 12.0 N/μm), with a wheel speed ranging from 32 to 91 m/s. A comparative assessment of grinding processes with a fixed and controllable levels of forced vibrations at idling speed has demonstrated that the weakening of the positive influence of the speed factor on the surface roughness and thickness of cut with increasing workpiece speed proper and/or wheel speed is due to the vibration-induced suppression—an increasing amplitude of the wheel-workpiece vibration displacements at rotational speeds of the wheel, workpiece, and other elements of the grinder-workpiece-tool system. Some recommendations are offered of how to improve the ergonomics of cylindrical up grinding with cBN tools by increasing the workpiece speed proper.     

Comparative analysis of performance of cubic boron nitride and microcrystalline alumina tools in profile grinding of form cutters

A comparative analysis of performance of cubic boron nitride, microcrystalline alumina, and white fused alumina wheels has been carried out in profile grinding of long-length HSS profile broach under production conditions. In a particular case, where grinding is accompanied by impact loading and the wheel has to be frequently dressed to provide precise profiling, the vitrified cubic boron nitride wheel has demonstrated an essentially better performance in terms of removal rate, minimum time for dressing cycles, and overall labor input in shaping a working profile of the broach.     

Spark plasma sintering of Co-WC cubic boron nitride composites

25 vol.% cubic boron nitride (cBN) added tungsten carbide (WC) powders containing 6 wt.% Co (WC-6Co) were densified by spark plasma sintering (SPS) technique under different experimental conditions and the effect of cBN addition on the microstructure, mechanical properties and thermal conductivity were investigated. Over 99.5% theoretical density was achieved for WC-6Co-cBN composites sintered at 1300 °C, under 75 MPa pressure for 7.5 min. Under these conditions, cBN → hBN phase transformation was not observed.     

High pressure synthesis of cubic boron nitride from amorphous state

Cubic-zincblende-type boron nitride was synthesized from amorphous state at pressures higher than 6 GPa and at temperatures higher than 800°C, without any planned addition of catalysts. Also, wurtzite type boron nitride was formed although the amount was small.     

Molecular dynamics study of deposition mechanism of cubic boron nitride

We have performed molecular dynamics simulations of bombardment of graphitic boron nitride (gBN) by energetic boron and nitrogen particles in order to examine the roles of ion bombardment in ion/plasma-assisted deposition of cubic boron nitride (cBN) thin films. We have found that the interaction of the energetic particles with gBN creates four-fold coordinated local structures (sp³-formation) inside gBN. We have also found that clusters of sp³-formations are created as a result of successive bombardment, some of which have cBN-like structures. On the basis of these results, we propose an atomic-scale model of cBN nucleation in which successive sp³-formation converts gBN into cBN.     

Laser physico-chemical vapour deposition of cubic boron nitride thin films

A laser physico-chemical vapour deposition (LPCVD) technique was developed based on the interaction of an ultraviolet laser beam with a boron nitride target and borazine gas to synthesize cubic boron nitride (CBN) thin films on silicon substrates. The process involved a hybrid of pulsed laser ablation (PLA) of a solid HBN target and chemical vapour deposition (CVD) using borazine as a feed stock. The films were characterized with scanning electron microscopy, X-ray diffraction and infrared spectroscopy. Results indicate that the thin films consisted of almost single-crystalline CBN structures and that the film quality in terms of adherence, particulate density and smoothness was excellent. The purity and crystal structure of target material, laser beam wavelength and energy fluence were the key variables that controlled the film characteristics. In contrast to LPCVD, the conventional PLA method did not generate CBN films.     

Molecular dynamics study of deposition mechanism of cubic boron nitride

We have performed molecular dynamics simulations of bombardment of graphitic boron nitride (gBN) by energetic boron and nitrogen particles in order to examine the roles of ion bombardment in ion/plasma-assisted deposition of cubic boron nitride (cBN) thin films. We have found that the interaction of the energetic particles with gBN creates four-fold coordinated local structures (sp³-formation) inside gBN. We have also found that clusters of sp³-formations are created as a result of successive bombardment, some of which have cBN-like structures. On the basis of these results, we propose an atomic-scale model of cBN nucleation in which successive sp³-formation converts gBN into cBN.     

Growth conditions of the cubic phase cBN in boron nitride films

The nucleation and the subsequent coalescence period of the cubic phase cBN in sputter deposited BN-films is characterized by a shrinking of the film thickness. This is due to the transition of hBN into the denser cBN-phase which occurs inside a highly textured hBN base layer. The corresponding variation of the film thickness with the deposition time is described by a quantitative model. Full BN-stoichiometry in the hBN base layer is shown to be a mandatory condition for the nucleation process and the following growth of the cubic BN-phase. An increase of the substrate temperature fosters the incorporation of nitrogen into the growing film and, thus, the achievement of the stoichiometry condition.     

Epitaxy of cubic boron nitride on (001)-oriented diamond

Cubic boron nitride (c-BN), although offering a number of highly attractive properties comparable to diamond, like hardness, chemical inertness and a large electronic bandgap, up to now has not found the attention it deserves. This mostly has to do with preparational problems, with easy chemical routes not available and, instead, the necessity to apply ion-bombardment-assisted methods. Hence, most of the c-BN samples prepared as thin films have been nanocrystalline, making the prospect of using this material for high-temperature electronic applications an illusion. Although heteroepitaxial nucleation of c-BN on diamond substrates has been demonstrated using the high-pressure-high-temperature technique, none of the low-pressure methods ever succeeded in the epitaxial growth of c-BN on any substrate. Here, we demonstrate that heteroepitaxial c-BN films can be prepared at 900 degrees C on highly (001)-oriented diamond films, formed by chemical vapour deposition, using ion-beam-assisted deposition as a low-pressure technique. The orientation relationship was found to be c-BN(001)[100]||diamond(001)[100]. High-resolution transmission electron microscopy additionally proved that epitaxy can be achieved without an intermediate hexagonal BN layer that is commonly observed on various substrates.     

Mechanism of nucleation and growth of cubic boron nitride thin films

The mechanism and the crystallography of the nucleation and growth of cubic boron nitride (c-BN) films deposited on 100-oriented silicon substrate by RF bias sputtering have been studied by means of cross-sectional high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy. Both methods provide experimental information showing no sp²-bonded BN layer formation in the subsurface region of c-BN phase. This is clear evidence for layer-by-layer homoepitaxial growth of cubic boron nitride without graphitic monolayers in the near-surface region of the film. The turbostratic boron nitride (t-BN) consists of thin sub-layers, 0.5–2 nm thick, growing in such a way that a sub-layer normal is almost parallel to the growth direction. t-BN also comprises a large volume fraction of the grain boundaries with high interface energies. The present result and the finding by Shtansky et al. [Acta Mater. 48, 3745 (2000)], who showed that an individual sub-layer consists of parallel lamellae in both the hexagonal (h-BN) and rhombohedral (r-BN) configurations, demonstrate that high intrinsic stress in the films is due to the complex structure of sp²-bonded BN. The crystallography of c-BN films indicates heteroepitaxial nucleation of cubic phase on the graphitic BN structural precursor. The present results are consistent with stress-induced c-BN formation.