涂覆类cBN磨料性能研究

对比了几种涂覆类cBN磨料的性能及其应用于陶瓷结合剂磨具的性能,通过扫描电子显微镜(SEM)、差热—热重分析仪(DSC-TG)及力学性能测试仪对其进行表征,结果发现:刚玉涂覆cBN磨料的力学性能和热稳定性没有劣化,但与陶瓷结合剂制成磨具抗折强度降低;钛涂覆cBN磨料陶瓷结合剂磨具抗折强度提高,但钛涂覆后cBN磨料力学性能和热稳定性变差;玻璃涂覆cBN磨料的力学性能和热稳定性有所提高,其与陶瓷结合剂在界面处结合紧密,增强了二者之间的把持力,提高了其磨削性能.     

切削刀具用立方氮化硼涂层的研究进展

立方氮化硼( cubic boron nitride,cBN)具有优异的物理力学性能和极高的化学稳定性,可以胜任铁系金属的加工,在高性能切削刀具等领域有着广泛的应用前景.cBN涂层在复杂刀具应用中有着不可替代的作用,由于气相生长高纯度和结合性能的cBN刀具涂层仍存在着较多的技术难题,因此仍然难以得到广泛的应用.本文综述了近20年来气相沉积cBN涂层的研究进展,阐述了物理气相沉积与化学气相沉积cBN涂层的方法与机理,分析了cBN刀具涂层制备与应用的关键问题,结合研究现状指出了cBN刀具涂层研究的发展方向.     

Preparation and characterization of vitrified CeO2 coated cBN composites

The performances of vitrified cBN composites are deeply affected by the wettability of vitrified bonds on cBN particles. CeO₂ coated cBN particles were successfully prepared for the further improvement of the covering and wetting of cBN by vitrified bonds. The microstructure and properties of vitrified cBN composites were characterized by scanning electron microscope (SEM), hot stage microscope (HSM), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and flexural strength. Results showed that the prepared CeO₂ coating on the surface of cBN was uniform and dense. Besides, the improved wettability of vitrified bonds on CeO₂ coated cBN particles accompanied with the formation of Ce–O–Al and N–Si confirmed by XPS were supposed to conduce to enhancing the holding power of the vitrified bonds to cBN particles, which resulted in increasing the flexural strength of vitrified cBN composites by 9.16%. Thus, coating cBN with CeO₂ was a potential and effective method to obtain vitrified cBN composites with higher flexural strength.     

Hydrothermal-Reaction-Assisted Laser Machining of Cubic Boron Nitride

Although the high hardness and chemical stability of cubic boron nitride (cBN) complicate its machining, the mass decrease of cBN in a steam environment at high temperature has been reported. In this study, we investigated hydrothermal-reaction-assisted laser drilling of cBN in various environments. A single-crystalline cBN grain, binder-containing sintered cBN, and binderless sintered cBN were irradiated with an Ar ion laser in water and steam, and in gas atmospheres. The cBN reacted with water, and NH₄⁺ or NH₄⁺-N and boric acid were produced. Hydrothermal-reaction-assisted machining was not effective for binder-containing sintered cBN, but was effective for single-crystalline cBN and binderless sintered cBN.     

Cubic boron nitride films for industrial applications

The effects of kinetic energy, chemical nature of substrates and temperature on the synthesis of cBN films are explored to obtain cBN films with industrial quality. Carbon including amorphous carbon, nanocrystalline and polycrystalline diamond enables deposition of stable, thick and adherent cBN films with characteristic Raman signature. Although temperature has been designated as an unimportant parameter, the deposition at higher temperatures yields higher quality of cBN films. The higher temperature (800 °C) was also employed at cBN deposition on diamond coated tungsten carbide (WC) cutting inserts using plasma enhanced chemical vapor deposition (PECVD). The quality of cBN films grown by PECVD significantly overcomes that prepared by physical vapor deposition (PVD) which is affected in large extent by the lower kinetic energies of particles used in PECVD. The low kinetic energy of particles induces surface growth mechanism which differs from the growth models previously proposed.     

High pressure synthesis of cubic boron nitride from Si-hBN system

Cubic boron nitride (cBN) was synthesized from hexagonal boron nitride (hBN) in the presence of silicon. The cBN forming pressure-temperature region was determined at pressures up to 7.7 GPa. Near perfect octahedral cBN crystals could be synthesized under the P-T conditions near the low temperature boundary of the cBN-forming region. When the temperature was above 1700°C at 6.5 GPa, the transformation rate of cBN from hBN was very high and the cBN crystals had an oriented columnar morphology. This suggests that silicon has strong catalytic ability for cBN formation from hBN. The energy-disperse X-ray analysis (EDXA) identified that silicon was homogeneously distributed in the cBN crystals.     

Densification and Phase Transformation of β-SiAlON–Cubic Boron Nitride Composites Prepared by Spark Plasma Sintering

β-SiAlON–cubic boron nitride (cBN) composites were prepared from β-SiAlON and cBN powders at 1600°–1900°C under a pressure of 100 MPa by spark plasma sintering. The effects of cBN content and sintering temperature on densification and phase transformation of the β-SiAlON–cBN composites were studied. When 10–30 vol% cBN was added to β-SiAlON, the shrinkage rate of the compacts increased. The compacts of β-SiAlON–BN composites originally containing 10–30 vol% cBN ceased to shrink at a temperature lower than that of β-SiAlON and the density of the composites increased. The densification of β-SiAlON–BN composites originally containing >40 vol% cBN was suppressed. The phase transformation of cBN to hexagonal BN in the β-SiAlON–BN composite was inhibited to a greater degree than that in the cBN body.     

Self-sharpening property of porous metal-bonded aggregated cBN wheels during the grinding of Ti–6Al–4V alloys

Grinding process with cubic boron nitride (cBN) superabrasive wheels has been the subject of extensive research during high efficiency and precision machining difficult-to-cut materials in aerospace and aviation industries. However, the grinding performance and tool-life of conventional cBN abrasive wheels are severely affected by the probable macro-fracture and pull-out of cBN grains owing to their anisotropic crystalline structure. In this case, porous metal-bonded grinding wheels coupled with high-performance aggregated cBN abrasive grains were developed to improve tool performance and machined surface integrity. Characterisation of morphologies, including as-sintered aggregated cBN abrasive grains, pore structures and grain wear evolutions, was performed. The grinding ratio, grinding forces, force ratio and ground surface roughness were evaluated through single-grain grinding of Ti–6Al–4V alloys. Experimental results indicated that the porous aggregated cBN wheels had abundant chip storage space and excellent wear resistance. A stable grinding force ratio and small ground surface roughness were obtained during the tool wear tests due to the combined characteristics of microfracture and partial macrofracture of multi-layer cBN particles.     

Synthesis and Characterization of TiN‐coated Cubic Boron Nitride Powders

Titanium nitride‐coated cubic boron nitride (TiN/cBN) composite powders were prepared by nitridizing TiO₂/cBN powders in a NH₃ flow at 950°C. The TiO₂/cBN powders were synthesized via a sol‐gel process using tetra‐butyl titanate and concentrated‐HNO₃‐treated BN powders as starting materials. The techniques of XRD, SEM, TEM, FT‐IR, and TG‐DTA were used to characterize the products and their intermediates. The cBN powders were uniformly coated with TiN nanoparticles. During the nitridization, the morphology of the TiO₂/cBN powders is unchanged. The TiN/cBN powders can be used as starting materials to prepare polycrystalline cBN compacts, or as reinforcements to strengthen metal‐matrix composites.     

Recrystallization behaviour of cubic boron nitride under high pressure

The recrystallization behaviour of micron-sized cubic boron nitride (cBN) was studied by analysing the grain size and morphology of samples treated at 8−16 GPa/1500–2200 °C. The results show that the recrystallization temperature of cBN under a pressure of 8 GPa is approximately 1650 °C and increases by approximately 100 °C with every 2 GPa increase in pressure. Once grain recrystallization starts, the grains grow abnormally quickly as the temperature rises, and the strengthening effects of grain refinement and defect structure are greatly weakened. The recrystallization behaviour of cBN at high pressure is helpful to understand the sintering mechanism and control the microstructure and mechanical properties of sintered polycrystalline cBN compacts. In addition, the melting curve for cBN under high pressure is inferred according to the empirical relationship between recrystallization temperature and melting temperature, and the phase diagram for boron nitride is revised based on this new melting curve.     

On the low-temperature threshold for cubic boron nitride formation in energetic film deposition

The sharp threshold in substrate temperature below which cubic boron nitride (cBN) cannot be formed in energetic film-deposition processes was investigated. We found that cBN could be synthesized below the threshold temperature on top of cBN that had been previously formed above the threshold temperature. That the initial nucleation of cBN is more strongly dependent on temperature than its subsequent growth is suggested. How the structure of the sp²-bonded BN that accompanied cBN growth changed with temperature was also investigated. Lowering the substrate temperature decreased the local ordering within the graphitic planes, and below the threshold temperature the separation of the graphitic planes increased dramatically. How these structural changes may influence the nucleation of cBN is discussed.     

Effect of time on microstructure and hardness of βSiAlON-cubic boron nitride composites during spark plasma sintering

βSiAlON-cubic boron nitride (cBN) composites were consolidated by spark plasma sintering, and the effects of holding time and heating rate on the phase transformation of cBN and Vickers hardness were investigated. The cBN phase transformed into hexagonal BN (hBN) and the hardness decreased with increasing holding time. The phase transformation from cBN to hBN was retarded by increasing the heating rate, resulting in increased hardness.     

High pressure synthesis of tungsten carbide–cubic boron nitride (WC–cBN) composites: Effect of thermodynamic condition and cBN volume fraction on their microstructure and properties

Tungsten carbide (WC) with different amounts of Cubic boron nitride (cBN) were synthesized by High Pressure-High Temperature (HPHT) method. The mapping correlation between thermodynamic condition, cBN addition, and microstructure, mechanical properties of WC–cBN composites was established and analyzed by response surface methodology. The main factors affecting the properties of composites were identified by ANOVA. The optimum thermodynamic condition was calculated. It was found that a minor phase transformation of cBN into the low-hardness hBN occurred at a temperature of 1300 °C and intensified at 1500 °C. The homogeneously dispersed cBN particles in the WC matrix promoted an improvement of hardness and fracture toughness, but the phase transition of cBN and its truss effect can dramatically reduce the mechanical properties. The Vickers hardness and fracture toughness of the well-sintered WC-cBN bulks reached a high value of 34 GPa and 13.6 MPa·m^1/2, which are improved by 17% and 52% respectively compared with the pure WC samples sintered under similar high-pressure level.     

Densification of SiO2–cBN composites by using Ni nanoparticle and SiO2 nanolayer coated cBN powder

Cubic boron nitride (cBN) powder was coated with Ni nanoparticle and SiO₂ nanolayer (abbreviated as cBN/Ni and cBN/SiO₂, respectively) by rotary chemical vapor deposition (RCVD), and compacted with SiO₂ powder by spark plasma sintering at 1473–1973 K for 0.6 ks. The effects of Ni and SiO₂ coatings on the densification, phase transformation of cBN and hardness of SiO₂–cBN composites were compared. The phase transformation of cBN to hBN was identified at 1973 K in SiO₂–cBN/SiO₂ composites, 300 K higher than that in SiO₂–cBN/Ni composites, indicating that SiO₂ retarded the transformation of cBN. The relative density of SiO₂–cBN/SiO₂ with 50 vol% cBN sintered at 1873 K was 99% with a hardness of 14.5 GPa.     

Improvement of the adhesion of sputter-deposited cubic boron nitride films

Cubic boron nitride (cBN) thin films were grown on Si(100) and high-speed steel substrates by reactive r.f. sputtering in an Ar/N₂ discharge using an electrically conducting boron carbide (B₄C) target. The substrate electrode was grounded or operated either with a d.c. or an r.f. power supply. The deposition of cBN can be subdivided into three steps: (1) the growth of a thin, textured, hexagonal boron nitride (hBN) film, (2) the nucleation of cBN and (3) the growth of the cBN phase. As a measure of the cBN content, the ratio of the infrared absorption bands near 1100 cm⁻¹ (cBN) and 1400 cm⁻¹ (hBN) was used. The adhesion of cBN films is still an unsolved problem. Two aspects have to be considered: (1) the high intrinsic stress of the film and (2) the reactivity under humid conditions. We investigated the influence of the thickness, structure and surface roughness of hBN on the adhesion of cBN films. To modify the hBN films, the pressure, substrate bias and Ar/N₂ mixture was varied. Another way of improving the adhesion is plasma treatment of the cBN film directly after deposition. The process variations mentioned above increase the thickness of the adhering cBN films.     

Electron-assisted deposition of cubic boron nitride by r.f. magnetron sputtering

Cubic boron nitride (cBN) has been deposited on silicon (100) substrates by means of radio frequency (r.f.) magnetron sputtering in nitrogen using a hexagonal boron nitride target with the assistance of a simultaneous electron bombardment of the growing surface. Unlike most thin-film deposition processes for cBN, intentional bombardment of the growing surface by ion beams within specific ranges in energy and flux is not required for this process to achieve high-purity cBN films. Fourier transform infra-red (FTIR) spectra of cBN films show a strong absorption band around 1070 cm⁻¹. With electrons bombarding the growing surface at a current density of 140 mA cm⁻² or higher, pure (according to FTIR spectra) cBN films are deposited on silicon substrates at temperatures above 750°C. The effects of electron current density and nitrogen gas pressure on the synthesis of cBN films will be discussed.     

cBN reinforced Y-α-SiAlON composites

Dense α-Sialon–cBN composites were produced by FAST/SPS–sintering at 1575–1625 °C. The hardness of the materials increases only up to 21 GPa for materials with 10 vol.% cBN. On the other hand the fracture toughness increases up to nearly 8 MPa m^0.5 with 30 vol.% cBN. The reason for the increase in fracture toughness is attributed to crack deflection at cBN grains due to the weak bonding of the grains in the matrix. The weak interfaces are also responsible for the moderate increase in hardness. Detailed investigation of the interface between cBN and the matrix was carried out by TEM.     

Real indentation hardness of nano-polycrystalline cBN synthesized by direct conversion sintering under HPHT

The hardness characteristic of nano-polycrystalline cBN synthesized by direct conversion sintering was thoroughly investigated using Vickers and Knoop indenters. It was found that nano-polycrystals consisting of smaller cBN grains increase the elastic recovery of indentations during unloading of the indenters and the diagonal of Vickers indentations and the minor diagonal of Knoop indentations significantly decrease in length. Thus, if a Vickers indenter is used, the apparent hardness value increases, making it impossible to perform an accurate evaluation, e.g. incorrect Vickers hardness values in excess of 80 GPa were obtained from nano-polycrystalline cBN with a grain size of 50 nm or less. On the other hand, it was verified that a Knoop indenter ensures an accurate hardness evaluation even if the constituent grains are fine because its major diagonal length which is used for measurement is less susceptible to elastic recovery. In an accurate evaluation of the hardness of different types of nano-polycrystalline cBN using a Knoop indenter, the hardness of each type of cBN was around 45 GPa, and there was no clear Hall–Petch relationship between hardness and grain size without a slight bell-like correlation. These results suggest that reported hardness values higher than 80 GPa of similar nano-polycrystalline cBN evaluated using a Vickers indenter are incorrect values caused by elastic recovery occurring at the indentation.     

Stoichiometric cBN films deposited by electron-cyclotron-wave-resonance plasma sputtering of hBN

Electron-cyclotron-wave-resonance (ECWR) plasma of pure nitrogen was employed to deposit cBN thin films onto Si(100) substrates by hBN target sputtering. The ion current density and ion energy can be varied fairly independently. Deposition was achieved with ion energies between 70 and 230 eV. The cBN phase was identified with both Fourier transform-infrared spectroscopy (FT-IR) and high-resolution transmission electron microscopy (TEM). Thicker films with nearly 100% cBN phase in the upper layer appeared bright gray or transparent. A maximum thickness of 350 nm for the cBN layer was measured. The film growth follows the aBN→hBN→cBN sequence, with nanoarches being located at the hBN–cBN interface. Energy-dispersive X-ray (EDX) analysis verified the perfect stoichiometry of the deposits. Depending on the processing parameters the films displayed varied morphology. In particular, the surface of the film deposited at 74 eV exhibited large islands with diffuse boundaries on a compact base plane, suggesting the reduction of compressive stress at low ion energy.     

Microstructure and tribological properties of cubic boron nitride films on Si3N4 inserts via boron-doped diamond buffer layers

Cubic boron nitride (cBN) coatings were deposited on silicon nitride (Si₃N₄) cutting inserts through conductive boron-doped diamond (BDD) buffer layers in an electron cyclotron resonance microwave plasma chemical vapor deposition (ECR MPCVD) system. The adhesion and crystallinity of cBN coatings were systematically characterized, and the influence of doping level of BDD on the phase composition and microstructure of the cBN coatings were studied. The nano-indentation tests showed that the hardness and elastic modulus of the obtained cBN coatings were 78 GPa and 732 GPa, respectively. The tribological properties of the cBN coatings were evaluated by using a ball-on-disc tribometer with Si₃N₄ as the counterpart. The coefficient of the friction and the wear rate of the cBN coatings were estimated to be about 0.17 and 4.1 × 10^− 7 mm³/N m, respectively, which are remarkably lower than those of titanium aluminum nitride (TiAlN) coatings widely used in machining ferrous metal. The results suggest that cBN/BDD coated Si₃N₄ inserts may have great potentials for advanced materials machining.