Boron Nitride Grains Research Articles

Thermal Conductance along Hexagonal Boron Nitride and Graphene Grain Boundaries

We carried out molecular dynamics simulations at various temperatures to predict the thermal conductivity and the thermal conductance of graphene and hexagonal boron-nitride (h-BN) thin films. Therefore, several models with six different grain boundary configurations ranging from 33–140 nm in length were generated. We compared our predicted thermal conductivity of pristine graphene and h-BN with previously conducted experimental data and obtained good agreement. Finally, we computed the thermal conductance of graphene and h-BN sheets for six different grain boundary configurations, five sheet lengths ranging from 33 to 140 nm and three temperatures (i.e., 300 K, 500 K and 700 K). The results show that the thermal conductance remains nearly constant with varying length and temperature for each grain boundary.

Laser surface texturing of a WC-CoNi cemented carbide grade: Surface topography design for honing application

Abstract Abrasive effectiveness of composite-like honing stones is related to the intrinsic surface topography resulting from the cubic boron nitride (CBN) grains protruding out of the metallic matrix. Within this framework, Laser Surface Texturing (LST) is implemented for replicating topographic features of a honing stone in a WC-base cemented carbide grade, commonly employed for making tools. In doing so, regular arrays of hexagonal pyramids (similar to CBN grains) are sculpted by a laser micromachining system. Micrometric precision is attained and surface integrity does not get affected by such surface modification. Finally, potential of laser-patterned cemented carbide tools, as alternative to conventional honing stones, is supported by successful material removal and enhanced surface smoothness of a steel workpiece in the abrasive testing.

Abrasive wear of a single CBN grain in ultrasonic-assisted high-speed grinding

Research into the single grain cutting mechanism is important for understanding complex grinding mechanisms. Based on the characteristics of ultrasonic vibration, the motion equation of the grain is established, and the generated trajectory is theoretically analyzed. By adopting the method of combining high-speed grinding technology with ultrasonic vibration, abrasive wear forms of single cubic boron nitride (CBN) grains under common and ultrasonic conditions are studied. Further studies are conducted on the influence of the grain itself and the main grinding parameters on abrasive wear. Research shows that the main forms of abrasive wear during ultrasonic-assisted grinding are shearing wear and removing wear. However, common grinding leads to micro-crushing wear and a small amount of abrasion wear; the different forms of wear correspond to different grinding force signals. The greater the initial grain protrusion height, the greater is the abrasion of the protrusion; for the same grain protrusion height, the abrasive wear due to ultrasonic-assisted grinding is larger than that due to common grinding. As the grinding depth increases, the abrasive wear of both processing modes increases; however, in the case of ultrasonic machining, the abrasive wear increases slowly and is larger than that under common grinding. This study provides a certain decision basis for real-time monitoring of the ultrasonic-assisted high-speed grinding process. Additionally, it provides guidance and reference for the manufacture and selection of the grinding wheel and for the selection of reasonable processing parameters.

Compressive Strength and Interface Microstructure of PCBN Grains Brazed with High-Frequency Induction Heating Method

In order to prepare monolayer brazed superabrasive wheels, the polycrystalline cubic boron nitride (PCBN) grains were brazed to AISI 1045 steel matrix with Ag–Cu–Ti filler alloy using the high-frequency induction heating technique. The compressive strengths of brazed grains were measured. Morphology, chemical composition and phase component of the brazing resultant around PCBN grain were also characterized. The results show that the maximum compressive strength of brazed grains is obtained in the case of brazing temperature of 965 °C, which does not decrease the original grain strength. Strong joining between Ag–Cu–Ti alloy and PCBN grains is dependent on the brazing resultants, such as TiB2, TiN and AlTi3, the formation mechanism of which is also discussed. Under the given experimental conditions, the optimum heating parameters were determined to be current magnitude of 24 A and scanning speed of 0.5 mm/s. Finally, the brazing-induced residual tensile stress, which has a great influence on the grain fracture behavior in grinding, was determined through finite element analysis.

Electrical and Thermal Transport in Coplanar Polycrystalline Graphene-hBN Heterostructures.

We present a theoretical study of electronic and thermal transport in polycrystalline heterostructures combining graphene (G) and hexagonal boron nitride (hBN) grains of varying size and distribution. By increasing the hBN grain density from a few percent to 100%, the system evolves from a good conductor to an insulator, with the mobility dropping by orders of magnitude and the sheet resistance reaching the MΩ regime. The Seebeck coefficient is suppressed above 40% mixing, while the thermal conductivity of polycrystalline hBN is found to be on the order of 30-120 Wm-1 K-1. These results, agreeing with available experimental data, provide guidelines for tuning G-hBN properties in the context of two-dimensional materials engineering. In particular, while we proved that both electrical and thermal properties are largely affected by morphological features (e.g., by the grain size and composition), we find in all cases that nanometer-sized polycrystalline G-hBN heterostructures are not good thermoelectric materials.

Experimental study of single cubic boron nitride grains in the ultrasound-assisted high-speed grinding

The kinematic characteristics, grinding force, surface quality of workpiece surface, and wear of abrasive particles were studied by theoretical analysis and experimental study on the single cubic boron nitride abrasive particles under ultrasonic-assisted high-speed grinding. Under the condition of the same grinding parameters, the motion characteristics and the grinding forces of the two machining modes of general grinding and ultrasonic-assisted grinding are compared and analyzed. Research shows that the ultrasonic vibration is applied in the common external circular grinding on grinding particle movement characteristics changed obviously, grinding particle trajectory of variable length, cutting groove width wider, thereby improving the grinding efficiency and the grinding removal rate; ultrasonic assisted under high speed grinding, the grinding force is higher than that of common grinding force is small, efficiency of grinding under ultrasonic processing mode is much higher than ordinary grinding, the surface quality of the workpiece has improved markedly.

Water-assisted growth of large-sized single crystal hexagonal boron nitride grains

A water-assisted chemical vapor deposition method is developed for the fast growth of large-sized h-BN single crystal domains.

Surface Topography Quantification of Super Hard Abrasive Tools by Laser Scanning Microscopy

Abstract Non-conventional super hard abrasive tools are made of composite materials containing super hard grains, e.g., diamond or cubic boron nitride (CBN) grains, bound by a metallic constitutive phase. These tools are usually produced by means of sintering, and are widely applied in the abrasive machining processes of modern manufacturing, especially in precision machining. The abrasive grains, which induce the material removal processes, are embedded in the metallic binder. They emerge as a consequence of self-dressing, resulting in a self-sharping effect. Therefore, the cutting surface of the tool displays an irregular topography. Quantification of surface topography scenario may supply valuable information to evaluate and understand its correlation to wear mechanisms. In this study, an experimental protocol consisting of five steps: specimen preparation, surface scanning, image assembly, image digital processing and surface quantification, was proposed and validated by characterizing two CBN honing tools used for precision machining: B151/L2/2010/50 (B151) and B91/128/x44/35 (B91) CBN honing stones. It involved the use of laser scanning microscopy and digital imaging processing for assessing significant dimensional, geometrical, and positional properties of CBN grains at the surface of super hard abrasive tools. It was shown that surface topography quantification is an effective method to evaluate and obtain the defined parameters. However, smaller grains may require images with higher resolution; thus, scanning must be refined. Finally, a critical comparative analysis of the experimental results attained for the studied tools pointed out honing stone B91 as more appropriated than B151 one for achieving a higher machining quality of the workpiece.

Szlifowanie jednostronne ściernicami o spoiwie niklowym z ziarnami z regularnego azotku boru

Szlifowanie jednostronne ściernicami o spoiwie niklowym z ziarnami z regularnego azotku boru

Joining interface and compressive strength of brazed cubic boron nitride grains with Ag-Cu-Ti/TiX composite fillers

Ag-Cu-Ti/TiX (TiX=TiB2, TiN or TiC) composite filler materials, instead of pure Ag-Cu-Ti alloy, were developed to improve the comprehensive mechanical performance of brazed joints of cubic boron nitride (CBN) grains/bonding layer/steel matrix. This article mainly concerns the effects of TiX addition on the joining interface and compressive strength of brazed CBN grains. The results demonstrate that, due to the variation of chemical activity of Ti atoms induced by TiX addition into the brazing system, the brazing reactions, especially chemical resultants produced at the joining interface between CBN grain and Ag-Cu-Ti alloy, are restrained to some extents. In general, the TiN particles show the greatest suppression effect on the brazing resultants, while the TiC particles have the weakest effect, and TiB2 particles have the medium effect. The optimum reinforcement of the composite filler is finally determined as the TiB2 particles with the content of 8wt%, with which the average compressive strength of brazed CBN grains reaches 15 N, which is nearly the same high as that of original CBN grains.

Preparation and Characterization of Ni Spines Grown on the Surface of Cubic Boron Nitride Grains by Electroplating Method.

Cubic boron nitride (cBN) is widely applied in cutting and grinding tools. cBN grains plated by pure Ni and Ni/SiC composite were produced under the same conditions from an additive-free nickel Watts type bath. The processed electroplating products were characterized by the techniques of scanning electron microscopy (SEM), X-ray diffraction (XRD) and thermoanalysis (TG-DTA). Due to the presence of SiC particles, there are some additional nodules on the surface of Ni/SiC plated cBN compared with the pure Ni plated cBN. The unique morphology of Ni/SiC plated cBN should attain greater retention force in resin bond. Moreover, the coating weight of cBN grains could be controlled by regulating the plating time. cBN grains with 60% coating weight possess the optimum grinding performance due to their roughest and spiniest surface. In addition, Ni spines plated cBN grains show good thermal stability when temperature is lower than 464 °C. Therefore, the plated cBN grains are more stable and suitable for making resin bond abrasive tools below 225 °C. Finally, the formation mechanism of electroplating products is also discussed.

Surface and subsurface characterisation in micro-milling of monocrystalline silicon

Abstract This paper presents an experimental investigation on surface and subsurface characterisation of micro-machined single-crystal silicon with (100) orientation. Full immersion slot milling was conducted using solid cubic boron nitride (CBN) and diamond-coated fine grain tungsten carbide micro-end mills with a uniform tool diameter of 0.5 mm. The micro-machining experiments were carried out on an ultra-precision micro-machining centre. Formal design of experiments (DoE) techniques were applied to design and analysis of the machining process. Surface roughness, edge chipping formation and subsurface residual stress under varying machining conditions were characterised using white light interferometry, SEM and Raman microspectroscopy. Tens of nanometre-level surface roughness can be achieved under the certain machining conditions, and influences of variation of cutting parameters including cutting speeds, feedrate and axial depth of cut on surface roughness were analysed using analysis of variance (ANOVA) method. Raman microspectroscopy studies show that compressive subsurface residual stress and amorphous phase transformation were observed on most of the micro-machined subsurface, which provides evidence of ductile mode cutting. Surface and subsurface characterisation studies show that the primary material removal mode is ductile or partial ductile using lower feedrate for both tools, and diamond-coated tools can produce better surface quality. Silicon brain implants were fabricated with good dimensional accuracy and edge quality using the optimised machining conditions, which demonstrated that micro-milling is an effective process for fabrication of silicon components at a few tens to a few hundreds of micron scale.

Synthesis of large single-crystal hexagonal boron nitride grains on Cu-Ni alloy.

Hexagonal boron nitride (h-BN) has attracted significant attention because of its superior properties as well as its potential as an ideal dielectric layer for graphene-based devices. The h-BN films obtained via chemical vapour deposition in earlier reports are always polycrystalline with small grains because of high nucleation density on substrates. Here we report the successful synthesis of large single-crystal h-BN grains on rational designed Cu-Ni alloy foils. It is found that the nucleation density can be greatly reduced to 60 per mm(2) by optimizing Ni ratio in substrates. The strategy enables the growth of single-crystal h-BN grains up to 7,500 μm(2), approximately two orders larger than that in previous reports. This work not only provides valuable information for understanding h-BN nucleation and growth mechanisms, but also gives an effective alternative to exfoliated h-BN as a high-quality dielectric layer for large-scale nanoelectronic applications.

Comparative investigation on wear behavior and self-sharpening phenomenon of polycrystalline cubic boron nitride and monocrystalline cubic boron nitride grains in high-speed grinding

High-speed grinding experiments were conducted on a nickel-based superalloy Inconel718 with the monolayer brazed wheel containing monocrystalline cubic boron nitride grains and polycrystalline cubic boron nitride grains. Comparative investigation on the wear behavior and self-sharpening phenomenon of polycrystalline cubic boron nitride and monocrystalline cubic boron nitride grains was carried out based on the fractal analysis. The results obtained indicate that the wear process of the monocrystalline cubic boron nitride grain cutting edges is, in order, attritious wear → large fracture → micro fracture → large fracture → attritious wear, while that of the polycrystalline cubic boron nitride grain cutting edges is micro fracture → attritious wear → micro fracture. Micro fracture of polycrystalline cubic boron nitride grain occurs easily in the particular zone where large impact load is formed due to the first contact between the grain cutting edges and the workpiece material. The fractal dimension of monocrystalline cubic boron nitride wheel is 2.040–2.047, while that of the polycrystalline cubic boron nitride wheel is 2.049–2.054, which indicates that the polycrystalline cubic boron nitride grain cutting edges are finer than that of the monocrystalline cubic boron nitride counterparts. Compared to monocrystalline cubic boron nitride grains, better performance, that is, smaller radial wheel wear, lower grinding force and forces ratio, is obtained for polycrystalline cubic boron nitride grains due to micro fracture behavior and self-sharpening phenomenon in high-speed grinding.

Effect of cubic boron nitride grain size in the reinforcing of α-Sialon ceramics sintered via SPS

Abstract Utilising SPS, fully dense α-Sialon ceramics were obtained during sintering at temperatures as low as 1550 °C. This enabled the full densification α-Sialon composites with 10 and 100 μm cubic boron nitride (cBN) grains. The detailed microstructural analysis of these materials with SEM and Raman spectroscopy revealed different cBN to hexagonal BN (hBN) conversion mechanisms. The smaller cBN grains only show a conversion into hBN on their surface whereas the large grains show not only conversion on the surface but also conversion at the internal grain boundaries between the twins detected inside the cBN grains. The increasing degree of formation of hBN results in decreasing hardness and increasing fracture toughness. Therefore the properties of materials with 100 μm cBN grains are more sensitive to the sintering temperature.

Microstructure and mechanical properties of ZTA/BN machinable ceramics fabricated by reactive hot pressing

Abstract Zirconia-toughened alumina (ZTA)/boron nitride (BN) machinable ceramics have been successfully fabricated by reactive hot pressing. BN was in-situ introduced into the composite by the reaction of aluminum nitride (AlN) and boric acid (H3BO3). The microstructure of ZTA/BN composite was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) techniques. The results indicated that BN grains were uniformly and compactly distributed among Al2O3 and ZrO2 grains. The relative density, flexural strength, fracture toughness, and Vickers hardness increased with increasing temperature. The flexural strength and fracture toughness of the ZTA/BN composite with BN content of 12.5 vol% sintered at 1800 °C achieved high values of 731 MPa and 7.48 MPa m1/2, respectively. These values were much higher than those of the composite with the same composition prepared by conventional powder sintering method, and even higher than those of the monolithic ZTA ceramics. The enhancements of the fracture strength and fracture toughness were primarily ascribed to the reactive hot pressing method which introduced BN in-situ and prevented the aggregation of BN and reduced the flaw sizes. The ZTA/BN composite exhibited excellent machinability and could be drilled using conventional cemented carbide drill bits.

Ion sputtering erosion mechanisms of h-BN composite ceramics with textured microstructures

Since the hexagonal boron nitride (h-BN) grain shows typical lamellar structures, textured materials can be obtained by arranging h-BN grains along one direction. In this work, textured h-BN composite ceramics with the c-axis orientation arranged along the pressure direction are manufactured by hot-press sintering using mullite as the sintering additive. The results show that sintering pressures not only play a major role in the density and the textured degrees of composite ceramics, but also influence Xe ion erosion resistance performances. After Xe ion sputtering, compositions of both h-BN and mullite stay stable, while the elemental compositions have changed due to the so-called “preferential sputtering”. Sputtered surfaces along different orientations show diverse morphologies attributed to the textured microstructures. The erosion mechanisms of h-BN grains during Xe ion sputtering are breaking of B–N bonds and delamination of BN layers. While the mass loss of composite ceramics is due to the erosion of h-BN grains and mullite coupled with partial detachment of h-BN grains from the surface.

Anisotropic mechanical properties and fracture mechanisms of textured h-BN composite ceramics

The hexagonal boron nitride (h-BN) grain shows typical lamellar structures, so textured materials can be achieved by arranging h-BN grains along the same direction. In this work, textured h-BN composite ceramics with the c-axis orientation arranged along pressure direction were manufactured by hot-press sintering using mullite as the sintering additive. The results show that sintering pressure is an important factor that affects not only the density and the textured degrees of composite ceramics, but also the mechanical properties. Based on the textured microstructure features, the composite ceramics show the obviously anisotropically characterized mechanical properties, such as elastic modulus, flexural strength and fracture toughness, together with various fracture mechanisms.

Joining interface and grain fracture of single-layer brazed grinding wheels with binderless CBN grains

Fabrication experiments of single-layer brazed grinding wheels were carried out using binderless cubic boron nitride (CBN) grains, Cu–Sn–Ti alloy and AISI 1045 steel. The brazing temperature was 900 °C and the dwelling time was 8 min. The microstructure of the joining interface was characterized. The performance of the grinding wheels was evaluated during high-speed grinding nickel superalloy. The fracture behavior of the abrasive grains versus the embedding depth was studied quantificationally. Results obtained show that good joining interface were formed among binderless CBN grains, Cu–Sn–Ti alloy and AISI 1045 steel dependent on the elemental diffusion and chemical reaction during brazing, which ensures firm hold to the abrasive grains. The grinding wheel with binderless CBN grains has exhibited evident advantages upon that with monocrystalline CBN grains in terms of grinding force and force ratio. The critical force acting on the binderless CBN grain with the exposing height of 50 and 70 % is determined when the grain fracture takes place.

Reconstruction and Analysis of Wear Topography of PcBN Abrasive Grain

This paper mainly deals with the reconstruction and fractal analysis of wear topography of brazed polycrystalline cubic boron nitride (PcBN) grain in grinding process. On the basis of fractal analysis, the uncertainty in shape and complexity in wear mechanism are analyzed by means of the construction of micro fracture of PcBN grain after producing the brazed abrasive tool with PcBN grains and carrying out grinding test. The main results are summarized as follows: (1) Actual behavior of self-sharpening phenomenon due to micro fracture in the grinding process can be grasped and reconstructed using SEM and 3D video microscope. The reconstruction model can clearly express the complicated wear features and characteristics of the grain. (2) The fractal dimension can relate the performance of PcBN grain to its micro fracture, and then the self-sharpening phenomenon due to micro fracture can be evaluated quantitatively on the basis of fractal analysis.