Boron Nitride Grains Research Articles

Investigations on microstructure and mechanical properties of boron nitride fiber using experimental and numerical methods

Boron nitride (BN) fiber has attracted increasing attentions in the aerospace and other engineering fields due to its outstanding thermal, mechanical and electromagnetic properties. In this paper, the relationship between the microstructure and mechanical properties of BN fiber is investigated using the experimental and numerical methods. The microstructure of BN fiber is characterized by XRD, SEM, and TEM, subsequently a representative model is established via classical molecular dynamics (MD) for revealing the relationship between the mechanical properties and the structures of grain size, grain orientation, and crystallinity. Experimental observations show that the BN fiber is composed of amorphous boron nitride ( a -BN), turbostratic boron nitride ( t -BN) and hexagonal boron nitride ( h -BN), where the h -BN grains with the AB stacking sequences are embedded in a -BN or t -BN. Molecular dynamics (MD) simulation results show that the BN fiber which h -BN grains arranged along the fiber axis has higher Young's modulus and fracture strength. With the increase of crystallinity, the Young's modulus increases and the fracture strength decreases for all the simulation results. In terms of the grain size, there should be a trade-off between the high modulus and high strength of BN fiber, i.e., the former requires larger size of h- BN grains, and the latter needs smaller ones. These findings could not only bridge the microstructure and mechanical properties but also be useful for the synthesis of high-performance BN fiber.

Effect of the Granularity of Cubic Boron Nitride Vitrified Grinding Wheels on the Planar Technical Blades Sharpening Process.

The most widely used method for shaping technical blades is grinding with abrasive tools made of cubic boron nitride (cBN) grains and vitrified bond. The goal of this work was to determine the effect of grinding wheel grain size (cBN grain number according to FEPA standards: B126, B181 and B251), kinematics (grinding with the circumference, face and conical surface of the wheel) and feed rate (vf = 100; 150; 200 mm/min) on the effects of the grinding process evaluated by the cutting force of the blade after machining F, blade surface texture parameters (Sa, St, Smvr, Str, Sdq, Sdr and Sbi) as well as blade surface morphology. An analysis of output quantities showed that grinding wheels made of B181 cBN grains are most favorable for shaping planar technical blades of X39Cr13 steel in the grinding process.

Tribological Behaviors of Polycrystalline Cubic Boron Nitride Sliding against Bearing Steel in Vacuum Conditions

In order to understand the surface and interface conditions of polycrystalline cubic boron nitride (PcBN) sliding against bearing steel in vacuum environments, the effects of different loads on the tribological behaviors of PcBN and bearing steel AISI 52100 were studied deeply in a vacuum tribometer. Furthermore, the wear tracks of PcBN and the wear scars of AISI 52100 were characterized by scanning electron microscopy and energy dispersive X-ray spectroscopy. The results show that the stable coefficient of friction (CoF) of the tribopair experiences a decrease first and then an increase with the increase in loads from 2 N to 15 N. The adhesive layer increases with the increase in loads, and the formation of adhesive layer contributes to the change of CoF and wear rate of counterpart balls. The adhesive layer is formed due to the combination of high contact stress and high temperature. Meanwhile, the exfoliated cubic boron nitride grains are embedded into the adhesive layer as abrasive grains, resulting in abrasive wear. Thus, the main wear mechanisms are adhesive wear and abrasive wear.

Growth and characterization of uniformly distributed triangular single-crystalline hexagonal boron nitride grains on liquid copper surface

Single-layer hexagonal boron nitride (h-BN) is a two-dimensional (2D) material with a wide band gap and extraordinary mechanical, thermal and optical properties. It has promising applications in optoelectronics, electronics and photovoltaics. Low-pressure (150 Pa) chemical vapor deposition (LPCVD) was used to prepare uniformly distributed single-crystalline triangular h-BN grains and continuous film on liquid copper surface. A series of microscopic and spectroscopic methods were performed to characterize the morphology, crystalline structure and quality, component and thickness. It was found that h-BN grains turned into circles at higher background pressure (1000 Pa), which was due to the change in the growth mechanism from equilibrium controlled to deposition controlled. Small islands formed on primary h-BN grains because of higher concentration of precursor product. Compared to the higher background pressure, the synthetic h-BN films at low background pressure exhibited larger domain size, lower nucleation density, and no adlayer growth. Due to randomly distributed polar h-BN grains, the grain boundaries formed along the h-BN domains after merging. Typical patterns formed via the coalescence of triangular h-BN grains indicating the merging mode of growth, including edge-to-edge and edge-to-point modes. This work provides a pathway for the preparation of uniformly distributed single-crystalline h-BN grains and an in-depth understanding of the growth and merging process on liquid Cu surface.

Effects of open pores on grinding performance of porous metal-bonded aggregated cBN wheels during grinding Ti–6Al–4V alloys

Porous metal-bonded wheels coupled with the aggregated cubic boron nitride (cBN) grains and water-soluble carbamide particles as pore-forming agents were fabricated, aiming to the improvement of grinding wheel performance and ground surface quality in grinding of Ti–6Al–4V alloys. Grinding forces and force ratio, ground surface roughness and microhardness were investigated to evaluate wheel performance. In addition, the wear evolutions of cBN grains and macropores were performed as the material removal volume increases during wheel wear tests. Findings show that the dynamic changing behavior of the coverage and exposure of open pores attributes to the improvement of grinding performance and ground surface quality of porous AcBN wheels. Meanwhile, the promising self-sharpening property of wheels can be guaranteed in basis of dynamic wear variations of multiple cBN abrasive grains layer by layer.

Controlled Lay-By-Layer Etching of CVD Grown Multilayer h-BN Single Crystal

Being a 2D insulator with alternating sp2 hybridized B and N atoms in a honeycomb structure, Hexagonal Boron Nitride(hBN) not only have uniqure properties like high transparency, large thermal conductivity, high mechanical strength and superior chemical inertness by itself, but also be able to promote other 2D materials’ performance when serving as the ultra-smooth dielectrilayer.[1,2] Since CVD process is established as the most effective way to grow large area hBN materials, it is critical to understand and control the growth process so as to making them ready for the large scale industrial applications. Etching of 2D materials should provide a scalable route for fabrication of highly desired various nanostructures such as nanoribbons with controlled termination providing a simple and direct way for 2D material property design, such as controlling the band gap and optical properties. [3,4] However, it is often easy to initiate the etching, but difficulty the control the location, shape, and size of the etching pattern. Previously reported etching patterns for 2D materials can be generally divided into four different categories: domain, boundary, fractal, and adlayer etching. [3-6] These etching patterns may co-exist in the etching process, but the dominant pattern depends on competition between various factors such as atmosphere, temperature, substrate, time etc. Layer number increase can often be achieved by increase the reaction temperature, time, and precursor concentration.While the layer number reducing after the multilayer hBN growth process remain challenging. Here we use the as-grow multilayer hBN single crystal from CVD process for controllable etching experiment, then both temperature related hydrogen etching and electronic energy related e-beam etching will be used to remove single layer of as-grown multilayer hBN. References Lu, G.; Wu, T; Yuan, Q.; Wang, H.; Wang, H.; Feng Ding, F.; Xie, X.; Jiang, M. Synthesis of large single-crystal hexagonal boron nitride grains on Cu–Ni alloy. Nat. Commun. 2014, 6, 6160.Liu, Z.; Gong, Y.; Zhou, W.; Ma, L.; Yu, J.; Idrobo, J. C.; Jung, J.; MacDonald, A.H.; Vajtai, R.; Lou, J.; Ajayan, P.M. Ultrathin high-temperature oxidation-resistant coatings of hexagonal boron nitride. Nat. Commun. 2013, 4, 2541.Vlassiouk, I.; Regmi, M.; Fulvio, P.; Dai, S.; Datskos, P.; Eres, G.; Smirnov, S. Role of hydrogen in chemical vapor deposition growth of large single-crystal graphene. ACS nano 2011, 5, 6069-6076.Wang, L.; Wu, B.; Jiang, L.; Chen, J.; Li, Y.; Guo, W.; Hu, P.; Liu, Y. Growth and Etching of Monolayer Hexagonal Boron Nitride. Adv. Mater. 2015, 27, 4858–4864.Rong, Y.; He, K.; Pacios, M.; Robertson, A. W.; Bhaskaran, H.; Warner, J. H. Controlled Preferential Oxidation of Grain Boundaries in Monolayer Tungsten Disulfide for Direct Optical Imaging. ACS Nano, 2015, 9, 3695–3703.Stehle, Y.; Sang, X.; Unocic, R.; Voylov, D.; Jackson, R.; Smirnov, S.; Vlassiouk, I., Anisotropic etching of hexagonal boron nitride and graphene: question of edge terminations Nano letters, 2017, 17 (12), 7306-7314

Method for recovering cubic boron nitride abrasive grains from vitrified cBN stones – A preliminary study

In this work, a process is established for recovering cubic boron nitride (cBN) grains from fired vitrified cBN products. The abrasive grains are recovered by an acid washing technique. Two methods were used to recover the abrasive grains, namely with and without pre-heating. Shape and surface characteristics of the recovered grains were analysed through optical microscopy and Scanning Electron Microscopy (SEM). The recovery yield of abrasive grain and bond percentage present in the abrasive tool was measured experimentally. From the experimental results, the maximum recovery yield of 81 percentage was obtained in the pre-heated sample. Bond free abrasive grains are also obtained from the pre-heated samples and confirmed through elemental analysis.

Effect of wear behaviour of single mono- and poly-crystalline cBN grains on the grinding performance of Inconel 718

Polycrystalline cubic boron nitride (PcBN) grains were fabricated by combining the monocrystalline cBN (McBN) nanoparticles and inter-abrasive ceramic materials via high temperature and pressure techniques. Grinding performance of Inconel 718 with single McBN and PcBN grains, including grinding force, force ratio, ground surface quality was investigated. Characterization of the wear morphology evolution of worn grains and scratches of PcBN grains were discussed. In addition, the fracture behaviour of PcBN grains was evaluated as the varying of the undeformed chip thickness. Results show that PcBN grains have the smaller grinding force and force ratio, more stable grain wear rate in comparison to McBN grains. Additionally, the better wear-resistance and grinding performance owing to its multi-cutting edges structure in terms of the grain wear morphology evolution were achieved for PcBN grains regardless of the undeformed chip thickness.

Effects of heat treatment on microstructure and mechanical properties of 2.5D Cf/BN/pyrocarbon composites by chemical vapor infiltration

In this study, 2.5D carbon fiber reinforced pyrocarbon matrix composites with a BN interphase (Cf/BN/PyC) were prepared by chemical vapor infiltration. Through comparing the distribution of thermal residual stress and the evolution of the interface microstructure of composite before and after heat treatment, the reason why the structure and orientation of the BN interphase in the Cf/BN/PyC composite deviate from the layered structure required for the expected compliant interface was analyzed. The results show that the crystalline skin of Cf affects the order of BN near the Cf/BN interface, thereby forming BN interphase with a structural gradient from the beginning. The infiltration process of pyrocarbon interacts with the growth and rearrangement process of BN, resulting in incomplete crystallization of BN. After heat treatment at 1600°C, BN interphase with high crystallinity in the nano-domain but disordered orientation is formed, although it can also debond the interface according to He and Hutchinson's model. The BN grain orientation deviates from the fiber axis mainly due to multi-factor coupling, in which the axial residual thermal stress generated at the Cf/BN/PyC interface is the main influencing factor. Understanding the influence of the heat treatment (HT) on the atomic-domain orientation change and structural evolution during the preparation of boron nitride (BN) interphase by chemical vapor infiltration paves us an effective way to modify the preparation of interphase with low fracture energy.

Cubic boron nitride wheel topography effects on phase transformation of maraging C250 steel and grinding surface quality

It is likely for the interaction between the cubic boron nitride grains on the working surface of grinding wheels and maraging C250 steel workpiece to cause phase transformation, thus reducing the material removal rates and affecting the quality of the workpiece surface generated during grinding. Up to now, there remain few studies focusing on the relationship between grinding wheel topography and phase transformation. In this study, an analysis was conducted of the effects that the topography of cubic boron nitride grinding wheel has on the distribution of phase transformation and the surface quality of maraging steel during grinding. The phase transformation diffraction curve was plotted based on the results of X-ray diffraction (XRD) analysis, and then, it was compared with the known data under different grinding wheel conditions. The topography of the cubic boron nitride grinding wheel was characterized using 3D digital microscope analysis, and the dynamic abrasive grit density coefficient λd for micro-grinding was proposed to clarify the effect of relevant parameters on phase transformation. The surface quality of grinding wheel and workpiece after grinding was analyzed, while the combination of different grinding wheel topography parameters and the process optimization scheme were proposed to achieve the optimal effect of grinding.

Characterisation of the wear properties of a single-aggregated cubic boron nitride grain during Ti–6Al–4V alloy grinding

Single-grain grinding the Ti–6Al–4V alloys is performed to investigate the wear characteristics and material removal mechanism of aggregated cubic boron nitride (AcBN) grains fabricated by multiple monocrystalline cBN grains. An innovative approach entailing the tracking of grains and scratches by using the 3D reconstruction is proposed to investigate accurately the wear characteristics and material removal ability of such grains. Grain wear, which is affected by grinding wheel speed vs and undeformed chip thickness agmax, is characterised. The variations in grinding force and grinding force ratio, volumetric pile-up ratio and volumetric groove removal rate are also studied. Results show that the as-sintered AcBN grains have better self-sharpening ability, higher grinding ratio, and more stable grain wear rate than the McBN grains. In addition, the wear rate of AcBN grains is more sensible in determining the effect of vs compared with the use of agmax. The AcBN grains attained a higher wear volume (nearly thrice the value), a more stable wear rate and a more stable grinding force ratio when vs increases to 120 m/s as opposed to the other smaller vs values regardless of the agmax values.

Effect of grain contents of a single-aggregated cubic boron nitride grain on material removal mechanism during Ti–6Al–4V alloy grinding

Aggregated cubic boron nitride (AcBN) grains were developed by sintering monocrystalline cBN (McBN) grains and inter-abrasive metallic phase. The sintering parameters, including sintering temperature and dwelling time, and volumetric proportions of McBN grains are optimized and correlated with the fracture strength of AcBN grains. Quantitative characterization of the grain and scratches were tracked based on the 3D reconstruction technique to study the grain wear characteristics and material removal ability of AcBN grains. The grinding performance for different AcBN grain types, including grinding force, force ratio, volumetric pile-up ratio and volumetric groove removal rate were investigated. Experimental results indicate that the optimized sintering parameters are 800 °C with a dwelling time of 10 min. The AcBN grains with 90% volumetric proportion of McBN grains possess the more stable grinding force ratio and grain wear rate, better self-sharpening ability, and higher grinding ratio. In addition, the material removal efficiency tends to decline and then increase due to the self-sharpening characteristics through the analysis of the volumetric pile-up ratio regardless of AcBN grain types.

Grain-orientation dependence of the anisotropic thermal shock performance of hexagonal boron nitride ceramics

The anisotropic properties of hexagonal boron nitride (h-BN) grain are always ignored when evaluating the thermal shock performance of h-BN ceramics. In this work, h-BN ceramics with different orientation degree were selected, and effects of orientation of anisotropic h-BN grain on their physical properties were investigated. Thereafter, the anisotropic thermal shock performance was judged in macroscopic scale according to thermal stress fracture resistance parameter (R) and thermal stress damage resistance parameter (R⁗) and in mesoscopic scale based on stress distribution. The results show that orienting h-BN grains slightly can improve the thermal shock performance of h-BN ceramics in both directions.

Effect of temperature on the growth of boron nitride interfacial coatings on SiC fibers by chemical vapor infiltration

In order to improve bonding property between SiC fibers and matrix of SiC f /SiC composites, boron nitride (BN) interfacial coatings were synthesized by chemical vapor infiltration. BN coatings were fabricated from BCl 3 –NH 3 gaseous mixtures at four different temperatures (843 °C, 900 °C, 950 °C and 1050 °C) with different deposition times. Growth kinetics, nucleation and growth processes, microstructure and chemical composition of boron nitride coatings were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and Raman spectrometry. Results showed that deposition rate increased as the temperature increased from 843 °C to 950 °C. However, deposition rate decreased slightly from 23.10 ± 0.85 nm/min (950 °C) to 21.39 ± 0.67 nm/min when the temperature was increased further to 1050 °C. It could be due to the nucleation occurring in the gas and the consumption of a large amount of BCl 3 and NH 3 . When deposition temperature was 843 °C, BN grains deposited on top layer of the coating could not completely cross Ehrlich-Schwoebel barrier and grew in island growth mode. On the other hand, the deposition pattern followed a layer-by-layer growth mode when deposition temperature was 1050 °C. Deposition temperature significantly affected the microstructure of as-deposited BN coatings. At 843 °C, 950 °C and 1050 °C, the coatings presented amorphous, polycrystalline and hexagonal structures, respectively.

Effect of grain wear on material removal behaviour during grinding of Ti-6Al-4V titanium alloy with single aggregated cBN grain

Abstract This study aims to explore the grinding ability of as-sintered aggregated cubic boron nitride (AcBN) grains. Single-grain grinding tests of Ti-6Al-4V titanium alloy are conducted, and the wear characteristics of AcBN grains and their corresponding effects on material removal behaviour are investigated. Characterisation of the morphologies and micro-structure of AcBN grains and scratch morphologies is also performed. The grinding force ratio, material removal efficiency and grain wear characteristics of AcBN abrasives are discussed. Results show that the developed AcBN grains have multiple cutting edges and better self-sharpening ability than conventional monocrystalline cBN grains. In addition, AcBN grains feature a more stable grinding force ratio, larger material removal volume, higher material removal efficiency and longer steady-wear regions.

Oxidation‐led decomposition of hexagonal boron nitride coatings on alloy substrates at 900 °C: Chromia‐formers

In contrast to a mass of powder in air at 900 °C, the oxidation rate of hexagonal boron nitride does not tend to a plateau when present as a coating on austenitic stainless steel type 304 and type 310, and a real‐world hot forming tool alloy. As with a mass of powder, a molten boron trioxide diffusion barrier encapsulates each hexagonal boron nitride grain around the basal plane. These subsequently merge to form a dispersion of remnant grains throughout a molten matrix. Boron trioxide is semipermeable, and as such, the substrate oxidizes concurrently. As it does so, outwardly diffusing chromium (III) ions react with boron trioxide to form chromium borate, which depletes the diffusion barrier. This causes the oxidation of hexagonal boron nitride to be sustained at an enhanced rate and the overlayer ultimately undergoes complete decomposition. During this process, the scale comprises chromium borate and chromia only. While manganese chromium spinel and iron chromium spinel form readily at the oxide‐atmosphere interface of equivalently heat‐treated uncoated substrates, they do not form until after the coating has completely decomposed.

Influence of TiH2 on Interfacial Reaction Features Between Cubic Boron Nitride and Cu-Sn-Ti Active Filler Metals

Cubic boron nitride (CBN) grains are brazed by Cu-Sn-Ti filler metal in vacuum at 1223 K. Wetting behaviors, reaction layer features, and elemental distribution characteristics have been systematically investigated through scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). The results show that when the TiH2 content is 5 wt pct, virtually no sharp-edged, partially coated, and uncoated CBN grains can be observed, indicating that adequate wetting could be achieved. Moreover, a continuous reaction layer with uniform thickness (1.30 μm) consisting of TiN, TiB2, and TiB is obtained, which indicates that large numbers of TiN, TiB2, and TiB compounds could have been formed at the interface. However, when the TiH2 content exceeds 5 wt pct, the thickness of the reaction layer is no longer significantly expanding and excessive Ti present in the matrix will lower the volume percentage of Cu3Sn1 phase, which will reduce the bonding strength of the brazing samples. Wear tests indicate that the minimum weight loss also occurs at 5 wt pct TiH2.

The influence of BN additives on the phase composition, microstructure and mechanical properties of 316L steel consolidated by spark plasma sintering

Blends of commercial 316L steel and boron nitride (BN) were consolidated using Spark Plasma Sintering (SPS). The components were mixed by high energy milling (using attritor) before sintering. The aim was to study the effect of BN on the microstructure and the mechanical properties. Two composites containing 0.5 and 2 wt.% BN were produced beside a pure 316L specimen. It was found that bimodal grain size distribution formed during the production. A bcc α’-phase formed during milling beside the main fcc γ-phase. The addition of BN resulted in a slightly higher dislocation density and a smaller grain size in the fine grained parts of the samples. At the same time, the elastic modulus and the strength determined by bending test were reduced, most probably due to the weak bonding between the 316L and the BN grains, and the lower fraction of α’-phase.

Spark Plasma Sintering and Characterization of Mixed <i>h</i>-BN Powders with Different Grain Sizes

Mixtures of two grades of hexagonal boron nitride (h-BN) powder differing in their grain size and crystallinity were spark plasma sintering in a vacuum. The influences of the mixing ratio of h-BN powders on the densification, microstructure and mechanical properties were investigated in detail. The results show that the smaller grain size, the lower crystallization degree and the higher density and mechanical properties. While the orientation degree of flake h-BN grains can be greatly enhanced by using h-BN powder with larger particle size. As the increasing of nanometer h-BN powder adding amount, the relative density has the same “V” shape changing trends with mechanical properties. In addition, the orientation of boron nitride grains varied widely, and a nearly isotropic material was prepared from a mixture containing 90wt.% nanometer powder.

Laser Sintering of Abrasive Layers with Inclusions of Cubic Boron Nitride Grains

One of the mainstream technologies for industrial tools manufacturing is the high-speed local heating technique that drastically increases the productivity of tools manufacturing. With further optimization of sintering processes, it is possible to manufacture industrial processing tools with high density and relatively small grain size. The use of a focused laser beam as a heating source also contributes significantly to the increase in productivity of tool manufacturing and broadens the range of bonding materials that could be used to form the Cubic boron nitride (CBN) containing abrasive layers. The research is mainly focused on the irradiation of CBN grains with lasers of two different wavelengths - λ = 1.06 μm and λ = 10.6 μm. Technological parameters of laser processing were as follows: Laser power P – 200 - 700 W, focal spot diameter d – 3 mm, processing speed V – 0.01 - 2.0 m/min. The density of CBN grains was measured before and after laser irradiation using DA-2 measuring unit according to standard procedure. Bronze and Nickel powders (BrО10 and PS-12 N-WC) were used as a bonding material. They were mixed with CBN grains and were applied onto the steel base plates followed by laser irradiation of the prepacked layer. Scanning electron microscopy (SEM) was used to investigate the microstructure of samples along with local and integral X-ray element analysis. Microhardness of irradiated samples was measured as well. The interaction of laser beam and CBN grains was studied in detail and relationships between key technological parameters and quality of composites (grains durability, wear resistance and bonding strength) was established.