BN Grains Research Articles

Mechanical properties and thermal shock resistance of BaAl2Si2O8/BN ceramic composites

BaAl2Si2O8/BN (BAS/BN) ceramic composites were prepared using hot press sintering. The amorphous state of BAS ceramics exhibits a low melting point, high liquid-phase mobility, and good infiltration with BN grains, facilitating the sintering process. Conversely, the crystalline state of BAS has a high melting point and strength, contributing to the strengthening and toughening of the composite. The mechanical properties, thermal properties and thermal shock resistance of BAS/BN ceramic composite was investigated. Ceramic composites containing 40 vol% BAS have the best overall mechanical properties with bending strength, elastic modulus, fracture toughness, and hardness of 175 ± 3 MPa, 114.0 ± 1.2 GPa, 2.02 ± 0.10 MPa·m1/2, and 1.08 ± 0.01 GPa, respectively. The BAS/BN ceramic composite has excellent thermal shock resistance, and the residual bending strength shows a pattern of increasing and then decreasing with increasing thermal shock temperature difference. Finite element calculation results show that the cooling process is basically completed after 20 s of water quenching, the maximum thermal stress generated is about 22 MPa.

Homoepitaxy of Boron Nitride on Exfoliated Hexagonal Boron Nitride Flakes.

Although large efforts have been made to improve the growth of hexagonal boron nitride (hBN) by heteroepitaxy, the non-native substrates remain a fundamental factor that limits the quality. This problem can be solved by homoepitaxy, which is the growth of hBN on hBN substrates. In this report, we demonstrate the homoepitaxial growth of triangular BN grains on exfoliated hBN flakes by Metal-Organic Vapor Phase Epitaxy and show by atomic force microscopy and photoluminescence that the stacking of these triangular islands can deviate from the AA' stacking of hBN. We show that the stacking order is enforced by the crystallographic direction of the edge of the exfoliated hBN flakes, with armchair edges allowing for centrosymmetric stacking, whereas zigzag edges lead to the growth of noncentrosymmetric BN polytypes. Our results indicate pathways to grow homoepitaxial BN with tunable layer stacking, which is required to induce piezoelectricity or ferroelectricity.

Stacking-Controlled Growth of rBN Crystalline Films with High Nonlinear Optical Conversion Efficiency up to 1.

Nonlinear optical crystals lie at the core of ultrafast laser science and quantum communication technology. The emergence of 2Dmaterials provides a revolutionary potential for nonlinear optical crystals due to their exceptionally high nonlinear coefficients. However, uncontrolled stacking orders generally induce the destructive nonlinear response due to the optical phase deviation in different 2D layers. Therefore, conversion efficiency of 2D nonlinear crystals is typically limited to less than 0.01% (far below the practical criterion of >1%). Here, crystalline films of rhombohedral boron nitride (rBN) with parallel stacked layers are controllably synthesized. This success is realized by the utilization of vicinal FeNi (111) single crystal, where both the unidirectional arrangement of BN grains into a single-crystal monolayer and the continuous precipitation of (B,N) source for thick layers are guaranteed. The preserved in-plane inversion asymmetry in rBN films keeps the in-phase second-harmonic generation field in every layer and leads to a record-high conversion efficiency of 1% in the whole family of 2D materials within the coherence thickness of only 1.6µm. The work provides a route for designing ultrathin nonlinear optical crystals from 2D materials, and will promote the on-demand fabrication of integrated photonic and compact quantum optical devices.

Effects of boron source on properties of pressureless solid-state sintered silicon carbide ceramics

The effects of the boron source (B, BN, and B4C) on the thermal, electrical, and mechanical properties of pressureless solid-state sintered SiC ceramics were investigated. A high relative density of 99.9 % was successfully achieved for all ceramic specimens using an appropriate additive composition and ideal sintering conditions. SiC ceramics sintered with B and B4C additives displayed higher electrical and thermal conductivities and higher flexural strength than those sintered with the BN additive at the same relative density. The presence of intrinsically-weak, electrically-insulating BN grains at the junction and grain boundaries increased both the fracture toughness and electrical resistivity, while the flexural strength and thermal conductivity of the ceramics decreased. The electrical resistivity, thermal conductivity, and flexural strength of the SiC ceramics were able to be adjusted with values ranging from 3.2 × 104–1.6 × 106 Ω‧cm, 72.4–147.5 W‧m−1‧K−1, and 386–545 MPa, respectively, depending on the boron source used.

Etching resistance and etching behavior of h-BN textured ceramics under Xe plasma condition

The h-BN ceramic with two-dimensional laminar structure is an important constituent material of Hall thruster channel, and the differences in its chemical bonding structures lead to anisotropy of BN grains in different orientations. In this paper, h-BN matrix textured ceramics are prepared by hot-press sintering. Xe plasma is used to etch the textured ceramics with different preparation processes and orientations. The effects of microstructure and h-BN grain orientation on plasma etching resistance of the materials are investigated. The results show that the intergranular phase has superior plasma etching resistance compared to the h-BN material. During the etching, the sample surface maintains dynamic equilibrium on the macroscopic scale, while preferential etching exists on the microscopic scale. The h-BN grains exhibit point etching, line etching and composite etching behavior during the etching process. The etching resistance of textured ceramics is related to the intrinsic plasma etching resistance of the matrix material, but less correlation to the preparation process, microstructure, and orientation of the BN grains of the matrix material, moreover, there is no anisotropic etching behavior.

Pressureless solid-state sintering of SiC ceramics with BN and C additives

ABSTRACT This study proposes BN as a new sintering aid for pressureless solid-state sintering of SiC ceramics. The full densification of SiC ceramics for 0.5–2.7 wt% BN addition showed the composition tolerance of the newly developed ceramics. The electrical resistivity decreased by an order of magnitude (107→106 Ω·cm) as BN content increased from ~0.5 to ~0.9 wt% because of the increased BN-derived B doping in the SiC lattice. A further increase in BN content had no significant effect on the electrical resistivity, which is attributed to the limited solubility of B in the SiC lattice. The thermal conductivity decreased with increasing BN content owing to increased phonon scattering at B-doped sites and the thermally insulating BN phase located at the grain boundaries. The fracture toughness increased with increasing BN content owing to interfacial debonding at the weak SiC-BN interfaces. However, intrinsically weak BN grains with low hardness were responsible for reduced flexural strength and hardness with increasing BN content.

Facile preparation of BCN nanofibers with tunable band gaps

As a new synthetic metal-free material, BCN has aroused great interests in the field of photocatalyst materials due to its lower cost as well as higher reliability and better sustainability. In this work, we prepared BCN nanofibers by electrospinning of precursor solution containing polyacrylonitrile and ammonia borane, and then pyrolyzed the polymer fibers in NH3. Infrared spectroscopy (FT-IR), Scanning emission microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were used to analyse the composition and structure of the samples. The results show that the BCN nanofibers have an average diameter of 50-100 nm and mainly compose of BN nanocrystals. C atoms are distributed at the BN grain boundary in terms of B-C and B-N bonds. The band gap of the fibers is between 2.1-2.7 eV and can be adjusted by changing the ratio of PAN and AB in the raw materials.

Wafer-scale MOVPE growth and characterization of highly ordered h-BN on patterned sapphire substrates

Layered h-BN were grown on sub-micron size dome patterned sapphire substrates (PSS) by metal organic vapor phase epitaxy (MOVPE) to study the effects of substrate orientations, strain and subsequent thermal stress evolution. Electron micrograph images on the surface of the c-plane sapphire in-between the dome structures show typical wrinkled morphology of layered h-BN, similar to the reference sample surface confirming the high quality h-BN growth. However, the h-BN morphology on the domes exhibited pattern like folding which were drastically different from the h-BN on the c-plane sapphire. Detailed TEM studies show that the h -BN is layered and covers the dome patterns of the sapphire substrates like a “blanket of snow” continuously with localized strain variations induced turbostratic phase insertions. In addition, the local variation in optical emission properties of layered materials, which are dramatically affected by the rippled morphology were also investigated using depth resolved cathodoluminescence. The intensity variation of the 320 nm peak confirms that the BN on dome structures have more strain related defects than the c-plane h-BN. Thus, these results give better insight on the origin of turbostratic BN grains and local control over the structural and optical emission characteristics.

Effect of synthesis conditions of BCNO on the formation and structural ordering of BN at 1200 °C and 1 GPa

We investigated the correlation between the synthesis conditions of BCNO compounds and the formation and structure ordering of BN by subjecting BCNO compounds to a post heat-pressure treatment at 1200 °C and 1 GPa. We made a direct observation that BN grains form from BCNO matrix. We found that the formation and structural ordering of BN strongly depend on the synthesis conditions of BCNO compounds. An increase in synthesis temperature promotes the formation of BN since the reaction process involved is endothermic. An increase in boric acid to melamine ratio promotes the formation and structural ordering of BN since i) it reduces the theoretical temperature for BN formation and ii) it enhances mass transfer and mobility of BN layers by forming additional amount of B2O3 phase. Further, loading pressure of 1 GPa resulted in a more compressed layered structure of the resultant BN.

Degradation mechanisms of pcBN tool material during Friction Stir Welding of Ni-base alloy 625

In Friction Stir Welding (FSW), interactions between the plasticized stirred material and the tool significantly affect resulting weld properties. When welding metals with high strength and melting point, the tribological load on the tool is severe, and poses the main limiting factor for the technology's industrial exploitation. Since tool materials are loaded to their limits, it is essential to understand the interactions of specific tool material and welded metal combinations.In the present study 3.2mm alloy 625 sheets were joined using a pcBN tool with W-Re binder phase. Wear lead to a change in tool geometry followed by tool fracture. In SEM investigations the welds revealed typical banded structures, composed of small grains and non-metallic phases containing W from the tool material. The tool surface is extensively covered by adhering sheet metal. Further, BN grain pull-outs and appearances of diffusive wear are visible on the worn tool surface.Tool wear is mainly caused by detachment of BN grains due to thermal softening of the metallic binder phase and dissolution of BN in the hot material in the stirred zone. Using low rotational speeds resulting in lower process temperatures reduces tool wear and results in a homogeneous stirred zone.

Densification, microstructural evolution and mechanical properties of Si-B-C-N monoliths with LaB6 addition

The mixture of 5 wt% LaB6 and mechanically alloyed Si-B-C-N powders was conventionally ball-milled and then compacted by spark plasma sintering at 1250–2000 °C and 50 MPa. Densification, microstructural evolution and mechanical properties of the as-prepared La/Si-B-C-N monoliths were investigated. At 1360 °C, sintering initiates coupled with rapid densification caused by particle rearrangement and plastic deformation. Grain-boundary sliding and atomic diffusion lead to further densification when sintering continues. Only SiC and La-containing BN(C) phases are detected in La/Si-B-C-N monoliths sintered at temperatures higher than 1900 °C. The monoliths sintered at 2000 °C show a high fracture toughness (4.6 MPa m−1/2) attributed mainly to lamellar La-containing BN(C) grains.

Microstructural evolution, mechanical and thermal properties of LaB6 embedded in Si-B-C-N prepared by spark plasma sintering

Si-B-C-N monoliths with 5wt% LaB6 additives were prepared by spark plasma sintering at 1250–2000°C and 50MPa using a mechanically alloyed mixture of graphite, c-Si, h-BN and LaB6 powders as the starting materials. Microstructural evolution, mechanical and thermal properties of the as-prepared La/Si-B-C-N monoliths were investigated. The densification of the ceramics starts at 1160° and ends at 1800°C with the formation of La-containing compounds coupled with SiC and BN(C) phases. La-containing BN(C) grains develop into a lamellar structure at 1900°C offering improved fracture toughness and decreased Vickers hardness, flexural strength and elastic modulus. The formation of lamellar BN(C) is also responsible for a high thermal expansion coefficient of 4.2×10−6 /°C.

Si-B-C-N monoliths with LaB6-induced well-developed BN(C) flakes

Si-B-C-N ceramic monoliths with LaB6 additive were prepared by mechanical alloying followed by spark plasma sintering. The microstructures and mechanical properties were investigated compared to LaB6-free Si-B-C-N counterparts. After sintered at 1900°C/50MPa, the La/Si-B-C-N monoliths provide well-developed BN(C) flakes (≤100nm in thickness) and improved fracture toughness, while the BN(C) ones show no lamellar microstructures in Si-B-C-N systems. The solid-solution of La in BN(C) grains promotes ordered arrangement of BN(C) along the (0002) plane causing in situ growth of BN(C) thus leading to an increase in toughness by 40% (from 5.6 to 7.8MPam−1/2).

Microstructural and mechanical properties of cBN-Si composites prepared from the high pressure infiltration method

The grain size dependence of the mechanical properties of cBN-Si composites prepared using the high pressure infiltration method has been investigated. Indentation testing indicates that cBN-Si composites have hardness values of 38–43GPa, which increase with increasing grain size and are harder than traditional polycrystalline cBN composites (PcBNs). Thermostability analyses display that cBN-Si composites with a grain size of >9μm also possess a higher temperature of oxidation, compared to traditional PcBNs, and the thermostability increases with increasing cBN grain size. Fracture toughness tests show that almost no cracks appear on the polished cBN-Si samples when the loading forces are increased to 294N and the fracture toughness is better than for commercial samples. Scanning electron microscopy illustrates that deformations and close pores occurred easily between coarse BN grains, leading to denser cBN-Si compacts with better mechanical performances.

Microstructure and erosion resistance of in-situ SiAlON reinforced BN-SiO2 composite ceramics

BN-SiO2-SiAlON composite ceramics were successfully prepared by the means of hot pressed sintering. Xe plasma flow generated by Hall Thruster was used for sputtering the surface of the samples in order to evaluate the plasma erosion resistance. XRD, TEM, SEM, and LSCM were used to characterize the phase composition and morphologies of as-made composite ceramics before and after Xe plasma erosion. The ceramics were composed of h-BN, fused silica, and SiAlON, which maintained structural stability during the process of Xe plasma sputtering. In conclusion, comparing with BN-SiO2 composite ceramics, the plasma erosion rate of BN-SiO2-SiAlON composite ceramics decreases significantly at first then rises with the increase of AlN addition. Erosion pits can be observed by using SEM on the surface after plasma sputtering, which demonstrates that the BN grains have dropped off the surface. In addition, mechanical denudation by high-speed Xe ions is recognized as the injury mechanism for the BN-matrix composite materials.

Fabrication and microstructures of functional gradient SiBCN–Nb composite by hot pressing

A functional gradient material with five layers composed of SiBCN ceramic and niobium (Nb) was prepared successfully by hot pressing. The phase composition, morphology features and microstructures were investigated in each layer of the gradient material. The Nb-containing compounds involving NbC, Nb6C5, Nb4C3, Nb5Si3 and NbN increase with the volume fraction of Nb increasing in the sub-layer. They are randomly scattered (≤25vol.% Nb), then strip-like, and finally distribute continuously (≥75vol.% Nb). The size of BN(C) and SiC grains in Nb-containing layers is larger than in 100% SiBCN layer due to the loss of the capsule-like structures. No distinct interfaces form in the transition regions indicating the gradual changes in phase composition and microstructures.

Electrical and thermal properties of silicon carbide–boron nitride composites prepared without sintering additives

Composites of SiC–BN consisting of uniformly distributed plate-shaped hexagonal BN grains and nitrogen-doped α-SiC grains were fabricated by conventional hot-pressing in an argon atmosphere without sintering additives. The effects of the BN content on the electrical, structural, thermal, and mechanical properties of the SiC–BN composites were investigated. The electrical resistivity showed a minimum at a BN content of 10vol%, which is attributable to N substitution for C sites in the 6H–SiC lattice, which acts as donors for supplying carriers to the conduction band. The thermal conductivity of the composites showed a decreasing trend with increasing BN content, indicating that the presence of the SiC/BN interface discourages phonon conduction in the composites. The electrical resistivity, thermal conductivity, flexural strength, fracture toughness, and hardness of the SiC–10vol% BN composite were 9.3×105Ω×cm, 74.4W/mK, 559MPa, 3.6MPam1/2, and 23GPa, respectively.

High pressure sintering of cubic boron nitride using Co–V–Al alloy as bonding media

Fine-grained polycrystalline cubic boron nitride compacts were prepared using the mixture of cubic BN and Co–V–Al powders at 5.6GPa and 1700°C for 0.5h. The weight percent of Co, V and Al to cubic BN powder was 10wt.% or 20wt.%. Results of electron dispersive spectrum were suggested that binder composition was changed from metallic binder of Co and V was dominant to VN and AlN mixture was dominant with increase of atomic percent of Al to BN in the sample. Microstructures of the sintered samples were observed by scanning electron microscope and back scattered electron image. The region of atomic weight ratio of Al to V that was about 1–5 showed homogeneously sintered microstructure of less than 3μm of cubic BN grain size. The Vickers hardness trend that showed higher values in the region of atomic weight ratio of Al to V was about 1–5. The highest Vickers hardness resulted in the present study was about 40.7GPa for the 20wt.% metallic binder that contained a 1.39 atomic weight ratio of Al to V and 0.5–2μm size of starting cubic BN powder was used.

Effect of Pyrolysis Temperature on Properties of Porous Si3N4-BN Composites Fabricated Via PIP Route

The Si3N4-BN composites have been prepared via die pressing and the precursor infiltration and pyrolysis route using borazine as the precursor. The Si3N4-BN composites are composed of h-BN, α-Si3N4, and β-Si3N4 produced at a pyrolysis temperature from 1200 to 1750 °C with only 0.17-3.9 wt.% phase transition of Si3N4. The effect of pyrolysis temperature on properties of the composites has been investigated. The density and mechanical properties of the composites, at both room temperature and 1000 °C, increase along with the elevating of the pyrolysis temperature. The density of the composites achieves 2.33 g/cm3 at 1750 °C with the porosity of 14.1%. The flexural strength, elastic modulus, and fracture toughness at room temperature of the Si3N4-BN composites pyrolyzed at 1750 °C are 219.1 MPa, 75.5 GPa, and 2.62 MPa m1/2, respectively. A desirable flexural strength of 184.9 MPa with a residual ratio of 84.4% has been obtained when the composites are exposed at 1000 °C in the air. Micrographs of SEM and TEM illustrate the bonding structure of the pyrolysis BN and Si3N4 grains.

Synthesis of (Ca, Mg)–α′-Sialon–AlN–BN powders from boron-rich blast furnace slag by microwave carbothermal reduction-nitridation

(Ca, Mg)–α′-Sialon–AlN–BN powders were synthesized by the carbothermal reduction and nitridation (CRN) method using boron-rich slag, one of the intermediate products from pyrometallurgy separation of pageit, as the staring material. The influences of synthesis temperature and holding time on the phase composition and microstructure during the microwave CRN were studied by XRD, SEM and EDS. The comparison between two heating techniques, conventional and microwave heating, on the synthesized powder was presented as well. The experimental results revealed that the phase compositions and microstructures of the synthesized products were greatly affected by the synthesis temperature and holding time. With an increase in the synthesis temperature or holding time, the relative amount of α′-Sialon increased and α′-Sialon became the main crystalline phase at 1400 °C for 6 h. The synthesized products also contained AlN, BN and a small amount of β-SiC. Elongated α'-Sialon grains, short rod AlN grains, aggregate nanoscale BN grains were observed in the synthesized powders. The reaction temperature of microwave heating method was reduced by 80 °C, the reaction time was shortened by 2 h, and more elongated α′-Sialon grains with large aspect ratio were observed.