Microstructural and microtextural investigations of boron nitride deposited from BCl3–NH3–H2 gas mixtures

The structural, morphological and textural characteristics of BN coatings processed by CVD from (BCl₃, NH₃, H₂) gas mixtures, at low pressure (P=1.3 kPa) and low temperature (T=800 °C), with different Q_NH3/Q_BCl3 gas flow rate ratios, have been investigated. Whereas the as-processed coatings are amorphous, a high degree of crystallisation can be achieved after a post-deposition heat treatment. The sole post-elaboration heat treatment does not allow the improvement of the crystallisation degree of the boron nitride. The presence of a small amount of oxygen, resulting from a simple exposure of the coating to a controlled atmosphere (temperature, moisture rate), is also necessary. For given temperature and pressure, a wide range of microstructures of the heat-treated BN coatings, from isotropic to anisotropic, can be observed by varying the Q_NH3/Q_BCl3 ratio.     

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 BCl₃–NH₃ 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 BCl₃ and NH₃. 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.     

Influence of hydrogen on growth and microstructure of boron nitride coatings obtained from BCl3–NH3–H2 system by chemical vapor infiltration

Hexagonal boron nitride (h-BN) interfacial coatings were prepared by chemical vapor infiltration (CVI) process from BCl₃–NH₃–H₂ system with different hydrogen contents for improving the toughness of ceramic matrix composites. In this study, the yield of BN was found to be 94.90% without hydrogen present in the reactant system as calculated via FactSage, while it reached 99.95% at the [H₂]/[BCl₃] ratio of 10 and the [NH₃]/[BCl₃] ratio of 1, when chemical equilibrium was reached. BN interfacial coatings containing mixture of hexagonal and turbostratic phases were obtained. The deposition rate of coating increased from 18.2 ± 0.4 nm min⁻¹ (β = 0) to 23.0 ± 0.4 nm min⁻¹ (β = 5) with the increase of hydrogen content in reactants, then it significantly decreased when β was 10. Owing to different nucleation amounts on the surface of fibers, samples S2 (β = 2) and S3 (β = 5) exhibited particles with circular shapes and smooth surfaces, while the other coatings presented particles with polygonal shapes and rough surfaces. Moreover, the onset temperature of weight gain of sample S2 was 102 °C higher than that of sample S4, thus indicating the enhancement of the high-temperature oxidation resistance of BN coating.     

Crystallization of hexagonal boron nitride exhibiting excitonic luminescence in the deep ultraviolet region at room temperature via thermal chemical vapor phase deposition

Boron nitride exhibiting intense exciton-related luminescence at 216–227 nm in the ultraviolet (UV) region was synthesized on nickel substrates by chemical vapor phase deposition using the BCl₃-NH₃ system. We investigated the effects of the deposition temperature and flow rate ratio of source gases on the cathodoluminescence property in the deep ultraviolet (DUV) light region measured at room temperature. UV-luminous hBN can form at temperatures of 1170 °C and above with a ratio of NH₃ gas flow to BCl₃ flow of 2 or below. Drastic surface roughening accompanies the formation of UV-luminous hBN with high crystallinity, showing that the etching process of nickel substrates by BCl₃ at high temperatures is related to the formation of UV-luminous hBN.     

SiCn/SiC oxidation protective coating for carbon/carbon composites

Nano-SiC (SiC_n) coating was deposited on SiC pre-coated C/C composites by a hydrothermal electrophoretic deposition. The phase compositions, surface and cross-section microstructures, and anti-oxidation properties of the multilayer coatings were investigated. Results show that hydrothermal electrophoretic deposition is an effective route to prepare smooth and homogeneous SiC_n coating on SiC-C/C composites. The as-prepared SiC_n/SiC multilayer coatings can effectively protect C/C composites from oxidation in air at 1773 K for 202 h with a weight loss of 0.79% and at 1873 K for 64 h with a weight loss of 1.3%.     

Wet chemical deposition of BN,SiC and Si3N4 interphases on SiC fibers

Fiber coatings based on BN, BN/SiC and BN/Si₃N₄ were deposited on Hi Nicalon type S SiC fibers. The coating parameters were optimized using a design of experiments study. With optimized parameter sets, the coatings exhibited a high degree of coverage on the fibers and almost no fiber bridging could be observed. The coated fiber bundles are flexible and can be processed further by techniques such as filament winding. In comparison to a non-processed reference sample, the maximum tensile load of the fiber bundles with BN, BN/SiC and BN/Si₃N₄ coatings was reduced by only 5 %, 13 % and 10 %, respectively. The coated fiber bundles retained their tensile strength after thermal annealing up to 1650 °C in a nitrogen atmosphere for 0.5 h. SiC_f/SiC samples with BN/SiC fiber coatings exhibited higher values of bending strength and strain-to-failure as a reference sample without fiber coating indicating the functionality of the fiber coatings.     

Synthesis and microstructure of the Ti3SiC2 in SiC matrix grown by chemical vapor deposition

Ti₃SiC₂ + SiC and TiC + SiC were deposited on graphite substrate at 1300–1600 °C by chemical vapor deposition with TiCl₄, SiCl₄, C₃H₈, H₂ as reactive gases. Process parameters such as temperature, pressure, concentration of C₃H₈ were varied to study their effects on the phases and microstructure of the deposited layers. The results show that binary phases of Ti₃SiC₂ + SiC are formed at temperature less than 1400 °C. For temperature above 1500 °C, TiC + SiC phases are formed. Increase of the process pressure causes the disappearance of Ti₃SiC₂ and the formation of TiC. The surface morphology of Ti₃SiC₂ shows a plate-like structure. The hardness of Ti₃SiC₂ + SiC and TiC + SiC is HV4251 and HV4612 respectively for a load of 10 g.     

Cathode plasma electrolytic deposition of Al2O3 coatings doped with SiC particles

Semiconductor particles doped Al₂O₃ coatings were prepared by cathode plasma electrolytic deposition in Al(NO₃)₃ electrolyte dispersed with SiC micro- and nano-particles (average particle sizes of 0.5–1.7?µm and 40?nm respectively). The effects of the concentrations and particle sizes of the SiC on the microstructures and tribological performances of the composite coatings were studied. In comparison with the case of dispersing with SiC microparticles, the dispersion of SiC nanoparticles in the coatings was more uniform. When the concentration of SiC nanoparticles was 5?g/L, the surface roughness of the composite coating was reduced by 63%, compared with that of the unmodified coating. Friction results demonstrated that the addition of 5?g/L SiC nanoparticles reduced the friction coefficient from 0.60 to 0.38 and decreased the wear volume under dry friction. The current density and bath voltage were measured to analyze the effects of SiC particles on the deposition process. The results showed that the SiC particles could alter the electrical behavior of the coatings during the deposition process, weaken the bombardment of the plasma, and improve the structures of the coatings.     

The Microstructure and Dielectric Properties of SiBCN Ceramics Fabricated Via LPCVD/CVI

Low‐pressure chemical vapor deposition and infiltration was used for the first time to fabricate SiBCN ceramics with proper dielectric properties to lower the preparation temperature. SiBCN ceramics indicated by thermodynamic phase diagram with different phase compositions from CH₃SiCl₃–NH₃–BCl₃‐H₂ could be obtained through adjustable parameters. The as‐prepared SiBCN ceramics fabricated above 900°C showed proper dielectric properties with dielectric loss of about 0.1, which was due to the generation of amorphous carbon and SiC nanocrystals.     

Ammonia synthesis over the Ba-promoted ruthenium catalysts supported on boron nitride

Barium promoted ruthenium catalysts deposited on the boron nitride supports were characterised (XRD, O₂ and CO chemisorption) and tested in NH₃ synthesis. Prior to use, the raw BN materials marked as BNS (Starck, 96 m²/g) and HCV (Advanced Ceramics Corporation Cleveland USA, 40 m²/g) were heated in an ammonia stream at 700–800 °C for 120 h. As a result, the oxygen content was reduced from 7.0 at% (BNS) to 3.5 at% (BNS_NH3) and from 3.8 to 2.7 at% (HCV_NH3), as evidenced by XPS. The kinetic studies of NH₃ synthesis (63 or 90 bar; H₂:N₂ = 3:1) revealed that the catalysts based on the modified supports were more active, respectively, than those derived from starting nitrides, the difference being especially pronounced in the case of BNS and BNS_NH3. Studies of the catalysts activation have shown, in turn, that the stabilisation in a H₂:N₂=3:1 mixture at 1bar is very slow, i.e. the reaction rate increases slowly versus time on stream even at a high temperature of 550 – 600°C. Stabilisation is faster and the NH₃ synthesis rates are higher when the activation is performed with an ammonia rich mixture (10% NH₃ in H₂:N₂=3:1) flowing under high pressure of 90 bar. It is suggested that boron oxide (an impurity) acts as a deactivating agent for Ba–Ru/BN and that the reaction between NH₃ and B₂O₃ (B₂O₃+2NH₃=2BN +3H₂O) is responsible for the activity increase. A poisoning mechanism of B₂O₃ is discussed.     

Influence of H2/NH3 mixtures on the composition of SiCNYO and SiCNAl(O) nanopowders

We performed pyrolysis of SiCNAlH and SiCNYOH nanopowder precursors under a reactive atmosphere (Ar/NH₃/H₂) with various compositions of ammonia (NH₃) and dihydrogen (H₂) to diminish C content, which is deleterious for thermal stability and sintering of the powders. This paper continues a previous work on the fabrication of an Si₃N₄/SiC composite without free C by studying the effect of H₂ on the C/N atomic ratio of the powder. We studied the influence of the nature of the gaseous mixture (Ar/NH₃/H₂) on the powder composition. Elemental analysis showed that the introduction of H₂ in the pyrolysis atmosphere limited the decomposition of NH₃ and allowed for control of the C/N ratio. This behaviour can be explained by the structural evolution observed by ²⁹Si NMR spectrometry but also by Fourier transform infrared and Raman spectroscopy. An Si₃N₄/SiC composite, with traces of free C, was obtained after post-pyrolysis heat treatment of the powders synthesized with 10 wt.% of H₂ and 25 wt.% NH₃.     

Microstructure and deposition mechanism of CVD amorphous boron carbide coatings deposited on SiC substrates at low temperature

Amorphous boron carbide (α-B₄C) coatings were prepared on SiC substrates by chemical vapor deposition (CVD) from CH₄/BCl₃/H₂/Ar mixtures at low temperature (900–1050 °C) and reduced pressure (10 kPa). The deposited coatings were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), micro-Raman spectroscopy, energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The results showed that two kinds of α-B₄C coatings were deposited with different microstructures and phase compositions, and the effect of deposition temperature was significant. When deposited at 1000 °C and 1050 °C, the coatings exhibited a nodular morphology and had a relatively low content of boron. The free carbon was distributed in them inhomogeneously; in contrast, when deposited at 900 °C and 950 °C, the coatings presented a comparatively flat morphology and had a uniform internal structure and high boron content. They did not contain free carbon. At the last of this paper, the pertinent mechanisms resulting in differences in microstructure and phase composition were discussed.     

Microstructure and tensile behavior of (BN/SiC)n coated SiC fibers and SiC/SiC minicomposites

Interphase plays an important role in the mechanical behavior of SiC/SiC ceramic-matrix composites (CMCs). In this paper, the microstructure and tensile behavior of multilayered (BN/SiC)_n coated SiC fiber and SiC/SiC minicomposites were investigated. The surface roughness of the original SiC fiber and SiC fiber deposited with multilayered (BN/SiC), (BN/SiC)₂, and (BN/SiC)₄ (BN/SiC)₈ interphase was analyzed through the scanning electronic microscope (SEM) and atomic force microscope (AFM) and X-ray diffraction (XRD) analysis. Monotonic tensile experiments were conducted for original SiC fiber, SiC fiber with different multilayered (BN/SiC)_n interfaces, and SiC/SiC minicomposites. Considering multiple damage mechanisms, e.g., matrix cracking, interface debonding, and fibers failure, a damage-based micromechanical constitutive model was developed to predict the tensile stress-strain response curves. Multiple damage parameters (e.g., matrix cracking stress, saturation matrix crack stress, tensile strength and failure strain, and composite’s tangent modulus) were used to characterize the tensile damage behavior in SiC/SiC minicomposites. Effects of multilayered interphase on the interface shear stress, fiber characteristic strength, tensile damage and fracture behavior, and strength distribution in SiC/SiC minicomposites were analyzed. The deposited multilayered (BN/SiC)_n interphase protected the SiC fiber and increased the interface shear stress, fiber characteristic strength, leading to the higher matrix cracking stress, saturation matrix cracking stress, tensile strength and fracture strain.     

Investigation of 1,3,5-tris(2-metoxypropane)benzene/BCl3 initiated living isobutylene polymerization by C13 and B11 NMR spectroscopy

Summary The living carbocationic polymerization of isobutylene initiated by 1,3,5-tris(2-methoxypropane) benzene (TriCumOMe)/BCl₃ system was investigated by C¹³ and B¹¹ NMR spectroscopy. The reaction between the TriCumOMe and BCl₃ at-30°C in CH₂Cl₂ after 15 mins reaction time resulted in 1,3,5-tris(2-chloropropane) benzene (TriCumCl) and methyl-dichloroboronite (BCl₂OMe). The same system in the presence of isobutylene yielded three-arm star, chlorine terminated telechelic polyisobutylene and BCl₂OMe. No counterions, i.e., BCl₃OMe, BCl ₄ , or neutral boron complexes, e.g., TriCumOMe, 3BCl₃ could be detected. The simultaneous measurement of static permittivity (direct monitoring method) showed different reaction rate patterns in the case of AMI method, and when the TriCumOMe+BCl₃ mixture was aged and the polymerization was started by isobutylene.     

The wetting-to-nonwetting transition of CVD BN coatings deposited at different temperatures

Boron nitride (BN) coatings (thickness 20–40 μm) were prepared on graphite substrates by chemical vapor deposition, with precursors of BCl₃ and NH₃ (ratio of 1:4) and pressure of 500 ± 50 Pa. The influence of the deposition temperature (650°C–1250°C) on the wettability of BN coatings with deionized water was studied. The wetting angle rapidly increases at 1100°C–1250°C, and the wetting-to-nonwetting transition occurs. The crystal structure and surface morphology of the BN coatings were characterized by a stylus instrument, scanning electron microscopy, and transmission electron microscopy. Research shows that the contact angle or nonwettability increases with a higher degree of crystallinity and a lower surface roughness, which were both under the control of the deposition temperature since the pressure and gas flows were kept constant in this study. At a deposition temperature of 650°C–950°C, the increase in the degree of crystallinity dominates; at 950°C–1100°C, the increase in surface roughness takes over. At 1100°C–1250°C, the degree of crystallinity continues to increase, while the surface roughness decreases due to the advantage of nucleation and the breakage of large surface clusters into smaller clusters. This results in increases (650°C–950°C), then decreases (950°C–1100°C) and again fast increases (1100°C–1250°C) in the wetting angle between the BN coating and deionized water and finally in the wetting-to-nonwetting transition (1100°C–1250°C).     

Catalytic steam reforming of ethane and propane over CeO2-doped Ni/Al2O3 at SOFC temperature: Improvement of resistance toward carbon formation by the redox property of doping CeO2

Ni/Al₂O₃ with the doping of CeO₂ was found to have useful activity to reform ethane and propane with steam under Solid Oxide Fuel Cells (SOFCs) conditions, 700-900 °C. CeO₂-doped Ni/Al₂O₃ with 14% ceria doping content showed the best reforming activity among those with the ceria content between 0 and 20%. The amount of carbon formation decreased with increasing Ce content. However, Ni was easily oxidized when more than 16% of ceria was doped. Compared to conventional Ni/Al₂O₃, 14%CeO₂-doped Ni/Al₂O₃ provides significantly higher reforming reactivity and resistance toward carbon deposition. These enhancements are mainly due to the influence of the redox properties of doped ceria. Regarding the temperature programmed reduction experiments (TPR-1), the redox properties and the oxygen storage capacity (OSC) for the catalysts increased with increasing Ce doping content. In addition, it was also proven in the present work that the redox of these catalysts are reversible, according to the temperature programmed oxidation (TPO) and the second time temperature programmed reduction (TPR-2) results.During the reforming process, in addition to the reactions on Ni surface, the gas-solid reactions between the gaseous components presented in the system (C₂H₆, C₃H₈, C₂H₄, CH₄, CO₂, CO, H₂O, and H₂) and the lattice oxygen (O_x) on ceria surface also take place. The reactions of adsorbed surface hydrocarbons with the lattice oxygen (O_x) on ceria surface (C_nH_m+O_x→nCO+m/2(H₂)+O_x−n) can prevent the formation of carbon species on Ni surface from hydrocarbons decomposition reaction (C_nH_m⇔nC+m/2H₂). Moreover, the formation of carbon via Boudard reaction (2CO⇔CO₂+C) is also reduced by the gas-solid reaction of carbon monoxide (produced from steam reforming) with the lattice oxygen (CO+O_x⇔CO₂+O_x−1).     

SiC/SiC minicomposites with structure-graded BN interphases

BN interphases in SiC/SiC minicomposites were produced by infiltration of fibre tows from BF₃–NH₃–H₂ gaseous system. During interphase one-step processing, the tow travels through a reactor containing a succession of different hot areas. By TEM characterization, the BN interphases were found to be made of a structural gradient: from isotropic to highly anisotropic. The very first coating is poorly organised and allows to protect the fibre from a further chemical attack by the reactant mixture. The minicomposites were tensile tested at room temperature with unloading-reloading cycles. The BN interphases act as mechanical fuses; the fibre/matrix bonding intensity ranges from weak to rather strong depending on the tow travelling rate during interphase infiltration. The specimen lifetimes at 700°C under a constant tensile loading were measured in dry and moist air. Compared to a pyrocarbon reference interphase, the BN interphases significantly improve the oxidation resistance of the SiC/SiC minicomposites.     

Synthesis of coatings on SiC fibers and their effects on microstructure and mechanical properties of SiCf/SiBCN composites

In this study, the amorphous C, ZrB₂, and BN single-layer coatings as well as C/BN, C/ZrB₂, ZrB₂/BN, and C/ZrB₂/BN composite coatings were prepared on SiC fibers (SiC_f) by an in situ synthesis and solution impregnation–pyrolysis method. Subsequently, SiC_f/SiBCN composites were fabricated by hot-pressing sintering at 1900℃/60 MPa/30 min to explore the influence of different coatings on the microstructure and mechanical performance of resulting composites. After the preparation of single-layer-coated SiC_f, the SiC_f(BN) or SiC_f(ZrB₂) tended to be overlapped with each other, whereas the dispersion of amorphous C–coated SiC_f was satisfying. Besides, some uneven areas and attached particles have appeared on fiber surfaces of the SiC_f(BN) or SiC_f(ZrB₂), whereas smooth and dense surfaces of amorphous C–coated SiC_f were observed. Because the uniformity of ZrB₂ coatings can be partially damaged by the subsequent coating process of BN, the composite coatings of ZrB₂/BN and C/ZrB₂/BN were thereby not suitable for strengthening SiBCN matrix. The SiC_f/SiBCN composites with C/ZrB₂ coatings have desirable comprehensive mechanical properties. Nevertheless, the conventional toughening mechanisms such as fiber pull-out and bridging, and crack deflection are not available for these composites because the serious crystallization of SiC_f leading to great strength loss, resulting in catastrophic brittle fracture.     

Si/B/C/N/Al precursor-derived ceramics: Synthesis,high temperature behaviour and oxidation resistance

The Si/B/C/N/H polymer T2(1), [B(C₂H₄Si(CH₃)NH)₃]_n, was reacted with different amounts of H₃Al·NMe₃ to produce three organometallic precursors for Si/B/C/N/Al ceramics. These precursors were transformed into ceramic materials by thermolysis at 1400 °C. The ceramic yield varied from 63% for the Al-poor polymer (3.6 wt.% Al) to 71% for the Al-rich precursor (9.2 wt.% Al). The as-thermolysed ceramics contained nano-sized SiC crystals. Heat treatment at 1800 °C led to the formation of a microstructure composed of crystalline SiC, Si₃N₄, AlN(+SiC) and a BNC_x phase. At 2000 °C, nitrogen-containing phases (partly) decomposed in a nitrogen or argon atmosphere. The high temperature stability was not clearly related to the aluminium concentration within the samples. The oxidation behaviour was analysed at 1100, 1300, and 1500 °C. The addition of aluminium significantly improved the oxide scale quality with respect to adhesion, cracking and bubble formation compared to Al-free Si(/B)/C/N ceramics. Scale growth rates on Si/B/C/N/Al ceramics at 1500 °C were comparable with CVD–SiC and CVD–Si₃N₄, which makes these materials promising candidates for high-temperature applications in oxidizing environments.     

HfB2 coating on C/C composites prepared by chemical vapor deposition: Thermodynamics and experimental investigation

A thermodynamic calculation on the HfB₂ coating prepared by chemical vapor deposition (CVD) through HfCl₄-BCl₃-H₂-Ar system was performed, together with the relevant verification experiments. The calculation results indicated that HfB₂ coating could be obtained above 900 °C with the ratios of BCl₃/HfCl₄ and H₂/HfCl₄ higher than 1 and 12, respectively. The experimental results demonstrated that the deposition temperature, H₂ and BCl₃ flow rates had significant effects on the grain size, growth rate and phase composition of HfB₂ coatings. A dense and uniform HfB₂ coating was prepared at 1150 °C with a BCl₃/HfCl₄ ratio of 3 and a H₂/HfCl₄ ratio of 20, whose mass and linear ablation rates were 15.61 mg/s and 15.58 μm/s under oxyacetylene flame.