Influence of Isothermal Chemical Vapor Deposition and Chemical Vapor Infiltration Conditions on the Deposition Kinetics and Structure of Boron Nitride

An experimental study has been performed to gain some insight into the correlations between the deposition conditions and the structure of boron nitride (BN) coatings that are used in ceramic-matrix composites. BN has been deposited at 700°C from BCl₃-NH₃-H₂ mixtures on various substrates, by using chemical vapor deposition (CVD) and isothermal-isobaric chemical vapor infiltration (ICVI) processes, simultaneously in the same reactor. A kinetic study has shown that the CVD process is governed either by a combination of mass transfer with chemical kinetics at low flow rates or by the heterogeneous kinetics only at high flow velocities. In contrast, the limiting contribution of mass transfer always is observed for the ICVI process. The influence of diffusion cages that are positioned around the fibrous preforms is reported. The structure of BN deposits has been studied as a function of the various deposition conditions via transmission electron microscopy. The chosen CVD conditions lead to a poor organization of the BN deposits. Fairly well-organized BN coatings are deposited on all fibers of a fibrous preform via ICVI. The results are discussed in terms of supersaturation and deposition yields. The use of diffusion cages and the adjustment of the inlet composition and mass flow rate seem to be very important to obtain the best BN organization and thickness uniformity.     

Polymeric Cyanoborane, (CNBH2)n: Single Source for Chemical Vapor Deposition of Boron Nitride Films

Polymeric cyanoborane, (CNBH₂) _n , is a material readily prepared by passing hydrogen chloride through an ether suspension of sodium cyanotrihydroborate. This polymeric material was volatilized in a CVD reactor to produce, at 600°C, amorphous films containing boron, nitrogen, and carbon. Residual carbon present in the films was removed by ammonia treatment at 800°C, producing nearly stoichiometric boron nitride films that were adherent to a variety of substrates including silicon.     

Characterization of Dip-Coated Boron Nitride on Silicon Carbide Fibers

Amorphous boron nitride thin coatings (∼0.2 μm) have been formed on Nicalon and C-Nicaion (pre-carbon-coated Nicalon) yarns via dip coating in boric acid solution followed by heating and nitriding in NH₃gas at 1000°C. X-ray photoelectron spectroscopy (XPS) and Auger electron spec-troscopy (AES) studies have shown the formation of boron nitride. The coating was boron rich and contains oxygen. The N/B and O/B ratios range from 0.6 to 0.8 and from 0.1 to 0.25, respectively. Tensile strength measurements revealed that the BN-coated C-Nicalon yarn maintained ∼85% of its original strength while BN-coated Nicalon lost ∼85% of its original strength. Auger depth profiles showed that there was a consumption of carbon during the heating and nitridation process for both BN-coated Nicalon and C-Nicalon fibers. However, the depletion of carbon in BN-coated Nicalon fibers was much more severe than that in BN-coated C-Nicalon fibers.     

Influence of Microstructure and Temperature on the Interfacial Fracture Energy of Silicon Nitride/Boron Nitride Fibrous Monolithic Ceramics

The microstructure and interfacial fracture energy of silicon nitride/boron nitride fibrous monoliths, Gamma_BN, were determined as a function of starting silicon nitride composition and temperature using the method described by Charalambides. The glassy phase created by the sintering aids added to the silicon nitride cells was shown to migrate into the boron nitride cell boundaries during hot-pressing. The amount of glassy phase in the boron nitride cell boundaries was shown to strongly influence Gamma_BN at room temperature, increasing the fracture energy with increasing amounts of glass. Similar trends in the interfacial fracture energy as a function of temperature were demonstrated by both compositions of fibrous monoliths, with a large peak in Gamma_BN observed over a narrow temperature range. For silicon nitride cells densified with 6 wt% yttria and 2 wt% alumina, the room-temperature interfacial fracture energy was 37 J/m², remaining constant through 950°C. A sharp increase in Gamma_BN, to 60 J/m², was observed between 1000° and 1050°C. This increase was attributed to interactions of the crack tip with the glassy phase in the boron nitride cell boundary. Measurements at 1075°C indicated a marked decrease in Gamma_BN to 39 J/m². The interfacial fracture energy decreased with increasing temperature in the 1200° to 1300°C regime, plateauing between 17 to 20 J/m². A crack propagation model based on linkup of existing microcracks and peeling/cleaving boron nitride has been proposed.     

Preparation and Use of a Boron Nitride Precursor as Binder for the Densification of Boron Nitride

A precursor of boron nitride was prepared through the partial condensation of 2,4,6-trichloroborazine and bis-(trimethylsilyl)acetylene. This reaction was conducted at 100°C and is catalyzed by AlCl₃. The condensation product pyrolyzed at 800°C, producing trimethylsilyl chloride as a volatile product and a boron nitride rich residue containing 54 wt% of the initial weight. Mixtures of the precursor and commercial boron nitride were made and hot-pressed at 800°C and 27.6 MPa. A maximum density of 1.84 g/cm³ is reached at a loading corresponding to the deposition of 13 wt% residue derived from the precursor. Examination by analytical electron microscopy, including X-ray energy dispersive spectroscopy and electron energy loss spectroscopy analyses, revealed the location of material derived from the precursor in BN-binder composites through the presence of residual aluminum, silicon, and carbon. Crystallization of boron nitride from the precursor appears to have taken place, as deduced from the morphology of the phases observed and association with residual elements present in the binder.     

Boron Nitride Powders Formed by Aerosol Decomposition of Poly(borazinylamine) Solutions

Micrometer-sized aerosol droplets of a liquid ammonia solution of poly(borazinylamine) were formed in a nitrogen carrier gas by using an aerosol generator. The resulting droplets were thermally decomposed in a flow reactor at 1000°C. The white powder obtained was amorphous to X-rays and spherical with particle sizes on the order of 0.5 μm. The particles at this stage were porous, and some appeared to exhibit a hollow-shell morphology. Subsequent calcining at 1600°C yielded dense, crystalline boron nitride powder with partial sintering.     

Synthesis of Nanocrystalline Cubic Tantalum(III) Nitride Powders by Nitridation–Thermal Decomposition

Tantalum(V) nitride, prepared by nitridation of nanosized Ta₂O₅ at 800°C for 8 h under ammonia flow, was thermally decomposed to cubic nanocrystalline TaN at a temperature of 1000°C for 3 h under argon atmosphere. The resulting powders have been characterized using X-ray diffraction (XRD) and transmission electron microscopy. XRD-pure cubic TaN nanoparticles with a diameter of 50–100 nm can be obtained by the process. The decomposition process was found to depend on the temperature. Mechanisms that account for the decomposition of Tantalum(V) nitride are discussed. The results indicate that the method can permit formation of cubic-phase Tantalum(III) nitride under ambient pressure and moderate temperatures.     

Characteristic Study of Silicon Nitride Films Deposited by LPCVD and PECVD

This paper analyzes and compares the characteristics of silicon nitride films deposited by low pressure chemical vapor deposition (LPCVD) and plasma enhanced chemical vapor deposition (PECVD), with special attention to the hydrogenation and chemical composition of silicon nitride films. Three different LPCVD processes at various DCS and NH₃ gas flow rates and deposition temperatures, together with PECVD using SiH₄ and NH₃ and ICP CVD using SiH₄ and N₂, were compared. The silicon nitride film deposition rate decreases with an increasing NH₃/DCS ratio in LPCVD, which also leads to an increase in the refractive index and a decrease in the residual stress in the film. There is nearly no hydrogen incorporated in the LPCVD films, which differs from PECVD and ICP CVD that show significant Si-H and N-H bonds. The chemical composition of silicon nitride films is mostly Si-rich, except for the LPCVD process at high NH₃/DCS ratio with near stoichiometric chemistry.     

Sintering of the Mechanochemically Activated Powders of Hexagonal Boron Nitride

Pressureless sintering of hexagonal boron nitride (BN) was performed using a powder activated by mechano-chemical treatments. Physical properties of the sintered BN bodies depend on the type of starting powder and the conditions of the treatments. The BN body, which was obtained at 2000°C using an appropriate activated powder, was 99 wt% pure and was excellent in mechanical and physical properties, in spite of its low density (1.64 g/cm³).     

Low-Temperature Chemical Vapor Deposition Tungsten Carbide Coatings for Wear/Erosion Resistance

A new chemical vapor deposition (CVD) process has been developed to deposit hard coatings, containing tungsten carbide, at temperatures below 500°C. These coatings, which have been applied to both ferrous and nonferrous alloys, exhibit excellent resistance to wear and erosion. The coatings comprise a mixture of tungsten and the tungsten carbide, the latter being present as W₂C, W₂C + W₃C, or W₃C. The coatings' composition and properties can be controlled by varying the CVD process parameters. The unique lamellar, fine-grained microstructures of these coatings contribute to their good tribological properties.     

Bilayered coatings of BN/diamond grown on Si3N4 ceramic substrates

Cubic boron nitride (c-BN) is a well known material to be used in machining of ferrous metallic alloys, namely as a coating. However, in most practical cases, there is a lack of adhesion to the substrate material. In this work, BN coatings were deposited by magnetron sputtering on silicon nitride (Si₃N₄) ceramic substrates using an intermediate layer of CVD microcrystalline (MCD) or nanocrystalline diamond (NCD). The goal was to improve the c-BN content by using diamond interlayers, and to optimize film adhesion to the substrate by employing such ceramic, which is known to provide very high adhesion strength to CVD diamond. The BN/NCD/Si₃N₄ combination demonstrated to be unique regarding the absence of delamination at both the BN/diamond and diamond/substrate interfaces, also providing the highest c-BN phase content.     

Electrical Resistivity and Microwave Transmission of Hexagonal Boron Nitride

The dc conductivity of hexagonal boron nitride (BN) and BN-containing composites was measured as a function of temperature up to 2400°C. The results confirm that at high temperatures BN is an intrinsic semiconductor with an energy gap of 0.99 ± 0.06 aJ (6.2 ± 0.4 eV) at T = 0 K. Extrapolated values for the resistivity of BN in the range 2600° to 3000°C are used to analyze the absorption, reflectivity, and transmissivity of a BN window when subjected to microwave radiation under atmospheric reentry conditions. It appears that the transmissivity is of the order of 1 to 10% at these temperatures due mainly to the high conductivity in a very thin, very hot surface layer. The transmissivity can be improved by using a composite made of boron nitride and silica.     

Selective etching of boron nitride phases

Cubic boron nitride (c-BN) films can be used as hard coatings and for electronic devices due to their outstanding material properties, but the gas phase deposition of c-BN is still a challenging task. Until now it has only been possible to achieve nanocrystalline c-BN layers via physical vapor deposition (PVD) methods with rather weak film qualities. Only a chemical vapor deposition (CVD) process for c-BN can produce high quality films with material properties similar to those of the product achieved by high pressure, high temperature processes (HPHT) conventional routes. Therefore it is essential to tune the individual steps in the CVD process (nucleation, growth and selective etching) in a similar manner to that for diamond CVD to enable continuous growth of c-BN.Since selective etching of hexagonal boron nitride (h-BN) and sp² phases is still a major problem, we investigated the interaction of h-BN and c-BN with different reactive gases — ammonia (NH₃), chlorine (Cl₂), hydrogen chloride (HCl) and boron trifluoride (BF₃) — regarding their etching behaviour and surface stabilisation properties. Etching ratios from ≈10:1 up to 450:1 were found in the temperature range 600–1300°C for the h-BN/c-BN system, clearly indicating a high selectivity due to kinetic effects.The reaction mechanisms will be discussed with respect to the kinetic differentiation of the degradation of c-BN and h-BN (selective etching). The morphological changes and the quality of the remaining BN phases was studied by scanning electron microscopy (SEM), X-ray diffraction (XRD), and infrared and Raman spectroscopy and these indicated a homogeneous decay of the individual phases. Since a homogeneous decay of c-BN resembles the reversed growth, the study of the interaction of both BN phases with reactive gases allowed us to collect more detailed information of the molecular mechanisms involved in the formation of the individual phases. These results will provide new routes for growing c-BN in a CVD process.     

High-Thermal-Conductivity AlN Filler for Polymer/Ceramics Composites

To increase the thermal conductivity of polymer/ceramic composites, aluminum nitride (AlN) granules were added as a ceramic filler. Granules, sintered at 1850°C for 24 h, showed a very high conductivity of 266±26 W (m·°C)⁻¹, as measured by a thermal microscope equipped with thermoreflectant and periodic heating techniques. This conductivity exceeds 80% of the theoretical value of AlN. Ceramic fillers consisting of the obtained AlN granules and commercially available hexagonal boron nitride particles (h-BN) powder plus polyimide resins were mixed and then molded at 100 MPa and 420°C in a vacuum. The resultant composite showed a high conductivity of 9.3 W (m·°C)⁻¹. This study demonstrates that a high-thermal-conductivity filler effectively enhances the conductivity of polymer/ceramic composites.     

Thermal Oxidation of Aluminum Nitride Powder

The oxidation kinetics, morphology, and crystallinity of aluminum nitride (AlN) powder thermally oxidized in flowing oxygen were determined from 800° to 1150°C. At 800°C the oxidation became detectable with weight change. AlN powder was almost completely oxidized at 1050°C after only 0.5 h. Amorphous aluminum oxide formed at relatively low temperatures (800°–1000°C), with a linear oxidation rate governed by the oxygen–nitride interfacial reaction. Transmission electron microscopy displayed individual aluminum oxide grains which formed a discontinuous oxide layer at this temperature range. The aluminum oxide was crystalline at higher temperatures (>1000°C), as detected by X-ray diffraction, and the density of oxide grains increased with temperature.     

Elevated-Temperature Mechanical Properties of Silicon Nitride/Boron Nitride Fibrous Monolithic Ceramics

A unique, all-ceramic material capable of nonbrittle fracture via crack deflection and delamination has been mechanically characterized from 25° through 1400°C. This material, fibrous monoliths, was comprised of unidirectionally aligned 250 μm diameter silicon nitride cells surrounded by 10 to 20 μm thick boron nitride cell boundaries. The average flexure strengths of fibrous monoliths were 510 and 290 MPa for specimens tested at room temperature and 1300°C, respectively. Crack deflection in the BN cell boundaries was observed at all temperatures. Characteristic flexural responses were observed at temperatures between 25° and 1400°C. Changes in the flexural response at different temperatures were attributed to changes in the physical properties of either the silicon nitride cells or boron nitride cell boundary.     

Machinable α-SiAlON/BN Composites

Dense machinable α-SiAlON/BN composites were fabricated by hot-pressing using turbostratic boron nitride (tBN) obtained from nitridation of melamine diborate. The tBN was added to the starting powders, or introduced as a coating that formed in situ on α-Si₃N₄ carrier powders during nitridation, and was subsequently converted to hexagonal boron nitride (hBN) during hot pressing by solution reprecipitation. These composites maintain high strength at 1000°C and their strength/hardness are much higher than similar composites prepared using commercial hBN powder, which yielded a coarser microstructure. Good machinability was achieved despite a flat R curve.     

Carbothermal synthesis of boron nitride coating on PAN carbon fiber

Boron nitride (BN) thin coating has been formed on the surface of chemically activated polyacrylonitrile (PAN) carbon fiber by dip coating method. Dip coating was carried out in saturated boric acid solution followed by nitridation at a temperature of 1200 °C in nitrogen at atmospheric pressure to produce BN coating. Chemical activation improved surface area of PAN fiber which favours in situ carbothermal reduction of boric acid. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) have shown the formation of boron nitride. The X-ray photoelectron spectroscopy reveals that the coating forms a composite layer of carbon, BN/BO_xN_y and some graphite like BCN with local structure of B–N–C and B(N–C)₃. The oxidation resistance of the coated fiber was significantly higher than uncoated carbon fiber. Tensile strength measurement reveals that the BN coated fiber maintained 90% of its original strength. As compared to chemical vapor deposition (CVD), this process is simple, non-hazardous and is expected to be cost effective.     

Breath Figure Patterns in the Oxidation of Boron Nitride

Liquids condensing on solids can form a pattern known as "breath figures." Here we report similar patterns for liquid boron oxide droplets formed by high temperature oxidation reaction of boron nitride. Boron oxide does not wet boron nitride, so the liquid B₂O₃ oxide forms small droplets. As oxidation proceeds at 1200°C, the oxide created by reaction on the surface migrates to existing droplets or forms new droplets. The average diameter and maximum diameter of the boron oxide droplets increase with time following the kinetics of breath figures, similar to water condensation breath figures for dew drops.     

Wettability of Pyrolytic Boron Nitride by Aluminum

The wetting of pyrolytic boron nitride by molten 99.9999% pure aluminum was investigated by using the sessile drop method in a vacuum operating at approximately 660 μPa at temperatures ranging from 700° to 1000°C. The equilibrium contact angle decreased with an increase in temperture. For temperatures at 900°C or less the equilibrium contact angle was greater than 90°. At 1000°C a nonwetting-to-wetting transition occurred and the contact angle stabilized at 49°.