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.     

Crystallization Behavior of Amorphous Si2BC3N Ceramic Monolith Subjected to High Pressure

The crystallization behavior of amorphous Si₂BC₃N monoliths by heating at 1000°C–1400°C and 5 GPa was investigated with the special attention to the nucleation mechanisms of β‐SiC and BN(C) phases. Nanoscale puckered structures arising in particle bridging areas were found and its evolution behavior well reflected the nucleation process of nanocrystallites. The temperature‐dependent crystallization of amorphous Si₂BC₃N monoliths at 5 GPa passes through four stages: The material remains amorphous below 1100°C. It undergoes partial phase segregation (1100°C–1200°C), followed by initiation of nucleation (1200°C–1250°C), and then nucleation and growth of β‐SiC and turbostratic BN(C) crystallites (>1250°C). The first principles calculation indicates the nucleation precedence of BN(C) phase over β‐SiC. BN(C) nucleates preferentially at bridges between ceramic particles causing SiC to concentrate in particle interiors thus forming capsule‐like structures.     

Characterization of Yb2SiO5-based environmental barrier coating prepared by plasma spray–physical vapor deposition

Due to the high-input power compared to atmospheric plasma spraying (APS), plasma spray-physical vapor deposition (PS-PVD) can primarily achieve a splat-like deposition, allowing for the preparation of high-density environmental barrier coatings (EBCs). In this paper, dense Yb₂SiO₅-based coatings are prepared by PS-PVD at different substrate temperatures. It was found that the coating deposited at the substrate temperature of 700 °C contained a large amount of silicon-rich amorphous phase. When the substrate temperature increased to 1100 °C and a slow cooling process after deposition was involved, a coating with high crystallinity of ～77% and low porosity of less than ～2% was achieved. Phase evolution of the coatings was studied by a semi-in-situ high-temperature X-ray diffractometer. During the heating process, metastable phases X1-Yb₂SiO₅ and α-Yb₂Si₂O₇ emerged and transformed into stable phases following high-temperature treatment. Furthermore, the effects of long-term thermal aging at 1300 °C on the microstructure, phase composition, thermal conductivity, and hardness of the coating prepared at the substrate temperature of 1100 °C were found to be limited.     

Low temperature atomic layer deposition of SiO2 thin films using di-isopropylaminosilane and ozone

Silicon dioxide (SiO₂) films are deposited by atomic layer deposition (ALD) at low temperatures from 100 to 200 °C using di-isopropylaminosilane (SiH₃N(C₃H₇)₂, DIPAS) as the Si precursor and ozone as the reactant. The SiO₂ films exhibit saturated growth behavior confirming the ALD process, showing a growth rate of 1.2 Å/cycle at 150 °C, which increases to 2.3 Å/cycle at 250 °C. The activation energy of 0.24 eV, extracted from temperature range of 100–200 °C, corresponds to the reported energy barrier for reaction between DIPAS and surface –OH. The temperature dependence of the growth rate can be explained in terms of the coverage and chemical reactivity of the thermally activated precursor on the surface. The ALD-SiO₂ films deposited at 200 °C show properties such as refractive index, density, and roughness comparable to those of conventionally deposited SiO₂, as well as low leakage current and high breakdown field. The fraction of Si–O bond increases at the expense of Si–OH at higher deposition temperature.     

Stressed-oxidation behavior of 2D CVI SiC/BN/SiC at elevated temperature in air

The stressed-oxidation behaviors of 2D woven SiCf/BN/SiC composites were investigated at 950 °C and 1100 °C in air. The different proportions (60%–90%) of the ultimate tensile strength (UTS) at corresponding temperatures were chosen as constant stress. The stressed-oxidation experiments were taken to failure or interrupted (240h). The UTS decreases by 20.75% at 950 °C and 30.71% at 1100 °C. The composites did not fail during stressed oxidation when subjected to constant stress corresponding to the initial linear and the beginning of nonlinear segments of the tensile curve, above which the composites failed with a maximum failure life of about 10h. Fiber degradation due to the thermal exposure and the fiber cracks caused by the oxidation of BN interface coating and SiC fiber could be responsible for the strength degradation and failure of the composites during stressed oxidation.     

The effects of deposition temperature on structure and dielectric properties of Ba0.6Sr0.4TiO3 thin films produced by pulsed laser deposition

The effects of deposition temperature on orientation, surface morphology and dielectric properties of the thin films for Ba_0.6Sr_0.4TiO₃ thin films deposited on Pt/Ti/SiO₂/Si substrates by pulsed laser deposition were investigated. X-ray diffraction patterns revealed a (2 1 0) preferred orientation for all the films. With rising substrate temperature from 650 °C to 700 °C, the crystallinity and crystal grain size of the films increase, the relative dielectric constant increases, but the dielectric losses have not obvious difference. The film deposited at 350 °C and annealed at 700 °C has strongly improved roughness and dielectric permittivity compared with the film only deposited directly at 700 °C. Three distinct relaxation processes within tan(δ) were found for the Ba_xSr_1?xTiO₃ film: a broadened process of the film relaxation, an intermediate peak which originates from Maxwell–Wagner–Sillars polarization, and an extremely slow process ascribed to leak current. The complex dielectric permittivity and loss can be fitted by an improved Cole–Cole model corresponding to a stretched relaxation function.     

Thermostability,oxidation, and high-temperature tribological properties of nano-multilayered AlCrSiN/VN coatings

In this study, monolithic AlCrSiN, VN, and nano-multilayered AlCrSiN/VN coatings were deposited using a hybrid deposition system combining arc ion plating and pulsed direct current magnetron sputtering. The microstructure, thermostability, mechanical, oxidation and tribological properties of the coatings were comparably investigated. The multilayered AlCrSiN/VN coating exhibited a face-centered cubic (fcc) structure with (200) preferred orientation and showed the highest hardness (30.7 ± 0.5 GPa) among these three coatings due to the multilayer interface enhancement mechanism and higher compressive stress. The AlCrSiN sublayers effectively prevented the V element from rapid outward diffusion to the surface of AlCrSiN/VN coating at elevated temperatures, which improved the oxidation resistance of the coating. Decomposition of V (Cr)–N bonds occurred at annealing temperatures from 800 °C to 1000 °C and V₂N phase appeared at 1100 °C. The AlCrSiN/VN coating showed excellent tribological performance at high temperatures by combining the merits of VN layers for low friction coefficient and AlCrSiN layers for superior oxidation resistance. Compared to VN and AlCrSiN coatings, AlCrSiN/VN coating showed the lowest wear rate of 2.6×10^-15 m³/N·m at 600 °C and lowest friction coefficient of 0.26 at 800 °C with a relativity low wear rate of 39.4×10^-15 m³/N·m.     

Remote plasma enhanced metal organic chemical vapor deposition of TiN for diffusion barrier

TiN films were deposited with remote plasma metal organic chemical vapor deposition (MOCVD) from tetrakis-diethyl-amido-titanium (TDEAT) at substrate temperature of 250–500°C and plasma power of 20–80 W. The growth rate using N₂ plasma is slower than that with H₂ plasma and showed 9.33 kcal/mol of activation energy. In the range of 350–400°C., higher crystallinity and surface roughness were observed and resistivity was relatively low. As the temperature increased to 500°C., randomely oriented structure and smooth surface with higher resistivity were obtained. At low deposition temperature, carbon was incorporated as TiC phase, as the deposition temperature increases, carbon was found as hydrocarbon. At 40 W of plasma power, higher crystallinity and rough surface with lower resistivity were obtained and increasing the plasma power to 80 W leads to low crystallinity, smooth surface and higher resistivity. It may be due to the incorporation of hydrocarbon decomposed in the gas phase. Surface roughness was found to be related to the crystallinity of the film.     

Titanium oxide coatings formed by plasma spraying followed by induction heat treatment

Porous titanium-containing coatings were formed on titanium samples by atmospheric plasma spraying. The resulting samples were subjected to induction heat treatment in an oxygen-containing (air) atmosphere at normal pressure and treatment temperature of 650–1250 °C. As a result of experimental studies, it was established that thermal modification allowed an increase in the oxygen content in the coatings from 49.6 ± 9.2 to 71.7 ± 1.1 at.%. The change in the elemental chemical composition was accompanied by the formation of titanium oxide coatings, the surface of which was distinguished by the presence of acicular crystals of titanium oxide (TiO), anatase, and rutile (TiO₂) with an average length of 150–200 nm and a width of about 50–100 nm. Induction heat treatment also led to an increase in the microhardness of titanium oxide coatings from 1530 ± 55 to 1825 ± 191 HV_0.98 and an increase in adhesive strength.     

CMAS (CaO–MgO–Al2O3–SiO2) resistance of Y2O3-stabilized ZrO2 thermal barrier coatings with Pt layers

Calcium–magnesium–alumina–silicate (CMAS) corrosion significantly affects the durability of thermal barrier coatings (TBCs). In this study, Y₂O₃ partially stabilized ZrO₂ (YSZ) TBCs are produced by electron beam-physical vapor deposition, followed by deposition of a Pt layer on the coating surfaces to improve the CMAS resistance. After exposure to 1250 °C for 2 h, the YSZ TBCs were severely attacked by molten CMAS, whereas the Pt-covered coatings exhibited improved CMAS resistance. However, the Pt layers seemed to be easily destroyed by the molten CMAS. With increased heat duration, the Pt layers became thinner. After CMAS attack at 1250 °C for 8 h, only a small amount of Pt remained on the coating surfaces, leading to accelerated degradation of the coatings. To fully exploit the protectiveness of the Pt layers against CMAS attack, it is necessary to improve the thermal compatibility between the Pt layers and molten CMAS.     

Oxidation of SiC/BN/SiC ceramic matrix composites in dry and wet oxygen at intermediate temperatures

The oxidation behavior of SiC/BN/SiC ceramic matrix composites (CMCs) was evaluated from 400° to 800 °C in 100% O₂ and 50% H₂O/50% O₂ gas mixtures. Thermogravimetric analysis (TGA) was utilized to measure weight change during controlled environment exposures at elevated temperatures for 1 and 50 hours. Oxidized CMCs and their oxides were studied post-exposure with scanning electron microscopy and energy dispersive spectroscopy. The oxidation onset and composition transition temperatures were evaluated. Key observations include oxide composition, oxide wetting, oxygen solubility in Hi-Nicalon SiC fibers and BN fiber coating oxidation and volatility behavior as a function of temperature. Degradation in wet environments at 600 °C was most extensive due to the formation of a non-wetting, non-protective surface oxide, allowing oxidant access to the BN fiber coatings followed by oxidation and volatilization. Implications of the CMC oxidation behavior are discussed for CMCs in service.     

Preparation of barium hexaferrite coatings using atmospheric plasma spraying

Thick coatings of barium hexaferrite with the compositions BaFe₁₂O₁₉ and BaCoTiFe₁₀O₁₉ were prepared using atmospheric plasma spraying (APS) technology. The coatings were prepared from pre-reacted powders of the desired composition. The as-deposited coatings were poorly crystallized, but their crystallinity was improved with a subsequent annealing. The crystallization mechanism of the sprayed hexaferrites was studied during annealing up to 1300 °C, using X-ray powder diffraction combined with thermal analysis and with electron microscopy including microanalysis. Single-phase coatings were obtained after annealing treatments at 1100–1300 °C. Their magnetic properties showed that they would be suitable for absorbers at microwave and mm-wave frequencies, depending on the coating phase's composition, the crystallinity and the thicknesses.     

Deposition Rate,Texture, and Mechanical Properties of SiC Coatings Produced by Chemical Vapor Deposition at Different Temperatures

Silicon carbide (SiC) coatings were produced on carbon/carbon (C/C) composites substrates using chemical vapor deposition (CVD) at different temperatures (1100°C, 1200°C, and 1300°C). The deposition rate was found to increase with deposition temperature from 1100°C to 1200°C. From 1200°C to 1300°C, the deposition rate decreased. SiC coating produced at 1200°C exhibited a strong (111) texture compared with the coatings produced at other temperatures. Both hardness and Young's modulus were also found to be higher in the coating produced at 1200°C. The variation in mechanical properties with the increase in temperature from 1100°C to 1300°C showed a direct correlation with the change in deposition rate and (111) texture. Microstructure analysis shows that the change in CVD temperature leads to the change in grain size, crystallinity, and density of stacking faults of SiC coatings, which appears to have no significant effect on mechanical properties of SiC compared with the texture observed in SiC coating. For the coating deposited at 1200°C, both the hardness and Young's modulus increased gradually from the substrate/coating interface to the top surface. The nonuniformity of mechanical properties along the cross‐section of the coating is attributed to the nonuniform microstructure.     

Microstructure evolution of CMAS glass below melting temperature and its potential influence on thermal barrier coatings

CaO–MgO–Al₂O₃–SiO₂ (CMAS) deposition significantly degrades the performance of thermal barrier coatings (TBCs). In this study, the microstructure evolution of CMAS glass at temperatures below its melting point was investigated in order to study the potential influence of temperature on the applicability of CMAS glass in TBCs. The CMAS glass fabricated in this study had a melting point of 1240 °C, became opaque, and underwent self-crystallization when the temperature reached 1000 °C. After heat treatment at 1050 °C, diopside and anorthite phases precipitated from the glass; at a higher temperature (1150 °C), diopside, anorthite, and wollastonite were formed as the self-crystallization products. An increase in the dwelling time resulted in the transformation of diopside to wollastonite and anorthite. At 1250 °C, all products formed a eutectic microstructure and melted. The results indicate that even at low temperatures, CMAS glass underwent microstructure evolution, which could influence the coating surface and stress distribution when deposited on TBCs.     

Electrophoretic deposition of ZrB2 coatings in NaCl-KCl-AlF3 melt containing synthesized ZrB2 nanoparticles

A novel method for preparation of ZrB₂ coatings has been proposed by combination of molten salt synthesis of ZrB₂ nanoparticles and subsequent electrophoretic deposition of the as-synthesized ZrB₂ nanoparticles in the same molten system in this paper. Nanoscale ZrB₂ particles have been produced by borothermal reduction of ZrO₂ in NaCl-KCl-AlF₃ melt at 980°C. Then electrophoretic deposition of the as-synthesized ZrB₂ nanoparticles has been achieved in the resulting molten suspension at a reduced temperature of 900°C, yielding relatively dense ZrB₂ coatings on graphite substrates with a thickness of around 25 μm. Moreover, the effect of different cell voltages ranging from 0.8 V to 1.4 V (i.e., electric field 0.4–0.7 V/cm) on the prepared ZrB₂ coatings has been investigated. Finally, during the tests at 600°C–800°C in air, ZrB₂ coatings deposited at a cell voltage of 1.2 V have exhibited good high-temperature oxidation resistance.     

Low‐Temperature Deposition of Hexagonal Boron Nitride via Sequential Injection of Triethylboron and N2/H2 Plasma

Hexagonal boron nitride (hBN) thin films were deposited on silicon and quartz substrates using sequential exposures of triethylboron and N₂/H₂ plasma in a hollow‐cathode plasma‐assisted atomic layer deposition reactor at low temperatures (≤450°C). A non‐saturating film deposition rate was observed for substrate temperatures above 250°C. BN films were characterized for their chemical composition, crystallinity, surface morphology, and optical properties. X‐ray photoelectron spectroscopy (XPS) depicted the peaks of boron, nitrogen, carbon, and oxygen at the film surface. B 1s and N 1s high‐resolution XPS spectra confirmed the presence of BN with peaks located at 190.8 and 398.3 eV, respectively. As deposited films were polycrystalline, single‐phase hBN irrespective of the deposition temperature. Absorption spectra exhibited an optical band edge at ~5.25 eV and an optical transmittance greater than 90% in the visible region of the spectrum. Refractive index of the hBN film deposited at 450°C was 1.60 at 550 nm, which increased to 1.64 after postdeposition annealing at 800°C for 30 min. These results represent the first demonstration of hBN deposition using low‐temperature hollow‐cathode plasma‐assisted sequential deposition technique.     

Electron microscopy study on the high-temperature oxidation of Si3N4–TiN ceramics: in situ and ex situ investigations

The high-temperature oxidation of Si₃N₄–TiN particulate composites with different amounts of the glass forming sinter additives Al₂O₃ and Y₂O₃ has been studied in order to reveal the oxidation mechanism with its different reaction steps and kinetics and especially identify the role of the glass phase in the course of oxidation. The initial stages of oxidation have been observed in situ in an environmental scanning electron microscope while exposing the materials to dry or humid oxidation environment at temperatures between 600 and 1100 °C. For the characterization of the later oxidation stages, materials were oxidized ex situ for longer times. The oxidation scales were characterized by X-ray diffraction, field emission scanning electron microscopy and transmission electron microscopy.Oxidation of the composites starts at 650 °C, when TiN surface particles begin to oxidize and form on their exposed surface islands of nanocrystalline TiO₂. At around 950 °C, the glass transition temperature of the intergranular glass phase, these nanocrystals start to grow laterally on the surface. At the same time, oxidation progresses into the depth of the material, forming thereby several distinguished oxidation subscales. The intergranular glass plays a crucial role for the oxidation in the temperature range between 950 and 1100 °C. Depending on the glass quantity in presence, different reaction mechanisms dominate; the oxidation kinetics are strongly controlled by the transport within the intergranular glass.     

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.     

Surfactant-catalyzed SiO2 thin films preparation and characterization

SiO₂ thin films are in high demand for wide range of applications including microelectronics, optoelectronics, solar energy conversion, photocatalysis, and self-cleaning coatings. The performance of thin film is strongly influenced by surface properties like surface roughness, thickness, morphology, wetting behavior, and thermal stability. In these applications, the SiO₂ sols were prepared using tetraethylorthosilicate as a source of SiO₂ and deposited on 100?×?40?×?2?mm³ glass slide using dip-coating method for 2?min and calcined at 250?°C for 30?min. The SiO₂ thin films were obtained using DTAB, SDS, and Tween 20 (Tw 20) surfactants with the thickness of 36.92, 47.15, and 52.39?nm, respectively. Surface morphology was studied with AFM and surface roughness was depicted with 0.9528, 3.6534, and 0.9294?nm. Contact angle measurements have been performed with goniometer to evaluate the wetting behavior of the film. The contact angle of 58.01°, 48.40°, and 37.88° was observed with SDS, DTAB, and Tw 20 film, respectively. The SiO₂ thin films with SDS showed more surface roughness and water repelling ability when compared to DTAB and least with Tw 20.     

Constrained sintering of YSZ/Al2O3 composite coatings on metal substrates produced from eletrophoretic deposition

Yttria stabilized zirconia/alumina (YSZ/Al₂O₃) composite coatings were prepared from electrophoretic deposition (EPD), followed by sintering. The constrained sintering of the coatings on metal substrates was characterized with microstructure examination using electron microscopy, mechanical properties examination using nanoindentation, and residual stress measurement using Cr³⁺ fluorescence spectroscopy. The microstructure close to the coating/substrate interface is more porous than that near the surface of the EPD coatings due to the deposition process and the constrained sintering of the coatings. The sintering of the YSZ/Al₂O₃ composite coating took up to 200 h at 1250 °C to achieve the highest density due to the constraint of the substrate. When the coating was sintered at 1000 °C after sintering at 1250 °C for less than 100 h, the compressive stress was generated due to thermal mismatch between the coating and metal substrate, leading to further densification at 1000 °C because of the ‘hot pressing’ effect. The relative densities estimated based on the residual stress measurements are close to the densities measured by the Archimedes method, which excludes an open porosity effect. The densities estimated from the hardness and the modulus measurements are lower than those from the residual stress measurement and the Archimedes method, because it takes account of the open porosity.