Ablation behavior of laminated Graphite/ZrB2-SiC ceramics in two different directions

Laminated Graphite/ZrB₂-SiC ceramics were fabricated by tape casting and hot pressing. The ablation properties of the ceramics in the parallel and the perpendicular directions were studied using an oxyacetylene torch. The mass ablation rates were 8.1?±?0.4?mg/s in the parallel direction and 0.2?±?0.1?mg/s in the perpendicular direction. The linear ablation rates were 3.1?±?0.2?µm/s in the parallel direction and 1.2?±?0.1?µm/s in the perpendicular direction. Thus, the ablation resistance of the laminated Graphite/ZrB₂-SiC ceramics in the perpendicular direction was higher than that in the parallel direction. This anisotropy was mainly attributed to the lower surface temperature in the perpendicular direction resulted from higher thermal conductivity, as well as the orientation of the weak graphite interface layer perpendicular to the ablation surface.     

Formation of Silica-Coated Carbon Powder and Conversion to Spherical β-Silicon Carbide by Carbothermal Reduction

Spherical β-SiC powders that are a few micrometers in size have been prepared by heating a mixture of phenolic resin powder and fine-grained fumed silica at 1600°C in argon. The overall process is composed of two consecutive steps: (i) the formation of silica-coated spherical carbon powder and (ii) carbothermal reduction. The irregularly shaped resin powder transforms to a spherical-shaped morphology in the first step, and the resulting silica-coated spherical carbon powder is converted to β-SiC in the second step. The key factor in the first step is the utilization of fumed silica that has hydrophobic surface functional groups. Hydrophobic interactions at the point of intimate contact between the resin powder and the silica likely reduce the surface energy of the resin powder, thereby discouraging interparticle coalescence. The resulting β-SiC powder exhibits a radially developed columnar microstructure. Hollow β-SiC spheres also can be prepared by controlling the reaction conditions in the carbothermal reduction step.     

Scanning Tunneling Microscopy of Cubic Silicon Carbide Surfaces

Scanning tunneling microscopy is used to study β-SiC(001) surfaces. The β-SiC(001) single crystals were epitaxially grown by a two-steep chemical vapor deposition process on Si(001) wafer substrates. The overall surface topography of β-SiC is generally much rougher than that of Si wafers. Atomically resolved images corresponding to 3 × 2 and c (2 × 2) geometries of the β-SiC(001) surface are presented. Our results agree with models constructed from Si dimers for these structures. The larger-scale images show that the surface is under compressive stress and exhibits high density of defects, e.g., antiphase boundaries (APB's), in some areas. Images with unusual superstructures are also shown.     

Oxidation behavior of ZrB2-SiC-ZrC at 1700 °C

The oxidation behaviors of four compositions of ZrB₂-SiC-ZrC and one composition of ZrB₂-SiC were studied at 1700 °C in air and under low oxygen partial pressure. Volatility diagrams for ZrB₂-SiC-ZrC and ZrB₂-SiC were used to thermodynamically elucidate the oxidation mechanisms. SiO₂ and ZrO₂ layers formed on the surfaces of ZrB₂-SiC-ZrC and ZrB₂-SiC oxidized at 1700 °C. A SiC-depleted layer only formed on the surface of the ZrB₂-SiC oxidized under low oxygen partial pressure. The oxide layer thickened with increasing ZrC volume content during oxidation in air and under low oxygen partial pressure. The ZrB₂-SiC-ZrC oxide surface exploded in air when the ZrC volume content was more than 50%. Under low oxygen partial pressure, the oxide surfaces of all the ZrB₂-SiC-ZrC specimens bubbled.     

Effect of powder particle size on vibration damping behaviour of plasma sprayed alumina (Al2O3) coating on AISI 304 stainless steel substrate

The damping capacity of plasma sprayed alumina (Al₂O₃) coatings on AISI 304 stainless steel was investigated in this study as a function of particle size of the starting alumina powder. The coatings were prepared from different sizes alumina powder using commercial air plasma spraying (APS) technique. The damping properties of coated samples were characterized by damping capacity (Q^?1) measured experimentally using dynamic mechanical analyzer (DMA). The surface morphology of the coatings was studied using scanning electron microscope (SEM). The results revealed that the coating was porous and was able to improve the damping capacity of bare substrate. It was also observed that the powder particle size had a significant effect on the damping characteristics of the coatings. The damping values were found to be increased with the increase in particle size in the measured strain range. This behaviour was correlated with the microstructure investigated by SEM.     

In Situ Growth of β-SiC Nanowires in Porous SiC Ceramics

Polycarbosilane (PCS) was used as a precursor to prepare porous silicon carbide (SiC) ceramics with in situ growth of β-SiC nanowires. The pore size of the as-prepared porous ceramics was 1.37 μm in average, and had a narrow distribution. The nanowires with diameters ranging from ∼10 to 50 nm existed in the channels of the porous body. Their morphology, microstructure, and composition were characterized by field emission scanning electron microscopy, transmission electron microscopy, and energy-dispersive X-ray spectroscopy, which confirmed that the nanowires had a single-crystal β-SiC structure with the 〈111〉 growth direction. A vapor–liquid–solid process was discussed as a possible growth mechanism of the β-SiC nanowires.     

Effect of the surface microstructure of SiC inner coating on the bonding strength and ablation resistance of ZrB2-SiC coating for C/C composites

The present study has been conducted in order to investigate the effect of the surface morphology of SiC inner coating on the bonding strength and ablation resistance of the sprayed ZrB₂-SiC coating for C/C composites. The microstructure of SiC inner coatings prepared by chemical vapor deposition and pack cementation at different temperatures were analyzed by X-ray diffraction, scanning electron microscopy, and 3D Confocal Laser Scanning Microscope. Tensile bonding strength and oxyacetylene ablation testing were used to characterize the bonding strength and ablation resistance of the sprayed ZrB₂-SiC coating, respectively. Results show that SiC inner coating prepared by chemical vapor deposition has a smooth surface, which is not beneficial to improve the bonding strength and ablation resistance of the sprayed ZrB₂-SiC coating. SiC inner coating prepared by pack cementation at 2000 °C has a rugged surface with the roughness of 72.15 µm, and the sprayed ZrB₂-SiC coating with it as inner layer exhibits good bonding strength and ablation resistance.     

Effects of Boron, Carbon, and Iron Content on the Stacking Fault Formation during Synthesis of β-SiC Particles in the System SiO2-C-H2

Through the systematic addition of B, C, and Fe, additive effects on the stacking fault formation and the morphology of the particles formed during reaction synthesis of β-SiC were investigated in the present study. The whisker content of the synthesized product and the formation mechanism of whiskers were closely related to the stacking fault density. The addition of B inhibited whisker formation probably because of isotropic imperfection and the suppression of surface diffusion. Increase in the reduction force by using an active carbon source and also adding excess carbon led to an increase in stacking fault density through enhanced whisker formation. In the presence of Fe, the synthesized β-SiC whiskers appeared to possess only a small amount of stacking faults. The growth mechanism was different with Fe; i.e., isotropic growth occurred via a dissolution-precipitation reaction through a liquid phase in the Fe-Si system.     

Effect of Substrate Orientation on Interfacial and Bulk Character of Chemically Vapor Deposited Monocrystalline Silicon Carbide Thin Films

The high breakdown electric field, saturated electron drift velocity, and melting (decomposition) point of SiC have given continual impetus to research concerned with the development of thin films having minimum concentrations of line and planar defects and electronic devices for severe environments. To this end, epitaxial growth via chemical vapor deposition of monocrystalline films of β-SiC on Si (100) and 6 H -SiC {0001} substrates and 6 H -SiC on vicinal 6 H -SiC {0001} substrates have been conducted. High concentrations of stacking faults, microtwins, and inversion domain boundaries were produced in films grown directly on Si (100) as a result of a lattice parameter difference of ∼ 20% and the presence of single (or odd number) atomic steps on the substrate surface. Growth on Si (100) oriented 3° to 4° toward [011] completely eliminated the IDBs (but not the other defects) due to the preferential formation of double steps with dimerization axes on the upper terraces parallel to the step edges. Growth of β-SiC films on 6 H {0001} lowered the density of all defects but resulted in the formation of a new defect, namely, double positioning boundaries. The latter were eliminated by using 6 H {0001} oriented 3° toward [1120]. The defect density in these last films, relative to those grown on on-axis Si (100), was reduced substantially (to ∼10⁵ cm/cm³). However, the resulting film was 6 H -SiC. Significant improvements in electrical properties of simple devices were obtained as the defect density was progressively decreased.     

Mechanism of Material Removal from Silicon Carbide by Carbon Dioxide Laser Heating

The mechanism of material removal from SiC by CO₂ laser heating was studied using sintered and single-crystal α-SiC. Removal rate and width of the groove showed maxima when plotted as a function of translation speeds. Groove depth decreased as the translation speed of samples increased. Similar results were obtained if argon or air was used as gas assist, which indicated that the material removal mechanism is induced dissociation of SiC. Microstructure of the material deposited in and outside of the groove was studied by SEM. At low scanning speeds, columnar grains 10 to 50 μm long appeared. As the scanning speed increased, columnar grains became smaller and finally only irregular polycrystalline particles were observed. By using Raman spectroscopy, Auger analysis, and X-ray diffraction, phases inside and outside the groove were identified as Si, β-SiC, C, and SiO₂. Columnar grains were identified as β-SiC covered with thin layers of C, Si, and SiO₂. Slow scanning speeds enhanced the growth of β-SiC. At slow scanning speed, free silicon was always found in the grooves of lased single crystals but not in the grooves of lased sintered SiC. It can be concluded that the mechanism of material removal from silicon carbide by CO₂ laser heating is a vaporization process, and material found in the groove and on the surface near the groove is formed by condensation from the vapor.     

SiC Crystals Derived from β-SiC Powder Using a Conically Converging Shock-Wave Technique

α-SiC and β-SiC crystals are prepared from vapor by sublimation of β-SiC powder subjected to a conically converging shock-wave generated by detonating an explosive charge. α-SiC crystals are mainly 6H modifications and their features are plates, pyramids, needles, and filaments grown in the [0001] direction; 4H-SiC filaments are also obtained. α-SiC rods having a triangular cross section develop parallel to the [111] direction.     

Bond Strength of Porcelain Bonded to Enamel and Dentin Surfaces Prepared with Different Surface Treatments

Although acid etching is routinely used to condition tooth surfaces, it increases the caries susceptibility of enamel and enhances enamel demineralization; thus the role of alternative surface treatments such as alumina air abrasion and erbium-doped yttrium aluminium garnet (Er:YAG) laser irradiation for tooth conditioning is controversial. This study was undertaken to compare the effects of different conditioning methods on the shear bond strength (SBS) of resin cement on enamel and dentin. Prepared permanent human dentin and enamel samples (N = 210) embedded in clear acrylic resin were conditioned by 37% phosphoric acid etching, 50-µm alumina air abrasion, Er:YAG laser irradiation (120 mJ, 10 Hz, medium short pulse mode), or their combinations. Porcelain laminates were cemented by using photopolymerizing luting composite. SBS was evaluated after thermal cycling (10,000 cycles, 5–55°C) and fracture types (adhesive, cohesive, or mixed) were observed by stereomicroscopy. Data were analyzed by Kruskal–Wallis analysis of variance and followed by Tamhane's test (p < 0.05). Enamel and dentin specimens showed significant differences in SBS (p < 0.000). Er:YAG laser etching presents successful alternatives to acid etching on dentin surfaces; it does not enhance adhesion of the resin cement on enamel surfaces.     

The Wulff Shape of Alumina: II, Experimental Measurements of Pore Shape Evolution Rates

The rate at which a facetted tetragonal cavity of nonequilibrium shape approaches a cubic equilibrium (Wulff) shape via surface diffusion was modeled. The shape relaxation rate of a facetted "stretched cylinder" was also modeled. For the first geometry, only an approximate solution based on linearizing the mean potential difference between the source and sink facets was obtained. For the stretched cylinder, both an approximate and an exact solution can be obtained; the approximate solution underestimates the evolution rate by a factor of ∼2. To assess the applicability of the models, nonequilibrium shape pores of identical initial geometry (∼20 μm × 20 μm × 0.5 μm) were introduced into (0001), {10[Onemacr]2}, {1120}, and {100} surfaces of sapphire single crystals using microfabrication techniques, ion-beam etching, and hot pressing. The large (∼20 μm × 20 μm) faces of the pore are low-index surfaces whose nature is dictated by the wafer orientation. A series of anneals was performed at 1900°C, and the approach of the pore shape to an equilibrium shape was monitored. The kinetics of shape evolution are highly sensitive to the crystallographic orientation and stability of the low-index surface that dominates the initial pore shape. The measured variations of the pore aspect ratio were compared to those predicted by the kinetic model. The observations suggest that when the initial bounding surface is unstable, shape relaxation may be controlled by diffusion. However, surface-attachment-limited kinetics (SALK) appears to play a major role in determining the pore shape evolution rate in cases where the initial bounding surfaces have orientations that are part of the Wulff shape.     

Methylene-bridged carbosilanes and polycarbosilanes as precursors to silicon carbide—from ceramic composites to SiC nanomaterials

Polymeric and oligomeric carbosilanes having Si atoms linked by methylene (CH₂) groups were used to prepare nano-sized tubules and bamboo-like SiC structures by both CVD and liquid precursor infiltration and pyrolysis inside of nanoporous alumina filter disks, followed by dissolution of the alumina template in HF_(aq). These initially amorphous SiC structures were characterized by SEM, EMPA, TEM, and XRD. Typical outer diameters of the SiC nanotubes (NTs) were 200–300 nm with 20–40 nm wall thicknesses and lengths up to the thickness of the original alumina templates, ca. 60 μm. In the case of the CVD-derived SiC NTs, annealing these structures up to 1600 °C in an Ar atmosphere yielded a nanocrystalline β-SiC or β-SiC/C composite in the shape of the original NTs, while in the case of the liquid precursor-derived nanostructures, conversion to a collection of single crystal SiC nanofibers and other small particles was observed.     

Model of morphology evolution in the growth of polycrystalline β-SiC films

The growth of β-SiC films via chemical vapor deposition (CVD) has been under intensive investigation because this is viewed to be an enabling material for a variety of new semiconductor devices in areas where silicon cannot effectively compete. However, the difficulty in achieving single-crystal or highly textured surface morphology in films with low bulk defect density has limited the use of β-SiC films in electronic devices. Although several researchers have reported results relating the morphology of β-SiC films to deposition parameters, including substrate temperature and gas composition, detailed knowledge of the effects of deposition parameters on film morphology and crystallographic texture is still lacking. If these relationships between deposition parameters and film morphology can be quantified, then it may be possible to obtain optimal β-SiC film morphologies via CVD for specific applications such as high-power electronic devices.The purpose of this study is to predict the dependence of the surface morphology of β-SiC films grown by CVD on substrate temperature and inlet atom ratio of Si:C, and to model the morphological evolution of the growing polycrystalline film. The Si:C ratio is determined by the composition of the reactant gases, propane (C₃H₈) and silane (SiH₄). A two-dimensional numerical model based on growth rate parameters has been developed to predict the evolution of the surface morphology. The model calculates the texture, surface roughness, and grain size of continuous polycrystalline β-SiC films resulting from growth competition between nucleated seed crystals of known orientation. Crystals with the fastest growth direction perpendicular to the substrate surface are allowed to overgrow all other crystal orientations. When a continuous polycrystalline film is formed, the facet orientations of crystals are represented on the surface. In the model, the growth parameter α_2D, the ratio of the growth rates of the {10} and {11} faces, determines the crystal shapes and, thus, the facet orientations of crystals. The growth rate parameter α_2D used in the model has been derived empirically from the textures of continuous β-SiC films reported in the literature.     

Synthesis and characterization of balloons and porous blocks of β-SiC using silicone and urethane foam

A process to form different shapes of β-SiC was developed using precursors composed of silicone compounds and urethane foam. Two types of low molecular weight silicone, a Pt catalyst for curing, and a catalyst regulator were impregnated into urethane foam chips with different porosities and bulk densities, and cured at 200 °C for 1 h in air. Formed precursors were converted into β-SiC by pyrolysis at 1600 °C for 1–5 h in argon. Depending on the type of urethane foam, β-SiC balloons and β-SiC blocks with high porosities and low bulk densities were obtained. In addition, fine particles of β-SiC smaller than 100 nm were also obtained by crushing the blocks. For a disk formed by 5 h pyrolysis at 1500 °C, N₂ gas permeability, electric conductivity, and oxidation resistance at 900 °C in air were measured. The results indicated that the disk may be used as a gas permeable heater. The process is simple, and the materials for this process are not hazardous and commercially available at low cost.     

Time‐dependent changes of surface properties of polyether ether ketone caused by air plasma treatment

The effect of air plasma treatment on wetting and energy properties, surface composition and morphology of polyether ether ketone (PEEK) was investigated. The influence of the storage time on the surface properties of plasma‐treated polymer plate was also examined. The properties were determined by advancing and receding contact angle measurements, Fourier transform infrared spectroscopy supported by theoretical spectrum modelling, X‐ray photoelectron spectroscopy and optical profilometry. Three theoretical approaches were used in the determination of the apparent surface free energy of the untreated and plasma‐treated PEEK samples from the measured contact angles of probe liquids (water, formamide, diiodomethane): the contact angle hysteresis method, the Owens and Wendt approach and the Lifsthitz ? van der Waals acid–base approach. It was found that air plasma treatment of PEEK causes significant chemical and morphological changes of the polymer surface, which are reflected in the decrease of contact angles from 83.4° to 11.7° for water after 180 s plasma treatment. This is due to the formation of polar functional groups resulting in the increase of the surface hydrophilicity. After plasma treatment the apolar component of the surface free energy practically does not change, while the polar component increases significantly, especially for plates treated for 180 s, from 0 to 19.6 mJ m^?2. In addition, the modified PEEK surface is not stable during storage and it acquires more hydrophobic character. © 2016 Society of Chemical Industry     

Effect of chromium additive on sintering and oxidation behavior of HfB2-SiC ceramics

The effect of chromium admixture on the processes in the HfB₂-SiC ceramic powder system during its pressureless sintering at 1600?°C was studied. It was shown that an increase in chromium content from 0% to 15.5% in the HfB₂-SiC ceramic powder mixture leads to a continuous increase in its relative density up to 90%. A transient liquid phase Cr-Si-C-B is formed at 1600?°C, and it promotes intense sintering of HfB₂ and SiC powders. The oxidation resistance of HfB₂-SiC-Cr ceramics was studied in static air at 1000–1500?°C. It was shown that the oxidation resistance is greatly improved due to a decrease in the porosity of the sintered ceramic system because of chromium additive. The presence of chromium oxide in the formed surface glassy layer can also lead to the increase in the oxidation resistance. These results suggest that chromium can be considered as a promising sintering additive for HfB₂-SiC and similar systems.     

Boehmite derived surface functionalized carbon nanotube-reinforced macroporous alumina ceramics

Carbon nanotube (CNT)-reinforced macroporous alumina ceramics with tailored porosity were fabricated using hydrothermally synthesized (200 °C for 2 h) boehmite–CNT starting composite powders. Multi-wall CNTs were first mixed with a mixture of chemicals suitable to synthesize stoichiometric boehmite powders and then put in an autoclave. During hydrothermal synthesis, the formation of fine particles of boehmite was accompanied by the functionalization of CNTs. Subsequently, CNT–boehmite powders were used to produce bulk ceramics and sintering took place in a vacuum furnace at 1450 °C for 3 h for the formation of CNT-reinforced alumina ceramics. The pore network in various dimensions occurred as a consequence of the reconstructive transformation and dehydration of boehmite during the transformation to alumina. FEG-SEM and TEM analysis were used to determine the CNT distribution in the matrix, the morphology and size of particles, as well as the visual properties of the pores. The final macroporous alumina ceramics can be considered to be ideal for the separation and filtration of contaminants in liquid or air environment.     

SiAlC ceramics from a new polyaluminocarbosilane: Synthesis,structure and oxidation behaviors

SiAlC ceramics were prepared with a new polyaluminocarbosilane (PACS-N01) which was synthesized using methylaluminoxane (MAO) and liquid hyperbranched polycarbosilane (LPCS), and had a quite high ceramic yield (around 82.7%) at 900 °C in argon after curing. The obtained SiAlC ceramics consisted of β-SiC as major building units, small amounts of α-SiC and Al-containing phases with Al–O and Al–C bonds, and stayed partially amorphous till 1600 °C. Oxidation behaviors of SiAlC ceramics and SiC ceramics prepared through the same procedure were studied in the air at 1200 °C for different times. Dense oxide scales free of pores were formed on the surface of both samples. Measurements of oxide scale thickness revealed that the oxidation followed parabolic reaction kinetics and that the oxidation rate constant was apparently smaller for SiAlC than that for SiC. Besides, –Al–O–Si– network structure could be formed in SiAlC's oxide scale, which was supposed to block the pathway of oxygen diffusion and thus lowered the oxidation rates.