Influence of oxygen partial pressure on the wetting of SiC by a Co–Si alloy

In this investigation the influence of oxygen partial pressure P_O2 on the wetting of SiC by a Co–Si alloy was studied. Wetting experiments were carried out in argon with different oxygen contents (from 5 to 1000 ppm). The relationship between wetting and deoxidation of surfaces (SiC and Co–Si alloy) was investigated. Calculations were performed to evaluate the temperature range over which deoxidation is possible. These calculations are in agreement with the experimental results.     

Heteroepitaxy of zinc-blende SiC nano-dots on Si substrate by organometallic ion beam

The self-assembled SiC nano-dots were fabricated on Si(111) substrate at low-temperatures using the organometallic ion beam deposition technique. The single precursor of methylsilicenium ions (SiCH₃⁺) with the energy of 100 eV was deposited on Si(111) substrate at 500, 550 and 600 °C. The characteristics of the self-assembled SiC nano-dots were analyzed by reflection high-energy electron diffraction (RHEED), Raman spectroscopy and atomic force microscope (AFM). The RHEED patterns showed that the crystal structure of the SiC nano-dots formed on Si(111) substrate was zinc-blende SiC (3C-SiC) and it was heteroepitaxy. The self-assembled SiC nano-dots were like a dome in shape, and their sizes were the length of 200-300 nm and the height of 10-15 nm. Despite the low-temperature of 500 °C as SiC crystallization the heteroepitaxial SiC nano-dots were fabricated on Si(111) substrate using the organometallic ion beam.     

Wetting of ceramics by molten silicon and silicon alloys: a review

The surface of silicon is very sensitive to interactions with oxygen present as impurity in furnace atmosphere. It is shown that three types of Si surfaces can be obtained depending on the oxygen’s partial pressure in the furnace and on temperature: oxidized, oxide-free but containing adsorbed oxygen and adsorption free. The influence of oxygen on the surface tension of molten Si is also discussed. Wetting by Si and Si alloys is then described and analysed for three types of ceramics: (i) ionocovalent oxides (Al₂O₃, SiO₂, MgO, etc.), with a particular emphasis on the Si/silica couple, (ii) the different types of carbons where wetting is assisted by the reaction between Si and carbon, and (iii) the predominantly covalent ceramics (SiC, Si₃N₄, AlN, BN, etc). The role of wetting in the processing of silicon or silicon-based multimaterials is also illustrated.     

The oxidation of SiC particles and its interfacial characteristics in Al-matrix composite

The passive oxidation behavior of SiC particles has been studied in an electric furnace at atmospheric pressure and in dry air, the weight change due to the transformation from SiC into SiO₂ is descibed as a function of exposed temperature and holding time. According to the oxidation data of SiC particles, the oxidation parameters and the degree of oxidation for SiC particles can be controlled. Controllable preoxidation of SiC particles is one of the keys for designing interface and interphase to achieve high performance aluminum composite. Consequently, the evolution of interfacial reaction products in 2014 aluminum alloy composite reinforced with oxidized-SiC particles after extended thermal exposure at elevated temperatures were further characterized by Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), and X-ray diffraction. While it could act to prevent the interfacial reaction between SiC particles and aluminum alloy, the preoxidation of SiC particles led to the formation of other interfacial reaction products. The observation of the microstructure revealed that at elevated temperatures nano-MgO formed initially on the surface of the oxidized SiC particles and then turned into nano-MgAl₂O₄ crystal due to the reaction between the SiO₂ and aluminum alloy containing Mg. TEM observations indicated that the oxidized layer on SiC particles was uniform and had a good bonding with SiC and aluminum alloy.     

Pressure dependence of morphology and phase composition of SiC films deposited by microwave plasma chemical vapor deposition on cemented carbide substrates

SiC films were deposited on cemented carbide substrates by employing microwave plasma chemical vapor deposition method using tetramethylsilane (Si(CH₃)₄) diluted in H₂ as the precursor. Scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction and scratching technique were used to characterize morphology, composition, phases present and adhesion of the films. Experimental results show that the deposition pressure has great influence on morphologies and phase composition of the films. In sequence, SiC films with a cauliflower-like microstructure, granular films with terrace-featured SiC particles coexisting with Co₂Si compound and clusters of nanometer SiC nanoplatelets appear as a function of the deposition pressure. In terms of plasma density and substrate temperature, this sequential appearance of microstructures of SiC films was explained. Adhesion tests showed that among the three types of films studied, the films with the terrace-featured SiC particles have relatively higher adhesion. Such knowledge will be of importance when the SiC films are used as interlayer between diamond films and cemented carbide substrates.     

Wear and Creep Characteristics of a Carbon‐Derived Si3N4/SiC Micro/Nanocomposite

In the presented work some properties of a recently developed Si₃N₄/SiC micro/nanocomposite have been investigated. The material was tested using a pin on disc configuration. Under unlubricated sliding conditions using Si₃N₄ pin at 50 % humidity, the friction coefficient was in the range of 0,6 ‐ 0,7. The reduction of humidity resulted in a lower coefficient of friction, in vacuum the coefficient of friction had a value of about 0,6. The wear resistance in vacuum was significantly lower then that in air. The wear patterns on the Si₃N₄+SiC disc revealed that mechanical fracture was the wear controlling mechanism. Creep tests were realized in four point bending configuration in the temperature interval 1200‐1400 °C at stresses 50,100 and 150 MPa and the minimal creep deformation rate was established for each stress level. The activation energy, established from the minimal creep deformation had a value of about 360 kJ/mol and the stress exponent values were in the range of 0.8‐1.28. From the achieved stress exponents it can be assumed that under the studied load/temperature conditions the diffusion creep was the most probable creep controlling mechanism.     

Gaseous corrosion mechanisms of silicon carbides in Na2SO4 and V2O5 environments

Hot-pressed polycrystalline SiC (HPSiC) and single crystal SiC (SCSiC) were exposed to Na₂SO₄ and V₂O₅ vapours at 1000 °C. Vapours were carried by the argon gas over the specimens where corrosion studies were conducted. Mixed-kinetics mechanisms for HPSiC under gaseous environments have been developed in the form of corrosion weight loss per unit area with time. The decomposed gases diffusing through the SiO₂ film and the reaction products at the substrate surfaces are the two major steps that contribute to the overall reaction for this material. SCSiC was observed to have severe corrosion attacks on its surfaces and corrosion pits with honeycomb shapes were left.     

Processing of Al–SiCp Metal Matrix Composites by Pressureless Infiltration of SiCp Preforms

An optimum method for producing Al-SiC_p metal matrix composites was developed by determining the optimum conditions for wetting SiC by aluminum and the optimum parameters for pressureless infiltration of SiC_p preforms. The quantitative effect of magnesium and silicon additions to aluminum, free silicon on the SiC substrate, nitrogen gas in the atmosphere, and process temperature on the wetting characteristics of SiC by aluminum alloys was investigated using the sessile drop technique. The contribution of each of these parameters and their interactions, in terms of a relative power, to the contact angle, surface tension, and driving force for wetting were determined. In addition, an optimized process for enhanced wetting was suggested and validated. The optimum conditions for wetting SiC by aluminum that were arrived at were used to infiltrate SiC_p preforms and the mechanical properties of the resulting metal matrix composites were measured. The effect of SiC particle size, infiltration time, preform height, vol.% SiC in the preform, and Si coating on the SiC particles on the pressureless infiltration of SiC_p compacts with aluminum was investigated and quantified. The contribution of each of these parameters and their interactions to the retained porosity in the composite, the modulus of elasticity, and the modulus of rupture were determined. Under optimum infiltration conditions, metal matrix composites with less than 3% porosity, over 200 GPa modulus of elasticity, and about 300 MPa modulus of rupture were routinely produced.     

Microstructure and microchemistry of the Al/SiC interface

The characteristics of the Al/SiC interface play a critical role in controlling the properties of SiC-reinforced aluminium composites and aluminium-brazed SiC ceramic joints. Recently, a detailed investigation on the wettability of SiC single crystals by aluminium and several of its alloys was conducted. In order to understand further the nature of the Al/SiC interface, high resolution and conventional transmission electron microscope techniques have now been used to investigate its microchemistry and microstructure. The results revealed the coexistence of two polytype structures, rhombohedral and hexagonal, in the SiC single crystal structure. Aluminium carbide (Al₄C₃) and silicon were the reaction products found at the Al/SiC interface. From diffraction patterns, epitaxial orientation relationships between the SiC substrate and Al₄C₃, Si were determined.     

Oberflächenmodifizierung von Titan zur Verbesserung der Grenzflächenverträglichkeit

Surface Modification of Titanium for Improvement of the Interfacial Biocompatibility We report the CVD‐polymerisation of amino‐functionalized [2,2]‐paracyclophane for polymer coating and functionalization of titanium surfaces. Additionally, the functionalization was carried out by silanization with 3‐aminopropyl‐triethoxysilane. The generated amino‐groups were used for covalent immobilization of bioactive substances to stimulate the adhesion and growth of osteoblasts. As bioactive substances the pentapeptide GRGDS and the growth factor BMP‐2 were chosen. The covalent bonding was achieved by activation with hexamethylene diisocyanate. Each modification step was characterized by X‐ray‐photoelectron‐spectroscopy (XPS), atomic force microscopy (AFM) and contact angle measurements. The covalent bonding of the bioactive substances was proven by radiolabelling and surface‐MALDI‐ToF‐MS. In vitro‐biocompatibility tests with primary, human osteoblasts demonstrated the improved cell adhesion and spreading on the bioactive modified titanium surfaces.     

Pressure-pulsed chemical vapour infiltration of SiC to porous carbon from a gas system SiCl4-CH4-H2

SiC matrix was deposited into porous carbon from a gas system SiCl₄-CH₄-H₂ in the temperature range 900–1200 °C using pressure-pulsed chemical vapour infiltration (PCVI) process. At 1000 °C, silicon single phase, a mixed phase of (Si + SiC), and SiC single phase, were detected by X-ray diffractions for specimens obtained with the reaction time per pulse of 1, 2–3, and 5 s, respectively. At 1100 °C, SiC single phase was obtained with a reaction time of only 0.3s. Between 1050 and 1075 °C, deposition rate accelerated suddenly. The increase of SiCl₄ concentration increased the deposition rate linearly up to 4%–6%. The residual porosity decreased from 29% to 6% after 2×10⁴ pulses of CVI at 1100 °C, and the flexural strength was 110 MPa.     

Orientation dependent growth of SiC nanocrystals at the SiO2/Si interface

Cubic SiC nanocrystals are formed epitaxially and void-free on single crystal Si substrate by reactive annealing in CO. In this study characterization of the nucleation, growth and morphology is presented on differently oriented single crystal Si substrates. It is found that SiC nanocrystals of various shape can be grown in different densities on the (100), (110) and (111) Si surfaces with an average size of 30-60 nm. Effect of annealing time, CO concentration, substrate orientation and crystal size on crystallite growth is discussed. Parameters to obtain increased SiC nucleation density are determined.     

Aligned Single‐Crystalline β‐Si3N4 Whiskers Prepared with SHS Process

Aligned single‐crystalline β‐Si₃N₄ whiskers with high aspect ratio were first prepared via a Self‐propagating high temperature‐synthesis (SHS) process, by using tungsten powders as catalysts. The as‐synthesized Si₃N₄ whiskers typically have uniform diameters of 400 nm, length about 200 µm, and exhibit smooth and straight surfaces. Above all, the products possesses a perfect aligned structure, which is quite different from the reported β‐Si₃N₄ whiskers. Elastic bending modulus of individual whiskers was measured by in‐situ TEM process, the average value of elastic bending modulus of individual as‐synthesized whiskers was 488 GPa. Results revealed that tungsten powders plays an significant effects on the morphology of Si₃N₄ whiskers.     

Template Engineering of CuBi2O4 Single‐Crystal Thin Film Photocathodes

To develop strategies for efficient photo‐electrochemical water‐splitting, it is important to understand the fundamental properties of oxide photoelectrodes by synthesizing and investigating their single‐crystal thin films. However, it is challenging to synthesize high‐quality single‐crystal thin films from copper‐based oxide photoelectrodes due to the occurrence of significant defects such as copper or oxygen vacancies and grains. Here, the CuBi₂O₄ (CBO) single‐crystal thin film photocathode is achieved using a NiO template layer grown on single‐crystal SrTiO₃ (STO) (001) substrate via pulsed laser deposition. The NiO template layer plays a role as a buffer layer of large lattice mismatch between CBO and STO (001) substrate through domain‐matching epitaxy, and forms a type‐II band alignment with CBO, which prohibits the transfer of photogenerated electrons toward bottom electrode. The photocurrent densities of the CBO single‐crystal thin film photocathode demonstrate ?0.4 and ?0.7 mA cm^?2 at even 0 V_RHE with no severe dark current under illumination in a 0.1 m potassium phosphate buffer solution without and with H₂O₂ as an electron scavenger, respectively. The successful synthesis of high‐quality CBO single‐crystal thin film would be a cornerstone for the in‐depth understanding of the fundamental properties of CBO toward efficient photo‐electrochemical water‐splitting.     

Chemische Zusammensetzung und Mikrostruktur polymerabgeleiteter Gläser und Keramiken im System Si–C–O. Teil 2: Charakterisierung der Gefügeausbildung mittels hochauflösender Durchstrahlungselektronenmikroskopie und Feinbereichsbeugung

Chemical Composition and Microstructure of Polymer‐Derived Glasses and Ceramics in the Si–C–O System. Part 2: Characterization of microstructure formation by means of high‐resolution transmission electron microscopy and selected area diffraction Liquid or solid silicone resins represent the economically most interesting class of organic precursors for the pyrolytic production of glass and ceramics materials on silicon basis. As dense, dimensionally stable components can be cost‐effectively achieved by admixing reactive filler powders, chemical composition and microstructure development of the polymer‐derived residues must be exactly known during thermal decomposition. Thus, in the present work, glasses and ceramics produced by pyrolysis of the model precursor polymethylsiloxane at temperatures from 525 to 1550 °C are investigated. In part 1, by means of analytical electron microscopy, the bonding state of silicon was determined on a nanometre scale and the phase separation of the metastable Si–C–O matrix into SiO₂, C and SiC was proved. The in‐situ crystallization could be considerably accelerated by adding fine‐grained powder of inert fillers, such as Al₂O₃ or SiC, which permits effective process control. In part 2, the microstructure is characterized by high‐resolution transmission electron microscopy and selected area diffraction. Turbostratic carbon and cubic β‐SiC precipitate as crystallization products. Theses phases are embedded in an amorphous matrix. Inert fillers reduce the crystallization temperature by several hundred °C. In this case, the polymer‐derived Si–C–O material acts as a binding agent between the powder particles. Reaction layer formation does not occur. On the investigated pyrolysis conditions, no crystallization of SiO₂ was observed.     

Thermal Conductivity of ZrB2?SiC?B4C from 25 to 2000 °C

The thermal diffusivities of ZrB₂–SiC (10.7, 21.9, or 48.7 vol% SiC) with B₄C sintering aid were measured over 25–2000 °C using laser flash. The composition with the highest SiC showed the highest thermal conductivity (k) at 25 °C, but the lowest above ≈400 °C, because of the greater k temperature sensitivity of the SiC phase. Finite difference calculations of k, using selected literature data for the individual phases, and the concentration of phases from microstructures, correctly predicted temperature and phase concentration dependencies, but were lower than experimental results. The k of pure ZrB₂ and SiC as a function of temperature were back‐calculated from the experimental results for the multi‐phase materials; they were in good agreement with specific literature values.     

Intrinsic toughness of interfaces between SiC coatings and substrates of Si or C fibre

The critical energy release rate for separation of SiC coatings from single crystal Si substrates or surfaces of carbon fibres, along their well-defined interfaces can be determined quite accurately from analysis of the spontaneous delamination of coatings under bi-axial stress, when such coatings exceed a critical thickness. Direct evaluations have been made of the specific work of delamination along the interface for SiC coatings from single crystal Si substrates, for both the case of coatings under bi-axial compression, as well as under bi-axial tension. The critical energy release rate for coatings in tension was 5.1 J m², and that for coatings under compression was 5.9 J m². The higher value of the latter is attributed to relative slippage between coating and substrate before lift-off of the former. Corresponding determination of the critical energy release rate for delamination of SiC coatings under bi-axial tension from surfaces of anisotropic Pitch-55 carbon fibres gave an answer of 5.5 J m². These values compare very well with expectations from surface energies of strong solids.     

Optimization of the Carbosilicothermic Synthesis of the Ti<Subscript>4</Subscript>SiC<Subscript>3</Subscript> MAX Phase

We have studied the formation of the Ti4SiC3 MAX phase during the vacuum carbosilicothermic reduction of TiO₂ with a combined reducing agent consisting of SiC and elemental Si and analyzed the effects of the synthesis temperature, heat treatment time, and percentage of elemental silicon in the starting mixture on the Ti₄SiC₃ yield. Optimal Ti₄SiC₃ synthesis conditions are as follows: temperature from 1550 to 1650°C, isothermal holding time of 360 min, and the starting-mixture composition TiO₂ + 1.2SiC + 0.6Si. The Ti₄SiC₃ yield then reaches 92 wt %.     

Pretreatment process of SiC particles and fabrication technology of SiC particulate reinforced Zn–Al alloy matrix composite

Abstract

In the present paper, a novel pretreatment process for SiC particulate and a new mechanical–electromagnetic combination stirring process for fabricating Zn–Al(ZA27)/SiC_p composites are described. The optimal pretreatment route and the most appropriate SiC particle parameters were experimentally determined. The pretreated SiC particles were easily incorporated and dispersed in the ZA27 alloy melt and were not agglomerated before addition to the melt. The surface status of the SiC particles before and after pretreatment was observed and analysed by scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction, and transmission electron microscopy. It was found that gas existing on the SiC particle surfaces by physical and chemical absorption was a significant hindrance to the incorporation and dispersion of SiC particles in the alloy melt. The gas absorption was induced by ultrafine SiC powders, fracture steps, and ions existing on the SiC particle surfaces. The carbon, silicon, and oxygen contents on the SiC surface were varied with different pretreatment techniques. Moreover, a dense layer of amorphous SiO₂, which improves wetting of SiC particles in the ZA27 melt, was formed owing to calcination of SiC particles in air. The new combined stirring process exploits the advantages of both mechanical and electromagnetic stirring of the melt at the different processing stages during fabrication. The microstructural characteristics of the resulting composites are: homogeneously distributed SiC particles, fewer macro gas blows and inclusions, and little shrinkage porosity in comparison to composites fabricated by a mechanical stirring process. Finally, the mechanisms of degassing and reducing the porosity and the number of oxide inclusions are discussed.     

A TEM study of the interfaces and matrices of SiC-coated carbon fibre/aluminium composites made by the K2ZrF6 process

Carbon fibre-reinforced aluminium composites were pressurelessly cast by using K₂ZrF₆ as the wetting promotion agent. Transmission electron microscopy (TEM) and energy dispersed analysis of X-rays, (EDAX) were used. The results showed that interfacial reactions were very active after K₂ZrF₆ treatment. This was caused by the diffusion and reaction of zirconium in the surface of carbon fibres or in the SiC coating. Silicon alloying of aluminium could suppress the interfacial reactions by decreasing the activity of zirconium and changing intermetallic Al₃Zr to Zr₃Al₄Si₅, and building up the phase equilibrium between SiC, aluminium and silicon. The requested silicon content was higher than the equilibrium content of Al-Si-SiC system to suppress the SiC/Al interfacial reaction. A perfect interface was achieved in SiC-coated carbon fibre Al-12 wt% Si composite.