Interfacial reaction and adhesion between SiC and thin sputtered nickel films

Thin sputtered nickel films grown on SiC were annealed in an Ar/4 vol % H2 atmosphere at temperatures between 550 to 1450 °C for various times. The reactivity and the reaction-product morphology were characterized using optical microscopy, surface profilometry, X-ray diffraction, scanning electron microscopy and electron probe microanalysis. The reaction with the formation of silicides and carbon was observed to first occur above 650 °C. Above 750 °C, as the reaction proceeded, the initially formed Ni3Si2 layer was converted to Ni2Si and carbon precipitates were observed within this zone. The thin nickel film reacted completely with SiC after annealing at 950 °C for 2 h. The thermodynamically stable Ni2Si is the only observed silicide in the reaction zone up to 1050 °C. Above 1250 °C, carbon precipitated preferentially on the outer surface of the reaction zone and crystallized as graphite. The relative adhesive strength of the reaction layers was qualitatively compared using the scratch test method. At temperatures between 850 to 1050 °C the relatively higher critical load values of 20–33 N for SiC/Ni couples are formed. This revised version was published online in November 2006 with corrections to the Cover Date.     

Interface structure and reaction kinetics between SiC and thick cobalt foils

Reaction couples of SiC with thick cobalt foils were annealed in an Ar-4 vol% H₂ atmosphere at temperatures between 950 and 1250 °C for times between 4 and 100 h. At temperatures above 950 °C, solid-state reactions lead to the formation of various silicides with carbon precipitates. The typical layer sequence in the reaction zone was determined by quantitative microanalysis to be SiC/CoSi+C/Co₂Si+C/Co₂Si/Co₂Si+C/ ... /Co₂Si/Co(Si)/Co. The mechanism of the periodic band structure formation with the carbon precipitation behaviour was discussed in terms of reaction kinetics and thermodynamic considerations. Two ternary phases, CoSiC₂ and Co₂SiC₃, unstable at room temperature, may exist in the system Co-Si-C. The growth of the reaction zone is dependent on the square root of time. The reaction kinetics are proposed to estimate the effective reaction constant from the parabolic growth of the reaction zone. The mechanical properties of the reaction zones were determined by the microhardness test.     

Pressure pulsed chemical vapour infiltration of SiC to two-dimensional-Tyranno/SiC-C preforms

Preforms of two-dimensional Tyranno fibre (SiC base) of 7×20×1.3 mm³ were chemically vapour infiltrated with SiC at 850–1050 °C from a gas mixture of CH₃SiCl₃ (6%)-H₂ using pressure pulses between below 0.3 kPa and 0.1 MPa. Above 900 °C, films grew on the macrosurface dominantly. At 850 °C, residual porosity decreased to about 10% after 10⁵ pulses, and three point flexural strength reached about 200 MPa. X-ray diffractograms (XRDs) on the surface showed the deposits to be -SiC only.     

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.     

In-situ high temperature X-ray diffraction study of Co/SiC interface reactions

In situ experiments on the Co/SiC interface reaction were carried out with a high temperature X-ray diffractometer capable of measuring the X-ray diffraction pattern in 1–4s using an imaging plate. The kinetic formation processes of the interface reaction layer were measured in short-period exposure experiments with the apparatus. The time-temperature phase diagram of Co/SiC in N₂was determined. Co₂Si and CoSi were formed at the Co/SiC interface between 921 and 1573 K in N₂. The formation of CoSi obeyed the parabolic rate law. The value of the activation energy was 95 kJ/mol. The results of thermal expansion coefficient measurements suggest that when a sample is cooled to room temperature, compressive strain caused by CoSi occurs on SiC.     

The effects of Al and Si additions on the microstructures of precipitates in low carbon steels containing Sn

The microstructures of low carbon steels with Sn additions were investigated using scanning electron microscopy, electron probe microanalysis, transmission electron microscopy and energy dispersive X-ray spectroscopy. Four steels based on Fe-0.9Nb-0.3Sn-0.05C (wt%) with different levels of Al and Si additions were prepared by arc melting under an argon atmosphere. The effects of heat treatment and the level of alloying elements Al and Si on the precipitation of Sn-rich phases were studied. After ageing at 1150°C and 850°C, NbC precipitates were found in all samples, as well as AlN in the higher Al content steels. The concentration of Al in steel was also found to affect the formation of Sn-rich compounds after heat treatment at 850°C for 96 hours. In the lower Al or Al-free steels, a -Fe₂Nb₃ phase, which dissolves a significant amount of Si, was observed. In the higher Al steels, a Fe₂Nb-based Laves phase, which dissolves both Si and Sn was detected. A mechanism based on both size factors and thermodynamic considerations is described, which accounts for the experimental observations.     

Continuous synthesis of silicon carbide whiskers

A two-step reaction scheme has been employed for the synthesis of SiC whiskers at 1450 °C under an argon or hydrogen flow. First, SiO vapour was generated via the carbothermal reduction of silica in a controlled manner. Second, the generated SiO vapour was reacted with carbon-carrying vapours such as CO and CH₄, which resulted in the growth of SiC whiskers on a substrate away from the batch. A higher growth rate was observed in the hydrogen atmosphere due to the formation of CH₄ which provides a more favourable reaction route. By the use of thermodynamic calculations, the preferred reaction routes have been selected for an efficient synthesis of SiC whiskers, and a continuous reactor has been designed. The system consists of a boat-train loaded with the silica-carbon mixture and iron-coated graphite substrate above it in an alumina-tube reactor. By pushing the boat-train into the hot zone at a fixed speed, SiO vapour is constantly generated. High-quality SiC whiskers have been grown on the substrate with diameters of 1–3 m. The yield was about 30% based on the silicon input as SiO₂ and silicon output as SiC whiskers. This demonstrates the feasibility of continuous production of high-quality SiC whiskers which does not require additional processes such as purification and classification.     

High temperature reactions between SiC and platinum

Solid state reactions between SiC and platinum have been studied at temperatures between 900 and 1100 °C. In the reaction zones, alternating layers of Pt₃Si and carbon, and Pt₂Si and carbon were formed at 900 and 1000 °C, respectively. Both the Pt₃Si and Pt₂Si phases were stable at respective temperatures. Annealings at 1100 °C, however, produced alternating layers of mixed Pt-silicides and carbon. The formation of platinum silicides gave rise to interfacial melting between SiC and platinum at all the temperature regimes. Laser Raman microprobe indicates that SiC decomposes into carbon and silicon at all the temperatures. The silicon reacts with platinum and forms platinum silicides, while the carbon forms clusters and stays unreacted. Based on the Raman results, the carbon exists in two different crystalline states depending upon its location from the SiC reaction interface. The reaction kinetics between SiC and platinum and the formation of periodic structure, respectively, are discussed based on the decomposition of the SiC and the phase separation of carbon from platinum silicides.     

Oxidation behavior and mechanical properties of C/SiC composites with Si-MoSi2 oxidation protection coating

A new kind of oxidation protection coating of Si-MoSi₂ was developed for three dimensional carbon fiber reinforced silicon carbide composites which could be serviced upto 1550 °C. The overall oxidation behavior could be divided into three stages: (i) 500 °C < T < 800 °C, the oxidation mechanism was considered to be controlled by the chemical reaction between carbon and oxygen; (ii) 800 °C < T < 1100 °C, the oxidation of the composite was controlled by the diffusion of oxygen through the micro-cracks, and; (iii) T > 1100 °C, the oxidation of SiC became significant and was controlled by oxygen diffusion through the SiC layer. Microstructural analysis revealed that the oxidation protection coating had a three-layer structure: the out layer is oxidation layer of silica glass, the media layer is Si + MoSi₂ layer, and the inside layer is SiC layer. The coated C/SiC composites exhibited excellent oxidation resistance and thermal shock resistance. After the composites annealed at 1550 °C for 50 h in air and 1550 °C 100 °C thermal shock for 50 times, the flexural strength was maintained by 85% and 80% respectively. The relationship between oxidation weight change and flexural strength revealed the criteria for protection coating was that the maximum point of oxidation weight gain was the failure starting point for oxidation protection coating.     

Deposition and microhardness of SiC from the Si2Cl6-C3H8-H2-Ar system

We obtained SiC coating layers on a graphite substrate using hexachlorodisilane (Si₂Cl₆, boiling point 144° C) as a silicon source and propane as a carbon source. We examined the deposition conditions, contents of carbon, silicon and chlorine in the deposits, and the microhardness. Mirror-like amorphous silicon layers were deposited in the reaction temperature range 500 to 630° C. well-formed silicon carbide layers with good adherency to the substrate were obtained above 850° C. The lowest deposition temperature of SiC was estimated to be 750 to 800° C. The Vickers microhardness of the SiC layer was about 3800 kg mm^–2 at room temperature and 2150 kg mm^–2 at 1000° C.     

Precipitation processes in Cu-Co-Si alloys

The precipitation of cobalt and silicon atoms from supersaturated solid solutions of Cu-Co-Si alloys was studied during ageing between 400 and 600° C by electrical resistivity, thermoelectric power and calorimetric measurements, by mechanical testing and by transmission electron microscopy investigations. It has been found that the decomposition begins with the appearance of concentration fluctuations from which cobalt precipitates first. The clustering of cobalt atoms initiates the precipitation of silicon, and so particles with the stoichiometric Co₂Si composition are finally formed. The silicon that is in excess with respect to the stoichiometry of Co₂Si is retained in the solid solution but its increase in the initial supersaturated solid solution strongly enhances the nucleation of the particles and, therefore, results in a finer precipitate structure. On the basis of calorimetric measurements the effective activation energy of both the cobalt and silicon precipitation was determined to be around 1.5 eV.     

Microstructure and oxidation resistance of SiC coated carbon-carbon composites via pressureless reaction sintering

The effects of processing parameters on the microstructure and oxidation resistance of silicon carbide (SiC) coated carbon-carbon (C-C) composites were investigated. C-C composites were made from plain woven carbon cloths and phenolic derived carbon matrices in the laboratory. Pressureless reaction sintering has been used to apply SiC coating to C-C composites using epoxy resin and silicon powder as the precursor. Results showed that the oxidation resistance of C-C composites was enhanced by coating with SiC. The pressureless reaction sintering process exhibits good processability. -SiC was formed after heat treatment at 1800 °C and the -SiC formed after heat treatment at 2200 °C. The SiC coated C-C composites exhibit good oxidation resistance at 1000 °C for 100 h under the test conditions.     

Aging and wear resistant characterization of al-5Mg-X(Si, Cu, Ti)/SiCp composites produced by pressureless infiltration method

The effects of SiC particle size and alloy elements such as Si, Cu and Ti on the response to aging treatment and wear resistance in Al-5Mg-X(Si,Cu,Ti)/SiCp composites fabricated by pressureless infiltration method have been investigated by hardness tester, scanning electron microscope (SEM), X-ray diffractometer (XRD), differential scanning calorimetry (DSC) and wear tester. The Al-5Mg-0.3Si-0.1Cu-0.1Ti/SiC_P composites had better wear resistance property among Al-5Mg-X(Si,Cu)/SiC_P composites. The wear resistance property of all the composites was enhanced after aging at 170°C for 8 hrs due to precipitates of '(Mg₂Si) phase. The wear resistance property of the composite as-fabricated with 50 m size of SiC particle is superior to that of the composite as-fabricated with 100 m SiC size of particle. In Al-5%Mg alloy aged at 170°C for 8 hrs, the frictional seizure appeared more than abrading speed of 1.90 m/s, but in Al-5Mg-(Si,Cu,Ti)/SiC_P composites aged at 170°C for 8 hrs, the frictional seizure was not found at abrading speed ranging from 0.5 m/s to 4.3 m/s.     

An internal synthesis method for mullite-chromium carbide composite in a vacuum system

In situ formation of chromium carbide in a mullite matrix through reaction of Cr₂O₃, SiC and A₂O₃ has been studied. Three different chromium compounds, Cr₃Si, Cr₃C₂, Cr₇C₃, and mullite were formed. In a vacuum environment, the Cr₃Si particles formed first and were retained below 1550 °C, while the Cr₇C₃ phase was only dominant above 1600 °C. The Cr₃C₂ phase was the dominant dispersed phase at temperatures of 1450–1500 °C. In an argon environment, the Cr₃C₂ phase was the main product component at temperatures ranging from 1450 to 1550 °C. The mullite phase formed concurrently through the diffusion of the SiO₂ phase into the Al₂O₃. SiO₂ was the product of the reaction between Cr₂O₃ and SiC. The composite hot-pressed at 1450 °C in vacuum gave a flexural strength and fracture toughness of up to 457 MPa and 4.1 MPa m^1/2, respectively.     

Carbothermal production of β′-sialon from alumina,silica and carbon mixture

Mixtures of pure nanometer-sized amorphous silica and -alumina with the atomic ratio SiAl=1 were reduced by a stoichiometric amount of carbon between 1100 and 1450 °C in flowing nitrogen in order to produce -sialon powder. Using aqueous suspensions of starting materials, compacts with different microstructures were prepared for reaction. Silica reduction to SiO occurred at a temperature as low as 1300 °C and part of it was removed with flowing nitrogen. Carbothermal reaction involving nitrogen stated at 1350 °C and Si₂N₂O was found as an intermediate together with SiC, resulting in -sialon formation. Loss of silica from the system led to AlN formation. Decomposition of -sialon into sialon polytypoids (15R, 12H) was observed as a result of sialon and AlN reaction at 1450 °C. The reaction rate of sialon formation was slowed down compared to the carbothermal reduction of kaolin because of the lack of impurities. The microstructure of the reacted pellets influenced the reaction products, and the narrow pore size distribution as well as good homogeneity enhanced -sialon formation.On leave, from Silesian Technical University, Krasiskiego 8, 40-019 Katowice, Poland.     

An electron microscope study of thin ferrite films prepared by the oxidation of high purity vacuum-evaporated metal thin films

The oxidation of thin films of nickel, cobalt, iron, NiFe₂ and CoFe₂ has been investigated between 200 and 1200° C. The oxidation products for the elemental metals differ from the oxidation products observed in previous work upon bulk material. The oxidation mechanism proposed for bulk material is in general still valid in the thin film situation. Above oxidation temperatures of approximately 850° C both NiFe₂ and CoFe₂ form the respective ferrites, although in the case of nickel ferrite, traces of the -Fe₂O₃ tetragonal superstructure can still be detected at oxidation temperatures of 1200° C. Films of nickel ferrite and cobalt ferrite upon single-crystal magnesium oxide substrates, produced by oxidation of vacuum-deposited NiFe₂ and CoFe₂ thin films, have been investigated by transmission and scanning electron microscopy. It has been found that (100) nickel ferrite prepared by this technique grows epitaxially upon (100) magnesium oxide.     

Binding energies of Si 2<Emphasis Type="Italic">p</Emphasis> and Co 3<Emphasis Type="Italic">p</Emphasis> electrons in cobalt silicides

The binding energies of Si 2p and Co 3p core-shell electrons in four stable cobalt silicides (Co₃Si, Co₂Si, CoSi, and CoSi₂) have been determined by high-resolution photoelectron spectroscopy using synchrotron radiation. The silicides were formed by solid-state epitaxy under identical conditions on Si(100), Si(110), and Si(111) faces of silicon single crystals.     

Formation of SiC whiskers from silicon nitride

Attempts have been made to produce SiC whiskers through vacuum pyrolysis of Si₃N₄ without any addition of extraneous carbon. Vacuum pyrolysis of Si₃N₄ granules and powder compacts, has been carried out at 1550 and 1700°C using a graphite resistance furnace. The products of pyrolysis have been identified through XRD and SEM as SiC whiskers and particles. Small amounts of elemental silicon at 1550°C and free carbon at 1700°C have been detected through X-ray diffraction. Detection of elemental silicon through X-ray diffraction and solidified silicon droplets at the whisker tips in the SEM provide important clues regarding the mechanism of SiC_w formation, as the one involving the reaction 2Si(l) + CO(g) SiC(s) + SiO(g) Silicon carbide whiskers, 3–4 mm long, have been grown from Si₃N₄ compacts at 1550°C over a short period of 0.5 h. It has been shown in the present study that Si₃N₄ can be completely converted to SiC_w, when a loose bed of Si₃N₄ in the form of granules is pyrolysed in the presence of CO at about 1550°C.     

Multicomponent toughened ceramic materials obtained by reaction sintering

Very dense zirconia-toughened ceramics with a mullite matrix based on the quaternary system ZrO₂-Al₂O₃-SiO₂-MgO in the temperature range as low as 1450 to 1500° C have been obtained by reaction sintering of zircon/alumina/magnesia mixtures. The shrinkage, advance of reaction, microstructure, densification mechanism and mechanical properties are reported. The results are explained in terms of transitory and permanent liquids that appear at 1425° C and ~ 1450° C respectively.     

Alumina-chromium carbide composite through an internal synthesis method

In situ formation of chromium carbide particles, through a solid state reaction between Cr₂O₃ and SiC, for strengthening AI₂O₃ has been studied. Three kinds of chromium compound, Cr₃Si, Cr₃C₂ and Cr₇C₃ and mullite were formed in the alumina matrix. The reaction behaviour during hot pressing depends on heating parameters such as temperature and atmosphere. In a vacuum environment, the Cr₃Si particles formed first and was the dominant dispersed phase below 1550°C, while the Cr₇C₃ phase was only dominant above 1600°C. The Cr₃C₂ phase emerged briefly then diminished at temperature 1500°C. In an argon environment, however, the Cr₃C₂ phase was the main product component at temperatures ranging from 1450–1550 °C. The mullite phase formed concurrently through the diffusion of SiO₂ phase into the Al₂O₃ matrix, which is a by-product from the reaction between Cr₂O₃ and SiC. Incorporating chromium carbide or suicide particles into the Al₂O₃ matrix induces a strengthening effect. However, only when the content of dispersed phases is low and is mainly of Cr₃C₂ particles, is the strengthening effect significant. For instance, the composite, containing 5 vol% chromium carbide and hot-pressed at 1500°C in argon, gives a flexural strength and fracture toughness up to 600 MPa and 6.1 MPam^1/2, respectively.