SiC Si Composites Research Articles

Mechanism of SiC crystals growth on {100} and {111} diamond surfaces upon microwave heating

The subject of this work is focused on characterization of the microstructures and orientations of SiC crystals synthesized in diamond–SiC–Si composites using reactive microwave sintering. The SiC crystals grown on the surfaces of diamonds have either shapes of cubes or hexagonal prisms, dependent on crystallographic orientation of diamond. The selection of a specified plane in diamond lattice for the TEM investigations enabled a direct comparison of SiC orientations against two types of diamond facets. On the {111} diamond faces a 200 nm layer of 30–80 nm flat β-SiC grains was found having a semi-coherent interface with diamond at an orientation: (111)[112]SiC║(111)[110]C. On the {100} diamond faces β-SiC forms a 300 nm intermediate layer of 20–80 nm grains and an outer 1.2 µm layer on top of it. Surprisingly, the SiC lattice of the outer layer is aligned with the diamond lattice: (111)[110]SiC║(111)[110]C.

Yttrium silicate coating system for oxidation protection of C/C–Si–SiC composites: Electrophoretic deposition and oxygen self-diffusion measurements

Yttrium silicate (YSI) coatings are promising complements to SiC coatings for protecting C/C–Si–SiC composites against oxidation owing to the pronounced chemical and mechanical properties of this material (low Young's modulus, low thermal expansion coefficient, low evaporation rate and oxygen permeability, good erosion resistance). Under control of deposition voltage and duration, yttrium silicate coatings of various thicknesses were electrophoretically deposited from powder suspensions. The protectiveness of these coatings was tested by means of thermogravimetric analysis in air. Diffusion data for oxygen in yttrium silicate needed to correlate the experimentally obtained oxidation rates in a phenomenological model were obtained by the IEDP (isotope exchange depth profiling) method from tracer diffusion measurements in single crystalline yttrium silicate and compared to diffusion data in mullite.

Oxidation protection of C/C–SiC composites by an electrophoretically deposited mullite precursor: Cyclic thermogravimetric analysis

Sufficiently thick and homogeneous layers which transform into mullite at T ≥ 1000 °C were synthesized by means of sol–gel technology and electrophoretically deposited onto the surface of C/C–Si–SiC composites. The protectiveness of the deposited mullite layers was isothermally tested in air in the temperature range 1200 ≤ T ≤ 1550 °C for up to 200 h. At temperatures above 1600 °C, the protectiveness of the deposited layer is reduced due to the existence of a liquid phase and the formation of CO bubbles above the cracks in the SiC layer. In order to prolong the protectiveness of our mullite layers at higher temperatures an additional layer from a suspension of mullite precursor with 5 wt.% of Al 2O 3 powder was deposited. The protectiveness of the obtained mullite and mullite/Al 2O 3 multilayer systems were also tested under cyclic conditions. These experiments yielded encouraging results, which will have to be confirmed at higher temperatures.

Raman mapping of local phases and local stress fields in silicon–silicon carbide composites

Silicon–silicon carbide (Si–SiC) composites have emerged as a new class of materials with superior properties like high resistance against oxidation and corrosion, low thermal expansion coefficient and high mechanical strength. This enabled the composite to be successfully utilized in different industrial applications, such as combustion chambers, gas turbines, heat exchangers, fusion reactors, seal rings, welding nozzles, valve discs and ceramic engine parts. The strength of such composites showed a dependence upon their internal granular structure, since the latter was found to affect local stress distribution in the composite. In this study, we developed a Raman spectroscopy based technique to monitor local composition and local stress fields in Si–SiC composites. The Raman technique was used to monitor local stress fields’ development as the composite is globally loaded in compression for two different composites with reinforcement volume fractions of 13 and 34%. Our results show that the Raman based technique is very suitable for monitoring the local stress fields and their development as the composite is globally loaded. Secondly, it was found that in composites with high volume fraction of reinforcement, local stress fields are more uniform and homogeneous than in composites with low fraction of reinforcement.

Bioinspired Rattan-Derived SiSiC/Zeolite Monoliths: Preparation and Characterisation

In the present article, a three-step process for the preparation of SiSiC/zeolite composites is presented. Rattan-derived SiSiC composites were obtained via a two-step biotemplating liquid silicon infiltration process (LSI). The LSI process, consisting of pyrolysis of the biotemplates (Rattan stems) followed by reactive infiltration of the carbon preforms with liquid silicon at 1550 °C, led to the formation of SiSiC ceramic composites. The SiSiC replicas (59 wt.% of SiC, 40 wt.% of solidified Si, 1 wt.% carbon) faithfully reproduced the macrostructure of Rattan and exhibited an open porosity between 20 and 40 vol.%, with unidirectional parallel microchannels in the range of ∼100–300 μm in diameter. In a third stage MFI-type zeolite (Silicalite-1 and ZSM-5) coatings were developed on the SiSiC ceramic supports via a partial conversion of the substrate into zeolite (support self-transformation method). The metallic Si in the support was partially dissolved under hydrothermal conditions in a reaction mixture consisting of deionised water, template (TPABr) and NaOH, but without any external Si-source. The influence of different synthesis parameters in the development of the zeolite coating is discussed in detail. The resulting products were characterised by X-ray diffraction, TGA, N 2 adsorption/desorption and SEM-EDX. The filtrates were analysed by ICP-OES. SEM and adsorption measurements revealed that biomorphic cellular SiSiC/zeolite composites possess bimodal (micro-/macro-) porosity. In the final SiSiC/zeolite composite, a maximum zeolite loading up to 40 wt.% was calculated on the basis of TGA and XRD analyses. Thermal shock tests showed that a good coating adherence to the SiSiC substrate was obtained. In addition, a SiSiC/ZSM-5 monolith was also tested as structured catalyst for n-hexane cracking.

Laminated Object Manufacturing (LOM) of SiSiC Composites

Carbon templates of laminar structure were manufactured from pyrolysed filter paper sheets by Laminated Object Manufacturing (LOM). The laminates were converted into SiSiC composites by post‐pressureless reactive Si‐infiltration. The microstructure and mechanical properties of the fabricated materials depends on the density, porosity and pore size distribution of the biocarbon preform, which can be varied by changing the parameters of the LOM‐processing.

Biomimetic synthesis of cellular SiC based ceramics from plant precursor

A novel biomimetic approach in designing and fabricating engineering ceramic materials has gained much interest in recent times. Following this approach, synthesis has been made of dense Si-SiC duplex ceramic composites and highly porous SiC ceramics in the image of the morphological features inherent in the caudex stem of a local monocotyledonous plant. The process route involves making of a carbonaceous biopreform and its subsequent reaction with an infiltrating silicon melt to yield the biomorphic Si-SiC ceramic composites with flexural strength and Young’s modulus of 264 MPa and 247 Gpa, respectively and loss in weight of only ∼9% during oxidative heating up to 1200°C in flowing air.The Si-SiC composites were transformed into porous (49 vol.%) SiC ceramics with complete preservation of microcellular anatomy of the parent plant, by depleting residual silicon phase in channel pores through reaction with carbon. SiC based materials so derived can be used in structural applications and in designing high temperature filters and catalyst supports.

Si-SiC material and SiC fiber-reinforced Si-SiC composites

Si-SiC material and SiC fiber-reinforced Si-SiC composites

Influence of resin content and compaction pressure on the mechanical properties of SiC–Si composites with sub-micron SiC microstructures

Abstract The influence of resin content (15–20, 25 wt.%) and compaction pressure (75, 100, 125 MPa) on the mechanical properties and microstructure of SiC–Si composites with sub-micron microstructure has been evaluated. To produce the composites three powders with particle sizes in the range of 0.22–0.70 μm were used. Before application the SiC powders were treated with hydrofluoric acid to remove the extent of SiO2. Due to this treatment a successful infiltration of green-bodies, even of those produced of SiC powder with a mean particle size of 0.22 μm, was possible. The fracture toughness and bending strength increased with increasing resin content. The highest value of strength was observed at 733 MPa for the composites produced of SiC powder with a mean grain size of 0.70 μm and a resin content of 25 wt.%. Within this composite the free silicon content was reduced to 12 vol.%. In regard to previous experiments this is a reduction of more than 50%. These results are a consequence of the lowering of the free silicon content of the composites when increasing the resin amount. With decreasing SiC starting particle size the mechanical properties decreased but they were independent on compaction pressure.

XRD investigation of Si–SiC composites with fine SiC microstructure

Si–SiC composite (reaction bonded SiC) with a submicron SiC microstructure (starting SiC particle size: 0.22 μm) was examined by XRD analysis to determine the amount and phase composition of the secondary SiC formed by the reaction between silicon and carbon during the sintering process. It was found that the secondary SiC has grown onto the original hexagonal α-SiC grains as well as into the porosity of the green body. An increase of 3C–SiC was found within the microstructure after infiltration (from 2.6 wt. % before infiltration to 8.8 wt. % after infiltration) whereas the 4H-ploytype content was reduced. This behavior may be explained by the very small original SiC grains which acted as seeds for disoriented SiC growth and were assumed to force the nonepitaxically deposition of secondary SiC. Solid state and fast transportation processes caused the observed transformation of the SiC. Examinations of the silicon source (infiltrant) after the infiltration procedure showed that most of the carbon was converted to SiC with cubic modification (3C stacking sequence)

Fabrication of functionally graded reaction infiltrated SiC–Si composite by three-dimensional printing (3DP™) process

Carbon performs have been fabricated using the three-dimensional printing (3DP™) process for reaction-infiltrated SiC–Si composites. Starting with glassy carbon powders of 45–105 μm sizes, the preform was produced by printing acetone-based furfuryl resin binder. The bulk density and open porosity of the resulting preform was 0.6 g cm −3 and 48%, respectively. The binder printing conditions during preform fabrication mostly determined the preform microstructure. Pressureless reactive infiltration of such preforms at 1450°C in nitrogen atmosphere formed a SiC–Si composite with a coarse-SiC grain structure. Some residual carbon remained inside the SiC grains in this reaction bonded SiC–C due to sluggish reactivity of the larger carbon powder particles. Relatively complex-shaped carbon preforms with overhang, undercut, and inner channel structures were produced, demonstrating the capability of the 3DP process. A functionally graded SiC–Si composite was also fabricated, by varying carbon-yielding binder dosage during the preform fabrication, in order to control the spatial SiC concentration within the SiC–Si composite.

Low-Pressure Fabrication of Diamond–SiC–Si Composites

A technique is proposed for fabricating diamond–SiC–Si composites via chemical reactions between silicon and carbon directly in the green compact at pressures no higher than 105Pa. An important advantage of the process is the ability to fabricate complex-shaped bodies from diamond-containing composites exhibiting unique mechanical properties.

Influence of annealing on the mechanical properties of SiC–Si composites with sub-micron SiC microstructures

The influence of annealing temperature (1000, 1100 and 1200°C) on the mechanical properties of SiC–Si composites has been evaluated. Three SiC powders with particle sizes in the range of 0.24 to 0.7 μm were used to produce the composites. Before application the SiC powders were treated with hydrofluoric acid to remove the extent of SiO 2. With this treatment a successful infiltration of green-bodies especially produced of SiC powder with a mean particle size of 0.24 μm was possible. The bending strength decreased with decreasing SiC starting particle size as well as with increasing annealing temperature. However, the fracture toughness was independent on SiC starting particle size and annealing temperature. XRD diffraction analysis showed that internal stress, expressed by broadening of XRD peaks, is low and had no effects on the mechanical properties of the composites.

The effect of SiC particles on the size and morphology of eutectic silicon in cast A356/SiC p composites

Properties of cast aluminium matrix composites are greatly influenced by the nature of distribution of reinforcing phase in the matrix and matrix microstructural length scales, such as grain size, dendrite arm spacing, size and morphology of secondary matrix phases, etc. Earlier workers have shown that SiC reinforcements can act as heterogeneous nucleation sites for Si during solidification of Al–Si–SiC composites. The present study aims at a quantitative understanding of the effect of SiC reinforcements on secondary matrix phases, namely eutectic Si, during solidification of A356 Al–SiC composites. Effect of volume fraction of SiC particulate on size and shape of eutectic Si has been studied at different cooling rates. Results indicate that an increase in SiC volume fraction leads to a reduction in the size of eutectic Si and also changes its morphology from needle-like to equiaxed. This is attributed to the heterogeneous nucleation of eutectic Si on SiC particles. However, SiC particles are found to have negligible influence on DAS. Under all the solidification conditions studied in the present investigation, SiC particles are found to be rejected by the growing dendrites.

Oxidation Behavior of Si-SiC at High Temperature

Oxidation behavior of high-purity Si-SiC composite (molar ratio 40/60) was studied during reactions at 1600K for 15h in Ar-O2 (PO2: 0.02-97kPa) atmospheres to evaluate the oxidation-resistant property. The oxidation rate was determined by measuring CO and CO2 concentrations during the oxidation using a quadrupole mass spectrometer. The oxidation kinetics obeyed the linear-parabolic law at high oxygen partial pressures (97kPa) and obeyed the linear law at low oxygen partial pressures (0.02kPa). The former case accompanied the weight gain, while the latter the weight loss. The oxygen partial pressure causing the transition from passive to active oxidation was estimated to lie between 0.1kPa and 0.02kPa. In passive oxidation, apparent low oxidizability of Si as compared with that of SiC was considered to result from the conversion of Si in the surface layer of the sample into SiC caused by its reaction with C released from SiC. Cooperative actions between Si and SiC were discussed in connection with the oxidation resistance of Si-SiC composite.

Growth and microstructure of Al 2O 3SiCSi(Al) composites prepared by reactive infiltration of silicon carbide preforms

Alumina matrix composites have been grown by directed melt oxidation of an AlSiZnMg alloy into sintered SiC preforms. Reaction between melt and the silica layer on the pre-oxidised SiC yields a matrix in which the residual alloy is principally silicon. However, there is no evidence of the formation of Al 4C 3. The coarsest particles display the least reaction while porosity increases as the particle size decreases .Thus, owing to the enrichment of silicon, growth rates are retarded compared to those in free space. The microstructure of the oxide is monocrystalline over distances of the order of the inter-particle spacing, however, a new type of growth fault that constitutes an inversion boundary in the Al 2O 3, has been found. Owing to the substantial reaction between the particle and the melt, the volume fraction of the alloy constituent in the final composite depends sensitively on particle size, in contrast to its relative invariance in directed melt oxidation into inert preforms.

Liquid-phase hot-pressing and WC-particle reinforcement of SiC-Si composites

Abstract The sintering behaviour of SiC-Si and SiC-Si-WC composites hot-pressed via silicon liquid phase has been studied. The results show that the increase of silicon makes the sintered materials densify at lower temperature and the grain growth is restricted due to the effect of the liquid. The strength of SiC-25 vol.% Si samples is higher than the strength obtained for SiC-5 vol.% Si with the same hot-pressing conditions. The addition of WC particles improves relative densities and strengths of SiC-Si ceramics.