Liquid Silicon Infiltration Process Research Articles

Effect of pyrocarbon content in C/C preforms on microstructure and mechanical properties of the C/C–SiC composites

Carbon fiber reinforced silicon carbide matrix (C/C–SiC) composites were prepared by liquid silicon infiltration (LSI) process, using four kinds of quasi-three-dimensional C/C preforms with the same fiber volume fraction (32%) but different pyrocarbon content (from ∼20% to ∼50%, in volume) filled by chemical vapor infiltration (CVI). The microstructure and mechanical properties of C/C–SiC composites have been investigated. X-ray diffractmeter (XRD) analysis results show the composites are composed of three phases of β-SiC, C and Si. With the increase of the pyrocarbon content in the C/C preforms, the content of formed silicon carbide in the C/C–SiC composites decreases but the residual pyrocarbon increases. Mechanical tests were performed to assess the role of the matrix and phase composition in the C/C–SiC composites. The results demonstrate that the flexural strength and shear strength of the composites increase gradually as the increase of the content of residual pyrocarbon. The flexural and shear strength of the C/C–SiC composite produced of the C/C preform with pyrocarbon content of ∼42 vol.% reached the highest value.

탄소섬유 배열이 LSI Cf-Si-SiC 복합체의 특성에 미치는 영향

Carbon fiber fabric-silicon carbide composites were fabricated by liquid silicon infiltration (LSI) process. The porous twodimensional carbon fiber fabric performs were prepared by 13 plies of 2D-plain-weave fabric in a three laminating method, [0/90], [±45], [0/90/±45] lay-up, respectively. Before laminating, a thin pyrolytic carbon (PyC) layer deposited on the surface of 2D-plain weave fabric sheets as interfacial layer with C₃H? and N₂ gas at 900℃. A densification of the preforms for Cf-Si-SiC matrix composite was achieved according to the LSI process at 1650℃ for 30 min. in vacuum atmosphere. The bending strength of the each composite were measured and the microstructural consideration was performed by a FE-SEM.

Carbon/Carbon–Silicon-Carbide Dual-Matrix Composites for Brake Discs

Composite ceramic materials reinforced with carbon fibers are interesting materials because of their excellent mechanical and thermo-physical properties, which make them appropriate for many applications, including re-entry shields, rocket nozzles, disc brakes, and prosthetic devices. For all these purposes, many different types of such materials have been developed, for example, carbon/silicon-carbide or carbon/carbon–silicon-carbide composites that are commonly produced from carbon/carbon composites impregnated with liquid silicon by the liquid-silicon infiltration (LSI) process. In this work, a new ceramic matrix composite friction material for brake discs was developed that combines a high fracture toughness with good oxidation and abrasion resistance, good thermal conductivity, and the appropriate friction characteristics. For the production of such materials, the “dual-matrix” approach combined with reactive sintering was used. The materials have suitable mechanical properties and the braking discs have appropriate coefficient of friction and very low wear rate.

The crack evolution on the atomistic scale during the pyrolysis of carbon fibre reinforced plastics to carbon/carbon composites

Abstract The development of the crack patterns during the pyrolysis of carbon fibre reinforced plastics (CFRP) to carbon/carbon composites as the second manufacturing step in the liquid silicon infiltration (LSI) process was investigated. In the basic examination reported previously, it was discovered that a substantial amount of cracking occurs beyond 650 °C, when the mesoscopic crack pattern has already fully developed. This additional cracking could not be visualized by using standard microscopy. Thus additional investigations were conducted by using conventional and high-resolution transmission electron microscopy to obtain information on the atomistic scale on the assumed cracking activity. It was found that the crack development starts at pores that develop as a compensation for the rough fibre surface. Crack propagation takes place by evolution of new nanoscopic cracks caused by fibre–matrix-debonding in the tensile stress field in front of the crack tip and subsequent connection with the main crack. Thus the interconnection mechanism – cracking as the connection of cracks on a subordinated scale to form a new crack – is the second main cracking mechanism beside transversal cracking (leading to a regular mesoscopic crack pattern) active during the carbonization of CFRP components.

Three-dimensional needled carbon/silicon carbide composites with high friction performance

Carbon fiber reinforced silicon carbide matrix composites (C/SiC) have the potential to be used in high performance brake applications because of low density and superior friction performances at elevated temperatures. In the present work, three-dimensional C/SiC composites were manufactured by chemical vapor infiltration combined with liquid silicon infiltration process. The preform was prepared by three dimension needling method and the volume fraction of carbon fiber was 30%. The preform was densified by chemical vapor infiltration to form porous carbon/carbon composites. Then, the porous carbon/carbon composites were converted into carbon/silicon carbide during liquid silicon infiltration, in which silicon carbide matrix was formed by the reaction of carbon and melt silicon. The density of C/SiC was 2.1 g/cm 3 and open porosity was 3%. Silicon carbide was mainly located among the carbon fiber yarns and some amount of free silicon was residual. Carbon/silicon carbide composites exhibited excellent mechanical and friction performances. The coefficient of friction (COF) curve was in the typical saddle shape. The average coefficient of friction was relatively high (0.34) and the wear rate was very low (1.9 um/cycle). Especially, the coefficient of friction in wet condition was 0.33 and was almost not decreased. During braking process, the continuous and smooth friction surface was formed and grain-abrasion was the main wear mechanism.

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.

C/C–SiC composites for space applications and advanced friction systems

Ceramic matrix composite materials are being considered the primary materials for hot structures of future launch vehicles. Melt infiltrated composites based on the liquid silicon infiltration process have proven their suitability under extreme thermo-mechanical environments in different structural parts like nose caps, nozzle jet vanes and engine flaps. Considerable progress has been achieved within the last few years to mature the manufacture technology and to tailor the properties of the materials. Among low densities and high damage tolerance behaviour C/C–SiC composites show superior tribological properties predestining these materials for advanced friction systems.

Biomorphic Cellular SiSiC/Zeolite Ceramic Composites: From Rattan Palm to Bioinspired Structured Monoliths for Catalysis and Sorption

The three-step manufacturing of bioinspired SiSiC/zeolite monoliths starting from native rattan stems is presented. Rattan-derived cellular SiSiC ceramics are prepared via a biotemplating liquid silicon infiltration process. MFI-type zeolite crystals are directly grown onto the ceramic substrate. SiSiC/zeolite composites (see Figure) possess hierarchical multimodal (micro-/macro-) porosity, biomorphous anatomy, unique architecture, and outstanding mechanical/thermal stability.

Carbon Fiber Reinforced CMC for High‐Performance Structures

The demands in space technology originally played the decisive role in the development of carbon fiber reinforced ceramic matrix composites (CMC). Within the last few years, the properties and the manufacturing methods were consistently improved, so that now the industry in general can share in the profits of this new class of composite materials. The liquid silicon infiltration (LSI) process is regarded as one of the most promising processes for industrial products, especially if the aspect of cost is considered. An overview of the design, manufacture, properties, and applications of C/C‐SiC ceramics, produced with the LSI process, is given in this paper.

Joining Techniques for Fibre Ceramic Structures

This paper shows three different joining methods for fibre ceramic materials. The so called in-situ joining method is an integral part of the manufacturing process for CMC structures via the liquid silicon infiltration (LSI) process. Stiffening elements, local patches within attachment areas, inserts etc. are permanently joined to shell structures, thus enabling highly integrated components to be realised with low manufacturing costs. Mechanical joining methods are required for the attachment of CMC thermal protection systems and the assembly of large structures which can not be manufactured as one part due to the limited size of manufacturing devices (e.g. autoclave, furnaces). For these cases, two different principles are available. The first method takes advantage of interlocking effects of hardened castable ceramics for permanent joints and the so called ceramic rivet, which has similar properties to metallic rivets, however using only elastic and frictional properties of the CMC basic material. The last joining method presented within this paper deals with the attachment of hot structures to a cold substructure. To solve the problems associated with thermal mismatch, elastic or kinematic attachment systems, well adapted to the thermal expansion behaviour are suitable candidates.

Thermal conductivity, specific heat capacity, and emissivity of ceramic matrix composities at high temperatures

Ceramic matrix composites (CMCs) with low porosity are obtained in one cycle via the well established liquid silicon infiltration (LSI) process, which is characterised by short processing times and fairly low manufacturing costs. Apart from aerospace applications, such as hot structures for re-entry vehicles, more and more applications beyond this classic field of CMCs are of increasing interest, eg brake discs, zero-expansion materials, high-temperature heat exchangers, heat-sink materials, etc. By applying special process parameters the microstructure as well as the physical properties can be tailor-designed to match specific requirements. The thermophysical properties of C/C-SiC, especially specific heat capacity, thermal conductivity as well as total and spectral emissivity were investigated in order to find out how the selection of fibres and pretreatment of the material affect them. Thermal conductivity of carbon fibres and therefore also of the composite considerably increases with increasing pyrolysis and graphitisation temperature.