Papers by Keyword: Liquid Silicon Infiltration

Abstract: Siliconized graphite was prepared by liquid silicon infiltration (LSI) of carbon preforms composed of mesocarbon microbeads (MCMBs), petroleum coke and graphite powder as the carbon source with binder of phenolic resin. Effects of the carbon source, binder contents, ball-milling time and moulding pressure on the properties of the porous carbon preforms and the siliconized graphite were investigated. The results showed that the moulding pressure was the main factor influencing the open porosity of the carbon preforms. The carbon preforms with porosity of above 45% could be infiltrated completely with Si, and maximum open porosity of 56% could be reached for the carbon preforms. For the siliconized graphite, high MCMBs contents contributed to high density, while high graphite content led to increased carbon remaining. The densities, open porosities, and the highest bending strength of the siliconized graphite were ranged between 2.90-3.01g·cm-3, less than 1.5%, and 317 MPa, respectively.

Abstract: SiC/SiC ceramics consist of silicon carbide fibres embedded in a silicon carbide matrix. As an alternative to classic CVI and PIP routes, Liquid Silicon Infiltration (LSI) was chosen as a technique with short process times to obtain composites with low porosity. Silicon carbide composites show good thermal shock resistance, a low coefficient of thermal expansion and excellent physical and chemical stability at elevated temperatures and are therefore regarded as promising candidates for various applications in jet engines and in power engineering. To build up the matrix, different phenolic resin based carbon precursors were infiltrated in fibre preforms and thermally cured, pyrolysed and siliconized. The aim is to obtain a high carbon yield during pyrolysis and to control the pore morphology in a way that the following liquid silicon infiltration leads to a complete reaction of the carbon matrix with silicon to SiC. The siliconization behaviour and conversion into SiC in dependence of pore morphology and chosen precursor is analysed.At the same time a functional fibre coating has to be developed which protects the fibres from liquid silicon and simultaneously provides a weak fibre matrix bonding. A LPCVD-SiN_x fibre coating has been chosen and is investigated in fibre composites especially in terms of protection and reactivity in different atmospheres during pyrolysis and siliconization.

Abstract: Three-dimensional short carbon fiber felt reinforced C/SiC composites were prepared by the liquid silicon infiltration (LSI) process. The influences of different porosity ratios, carbon coating, and heat treatment of C/C substrates, on the properties of C/SiC composites were studied. The optimized porosity ratio is calculated as 40.1% when the volume percent of carbon fiber (including carbon coating) is 23%, and after screening porosity ratio from ~55% to ~20%, the optimized experimental result (39.5%) is highly in accordance with the design value. The C/SiC composite after process parameter optimization, has a flexural strength and modulus of 125 MPa and 120 GPa, respectively. The C/SiC composite without carbon coating has a flexural strength of only 77 MPa, showing carbon coating plays a key role. The heat treatment of C/C substrate at 1600°C also improves the flexural strength of C/SiC composite for nearly 50%, and porosity rearrangement and interface weakening are believed to contribute such improvement.

Abstract: SiC whisker was introduced into reaction bonded silicon carbide to produce high performances composite by slip casting and reaction sintering. This study aimed at morphology of SiC whisker in the reaction bonded silicon carbide ceramics. The whisker was homogeneously dispersed into SiC/C suspension by ball-milling and ultrasonic dispersion. By chemical etching, the whisker displays the original burl profile on the polished surface of the composite. The raise of whisker fraction leads to an increase of porosity of the green body; and thus a decrease of density of the sintered body. For the specimen with 25 wt.% whisker, the rapid reaction of carbon with excess molten silicon leads to the missing of burl profile on the whisker surface. It is speculated that β-SiC on the whisker surface dissolved in the molten silicon during liquid silicon infiltration.

Abstract: Three-dimensional needle carbon fibre reinforced carbon and silicon carbide dual matrix composite (C/C-SiC) was a new type of high performance brake materials, and exhibited a series of outstanding advantages such as low density, good echanical properties, especially excellent toughness, to avoid catastrophic brittle fracture. In the present study, the C/C-SiC composites were fabricated by the combination of chemical vapor infiltration with liquid silicon infiltration. The preform was prepared by three dimension needling method. The preform was densified by chemical vapor infiltration to form porous carbon/carbon (C/C) composites. Then, the porous C/C composites were converted into C/C-SiC during LSI, in which silicon carbide matrix was formed by the reaction of carbon and melt silicon. The microstructure and mechanical properties of C/C-SiC has been investigated. The results indicated that the composites were composed of 55 wt%C, 39 wt%SiC and 6 wt%Si. The density of C/C-SiC was 2.2 g•cm^-3 and the open porosity was 6.4%, respectively. The C/C-SiC exhibited good echanical properties, especially excellent toughness, to avoid catastrophic brittle fracture. The flexural and compressive strength can reach to 214.6 MPa and 271.0 MPa, respectively. C/C-SiC had excellent impact damage tolerance of 25.2 kJ•m^-2.

Abstract: The concept of functionally graded materials (FGMs) was originated in the research field of thermal barrier coatings. Continuous changes in the composition, grain size, porosity, etc., of these materials result in gradients in such properties as mechanical strength and thermal conductivity. In recent years, functionally graded structural composite materials have received increased attention as promising candidate materials to exhibit better mechanical and functional properties than homogeneous materials or simple composite materials. Therefore the research area of FGMs has been expending in the development of various structural and functional materials, such as cutting tools, photonic crystals, dielectric and piezoelectric ceramics, thermoelectric semiconductors, and biomaterials. We have developed functionally graded structural ceramic/metal composite materials for relaxation of thermal stress, functionally graded anti-oxidation coatings for carbon/carbon composites, and functionally graded dielectric ceramic composites to develop advanced dielectric ceramics with flat characteristics of dielectric constant in a wide temperature range. This paper introduces functionally graded coatings for C/C composites with superior oxidation resistance at high temperatures.