Reaction-formed Silicon Carbide Research Articles

Evaluation of thermomechanical behavior in controlled atmospheres of silicon carbide obtained from sawdust residues of the Peruvian timber industry

The possibility of using biomass as a precursor of silicon carbide (SiC) has been studied for many years. In this research, reaction-formed silicon carbide was synthesized from sawdust residues of Peruvian wood. Residues were packed into cylindrical solid pieces through hot pressing and they were subsequently pyrolyzed in an inert atmosphere and infiltrated with metallic silicon in a vacuum atmosphere. SiC pieces were obtained with silicon remaining in their pores, so a chemical attack was performed to clean the samples. Mechanical and thermomechanical properties were evaluated in different environments to know the behavior of SiC through compression tests. The highest values were obtained at room temperature under normal conditions, a maximum stress of 686 MPa was achieved. In all cases, the samples had a fragile behavior as expected. In thermomechanical tests it was observed that resistance decreased according to the gradual rise in temperature. Finally, tests at 500 °C in oxidizing, inert and reducing atmospheres showed that the SiC maintained its maximum compression stress within the range of thermomechanical tests under normal conditions, which means that this material was not sensitive to chemical changes, being stable in all tests. At present, this work continues as research on thermomechanical behavior at temperatures above 1100 °C in different atmospheres.

Analysis of Pore Structure on Carbon Powder of Hemp Straw

Reaction-formed silicon carbide (RF-SiC) ceramics can be prepared by melt Si-infiltration of the carbon preform which fabricated by casting of carbon powder of hemp straw. In order to make clear the pore structure of the carbon powder of hemp straw which affect the mechanical property of RF-SiC ceramics, the morphology and inherent pore structure of carbon powder of hemp straw were characterized with powder X-ray diffraction, laser diffraction size analysis, scanning electron microscopy, transmission electron microscopy, mercury porosimetry and nitrogen adsorption. The results indicated that some parts of interconnected macroporous and mesoporous structures of hemp straw still remained after carbonization and the RF-SiC ceramics with outstanding performance can be fabricated with this carbon powder by melt Si-infiltration of the porous preform

Rapid manufacture of net-shape SiC components

Stereolithography was introduced into the net-shape SiC components fabrication process to produce the molds for the preparation of porous carbon preforms. The mixed resin was cast into the molds and then pyrolyzed to produce the porous carbon preforms which had a high porosity of 40.93% and were infiltrated and reacted with molten silicon to get reaction-formed silicon carbide (RFSC) components. To realize the complete infiltration of the thick wall parts, pipelines as the channels for the molten silicon were added into the components. The hierarchical structures of the porous carbon had been attained to realize the controllability of the microstructure and properties of RFSC. The samples had a high linear and volume shrinkage, 24.7% and 57.3%, respectively, during the pyrolysis process. Phase composition had been investigated by optical microscopy and X-ray diffraction analysis. The results indicate that no residual carbon was available in the final RFSC. The net-shape fabrication process for the RFSC components could promote the industry applications where SiC components with complex surface and inner structure were needed.

Microstructure and Properties of Reaction-Formed Silicon Carbide

The effects of microstructure on strength, toughness and corrosion resistance of reactionformed silicon carbide and heat treatment on the properties of reaction-formed silicon carbide have been investigated in this paper. The results show strength and corrosion resistance of reaction-formed silicon carbide decrease with increasing Si content. Fracture toughness shows no rule with free Si content. The average SiC grain size and bonding area of SiC grain increase during heat treatment at 1850°C. The strengths of the samples after heat treatment decrease compared with that of the samples before heat treatment due to the pores in the heat treatment bodies.

The effect of porous carbon preform and the infiltration process on the properties of reaction-formed SiC

The effect of porous carbon preform and the infiltration process on the properties of reaction-formed SiC have been investigated in this paper. Five different porous carbon preforms with porosity in a range 35–67% and pore size from 0.03 to 2.58 μm have been infiltrated with liquid or vapor Si. The porous carbon preform and infiltration process affect the reaction rate and infiltration rate, which are key issues in designing the final reaction-formed silicon carbide microstructure. Under suitable experimental conditions, vapor infiltration can lead to deeper infiltration of Si than liquid infiltration. The best result has been obtained by vapor Si infiltration with a relatively smaller scale porous carbon preform having a density of 0.92 g cm −3. The reaction-formed silicon carbide with a flexural strength 630 MPa and density 3.12 g cm −3 and Si-content 8 vol.%, which is superior to commercially obtained reaction-bonded silicon carbides, have been fabricated by Si vapor infiltration.

The fabrication of reaction-formed silicon carbide with controlled microstructure by infiltrating a pure carbon preform with molten Si

This paper describes the production of shaped silicon carbide based ceramic materials by in situ reaction of silicon with fine, ultra-uniform porous carbon skeletons that were produced from liquid polymer solutions without any particulate additions. Three typical types of carbon preforms (spherical carbon particulates, crosslinked network carbon, and lamellate carbon particulates) were infiltrated with molten Si. The morphologies and mechanical properties of corresponding reaction-formed silicon carbide (RFSC) ceramics have been investigated. The results revealed that the RFSC obtained by molten Si reaction with the porous carbon preform with discrete and spherical carbon particulates has the most uniform microstructure, the highest flexural strength (410±30 MPa) and the lowest amount of residual Si (approximately 12 wt.%). The general feature of a fracture surface showed the typically brittle failure in those materials.

FEM Analysis of Asymmetrical Bending Test for Smart Materials Joints

SiC/SiC composites with a thermal coating have been developed as smart composites for high temperature applications, where the joint techniques are indispensable for constructing large scale structures due to the fabrication and economical reasons. Since one of the important issues is the establishment of a design database for the joint strength, the testing method has to be carefully selected based on the theoretical background. In this study, the stress distribution of a SiC/SiC composite specimen containing a butt joint consisting of reaction-formed silicon carbide tested by the asymmetrical four-point bending test was precisely analyzed by the finite element method as a means to evaluate the applicability of analytical results. In the case without the effect of the thermal residual stresses, the shear stress distribution at the interface between the base and the joint almost agreed with the analytical theory. For the case with the residual stress, however, the shear stress near the surface was very large and the possibility of an initial crack induced by the residual stress was considered. Moreover, it was found that the residual shear stress distribution near the surface was significantly affected by the joint thickness.

High-temperature compressive strength of reaction-formed silicon carbide (RFSC) ceramics

High-temperature compressive strength of reaction-formed silicon carbide ceramics, fabricated by the melt infiltration of silicon, and silicon-5 at% niobium alloy into microporous carbon preforms, have been evaluated. The values of maximum strength ranged from 1000 MPa at 1250 °C to 500 at 1400 °C. These maximum strengths were reached for the ceramic fabricated by silicon infiltration into a low silicon yielding carbon preform. The ceramics fabricated by silicon infiltration into a higher volume fraction carbon preform showed a decrease on the strength. The infiltration of silicon-5 at% niobium alloy into a carbon preform also resulted in a decrease of strength compared to the ceramic fabricated by silicon infiltration into the same carbon preform. The dependence with temperature and composition of the high temperature compressive behavior is discussed.

Measurements of elastic and anelastic properties of reaction-formed silicon carbide ceramics

The piezoelectric ultrasonic composite oscillator technique (PUCOT) was used to measure dynamic Young's modulus, damping, strain amplitude, and the strain-amplitude dependence of damping on three reaction-formed silicon carbide (RFSC) specimens. The data were compared with results found on NC 203 SiC. The temperatures used to conduct the tests ranged from 595–1049°C, and this information complemented data collected earlier. The main results are as follows: the modulus for RFSC specimens decreases with increasing temperature over a range of approximately 350–260 GPa from room temperature to 1049°C, the modulus for NC203 ranges from 450–415 GPa over the same temperature range, and damping of RFSCs is independent of strain amplitude and has little temperature dependence.

High-temperature oxidation behavior of reaction-formed silicon carbide ceramics

The oxidation behavior of reaction-formed silicon carbide (RFSC) ceramics was investigated in the temperature range of 1100 to 1400 °C. The oxidation weight change was recorded by TGA; the oxidized materials were examined by light and electron microscopy, and the oxidation product by x-ray diffraction analysis (XRD). The materials exhibited initial weight loss, followed by passive weight gain (with enhanced parabolic rates, kp), and ending with a negative (logarithmic) deviation from the parabolic law. The weight loss arose from the oxidation of residual carbon, and the enhanced kp values from internal oxidation and the oxidation of residual silicon, while the logarithmic kinetics is thought to have resulted from crystallization of the oxide. The presence of a small amount of MoSi2 in the RFSC material caused a further increase in the oxidation rate. The only solid oxidation product for all temperatures studied was silica.

Microstructural characterization of reaction-formed silicon carbide ceramics

Microstructural characterization of two reaction-formed silicon carbide ceramics has been carried out by interference layering, plasma etching, and microscopy. These specimens contained free silicon and niobium disilicide as minor phases with silicon carbide as the major phase. In conventionally prepared samples, the niobium disilicide cannot be distinguished from silicon in light optical micrographs. After interference layering, all phases are clearly distinguishable. Backscattered electron imaging and energy-dispersive spectrometry in the scanning electron microscope confirmed the results obtained by interference layering. Plasma etching with CF 4 + 4% O 2 selectively attacks silicon in these specimens. It is demonstrated that interference layering and plasma etching are very useful techniques in the phase identification and microstructural characterization of multiphase ceramic materials.

Reactive melt infiltration of silicon-molybdenum alloys into microporous carbon preforms

Investigations on the reactive melt infiltration of silicon-1.7 and 3.2 at.% molybdenum alloys into microporous carbon preforms have been carried out by modeling, differential thermal analysis (DTA), and melt infiltration experiments. These results indicate that the pore volume fraction of the carbon perform is a very important parameter in determining the final composition of the reaction-formed silicon carbide and the secondary phases. Various undesirable melt infiltration results, e.g. choking-off, specimen cracking, silicon veins, and lake formation, and their correlation with inadequate preform properties are presented. The liquid silicon-carbon reaction exotherm temperatures are influenced by the pore and carbon particle size of the preform and the compositions of infiltrants. Room temperature flexural strength and fracture toughness of materials made by the silicon-3.2 at.% molybdenum alloy infiltration of medium pore size preforms are also discussed.

Microstructure and mechanical properties of reaction-formed silicon carbide (RFSC) ceramics

The microstructure and mechanical properties of reaction-formed silicon carbide (RFSC) ceramics fabricated by silicon infiltration of porous carbon preforms are discussed. The morphological characterization of the carbon preforms indicates a very narrow pore size distribution. Measurements of the preform density by physical methods and by mercury porosimetry agree very well and indicate that virtually all of the porosity in the preforms is open to infiltrating liquids. The average room temperature flexural strength of the RFSC material with approximately 8 at.% free silicon is 369±28 MPa (53.5±4 ksi). The Weibull strength distribution data give a characteristic strength value of 381 MPa (55 ksi) and a Weibull modulus of 14.3. The residual silicon content is lower and the strengths are superior to those of most commercially available reaction-bonded silicon carbide materials.

Reactive melt infiltration of silicon-niobium alloys in microporous carbons

Studies of the reactive melt infiltration of silicon-niobium alloys in microporous carbon preforms prepared by the pyrolysis of a polymer precursor have been carried out using modeling, DTA, and melt infiltration. Mercury porosimetry results indicate a very narrow pore size distribution with virtually all the porosity within the carbon preforms open to infiltrants. The morphology and amount of the residual phases (niobium disilicide and silicon) in the infiltrated material can be tailored according to requirements by careful control of the properties (pore size and pore volume) of the porous carbon preforms and alloy composition. The average room temperature four-point fiexural strength of a reaction-formed silicon carbide material (made by the infiltration of medium pore size carbon preform with Si–5 at. % Nb alloy) is 290 ± 40 MPa (42 ± 6 ksi) and the fracture toughness is 3.7 ± 0.3 . The fiexural strength decreases at high temperatures due to relaxation of residual thermal stresses and the presence of free silicon in the material.