Fiber Reinforced Silicon Carbide Composites Research Articles

Numerical prediction of the effective coefficient of thermal expansion of 3D braided C/SiC composite

This paper is focused on the microstructure modeling and evaluation of effective coefficient of thermal expansion (CTE) of 3D braided carbon fiber-reinforced silicon carbide composites (C/SiC). Regarding the multi-scale characteristics of the composite, the microstructure modeling is carried out sequentially from the fiber scale to tow scale. Effective elastic properties are obtained based on the sequential homogenization from the fiber scale to the tow scale. A stain energy model is developed for the prediction of the effective CTE of composite materials. This model is based on the relationship established between the strain energy of the microstructure and that of the homogenized equivalent model under specific thermo-elastic boundary conditions. Expressions of closed-form are derived for the effective CTE in terms of the strain energy and effective elastic tensor. Numerical results obtained by the proposed model show a good agreement with the results measured experimentally.

Emissivity and catalycity measurements on SiC-coated carbon fibre reinforced silicon carbide composite

The emissivity and the catalytic efficiency related to atomic oxygen recombination were experimentally investigated in the range 600–1600 °C for a carbon fibre reinforced silicon carbide composite produced by polymer vapour infiltration and then coated with SiC by chemical vapour deposition. High emissivity (about 0.7 at 1600 °C) and low recombination coefficients (on average 0.07 at 1500 °C) were found for all the samples tested. The experimental data showed a marked effect of the surface finish on the catalytic behaviour only: C/SiC samples belonging to the same production batch but having slightly different surface morphology, while exhibiting the same catalycity trend, are characterized by detectable differences in absolute values of the recombination coefficient. Micro-structural investigations performed on post-test samples have shown the formation of a silica-based glassy layer and have confirmed that the oxidation of C/SiC lies on the boundary line between the active and passive mechanism.

Numerical computing and experimental validation of effective elastic properties of 2D multilayered C/SiC composites

The effective elastic properties of carbon fibre reinforced silicon carbide composite (C/SiC) with multilayered matrix manufactured by chemical vapour infiltration method are studied in the present paper. Based on the material tomography of scanning electron microscopy, distributions of voids and microcracks are identified and taken into account in the finite element model of such C/SiC composite. It is shown the proposed two scale finite element models of multilayered C/SiC composites can characterise physically the real situation of involved voids and microcracks related to the chemical vapour infiltration process. The numerical results of effective moduli and experimental ones have a reasonable agreement.

Effect of stitch spacing on mechanical properties of carbon/silicon carbide composites

The effects of stitch spacing on the mechanical properties of the carbon fiber reinforced silicon carbide composites fabricated by chemical vapor infiltration (CVI) were investigated. The results showed that the stitch spacing had little effect on the density and open porosity of the composites. The stitches improved the shear and tensile strengths of all stitched C/SiC composites, by suppressing and delaying the delamination. However, flexural strength was not changed for the composites with stitch spacing of 10 mm or 5 mm, but was slightly reduced for densely stitched composites with spacing of 2.5 mm. The optimum stitch spacing was 5 mm, which improved apparently the interlaminar shear strength and tensile strength of the composites with little detrimental effect on the flexural strength.

Fabrication of toughened Cf/SiC whisker composites and their mechanical properties

In this paper, dense short carbon fiber reinforced silicon carbide matrix composites had been fabricated by hot-pressed (HP) sintering using Al 2O 3 and La 2O 3 as sintering additives. The results showed that the combination of Al 2O 3 and La 2O 3 system was effective to promote densification of short cut carbon fiber reinforced silicon carbide composites (Cf/SiC). The whisker structure of silicon carbide was formed during the annealed treatment at 2023 K for 1 h. However, it was noted that this structure was not observed in the as-received HP material. The mechanism of forming whisker structure was not clear, but this kind of whisker structure was helpful to improve mechanical properties. The combination of grain bridging, crack deflection and whisker debonding would improve the fracture toughness of the Cf/SiC composites.

Preparation of 3D-Cf/SiC composites at low temperatures

The conversion process of the polycarbosilane (PCS) into silicon carbide (SiC) was investigated by TG–DTA, IR, XRD, etc., which indicated the feasibility of transition from PCS to SiC ceramics at low temperatures under 1000 °C. The results show that the inorganic transition of PCS appears at about 750 °C, and the crystal emerges at 880 °C approximately. The carbon fiber reinforced silicon carbide composites (Cf/SiC) were hereby prepared at low temperatures as 700 °C, 800 °C, 900 °C, 1000 °C, respectively through precursor infiltration and pyrolysis (PIP). The mechanical properties of Cf/SiC composites prepared at low temperatures were almost all outstanding, with the flexural strength of 600.3 MPa for Cf/SiC composites prepared at 1000 °C, the shear strength of 64.2 MPa and the fracture toughness of 22.5 MPa m 1/2. And the flexural strength of composites at 1600 °C in vacuum was 411.2 MPa, which was prepared at 900 °C, while the flexural strength at room temperatures was 527.4 MPa.

Reaction sintering of two-dimensional silicon carbide fiber-reinforced silicon carbide composite by sheet stacking method

Two-dimensionally plain woven SiC fiber-reinforced SiC composite has been developed by reaction sintering using a sheet stacking method in order to further increase mechanical and thermal properties of the composite and to obtain flexibility of manufacturing process of 2D woven SiC/SiC composites which can be applied to the fabrication of larger parts. In addition, sinterability and mechanical properties of the composite were investigated. In this study, relative density of the composites was about 90–93% and a dense composite could be obtained by reaction sintering using the sheet stacking method. The bulk density and maximum bending strength of SiC/SiC composite with a C/SiC weight ratio of 0.6 were higher than that of the composite with C/SiC ratios of 0.5 or 0.7. The values were 2.9 g/cm 3 and 200 MPa, respectively. However, the composites obtained in this study fractured in almost brittle manner due to the lower fiber volume fraction.

Fabrication of C<sub>f</sub>/SiC Composites by Hot-Pressing

Continuous carbon fiber-reinforced silicon carbide (Cf/SiC) composite was fabricated by hot-pressing, via liquid phase sintering. Sintering conditions strongly affect the densification process, and therefore dominate the mechanical properties and fracture behavior. The composites under the lower sintering temperature behaves less densified matrix and it demonstrates a relatively weak fiber/matrix bonding allowing the longer fibers pull-out. Increasing sintering temperature could accelerate the densified matrix and make fiber/matrix bonding stronger. In this case, the shorter fibers pull-out was predominant fracture behavior and it could improve mechanical properties.

Effects of polycarbosilane infiltration processes on the microstructure and mechanical properties of 3D-C f/SiC composites

Three-dimensional braided carbon fiber-reinforced silicon carbide (3D-C f/SiC) composites were prepared through eight cycles of infiltration of polycarbosilane (PCS)/divinylbenzene (DVB) and subsequent pyrolysis under an inert atmosphere. The effects of infiltration processes on the microstructure and mechanical properties of the C f/SiC composites were investigated. The results showed that increasing temperature could reduce the viscosity of the PCS/DVB solution, which was propitious to the infiltration processes. The density and flexural strength of 3D-C f/SiC composites fabricated with vacuum infiltration were 1.794 g cm −3 and 557 MPa, respectively. Compared to vacuum infiltration, heating and pressure infiltration could improve the infiltration efficiency so that the composites exhibited higher density and flexural strength, i.e., 1.944 g cm −3 and 662 MPa. When tested at 1650 °C and 1800 °C in vacuum, the flexural strength reached 647 MPa and 602 MPa, respectively.

Effects of frequency on fatigue behavior of CVI C/SiC at elevated temperature

Effects of frequency on fatigue behavior of a chemical vapor infiltrated carbon fiber reinforced silicon carbide composite (C/SiC) were investigated at an elevated temperature of 550 °C. Tension–tension fatigue tests were conducted at three frequencies: 0.1, 10 and 375 Hz to establish stress versus cycles to failure ( S–N) relationships. There was an increase in cycles to failure at a given stress level as frequency increased from 0.1 to 375 Hz at elevated temperature. This trend was different at room temperature where cycles to failure decreased when frequency changed from 40 to 375 Hz but remained almost same below 40 Hz. There was a reduction in cycles to failure at frequencies less than 40 Hz but cycles to failure remained same at a higher frequency of 375 Hz when test environment changed from room temperature to 550 °C. Analysis of damage mechanisms showed that the oxidation of carbon fibers was the major difference between the room and elevated temperatures, which caused a reduction in cycles to failure with lower frequencies at elevated temperature in comparison to that at room temperature. However, oxidation of carbon fibers was almost absent or negligible at higher frequency at elevated temperature, which caused practically no reduction in cycles to failure at elevated temperature in comparison to their counterparts at room temperature.

Mechanical and thermal properties of SiC f/SiC composites irradiated with neutrons at high temperatures

A high fluence irradiation of SiC f/SiC composites has been performed to study the effect of neutron irradiation on the mechanical and physical behaviour of those composites. The fibre reinforced silicon carbide composites have been irradiated at two temperature levels of 600 °C and 900 °C up to a fluence of 3.5 × 10 25 n/m 2 ( E > 0.1 MeV). The stiffness of the bending bars after irradiation changed with factors between 0.75 and 1.04 for the different composites while the bending strength after irradiation was reduced with a factor of 2 in some cases. The laser flash thermal diffusivity ratio's ( α irr/ α o) of the composites measured at 600 °C are from 0.1 to 0.5 and from 0.25 to 0.75 for an irradiation temperature of 600 °C and 900 °C, respectively. The dimensional changes observed are small.

Effects of pyrolysis processes on the microstructures and mechanical properties of Cf/SiC composites using polycarbosilane

Abstract Three-dimensional braided carbon fiber reinforced silicon carbide composites (3D-B Cf/SiC) were prepared through eight cycles of vacuum infiltration of polycarbosilane (PCS) and subsequent pyrolysis under an inert atmosphere. The influences of heating rate and pyrolysis temperature on the microstructure and mechanical properties of Cf/SiC were discussed. It was found that the heating rate had great effect on the mechanical properties of Cf/SiC composites. With the increase of heating rate, the density of Cf/SiC composites increased and the interfacial bonding was weakened. As a result, the flexural strength of Cf/SiC was enhanced from 145 to 480 MPa when the heating rate was increased from 0.5 to 15 °C/min. The results showed that the flexural strength of the Cf/SiC composites fabricated at a heating rate of 15 °C/min could be increased from 480 to 557 MPa if the pyrolysis temperature of the sixth cycle was elevated from 1200 to 1600 °C, which was also attributed to the desirable interfacial structure and increased density. When tested at 1300 °C in vacuum, the Cf/SiC showed higher flexural strength (680 MPa) than that (557 MPa) at room temperature.

Displacement damage in silicon carbide irradiated in fission reactors

Calculations are performed for displacement damage in SiC due to irradiation in the neutron environments of various types of nuclear reactors using the best available models and nuclear data. The displacement damage calculations use recently developed damage functions for SiC that are based on extensive molecular dynamics simulations of displacement events. Displacements per atom (DPA) cross sections for SiC have been calculated as a function of neutron energy, and they are presented here in tabular form to facilitate their use as the standard measure of displacement damage for irradiated SiC. DPA cross sections averaged over the neutron energy spectrum are calculated for neutron spectra in the cores of typical commercial reactors and in the test sample irradiation regions of several materials test reactors used in both past and present irradiation testing. Particular attention is focused on a next-generation high-temperature gas-cooled pebble bed reactor, for which the high-temperature properties of silicon carbide fiber-reinforced silicon carbide composites are well suited. Calculated transmutations and activation levels in a pebble bed reactor are compared to those in other reactors.

Frequency dependence of high-cycle fatigue behavior of CVI C/SiC at room temperature

Effects of loading frequency on high-cycle fatigue behavior of a chemical vapor infiltrated carbon fiber reinforced silicon carbide composite were investigated. Tension–tension fatigue tests were conducted at three frequencies, 4, 40 and 375 Hz. Fatigue run out was set to 10 7 cycles. Applied stress versus cycles to failure (S–N) relationships were developed for these three frequencies. At 4 and 40 Hz, fatigue run out was achieved at a stress level of 375 MPa. At 375 Hz, stress level for run out was 350 MPa. Frequency dependence was observed between the two lower frequencies (4 and 40 Hz) and the higher frequency (375 Hz), but not between two lower frequencies (4 and 40 Hz). This manifested as a reduction in cycles to failure at 375 Hz compared to 4 and 40 Hz at a given stress level. Specimen surface temperature increased due to internal heat generation from sliding friction between constituents of the composite under cyclic loading. This increase was directly related to frequency and/or applied cyclic stress level. There was no clear indication that frequency greatly impacted either the stress-strain response or the overall appearance of fracture surfaces. However, a closer examination of specimens cycled at the highest frequency (375 Hz) showed evidence of the localized oxidation at fiber surfaces that might have attributed to the reduction in fatigue life at this frequency.

Mechanical properties of 3D fiber reinforced C/SiC composites

High toughness and reliable three dimensional textile carbon fiber reinforced silicon carbide composites were fabricated by chemical vapor infiltration. Mechanical properties of the composite materials were investigated under bending, shear, and impact loading. The density of the composites was 2.0–2.1 g cm −3 after the three dimensional carbon preform was infiltrated for 30 h. The values of flexural strength were 441 MPa at room temperature, 450 MPa at 1300°C, and 447 MPa at 1600°C. At elevated temperatures (1300 and 1600°C), the failure behavior of the composites became some brittle because of the strong interfacial bonding caused by the mis-match of thermal expansion coefficients between fiber and matrix. The shear strength was 30.5 MPa. The fracture toughness and work of fracture were as high as 20.3 MPa m 1/2 and 12.0 kJ·m −2, respectively. The composites exhibited excellent uniformity of strength and the Weibull modulus, m, was 23.3. The value of dynamic fracture toughness was 62 kJ·m −2 measured by Charpy impact tests.

Preparation and mechanical properties of unidirectional Hi-nicalon fiber reinforced silicon carbide composites

Preparation and mechanical properties of unidirectional Hi-nicalon fiber reinforced silicon carbide composites

High performance 3D textile Hi-Nicalon SiC/SiC composites by chemical vapor infiltration

Three dimensional textile Hi-Nicalon silicon carbide fiber reinforced silicon carbide composites with high toughness and reliability were fabricated by chemical vapor infiltration. The mechanical properties of the composite materials were investigated under bending, shear, and impact loading. The density of the composites was 2.5 g cm −3 after the three dimension silicon carbide perform has been infiltrated for 30 h. The values of flexural strength were 860 MPa at room temperature and 1010 MPa at 1300°C in vacuum. Above the infiltration temperature, the failure behavior of the composites became brittle because of the strong interfacial bonding and the mis-match of thermal expansion coefficients between fiber and matrix. The obtained value of shear strength was 67.5 MPa. The composites exhibited excellent impact resistance and the value of dynamic fracture toughness is 36.0 kJ m −2 was measured with Charpy impact tests.

Room- and High-Temperature Thermal Conductivity of Silicon Carbide Fiber-Reinforced Silicon Carbide Composites with Oxide Sintering Additives.

Green sheets of SiC containing Al2O3-Y2O3-CaO sintering additives prepared by the doctor-blade method and polycarbosilane-impregnated 2D woven Hi-Nicalon cloths with BN-coating were used for the fabrication of SiC fiber-reinforced SiC composite by hot-pressing. Thermal conductivity of the composite was measured between room temperature and 1000°C. It increased with increasing bulk density, and was 5-11W/m·K at room temperature. The thermal conductivity of the composites showed a maximum at approximately 500°C or slightly increased with test temperature up to 1000°C. The composite fabricated in this study showed relatively low thermal conductivity in spite of low porosity. The reasons for the low thermal conductivity could be attributed to the SiC matrix consisting of fine SiC grains, the phase transformation of 3C-SiC to 4H-SiC, and the low thermal conductivity grain-boundary phases, in addition to the large amount of SiC fibers with low thermal conductivity.

Microstructure and oxidation resistance of carbon/silicon carbide composites infiltrated with chromium silicide

In order to improve the mechanical properties and oxidation resistance of three-dimensional carbon fiber-reinforced silicon carbide composites prepared by Low-pressure chemical vapor infiltration (LCVI), electrodeposition chromium combined with silicon melt infiltration method (CSI) is developed for filling the pores of CVI C/SiC composites with chromium silicide. By this method, not only the large pores between bundles but also the small pores between fibers in a bundle can be filled. In the chemical vapor infiltration process, pyrolytic carbon interfacial layer and silicon carbide matrix layer are deposited on the surface of the carbon fiber. The open porosity of as-received CVI C/SiC is as high as 20%. Then the porous composite is infiltrated with chromium. Finally, the composite is infiltrated with silicon melt to react with chromium in the pores, thereby chromium silicide is in-situ formed and C/SiC–Cr 3Si composites with open porosity of 5% is obtained. Compared with CVI C/SiC composites, C/SiC–Cr 3Si composites exhibit better mechanical property and oxidation resistance. The room-temperature strength of the C/SiC–Cr 3Si is 586MPa, much higher than that of the CVI C/SiC. After the CVI C/SiC–Cr 3Si composite is exposed in the flame of burner rig for 20 h at 1300°C, the flexural strength of composite decreases only by 4.3%.

Oxidation behavior of C–SiC composites with a Si–W coating from room temperature to 1500°C

Oxidation tests of carbon fiber reinforced silicon carbide composites with a Si–W coating were conducted in dry air from room temperature to 1500°C for 5 h. A continuous series of empirical functions relating weight change to temperature after 5 h oxidation was found to fit the test results quite well over the whole temperature range. This approach was used to interpret the different oxidation mechanisms. There were two cracking temperatures of the matrix and the coating for the C–SiC composite. Oxidation behavior of the C–SiC composite was nearly the same as that of the coated C–C composite above the coating cracking temperature, but weight loss of the C–SiC composite was half an order lower than that of the coated C–C composite below the cracking temperature. As an inhibitor, the SiC matrix increased the oxidation resistance of C–SiC composites by decreasing active sites available for oxidation. As an interfacial layer, pyrolytic carbon decreased the activation energy below 700°C. From 800°C to 1030°C, uniform oxidation took place for the C–SiC composite, but non-uniform oxidation took place for the coated C–C composite in the same temperature range. The Knudsen diffusion coefficient could be used to explain the relationship between weight loss and temperature below the coating cracking temperature and the matrix cracking temperature.