Evolution of microstructure and tensile strength of Cansas-II SiC fibers under air oxidizing atmosphere

The influence of oxidation on the microstructure and tensile strength of Cansas-II SiC fibers at 900–1500 ℃ in the air was investigated in depth. The growth of β-SiC grains ordering as well as the increase of the size of free carbon in the SiC core occurred due to the thermal exposure. The thickness of the amorphous SiO₂ layer increases with the temperature, starting to transform to cristobalite at 1200 ℃. The activation energy in the ambient air is determined as 148KJ/mol, similar to that of Hi-Niaclon fibers (107∼151 KJ/mol). With the growth of the SiO₂ layer, lots of bubbles appeared in the SiO₂ layer due to the release of excess CO gas. Moreover, many cracks occurred on the fiber surface caused by the residual stress. The mean tensile strength decreased from initial 2.7 GPa to 0.3 GPa after the treatment at 1500 ℃, which could be mainly attributed to the SiO₂ layer.     

Microstructure and mechanical properties of SiC fibres after exposure at elevated temperature

To understand the service behaviour of SiC fibres, the effects of ambient environment and temperature on the microstructure, mechanical property and oxidation behaviour of these fibres were investigated. The result shows that, the surface of SiC fibres becomes rough after exposure in air from 973 to 1573?K due to the formation of small SiO₂ particles, and a smooth SiO₂ film will be formed on the SiC fibre at 1773?K. In Ar atmosphere, SiC fibres will change into clusters of large SiC crystals after heat treatment for 2?h at 2373?K. The tensile strength of SiC fibres decreased by 66 and 95% when the fibres were exposed at 1773?K for 5?min in air and 2373?K for 2?h in Ar, respectively. This degradation is associated with the evaporation of CO and SiO from the fibres as well as with SiC grain growth in the fibres.     

Ti3SiC2 interphase coating in SiCf/SiC composites: Effect of the coating fabrication atmosphere and temperature

The SiC fibers were coated with Ti₃SiC₂ interphase by dip-coating. The Ti₃SiC₂ coated fibers were heat-treated from 900 °C to 1100 °C in vacuum and argon atmospheres to comparatively analyze the effect of temperature and atmosphere on the microstructural evolution and mechanical strength of the fibers. The results show that the surface morphology of Ti₃SiC₂ coating is rough in vacuum and Ti₃SiC₂ is decomposed at 1100 °C. However, in argon atmosphere, the surface morphology is smooth and Ti₃SiC₂ is oxidized at 1000 °C and 1100 °C. At 1100 °C, Ti₃SiC₂ oxidized to form a thin layer of amorphous SiO₂ embedded with TiO₂ grains. Meanwhile, defects and pores appeared in the interphase scale. As a result, the fiber strength treated in the argon was lower than that treated in vacuum. The porous Ti₃SiC₂ interphase fabricated under vacuum was then employed to prepare the SiC_f/SiC mini composite by chemical vapor infiltration (CVI) combined with precursor infiltration pyrolysis (PIP), and can effectively improve the toughness of SiC_f/SiC mini composite. The propagating cracks can be deflected within the porous interphase layer, which promotes fiber pull-outs under the tensile strength.     

Fabrication of nanoporous silicon carbide fibres by thermal treatment

Abstract

Nanoporous silicon carbide fibres were prepared by curing and heat treatment of melt spun polycarbosilane (PCS) fibres. During the curing process, green PCS fibres were thermally oxidised at the temperature between 180 and 220°C and time between 2 and 10 h for cross-linking among the molecule chains in the PCS and controlling the oxygen concentration and distribution. After thermal oxidation, fibres were heat-treated between 1200 and 1600°C for the conversion to SiC phase. About 15–20 wt-% of oxygen was analysed after heat treatment at 1200°C and it can be possible to pyrolyse without melting or deformation of fibre. At a temperature above 1400°C, the uniform distribution of nanopores was observed on the fibre surface, and the size of pores was increased with curing and heating condition. This type of nanoporous SiC fibre is expected to be a good candidate for high temperature catalyst or catalytic supports.     

Effect of SiC coating and heat treatment on the thermal radiation properties of C/SiC composites

The effects of SiC coating and heat treatment on the emissivity were investigated for 2D C/SiC composites prepared by CVI in the 6–16 μm range. SiC coating had an obvious effect on the spectral emissivity of the composites but caused just 5% difference in the total emissivity. A radiation transport model was applied to explain those changes caused by SiC coating. Heat treatment affected the thermal radiation properties of the composites through the microstructure evolution. Base on the complementary analytical techniques, the changes in the emissivity were attributed to a good graphitization degree of carbon phases, large β-SiC grain sizes and high α-SiC content resulting in high emissivity.     

Improved thermal conductivity and mechanical properties of SiC/SiC composites using pitch-based carbon fibers

Pitch-based carbon fibers were assembled in horizontal and thickness directions of SiC/SiC composites to form three-dimensional heat conduction networks. The effects of heat conduction networks on microstructures, mechanics, and thermal conductivities were investigated. The results revealed the benefit of introducing heat conduction networks in the densification of composites. The maximum bending strength and interlaminar shear strength of the modified composites reached 568.67 MPa and 68.48 MPa, respectively. These values were equivalent to 18.6% and 69.4% increase compared to those of composites without channels. However, channels in thickness direction destroyed the continuity of fibers and matrix, creating numerous defects. As the volume fraction of heat conduction channels rose, the pinning strengthening effect of channels and influence of defects competed with each other to result in first enhanced mechanical properties followed by a decline. The in-plane thermal conductivity was found anisotropic with a maximum value reaching 86.20 W/(m·K) after introducing pitch-based carbon unidirectional tapes. The thermal conductivity in thickness direction increased with volume fraction of pitch-based carbon fibers and reached 19.13 W/(m·K) at 3.87 vol% pitch-based carbon fibers in the thickness direction. This value was 90.75% higher than that of composites without channels.     

Effect of interface types on the heat-resistance of Cansas-II SiCf /SiC composites

The heat-resistance of the Cansas-II SiC/CVI-SiC mini-composites with a PyC and BN interface was studied in detail. The interfacial shear strength of the SiC/PyC/SiC mini-composites decreased from 15 MPa to 3 MPa after the heat treatment at 1500 °C for 50 h, while that of the SiC/BN/SiC mini-composites decreased from 248 MPa to 1 MPa, which could be mainly attributed to the improvement of the crystallization degree of the interface and the decomposition of the matrix. Aside from the above reasons, the larger declined fraction of the interfacial shear strength of the SiC/BN/SiC mini-composites might also be related to the gaps in the BN interface induced by the volatilization of B₂O₃·SiO₂ phase, leading to a significant larger declined fraction of the tensile strength of the SiC/BN/SiC mini-composites due to the obvious expansion of the critical flaws on the fiber surface. Therefore, compared with the CVI BN interface, the CVI PyC interface has better heat-resistance at high temperatures up to 1500 °C due to the fewer impurities in PyC.     

Effects of fabrication processes on the properties of SiC/SiC composites

Precursor infiltration and pyrolysis (PIP) and chemical vapor infiltration (CVI) were used to fabricate SiC/SiC composites on a four-step 3D SiC fibre preform deposited with a pyrolytic carbon interface. The effects of fabrication processes on the microstructure and mechanical properties of the SiC/SiC composites were studied. Results showed the presence of irregular cracks in the matrix of the SiC/SiC composites prepared through PIP, and the crystal structure was amorphous. The room temperature flexural strength and modulus were 873.62 MPa and 98.16 GPa, respectively. The matrix of the SiC/SiC composites prepared through CVI was tightly bonded without cracks, the crystal structure had high crystallinity, and the room temperature bending strength and modulus were 790.79 MPa and 150.32 GPa, respectively. After heat treatment at 1300 °C for 50 h, the flexural strength and modulus retention rate of the SiC/SiC composites prepared through PIP were 50.01% and 61.87%, and those of the composites prepared through CVI were 99.24% and 96.18%, respectively. The mechanism of the evolution of the mechanical properties after heat treatment was examined, and the analysis revealed that it was caused by the different fabrication processes of the SiC matrix. After heat treatment, the SiC crystallites prepared through PIP greatly increased, and the SiOxCy in the matrix decomposed to produce volatile gases SiO and/or CO, ultimately leading to an increase in the number of cracks and porosity in the material and a decrease in the material load-bearing capacity. However, the size of the SiC crystallites prepared through CVI hardly changed, the SiC matrix was tightly bonded without cracks, and the load-bearing capacity only slightly changed.     

Effects of corrosion in liquid fluoride salt on the tensile strength of domestic SiC fibers

The relationship between the tensile strength of corroded domestic second-generation (2ed-gen) SiC fibers at various temperatures for 500 h in 46.5LiF-11.5NaF-42.0KF (mol. %) eutectic salt and the typical microstructure was studied. Weibull theory was used to analyze the critical defects that caused the tensile fracture, and the microstructure of fibers before and after corrosion was characterized. It is concluded that the decrease of tensile strength after corrosion at 800 °C is caused by the surface injury of fibers, which led to the shift of critical defects from the internal defects of virgin fibers to surface defects. Moreover, corrosion at higher temperature accelerates the corrosion process and dissolve the surface O-contained layer thoroughly. This shifts the critical defects back to the internal defects and will be helpful for the recovery of tensile strength of corroded fibers at the higher temperature.     

Strength degradation of SiC fibers with a porous ZrB2-SiC coating: Role of the coating porous structure

ZrB₂-SiC coatings with varied porous structures were deposited on SiC fiber tows using the sol-gel method and cured at 1400 ℃ in vacuum. Tensile strength of the coated SiC fibers were much lower than that of the uncoated fibers. The bimodal distribution in the Weibull plot of the coated SiC fibers demonstrated that the fracture of the coated fiber can be attributed to two types of defects: the porous structure of the coating and the fiber defects. Detailed morphology and microstructure characterization of the coating and fiber combined with strength calculation were carried out to investigate the individual contribution of the fiber defects and the porous coating layer respectively. The results revealed that apart from the fiber damage during the coating process the porous structure of the fiber coating has a non-negligible effect on the fiber strength, presumably due to a relatively strong bonding between the fiber and coating.     

Microstructure and mechanical properties of hot-pressed SiC nanofiber reinforced SiC composites

This study reports on the preparation and mechanical properties of a novel SiC_nf/SiC composite. The single crystal SiC nanofiber(SiC_nf) reinforced SiC ceramic matrix composites (CMC) were successfully fabricated by hot pressing the mixture of β-SiC powders, SiC_nf and Al–B–C powder. The effects of SiC_nf mass fraction as well as the hot-pressing temperature on the microstructure and mechanical properties of SiC_nf/SiC CMC were systematically investigated. The results demonstrated that the 15 wt% SiC_nf/SiC CMC obtained by hot pressing (HP) at 1850 °C with 30 MPa for 60 min possessed the maximum flexural strength and fracture toughness of 678.2 MPa and 8.33 MPa m^1/2, respectively. The nanofibers pull out, nanofibers bridging and cracks deflection were found by scanning electron microscopy, which are believed can strengthen and toughen the SiC_nf/SiC CMC via consuming plenty of the fracture energy. Besides, although the relative density of the prepared SiC_nf/SiC CMC further increased with the sintering temperature rose to 1900 °C, the further coarsend composites grains results in the deterioration of the mechanical properties for the obtained composites compared to 1850 °C.     

Fabrication and performance of mini SiC/SiC composites with an electrophoresis-deposited BN fiber/matrix interphase

The feasibility of fabricating a BN matrix/fiber interphase of SiC/SiC composites via electrophoresis deposition (EPD) was investigated based on the simplicity and non-destructiveness of the process and the excellent interfacial modification effects of BN. The BN suspension and SiC fiber surface properties were both adjusted to generate suitable conditions for the EPD process of the BN interphase. Next, the deposition dynamics and mechanism were studied under different deposition voltages and time, and the relationship between the deposition morphology of the BN interphase and mechanical properties of the fabricated mini SiC/SiC composites were also discussed. After oxidation at high temperature (600–1000 ℃), the mechanical properties of the mini SiC/SiC composites were studied to verify the oxidation resistance effect of the EPD-deposited BN interphase, whose oxidation resistance mechanism was briefly analyzed as well.     

Effect of intermediate heat treatment on mechanical properties of SiCf/SiC composites with BN interphase prepared by ICVI

SiC_f/SiC composites with BN interface were prepared through isothermal-isobaric chemical vapour infiltration process. Room temperature mechanical properties such as tensile, flexural, inter-laminar shear strength and fracture toughness (K_IC) were studied for the composites. The tensile strength of the SiC_f/SiC composites with stabilised BN interface was almost 3.5 times higher than that of SiC_f/SiC composites with un-stabilised BN interphase. The fracture toughness is similarly enhanced to 23 MPa m^1/2 by stabilisation treatment. Fibre push-through test results showed that the interfacial bond strength between fibre and matrix for the composite with un-stabilised BN interface was too strong (>48 MPa) and it has been modified to a weaker bond (10 MPa) due to intermediate heat treatment. In the case of composite in which BN interface was subjected to thermal treatment soon after the interface coating, the interfacial bond strength between fibre and matrix was relatively stronger (29 MPa) and facilitated limited fibre pull-out.     

The microstructure and electromagnetic wave absorption properties of near-stoichiometric SiC fibre

The microstructure and electromagnetic (EM) properties of near-stoichiometric SiC fibres (with C/Si ratio of 1.125) were analyzed and evaluated in detail. The SiC fibres consisted of β-SiC nanocrystallines and free carbon, and exhibited a uniquely specific skin-core structure with thin carbon layer of 5 nm on their surfaces. The relative complex permittivity increased with the increasing fibre volume fraction from 13 vol% to 27.5 vol%. The imaginary part of permittivity increased from 1.36 to 2.13 at 10 GHz, due to more SiC nanocrystallines and interfaces generating. The EM wave absorption properties were enhanced by the increasing fibre volume fraction and the effective absorption bandwidth was approximately 2.6 GHz when the fibre volume fraction was 27.5 vol%.     

Effect of pyrolysis temperature on the mechanical evolution of SiCf/SiC composites fabricated by PIP

Three types of SiC_f/SiC composites with a four-step three-dimensional SiC fibre preform and pyrocarbon interface fabricated via precursor infiltration and pyrolysis at 1100 °C, 1300 °C, and 1500 °C were heat-treated at 1300 °C under argon atmosphere for 50 h. The effects of the pyrolysis temperature on the microstructural and mechanical properties of the SiC_f/SiC composites were studied. With an increase in the pyrolysis temperature, the SiC crystallite size of the as-fabricated composites increased from 3.4 to 6.4 nm, and the flexural strength decreased from 742 ± 45 to 467 ± 38 MPa. After heat treatment, all the samples exhibited lower mechanical properties, accompanied by grain growth, mass loss, and the formation of open pores. The degree of mechanical degradation decreased with an increase in the pyrolysis temperature. The composites fabricated at 1500 °C exhibited the highest property retention rates with 90% flexural strength and 98% flexural modulus retained. The mechanism of the mechanical evolution after heat treatment was revealed, which suggested that the thermal stability of the mechanical properties is enhanced by the high crystallinity of the SiC matrix after pyrolysis at higher temperatures.     

Influence of tensile pre-fatigue stress on residual strength of SiC/(PyC/SiC)2/SiC minicomposites

The tensile-tension fatigue behavior of minicomposite SiC/(PyC/SiC)2/SiC at room temperature was studied, and the residual mechanical properties of specimens were tested after 10⁶ pre-fatigue cycles under different levels of stress. The results show that the residual strength first increases owing to the release of stress concentration and then decreases owing to excessive fiber wear. In addition, it is worth noting that the tensile curve of pre-fatigue specimens deflects twice; the first occurrence correlates with matrix crack reopening, and the second occurs when the uniaxial tensile load exceeds the pre-fatigue stress, and the degree of deflection also gradually decreases or almost disappears.     

Effects of channel modification on microstructure and mechanical properties of C/SiC composites prepared by LA-CVI process

Carbon fiber reinforced SiC matrix composites (C/SiC) with four different deposition channel sizes were fabricated via a novel laser-assisted chemical vapor infiltration (LA-CVI) method. Effects of infiltration channel sizes on microstructure and mechanical properties of C/SiC composites were investigated. The results showed that increasing the size of channels could expand infiltration passages and densification bands, which was consistent with theoretical calculations. Due to the presence of channels, the flexural strength of C/SiC composite increased by 14.47% when the channel diameter was 0.3?mm, compared to C/SiC composites prepared via conventional CVI process. Characteristics of matrix cracking and crack propagation on fracture surface were analyzed by using scanning electron microscopy. LA-CVI C/SiC composites displayed significantly improved damage-tolerant fracture behavior. Thus, findings of this work demonstrate that LA-CVI fabricated C/SiC composites are promising for a wide range of applications, particularly for enclosed-structure and thick-section C/SiC composites.     

An alternative composite approach to tailor the thermoelectric performance in SiAlON and SiC

The thermoelectric properties of SiAlON and SiC based composites prepared by adding TiCN (5 and 10 vol.%) as a particulate or as a segregated three-dimensional network were investigated. Although the volume fraction of the TiCN phase was the same in both kinds of SiAlON-based composites, the one containing the segregated three-dimensional network exhibited larger power factor and higher figure of merit than the particle-reinforced composite. Such an enhancement was not observed in SiC. This outcome highlights the importance of the microstructure design and indicates that the segregated network approach may be an effective method to tailor the thermoelectric properties of composite systems.     

XCT damage evaluation and analysis of SiC/SiC turbine guide vanes after thermal shocks

In this paper, the two-dimensional (2D) (0°/90°) plain-woven Amosic-3 SiC/SiC turbine guide vane (TGV) was fabricated using the chemical vapor infiltration method. Thermal and stress analysis of the TGV was conducted using the finite element method analysis. Multiple thermal shock tests at T = 1250, 1350, 1400, 1420, 1450, 1470, and 1480°C were conducted for N = 100, 100, 400, 300, 200, 200, and 700 cycles. After thermal shock tests, the surface damage of the TGV was observed visually, and the micro damage mechanism was analyzed using the scanning electronic microscopy. Micro X-ray computed tomography was adopted to characterize the internal damages in the SiC/SiC guide vanes. The delamination occurred at the positions approaching internal hollow, due to the weak binding force along the thickness direction and the high thermal shock stress caused by the temperature change. The diameter, area, volume, and sphericity distributions of the pores inside of the guide vanes were also obtained.     

Structural evolution and mechanical properties of Cansas-III SiC fibers after thermal treatment up to 1700 °C

The effects of thermal treatment on the Cansas-Ⅲ SiC fibers were investigated via heating at temperatures from 900 to 1700 ℃ for 1–5 h in argon atmosphere. The composition and morphology of the SiC fibers were characterized and the tensile strength of the SiC fiber bundles was analyzed via two-parameter Weibull distribution analysis. The results showed that the thermal treatment has negligible influence on the microstructure of the SiC fibers at temperatures ≤ 1100 ℃. At temperatures ≥ 1300 ℃, the surface of the fibers became rough with some visible particles. Particularly, at 1700 °C, numbers of holes appeared. With the increasing of heating temperature and holding time, the average tensile strength of the SiC fibers decreased gradually from 1.81 to 1.01 GPa. The decreasing of tensile strength can be attributed to the increase of critical defect sizes, grain growth and phase transformation (β→α) of SiC.