High‐temperature tensile strength and fracture behavior of ZrB2‐SiC‐graphite composite

The tensile behavior of ZrB₂‐SiC‐graphite composite was investigated from room temperature to 1800°C. Results showed that tensile strength was 134.18 MPa at room temperature, decreasing to 50.34 MPa at 1800°C. A brittle‐ductile transition temperature (1300°C) of ZrB₂‐SiC‐graphite composite was deduced from experimental results. Furthermore, the effect of temperature on the fracture behavior of ZrB₂‐SiC‐graphite composite was further discussed by microstructure observations, which showed that tensile strength was controlled by the relaxation of thermal residual stress below 1300°C, and was affected by the plastic flow during 1300°C and 1400°C. At higher temperature, the tensile strength was dominated by the changes of microstructures.     

Microstructure and EMW absorption properties of CVI Si3N4–SiCN ceramics with BN interface annealed in N2 atmosphere

A kind of chemical vapor infiltration (CVI) Si₃N₄–BN–SiCN composite ceramic with excellent electromagnetic wave (EMW) absorbing properties is obtained by CVI BN interface and SiCN matrix on porous Si₃N₄ ceramics, and then annealed at high temperatures (1200°C‐1500°C) in N₂ atmosphere. The crystallization behavior, EMW absorbing mechanism and mechanical properties of the composite ceramics have been investigated. Results showed CVI SiCN ceramics with BN interface were crystallized in the form of nanograins, and the crystallization temperature was lower. Moreover, both EMW absorbing properties and mechanical properties of CVI Si₃N₄–BN–SiCN composite ceramics firstly increased and then decreased with the increase in annealing temperature due to the influence of BN interface on the microstructure and phase composition of the composite ceramics. The minimum reflection coefficient (RC) and maximum effective absorption bandwidth (EAB) of the composite ceramics annealed at 1300°C were ?47.05 dB at the thickness of 4.05 mm and 3.70 GHz at the thickness of 3.65 mm, respectively. The flexural strength and fracture toughness of the composite ceramics annealed at 1300°C were 94 MPa and 1.78 MPa/m^1/2, respectively.     

Thermal shock response behaviour of carbon fibre reinforced silicon carbide matrix composites prepared by chemical vapour infiltration at different heating rates

The present work investigated the effects of thermal cycles in air on the tensile properties of a two-dimensional carbon fibre reinforced silicon carbide composite (2D C/SiC) prepared by chemical vapour infiltration at different heating rates. The composite was exposed to different cycles of thermal shock between 20 °C and 1300 °C in air. The damage mechanisms were investigated by AE online monitoring and fractured morphology offline analysis. The tensile strength of 2D-C/SiC decreases with increasing thermal cycles. However, the modulus only decrease within 40 cycles. Due to oxidation, with the decrease in heating rate, the residual properties of the material decrease more obviously. Meanwhile, the results of AE online monitoring and fracture analysis show that the matrix damage is more serious at higher heating rate and that more delamination occours in tensile fractures. The above results indicate that for the thermal shock of 2D C/SiC composites in air, oxidative damage plays a key role in the residual properties.     

Optimization of SiC–ZrC high emissivity composite flexible coating for thermal protection with high interfacial bond strength and temperature resistance

Aluminum silicate fiber fabric (ASFF) has been widely used in the outer surface of flexible insulation felt on the leeward side of aerospace vehicle. In order to improve the temperature resistance of ASFF, a kind of SiC–ZrC composite coating was prepared on the surface of fiber fabric via spraying method with SiC as emittance agent and ZrC as additive. The surface morphology and mechanical properties of the coating were studied. Compared with the single-component SiC coating, the composite coating could effectively avoid coating spalling and improve the surface integrity at high temperature. After thermal treatment at 1100 °C for 2 h, the interface bond strength of the composite coating/substrate was 52.41% higher than that of SiC coating/substrate. The tensile strength of fiber fabric with SiC–ZrC composite coating could reach 91.75 MPa, which was 101.76% higher than that of raw ASFF. Therefore, the SiC–ZrC coating could greatly improve the temperature resistance of ASFF, and has an attractive application prospect in the field of thermal protection system.     

Fabrication of nearly stoichiometric polycrystalline SiC fibers with excellent high‐temperature stability up to 1900°C

Due to the extensive applications of SiC fiber‐reinforced composite materials in the fields of aviation, aerospace, and nuclear power, there are increasing demands for SiC fibers with both excellent mechanical performance and high‐temperature stability. In this work, nearly stoichiometric polycrystalline SiC fibers were fabricated using amorphous Si–C–Al–O fibers with excess carbon and oxygen (C/Si = 1.34, O content: 7.74 wt%). The nearly stoichiometric composition (C/Si = 1.05) of the product fibers was achieved by thermal decomposition of the starting fibers. The fibers were well‐crystallized with grain sizes of ~200 nm due to sintering at a high temperature of 1900°C. The fibers exhibited a high tensile strength and a high elastic modulus and were composed of SiC grains with twins and stacking‐faults, exhibiting intragranular fracture behavior. Furthermore, the fibers maintained their original tensile strength after being maintained at 1800°C for 5 hour or at 1900°C for 1 hour under an inert atmosphere, and they exhibited a high strength retention (97%) after exposure at 1300°C for 1 hour under air. The high‐temperature stability and creep resistance of the fibers were comparable to that of commercial Hi‐Nicalon S and Tyranno SA fibers.     

Effects of heat treatment on the mechanical properties at elevated temperatures of plain-woven SiC/SiC composites

The effects of heat treatment on the mechanical properties of plain-woven SiC/SiC composites at 927 °C and 1200 °C in argon were evaluated through tensile tests at room temperature and at elevated temperature on the as-received and heat-treated plain-woven SiC/SiC composites, respectively. Heat treatment can improve the mechanical properties of composites at room temperature due to the release of thermal residual stress. Although heat treatment can damage the fiber, the effect of this damage on the mechanical properties of composites is generally less than the effect of thermal residual stress. Heat treatment will graphitize the pyrolytic carbon interface and reduce its shear strength. Testing temperature will affect the expansion or contraction of the components in the composites, thereby changing the stress state of the components. This study can provide guidance for the optimization of processing of ceramic matrix composites and the structural design in high-temperature environments.     

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.     

Effect of the talc filler content on the mechanical properties of polypropylene composites

This research examines the effect of a microsize/nanosize talc filler on the physicochemical and mechanical properties of filled polypropylene (108MF10 and 33MBTU from Saudi Basic Industries Corp. and HE125MO grade from Borealis) composite matrices. A range of mechanical properties were measured [tensile properties, bending properties, fracture toughness, notched impact strength (at the ambient temperature and ?20°C), strain at break, and impact strength] along with microhardness testing and thermal stability testing from 40 to 600°C as measured by differential thermal analysis and thermogravimetric analysis. Increasing filler content lead to an increase in the mechanical strength of the composite material with a simultaneous decrease in the fracture toughness. The observed increase in tensile strength ranged from 15 to 25% (the maximum tensile strength at break was found to be 22 MPa). The increase in mechanical strength simultaneously led to a higher brittleness, which was reflected in a decrease in the mean impact strength from the initial 18 kJ/m² (for the virgin polypropylene sample) to 14 kJ/m², that is, a 23% decrease. A similar dependency was also obtained for the samples conditioned at ?20°C (a decrease of 12.5%). With increasing degree of filling of the talc–polypropylene composite matrix, the thermooxidative stability increased; the highest magnitude was obtained for the 20 wt % sample (decomposition temperature = 482°C, cf. 392°C for the virgin polymer). © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Environment-friendly deposition of SiCN interlayer for reinforcing carbon fibers in C/SiC composites

Several C/SiC composites with no interlayer, single pyrocarbon (PyC) interlayer and PyC/SiCN interlayer were fabricated by polymer infiltration and pyrolysis process. The microstructure and mechanical properties were investigated. The results verified that SiCN interlayer was formed on carbon fibers. Both bulk density and flexural stress of C/SiC composite with PyC/SiCN interlayer were slightly higher than composite fabricated with single PyC interlayer. When the weight fraction of SiCN interlayer in the composite was about 18 wt%, the flexural stress of the composite was enhanced to 416 MPa from 352 MPa for composite with single PyC interlayer. The observations of pulled-out fibers on fracture surfaces revealed non-catastrophic fracture features for PyC/SiCN deposited C/SiC composite.     

Tensile properties of two-dimensional carbon fiber reinforced silicon carbide composites at temperatures up to 2300 °C

Carbon fiber reinforced silicon carbide (C/SiC) composites are of the few most promising materials for ultra-high-temperature structural applications. However, the existing studies are mainly conducted at room and moderate temperatures. In this work, the tensile properties of a two-dimensional plain-weave C/SiC composite are studied up to 2300 °C in inert atmosphere for the first time. The study shows that C/SiC composite firstly shows linear deformation behavior and then strong nonlinear characteristics at room temperature. The nonlinear deformation behavior rapidly reduces with temperature. The Young’s modulus increases up to 1000 °C and then decreases as temperature increases. The tensile strength increases up to 1000 °C firstly, followed by reduction to 1400 °C, then increases again to 1800 °C, and lastly decreases with increasing temperature. The failure mechanisms being responsible for the mechanical behavior are gained through macro and micro analysis. The results are useful for the applications of C/SiC composites in the thermal structure engineering.     

Mechanical,thermal, and dielectric properties of SiCf/SiC composites reinforced with electrospun SiC fibers by PIP

Electrospun unidirectional SiC fibers reinforced SiC_f/SiC composites (e-SiC_f/SiC) were prepared with ∼10% volume fraction by polymer infiltration and pyrolysis (PIP) process. Pyrolysis temperature was varied to investigate the changes in microstructures, mechanical, thermal, and dielectric properties of e-SiC_f/SiC composites. The composites prepared at 1100 °C exhibit the highest flexural strength of 286.0 ± 33.9 MPa, then reduced at 1300 °C, mainly due to the degradation of electrospun SiC fibers, increased porosity, and reaction-controlled interfacial bonding. The thermal conductivity of e-SiC_f/SiC prepared at 1300 °C reached 2.663 W/(m∙K). The dielectric properties of e-SiC_f/SiC composites were also investigated and the complex permittivities increase with raising pyrolysis temperature. The e-SiC_f/SiC composites prepared at 1300 °C exhibited EMI shielding effectiveness exceeding 24 dB over the whole X band. The electrospun SiC fibers reinforced SiC_f/SiC composites can serve as a potential material for structural components and EMI shielding applications in the future.     

Effect of off-axis angle on mesoscale deformation and failure behavior of plain-woven C/SiC composites with digital image correlation

In this study, the deformation response and failure behavior of a plain-woven C/SiC composite were investigated under on-axis and off-axis tensile loading. Digital image correlation (DIC) was utilized to characterize the full-field deformation and mesoscale strain distribution. The test results indicate a strong influence of the woven architecture on the mechanical properties and strain distribution, and the materials exhibit failure modes dependent on the loading directions or off-axis angles: the fracture positions of different layers are the same under off-axial load, while for on-axil loading, the fracture positions of different layers do not affect each other. SEM results provide direct evidence that the width of the off-axis specimen has a great influence on the mechanical properties. The reduction of the modulus and strength of off-axis specimen, is not only due to the off-axis loading, but also due to the reduction of effective bearing area or effective bearing fiber.     

Improvement of the mechanical properties of SiC reticulated porous ceramics with optimized three-layered struts for porous media combustion

Silicon carbide reticulated porous ceramics (SiC RPCs) with three-layered struts were fabricated by polymer replica method, followed by infiltrating alumina slurries containing silicon (slurry-Si) and andalusite (slurry-An), respectively. The effects of composition of infiltration slurries on the strut structure, mechanical properties and thermal shock resistance of SiC RPCs were investigated. The results showed that the SiC RPCs infiltrated with slurry-Si and slurry-An exhibited better mechanical properties and thermal shock resistance in comparison with those of alumina slurry infiltration, even obtained the considerable strength at 1300 °C. In slurry-Si, silicon was oxidized into SiO₂ in the temperature range from 1300 °C to 1400 °C and it reacted with Al₂O₃ into mullite phase at 1450 °C. Meantime, the addition of silicon in slurry-Si could reduce SiC oxidation of SiC RPCs during firing process in contrast with alumina slurry. With regard to slurry-An, andalusite started to transform into mullite phase at 1300 °C and the secondary mullitization occurred at 1450 °C. The enhanced mechanical properties and thermal shock resistance of SiC RPCs infiltrated alumina slurries containing silicon and andalusite were attributed to the optimized microstructure and the triangular zone (inner layer of strut) with mullite bonded corundum via reaction sintering. In addition, the generation of residual compressive stress together with better interlocked needle-like mullite led to the crack-deflection in SiC skeleton, thus improving the thermal shock resistance of obtained SiC RPCs.     

Improvement of high-temperature stability of PIP SiCf/SiC material through in situ grown BNNTs

In this paper, the effect of in situ grown boron nitride nanotubes (BNNTs) and preparation temperature on mechanical behavior of PIP (Precursor Infiltration and Pyrolysis) SiC_f/SiC minicomposites under monotonic and compliance tensile is investigated. In situ BNNTs are grown on the surface of SiC fibers using ball milling–annealing process. Composite elastic modulus, tensile strength, fracture strain, tangent modulus, and loading/unloading inverse tangent modulus (ITM) are obtained and adopted to characterize the mechanical properties of the composites. Microstructures of in situ grown BNNTs and tensile fracture surfaces are observed under scanning electronic microscopic (SEM). For SiC_f/SiC minicomposites with BNNTs, the elastic modulus, tensile strength, and fracture strain are all lower than those of SiC_f/SiC minicomposites without BNNTs, mainly due to high preparation temperature and the oxidation of the PyC interphase during the annealing process. Tensile stress–strain curves of SiC_f/SiC minicomposites with and without BNNTs are predicted using the developed micromechanical constitutive model. The predicted results agreed with experimental data. This work will provide guidance for predicting the service life of SiC_f/SiC composite materials and may enable these materials to become a backbone for thermal structure systems in aerospace applications.     

Influence of pyrolysis and melt infiltration temperatures on the mechanical properties of SiCf/SiC composites

In order to improve the degree of matrix densification of SiC_f/SiC composites based on liquid silicon infiltration (LSI) process, the microstructure and mechanical properties of composites according to various pyrolysis temperatures and melt infiltration temperatures were investigated.Comparing the microstructures of SiC_f/C carbon preform by a one-step pyrolysis process at 600 °C and two-step pyrolysis process at 600 and 1600 °C, the width of the crack and microcrack formation between the fibers and matrix in the fiber bundle increased during the two-step pyrolysis process. For each pyrolysis process, the density, porosity, and flexural strength of the SiC_f/SiC composites manufactured by the LSI process at 1450–1550 °C were measured to evaluate the degree of matrix densification and mechanical properties. As a result, the SiC_f/SiC composite that was fabricated by the two-step pyrolysis process and LSI process showed an 18% increase in density, 16%p decrease in porosity, and 150% increase in flexural strength on average compared to the composite fabricated by the one-step pyrolysis process.In addition, among the SiC_f/SiC specimens fabricated by the LSI process after the same two-step pyrolysis process, the specimen that underwent the LSI process at 1500 °C showed 30% higher flexural strength on average than those at 1450 or 1550 °C. Furthermore, under the same pyrolysis temperature, the mechanical strength of SiC_f/SiC specimens in which the LSI process was performed at 1500 °C was higher than that of the 1550 °C although both porosity and density were almost similar. This is because the mechanical properties of the Tyranno-S grade SiC fibers degraded rapidly with increasing LSI process temperature.     

Fabrication and mechanical behavior of ZrB2 strengthened SiCf/SiC composites prepared by hybrid processing route

A SiC fiber-reinforced composite containing a SiC-ZrB₂ mixed matrix (SiC_f/(SiC-ZrB₂)) with high density and enhanced mechanical properties was fabricated. ZrB₂ at 5 or 40?vol% was added to a (SiC + C) slurry to be infiltrated into the voids of 2D woven Tyranno?-SA grade-3 fabrics by electrophoretic deposition. Subsequent hot pressing at 1300?°C and 10?MPa for 1?h, followed by liquid silicon infiltration (LSI) at 1600?°C for 5?h in an Ar atmosphere resulted in the formation of the reaction-bonded SiC matrix, which revealed a composite density close to 97%. SiC_f/(SiC-ZrB₂) having open porosities of 0.2–0.6% showed peak strengths of 398 and 320?MPa for 5 and 40?vol% ZrB₂ addition, respectively. The large mismatch in the coefficient of thermal expansion and Young's modulus between the SiC and ZrB₂ phases was attributed to a reverse trend in the strength of composites. Brittle behavior of the composites in flexure can be explained by the strong bonding between the matrix and fibers formed by the reaction of interphase with molten Si during LSI. Strength retention after oxidation at 1000 and 1400?°C for 2?h was also compared in terms of ZrB₂ amount contained in the composites.     

Mechanical properties and microstructure evolution of KD-SA SiCf/BN/SiC CMCs oxidized at different temperatures

Silicon carbide fiber-reinforced SiC ceramic matrix composites (SiC_f/SiC CMCs) based on a domestic KD-SA SiC fiber were exposed to a wet oxygen atmosphere for 135 h at 800, 1100, and 1300°C. The evolution of the microstructure and mechanical properties of SiC_f/SiC CMCs have been systematically investigated following oxidation. For weight change, CMC-1300 showed the greatest gain (0.394%), followed by CMC-1100 (0.356%) and CMC-800 (0.149%). The volatilization of boron oxide (B₂O₃) combined with the slight oxidation of the SiC matrix at 800°C caused crack deflection and fiber pull-out. The complete dissipation of the interphase could be found when the oxidation temperature increases to 1100°C, generated a fracture surface with brittle fracture characteristics. At 1300°C, crystalline SiO₂ hindered oxygen diffusion, with evidence of fiber pull-out. Based on thermodynamic calculations and microscopic observations, we propose a mechanism to explain the thermal degradation of SiC_f/SiC CMCs. This work offers valuable guidance for the fabrication of SiC_f/SiC CMCs that are suitable for high-temperature applications.     

Effects of Fe addition on the mechanical and thermo-mechanical properties of SiC/FeSi2/Si composites produced via reactive infiltration

Remaining silicon in SiC-based materials produced via reactive infiltration limits their use in high-temperature applications due to the poor mechanical properties of silicon: low fracture toughness, extreme fragility and creep phenomena above 1000 °C. In this paper SiC–FeSi₂ composites are fabricated by reactive infiltration of Si–Fe alloys into porous Cf/C preforms. The resulting materials are SiC/FeSi2 composites, in which remaining silicon is reduced by formation of FeSi₂. For the richest Fe alloys (35 wt% Fe) a nominal residual silicon content below 1% has been observed. However this, the relatively poor mechanical properties (bending strength) measured for those resulting materials can be explained by the thermal mismatch of FeSi2 and SiC, which weakens the interface and does even generate new porosity, associated with a debonding phenomenon between the two phases.     

Thermal stability of CVI and MI SiC/SiC composites with Hi-Nicalon™-S fibers

The influence of high-temperature argon heat-treatment on the microstructure and room- temperature in-plane tensile properties of 2D woven CVI and 2D unidirectional MI SiC/SiC composites with Hi-Nicalon?-S SiC fibers was investigated. The 2D woven CVI SiC/SiC composites were heat-treated between 1200 and 1600 °C for 1- and 100-hr, and the 2D unidirectional MI SiC/SiC composites between 1315 and 1400 °C for up to 2000 hr. In addition, the influence of temperature on fast fracture tensile strengths of these composites was also measured in air. Both composites exhibited different degradation behaviors. In 2D woven CVI SiC/SiC composites, the CVI BN interface coating reacted with Hi-Nicalon?-S SiC fibers causing a loss in fast fracture ultimate tensile strengths between 1200 and 1600 °C as well as after 100-hr isothermal heat treatment at temperatures > 1100 °C. In contrast, 2D unidirectional MI SiC/SiC composites showed no significant loss in in-plane tensile properties after the fast fracture tensile tests at 1315 °C. However, after isothermal exposure conditions from 1315° to 1400°C, the in-plane proportional limit stress decreased, and the ultimate tensile fracture strain increased with an increase in exposure time. The mechanisms of strength degradation in both composites are discussed.     

The effects of preparation temperature on the SiCf/SiC 3D4d woven composite

Continuous silicon carbide fiber reinforced silicon carbide matrix (SiC_f/SiC) composites have promising applications in aero-engine due to their unique advantages, such as low density, high modulus and strength, outstanding high temperature resistance and oxidation resistance. As SiC fibers are main reinforcements in SiC_f/SiC composites, the crystallization rate and initial damage degree of SiC fibers are seriously influenced by preparation temperatures of SiC_f/SiC composites, namely mechanical properties of SiC fibers and SiC_f/SiC composites are influenced by preparation temperatures. In this paper, KD-II SiC fibers were woven into 3D4d preforms and SiC matrix was fabricated by PIP process at 1100 °C, 1200 °C, 1400 °C and 1600 °C. Digital image correlation (DIC) method was adopted to measure the uniaxial tensile properties of these SiC_f/SiC composites. In addition, finite element method (FEM) based on representative volume element (RVE) was adopted to predict the mechanical properties of SiC_f/SiC composites. The good agreements between numerical results and experimental results of uniaxial tensile tests verified the validity of the RVE. In last, the transverse tensile, transverse shear, uniaxial shear properties were predicted by this method. The predicted results illustrated that axial tensile, transverse tensile and axial shear properties were greatly influenced by the preparation temperatures of SiC_f/SiC composites while transverse shear properties were not significantly various. And the mechanical properties of SiC_f/SiC composites peaked at 1200 °C among these four temperatures while their values reached their lowest points at 1600 °C because of thermal damage and brittle failure of SiC_f/SiC composites.