Size effect of carbon fiber-reinforced silicon carbide composites (C/C-SiC): Part 1 – bending load and statistical effects

This study analyzed the influence of the sample volume, number of tested specimen, and testing method on the flexural strength of fabric-reinforced ceramic matrix composites. For this purpose, seven different batches of C/C-SiC were prepared with four different sample thicknesses to determine the flexural strengths and Weibull moduli by three- and four-point flexural tests. The result showed that C/C-SiC exhibits a size effect of strength under bending load because a decrease of measured flexural strength with increased specimen size was observed. This size effect was discussed regarding the Weibull weakest link approach and the concept of quasi-brittle materials.The determined Weibull moduli were comparable for the same load condition but dissimilar for the identical material if the load condition were changed from three- to four-point bending. Hence, the Weibull modulus was found to be not an inherent material constant for C/C-SiC and the Weibull weakest link approach seems not appropriate.     

Damage development during flexural loading of a 5-directional braided C/C-SiC composite,characterized by X-ray tomography and digital volume correlation

In situ observations of damage development within 3-dimensional 5-directional braided carbon fiber reinforced carbon and silicon carbide (C/C-SiC) ceramic composites, fabricated by gaseous silicon infiltration (GSI) and precursor infiltration pyrolysis (PIP), have been obtained using laboratory X-ray computed tomography during in situ flexural tests. The GSI composite has a denser structure than that fabricated by PIP, but contains initial defects within the fiber bundles. The GSI composite ultimately failed due to fracture across the fiber bundles, while failure of the higher strength PIP composite propagated along the interface between the fiber bundle and matrix with a greater degree of fiber pullout. These differences arise from the higher process temperature and greater degree of matrix-fiber reaction of GSI compared to PIP. Digital volume correlation (DVC), applied to the tomographs, measured the 3-dimensional deformations and hence the specimen curvature. This demonstrated the significant reduction in elastic modulus caused by the development of internal cracking with tensile strain in both materials.     

Resonant frequency study of tensile and shear elasticity moduli of carbon fibre reinforced composites (CFRC)

The dynamic elastic properties are important characteristics of composite materials. They control the vibrational behaviour of composite structures and are also an ideal tool for monitoring of the development of CFRCs’ mechanical properties during their processing (heat treatment, densification). The present studies have been performed to explore relations between the dynamic tensile and shear moduli and some structural features (viz., fibre fraction, fibre type, porosity, weave pattern of woven reinforcement) of various unidirectional or bi-directional fibre reinforced carbon/carbon composites, made out of PAN- or pitch-based fibres as reinforcements and phenolic resin or coal tar pitch as matrix precursors. The dynamic tensile and in-plane shear moduli were determined from resonant frequencies of a beam with free ends. The longitudinal dynamic Young’s modulus of unidirectional CFRC composites – besides its dependence on the original fibre modulus and fibre volume contents – also reflects changes induced in matrix and fibres by heat treatment. The in-plane shear modulus does not depend on the fibre type but there exists its distinct tendency to increase with increasing fibre fraction. For bi-directionally reinforced composites, the longitudinal tensile modulus is more sensitive to the fabric weave pattern than to the fibre type. Tensile modulus of diagonally cut specimens and in-plane shear modulus of longitudinally cut ones are mutually correlated and, therefore, simultaneously controlled by densification steps and graphitisation heat treatment.     

Microstructure characterization and compressive performance of 3D needle-punched C/C–SiC composites fabricated by gaseous silicon infiltration

3D needle-punched C/C-SiC composites were fabricated from carbon fiber reinforced carbon (C/C) preforms, with densities of 1.05?g/cm³ and 1.28?g/cm³, by the gaseous silicon infiltration (GSI) method at fabrication temperatures from 1500?°C to 1800?°C. The compressive strengths and elastic moduli in transverse direction are larger than those measured under longitudinal compression except that samples fabricated from 1.28?g/cm³ density exhibit lower elastic moduli in transverse direction than in longitudinal direction. The compressive strength and modulus increase with fabrication temperature at 1500?°C and 1600?°C, and then decrease with higher fabrication temperature. Samples fabricated from the lower density C/C preforms have greater compressive strength and modulus. X-ray tomography was applied before and after the mechanical tests to characterize the microstructure and damage patterns, and the results indicated that for C/C-SiC composites fabricated at 1700?°C from 1.28?g/cm³ density C/C preform the matrix has a volume fraction (vol%) of 36.9%, and the initial intra-bundle cracks (0.6?vol%) display a space crossing structure while the inter-bundle pores (6.0?vol%) are special irregularly distributed.     

Manufacture and characterisation of a low cost carbon fibre reinforced C/SiC dual matrix composite

A porous two-dimensional C/C composite was produced via the polymer pyrolysis route using phenolic resin as the matrix precursor and polyacrilonitrile- (PAN-) or pitch-based carbon fibres as reinforcement. The resulting C/C composites were then densified using a modified polysilane followed by pyrolysis to convert the polymer into silicon carbide, sealing the pores in the C/C composite. Aiming to increase the ceramic yield of the infiltrated polysilane and to reduce its volumetric shrinkage during pyrolysis the polymer’s curing behaviour was modified by catalytic addition of 0.1% dicobaltoctacarbonyl [Co₂(CO)₈]. The densification procedure is very efficient in sealing cracks in the C/C composite with SiC. The obtained carbon fibre reinforced C/SiC dual matrix composites were subjected to flexural tests and dynamic mechanical analysis. The flexural and visco-elastic properties of the composite are dominated by the strength of the fibre/matrix interface rather than by the fibre strength or modulus. A correlation between the mechanical loss factor (tan δ) and the fracture behaviour of the composite is suggested.     

Fracture Strength and Dynamic Elastic Modulus of Ce- and Er-TZP at Low Temperature

Fracture strength of TZP containing 14.5, 15.5, and 16.5 mol % CeO₂ is studied using static three-point bending at low temperature. There is 22 %–46 % increase from 300 to 77 K and 47 %–92 % increase from 300 to 4.2 K in fracture strength of Ce-TZP. The temperature dependencies of dynamic elastic modulus and damping capacity of CeO₂/Er₂O₃-alloyed tetragonal zirconia ceramics (Ce-/Er-TZP) between 300 and 77 K are characterized using flexural vibration. System identification techniques in time domain are used to identify dynamic parameters from strain vibration signals of the ceramic samples. The results indicate that there is 15 %–22 % increase of Ce-TZP and 9%–22% increase of Er-TZP in dynamic elastic modulus from 300 to 77 K. The microhardness of Ce-/Er-TZP was measured at room temperature.     

Mechanical characterisation of mullite-based ceramic matrix composites at test temperatures up to 1200°C

Nextel 610 fibre-reinforced mullite-based matrix fabricated by Dornier Forschung was characterised at DLR Institute of Materials Research. The material was produced by the polymer route after coating the fibres with a 0.1 μm thick carbon layer. The composite was manufactured by infiltrating the fibres with a slurry containing a diluted polymer and mullite powder, curing in an autoclave and subsequently heat treating and pyrolysis of the polymer. A final heat treatment in air is performed to remove the carbon coating and to reduce the residual stresses. A (0/90/0/90/0/90)_s-laminate was produced with an average fibre volume fraction of 45.6% and a porosity of 15.9%. Dog-bone-type tensile specimens with a width of 10 mm were cut from the plate by water jet and tested at temperatures up to 1200°C in air. The tensile strength at room temperature measured 177.4 MPa and linearly decreased to 145.2 MPa at a temperature of 800°C. A stronger decrease occurred at 1000 and 1200°C. In contradiction to ceramic matrix composites manufactured by the CVI-route the stress–strain behaviour is nearly linear up to failure. The modulus of the composite (at room temperature 108.8 GPa) is analysed on the basis of the expected moduli of the fibres and the mullite matrix. It can be concluded that the contribution of the matrix to the modulus of the composite is low, caused by porosity and components other than mullite. The intralaminar shear strength at room temperature measured 36 MPa. This value reflecting shear transfer capability of fibre to matrix limits the amount of fibre pull-out.     

Effect of thermal shock cycling on the quasi-static and dynamic flexural properties of flax fabric-epoxy matrix laminates

The aim of the present work is to investigate the influence of thermal shock cycling on the quasi-static and dynamic flexural properties of epoxy matrix composites reinforced with natural flax fibers fabric. Polymer composite laminates reinforced with four plies of natural flax fiber fabric have been manufactured. The samples have been exposed to different number of thermal shock cycles (0, 50, 100, 200, 300, 400), at a temperature range from −40 °C to +28 °C. Dynamic mechanical analysis (DMA) tests were performed on both pristine and thermally shocked specimens in order to determine their viscoelastic response. Due to the thermal shock cycling and after 100 thermal shock cycles, a maximum decrease in storage and loss modulus on the order of 50% was observed. After 100 thermal shock cycles, no further degradation of dynamic properties was observed. On the contrary, damping factor and glass transition temperature values showed a minor variation as number of thermal shock cycles increased. In addition, the time–temperature superposition principle (TTSP) was successfully applied, confirming the fact that the flax fiber fabric-epoxy laminate is a thermo-rheologically simple material. Likewise, quasi-static three-point bending tests were executed and a maximum decrease of 20% in flexural strength was observed after 400 thermal shock cycles. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48529.     

Microstructure and properties of Cu-Ti alloy infiltrated chopped Cf reinforced ceramics composites

Novel friction composites (C/C-Cu₅Si-TiC) were prepared via reactive melt infiltration (RMI) of Cu-Ti alloy into porous C/C-SiC composites. The microstructure, physical properties and tribological behaviors of the novel material were studied. Results were compared to conventional C/C-SiC composites produced by liquid silicon infiltration(LSI). The resultant composite showed the microstructure composed of Cu₅Si matrix reinforced with TiC particles and intact C/C structures. Most importantly, the composite did not present traces of free Si. As a result, the C/C-Cu₅Si-TiC composite showed higher flexural strength, impact toughness and thermal diffusivity in comparison to C/C-SiC composites. Tribological properties were measured using 30CrSiMoVA as a counterpart. In general, the C/C-Cu₅Si-TiC composites showed lower coefficient of friction(COF), but higher wear resistance and frictional stability. The improved wear resistance of the C/C-Cu₅Si-TiC composites is credited to the formation of friction films from Cu₅Si matrix. Other deformation and wear mechanisms are also described considering the microstructural observations.     

Time-Dependent Strength Degradation of a Siliconized Silicon Carbide Determined by Dynamic Fatigue

Both fast-fracture strength and strength as a function of stressing rate at room temperature, 1100°, and 1400°C were measured for a siliconized SiC. The fast-fracture strength increased slightly from 386 MPa at room temperature to 424 MPa at 1100°C and then dropped to 308 MPa at 1400°C. The Weibull moduli at room temperature and 1100°C were 10.8 and 7.8, respectively, whereas, at 1400°C, the Weibull modulus was 2.8. The very low Weibull modulus at 1400°C was due to the existence of two exclusive flaw populations with very different characteristic strengths. The data were reanalyzed using two exclusive flaw populations. The ceramic showed no slow crack growth (SCG), as measured by dynamic fatigue at 1100°C, but, at 1400°C, an SCG parameter, n , of 15.5 was measured. Fractography showed SCG zones consisting of cracks grown out from silicon-rich areas. Time-to-failure predictions at given levels of failure probabilities were performed.     

C/C多孔体对C/C-SiC复合材料微观结构和弯曲性能的影响

以4种纤维含量相同(32%,体积分数,下同),用化学气相渗透(chemical vapor infiltration,CVI)法制备了4种密度的碳纤维增强碳(carbon fiber reinforced carbon,C/C)多孔体,基体炭含量约20%～50%.利用液相渗硅法(liquid silicon infiltration,LSI)制备了C/C-SiC复合材料,研究了C/C多孔体对所制备的C/C-SiC复合材料微观结构和弯曲性能的影响.结果表明:不同密度的C/C多孔体反应渗硅后,复合材料的物相组成均为SiC,C及单质Si;随着C/C多孔体中基体炭含量的增加,C/C-SiC复合材料中SiC含量逐渐减少而热解炭含量逐渐增加.C/C-SiC复合材料弯曲强度随着材料中残留热解炭含量增加而逐渐增加,热解炭含量为约42%的C/C多孔体所制备的C/C-SiC复合材料的弯曲强度最大,达到320 MPa.     

Nanocomposite matrix for increased fibre composite strength

A new type of three-phase thermoplastic composite has been made, consisting of a main reinforcing phase of woven glass or carbon fibres and a PA6 nanocomposite matrix. Nanocomposites have the potential to improve the matrix dominated flexural and compressive strength by increasing the matrix modulus. Good quality fibre composites have been made with several types of PA6 nanocomposite and unfilled PA6 in combination with glass and carbon fibre reinforcement. Flexural tests on commercial PA6 fibre composites have shown the decrease of the flexural strength upon increasing temperature and this has been compared with the decrease of the matrix modulus. The nanocomposites used in this research have moduli that are much higher than unfilled PA6, also above T_g and in moisture conditioned samples. The strength of glass fibre composites can be increased by more than 40% at elevated temperatures and the temperature range at which a certain minimum strength is present can be increased by 40-50 °C. Carbon fibre composites also show significant improvements at elevated temperatures, although not at room temperature. The advantage of the use of nanocomposites instead of other polymers to improve the fibre composite properties is that the properties can be improved without any change in the processing conditions.     

Mechanical behaviour of carbon fibre reinforced TaC/SiC and ZrC/SiC composites up to 2100°C

The microstructure and elevated temperature mechanical properties of continuous carbon fibre reinforced ZrC and TaC composites were investigated. Silicon carbide was added to both compositions to aid sintering during hot pressing. Fibres were homogeneously distributed and no fibre degradation was observed at the interface with the ceramic matrix even after testing at 2100 °C. The flexural strength increased from 260 to 300 MPa at room temperature to ∼450 MPa at 1500 °C, which was attributed to stress relaxation. At 1800 °C, the strength decreased to ∼410 MPa for both samples. At 2100 °C plastic deformation resulted in lower strength at the proportional limit (210–320 MPa), but relatively high ultimate strength (370–440 MPa). The sample containing ZrC had a lower ultimate strength, but higher failure strain at 2100 °C due to the weak fibre/matrix interface that resulted in fibre-dominated composite behaviour.     

Mechanical properties of ceramic matrix composites with siloxane matrix and liquid phase coated carbon fiber reinforcement

In order to evaluate the benefits of continuous liquid phase coating (CLPC) for carbon fibers, coated fibers as well as uncoated fibers were applied in the preparation of unidirectionally reinforced ceramic matrix composites (CMCs) with polysiloxane based matrix. Fibers coated with precursor based ceramic or carbon coatings were transferred into prepregs by continuous fiber impregnation with liquid polysiloxane and filament winding. The wet prepregs were cut to shape, laminated and then pressed and cured in the mold at 150 °C for 1 h. The cured polymeric matrix composites were calcined and densified by subsequent precursor infiltration/calcination cycles. The flexural strength of the CMCs was measured by 4-point bending tests, the microstructure was determined by optical and scanning electron microscopy. The application of CLPC coated fibers led to a significant improvement in composite strength and young's modulus compared to identical reference samples with uncoated carbon fibers.     

Flexural Properties of PAN- and Pitch-Based Carbon Fibers

The flexural properties of ultrahigh tensile strength polyacrylonitrile-based (T1000GB), ultrahigh modulus pitch-based (K13D), and high ductility pitch-based (XN-05) carbon fibers have been investigated using a three-point bending test at various span lengths ranging from 200 to 1500 μm. The flexural modulus and flexural strength of these carbon fibers were measured at room temperature. The fracture surfaces under bending were examined using a high-resolution scanning electron microscope to identify the origin of the failure. Statistical distributions of the flexural strength (maximum flexural stress at 200 μm span length) were then characterized. The Weibull modulus for the T1000GB, K13D, and XN-05 fibers were calculated to be 11.7, 11.8, and 11.7, which is higher than those obtained from the tensile test.     

Fabrication of C/C-SiC composites using high-char-yield resin

Carbon fiber reinforced ceramic matrix composites (C/C-SiC composites) were fabricated using a type of high-char-yield phenolic resin with the char yield of 81.17 wt.%. Firstly, the fabric prepreg was prepared by spreading the phenolic resin solution onto the two dimensional carbon fiber plain weave fabric and dried consequently. Afterward, the resin was cured and then the carbon fiber reinforced polymer (CFRP) was pyrolyzed to get amorphous carbon. Finally, C/C-SiC composites were obtained through liquid silicon infiltration (LSI) process. SEM micrographs showed that the Si/SiC area was homogeneously dispersed in the matrix, and during the siliconization process, a layer of SiC was formed along the surface of carbon fibers or carbon matrix. The fiber volume of CFRP was about 40 vol.%, which was much lower than other studies. XRD result indicated that only β-SiC type was formed. The result of X-ray computed tomography clearly showed the structure changes before and after the melt infiltration process. Mechanical property test showed that the composites had fracture strength of 186 ± 23 MPa, and a flexural modulus of 106 ± 8 GPa.     

Effect of thermo-mechanical and low-cycle preloading on the strength of carbon fiber-reinforced ceramic matrix composites

Fiber-reinforced ceramic matrix composites (CMCs) exhibit excellent thermo-mechanical properties including outstanding resistance against damage and fatigue. Some CMCs show occasionally even a strength enhancement after fatigue, often interpreted with relieve of internal stresses and interfacial degradation. This study reports the influence of low-cycle thermo-mechanical preloading on the bending and tensile strength of carbon fiber-reinforced silicon carbon (C/C-SiC). For this purpose two C/C-SiC materials with the same fiber architecture but different assumed internal stress states were subjected to single and cyclic mechanical preloads up to 90% of their ultimate strength level at room temperature and at 350 °C. Statistical evaluations of the experiments show that the ultimate strength values were surprisingly unchanged after preloading. The results are discussed regarding the thermal residual stresses (TRS).     

Experimental investigation of mechanical,thermal, and ablation performance of ZrO2/SiC modified C-Ph composites

In this study, an effort has been made to improve the mechanical, thermal, and ablation performance of carbon-phenolic (C-Ph) composites. The ZrO₂, SiC, and ZrO₂/SiC hybrid fillers were synthesized using sol-gel method followed by individual incorporation into C-Ph composites. The thermal stability and flexural strength of these C-Ph composites were analyzed using thermogravimetry analysis and three-point bending test, respectively. A significant improvement in the flexural strength and modulus of the reinforced C-Ph composites was observed and also exhibited the higher thermal stability. The oxyacetylene flame test was conducted to measure the ablation behavior of these filler reinforced C-Ph composites under a heat flux of 4.0 MW/m² for 60 seconds. ZrO₂/SiC0.5 reinforcement in the C-Ph composite decreased the linear and mass ablation rates by 46% and 22%, respectively when compared with pure C-Ph composite. The surface morphology analysis revealed that the burnt composite covered with the ZrC ceramic phase and SiO₂ bubble-like structure, which could have improved the ablation resistance of composites. These results were found well within the acceptable range when using the surface energy dispersive spectroscopy and X-ray diffraction analysis.     

High temperature bending properties and failure mechanism of 3D needled C/SiC composites up to 2000 ℃

Three-dimensional (3D) needled C/SiC composites were prepared and subjected to three-point bending tests from room temperature (RT) to 2000 ℃ under vacuum. The results show that the flexural strength and modulus increase in the range of RT to 800 °C due to the release of thermal residual stress (TRS). At 800–1700 °C, the modulus further increases for the further release of TRS, while the destruction of the pyrolytic carbon (PyC) coating reduces the flexural strength. Up to 2000 ℃, the thermal mismatch stress in the composites cause fiber slippage and matrix crack deflection to be zigzag, which increase the fracture strength. The change of components properties mediated by high temperature and the release of TRS play a leading role in the flexural strength and fracture mode. The results provide important support for the mechanical behavior of 3D needled C/SiC composites at ultra-high temperature.     

Effective elastic moduli for thin-walled structural foam beams

Because of the nonhomogeneous morphology of rigid structural foams, the elastic moduli determined from tension and bend tests are different, the latter being larger. These moduli also depend on the geometry of the specimen. In general, the elastic bending stiffness of foams is determined by the rigidity tensor, which combines geometry and material information. Although the bending problem for nonhomogeneous materials is more complex than the equivalent homogeneous problem, the analysis simplifies considerably for thin-walled beams. The effective flexural modulus for a thin-walled foam beam is shown to be the tension modulus that would be measured on a flat foam specimen of the same thickness. The flexural modulus measured by bend tests on flat bars is shown to have very little effect on the stiffness of most thin-walled sections. This conclusion is independent of how the “true” material modulus varies across the thickness of the foam part.