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.     

Evaluation of elastic constants and high temperature tensile behaviour with damage assessment of the SiC seal-coated 2.5D Cf/SiC composites having multilayered interphase and Si-B-C added matrix

Elastic constants and tensile behaviour of chemical vapour infiltration processed 2.5D C_f/SiC composites possessing multilayered (PyC/SiC)_n=4 interphase, Si-B-C containing matrix and SiC seal-coating have been evaluated with microstructural examination and damage assessment. The strength obtained as ～187 ± 2 MPa in tensile tests at 27 °C is increased by ～18% and ～22% at 1000 °C and 1250 °C, respectively due to reduced thermal stress and increased strength of load-sharing C-fibres, which are protected from oxidation till failure by a self-healing borosilicate layer. The damage evolving during tension tests has been quantified by relating it to decrease of stress-strain slope with strain. Higher (6–8 times) elastic constants measured along fibre-axes than that obtained transversely, indicate significant anisotropy. Owing to matrix cracking with fibre-debonding and pull-out, the fibre-oriented elastic constants of tensile-fractured samples are significantly lower than those of as-received composites, and the difference scales with temperature, whereas negligible change is observed perpendicular to the fibre axes.     

Tensile behavior of ceramic matrix minicomposites with engineered interphases fabricated by chemical vapor infiltration

Ceramic matrix composites (CMCs) exhibit quasi-ductile behavior beyond the initial elastic region driven by a weak fiber-matrix interface that can be further engineered by introducing a finite thickness interphaseleading to enhanced strength and toughness. The current work explores the engineering of interphases in CMCs by a controlled variation of fabrication process parameters. C/BN/SiC minicomposite configurations have been fabricated by chemical vapor infiltration (CVI) with the intent of varying interphase thickness and constituent volume fractions by varying the interphase and matrix infiltration durations. The effect of processing durations on the resulting microstructure, tensile response, and damage mechanisms up to and during ultimate failure of CMC minicomposites have been investigated. The presented results highlight the significant influence of processing duration on the tensile and failure behavior of CMC minicomposites thereby providing an insight into the processing-microstructure-tensile response relationship in CMCs.     

Effect of diffusion-assisting holes on the tensile properties of a thick-section two dimensional C/SiC composite fabricated by chemical vapor infiltration

The concept of diffusion-assisting holes (DAHs) has been developed to increase matrix deposition in the middle layers of the thick-section ceramic matrix composites (CMCs) that are fabricated by chemical vapor infiltration (CVI). However, the effect of DAHs on the tensile properties of CMCs has not been studied. Here, the tensile properties and the state of matrix deposition of a 10-mm-thick two dimensional (2D) C/SiC with DAHs are investigated. Results showed, with DAHs, a zone of increased deposition with a radius of ca. 1.4 mm around a hole was introduced and the net-section strength of the 10-mm-thick 2D C/SiC was increased by 48.1%. In addition, the tensile load bearing capacity was also increased by 34.1%, although the load bearing section decreased with DAHs. The increased net-section strength and tensile load bearing capacity of the C/SiC are attributed to the increased matrix deposition in the middle layers of the thick-section composite.     

The design of zirconium and hafnium germanate interphase in SiCf/SiC composites

Unidirectional SiC_f/SiC minicomposites with SiC matrix derived by polymer-impregnation pyrolysis (PIP), reinforced with SiC fibers coated with zirconium or hafnium germanate were fabricated. Microdebonding indentation tests for SiC_f/SiC composites with one- and multilayered germanate interphase were performed. Interfacial shear stress depending on the number of germanate interfacial layers and morphology was determined. The microstructure of the minicomposites and indented fracture surfaces were studied by scanning electron microscopy (SEM). It was stated that an increase in the number of interfacial coatings leads to a decrease in the interfacial frictional stress in SiC_f/SiC minicomposites with germanate interphases. The key factor of interphase weakening is the formation of a weak interlayer bonding within the interphase but not germanate layered crystal structure itself. Thus, bonding at the fiber/matrix boundary could be regulated by the number of layers of ZrGeO₄ or HfGeO₄ in the interphase zone.     

Porous SiCnw/SiC ceramics with unidirectionally aligned channels produced by freeze-drying and chemical vapor infiltration

The current study introduces a methodology for the fabrication of porous silicon carbide nanowire/silicon carbide (SiC_nw/SiC) ceramics with macroscopic unidirectionally aligned channels and reports on their microstructural and mechanical properties. The material was produced by freezing of a water-based slurry of β-SiC nanowires (SiC_nw) with control of the ice growth direction. Pores were subsequently generated by sublimation of the columnar ice during freeze-drying. Chemical vapor infiltration (CVI) of SiC into the open pore network of the SiC_nw aerogel with unidirectionally aligned channels, resulted in the formation of highly porous SiC_nw/SiC ceramics which exhibited a unique microstructure as identified by scanning electron microscopy. The pore size distribution and the mechanical properties of the as-fabricated porous ceramics were examined by mercury intrusion porosimetry and three-point bending and compression tests, respectively, while phase composition was investigated through X-ray diffraction.     

Theoretical prediction on structure evolution and optimal properties of silicon modified hexagonal boron nitride as interphase in SiCf/SiC composite

To predict the effects of Si doping on hexagonal boron nitride (h-BN) and to achieve a balance between mechanical and oxidation properties for the interphase modification in SiC_f/SiC composites, we herein calculate and analyze the crystal structures and mechanical properties of (BN)₆₄Si_x (x = 4, 8, 16, 32) models by means of density functional theory (DFT) calculations and ab initio molecular dynamics (aiMD) simulations. The possible trends of crack deflection and self-healing ability are discussed. The modeling shows an obvious transition of (BN)₆₄Si_x from the layered crystal structure and anisotropic mechanical property to amorphous structure and isotropic mechanical property as the Si doping content up to 36.1 wt%. Regarding to the application of interphase in SiC_f/SiC composites, (BN)₆₄Si₁₆ model structure possess the highest debonding potential according to Cook and Gordon’s criteria and illustrates the higher self-healing capacity at elevated temperature.     

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.     

Effects of CVI SiC amount and deposition rates on properties of SiCf/SiC composites fabricated by hybrid chemical vapor infiltration (CVI) and precursor infiltration and pyrolysis (PIP) routes

The SiC_f/SiC composites have been manufactured by a hybrid route combining chemical vapor infiltration (CVI) and precursor infiltration and pyrolysis (PIP) techniques. A relatively low deposition rate of CVI SiC matrix is favored ascribing to that its rapid deposition tends to cause a ‘surface sealing’ effect, which generates plenty of closed pores and severely damages the microstructural homogeneity of final composites. For a given fiber preform, there exists an optimized value of CVI SiC matrix to be introduced, at which the flexural strength of resultant composites reaches a peak value, which is almost twice of that for composites manufactured from the single PIP or CVI route. Further, this optimized CVI SiC amount is unveiled to be determined by a critical thickness t₀, which relates to the average fiber distance in fiber preforms. While the deposited SiC thickness on fibers exceeds t₀, closed pores will be generated, hence damaging the microstructural homogeneity of final composites. By applying an optimized CVI SiC deposition rate and amount, the prepared SiC_f/SiC composites exhibit increased densities, reduced porosity, superior mechanical properties, increased microstructural homogeneity and thus reduced mechanical property deviations, suggesting a hybrid CVI and PIP route is a promising technique to manufacture SiC_f/SiC composites for industrial applications.     

Analytical and numerical study of the densification of carbon/carbon composites by a film-boiling chemical vapor infiltration process

The film-boiling chemical vapor infiltration (CVI) process is a fast process developed for composite material fabrication, and especially carbon/carbon composites. In order to help define optimal conditions, a local 1D model has been developed to study the densification front which establishes itself during the processing of a carbon/carbon fibrous preform. The model features heat conduction, precursor gas diffusion, densification reactions and structural evolution of the porous medium. The effects of total mass flux, Thiele modulus, porous medium geometry on front behavior parameters such as width, velocity and residual porosity are presented as semi-analytical correlations. An existence criterion is produced, which involves a minimal heat flux. Comparison between process-scale experiments and simulation is then possible by inserting the semi-analytical results achieved in the local study of the front into a light numerical model involving the entire preform. The model has been validated with respect to previous experimental and numerical data.     

Electromagnetic interference shielding and dielectric properties of SiCf/SiC composites containing pyrolytic carbon interphase

The SiC_f/SiC composites containing various thickness of pyrolytic carbon (PyC) interphase were prepared and their properties were investigated for electromagnetic interference (EMI) shielding applications in the frequency of 8.2–12.4 GHz. The composites containing 310 nm thickness (3.3 vol%) PyC interphase show an about 25 dB shielding effectiveness in the whole frequency band. Interestingly, the contribution of reflection to the EMI shielding effectiveness increases and the contribution of absorption decreases as the PyC interphase thickness increases.     

Computational characterisation of microwave heating of fibre preforms for CVI of SiCf/SiC composites

Silicon carbide fibre reinforced silicon carbide matrix composites (SiC_f/SiC) are known as materials with high-performance mechanical properties for the aerospace industry. Microwave-enhanced (ME) chemical vapour infiltration (CVI) heating of ceramic matrix composites is potentially an energy efficient production technique capable of yielding near fully dense SiC_f/SiC composites in a much shorter time span. This paper reports on the output of computational analysis of electromagnetic (EM) and thermal characteristics of the ME CVI process occurring with thin circular SiC fibre preform in a Labotron microwave system from SAIREM. Computer simulation is performed with the use of the finite-difference time-domain technique implemented in QuickWave computational environment. Multiple puzzling phenomena observed in the earlier experimental work are illuminated in the present study and the causes for the formation of microwave-induced temperature fields are clarified. With the use of the developed EM model, resonant and non-resonant frequencies of the Labotron system for different temperatures of the processed samples are analysed to explain the differences and variability in heating rates. This showed that when microwave processing of small SiC samples, energy coupling is extremely sensitive to frequency: a change of the reflection coefficient from 0.05 (absorbing) to 0.75 (reflective) could be made by a drift as small as 0.003–0.005 GHz, respectively, indicating the importance of scaling the microwave cavity to the sample size and the ability to precisely control the frequency of the microwave source. Moreover, energy coupling is temperature-dependent: low reflections produces very high heating rates (greater than 550 ℃ min^-1); the opposite is true for high reflections where heating rates are significantly slower. Temperature fields in the SiC fibre preforms are computed with the coupled EM-thermal model at different frequencies. It is shown that while being highly non-uniform in the beginning of the process, temperature patterns evolve to being fairly homogeneous by its end. Overall, the results suggest a means for better control of the equipment to pave the way to more efficient, controlled, and repeatable implementations of the ME CVI technology to produce high quality SiC_f/SiC composites.     

In situ investigation of tensile behavior of Cf/SiC mini composites

This study presents an in depth analysis over the in situ tensile behavior of C_f/SiC mini composites. As part of the process, the matrix crack spacing at saturation was determined by the in situ x-ray microtomography tensile test and the test results were compared with others obtained by in situ optical microscope tensile test and scanning electron microscopy scan. Moreover, elastic modulus of fiber and matrix as well as interface shear stress were identified by the indirect method and in situ modulus of C fibers and SiC matrix were also measured by the nanoindentation test, showing outcomes much lower than those identified by indirect method. The in situ property parameters measured by in situ XCT tensile test and identified by the indirect method were substituted into the shear-lag model to predict the stress-strain responses of C_f/SiC mini composites and the predicted results agrees well with the experimental data, while there exists large deviation between the stress-strain response predicted by using the in situ modulus of C fibers tested by nanoindentation and the experimental data, which indicates that in situ modulus of C fibers tested by nanoindentation tests cannot be utilized to model the tensile stress-strain responses due to the possible asymmetry of tension and compression of C fibers.     

In situ growth of SiC wires on CVI densification of SiC foam

Silicon carbide (SiC) foam prepared by polymer infiltration and pyrolysis (PIP) process was further densified with β-SiC by chemical vapor infiltration (CVI) technique. Scanning electron microscopy and high-resolution transmission electron microscopy images confirmed the presence of highly entangled and branched in situ grown SiC wires of uniform diameter (∼500 nm) over the struts of open-cell SiC foam. A uniform rate increase in diameter from nanometer to micron range (∼11 μm) was observed with an increase in the CVI reaction period. X-ray diffraction results showed the formation of highly crystalline β-SiC structure along the <111> direction with stacking faults. The formation of SiC wires was explained by the vapor–liquid–solid mechanism and evenness of the surface and uniform growth rate of SiC confirmed the homogeneous concentration of gaseous species during CVI reaction. The compressive strength increased with relative density, with maximum values of 5.5 ± 1.26 MPa for ultimate SiC foam (ρ = 400 kg/m³) prepared by hybrid PIP/CVI technique. The thermo-oxidative stability of the resultant foam was evaluated up to 1650°C under air and shows excellent thermal stability compared to SiC foam prepared by PIP route. The densified SiC foam can find potential applications in the field of hot gas filters, catalyst supports, microwave absorption properties, and heat insulation for high-temperature applications.     

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.     

In-situ characterization on crack propagation behavior of SiCf/SiC composites during monotonic tensile loading

The microstructure and crack propagation path of 2.5D SiC_f/SiC composites were observed by synchrotron radiation x-ray computed micro tomography (SR-μCT) equipped with in-situ tensile device. The results showed that the pore morphologies of the SiC_f/SiC composites are mainly divided into three types in three-dimension space: interconnected pores, isolated pores and micro pores in fiber bundles. The crack initiation occurred at the root of the defects under in-situ tensile load and the crack was perpendicular, parallel to the stress axis or mixed mode to propagate. At the interface scale between fiber and matrix, the crack deflection will be controlled by physical parameters such as fracture energy release rate and the modulus of elasticity. At the fiber bundle scale, the crack is easy to shear propagate along the interface between weft and warp fiber bundles due to the existence of the mechanical bonding and residual tensile stress.     

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.     

3D-SiC decorated with SiC whiskers: Chemical vapor infiltration on the porous 3D-SiC lattices derived from polycarbosilane-based suspensions

SiC_w/3D-SiC composites were fabricated by chemical vapor infiltration (CVI) of the 3D SiC lattices, which were prepared via direct ink writing of polycarbosilane-based suspensions. Microstructure, composition and tensile strength of the composites were investigated. Curing and pyrolysis temperature greatly affected the shrinkage, weight loss, density and composition of the 3D SiC. Although sound structure with spanning feature was achieved, cracks and pores in 3D SiC were formed during the pyrolysis owing to the large shrinkage. CVI process decreased the porosity and led to fully dense surface of the SiC_w/3D-SiC composites. After 60h of CVI, short β-SiC fibres or long SiC whiskers were deposited in the structural spacing of 3D lattices or spherical pores inside the filaments, respectively. The tensile strength of the composites by CVI increased from 3.3 MPa to 15.7 MPa (20 h) and 47.3 MPa (60 h), due to the high strength of dense CVI layers and in-situ formed SiC whiskers in pores. This work showed a way to strengthen the 3D SiC with in-situ formed whiskers via the polymer precursor routes.     

Ring compression properties of SiCf/SiC composites prepared by chemical vapor infiltration

SiC fiber reinforced SiC matrix (SiC_f/SiC) composites prepared by chemical vapor infiltration are one of promising materials for nuclear fuel cladding tube due to pronounced low radioactivity and excellent corrosion resistance. As a structure component, mechanical properties of the composites tubes are extremely important. In this study, three kinds of SiC_f preform with 2D fiber wound structure, 2D plain weave structure and 2.5D shallow bend-joint structure were deposited with PyC interlayer of about 150–200?nm, and then densified with SiC matrix by chemical vapor infiltration at 1050?°C or 1100?°C. The influence of preform structure and deposition temperature of SiC matrix on microstructure and ring compression properties of SiC_f/SiC composites tubes were evaluated, and the results showed that these factors have a significant influence on ring compression strength. The compressive strength of SiC_f/SiC composites with 2D plain weave structure and 2.5D shallow bend-joint structure are 377.75?MPa and 482.96?MPa respectively, which are significantly higher than that of the composites with 2D fiber wound structure (92.84?MPa). SiC_f/SiC composites deposited at 1100?°C looks like a more porous structure with SiC whiskers appeared when compared with the composites deposited at 1050?°C. Correspondingly, the ring compression strength of the composites deposited at 1100?°C (566.44?MPa) is higher than that of the composites deposited at 1050?°C (482.96?MPa), with a better fracture behavior. Finally, the fracture mechanism of SiC_f/SiC composites with O-ring shape was discussed in detail.     

Influence of boron nitride nanotubes on the damage evolution of SiCf/SiC composites

As-grown and BN-coated boron nitride nanotubes (BNNTs) were incorporated into SiC_f/SiC composites to produce nanotube-based hierarchical composites. In-depth studies on damage evolution reveal that early damage development are delayed owing to the restriction effects on crack propagations from as-grown and BN-coated BNNTs. Moreover, this delay effect is more pronounced from BN-coated BNNTs because BN-coated BNNTs/matrix interfacial bonding strength is low. Final failure of composites with as-grown BNNTs still comes much earlier compared with virgin composite due to strong fibers/matrix bonding enhanced by as-grown BNNTs. This premature final failure is remedied in large part in composites with BN-coated BNNTs because fibers/matrix bonding enhanced by as-grown BNNTs is weaken after the deposition of an interphase on nanotube surface. Additionally, the type, the number and the released energy level of damage mechanisms during the whole damage evolution after the incorporation of as-grown and BN-coated BNNTs were also discussed elaborately compared with virgin composite.