Effect of SiC Particles on Mechanical Properties of Laminated (SiCw+SiCp)/SiC Ceramic Composites

Laminated (SiC_w+SiC_p)/SiC ceramic composites were fabricated by tape casting and chemical vapor infiltration (CVI), and the effect of SiC particles on strengthening/toughening of the composites was investigated. When the SiC particle content was constant, the mechanical properties of (SiC_w+SiC_p)/SiC composites were increased with increasing SiC whisker content. When the SiC particle content was varied, the mechanical properties of (SiC_w+SiC_p)/SiC composites were dependent on SiC particle content. The addition of SiC particles can increase the strength of the matrix and the crack propagation resistance, the former increased the strength and the latter increased the toughness.     

Preparation of SiC/SiC composites with high toughness by reaction of the SiCP+C double-cladding layer with liquid silicon

SiC/SiC composites prepared by liquid silicon infiltration (LSI) have the advantages of high densification, matrix cracking stress and ultimate tensile strength, but the toughness is usually insufficient. Relieving the residual microstress in fiber and interphase, dissipating crack propagation energy, and improving the crystallization degree of interphase can effectively increase the toughness of the composites. In this work, a special SiC particles and C (SiC_P +C) double-cladding layer is designed and prepared via the infiltration of SiC_P slurry and chemical vapor infiltration (CVI) of C in the porous SiC/SiC composites prepared by CVI. After LSI, the SiC generated by the reaction of C with molten Si combines with the SiC_P to form a layered structure matrix, which can effectually relieve residual microstress in fiber and interphase and dissipate crack propagation energy. The crystallization degree of BN interphase is increased under the effects of C-Si reaction exotherm. The as-received SiC/SiC composites possess a density of 2.64 g/cm³ and a porosity of 6.1%. The flexural strength of the SiC/SiC composites with layered structure matrix and highly crystalline BN interphase is 577 MPa, and the fracture toughness reaches up to 37 MPa·m^1/2. The microstructure and properties of four groups of SiC/SiC composites prepared by different processes are also investigated and compared to demonstrate the effectiveness of the SiC_P +C double-cladding layer design, which offers a strategy for developing the SiC/SiC composites with high performance.     

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.     

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.     

Effect of SiC nanowires on the mechanical properties and thermal conductivity of 3D-SiCf/SiC composites prepared via precursor infiltration pyrolysis

In this study, SiC nanowires (SiC_NWS) were grown in situ on the surface of PyC interface through chemical vapor infiltration (CVI) to improve the mechanical characteristics and thermal conductivity of three-dimensional SiC_f/SiC composites fabricated via precursor infiltration pyrolysis (PIP). The effect of SiC_NWS on the properties of the obtained composites was investigated by comparing them with conventional SiC_f/PyC/SiC composites. After the deposition of SiC_NWS, the flexural strength of the SiC_f/SiC composites was found to increase by 46 %, and the thermal conductivity showed an obvious increase at 25?1000 °C. The energy release of the composites in the damage evolution process was analysed by acoustic emission. The results indicated that the damage evolution process was delayed owing to the decrease in porosity, the crack deflection and bridging of the SiC_NWS. Furthermore, the excellent thermal conductivity was attributed to the thermally conductive pathways formed by the SiC_NWS in the dense structure.     

Mechanical properties and strengthening mechanism of SiCf/SiC mini-composites modified by SiC nanowires

The effects of the SiC nanowires (SiCNWs) and PyC interface layers on the mechanical and anti-oxidation properties of SiC fiber (SiC_f)/SiC composites were investigated. To achieve this, the PyC layer was coated on the SiC_f using a chemical vapour infiltration (CVI) method. Then, SiCNWs were successfully coated on the surface of SiC_f/PyC using the electrophoretic deposition method. Finally, a thin PyC layer was coated on the surface of SiC_f/PyC/SiCNWs. Three mini-composites, SiC_f/PyC/SiC, SiC_f/PyC/SiCNWs/SiC, and SiC_f/PyC/SiCNWs/PyC/SiC, were fabricated using the typical precursor infiltration and pyrolysis method. The morphologies of the samples were examined using scanning electron microscopy and energy dispersive X-ray spectrometry. Tensile and single-fibre push-out tests were carried out to investigate the mechanical performance and interfacial shear strength of the composites before and after oxidization at 1200 °C. The results revealed that the SiC_f/PyC/SiCNWs/SiC composites showed the best mechanical and anti-oxidation performance among all the composites investigated. The strengthening and toughening is mainly achieved by SiCNWs optimization of the interfacial bonding strength of the composite and its own nano-toughening. On the basis of the results, the effects of SiCNWs on the oxidation process and retardation mechanism of the SiC_f/SiC mini-composites were investigated.     

Microstructure and strengthening behavior in high content SiC nanowires reinforced SiC composites

In this study, the high-content SiC_nw reinforced SiC ceramic matrix composites (SiC_nw/SiC CMC) were successfully fabricated by hot pressing β-SiC and sintering additive (Al₂O₃-Y₂O₃) with boron nitride interphase modification SiC_nw. The effects of sintering additive content and mass fraction (5–25 wt%) of SiC_nw on the density, microstructure, and mechanical properties of the composites were investigated. The results showed that with the increase of sintering additives from 10 wt% to 12 wt%, the relative density of the SiC_nw/SiC CMC increased from 97.3% to 98.9%, attributed to the generated Y₃Al₅O₁₂ (YAG) liquid phase from the Al₂O₃-Y₂O₃ that promotes the rearrangement and migration of SiC grains. The comprehensive performance of the obtained composite with 15 wt% SiC_nw possessed the optimal flexural strength and fracture toughness of 524 ± 30.24 MPa and 12.39 ± 0.49 MPa·m^1/2, respectively. Besides, the fracture mode of the composites with 25 wt% SiC_nw content revealed a pseudo-plastic fracture behavior. It concludes that the 25 wt% SiC_nw/SiC CMC was toughened by the fiber pull-outs, debonding, bridging, and crack deflection that can consume plenty of fracture energy. The strategy of SiC nanowires worked as a main bearing phase for the fabrication of SiC/SiC CMC providing critical information for understanding the mechanical behavior of high toughness and high strength SiC nanoceramic matrix composites.     

The mechanical,thermophysical and electromagnetic properties of UD SiCf/SiC composites in different directions

Unidirectional (UD) silicon carbide (SiC) fiber-reinforced SiC matrix (UD SiC_f/SiC) composites with CVI BN interphase were fabricated by polymer infiltration-pyrolysis (PIP) process. The effects of the anisotropic distribution of SiC fibers on the mechanical properties, thermophysical properties and electromagnetic properties of UD SiC_f/SiC composites in different directions were studied. In the direction parallel to the axial direction of SiC fibers, SiC fibers bear the load and BN interphase ensures the interface debonding, so the flexural strength and the fracture toughness of the UD SiC_f/SiC composites are 813.0 ± 32.4 MPa and 26.1 ± 2.9 MPa·m^1/2, respectively. In the direction perpendicular to the axial direction of SiC fibers, SiC fibers cannot bear the load and the low interfacial bonding strengths between SiC fiber/BN interphase (F/I) and BN interphase/SiC matrix (I/M) both decrease the matrix cracking stress, so the corresponding values are 36.6 ± 6.9 MPa and 0.9 ± 0.5 MPa?m^1/2, respectively. The thermal expansion behaviors of UD SiC_f/SiC composites are similar to those of SiC fibers in the direction parallel to the axial direction of SiC fibers, and are similiar to those of SiC matrix in the direction perpendicular to the axial direction of SiC fibers. The total electromagnetic shielding effectiveness (EM SE_T) of UD SiC_f/SiC composites attains 32 dB and 29 dB when the axial direction of SiC fibers is perpendicular and parallel to the electric field direction, respectively. The difference of conductivity in different directions is the main reason causing the different SE_T. And the dominant electromagnetic interference (EMI) shielding mechanism is absorption for both studied directions.     

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.     

Strengthening/toughening of 2D C/SiC composites by coating

SiC and SiC_w/SiC coatings were prepared on two-dimensional carbon fiber reinforced silicon carbide ceramic matrix composites (2D C/SiC), and strengthening/toughening of the composite by the coatings was investigated. After coating, the density of the C/SiC composites was increased effectively and the mechanical properties were improved significantly. Compared with SiC coating, SiC_w/SiC coating showed the more significant effect on strength/toughness of the composites. Coatings had two effects: surface strengthening and matrix strengthening. The latter was the dominant effect. The surface strengthening can increase the crack initiation stress, while the matrix strengthening can enhance the crack propagation resistance. The former effect increased the strength and the latter effect increased the toughness.     

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 of highly dense three-layer SiC cladding tube by chemical vapor infiltration method

Three-layer silicon carbide (SiC) cladding architectures are considered to be promising materials for current light-water nuclear reactors. Herein, a novel processing approach was proposed to fabricate dense three-layer SiC tubes by introducing SiC nanowires (NWs) on the graphite rod, which resulted in change in the valley-peak structure of SiC_f tubular preform. A dense three-layer-NWs SiC cladding tube, consisting of a chemical vapor infiltration (CVI)-SiC inner layer, a CVI-SiC_f/SiC composite layer, and a CVI-SiC outer layer, was obtained through CVI process. Microstructure and hoop strength of the as-obtained three-layer-NWs SiC cladding tube were systematically investigated. By avoiding delamination of the layers and reducing the pores, the three-layer-NWs SiC cladding tube exhibited an average hoop strength of 316.6 MPa with a Weibull modulus of 11.55.     

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.     

Improved mechanical properties of reaction-bonded SiC through in-situ formation of Ti3SiC2

Reaction-bonded SiC is a ceramic with excellent thermal properties, good corrosion resistance and the characteristic of near-net-shape manufacturing. However, the poor fracture toughness of free Si limits the applications of reaction-bonded SiC. In this study, TiC was added to reaction-bonded SiC and reacted with free Si to form Ti₃SiC₂. The effects of TiC and carbon black on the mechanical properties of reaction-bonded SiC were investigated. The results demonstrated that the in-situ formation of Ti₃SiC₂ and decrease in the content and size of free Si improved the mechanical properties of reaction-bonded SiC ceramics. The mechanical properties of TiC-added reaction-bonded SiC with 17.5 wt% carbon black were superior to those of TiC-added reaction-bonded SiC with 15 wt% carbon black. Moreover, increasing the TiC content of reaction-bonded SiC with 17.5 wt% carbon black from 0 to 7.5 wt% caused an increase in its bending strength from 183.92 to 424.43 MPa and an increase in fracture toughness from 3.7 to 5.24 MPa m^1/2.     

Synthesis and properties of macroporous SiC ceramics synthesized by 3D printing and chemical vapor infiltration/deposition

Open porosity cellular SiC-based ceramics have a great potential for energy conversion, e.g. as solar receivers. In spite of their tolerance to damage, structural applications at high temperature remain limited due to high production costs or inappropriate properties. The objective of this work was to investigate an original route for the manufacturing of porous SiC ceramics based on 3D printing and chemical vapor infiltration/deposition (CVI/CVD). After binder jetting 3D-printing, the green α-SiC porous structures were reinforced by CVI/CVD of SiC using CH₃SiCl₃/H₂. The multiscale structure of the SiC porous specimens was carefully examined as well as the elemental and phase content at the microscale. The oxidation and thermal shock resistance of the porous SiC structures and model specimens were also studied, as well as the thermal and mechanical properties. The pure and dense CVI/CVD-SiC coating considerably improves the mechanical strength, oxidation resistance and thermal diffusivity of the material.     

Improving thermal stability of SiC whiskers via nanoscale Ti3SiC2 coatings

Effects of SiC whiskers (SiC_w) on the mechanical properties of composites largely depend on their thermal stability at high temperature. In this study, pure SiC_w and Ti₃SiC₂ coated SiC_w were thermal treated at 1600–1800°C for 1 h. Their phase assemblage, morphology, and structural evolution were investigated. Oxygen partial pressures in the graphite furnace resulted in the breakdown of SiC_w into particles at 1600°C, and the degradation became more pronounced with temperature increasing. The thermal stability of SiC whiskers at 1600–1700°C was significantly improved by a thin Ti₃SiC₂ coating on them, as both thermodynamic calculations and experimental observations suggest Ti₃SiC₂ coating could be preferentially oxidized/decomposed, prior to the active oxidation of SiC. At 1800°C, the protective role of the coating on the whiskers became weakened. SiC was converted into gaseous SiO and CO, with the remaining of interconnected TiC micro-rods and amorphous carbon.     

High-strength SiC joints with a novel in-situ formed SiC/Al4SiC4 joining filler

The in-situ formed SiC/Al₄SiC₄ joining layer was used to join monolithic SiC using an electric field-assisted sintering technique. A multiphase powder of Al₄C₃/SiC/Al₄SiC₄ was used as the initial joining material to obtain the in-situ reaction layer of SiC/Al₄SiC₄ via the appropriate interface reactions. The bending strength as high as 240.5 ± 6.6 MPa was obtained for the sample joined at 1800 °C, which was higher than the strength of the un-joined SiC matrix. Sound joints were obtained when Al₄C₃ was completely transformed to Al₄SiC₄, and a fully dense SiC/Al₄SiC₄ joining layer was consolidated. The integration of the joining layer with the SiC matrix was improved by a high amount of liquid phase formed at the interface. The proposed SiC/Al₄SiC₄ joining layer, with good thermal matching with SiC matrix, shows a great potential to be applied as a joining material for SiC-based ceramic matrix composites.     

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.     

Study on growth factors of SiC whisker in situ in SiCW/SiC composites based on selective laser sintering technology

In this study, SiC_W/SiC composite material was prepared through selective laser sintering (SLS), the effect of material parameters (pretreatment of PCS, content of PCS, type of catalyst and content of catalyst) on the growth number of SiC whisker was investigated. The results indicate that when the temperature variable of PCS pretreatment was 260 °C or 300 °C, the number of in situ generated SiC whisker was noticeably higher than that pretreated at 140/180/200/240 °C. The catalytic effect of ferrocene was superior than pure Fe, α-Al₂O₃ and Fe(NO₃)₃. Under this experimental condition, the best parameter was 10 wt.% of PCS and 4 wt.% of ferrocene. This find not only provides a research basis for the preparation of SiC_W/SiC composites prepared by SLS technology, but also perfects basic datas for the preparation process of SiC_W/SiC composites.     

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.