Fabrication method and mechanical properties of biomimetic Cf/ZrB2-SiC ceramic composites with bouligand structures

Herein, biomimetic C_f/ZrB₂-SiC ceramic composites with bouligand structures are fabricated by combining precursor impregnation, coating, helical assembly and hot-pressing sintering. First, C_f/ZrB₂-SiC ceramic films are achieved through a precursor impregnation method using polycarbosilane (PCS). Second, the PCS-C_f/ZrB₂-SiC ceramic films are coated with ZrB₂ and SiC ceramic layers. Finally, hot-pressing sintering is employed to densify helical assembly C_f/ceramic films with a fixed angle of 30°. The microstructures and carbon fiber content on the mechanical properties of biomimetic C_f/ZrB₂-SiC ceramic composites are analyzed in detail. The results show that the coated ceramic layer on PCS-C_f/ZrB₂-SiC films can heal the cracks formed by pyrolysis of PCS, and the mechanical properties are obviously improved. Meanwhile, the mechanical properties could be tuned by the contents of the carbon fiber. The toughening mechanisms of C_f/ZrB₂-SiC ceramic composites with bouligand structures are mainly zigzag cracks, crack deflection, multiple cracks, carbon fiber pulling out and bridging.     

Characterization and mechanical properties of Cf/ZrB2-SiC composites fabricated by a hybrid technique based on slurry impregnation,polymer infiltration and pyrolysis and low-temperature hot pressing

The continuous carbon fiber reinforced ZrB₂-SiC composite was fabricated successfully via a hybrid technique based on nano ceramic slurry impregnation, polymer infiltration and pyrolysis and low-temperature hot pressing. The C_f/ZrB₂-SiC composites exhibited non-brittle fracture modes and the chemical interaction at the fiber/matrix interfaces was effectively inhibited owing to the low sintering temperature. The S2-C_f/ZrB₂-SiC composite presented the highest mechanical properties with fracture toughness of 4.47?±?0.15?MPa?m^1/2 and the work of fracture of 877?J/m², which was attributed to the multiple length-scale toughening mechanisms including the macroscopic toughening mechanisms of crack deflection and crack branching, the micro toughening mechanisms of fiber bridging and fiber pull-out. This work presented a novel and effective method to fabricate high-performance continuous carbon fiber reinforced ceramic matrix composites.     

A novel vibration-assisted slurry impregnation to fabricate Cf/ZrB2-SiC composite with enhanced mechanical properties

The three dimensional needle-punched carbon fiber reinforced ZrB₂-SiC composite (C_f/ZrB₂-SiC) with highly uniform distribution was fabricated successfully via a novel vibration-assisted slurry impregnation and low-temperature (1450 °C) hot pressing technique using nanosized ZrB₂ powders. The carbon fiber/ceramic matrix interfaces were clear without obvious reaction products detected by the high resolution transmission electron microscopy (HR-TEM), indicating the degradation of carbon fiber was effectively inhibited. The C_f/ZrB₂-SiC composite exhibited a typical non-brittle fracture feature with a high work of fracture of 1104 J/m², which was approximately twice that of composite fabricated only by slurry impregnation and hot pressing. The enhancement in work of fracture was attributed to multiple toughening mechanisms of continuous carbon fibers such as extensive fiber bridging and pull-out accompanied by obvious crack deflection and branching. This work provides a valuable potential of preparing continuous carbon fiber reinforced ceramic composites with uniform component distribution and enhanced mechanical properties.     

Effect of pyrolytic carbon coating on the microstructure and fracture behavior of the Cf/ZrB2-SiC composite

The high sintering temperature and interface interaction seriously degraded the toughening effects of continuous carbon fiber in ZrB₂-SiC ceramic. The pyrolytic carbon coated carbon fiber reinforced ZrB₂-SiC composite (C_f-PyC/ZrB₂-SiC) with desirable properties was successfully achieved via brushing nano ZrB₂-SiC slurry followed by spark plasma sintering at relatively low sintering temperature. The fabricated C_f-PyC/ZrB₂-SiC composite presented a non-brittle fracture feature and a remarkable enhancement in comparison with the ZrB₂-SiC composite reinforced by the as-received carbon fiber (C_f-AS/ZrB₂-SiC). The fracture toughness and critical crack size were increased from 5.97?±?0.18–7.66?±?0.24?MPa?m^1/2 and from 91.6 to 164.5?µm, respectively. A high work of fracture of 1915?J/m² for C_f-PyC/ZrB₂-SiC composite was achieved, almost four times higher than that of the C_f-AS/ZrB₂-SiC composite (463?J/m²). Multiple toughening mechanisms contributed to such enhancement, such as crack deflection, fiber bridging, fiber pull-out and crack branching. This work provides a feasible approach to fabricate high-performance fiber reinforced ceramic composites having a high work of fracture.     

Design and optimization of the coating thickness on chopped carbon fibers and sintering temperature for ZrB2-SiC-Cf composites prepared by hot pressing

Interface designation and low-temperature densifying were expected to inhibit the performance attenuation of fiber in the preparation of ceramic matrix composites. Nevertheless, the optimization of interface thickness and sintering temperature were still rarely investigated systematically. Herein, the performance of carbon fiber varied with coating thickness and the heat-treat temperatures were investigated by quantitatively. The tensile strength of carbon fibers degraded dramatically (≥30%) when coating thickness above a critical value. A larger dimension and ordered graphite crystallite structure resulted in the severe performance deterioration of carbon fibers as the sintering temperature exceeding 1500 ℃. The PyC coatings with an appropriate thickness acted as a critical role of changing the fracture mode of the ZrB₂-SiC-C_f and noticeably increased the fracture strain of the composite. The regular trend provided a general design strategy in construction of ZrB₂-SiC-C_f composite with optimal properties and could be extended to other ceramic matrix composites.     

Enhanced mechanical properties and thermal shock resistance of Cf/ZrB2-SiC composite via an efficient slurry injection combined with vibration-assisted vacuum infiltration

An efficient slurry injection combined with vibration-assisted vacuum infiltration process has been developed to fabricate 3D continuous carbon fiber reinforced ZrB₂-SiC ceramics. Homogenous distribution between carbon fiber and ceramic was achieved successfully, leading to an enhancement in mechanical properties. The C_f-PyC/ZrB₂-SiC composite exhibited a typical non-brittle fracture mode with a superior fracture toughness of 6.72 ± 0.21 MPa·m^1/2 and an extraordinary work of fracture of 2270 J/m², respectively, increasing by nearly 14.8 % and 36 % as compared with those of a parent composite fabricated by only slurry injection and slurry infiltration. The enhancement in fracture toughness and work of fracture were attributed to multiple toughening mechanism including crack deflection, PyC coated fiber bundles pull-out and fiber bridging. Moreover, a critical thermal shock temperature difference of 814 °C was achieved, higher than that of traditional ZrB₂-based ceramics. This work presents an efficient approach to fabricate high-performance C_f/UHTCs with uniform architecture.     

Effect of PAN-based and pitch-based carbon fibres on microstructure and properties of continuous Cf/ZrB2-SiC UHTCMCs

In this paper the microstructure and mechanical properties of two different C_f/ZrB₂-SiC composites reinforced with continuous PyC coated PAN-derived fibres or uncoated pitch-derived fibres were compared.Pitch-derived carbon fibres showed a lower degree of reaction with the matrix phase during sintering compared to PyC/PAN-derived fibres. The reason lies in the different microstructure of the carbon. The presence of a coating for PAN-derived fibres was found to be essential to limit the reaction at the fibre/matrix interface during SPS. However, coated bundles were more difficult to infiltrate, resulting in a less homogeneous microstructure.As far as the mechanical properties are concerned, specimens reinforced with coated PAN-derived fibres provided higher strengths and damage tolerance than uncoated pitch-derived fibres, due to the higher degree of fibre pull-out. On the other hand, the weaker fibre/matrix interface resulted in lower interlaminar shear, off-axis strength and ablation resistance.     

Multiscale toughening of ZrB2-SiC-Graphene@ZrB2-SiC dual composite ceramics

The design of bioinspired architectures is effective for increasing the toughness of ceramic materials. Particularly, a dual composite equiaxial architecture is ideal for fabricating weak interface-toughened ZrB₂-SiC ceramics with isotropic performance. In this paper, ZrB₂-SiC-Graphene@ZrB₂-SiC dual composite ceramics were synthesized via an innovative processing technique of granulating-coating method. ZrB₂-20 vol.% SiC containing 30 vol.% Graphene was selected as weak interface to realize multiscale toughening and improve the thermal shock resistance of ZrB₂-SiC ceramic materials. The incorporation of ZrB₂-SiC-Graphene weak interface into the ZrB₂-SiC matrix improved the damage tolerance and critical thermal shock temperature difference. The design of equiaxial structures moderated the anisotropy of performance in different planes. The graphene sheets incorporated in the ZrB₂-SiC-Graphene interface phase played a key role in multiscale toughening, including macroscopic toughening of crack deflection and microcracks, and microscopic toughening of graphene bridging and pull-out.     

Supersonic atmospheric plasma sprayed ZrO2 and ZrB2-SiC/ZrO2 coatings on Cf/Mg composites for anticorrosion

To improve the corrosion resistance of the carbon fiber reinforced magnesium matrix composites (C_f/Mg composites), ZrO₂ and ZrB₂-SiC/ZrO₂ composite coatings were prepared by supersonic atmospheric plasma spraying (SAPS) on C_f/Mg composites. The microstructure and phase composition of the coatings before and after the corrosion test were investigated. Open circuit potential and potentiodynamic polarization tests were measured at room temperature. Results revealed that the corrosion current density ($i_{\mathit{corr}}$) of the ZrO₂ coated C_f/Mg composites decreased by one order while the ZrB_2-SiC/ZrO₂ coated C_f/Mg composites reduced by two orders. Compared with C_f/Mg composites, the corrosion potential ($E_{\mathit{corr}}$) of the ZrO₂ and ZrB₂-SiC/ZrO₂ coated C_f/Mg composites increased by 220.5?mV and 1021.8?mV respectively, indicating that the ZrB₂-SiC/ZrO₂ composite coatings greatly improve the corrosion resistance of C_f/Mg composites. The uniform distribution of the SiC particles with small grain size in ZrB₂ is responsible for the densification of the coating. The ZrB₂-SiC/ZrO₂ composite coatings provide a barrier for the substrate to impede the entry of Cl^- in the corrosion solution, thus exhibiting a better corrosion resistance than the ZrO₂ coating.     

Continuous carbon fiber reinforced ZrB2-SiC composites fabricated by direct ink writing combined with low-temperature hot-pressing

As one of additive manufacturing techniques, direct ink writing has significant advantages in the manufacture of ceramic matrix composites, nevertheless, the poor impregnability of ceramic slurry makes it difficult to fill the interior of fiber bundles, causing poor mechanical properties. Here, ultrasound-assisted fiber separation technique was introduced to impregnate ceramic slurry with a continuous carbon fiber bundle during direct ink writing of continuous carbon fiber/ceramic green body and subsequent low temperature hot-pressing was combined to improve its robustness. Suitable thickness of carbon coating could bring to high fracture resistance, whereas excessively thick carbon coating will adversely affect the mechanical properties. A carbon interface with thickness around 110 nm was incorporated, the flexural strength, fracture toughness and work of fracture of C_f/ZrB₂-SiC composite reached 388.3 MPa, 10.04 MPa·m^1/2 and 2380 J/m², respectively. Therefore, direct ink writing combined with low temperature hot-pressing, was effective to fabricate high-performance ceramic matrix composites.     

Microstructure and mechanical properties of quasi-isotropic chopped fiber-reinforced PyC–SiC matrix composite prepared by novel nondamaging method

In this work, quasi-isotropic chopped carbon fiber-reinforced pyrolytic carbon and silicon carbide matrix (C_f/C–SiC) composites and chopped silicon carbide fiber-reinforced silicon carbide matrix (SiC_f/SiC) composites were prepared via novel nondamaging method, namely airlaid process combined with chemical vapor infiltration. Both composites exhibit random fiber distribution and homogeneous pore size. Young's modulus of highly textured pyrolytic carbon (PyC) matrix is 23.01 ± 1.43 GPa, and that of SiC matrix composed of columnar crystals is 305.8 ± 9.49 GPa in C_f/C–SiC composites. Tensile strength and interlaminar shear strength of C_f/C–SiC composites are 52.56 ± 4.81 and 98.16 ± 24.62 MPa, respectively, which are both higher than those of SiC_f/SiC composites because of appropriate interfacial shear strength and introduction of low-modulus and highly textured PyC matrix. Excellent mechanical properties of C_f/C–SiC composites, particularly regarding interlaminar shear strength, are due to their quasi-isotropic structure, interfacial debonding, interfacial sliding, and crack deflection. In addition to the occurrence of crack deflection at the fiber/matrix interface, crack deflection in C_f/C–SiC composites takes also place at the interface between PyC–SiC composite matrix and the interlamination of multilayered PyC matrix. Outstanding mechanical properties of as-prepared C_f/C–SiC composites render them potential candidates for application as thermal structure materials under complex stress conditions.     

Effects of multilayer hydrothermal carbon interphases on mechanical properties and thermal shock resistance of CF/ZrB2-SiCBN

Multilayer hydrothermal carbon coatings (HTCCs) with various thicknesses were constructed on CFs using alternating hydrothermal carbonization and heat treatment approach. Well-established CF/ZrB₂-SiCBN composites with distinct multilayer interphases were processed via slurry injection and precursor infiltration pyrolysis method. The multilayer HTCCs with suitable thickness would be pulled-out stepwise from the matrix, which could absorb the impact energy and toughen the composites effectively. Although the outer HTCC would still react with the matrix to form a strong interface, the moderate interfacial binding force between CF and inner HTCC could effectively transfer the load and assure the fiber debonding when the composites deformed drastically. Moreover, the increasing of coating thickness could alleviate the damage of fibers in oxidation environment and improve the critical thermal shock temperature difference of composites, while the excessive thick coating would weaken the fiber strength and limit toughening abilities of the CFs.     

Microstructure and mechanical properties of continuous carbon fiber-reinforced ZrB2-based composites via combined electrophoretic deposition and sintering

A process combining electrophoretic deposition (EPD) with hot pressing (HP) was developed to fabricate continuous carbon fiber-reinforced ZrB₂-based composites (C_f/ZrB₂-based composites). ZrB₂-based ultra-high temperature ceramic (UHTC) particles were uniformly pre-coated on continuous carbon fibers via EPD. Then, the UHTC-coated carbon fibers were stacked and hot pressed to prepare the C_f/ZrB₂-based composites. Microstructure observations revealed that almost no micro-pores were found in the inter-bundle and intra-bundle regions of fibers after HP. The flexural strength, fracture toughness and the work of fracture of the C_f/ZrB₂-based composite were measured as 199 ± 26 MPa, 6.71 ± 1.29 MPa·m^1/2, and 754 ± 58 J/m², respectively. Based on the observations of non-brittle fracture behavior, fractured morphology and crack propagation, the enhanced fracture properties were mainly attributed to the multiple toughening mechanisms, such as fiber pull-out, fiber bridging, crack deflection and branching along the interfaces.     

Interphase degradation of three‐dimensional Cf/SiC–ZrC–ZrB2 composites fabricated via reactive melt infiltration

Layer‐structured interphase, existing between carbon fiber and ultrahigh‐temperature ceramics (UHTCs) matrix, is an indispensable component for carbon fiber reinforced UHTCs matrix composites (C_f/UHTCs). For C_f/UHTCs fabricated by reactive melt infiltration (RMI), the interphase inevitably suffers degradation due to the interaction with the reactive melt. Here, C_f/SiC–ZrC–ZrB₂ composite was fabricated by reactive infiltration of ZrSi₂ melt into sol‐gel prepared C_f/B₄C–C preform. (PyC–SiC)₂ interphase was deposited on the fiber to investigate the degradation mechanism under ZrSi₂ melt. It was revealed that the degraded interphase exhibited typical features of Zr aggregation and SiC residuals. Moreover, the Zr species diffused across the interphase and formed nanosized ZrC phase inside the fiber. A hybrid mechanisms of chemical reaction and physical melting were proposed to reveal the degradation mechanism according to characterization results and heat conduction calculations. Based on the degradation mechanism, a potential solution to mitigate interphase degradation is also put forward.     

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.     

Fabrication process and growth mechanism of novel micro-stack PyC interphase with different textures

Pyrolytic carbon (PyC) interphase plays a crucial role in the mechanical properties of fiber-reinforced ceramic matrix composites. In this research, a novel micro-stack PyC interphase with different PyC textures was designed and fabricated by changing the deposition parameters during the chemical vapor infiltration process. The growth mechanism of the micro-stack PyC interphase with different texture were also studied by experimental characterizations and kinetic calculations, and the results show that the content ratio of (C₂H₂ + C₂H₄) to C₆H₆ gas intermediate is a key parameter to control the texture types of PyC interphase. Furthermore, the value of orientation angle (OA) value, thickness, and modulus of the micro-stack PyC interphase were further characterized by high resolution TEM (HRTEM), scanning electronic microscopy, and nanoindentation. Finally, the tensile testing of mini-C_f/PyC/SiC composites was conducted, and the results showed that the tensile strength of mini-C_f/PyC/SiC composites with micro-stack PyC interphase is approximately 40% higher than that containing single high texture PyC interphase. The improvements on the tensile strength of C_f/PyC/SiC composites prove the significant advantages of micro-stack PyC interphase.     

Synergistic reinforcement effects of ZrB2 on the ultra-high temperature stability of Cf/SiC composite fabricated by liquid silicon infiltration

A carbon fiber-reinforced silicon carbide (C_f/SiC) composite was fabricated with ZrB₂ via the liquid silicon infiltration (LSI) method. A prepreg was prepared by impregnating the phenolic resin with the ZrB₂ powder. The as-LSIed composites were tested for 5 min with an oxyacetylene torch to evaluate their ablation and oxidation properties under an ultra-high temperature environment. The ZrB₂ powders and SiC matrix between carbon fiber bundles generated a dense ZrO₂-SiO₂ layer, which inhibited further oxygen diffusion into the composite and minimized the ablation and oxidation of the carbon fibers. Weight loss and linear ablation rate were further reduced with the addition of ZrB₂ to the C_f/SiC composite; moreover, the synergistic effect of ZrB₂ and SiC reinforced the ablation properties with increased ZrB₂ content. ZrB₂ also reduced the amount of residual silicon, which was detrimental to the mechanical properties of C_f/SiC composite.     

High-temperature flexural strength of I-CVI processed Cf/SiC composites with variable interphases

The effect of single-layer pyrocarbon (PyC) and multilayered (PyC/SiC)_n=4 interphases on the flexural strength of un-coated and SiC seal-coated stitched 2D carbon fiber reinforced silicon carbide (C_f/SiC) composites was investigated. The composites were prepared by I-CVI process. Flexural strength of the composites was measured at 1200 °C in air atmosphere. It was observed that irrespective of the type of interphase, the seal coated samples showed a higher value of flexural strength as compared to the uncoated samples. The flexural strength of 470 ± 12 MPa was observed for the seal coated C_f/SiC composite samples with multilayered interphase. The seal coated samples with single layer PyC interphase showed flexural strength of 370 ± 20 MPa. The fractured surfaces of tested samples were analyzed in detail to study the fracture phenomena. Based on microstructure-property relations, a mechanism has been proposed for the increase of flexural properties of C_f/SiC composites having multilayered interphase.     

Microstructure and damage evolution of SiCf/PyC/SiC and SiCf/BN/SiC mini-composites: A synchrotron X-ray computed microtomography study

SiC_f/PyC/SiC and SiC_f/BN/SiC mini-composites comprising single tow SiC fibre-reinforced SiC with chemical vapor deposited PyC or BN interface layers are fabricated. The microstructure evolutions of the mini-composite samples as the oxidation temperature increases (oxidation at 1000, 1200, 1400, and 1600?°C in air for 2?h) are observed by scanning electron microscopy, energy dispersive spectrometry, and X-ray diffraction characterization methods. The damage evolution for each component of the as-fabricated SiC_f/SiC composites (SiC fibre, PyC/BN interface, SiC matrix, and mesophase) is mapped as a three-dimensional (3D) image and quantified with X-ray computed tomography. The mechanical performance of the composites is investigated via tensile tests.The results reveal that tensile failure occurs after the delamination and fibre pull-out in the SiC_f/PyC/SiC composites due to the volatilization of the PyC interface at high temperatures in the air environment. Meanwhile, the gaps between the fibres and matrix lead to rapid oxidation and crack propagation from the SiC matrix to SiC fibre, resulting in the failure of the SiC_f/PyC/SiC composites as the oxidation temperature increases to 1600?°C. On the other hand, the oxidation products of B₂O₃ molten compounds (reacted from the BN interface) fill up the fracture, cracks, and voids in the SiC matrix, providing excellent strength retention at elevated oxidation temperatures. Moreover, under the protection of B₂O₃, the SiC_f/BN/SiC mini-composites show a nearly intact microstructure of the SiC fibre, a low void growth rate from the matrix to fibre, and inhibition of new void formation and the SiO₂ grain growth from room to high temperatures. This work provides guidance for predicting the service life of SiC_f/PyC/SiC and SiC_f/BN/SiC composite materials, and is fundamental for establishing multiscale damage models on a local scale.     

Effect of PyC interphase thickness on mechanical and ablation properties of 3D Cf/ZrC–SiC composite

Three-dimensional (3D) C_f/ZrC–SiC composites were successfully prepared by the polymer infiltration and pyrolysis (PIP) process using polycarbosilane (PCS) and a novel ZrC precursor. The effects of PyC interphase of different thicknesses on the mechanical and ablation properties were evaluated. The results indicate that the C_f/ZrC–SiC composites without and with a thin PyC interlayer of 0.15 µm possess much poor flexural strength and fracture toughness. The flexural strength grows with the increase of PyC layer thickness from 0.3 to 1.2 µm. However, the strength starts to decrease with the further increase of the PyC coating thickness to 2.2 µm. The highest flexural strength of 272.3±29.0 MPa and fracture toughness of 10.4±0.7 MPa m^1/2 were achieved for the composites with a 1.2 µm thick PyC coating. Moreover, the use of thicker PyC layer deteriorates the ablation properties of the C_f/ZrC–SiC composites slightly and the ZrO₂ scale acts as an anti-ablation component during the testing.