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.     

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.     

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.     

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.     

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.     

Study on toughening mechanisms of pyrolytic carbon interface layer in Cf/ZrB2-SiC composites using in-situ tensile experimental and numerical methods

The toughening mechanism in continuous fiber toughened ZrB₂-SiC ceramic matrix (C_f/ZrB₂-SiC) composites was studied upon introduction of pyrolytic carbon coating at the fiber/matrix interface. The real-time deformation behavior, surface crack initiation and evolution of C_f/ZrB₂-SiC composites under tensile load were studied using in-situ scanning electron microscopy (SEM) to determine the typical damage modes and toughening mechanisms. A refined microscopic representative volume element (RVE) inserting the cohesive zone elements was established to study the PyC interface layer damage by using finite element method. It was found that PyC interface layer damage induced by thermal residual stress (TRS) was one of critical factors affecting the mechanical performance of the C_f/ZrB₂-SiC composites. The critical thickness of the interface layer was also further determined by analyzing the effect of interface layer thickness on the distribution of TRS, which can guide the design of PyC interface layer for manufacturing the C_f/ZrB₂-SiC composites.     

Mechanical properties of biomimetic ceramic with Bouligand architecture

Biomimetic Bouligand architecture is constructed in the ceramic to improve its toughness. Firstly, unidirectional carbon fiber-reinforced ZrB₂-SiC ceramic films are achieved through a vacuum-assisted filtration method using graphene oxide. Then, ceramic films are helically assembled at a fixed angle of 30° in the graphite die based on the fiber orientation. Finally, the spark plasma sintering method was utilized to densify helical assembly carbon fiber/ceramic films. By constructing Bouligand structure, high fracture toughness (7.4 MPa·m^0.5) and work of fracture (∼1055 J/m²) are achieved in ZrB₂-based ceramic. The toughening mechanisms mainly are crack deflection, twisting and branching, carbon fiber pulling out, and bridging.     

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.     

Effect of multi-walled carbon nanotubes on microstructure and fracture properties of carbon fiber-reinforced ZrB2-based ceramic composite

Multi-walled carbon nanotubes (MWCNTs) were used to optimize the microstructure and improve the fracture properties of hot-pressed carbon fiber-reinforced ZrB₂-based ultra-high temperature ceramic composites. Microstructure analysis indicated that the introduction of MWCNTs effectively reduced the carbon fiber degradation and prevented fiber-matrix interfacial reaction during processing. Due to the presence of MWCNTs, the matrix contained fine ZrB₂ grains and in-situ formed nano-sized SiC/ZrC grains. The fracture properties were evaluated using the single edge-notched beam (SENB) test. The fracture toughness and work of fracture of the C_f/ZrB₂-based composite with MWCNTs were 7.0±0.4 MPa m^1/2 and 379±34 J/m², respectively, representing increases of 59% and 87% compared to those without MWCNTs. The excellent fracture properties are attributed to the moderate interfacial bonding between the fibers and matrix, which favour the toughening mechanisms, such as fiber bridging, fiber pull-out and crack deflection at interfaces.     

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.     

Microstructure and mechanical properties of multiwalled carbon nanotube toughened spark plasma sintered ZrB2 composites

ZrB₂ based composites containing 10 vol.-% carbon nanotubes (CNTs) are synthesised by spark plasma sintering at temperatures ranging from 1600 to 18008C and at an applied pressure of 25?MPa. The effects of sintering temperature on densification behaviour, microstructural evolutions and mechanical properties are presented. Results indicate that ZrB₂-CNT composites fabricated at 16508C have the optimal combination of dense microstructure and properties. The fracture toughness is sensitive to the temperature change and reaches 7.2?MPa m^1/2 for the CNT toughened ZrB₂ ceramics, which is higher than the measured result for monolithic ZrB₂ (3.3?MPa m^1/2). The crack deflection and CNT pullout are the dominant toughening mechanisms.     

Achieving synergy of load-carrying capability and damage tolerance in a ZrB2-SiC composite reinforced through discontinuous carbon fiber

Achieving synergy between load-carrying capability and excellent damage tolerance is one of the most concerned issues in the ultra-high temperature ceramic field. Herein, ZrB₂-SiC-C_f composite with homogenous architecture was constructed by a novel pressure filtration and slurry infiltration accompanied by low-temperature hot pressing. The composite exhibited a typical non-brittle fracture feature owing to crack deflection, crack bifurcation and fiber pull-out mechanisms, resulting in a reliable specific flexural strength of 75 MPa/(g/cm³) and superior work of fracture of 672 J/m².     

Microstructures and mechanical properties of B4C-TiB2-SiC composites fabricated by ball milling and hot pressing

B₄C-TiB₂-SiC composites were fabricated via hot pressing using ball milled B₄C, TiB₂, and SiC powder mixtures as the starting materials. The impact of ball milling on the densification behaviors, mechanical properties, and microstructures of the ceramic composites were investigated. The results showed that the refinement of the powder mixtures and the removal of the oxide impurities played an important role in the improvement of densification and properties. Moreover, the formation of the liquid phases during the sintering was deemed beneficial for densification. The typical values of relative density, hardness, bending strength, and fracture toughness of the composites reached 99.20%, 32.84?GPa, 858?MPa and 8.21?MPa?m^1/2, respectively. Crack deflection, crack bridging, crack branching, and microcracking were considered to be the potential toughening mechanisms in the composites. Furthermore, numerous nano-sized intergranular/intragranular phases and twin structures were observed in the B₄C-TiB₂-SiC composite.     

Synthesis of coatings on SiC fibers and their effects on microstructure and mechanical properties of SiCf/SiBCN composites

In this study, the amorphous C, ZrB₂, and BN single-layer coatings as well as C/BN, C/ZrB₂, ZrB₂/BN, and C/ZrB₂/BN composite coatings were prepared on SiC fibers (SiC_f) by an in situ synthesis and solution impregnation–pyrolysis method. Subsequently, SiC_f/SiBCN composites were fabricated by hot-pressing sintering at 1900℃/60 MPa/30 min to explore the influence of different coatings on the microstructure and mechanical performance of resulting composites. After the preparation of single-layer-coated SiC_f, the SiC_f(BN) or SiC_f(ZrB₂) tended to be overlapped with each other, whereas the dispersion of amorphous C–coated SiC_f was satisfying. Besides, some uneven areas and attached particles have appeared on fiber surfaces of the SiC_f(BN) or SiC_f(ZrB₂), whereas smooth and dense surfaces of amorphous C–coated SiC_f were observed. Because the uniformity of ZrB₂ coatings can be partially damaged by the subsequent coating process of BN, the composite coatings of ZrB₂/BN and C/ZrB₂/BN were thereby not suitable for strengthening SiBCN matrix. The SiC_f/SiBCN composites with C/ZrB₂ coatings have desirable comprehensive mechanical properties. Nevertheless, the conventional toughening mechanisms such as fiber pull-out and bridging, and crack deflection are not available for these composites because the serious crystallization of SiC_f leading to great strength loss, resulting in catastrophic brittle fracture.     

Ablation behaviour of Csf/ZrB2-SiC composites fabricated by direct ink writing and reactive melt infiltration

Pitch-based short carbon fibres reinforced C_sf/ZrB₂-SiC composites were fabricated by direct ink writing of short carbon fibres, followed by slurry impregnation and reactive melt infiltration. Ablation behaviour of the C_sf/ZrB₂-SiC composite was studied by air plasma test. It is indicated that the skeleton of the oriented short carbon fibres provides heat diffusion channels. Consequently, temperatures at the ablation surface are as low as ∼1730 ^oC and ∼2000 ^oC respectively at 4 MW/m² and 5 MW/m². The composite presents outstanding ablation-resistant performance with a linear recession rate of ∼ − 0.04 µm/s and mass recession rate of ∼ − 3.40 mg/s at 4 MW/m², ∼ − 0.17 µm/s and ∼ 3.58 mg/s at 5 MW/m². It is revealed that the fibres area and matrix area of the composite present different ablation mechanisms. The fibres area is ablated severely, while the matrix area presents excellent ablation-resistance with continuous ZrO₂-SiO₂ protective layer.     

The role of TiO2 incorporation in the preparation of B4C/Al laminated composites with high strength and toughness

We prepared B₄C/Al laminated composites via ice-templating and gas-aided pressure infiltration and investigated the effects of TiO₂ addition on the microstructures and mechanical properties of the composites. The incorporation of TiO₂ led to the formation of TiB₂ after sintering, reduced the formation of harmful phases and increased the strength of ceramic architectures. However, its excessive addition resulted in the cracking of ceramic layers and the formation of metal strips after Al infiltration. The bending strength, fracture toughness and work of fracture of the composites first increased and then decreased with increasing initial TiO₂ content, reaching maxima of 420?±?20?MPa, 44?±?2?MPa?m^1/2 and 5002?±?175?J?m^?2, respectively. The specific strength and toughness are comparable to those of titanium alloys. Furthermore, fracture modes and toughening mechanisms were thoroughly addressed by analyzing crack propagation paths and fracture surface morphologies. Crack deflection and metal bridging are two primary extrinsic toughening mechanisms.     

Comparative study of static and cyclic fatigue of ZrB2-SiC ceramic composites

Room temperature static and cyclic fatigue of ZrB₂-32?vol% SiC and ZrB₂-45?vol% SiC particulate ceramic composites has been studied. It was established that the presence of grain bridging plays an important role in the lifetime and time dependent mechanical performance of ZrB₂-SiC composites. It was also established that the cohesive strength of grain boundaries of the composites was a determining factor if grain bridging would occur during crack growth, as the grain boundaries strength would determine the pathway of the moving crack. Grain bridging was limited in ZrB₂-32?vol% SiC leading to the absence of a cyclic fatigue effect, while grain bridging indeed occurred in ZrB₂-45?vol%SiC contributing to a cyclic fatigue effect which limits the lifetime of the composite. Such differences were responsible for the occurrence of R-curve behavior in ZrB₂-SiC ceramic composites.     

Tough salami-inspired Cf/ZrB2 UHTCMCs produced by electrophoretic deposition

One of the biggest challenges of the materials science is the mutual exclusion of strength and toughness. This issue was minimized by mimicking the natural structural materials. To date, few efforts were done regarding materials that should be used in harsh environments. In this work we present novel continuous carbon fiber reinforced ultra-high-temperature ceramic matrix composites (UHTCMCs) for aerospace featuring optimized fiber/matrix interfaces and fibers distribution. The microstructures – produced by electrophoretic deposition of ZrB₂ on unidirectional carbon fibers followed by ZrB₂ infiltration and hot pressing – show a maximum flexural strength and fracture toughness of 330 MPa and 14 MPa m^1/2, respectively. Fracture surfaces are investigated to understand the mechanisms that affect strength and toughness. The EPD technique allows the achievement of a peculiar salami-inspired architecture alternating strong and weak interfaces.     

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.     

Influence of in-situ synthesized Zr-Al-C on microstructure and toughening of ZrB2-SiC composite ceramics fabricated by spark plasma sintering

Zr-Al-C was in-situ synthesized as a toughening component in ZrB₂-SiC ceramics by spark plasma sintering (SPS) ball-milled ZrB₂-based composite powders with SiC and graphite powders. The phase composition of Zr-Al-C toughened ZrB₂-SiC (ZSA) composite ceramics fabricated through the two-step process (ball milling and SPS) did not change dramatically with varying content of Zr-Al-C which shows a major phase of Zr₃Al₄C₆. With increasing Zr-Al-C content, the fracture toughness of the ZSA ceramics initially increased and then decreased when the content reached 40 vol%. The ZSA ceramic with 30 vol% Zr-Al-C exhibited a maximum fracture toughness value of 5.96 ± 0.31 MPa m^1/2, about 22% higher than that of the ZSA ceramic with 10 vol% Zr-Al-C. When the Zr-Al-C content goes beyond 30 vol%, the higher open porosity and component agglomeration led to the relatively lower fracture toughness. Crack deflection and bridging resulted from the weak interface bonding between Zr-Al-C and matrix phases and the weak internal layers of Zr-Al-C crystals, leading to longer crack paths and, hence, the toughened ZSA composite ceramics. Compared to the one-step in-situ synthesis process of Zr-Al-C and the direct incorporation process of synthesized Zr-Al-C grains, the two-step in-situ synthesis process not only led to the more uniform distribution of different components but also resulted in a much larger size of Zr-Al-C grains with a large aspect ratio causing longer crack propagation path as the result of crack deflection and bridging. The larger Zr-Al-C grains combined with the more homogeneous microstructure achieve the most substantial toughening of the ZSA composite ceramics. This work points out a promising approach to control and optimize the microstructure and improve the fracture toughness of ZrB₂-SiC composite ceramics by selecting the incorporation process of compound reinforcement components.