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.     

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.     

Tribological behavior of carbon fiber reinforced ZrB2 based ultra high temperature ceramics

The tribological behavior of ultra-high temperature ceramic matrix composites (UHTCMCs) was investigated to understand these materials in friction applications. Samples consisting of pitch-based randomly orientated chopped carbon fiber (CF) reinforced ZrB₂-10 vol% SiC were prepared (ZS). The tribological behavior was tested on a self-designed dynamometer, coupling the UHTCMC pads with either carbon fiber reinforced carbon−silicon carbide (C/C-SiC) or steel disks, with two applied contact pressures (1 and 3 MPa) and the surface microstructures were analyzed to unravel the wear mechanisms. Even at high mechanical stresses, tests against the C/C-SiC disk showed stable braking performance and wear. The abraded material from a steel disk formed a stable friction film by fusing together harder pad particles with abraded steel, which reduced wear and stabilized the braking performance. The high values of coefficient of friction obtained (0.5–0.7), their stability during the braking and the acceptable wear rate make these materials appealing for automotive brake applications.     

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.     

Effect of ZrB2 powders on densification,microstructure, mechanical properties and thermal conductivity of ZrB2-SiC ceramics

With the view to improve the densification behaviour and mechanical properties of ZrB₂-SiC ceramics, three synthesis routes were investigated for the production of ZrB₂, prior to the fabrication of ZrB₂-20 vol. % SiC via spark plasma sintering (SPS). Two borothermal reduction routes, modified with a water-washing stage (BRW) and partial solid solution of Ti (BRS), were utilised, alongside a boro/carbothermal mechanism (BRCR) were utilised to synthesise ZrB₂, as a precursor material for the production of ZrB₂-SiC. It was determined that reduction in the primary ZrB₂ particle size, alongside a diminished oxygen content, was capable of improving densification. ZrB₂-SiC ceramics, with ZrB₂ derived from BRW synthesis, exhibited a favorable combination of high relative density (98.6%), promoting a marked increase in Vickers hardness (21.4 ± 1.7 GPa) and improved thermal conductivity (68.7 W·m^-1K^-1).     

Anchorage of carbon nanotubes grown on carbon fibres

A novel reinforcing material based on the concept of an uniform 3-dimensional distribution of carbon nanotubes directly grown on yarns of carbon fibres has been developed. This material shows a potential for applications in polymeric matrix composites, combining the properties of carbon nanotubes with those of a traditional reinforcement.In view of the dipping process of the CNT coated fibres into a polymeric matrix, a good anchorage of CNT to the fibre surface is mandatory. Carbon fibres coated with metallic clusters and CNT were immersed into different liquids (deionised water, ethanol, n-butanol, acetone) and processed with different treatments (immersion, magnetic stirring, centrifugation and ultrasonic bath) in order to test their behaviour in different stressing environments. The morphological features of the samples were characterised by SEM both before and after the tests, demonstrating a good adhesion of the three-component material, which was not destroyed even after the most aggressive test.     

Microstructure and mechanical behaviors of 2D-Cf/ZrB2-SiC composites at elevated temperatures

Microstructure and mechanical behaviors of the 2D-C_f/ZrB₂-SiC composites at RT to 1800 ℃ were studied. It is indicated that the interface structure is critical, which affects the matrix phase composition and cracks deflection path in the intra-bundle area. It subsequently determines the load transmitting, cracks propagation behaviors and the mechanical properties of the composites. Flexural strength of the composites increases slightly at 1000 ℃ by the release of residual stress and self-healing of defects. Partial decomposition and crystallization of polycarbosilane (PCS) derived SiC result in the weakening of the SiC matrix at 1500 ℃. On the other hand, residual carbon reacts with ZrO₂ impurity above 1300 ℃. As a result, the matrix turns to porous structure at 1500 ℃, leading to the formation of short micro cracks. At higher temperature of 1800 ℃, quasi-creep mechanical behavior is presented in the composite.     

Effect of ZrB2 content on the densification,microstructure, and mechanical properties of ZrC-SiC ceramics

ZrC ceramics containing 30 vol% SiC-ZrB₂ were produced by high-energy ball milling and reactive hot pressing. The effects of ZrB₂ content on the densification, microstructure, and mechanical properties of ceramics were investigated. Fully dense ceramics were achieved as ZrB₂ content increased to 10 and 15 vol%. The addition of ZrB₂ suppressed grain growth and promoted dispersion of the SiC particles, resulting in fine and homogeneous microstructures. Vickers hardness increased from 23.0 ± 0.5 GPa to 23.9 ± 0.5 GPa and Young’s modulus increased from 430 ± 3 GPa to 455 ± 3 GPa as ZrB₂ content increased from 0 to 15 vol%. The increases were attributed to a combination of the higher relative density of ceramics with higher ZrB₂ content and the higher Young’s modulus and hardness of ZrB₂ compared to ZrC. Indentation fracture toughness increased from 2.6 ± 0.2 MPa⋅m^1/2 to 3.3 ± 0.1 MPa⋅m^1/2 as ZrB₂ content increased from 0 to 15 vol% due to the increase in crack deflection by the uniformly dispersed SiC particles. Compared to binary ZrC-SiC ceramics, ternary ZrC-SiC-ZrB₂ ceramics with finer microstructure and higher relative densities were achieved by the addition of ZrB₂ particles.     

Seal and wear properties of graphite from MCMBs/pitch-based carbon/phenolic-based carbon composites

Mesocarbon microbeads were molded at 100 MPa pressure, and then heat-treated. In order to seek optimum seal effect, the graphite from MCMBs were impregnated with pitch and resins, and then carbonizated at 500 °C The resulting graphite from MCMBs/pitch-based carbon/phenolic-based carbon composites were evaluated in terms of physical properties, textures and the seal properties to determine their potential of using this material in liquid rocket motor. Results indicated that the bending strength of the composites were elevated and the leakage of 60 Pa/min was obtained after treated by cycles of impregnation/carbonization with pitch and phenolic resins.     

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.     

Effect of carbon content on microstructure and mechanical properties of WC-10Co cemented carbides with plate-like WC grain

Four sets of WC-10Co cemented carbides with different carbon content were prepared by adding the ultrafine WC powders as seeds during the in-situ sintering reaction among W, Co and C. The effect of carbon content on microstructure and mechanical properties were studied. The results show that the microstructure, phase composition and mechanical properties of WC-10Co cemented carbides with plate-like WC grains were seriously affected by the carbon content. The fast growth of WC grains with high carbon content could proceed the prismatic plane preferentially along the <1 0 $1(-)$ 0> directions, resulting in the high content of plate-like WC grains. The density increased with the increment of the carbon content and reached the maximum value, then, followed by a decline. The hardness and the transverse rupture strength of the alloy in the two-phase zone with carbon content of 5.91 wt% reached the maximum value. The existence of plate-like WC grains could impede the propagation of the cracks due to the decrease of the weakest carbide regions and the increase of the basal facets of broken WC crystals. In this case, more fracture energy was required to crack propagation and further improved the transverse rupture strength. Additionally, the plate-like WC was benefit to reduce the wear volume and bring about a better wear resistance. Thus, the alloy with the appropriate proportion of carbon content can obtain higher mechanical properties and wear resistance.     

Preparation and properties of PAN-based carbon fiber-reinforced SiCO aerogel composites

To overcome the brittleness issue of SiCO aerogels, the polyacrylonitrile-based (PAN) carbon fiber was impregnated with SiCO sol to obtain carbon fiber-reinforced SiCO aerogel composites (C/SiCO). SiCO sol was prepared through an acid-alkaline two-step catalysis by using methyltrimethoxysilane (MTMS) and dimethyldiethoxysilane (DMDES) as precursors. C/SiCO-1, C/SiCO-2 and C/SiCO-3 were obtained after repeated impregnation of the SiCO sol and gelating, aging, supercritical drying and pyrolyzing one to three times. SEM images show that the SiCO aerogel fills the pores between the carbon fibers, and the nanoporous structure of the SiCO aerogel can effectively improve the thermal insulation of the composites. As the times of impregnation of the SiCO sol increased, the mechanical properties and oxidation resistance of C/SiCO have been improved significantly. The bending strength of C/SiCO-3 was 32.52 MPa, and the compressive strength (25%ε) was 51.98 MPa. After heating at 1600 °C, the linear shrinkage in the thickness direction of C/SiCO-1 was 20.72%, while that of C/SiCO-3 was only 1.85%. A dense SiO₂ molten oxide film formed on the surface of C/SiCO at high temperature, and its extremely low oxygen permeability effectively protected the inside of the composites.     

Oxygen plasma modification of pitch-based isotropic carbon fibres

An isotropic carbon fibre was surface-treated by microwave oxygen plasma at different conditions and characterised by scanning electron microscopy (SEM), scanning tunneling microscopy (STM), N₂/CO₂ adsorption, Raman spectrometry, X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD). It is shown that the structure of the fibre suffers only limited alterations upon plasma treatment in such a way that the local disorder on its surface, which was already large in the fresh material, barely increases after the plasma exposure, as detected by Raman measurements. At the nanometre scale, STM images revealed a moderate increase in surface roughness. Evidence for chemical changes undergone by the fibre following the etching was provided by XPS and TPD, showing that stable oxygen functionalities were introduced by the plasma exposure, a result of practical importance for the application of this treatment not only to this type of carbon fibre, but to carbon materials in general. It was also observed that very gentle plasma exposures were generally sufficient to provide the fibre surface with a large amount of oxygen functional groups and that more intense treatments had a negative effect in this respect (i.e. they were not able to supply oxygen to the surface in larger amounts than the softer treatments did).     

Effect of carbonization temperature on the structure and microwave absorbing properties of hollow carbon fibres

A series of polyacrylonitrile-based hollow carbon fibres (PAN-HCFs) were prepared by carbonizing polyacrylonitrile (PAN) hollow cured fibres at temperature ranging from 550 to 950 °C for 1 h in nitrogen. The effects of carbonization temperature on the structure, elemental compositions, surface electrical conductivity, electromagnetic parameters and reflectivity of PAN-HCFs were investigated. Results indicate that the obtained PAN-HCFs have not been graphitized and the C content and surface electrical conductivity increases as the carbonization temperature increases. The reflectivity of composites of PAN-HCFs and resin is −7.50 dB at 6.06 GHz and the band of the reflectivity under −5 dB is 6 GHz when the carbonization temperature is 750 °C.     

Effect of brazing parameters on microstructure and mechanical properties of Cf/SiC and Nb-1Zr joints brazed with Ti-Co-Nb filler alloy

Reliable brazing of carbon fiber reinforced SiC (C_f/SiC) composite to Nb-1Zr alloy was achieved by adopting a novel Ti45Co45Nb10 (at.%) filler alloy. The effects of brazing temperature (1270–1320 °C) and holding time (5–30 min) on the microstructure and mechanical properties of the joints were investigated. The results show that a continuous reaction layer (Ti,Nb)C was formed at the C_f/SiC/braze interface. A TiCo and Nb(s,s) eutectic structure was observed in the brazing seam, in which some CoNb₄Si phases were distributed. By increasing the brazing temperature or extending the holding time, the reaction layer became thicker and the amount of the CoNb₄Si increased. The optimized average shear strength of 242 MPa was obtained when the joints were brazed at 1280 °C for 10 min. The high temperature shear strength of the joints reached 202 MPa and 135 MPa at 800 °C and 1000 °C, respectively.     

Improved thermal conductivity and mechanical properties of SiC/SiC composites using pitch-based carbon fibers

Pitch-based carbon fibers were assembled in horizontal and thickness directions of SiC/SiC composites to form three-dimensional heat conduction networks. The effects of heat conduction networks on microstructures, mechanics, and thermal conductivities were investigated. The results revealed the benefit of introducing heat conduction networks in the densification of composites. The maximum bending strength and interlaminar shear strength of the modified composites reached 568.67 MPa and 68.48 MPa, respectively. These values were equivalent to 18.6% and 69.4% increase compared to those of composites without channels. However, channels in thickness direction destroyed the continuity of fibers and matrix, creating numerous defects. As the volume fraction of heat conduction channels rose, the pinning strengthening effect of channels and influence of defects competed with each other to result in first enhanced mechanical properties followed by a decline. The in-plane thermal conductivity was found anisotropic with a maximum value reaching 86.20 W/(m·K) after introducing pitch-based carbon unidirectional tapes. The thermal conductivity in thickness direction increased with volume fraction of pitch-based carbon fibers and reached 19.13 W/(m·K) at 3.87 vol% pitch-based carbon fibers in the thickness direction. This value was 90.75% higher than that of composites without channels.     

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.     

Mechanical properties of high-strength carbon fibres. Validation of an end-effect model for describing experimental data

The present contribution deals with the use of different models accounting for the mechanical response of high-strength (HT) carbon fibres. In particular, analytical models based on Weibull type of statistical distributions will be employed to analyse the dependence of the strength of carbon fibres on length. This dependence is relevant for interpreting the results of conventional fragmentation tests, which are traditionally performed to characterise the level of fibre/matrix adhesion in fibre reinforced composites. The objective of this work is to compare alternative models, such as the so-called “end-effect” model, for determining the tensile strength of HT carbon fibres at small gauge lengths. To validate these models, tensile tests were performed with five different HT, ex-PAN carbon fibres: a fresh, untreated sample, and four samples which were prepared after submitting the previous one to various surface treatments. Specifically, plasma oxidation was carried out under three different conditions of power and/or time of exposure. A sample oxidized by the manufacturer, presumably using an electrochemical treatment, was also included. Results showed that the end-effect model represents well the behaviour of the untreated and plasma treated under the mildest conditions carbon fibres. Overall, all treatments tend to decrease the tensile strength of the fibres, with the commercial treatment being the most damaging when compared to any of the plasma treatments carried out.     

Effect of short carbon fibre concentration on microstructure and mechanical properties of TiCN-based cermets

Short carbon fibre (C_f) reinforced TiCN-based cermets (C_f/TiCN composites) were produced by powder metallurgy method with pressureless sintering technology. The phase evolution, microstructure and fracture morphology of C_f/TiCN composites were investigated. The results showed that TiC, TiN, WC, Cr₃C₂ and Mo phases disappeared gradually and diffused into core and rim phases by dissolution–reprecipitation process, finally formed new hard TiCN core phases and complex compound (Cr, W, Mo, Ti)(CN) rim phases, with the sintering temperature increasing. The added C_f did not change the ‘core–rim’ microstructure but improved the mechanical properties of TiCN-based cermets. The C_f/TiCN composite containing 3?wt-% C_f achieved the best comprehensive mechanical properties, with fracture toughness and bending strength increasing by about 14.4% and 30.8%, respectively, when compared with the composite without C_f. Toughening and strengthening mechanisms of C_f/TiCN composite were concluded as crack deflection and branch, as well as the pull-out, fracture and bridging of carbon fibres.     

Preparation and properties of high-porosity ZrB2-SiC ceramics by water-based freeze casting

The development of novel cermet composites based on porous ceramics with high porosity, interconnected pore structure and good mechanical property has attracted considerable attention in engineering application. In this work, water-based freeze casting process was employed to fabricate ZrB₂-SiC porous ceramic with aligned lamellar-channels structure using PAA-NH₄ as the dispersant. The results revealed that the well-dispersed suspension with best rheological behavior was obtained using 1.0 wt% PAA-NH₄ at pH 9. The crack-free porous ceramic exhibited small volume shrinkage ranging from 2.59 % to 1.87 %. By varying the solid loading, the fabricated samples displayed a tailored porosity ranging from 76.12% to 59.37% and an excellent compressive strength of 7 MPa to 78 MPa. After oxidation, the samples displayed a decreased porosity and an increased compressive strength. The ZrB₂­SiC porous ceramic fabricated in this work will be a promising candidate for the framework of cermet composite.