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To further improve the oxidation resistance of PDC (Polymer Derived Ceramic) composites in harsh environments, Cf/SiC/SiHfBOC composites were prepared by CVI (Chemical Vapor Infiltration) and PIP (Precursor Impregnation Pyrolysis) methods. The weight retention change, mechanical properties, and microstructure of Cf/SiC/SiHfBOC before and after oxidation in air were studied in detail. Microscopic analysis showed that only the interface between the ceramic and fibers were oxidized to some extent and hafnium had been enriched on the composite surface, after oxidation at different oxidation environments. After Cf/SiC/SiHfBOC composites oxidized at 1500 ℃ for 60 min, it was mainly determined by the HfO2 and HfSiO4 phase. Moreover, the weight retention ratio and compressive strength of the Cf/SiC/SiHfBOC composites are 83.97 % and 23.88 ± 3.11 MPa, respectively. It indicates that the Cf/SiC/SiHfBOC composites can be used for a long time in the oxidation environment at 1500 ℃.

Herein, a hydrothermal carbonization coating/SiO2 (HTCC/SiO2) composite coating on 3D needled carbon fibers (Cf/HTCC-SiO2) was successfully designed and constructed through a combined hydrothermal carbonization and sol-gel process for the first time. The results revealed that the microstructure and morphology of HTCC and SiO2 coatings strongly depended on the key process parameters in the synthetic process. The optimal conditions for fabrication of Cf/HTCC-SiO2 were achieved as following: the hydrothermal temperature was 180 °C and holding time was 2 h; the TEOS mass fraction was 30 wt%, the distilled water/ethanol molar ratio was 6:1 and sol-gel time was 2 h, respectively. The relatively continuous and uniform HTCC/SiO2 composite coating with a single thickness about 350 and 300 nm effectively improved the mechanical and anti-oxidation of 3D needled Cf. Compared with the uncoated 3D needled Cf, the compressive strength was increased about 336% and the rapid oxidation temperature was improved about 100 °C after the incorporation of HTCC/SiO2 composite coating.

Tremendous efforts have been dedicated to developing sorbents for water remediation due to their high efficiency and non-secondary pollution. However, the majority of sorbents still face the challenges of complex processing, low mechanical strength and volume absorption. Hence, the functional hydrothermal carbonization coatings (HTCCs) were prepared on carbon fibers in carbon fiber braid via a facile hydrothermal carbonization process of widely sourced carbohydrate to obtain a robust sorbent, which possessed the controllable microstructure and composition for various requirements of water remediation. The gradient surface structure of carbon fiber braid with interior smooth coatings carbon fibers and exterior rough surface could be fabricated at pH value of 1. The HTCCs-carbon fiber braid had superior yield strength and compressive strength. By regulating the reaction process, the yield strength could range from 0.044 MPa to 0.235 MPa and the max compressive strength change from 0.198 MPa to 1.113 MPa. The HTCCs-carbon fiber braid showed excellent adsorption for Rhodamine B with a high removal degree of 98.5%, which kept more than 90% even after 10 squeezing adsorption cycles. The HTCCs-carbon fiber braid could be adjusted to effectively absorb oil pollutants from water by a facile heat treatment. After heat treatment, the HTCCs-carbon fiber braid exhibited excellent volume absorption capacity for contaminants, which could change from 83.9% to 88.5%. Thus, the HTCCs-carbon fiber braid prepared by a green, high-efficiency and low-cost process has great potential for sorption multiple contaminations in water by virtue of the combination of controllable carbonaceous coatings and robust carbon fiber braid.

A high-entropy (TiZrNbTaMo)C ceramic has been successfully fabricated by hot pressing the newly-synthesized quinary carbide powder to investigate its microstructure and mechanical properties. The carbothermal reduction process of equimolar quinary metallic oxides at 1500 °C for 1 h generates a carbide powder mixture, which consists mainly of TaC- and ZrC-based solid solutions. The as-synthesized powder was then sintered to form a single-phase high-entropy ceramic by a two-step hot pressing at 1850 °C for 1 h and 2100 °C for 0.5 h, respectively. The high-entropy ceramic exhibits a fine grain size of about 8.8 μm, a high compositional uniformity and a high relative density of 98.6% by adding Mo as the strategic main component. The measured nanohardness values of (TiZrNbTaMo)C ceramic are 25.3 GPa at 9.8 N and 31.3 GPa at 100 mN, respectively, which are clearly higher than those of other available high-entropy carbide ceramics.

SiOC ceramic modified carbon fiber needled felt preform composites (Cf/SiOC) with densities of 0.4 and 0.7 g/cm3 were prepared by precursor infiltration and pyrolysis (PIP) method using methyltrimethoxysilane (MTMS) and dimethyldimethoxysilane (DMDMS) as precursors. The densification behavior was investigated through scanning electron microscopy (SEM) analysis of microstructure of Cf/SiOC composites undergoing different PIP times. The results indicate that with increase of PIP times, a great amount of SiOC ceramic was introduced into the preform, completely covering on the carbon fibers and occupying the open pores. The thermal performance, mechanical properties, and oxidation resistance of the composites were studied via various tests. The results illustrate that after two-time PIP procedure, thermal conductivities of the composites are 0.41–2.54 and 1.28–4.04 W/(m·K) in z direction and x/y plane, respectively, at RT-1500 °C. The compressive strengths of the composite arrive at 2.1 MPa in z direction and 7.8 MPa in x/y plane, which are almost 3.5 times and 6.5 times, respectively, counterparts of the raw preform. The incorporation of SiOC ceramic can remarkably improve anti-oxidation ability of the composites at 600 °C. The oxidation weight loss is merely 2.1 wt% after 60-min oxidation at 600 °C.

(Ti, W)C–W composites have been successfully synthesized by in-situ precipitation of multiscale W-rich particles in (Ti, W)C solid solution incorporated with TiH 2 as inducer. The microstructure of (Ti, W)C–W composites is characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The variations in lattice parameter of (Ti, W)C solid solution with increasing the sintering temperature are dependent upon the in-situ precipitation process of W-rich phase. The low temperature sintering route is beneficial to the precipitation of W-rich phase due to high concentration gradient in (Ti, W)C solid solution. The intragranular W-rich precipitates in the (Ti, W)C matrix exhibit semi-coherent interface and an orientation relationship of [111] W //[011] (Ti, W)C . The in-situ precipitation of multiscale W-rich particles have a positive influence on the fracture toughness, and the value of (Ti, W)C–W composite obtained at 1700 °C is 4.23 MPa m 1/2 .

Axially aligned fibrous monolithic ceramics present non-catastrophic fracture behavior via crack deflection and delamination along cell boundaries. However, severe in-plane anisotropy and time-consuming preparation procedures prevent their extensive promotion. The introduction of high content of weak phase components with poor oxidation resistance in weak interface destroys the excellent oxidation resistance of ceramic matrix. In this work, ZrB2-based short fibrous monolithic (SFM) ceramics with in-plane isotropic mechanical properties and excellent oxidation resistance were easily prepared by hot pressing randomly assembled short ceramic fibers. The microstructure and mechanical behavior of ZrB2-based SFM ceramics densified at various temperatures were systematically investigated. The mechanical properties of ZrB2-based SFM ceramics slightly improved with the increase of sintering temperature. ZrB2-based SFM ceramics exhibited excellent oxidation resistance and remained intact without macroscopic cracks after ablation for 615 s in oxyacetylene flame with maximum temperatures exceeding 2150 °C. The oxidation behavior was analyzed in detail.

The Cf/ZrB2-SiC composite was successfully fabricated via brushing nano ZrB2-SiC slurry followed by low-temperature hot pressing technique, in which the UHTC phases were uniformly introduced into intrafascicular space of the subdivided fiber bundles. The fabricated composite exhibited a non-brittle fracture behavior and excellent mechanical properties with a high work of fracture of 1338 J/m2, one order of magnitude higher than that of the ZrB2-SiC ceramic, which was attributed to the hybrid toughening mechanisms such as crack deflection, fiber bridging, fiber pull-out and crack branching. The fiber/matrix interfaces were analyzed by the high resolution transmission electron microscopy, and no obvious chemical reaction was detected, suggesting the degradation of carbon fiber was effectively inhibited. Moreover, the Cf/ZrB2-SiC composite showed a good thermal shock resistance with a thermal shock temperature difference of 764 °C, almost twice those of the ZrB2-SiC ceramic. This work provides a facile and effective approach to fabricate high-performance continuous carbon fiber reinforced ceramic composites with uniform component distribution.

Dense TiC-ZrC-Cr3C2 composites with various TiC content from 19.6 mol% to 78.4 mol% have been fabricated by hot-pressing sintering at 1950 °C using 2.0 mol% Cr3C2 as sintering aid. The effect of TiC content on the microstructure, mechanical and thermal properties of TiC-ZrC-Cr3C2 composites are investigated systematically. The single (Zr, Ti, Cr)C solid solution is obtained when TiC content is 19.6 mol%, while with increasing TiC content, the composites begin to consist of Zr-rich (Zr, Ti)C solid solution and Ti-rich (Ti, Zr, Cr)C solid solution phase. SEM and EDS analysis confirm that Cr element is not favorable to diffuse into ZrC lattice to form (Zr, Cr)C solid solution. Flexural strength and Vickers hardness increase gradually with increasing TiC content, but fracture toughness does not improve significantly. Fracture toughness are in the range of 3.34–4.01 MPa∙m1/2 for all composites, and the optimum value reaches 4.01 MPa·m1/2 with 49.0 mol% TiC. Experimental results of the thermal expansion coefficient reveal that the addition of TiC raises the thermal expansivity of TiC-ZrC-Cr3C2 composites. Noticeably, the thermal conductivities of TiC-ZrC-Cr3C2 composites show a decrement trend with increasing TiC content, not as theoretical predicting by the rule of mixtures. For instance, the thermal conductivity at 25 °C ranges from 18.0 W/m∙K for 8Z2T2C composite down to 10.6 W/m∙K for 2Z8T2C composite.

Novel quenching-partitioning-tempering (Q-P-T) and traditional quenching-tempering (Q-T) processes are conducted for M50 ultra-high strength steel. An excellent combination of strength and toughness is obtained via the Q-P-T process. The highest impact absorption energy of the Q-P-T specimen increases by 93% compared to the Q-T specimen. However, the yield and ultimate tensile strength of Q-P-T specimens are almost the same as those of Q-T specimens. Microstructure analysis indicates that the maximum fraction of retained austenite in the Q-P-T specimen is 7.5%, which is much higher than that in the Q-T (2.0%) specimen. Meanwhile, more fine cracks appear in the fracture Q-P-T specimen. The precipitation of fine and diffuse ω phase, Fe3C and Cr23C6 is observed during the partitioning process. The present work also reveals that the Q-P-T/Q-P process can be applied to high‑carbon steel with little Si or Al.

The damage mechanism of carbon fiber and fiber-ceramic interface of C f /ZrC-SiC were investigated and revealed. The chemical reactions of C with ZrO 2 and SiO 2 impurities during the sintering resulted in the surface and grain etching of fiber. The ZrC-SiC ceramics diffused into interface to form a strong bonding under high temperature and press, leading to a failure of stress release and guiding the crack propagation direct across fiber. Both consequently impeded the fiber pull-out and bridging. By obvious contrast, the chemical etching of fiber was effectively inhibited utilizing PyC coating as transition layer, and the strong bonding of fiber with ceramic was relieved at the same time replaced by a relatively weak bonding. As a result, the fiber pull-out and fiber bridging were boosted with a longer length more than 50 µm, over 2 times of uncoated composite, accompanied with obvious crack branching and deflection, together resulting in a graceful mechanical feedback with 2.5 times of work of fracture and improved fracture toughness. This work not only revealed the microstructure forms and inherent mechanism on structure damage of carbon fiber, but also offered an effective method of protecting fiber and tailoring the interface bonding, thereby guiding the design of C f /ZrC-SiC with graceful mechanical performance.

A chopped carbon fibre (CF) reinforced silica-phenolic resin (Si/PR) aerogel nanocomposite was prepared through a simple one-pot sol-gel polymerization in a slurry of CF, PR, silane, hexamethylenetetramine and ethylene glycol. CFs were distributed homogeneously and randomly in the Si/PR aerogels. The Si/PR aerogels exhibit finer microstructure, higher thermal stability and better anti-oxidation resistance than PR aerogels. The compressive strength for the composites with low densities between 0.402 and 0.463 g/cm3 ranged from 0.33 to 2.44 MPa and thermal conductivities from 0.089 to 0.116 W/(mK), respectively. Furthermore, the CF/Si/PR aerogel nanocomposites could deliver linear ablation rates as low as 0.117 mm/s, and internal temperature peaks approximately 100 °C at 38 mm in-depth position as the surface temperature exceeded 1800 °C in oxyacetylene flame. From mentioned above, this lightweight composite presents huge application prospects in thermal protection and heat insulation field, especially in aerospace industry.

Crack initiation and propagation of ZrB2-SiC-Graphite (ZSG) sharp leading edges (SLEs) subjected to thermal shock were systematically evaluated by the water spraying method followed by a crack dyeing treatment. Distinct differences in the crack patterns among different test conditions were observed, and the cracking behavior of ZSG SLEs (including crack initiation time, crack number and critical failure temperature) was revealed to be strongly dependent on both the cooling rate and the microstructure. The crack propagation during thermal shock could be considered as a quasistatic process (crack speed was lower than 1 cm/s) that needed to be driven by continuous cooling.

Reliable evaluation of the thermal shock behavior of complex-shaped components is very important for ultra-high temperature ceramics. Unfortunately progress in this field is hampered by a lack of available experimental methods. Here, a novel water spraying method is proposed to investigate the thermal shock behavior of ZrB 2 -based sharp leading edges (SLEs). Results indicated that the introduction of graphite flakes significantly improved the thermal shock resistance of ZrB 2 -SiC (ZS) SLEs. However, owing to the anisotropies of microstructure and thermal conductivity of ZrB 2 -SiC-G (ZSG) ceramic, the thermal shock resistance and the failure mode of ZSG SLEs were sensitive to the sampling orientation, which also were remarkably influenced by the size and/or shape of SLEs, leading to the variability of critical failure temperature from 750 °C to 375 °C and crack patterns from deflection to wave-shaped.

Excellent microwave absorption properties of porous SiC ceramics were successfully synthesized using SiC/camphene slurries with various polycarbosilane (PCS) contents related to the SiC powder. The compositions of the nanowires (NWs) growth in the pore channels of porous SiC ceramics strongly depended on the pyrolysis atmosphere, with N2-generating Si3N4 NWs and Ar SiC NWs. With the increase of PCS content, the minimum reflection coefficient (RC) of porous SiC ceramics decreased from −7.6 dB to −67.4 dB in Ar and from −10.9 dB to −24.7 dB in N2, respectively. The effective absorption bandwidth (EAB) of porous SiC ceramics could be up to 8.1 GHz in Ar and 4.5 GHz in N2. The enhanced microwave absorption properties of porous SiC ceramics could be attributed to the formation of SiC nano-crystalline, nanosized carbon and the NWs, which would increase the amount of boundaries and defects, leading to the electronic dipole polarization and interfacial scattering.

ZrB 2 -SiC-C f composite with carbon coated carbon fibers was successfully prepared by low temperature hot pressing. This composite showed an excellent thermal shock resistance compared with those composites fabricated by hot pressing and spark plasma sintering using as-received carbon fibers. Toughening mechanisms such as crack deflection, crack branching, fibers pull-out and fibers bridging were obviously detected in ZrB 2 -SiC-C f owing to the inhibited degradation of carbon fibers and the relatively weak fiber/matrix interfacial bonding, leading to a non-brittle fracture mode of the composite. Moreover, the composite exhibited an excellent thermal shock resistance with a high critical thermal shock temperature difference of 773 °C which is about twice those of the reported ZrB 2 -based ultra-high temperature ceramics. This work provides valuable guidance in the preparation of ZrB 2 -based ultra-high temperature ceramics with a combination of excellent properties.

ZrB2–SiC–Cf composites containing 20–50 vol% short carbon fibers were hot pressed at low sintering temperature (1450 °C) using nanosized ZrB2 powders, in which the fiber degradation was effectively inhibited. The strain-to-failure values of such composites increased with increasing fiber content, and the value for the composite with 50 vol% Cf was even more than 3 times higher than that of the composite with 20 vol% Cf. Furthermore, the composite exhibited non-brittle fracture mode when the fiber content was above 30 vol%, and the thermal shock critical temperature difference of the composite with 30 vol% Cf was up to 727 °C, revealing excellent thermal shock resistance of this composite. Additionally, ZrB2–SiC–Cf composites displayed good oxidation resistance when the fiber content was below 40 vol%, suggesting that this method provides a promising way for preparation of high-performance ZrB2–SiC–Cf composites at low temperature.

ZrB2 grains were prepared on graphite by chemical vapor deposition using a gas mixture of ZrCl4, BCl3, H2 and Ar at 1150 °C. The microstructure and phase composition of the coating were investigated by SEM, XRD, XPS and TEM. The results indicated that ZrB2 grains were deposited and grown directly by a vapor–solid growth mechanism on graphite without intermediate phase. Granulated ZrB2 grains were composed by many sub–grains with different crystal orientation. The rod–like or pagoda–like ZrB2 grains with lamellar structures were grew on the granulated ZrB2 particles and composed of two parts: the internal part with axial stacking faults and annular part with radial stacking faults.

Multiscale hybrid method is reported to enhance thermal ablative and insulative properties of lightweight ablative materials in high temperature oxidation extreme environment. A novel lightweight hybrid carbon-quartz fiber fabric reinforced phenolic-silica (C-QF/PSi) aerogel nanocomposite was fabricated through impregnation using C-QF hybrid needled felt as three-dimensional reinforcement and PSi hybrid aerogel as matrix. The prepared aerogel nanocomposite perfectly inherits their nanoporous microstructure and fascinating properties, resulting in remarkable high compressive strength (5.96–17.01 MPa), low thermal conductivity (~0.112 W/(mK)). In particular, good thermal ablative and insulative properties in oxy-acetylene oxidizing flame (linear ablation rate as low as 0.017 mm/s and internal temperature peaks below 100 °C at 80 mm in-depth position when the surface temperature exceeds 2000 °C). From mentioned above, this lightweight composite presents huge application prospects in thermal protection and heat insulation field, especially in aerospace industry.

Porous ZrB 2 –SiC (ZS) ceramics with different volume fractions of porosity, i.e. 8.7%, 17.9% and 27.3% and different mean pore diameter, i.e. 2.21, 3.65 and 4.23 μm, were fabricated using partial hot pressing, in order to investigate the effect of the porosity and pore size on their mechanical and thermal properties as well as thermal shock performance. Particular emphasis was placed in the study on thermal shock fracture resistance. The critical temperature difference, Δ T c , of porous ZS herein was found to increase slightly with increasing porosity. This phenomenon is inconsistent with most previous researches on other material systems. However, it was well explained from the viewpoints of both physical properties and microstructures of the material, which provides new perspectives and a deeper understanding of how porosity and pore-size affect Δ T c . The Δ T c was also observed to increase as pore size decrease. It could tailor the thermal shock resistance of ZS ceramic and obtain porous ZS ceramic with both high thermal shock fracture resistance and high thermal shock damage resistance, using the above porosity and pore-size dependence of physical properties.

Several millimeters long SiC–SiOx nanowires with the diameters of 50–300 nm were successfully synthesized by carbothermal reduction using the raw materials of carbon black, silica sol and alumina at 1400 °C. The morphology and microstructure of nanowires have been studied in detail by X-ray diffraction (XRD), scanning electron microscopy (SEM), electron energy scattering (EDX), Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The nanowires were mainly in a straight wire-like morphology and consisted of 3C-SiC. Alumina played an important role in regulating the content of SiO2 in the reaction to produce SiO and keep nanowires subsequently growth along one-dimensional direction, and then an alumina-assisted growth of vapor–solid (VS) mechanism was proposed for the growth mechanism of several millimeters long SiC–SiOx nanowires.

Carbon nanosheet–coated micron zirconium diboride particles were achieved by an atmospheric chemical vapor deposition in the presence of methane gas without catalyst. The microstructures and carbon structure for the novel particles were characterized by scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. In addition, the growth process of hybrid particles was analyzed by thermodynamic theory. The results showed that zirconium diboride particles were coated by carbon nanosheets uniformly. The carbon nanosheets/zirconium diboride particles were employed to fabricate ceramic composite using spark plasma sintering process. The fracture toughness of the composite was improved up to 5.9 MPa·m0.5 with only 0.9 wt% of carbon nanosheets, which was 40.9% higher than that of the composite without carbon nanosheets. The toughening mechanisms were mainly known as carbon nanosheets crack bridging and pulling out which lead to the formation of crack deflection in the novel composite.

Lightweight carbon-bonded carbon fiber (CBCF) composites were fabricated with chopped carbon fibers and dilute phenolic resin solution by pressure filtration, followed by carbonization at 1000 °C in argon. The as-prepared CBCF composites had a homogenous fiber network distribution in xy direction and quasi-layered structure in z direction. The pyrolytic carbon derived from phenolic resin was mainly accumulated at the intersections and surfaces of chopped carbon fibers. The composites possessed compressive strengths ranged from 0.93–6.63 MPa in xy direction to 0.30–2.01 MPa in z direction with a density of 0.162–0.381 g cm−3. The thermal conductivity increased from 0.314–0.505 to 0.139–0.368 Wm−1 K−1 in xy and z directions, respectively. The experimental results indicate that the CBCF composites prepared by this technique can significantly contribute to improve the thermal insulation and mechanical properties at high temperature. Keywords: Carbon fiber, Phenolic resin, Microstructure, Compressive strength, Thermal properties

An original catalytic method has been proposed to synthesize carbon nanotubes (CNTs)–ZrB2–ZrO2 heterostructures using ZrB2 polymeric precursor. The pyrolysized gases from the ZrB2 polymeric precursor are identified to be the carbon sources for CNTs growth. A parametric study is conducted to control the CNTs growth by optimizing parameters such as synthesis temperature and catalyst content. Observations show that the in situ grown CNTs are homogeneously dispersed in the powders, and the structure and the amount of CNTs are significantly dependent on the synthesis parameters. There are two kinds of grown CNTs existed in the produced hybrid heterostructures: (i) the kinking structured CNTs that are disordered and incomplete graphitization; (ii) the improved and graphitized CNTs. The ZrB2 polymeric precursor during thermal pyrolysis provides capable of supplying substantial carbon source for CNTs nucleation and growth by homogeneous vapor–liquid–solid reactions.

ZrB 2 –SiC coatings were prepared on the SiC coated specimens by pack cementation method at 1900 °C for 2 h. The phase, microstructure, anti-oxidation and thermal shock resistance properties of these coatings were investigated. It was found that ZrB 2 –SiC coatings improved anti-oxidation and thermal shock resistance. ZS50 coating endured 15 thermal shock cycles between 1500 °C and room temperature with 20.2% weight loss, which was attributed to the dense glass phase formed after oxidation.

The temperature response of ZrB 2 –20 vol% SiC composites with different surface microstructures of ZrB 2 /SiC (ZS1), silica-rich glass (ZS2) and porous zirconia (ZS3) was investigated using an oxy-acetylene torch. The three ZrB 2 –SiC composites showed a large difference in their surface temperatures during ablation, resulting from the different surface component and microstructure. ZS1 and ZS2 exhibited low temperature and good ablation resistance owing to the protection from glass on surface. However, ZS3 displayed high surface temperature (>2300 °C) and underwent remarkable mass loss under the same ablation condition that could be attributed to the combination of oxidation, radiation and heat conduction associated with the porous zirconia exposing on surface.

Ultra-high temperature ceramics (UHTCs) are composed of ZrC, ZrB2, HfC, HfB2, HfN and TaC and their melting points are higher than 3000℃. They are a vital class of high temperature structural materials and have attracted a great deal of attention in the past two decades in fundamental research and in technological applications. The ZrB2-SiC and HfB2-SiC UHTC composites, in particular, have excellent oxidation/ablation resistance, moderate thermal shock resistance and high strength retention at elevated temperature. They can survive oxidizing environments at temperatures of 2000℃ and higher. These properties make them the most promising materials for use in extreme environments such as hypersonic flight, atmospheric re-entry, and rocket propulsion. ZrB2-based and HfB2-based UHTCs have similar oxidation and ablation behaviors however as ZrB2-based UHTCs are lighter and less expensive, they are used as aerospace materials. Here we provide a comprehensive review of UHTC composites including their preparation, mechanical properties, thermal shock resistance, oxidation/ablation properties and thermal response, with a particular focus on ZrB2-based UHTCs. UHTCs were generally fabricated by hot pressing, spark plasma sintering, pressureless sintering, reactive hot pressing and sinter forging. Hot pressing is the dominant densification technique for the preparation of UHTCs. The flexural strength of UHTCs decreases with an increase in grain size and it exhibits a strong correlation with the size of the SiC particulates. However, the fracture toughness trend is not consistent with that of strength and therefore the matching of grain size becomes important. UHTCs display plasticity at elevated temperature and their brittle-to-ductile transition temperature (1500℃) strongly depends on the grain size and the purity of the grain boundaries. Catastrophic failure occurs easily during the heating or cooling process. The thermal shock resistance of UHTCs is a major issue for future applications. The methods used to improve the thermal shock resistance of UHTCs are summarized from critical thermal shock temperature and strength retention rate after thermal shock testing points of view. The oxidation behavior of UHTCs significantly depends on the temperature and the temperature limits of the applications of these materials are analyzed. The effect of the additives on the overall performance of the resultant composites as well as the relationship between material composition, microstructure and performance are also discussed in detail. This provides useful insights into effective design principles to optimize the overall structural performance of ultra-high temperature ceramic composites according to special service environments. The remaining challenges and future outlook of this field are also addressed.

Ultra-long SiC nanowires with lengths ranging from several millimeters to one centimeter were successfully prepared from graphite, silicon, silica and alumina raw materials via a simple carbon thermal reduction method in a tube furnace at 1300 °C. Scanning electron microscopy (SEM), electron energy scattering (EDX), X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and Fourier transform infrared spectroscopy (FTIR) were employed to characterize the morphology and microstructure of the obtained products. The results showed that the nanowires mostly consisted of 3C–SiC and exhibited mainly a straight-line shape with diameters in the range of 30–150 nm. Alumina may be a novel and highly effective mediator playing an important role in controlling the concentration of SiO during the growth of ultra-long SiC nanowires and an alumina-assisted growth of the vapor–solid (VS) mechanism was proposed for the growth mode of ultra-long SiC nanowires.

ZrB 2 –10SiC, ZrB 2 –20SiC and ZrB 2 –30SiC ceramics were fabricated by hot pressing. The typical effects of volume fraction of SiC on the electrical properties were investigated. The electrical resistivities of as-prepared ZrB 2 –SiC ceramics increased as the volume fraction of SiC increased. The room temperature electrical resistivity values for ZrB 2 –10SiC, ZrB 2 –20SiC and ZrB 2 –30SiC ceramics were 6.54, 8.65 and 11.29 μΩ cm, respectively. ZrB 2 acted as an electrical conductor and SiC acted as an electrical insulator in the ZrB 2 –SiC ceramics. The microstructure of ZrB 2 –SiC ceramics was also analyzed, which further proved the electrical resistivity measurement results.

ZrB 2 –SiC ceramic composites were prepared through water-based gelcasting and pressureless sintering. Effects of the pressureless sintering temperature (1500–2000 °C), heating rate (5–15 °C/min) and soaking time (0.5–2 h) on the relative density, microstructure and mechanical properties of the ZrB 2 –SiC composites were investigated in detail. A sintering temperature of 2000 °C, a heating rate of 5 °C/min and a soaking time of 2 h were found to be the optimal pressureless sintering procedure. The relative density, flexural strength and fracture toughness of the ZrB 2 –SiC composite prepared under the optimum condition were 97.8%, 403.1 ± 27.8 MPa and 4.05 ± 0.42 MPa·m 1/2 , respectively.

TiB2doped B4C composite whichTiB2contents were 12.4% and 25.4% have been fabricated by the combination of the ball milling process and hot pressing technology. The phase analysis and microstructure of the composite materials are investigated through X-ray diffraction and scan electron microscope, respectively. In addition, thermoelectric properties of the composite materials are studied. The results show that the microcracks are easy to form in the interface between B4C and TiB2. It is also found that, the electrical and thermal conductivity increase wi2. Edit Paperth the increase of TiB2doping, while the Seebeck coefficient decreases with the increase of TiB2doping. The thermoelectric figure of merit of the B4C+25.4%TiB2composite material is 50% higher than that of B4C at 573K, while the ZT of B4C+25.4% TiB2composite material is lower than that of B4C at 873K.

To improve the oxidation-resistance of carbon bonded carbon fiber composites (CBCFs), the low density and porous CBCFs modified with ZrB2 and SiC ceramics with a density of 0.26, 0.40, 0.61 and 0.77 g cm−3 were prepared by precursor infiltration and a pyrolysis method. The pyrolysis behavior of the Zr and B-containing hybrid polymeric precursor was studied. The densification behaviour was investigated through the analysis of the microstructures of CBCFs–SiC–ZrB2 (denoted as CSZ) composites with different density. The mechanical properties, thermal properties and oxidation behavior of the CSZ composites were studied. The results show that the incorporation of a ZrB2 ceramic coating does not change the anisotropic properties of the CBCF composites. The CSZ composites with a density of 0.77 g cm−3 with a continuous ZrB2 ceramic coated layer on the surface of the carbon fibers which exhibit better mechanical properties and antioxidant properties. The composites show remarkable thermal properties which are good for thermal insulation applications.

ZrB2–20 vol.% nanoSiC–20 vol.% graphite (ZSnpG) ceramics that contained graphite flakes with different diameters were investigated to determine the effects of graphite flake diameter on the microstructure and mechanical properties, as well as thermal shock resistance. The results showed that the flexural strength and relative density of ZSnpG ceramics firstly increased and then decreased with increasing the diameter of graphite, whereas the fracture toughness did not change significantly. The improvement in the flexural strength of ZSnpG ceramics was strongly dependent on the controlling of graphite distribution, the reduction of the length of the microcrack that resulted from thermal residual stress and the presence of intragranular nano-sized SiC particles. The calculated thermal shock parameters revealed that the graphite with finer starting diameter had a beneficial effect on the blocking of the crack initiation and the graphite with larger diameter restrained the crack propagation of ZSnpG ceramics.

The purpose of the present study is to prepare ZrB2-based nanocomposites with limited or no obvious grain growth. For ZrB2 nanoparticles, the densification on-set temperature was about 1300°C, whereas the obvious grain growth on-set temperature for ZrB2 nanoparticles was found to be about 1500°C. Therefore, there was a temperature “window” 1300–1500°C where ZrB2 nanoparticles could be densified with limited or no grain growth. In this study, densification of ZrB2-based nanocomposite was finally realized by means of low-temperature hot-pressing at 1450°C for 60 min under a uniaxial pressure of 30 MPa using Cu as a sintering additive. After hot-pressing, the grain growth of ZrB2 nanoparticles was greatly suppressed, and the average grain size for ZrB2 was only 310 nm. The microstructure and mechanical properties were examined, and the flexural strength and fracture toughness were 472.8 ± 36.4 MPa and 12.4 ± 0.5 MPa · m1/2, respectively. We believe this paper can lay the foundation for the preparation of ZrB2-based nanocomposites.

ZrB 2 –SiC nanocomposite ceramics toughened by ZrO 2 fiber were fabricated by spark plasma sintering (SPS) at 1700 °C. The content of ZrO 2 fiber incorporated into the ZrB 2 –SiC nanocomposites ranged from 5 mass% to 20 mass%. The content, microstructure, and phase transformation of ZrO 2 fiber exhibited remarkable effects on the fracture toughness of the ZrO 2(f) /ZrB 2 –SiC composites. Fracture toughness of the composites greatly improved to a maximum value of 6.56 MPa m 1/2 ± 0.3 MPa m 1/2 by the addition of 15 mass% of ZrO 2 fiber. The microstructure of the ZrO 2 fiber exhibited certain alterations after the SPS process, which enhanced crack deflection and crack bridging and affected fracture toughness. Some microcracks were induced by the phase transformation from t-ZrO 2 to m-ZrO 2 , which was also an important reason behind the improvement in toughness.

The effects of cooling and heating velocity on the microstructure and mechanical properties of the hot-pressed ZrB2–20 vol.% SiC ceramics added with 15 vol.% Zirconia fiber were investigated in details. It was indicated that the appropriate cooling velocity could reduce the residual stress concentration, and the proper heating velocity could allowed the green compacts to get densified, which was favorable to enhance the flexural strength and fracture toughness to 1117 ± 98 MPa and 7.1 ± 0.5 MPa m1/2, respectively. Due to the introduction of the Zirconia fiber, the observed toughening mechanisms were attributed to the fiber debonding, fiber pull-out and phase transformation toughening. The results presented here point out a promising way for improving the mechanical properties of ZrB2-based ultra-high temperature ceramics.

The oxidation behaviour of ZrB2-20 vol.% SiC composites was investigated based on the microstructural evolution of oxide scale under different oxygen partial pressures at 1500 °C, and the similar experiment was performed in atomic oxygen for comparison. The thickness of the oxide scale increases first and then gradually decreases as the pressure decreases, which is strongly dependent on both total pressure and oxygen partial pressure. The atomic oxygen significantly enhances the oxidation of ZrB2–SiC composites, but has little effect on the microstructure of oxide scale. The oxidation mechanism of ZrB2–SiC composites is also discussed in detail.

The ablation behavior of ZrB 2 –SiC sharp leading edges with different radius ( R ) values of 0.15 ( R 1), 0.5 ( R 2), 1.0 ( R 3) and 1.5 mm ( R 4) was investigated using an oxy-acetylene torch. Under the same ablation condition, sharp leading edge with a lower R value underwent a severer ablation. The surface temperature reached a maximum of ∼2100 °C for R 1, ∼2040 °C for R 2, ∼2000 °C for R 3 and ∼1930 °C for R 4, respectively, and subsequently decreased to a state value of about 1910–1935 °C for all models. R 1 and R 2 underwent intensive mass loss and linear shrinkage, whereas R 3 had only a slight mass loss and linear shrinkage, and R 4 had a slight mass gain and linear expansion. The finite element analysis (FEA) also gave the simulated temperature distributions in the ZrB 2 –SiC sharp leading edge models, which were in good accordance with the experimental results. Considering the surface temperature, shape retention behavior and the lift to drag ratio, R 3 (1.0 mm) was therefore selected as the optimal radius value for ZrB 2 –SiC sharp leading edge. The microstructures of the cross-section and oxidation surface of R 3 were also investigated.

HfB 2 –20 vol.% MoSi 2 ultra high temperature ceramic composites were prepared through aqueous gelcasting route. The stability of HfB 2 and MoSi 2 suspensions were studied by zeta potential measurements, sedimentation tests and apparent viscosity measurements. The solids loading had significant effects on the green and sintered densities, microstructure and mechanical properties of HfB 2 –MoSi 2 composites. The values of flexural strength of the green and sintered bodies ranged from 18.3 to 38.7, and 111.5 to 415.9 MPa, respectively, which were strongly dependent on the solids loading. The values of fracture toughness of the sintered bodies ranged from 2.18 to 4.24 MPa m 1/2 . The highest relative density, mechanical properties and the most homogeneous microstructure was obtained when the solids loading was 45 vol.%. The highest green strength, flexural strength and fracture toughness were 38.7 ± 5.3 MPa, 415.9 ± 17.0 MPa and 4.24 ± 0.22 MPa m 1/2 , respectively.

Zirconium diboride toughened by silicon carbide and zirconia fiber (ZrB2SiCZrO2f) was prepared by using planetary ball mill and the effect of milling time was investigated. The results showed that both the length of fiber and particle size of ZrB2SiC-matrix were reduced as the ball milling time increased. When milling time varied from 8 h to 12 h, the accumulated fibers and agglomerated particles were observed. The production of a homogeneous ceramic could be successfully achieved by using a combination of 20 h milling time and hot-pressing at 1850 °C for 60 min under a uniaxial load of 30 MPa. The optimal flexural strength and fracture toughness of the hot-pressed ZrB2SiCZrO2f ceramics reached 1084 MPa and 6.8 MPa m1/2, respectively. The main toughening mechanisms were fiber debonding, fiber pull-out and transformation toughening. The results indicated that the ball milling technique was proposed as a potential and simple method to obtain usable quantities of ZrB2SiCZrO2f ceramic.

Camphene-based freeze casting technique was adopted to fabricate ZrB2–SiC porous ceramic with 3-dimensional (3D) pore network. ZrB2–SiC/camphene slurries (initial solid loading: 20 vol%, 25 vol% and 30 vol%) were prepared for freeze casting. Regardless of initial solid loading, the fabricated sample had dense/porous dual microstructure. The thickness of dense layer was about 200–300 μm. The microstructures of ZrB2–SiC porous ceramics were significantly influenced by the initial solid loading, which determines the pore size, porosity and mechanical properties of the final products.

Zirconium diboride based ultra-high temperature ceramics toughened by zirconia fiber (ZrB2–ZrO2f) were prepared by hot pressing and pressureless sintering, and the effect of two sintering techniques on the phase composition, microstructures and mechanical properties of ZrB2–ZrO2f ceramics were studied in detail. The densification behavior was investigated through the analysis of the density curves. The microstructures and mechanical properties of ZrB2–ZrO2f ceramics were analyzed and compared in order to research the influence of the two sintering techniques. Results indicated that the hot-pressing process was more suitable for preparing ZrB2–ZrO2f ceramics than pressureless sintering process. The comprehensive properties of ZrB2 plus 30 vol.% ZrO2f ceramics obtained at temperature 1950 °C by hot pressing for 2 h were optimal, the flexural strength and fracture toughness reached 633 MPa and 5.6 MPa·m1/2, respectively. The higher flexural strength was attributed to the smaller size of grains and higher relative density, furthermore, the toughening mechanisms were fiber debonding, fiber pull-out, crack deflection and transformation toughening.

Complex-shaped ZrB 2 –SiC ultra high temperature ceramic components were successfully fabricated by gelcasting technique and pressureless sintering. When 0.4 wt% poly(acrylic acid) (PAA) was used as dispersant and the pH value was 10, an aqueous ZrB 2 –SiC–B 4 C–C slurry with a high solid loading up to 50 vol% but low viscosity (3.1 Pa s at 60 s −1 ) was prepared. For gelcasting process, the suitable monomers content (AM and MBAM) and the mass ratio of monomers (AM/MBAM) were selected as 3 wt% and 15:1, respectively. After pressureless sintering at 2100 °C for 2 h, ZrB 2 –SiC ceramics (using B 4 C and C as sintering additives) finally achieved about 98% density. Investigation of the mechanical properties revealed a flexural strength of 405±27 MPa and a fracture toughness of 4.3±0.3 MPa m 1/2 . Complex-shaped ZrB 2 –SiC ceramic components with homogeneous microstructure were also successfully produced. This study confirms that gelcasting is a reliable process for the fabricating of complex-shaped ultra high temperature ceramic components with satisfied properties.

ZrB 2 –SiC ultra high temperature ceramics containing B 4 C and C as sintering additives were successfully prepared by aqueous gelcasting and pressureless sintering. Polyacrylic acid (PAA) was used as the dispersant throughout this research. The various effects of zeta potential, pH value, dispersant concentration, solid loading and ball-milling time on the rheology and fluidity behavior of ZrB 2 –SiC suspension were investigated in detail. A well-dispersed suspension with 50 vol.% solid loading was prepared at pH 10 with 0.4 wt.% PAA after ball milling at 240 rpm for 24 h. Then crack-free green ZrB 2 –SiC ceramics were obtained by gelcasting process and then pressureless sintered at 2100 °C to about 98% relative density. The microstructure and mechanical properties were examined, and the flexural strength and fracture toughness were 405 ± 27 MPa and 4.3 ± 0.3 MPa m 1/2 , respectively.

ZrB 2 -based hybrid ceramics with SiC whisker and ZrO 2 fiber (ZrB 2 –SiC w –ZrO 2f ) were hot-pressed at 1850 °C for 60 min under a uniaxial load of 30 MPa in Argon atmosphere. XRD analysis indicated no reaction of SiC whisker with ZrO 2 fiber, and the optimal flexural strength and fracture toughness of the ZrB 2 –SiC w –ZrO 2f ceramic were 680 ± 45 MPa and 8.0 ± 0.5 MPa m 1/2 , respectively. The improvement in toughness was mainly contributed to the whisker toughening, fiber toughening and the stress-induced transformation toughening, which could exhaust the fracture energy. The results presented in this paper indicated that ZrB 2 –SiC w –ZrO 2f hybrid ceramic was a good candidate for ultra-high temperature structural application.

The ZrB 2 –SiC–Graphite ceramic was immerged in the supercritical water for different times. The microcracks appeared on the surface of the specimen and the composition of the microcracks was confirmed by EDS analysis to be ZrO 2 . The obvious corrosion of SiC and graphite flake on the surface of the specimen was not observed, which was attributed to the dissolution of the oxides of silicon and carbon (graphite) into the supercritical water. The corrosion of the specimen was accelerated as the pressure and temperature of the water increased. XPS analysis was carried out on the specimen corroded in water of 40 ± 1 MPa and 500 ± 10 °C for 75 min, and the significant peak of B 1s was also measured, indicating the presence of the ZrB 2 phase on the surface of the specimen. For the specimen immerged in all conditions, the Vickers’ hardness did not reduce, the fracture toughness was improved and the minimal strength of the immerged specimen was still higher than 90% of the original strength of 480 MPa, which indicated that the ZrB 2 –SiC–Graphite ceramic has excellent resistance to corrosion of the supercritical water applied to reactor.

In this paper, zirconium diboride based ceramics added with 20 vol.% silicon carbide particle and 15 vol.% zirconia fiber (Z20S p 15Z f ) were prepared by hot-pressing at 1850 °C for 60 min under a uniaxial load of 30 MPa in Ar atmosphere. R -curves for Z20S p 15Z f ceramics were studied using the indentation-strength in bending technique and the envelope method. The results indicated that these two testing methods were consistent and viable for estimating R -curve. Z20S p 15Z f ceramics had high resistance to crack growth and damage tolerance with the 6.8 MPa m 1/2 of steady-state toughness. The toughening mechanism was fiber debonding, fiber pull-out, crack bridging, crack branching, crack deflection and transformation toughening.

The processing, microstructure and ablative properties of novel phenolic impregnated 3-D Fine-woven pierced carbon fabric ablator (PICA) with different bulk density were investigated. The density of PICA material ranges from 0.352 to 0.701 g/cm3 that having uniform resin distribution within the fibrous substrate. An oxyacetylene torch was used to explore the ablative characteristics in terms of linear/mass ablation rate and microscopic pattern of ablation. Surface and in-depth temperatures during ablation were measured by using optical pyrometers and thermocouples. The experimental results showed that the linear ablation rate varied between 0.019 and 0.036 mm/s and the mass ablation rate increased from 0.045 to 0.061 g/s for the tested PICA composites. It suggests that the PICA composites with lower density may significantly contribute to improving the thermal insulation and ablative properties.