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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 ℃.

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.

Solvothermal treatment was used to synthesize nanoscale 4TaC-HfC (Ta4HfC5) powder at a relatively low calcination temperature and in a short period (1400 °C, 2 h). The obtained powder had uniform size distribution and dispersion. Ta4HfC5 ceramics were then consolidated via spark plasma sintering at 2100 °C. Ceramics had a better densification and smaller mean grain size at a shorter sintering time compared with that of materials sintered using mechanical ball milling method. The densification behavior of ceramics deriving from synthesized or ball milled powders was analyzed and the mechanical properties of different samples were investigated. To further increase the mechanical properties, a nearly fully dense Ta4HfC5-MoSi2 ceramic was sintered using the synthesized powder. The mechanical properties of the ceramic composite doubled the strength values. This processing route demonstrated to be a viable approach to synthesize nanoscale Ta4HfC5 powder with high purity and uniformity, and obtain higher performances ceramics once sintered.

The research on the ceramic/metal composites with interpenetrating continuous lamellar structure has attracted much interest in structural materials field. In this work, we proposed an innovative approach to fabricate co-continuous ZrB2–SiC/Zn–5Al composites by combining freeze casting with ultrasonic-assisted spontaneous infiltration. The ZrB2–SiC porous ceramic with orderly lamellar channels (30 vol%) was fabricated by freeze casting using ammonium polyacrylate as the dispersant. The effects of ultrasonication duration on the microstructure and mechanical properties of the composites were investigated in detail. Results indicated that the ultrasonication improved the wettability and facilitated the spontaneous infiltration. The composites obtained the excellent comprehensive properties when the duration was 300 s, which can be attributed to not only the sufficient infiltration of lamellar channels but also the longer infiltration distance inside the ceramic walls induced by the ultrasonic capillary and cavitation.

The intrinsic brittleness and poor damage tolerance of ultrahigh-temperature ceramics are the key obstacles to their engineering applications as nonablative thermal protection materials. Biomimetic layered or "brick-and-mortar" hybrid composites composed of alternative strong/weak interfaces exhibit excellent strength and high toughness; however, the commonly used interfacial materials are weak and have poor thermal stability and ablation resistance, which strictly limit their use in high-temperature and oxidative environments. In this work, ZrB2-based "brick-and-mortar" hybrid ceramics were constructed with a hierarchical biomimetic design to improve the fracture resistance and damage tolerance. ZrB2-20vol %SiC ceramics containing 30 vol % reduced graphene oxide nanosheets were used as the weak interface to increase crack growth resistance without destroying the excellent ablation resistance. Finally, the ZrB2-based "brick-and-mortar" composites achieve the synergy of superior damage tolerance and ablation resistance.

Dense mullite ceramics with excellent mechanical properties were produced using three kinds of mullite powders, namely micro-, submicro-, and nano-powders. The influence of sintering temperature on the microstructural and mechanical properties of the produced mullite ceramics was investigated. As the sintering temperature increased from 1200 to 1500 °C, the relative density of the mullite ceramic produced using micro-powder varied from 65.3 to 99.2%, leading to an increase in the flexural strength and fracture toughness from 87.6 to 245.7 MPa, and 1.4 to 2.5 MPa m1/2, respectively. The densification temperature of mullite ceramics significantly reduced when submicro- and nano-powders were used as raw materials. The relative density and mechanical properties of mullite ceramics fabricated using submicro- or nano-powders increased at first, then decreased following an increase in the sintering temperature. The mullite ceramic produced using submicro-powder became compact after being sintered at 1200 °C, and exhibited a high flexural strength of 254.7 MPa and fracture toughness of 2.9 MPa m1/2, indicating that low-temperature densification of mullite ceramics can be realised by enhancing the sintering activity of the mullite powder.

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.

To improve ablation resistance of graphite, SiC/ZrB2-SiC/ZrB2/SiC coating was prepared by pack cementation and CVD. SiC/ZrB2–SiC/ZrB2/SiC coating showed excellent ablation resistance, its mass and linear ablation rate were only 0.27 mg/s and 0.57 μm/s after ablation under oxyacetylene flame for 298 s. CVD ZrB2/SiC layer significantly improved the ablation resistance of SiC/ZrB2–SiC/ZrB2/SiC coating. During ablation, the surface temperature gradually increased until it stabilized around 2220 °C. The surface temperature response in different stages was affected by layered microstructure evolution and emissivity of different layers.

Although axially aligned fibrous monolithic ceramics have superior mechanical properties in ceramic fiber axial direction, the obvious anisotropy in physical and chemical properties of fibrous monolithic ceramics predetermine the significant on-axis and off-axis discrepancy. The existing ZrB2-based fibrous monolithic ceramics were all arranged by axially aligned ceramic fibers. Non-axially aligned ZrB2-SiC/ZrB2-SiC-graphene short fibrous monolithic ceramics with isotropic in-plane properties were fabricated by hot pressing of disorderly-arranged short ZrB2-SiC/ZrB2-SiC-graphene fibers to overcome in-plane anisotropy of the ZrB2-based fibrous monolithic ceramics. ZrB2-SiC/ZrB2-SiC-graphene short fibrous monolithic ceramics have remarkable increase in fracture toughness, critical crack size and work of fracture compared with ZrB2-SiC ceramics. The toughening mechanism of ZrB2-SiC/ZrB2-SiC-graphene short fibrous monolithic ceramics include macroscopic toughening of crack deflection, crack branching, interface friction and ZrB2-SiC cell extraction, and microscopic toughening of graphene bridging and pull-out.

This article reports the fabrication of fiber cloth-reinforced mullite composite via an optimized layer-by-layer (LbL) assembly method involving the infiltration and pyrolysis of fiber cloth with mullite precursor (PIP) followed by brushing matrix onto the treated fiber cloth and hot pressing. The influence of fiber content on the microstructure and mechanical properties of composite are investigated and the crack propagation is discussed to explain the toughening mechanisms. The composite with 30 vol% fiber content exhibited a high density 2.89 g/cm 3 , flexural strength 135.5 MPa and fracture toughness 4.13 MPa m 1/2 . After PIP pretreatment, the gaps existed in the fiber bundle are filled with mullite matrix which transform from mullite precursor, improving the density from 2.89 g/cm 3 to 2.94 g/cm 3 , flexural strength from 135.5 MPa to 163.2 MPa and fracture toughness from 4.13 MPa m 1/2 to 4.55 MPa m 1/2 . The optimized LbL assembly method realizes the creation of fiber cloth-reinforced mullite composites with high density and excellent mechanical properties.

In this work, a ZrB2-SiC-Ta4HfC5/Ta4HfC5 oxidation-resistant dual-layer coating was fabricated to improve the oxidation resistance of carbon–carbon (C/C) composites. The coating was prepared by pack cementation combined with the slurry paste method. Spark plasma sintering was used in the procedure. The as-prepared dual-layer coating was approximately 110 μm, and no distinct interface was observed between the coating and the matrix. The isothermal oxidation test results showed that the dual coating effectively prevented C/C composites from oxidizing. The weight loss of the coated samples was only 3.3 and 9.5% after oxidation at 1773 K for 20 h and ten thermal shock cycles between 1773 K and room temperature in air, respectively. The pores and microcracks were the main reason for the failure of the coating.

Graphene is one of the important candidates in ceramic toughening due to its outstanding physical and chemical properties. For the weak interface toughening of large-diameter graphene sheet and alleviation of the interfacial reaction between ceramic precursors and graphene sheets during high-temperature pyrolysis, ZrC–SiC‒Graphene composite was synthesized via a facile technology of infiltrating ceramic slurry instead of ceramic precursor into macroporous graphene network and spark plasma sintering. The incorporation of the graphene network improved fracture toughness, critical crack size, and fracture energy of ZrC–SiC ceramic. The multiple length-scale toughening mechanisms of ZrC–SiC‒Graphene composite include the macroscopic toughening mechanism of crack deflection and bifurcation and the micro toughening mechanism of graphene bridging, ceramic micro zone tearing, graphene pull-out, graphene and ceramic brick slipping.

SiC nanowires (SiCnws) were incorporated in supercritical-ethanol-dried graphene aerogel (GA–S) via chemical vapor infiltration (CVI) to fabricate a lightweight, thermostable, high-performance electromagnetic (EM) absorbing composite. The SiCnws improved the yield strength of GA–S from 0.15 MPa to 0.47 MPa in the linear elastic region. The oxidation temperature of GA–S contained in situ grown SiCnw exceeded 660 °C in air. The double-scale surface roughness increases the water contact angle from 53° of GA–S to 134° of SiCnw/GA–S. The SiCnw/GA–S composite showed a minimum EM wave reflection loss (RL) value of −54.8 dB at 5.3 GHz with thickness of 3.63 mm. The SiCnw/GA–S sample demonstrated a wide effective absorbing bandwidth of 6.5 GHz from 9.3 to 15.8 GHz at thickness of 2.0 mm. In addition, the SiCnw/GA–S sample could achieve effective absorption covering the entire X-band in a wide thickness range of 2.2–2.7 mm. Three-dimensional networks and residual oxygen functional groups of GA–S, the high-density stacking faults, and the abundant interfaces between SiCnws and graphene sheets played important roles in enhancing the EM dissipation of SiCnw/GA–S composites. The SiCnw/GA–S composites are promising EM-wave-absorbing materials in extreme environment.

This article reports the fabrication of continuous fiber reinforced mullite matrix composite via layer-by-layer assembly method, involving the coating of mullite fiber with BN coating, followed by compositing the coated fibers with mullite matrix and hot pressing. The influences of the fiber coating, fiber content and sintering temperature on the microstructure and mechanical properties of the composite are investigated. By optimizing the sintering temperature and fiber content, the damage of fiber could be avoided and the microstructure and mechanical properties could be improved. The composite containing 30 vol% fibers coated by BN layer sintered at 1300 °C exhibits 90.9% theoretical density with flexural strength value of 203.2 MPa and fracture toughness value of 4.74 MPa•m1/2. Fracture behavior is investigated to explain the toughening mechanisms. The layer-by-layer assembly method realizes the achievement of an individual architecture featuring fibers distribution and weak interfaces.

To improve the thermal cycling resistance of ZrB 2 –SiC/SiC dual layer coating, MgO was added into the pack powders used to prepare ZrB 2 –SiC outer coating by pack cementation. The composition, microstructure and thermal cycling resistance of these coatings were studied. The results show that as-received coatings were mainly composed of SiC, ZrB 2 and Si. ZrB 2 –SiC outer coating contained 5 wt%, 7 wt%, 9 wt% MgO additive showed better thermal cycling resistance with 97.6%, 98.8% and 96.5% weight retention after 15 thermal shock cycles between 1500 °C and room temperature.

The measured values of fracture toughness for ceramics are closely correlated with the sharpness of notch tips, which in turn influences the accurate measurement of fracture toughness. Here, typical structural ceramics, i.e., 3 mol% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP), ZrB 2 , ZrB 2 -SiC and ZrB 2 -SiC-Grapite, were used for the measurement of fracture toughness, and the effect of notch tip radius on the fracture toughness values of these typical structural ceramics was investigated. Ultra-sharp notches with a tip radius less than 1 μm can be fabricated by laser, lower than the critical notch tip radius in ceramics below which the fracture toughness value almost remains constant, and improved accuracy and consistency of fracture toughness measurement can be obtained by this method compared with traditional method.

In order to improve the oxidation resistance of C/C composites, a ZrB2–SiC/SiC oxidation protective dual-layer coating was prepared by a pack cementation combined with the slurry paste method. The ...

Dense silicon nitride - Molybdenum disilicide (Si3N4-MoSi2) was prepared by spark plasma sintering (SPS) of the Si3N4 and Mo powder mixture with 7 mol% CeO2 sintering additives. The sintering time and sintering temperature were optimized, and the effect of in-situ formed MoSi2 on the microstructure, phase transformation and thermal diffusivity of Si3N4 ceramic was investigated. The α – β transformation ratio and the thermal diffusivity of Si3N4 ceramic composites were both higher than the monolithic Si3N4 that had a β:α ratio of 84.3:15.7 and the thermal diffusivity of 10.24 mm2/s. Furthermore, the Si3N4 ceramic composites densified by two-stage SPS exhibited 100% β phase ratio of Si3N4, and the highest thermal diffusivity of the composites was 23.17 mm2/s when the content of Mo particles was under 6 wt%. The in-situ synthesis of MoSi2 is proposed as explaining the improvement of α to β-phase transformation and thermal diffusivity of Si3N4 ceramic composites, due to the enhanced SPS process.

Excellent high temperature mechanical properties of melt-grown Al 2 O 3 -based eutectics have previously been demonstrated in samples prepared by directional solidification methods. In this study, the deformation behaviour of melt-grown Al 2 O 3 /YAG/ZrO 2 eutectic bulk prepared by a non-directional solidification method was investigated by means of compressive tests in a temperature range of 1200–1700 °C. The non-directionally solidified eutectic bulk ceramic has a colony structure and is polycrystalline. It begins to show ductility and has a compressive strength of 320 MPa at 1500 °C, which is much higher than that of the sintered ceramic with the same composition. However, its plastic deformability is insufficient, even at 1700 °C (just below the melting point of 1715 °C), and cracking occurs during compressive deformation.

Graphene nanoribbons (GNRs) were obtained by unzipping multiwall carbon nanotubes (MWCNTs). Three different silicon nitride-carbon nanostructures were prepared by spark plasma sintering (SPS): ceramic composites that contained 1 wt% carbon nanofibers (CNFs), 1 wt% MWCNTs and 1 wt% GNRs respectively. The α to β-Si 3 N 4 transformation ratio and thermal diffusivity of GNR/Si 3 N 4 composites were higher than both CNF/Si 3 N 4 composites and MWCNT/Si 3 N 4 composites. Furthermore, the higher thermal diffusivities of GNR/Si 3 N 4 composites can primarily be attributed to the higher number of elongate β-Si 3 N 4 grains.

Through design hierarchical architectures and interlayer interaction, multifunctional macroscopic graphene oxide papers tend to attain outstanding mechanical properties. Three dimensional architectures functionalized graphene oxide papers are self-assembled with zirconium diboride particles and poly(vinyl alcohol, PVA) using vacuum assisted filtration method. Firstly, ternary graphene oxide micro-scale structures are achieved by the addition of zirconium diboride particles. Then, PVA is introduced to enhance the interface interaction between graphene oxide nanosheets and zirconium diboride particles. The results show significant enhancements in fracture strength (~200%) and tensile modulus (20–100%) are attained through synergistic effect of ternary graphene oxide structures and interlayer interaction between PVA, graphene oxide and zirconium diboride particles. Keywords: Graphene oxide paper, Zirconium diboride, Mechanical behaviors

Silicon carbide (SiC) coating was fabricated on mullite fiber using polycarbosilane as SiC precursor. The concentration of the precursor solution and the dip-coating cycle were considered variables to determine optimum amounts. Results showed that the SiC coating prepared with 0.01 mol/L SiC precursor for one impregnation cycle was uniform and thin. It had a thickness of 62.2 nm, which completely covered the mullite fiber. This SiC coating inhibited the reaction between the fibers and the matrix. It also weakened interface bonding, thereby resulting in a toughened fracture behavior of the composite with SiC coating.

Tetraethyl orthosilicate was selected as a matrix of heat insulating materials among three silanes, and an anti-infrared radiation fiber was chosen as a reinforcement for silica aerogel insulation composite. The silica aerogel was combined well and evenly distributed in the anti-infrared radiation fiber. The heat insulation effect was improved with the increase in thickness of the aerogel insulation material, as determined by the self-made aerospace insulation material insulation performance test equipment. The 15 mm and 30 mm thick thermal insulation material heated at 250 &deg, C for 3 h, the temperatures at the cold surface were about 80 &deg, C and 60 &deg, C, respectively, and the temperatures at 150 mm above the cold surface were less than 60 &deg, C and 50 &deg, C, respectively. The silica aerogel composites with various thicknesses showed good thermal insulation stability. The silica insulation composite with a thickness of 15 mm exhibited good heat insulation performance, meets the thermal insulation requirements of general equipment compartments under low-temperature and long-term environmental conditions. The thermal conductivity of prepared silica aerogel composite was 0.0191 W&middot, m&minus, 1&middot, k&minus, 1 at room temperature and 0.0489 W&middot, 1 at 500 &deg, C.

Silicon nitride ceramic composites were densified by spark plasma sintering with different-sized Mo particles. In-situ synthesis of Mo-Si compound can accelerate the nucleation of β-Si3N4 grain, and the number of β-Si3N4 grain nucleation increases with decrease of Mo particle size during spark plasma sintering. The effect of Mo particle size on the grain growth of β-Si3N4 is significant, while that for the phase composition of Si3N4 samples is negligible. Si3N4 ceramic composites with 6 wt% nano-sized Mo addition are shown to have the highest bending strength (922.56 MPa) and fracture toughness (8.94 MPa m0.5), which is attributed primarily to more elongated β-Si3N4 grains obtained by more β-Si3N4 grain nucleation.

To improve the ablation resistance of graphite, ZrB 2 SiC/SiC coating was prepared by pack cementation. Ablation resistance of the coating was investigated by oxyacetylene torch. After ablation for ∼200 s, the linear and mass ablation rates of ZrB 2 SiC/SiC coated graphite at center area were 4.647 μm/s and 1.216 mg/s, respectively. During ablation process, chemical erosion and particle denudation occurred at the same time were the main factors leading to ablation.

Natural bioinspired hierarchically ordered architectures from the nanoscale to the macroscale are achieved in graphene/ZrB 2 hybrid composites to improve their toughness using graphene oxide. Two types of films containing different volumes of graphene oxide are self-assembled with ZrB 2 or SiC micro particles through a vacuum-assisted filtration method. Scanning electron microscopy images show that ceramic particles are homogeneously distributed in a continuous multilayer graphene oxide network, forming a nano-micro hierarchical structure. Tensile tests are employed to test the strength of these films. The spark plasma sintering method is utilized to construct micro-macro structural order through densifying two types of alternately stacked films containing different volumes of graphene oxide. Indentation tests reveal alternately compressive and tensile layers are achieved after the sintering process. By combining different structural features spanning several length scales, the composites exhibit a unique combination of high strength (522 MPa) and toughness (9.5 MPa m 0.5 ); in particular, the fracture toughness is more than double that of the composite without the hierarchical architecture. The toughening mechanisms are also analyzed at different length scales. This bioinspired material-independent approach should be employed in the designing and processing of materials for structural, high-temperature and energy related applications.

In order to further improve the oxidation protective ability of (ZrB 2 –SiC)/SiC coated graphite, ferrocene was introduced into the outer ZrB 2 –SiC coatings during pack cementation process. The ferrocene addition increased the porosity of the outer ZrB 2 –SiC coatings, which was favorable to thermal shock resistance of (ZrB 2 –SiC)/SiC coated specimens. The ZrB 2 –SiC coating with 3 wt% ferrocene (ZS50-3f) has optimum oxidation protective ability. Its weight retention after 15 thermal shock cycles between 1500 °C and room temperature was 96. 2%. The weight loss of ZS50-3f was only 0.2% after 19 h oxidation at 1500 °C.

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.

Hierarchical structure consisting of graphene oxide (GO) grafted onto carbon fiber (CF) has been synthesized to improve the interfacial properties between the CFs and polymer matrix. The modified CFs are coated with siliconborocarbonitride (SiBCN) preceramic polymer by in situ polymerization to enhance their antioxidant and ablation properties. X-ray photoelectron spectroscopy were used to monitor the composition of the composites. Scanning electron microscopy images revealed that GO was successfully grafted onto CF (CF-g-GO) and coated with SiBCN preceramic polymer (CF-g-GO/SiBCN). Loading of SiBCN preceramic polymer on the surface of CF-g-GO increased remarkably compared to that on the surface of untreated CFs. Pyrolysis of CF-g-GO/SiBCN preceramic polymer at 1400 °C in inert atmosphere led to the formation of SiBCN ceramics with the approximate elemental composition of Si3N4/SiC/BN. TG results show that the thermal stability of the CF with ceramic layer has improved noticeably at high temperature.

SiBOC ceramics with highly graphitized free carbon were prepared from sol–gel derived pre-ceramic gels followed by pyrolysis at 1000–1600 °C under nitrogen. The effects of boron incorporation on the precursor architecture, microstructure and crystallization of SiBOC ceramics were discussed. The experimental results showed that the incorporation of boron had little effect on the silicon bonding environment in SiBOC precursor and led to a higher ceramic yield of 89%. The crystallization of both β-SiC and free carbon was enhanced in SiBOC ceramics. Raman spectra showed that the intensity ratio of D to G band was very small for the SiBOC ceramics pyrolyzed at 1600 °C, indicating the highly ordering and graphitization of free carbon. The clear presence of D׳ band at 1610 cm −1 in Raman spectra suggested the partial substitution of C with B atoms in graphite layer, which significantly promoted the graphitization of free carbon. The turbostratic graphite with 10–20 layers graphene was observed by TEM.

Macroporous SiOC ceramics have been prepared by the pyrolysis of the preceramic precursor obtained from methyltrimethoxysilane and dimethyldimethoxysilane. The structural evolution of the sol–gel derived SiOC ceramics at high temperatures was investigated using SEM, TG, FTIR, XRD and Raman spectra. The SiOC xerogel exhibited a porous structure consisting of cross-linked spherical gel particles. The structural rearrangement appeared and the SiOC ceramic particles changed from spherical to irregular shape after thermal pyrolysis. The ceramic yield was found to be approximately 70% until 1400 °C, and decreased to 38% at 1600 °C owing to carbothermal reduction. The SiC nanorods were in situ formed from macroporous SiOC ceramic wall at 1600 °C. The β-SiC nanocrystals precipitated from the SiOC matrix at 1400 °C and grew up to 4.6 nm at 1600 °C. The graphitization degree of free carbon increased with the increase of pyrolysis temperature.

The damage resistance and R-curve behavior of laminated ZrB 2 –SiC ceramic fabricated by hot-pressing were investigated using indentation-strength technique. The difference in fracture resistance properties between the monolithic and laminated ZrB 2 –SiC ceramics was also analyzed and results showed that obvious R-curve behavior was found in laminated ZrB 2 –SiC ceramic, compared with monolithic ZrB 2 –SiC ceramics. Moreover, the indentation strength values of laminated ZrB 2 –SiC ceramic were observed to be insensitive to the increases in the indentation load, which indicated that laminated ZrB 2 –SiC ceramic has excellent damage resistance. The improvement in damage tolerance and R-curve behavior of laminated ZrB 2 –SiC ceramic was attributed to the macroscopic crack deflection due to the presence of residual compressive stresses.

In this work, the thermal shock behavior of laminated ZrB2–SiC ceramic has been evaluated using indentation-quench method based on propagation of Vickers cracks and compared with the monolithic ZrB2–SiC ceramic. The results showed that the laminated ZrB2–SiC ceramic exhibited better resistance to crack propagation and thermal shock under water quenching condition, and the critical temperature difference (ΔTc) of laminated ZrB2–SiC ceramic (ΔTc ≈ 590°C) was much higher than that of monolithic ceramic (ΔTc ≈ 290°C). The significant improvement in thermal shock resistance was attributed to residual stresses enhancing the resistance to crack growth during thermal shock loading.

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.

Micrometre-sized B4C powder added with ZrB2 powder or the mixed powders of ZrB2 and SiC were sintered by spark plasma sintering under the load of 30 MPa at 1950°C for 18 minutes. The microstructure and mechanical properties of the composites were investigated. The B4C-20 vol.-% ZrB2 composite showed the highest flexural strength and fracture toughness of 489 MPa and 4·8 MPa·m1/2, were obtained of the B4C-20 vol.-%ZrB2 composite and which were much higher than those of monolithic B4C ceramic. The Vickers hardness of B4C composites ranged from 28·9 to 34·6 GPa. The toughening mechanisms have been analysed because of the deflection of microcrack along with the grains boundary and the mismatch of coefficient of thermal expansion between matrix and additions. Besides, the addition of ZrB2 and SiC was observed to enhance the electrical conductivity and electron discharge machining performance of B4C composites.

A C-SiC(20 vol.-%)-ZrC(30 vol.-%) composite was fabricated through hot-pressed sintering using ball-milling dispersion. The static oxidation behaviour of C-SiC(20 vol.-%)-ZrC(30 vol.-%) composite in air from 1000 to 1200°C was analysed through differential thermal analysis. The formed surface oxide was characterised through scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. The oxidation law for the C-SiC(20 vol.-%)-ZrC(30 vol.-%) composite from 1000 to 1200°C was found based on the mass-loss rate. A general model and the mechanism of oxidation for C-SiC(20 vol.-%)-ZrC(30 vol.-%) composite were proposed.

A graphene nanosheet reinforced ZrB2–SiC ceramic composite (GNs/ZrB2–SiC) using graphene oxide (GO) was hot pressed at 1950 °C and 30 MPa for 1 h. Raman and XPS analysis showed multilayer GNs structures were successfully introduced into the composite by in situ thermal reduction of GO during the hot pressing process. The homogeneous dispersion of GO guaranteed the uniform distribution of GNs structures in the composite. Mechanical properties such as bending strength, fracture toughness and hardness were studied for the ZrB2–SiC composite with different volumes fraction of GO. The addition of approximately 5 vol% graphene nanosheets improved the fracture toughness of ZrB2–SiC up to 7.32 MPa m0.5, and the strength was also raised to 1055 MPa. The toughening mechanisms were graphene crack bridging and pulling out induced crack deflection in the GNs reinforced composite.

The manipulation of the heat flow in a ceramic matrix composite is of great importance in industrial and academic fields. Energy flow, as a typical behavior of heat motion in ceramic surfaces can be confined within specific sites during thermal shock experiments, which weakens the temperature gradient distribution, and hence suppresses the crack propagation. The heat flow can be rationally controlled by the introduction of a nanostructured surface with a diverse forced convection coefficient and heat transfer resistance. Taking inspiration from a nanofin surface, yttria-stabilized zirconia (YSZ) nanostructures were fabricated using the sol–gel method. This bio-inspired coating exhibits a high forced convection coefficient (2.884 times) and high heat transfer resistance (30 times) because of the existence of irregular nanowires arrays and a porous nanostructure. The introduction of the nanostructured coating resulted in the rapid depression of the thermal gradient and stress concentration, and the crack propagation was also effectively suppressed. This sol–gel coating method effectively enhanced the thermal shock resistance of the ceramic materials, and indicates the potential for the application of ceramics in extreme environments.

Graphene aerogels with high surface areas, ultra-low densities and thermal conductivities have been prepared to exploit their wide applications from pollution adsorption to energy storage, supercapacitor, and thermal insulation. However, the low mechanical properties, poor thermal stability and electric conductivity restrict these aerogels’ applications. In this paper, we prepared mechanically strong graphene aerogels with large BET surface areas, low thermal conductivities, high thermal stability and electric conductivities via hydrothermal reduction and supercritical ethanol drying. Annealing at 1500 °C resulted in slightly increased thermal conductivity and further improvement in mechanical properties, oxidation temperature and electric conductivity of the graphene aerogel. The large BET surface areas, together with strong mechanical properties, low thermal conductivities, high thermal stability and electrical conductivities made these graphene aerogels feasible candidates for use in a number of fields covering from batteries to sensors, electrodes, lightweight conductor and insulation materials.

Two ZrC-based composites, viz., ZrC + 20 vol.%SiC and ZrC + 20 vol.%SiC + 20 vol.%ZrB2, were prepared by hot pressing. The oxidation behavior of the composites was studied in the temperature range of 800 °C–1300 °C in air. From the mass gain tests, the surface and cross-sectional SEM observation and EDS analysis of the oxidized samples, it was found that the ZrC + 20 vol.%SiC composite seemed to have a high oxidation resistance below 1000 °C, but the ZrC + 20 vol.%SiC + 20 vol.%ZrB2 composite appeared a more excellent oxidation resistance up to 1200 °C. It is main reason that the oxidation rate of ZrC is higher than that of SiC below 1300 °C, meaning that SiO2 from SiC is insufficient to form a dense tight protection film; however, the addition of ZrB2 particles promotes the formation of more borosilicate above 1000 °C, which exerts more protective effect by self-healing mechanism.

In this paper, gelcasting and pressureless sintering of YAG gel coated ZrB2–SiC (YZS) composite were conducted. YAG gel coated ZrB2–SiC (YZS) suspension was firstly prepared through sol–gel route. Poly (acrylic acid) was used as dispersant. YZS suspension had the lowest viscosity when using 0.6 wt.% PAA as dispersant. Gelcasting was conducted based on AM–MBAM system. The gelcast YZS sample was then pressureless sintered to about 97% density. During sintering, YAG promoted the densification process from solid state sintering to liquid phase sintering. The average grain sizes of ZrB2 and SiC in the YZS composite were 3.8 and 1.3 μm, respectively. The flexural strength, fracture toughness and microhardness were 375 ± 37 MPa, 4.13 ± 0.45 MPa m1/2 and 14.1 ± 0.5 GPa, respectively.

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.

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.

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.