SiC-30vol%VB2 ceramic composite was pressureless densified at 2150 °C with excess B4C and C as sintering aids after in-situ formation of VB2 in SiC matrix. The sintered bulk gained a considerably high fracture toughness of 7.0 ± 0.4 MPa m1/2, which was ∼2.4 times as high as that of the monolithic SiC ceramic, owing to the existences of weak heterophase boundaries, thermal residual stresses and microcracks. Meanwhile, since the VB2 particle has a lower elastic modulus than SiC and significantly suppressed the grain growth of SiC, the composite exhibited a high flexural strength of 458 ± 36 MPa and a relatively low Young’s modulus of 356 ± 6 GPa, resulting in an increase of ∼59.3% in mechanical strain tolerance (1.29 × 10−3) compared with that of single-phase SiC ceramic. Besides, the residual stresses and microcracks also induced a lower-than-expected Vickers hardness of 20.8 ± 0.5 GPa in the composite.

•TiC/hastelloy composites were prepared by in-situ reactive infiltration.•The increase of graphite particle size reduces TiC particles size in composites.•Continuous TiO2/Cr2O3 oxide layer was formed by reducing TiC particles size.•Oxidation resistance was improved by increasing graphite particle size.•Composites kept proper CTE and high electrical conductivity.TiC/hastelloy composites with suitable thermal expansion and excellent electrical conductivity are promising candidates for IT-SOFC interconnect. In this paper, the TiC/hastelloy composites are fabricated by in-situ reactive infiltration, and the oxidation resistance of composites is optimized by increasing graphite particle size. Results show that the increase of graphite particles size from 1 μm to 40 μm reduces TiC particle size from 2.68 μm to 2.22 μm by affecting the formation process of TiC. Moreover, the decrease of TiC particles size accelerates the fast formation of dense and continuous TiO2/Cr2O3 oxide layer, which bring down the mass gain (800 °C/100 h) from 2.03 mg cm−2 to 1.18 mg cm−2. Meanwhile, the coefficient of thermal expansion decreases from 11.15 × 10−6 °C-1 to 10.80 × 10−6 °C-1, and electrical conductivity maintains about 5800 S cm−1 at 800 °C. Therefore, the decrease of graphite particle size is one simple and effective route to optimize the oxidation resistance of composites, and meantime keeps suitable thermal expansion and good electrical conductivity.

•TiC/hastelloy composites were fabricated by in-situ reactive infiltration method.•TiC particle size was altered by preheating preform and addition of Mo element.•The oxidation resistance was improved by the decrease of TiC particle size.•The electrical conductivity of composites was 5600–7500 S cm−1 at 800 °C for 100 h.•The TiC/hastelloy composites were promising candidates for IT-SOFC interconnects.Titanium carbide/hastelloy (TiC/hastelloy) composites are potential candidates for intermediate-temperature solid oxide fuel cell interconnects. In this work, TiC/hastelloy composites with suitable coefficient of thermal expansion are fabricated by in-situ reactive infiltration method, and their properties are optimized by adjusting TiC particle size (dTiC). The oxidation process of TiC/hastelloy composites is comprehensive performance of TiC and Ni-Cr alloy and determined by outward diffusion of Ti and Ni atoms and internal diffusion of O2. The oxidation resistance of composites could be improved by the decrease of dTiC through accelerating the formation of continuous and dense TiO2/Cr2O3 oxide scale. Moreover, the electrical conductivity of composites at 800 °C for 100 h is 5600–7500 S cm−1 and changes little with the prolongation of oxidation time. The decrease of dTiC is favorable for the properties optimization, and composites with 2.16 μm TiC exhibits good integrated properties.

ZrB2–SiC foams were prepared from composite poring mechanisms of foaming and solid poring (PAA beads) by the application of a gelcasting technique. The porosities and microstructures of sintered bulks were tailored through changing the solids loading of slurries and the amount of added PAA beads. The increase in solid loading caused high viscosities of slurries, and thus resulted in the reduction of porosity and a tendency to form close pores. With increasing the amount of PAA beads, high porosities (up to 91.4%) and obvious connectivity improvement between bubble-derived pores emerged owing to efficient foaming and presence of bead-derived pores. As the porosity increased, the compressive strength (0.4–8.2 MPa) decreased, and the gas permeability (9.7–62.1×10−12 m2) augmented because of the effects of bead-derived pores and the increase in window number and size of bubble-bead pores.

Phase composition and microstructures of SiC ceramics with Al2O3 and Er2O3 as sintering additives were compared, which were prepared by pressureless liquid-phase sintering (PLPS), hot pressing sintering (HPS), and spark plasma sintering (SPS), respectively. Despite the different sintering temperatures and pressures, all samples were almost fully densified and exhibited fine-grained microstructure. Only SiC crystallized phase was detected in the SPS SiC because of the low sintering temperature and high cooling rate, while the ErAlO3 phase accompanied with a designed Er3Al5O12 phase was detected in the HPS SiC because of the absence of powder bed, which led to partial volatilization of Al2O3 powder at a higher sintering temperature. No obvious orientational growth of SiC grains was observed in all these ceramics, except a few 4H SiC grains along <1 0 0> axis in the SPS SiC. In addition, SiC–SiC grain boundary films differed from each other in these SiC ceramics.

•The TiC/Ni-Cr composites are considered as one candidate for the IT-SOFC interconnects.•The TiC/Ni-Cr composites have been developed via pressureless infiltration.•The effect of Mo content on the microstructure and properties of the composites was analyzed.•The microstructure evolution process during the fabrication and the interfacial structure of the composites were also studied.The TiC/NiCr composites, potential candidates for intermediate-temperature solid oxide fuel cell (IT-SOFC) interconnects, were fabricated by pressureless infiltration process and the adjustment of their microstructure and properties by Mo addition was investigated in this work. The results show that the core-rim structure, concave surfaces and coalescence of the particles were observed in the microstructure, which was resulted from the “dissolution-reprecipitation” process and coherent strain energy. The addition of Mo significantly changes the microstructure by refining the particles size and increasing the contiguity. Meanwhile the properties of composites with similar metal volume fraction could be adjusted by altering the microstructure. The large particle size and high contiguity is beneficial to decline the thermal expansion and electrical resistivity; the Vickers hardness increases as the particle size decreases, while the small particle size and high contiguity is detrimental to the flexure strength. The optimized content of Mo was 11 wt% for TiC/NiCr composites.

In order to fabricate SiC–BN composites with enhanced electrical resistivity, boric acid and urea were used as reactants for synthesizing nano-sized BN particles on the surfaces of SiC powders in flowing ammonia atmosphere. The composites containing 10 wt% BN were pressureless densified at 2150 °C for 1 h with B4C and C as sintering additives. The chemical route was advantageous for obtaining finer BN dispersoids and microstructures of higher homogeneity, which significantly enlarged the SiC/BN interdiffusion area. Consequently, the concentrations of dissolved B and N in SiC grains became more equivalent and the carrier concentration was further decreased through impurity compensation mechanism. The chemically processed composite exhibited a considerably high electrical resistivity (1.03 × 1012 Ω cm), which was ∼15 times higher than that of the composite prepared by conventional powder mixing method. Besides, considerably low dielectric constant (13.88) and loss tangent (0.128) at 1 MHz, as well as good thermal conductivity (70.08 W m−1 K−1) were also achieved.

This study reports the influence of in situ synthesized AlN on the densification of SiC ceramic by pressureless sintering. Pore-free dense SiC ceramics were achieved by sintering at 2170 °C for 1 h with 4 wt% in-situ AlN and 3 wt% carbon as sintering additives. AlN was synthesized through a carbothermal reduction reaction with Al2O3 and carbon in N2 at 1700 °C for 4 h. The reaction process of the powders and densification mechanism for SiC ceramic were also investigated. The densification behavior of SiC ceramics was found to be greatly promoted by in situ introduction of AlN. The as-sintered SiC ceramics presented homogenous microstructure with clean grain boundaries and exhibited transgranular fracture mode, indicating a strong bonding between silicon carbide grains. The flexural strength and fracture toughness of the sintered samples were 394±48 MPa and 3.07±0.20 MPa m1/2, respectively.

•The wettability of Hastelloy on the SiC substrate was analyzed.•Hastelloy matrix composites have been developed via pressureless infiltration.•Micron SiC particles were used as reinforcement.•Al2O3 coating was adopted to restrain the interfacial reaction of SiCp/Hastelloy composite.SiCp/Hastelloy composites were fabricated by pressureless Ti-activated infiltration process. The wetting and infiltration behaviors of Hastelloy on the SiC substrates and the interfacial reaction between the SiC particles and Hastelloy were investigated by real-time observation system, X-ray diffraction (XRD) and scanning electron microscopy (SEM) equipped with energy X-ray dispersive spectroscopy (EDS) system. The results demonstrated that the Hastelloy had a good wettability on SiC ceramic and could spontaneously infiltrate into the Ti-activated SiC preform. Moreover, intensive interfacial reaction similar to SiC/Ni system was found between the SiC particles and Hastelloy, which induced defects in the microstructure. In order to inhibit the interfacial reaction, Al2O3 coating on SiC particles was adopted as a diffusion barrier, which effectively reduced the extent of the interfacial reactions.

Electrical resistivity as high as 3.52 × 1011 Ω cm of SiC ceramic was obtained by spark plasma sintering with Al2O3 and Er2O3 as sintering additives. The interconnection and the amorphous nature of the grain boundary phase resulted from spark plasma sintering both contributed to the improvement of the electrical resistivity. The thermal conductivity of the SPS SiC ceramic was comparable with that of the pressureless liquid phase sintered (PLPS) SiC ceramic.

•Friction and wear behavior of LPS SiC ceramic was investigated.•Wear mechanism of LPS SiC ceramic was investigated.•Microstructure evolution during friction was investigated.Unlubricated friction properties of the pressureless liquid phase sintered SiC ceramics (PLPS SiC) at room temperature were investigated by a bar-on-ring tribometer. The results showed that the friction and wear behavior of PLPS SiC ceramics were closely related to the normal load and the microstructure. The friction coefficient increased with the rotation speed and normal load. Comparing to the rotation speed, the normal load had bigger influence on the friction and wear behavior of the PLPS SiC ceramics. The amorphous films existing between the SiC grains led to the easy spalling of SiC grains. The spalled SiC grains acted as abrasives, resulting in the formation of furrows on the surface first and causing severe wear subsequently. On the other hand, the spalled SiC grains were easily oxidized by the heat generated during the friction process due to their nanometer size. The worn process was the interaction of multiple wear mechanisms, and different worn mechanism was found under certain friction condition.

Silicon carbide (SiC) ceramics have been fabricated by pressureless liquid phase sintering with Al2O3 and rare-earth oxides (Lu2O3, Er2O3 and CeO2) as sintering additives. The effect was investigated of the different types of rare earth oxides on the mechanical property, thermal conductivity and microstructure of pressureless liquid phase sintered SiC ceramics. The room temperature mechanical properties of the ceramics were affected by the type of rare earth oxides. The high temperature performances of the ceramics were influenced by the triple junction grain boundary phases. With well crystallized triple junction grain boundary phase, the SiC ceramic with Al2O3–Lu2O3 as sintering additive showed good high temperature (1300 °C) performance. With clean SiC grain boundary, the SiC ceramic with Al2O3–CeO2 as sintering additive showed good room temperature thermal conductivity. By using appropriate rare earth oxide, targeted tailoring of the demanding properties of pressureless liquid phase sintered SiC ceramics can be achieved.

Porous solid-state-sintered SiC ceramics with controllable structures and excellent mechanical performances were prepared by aqueous gelcasting, combined with direct protein foaming technique. Dispersion of SiC and carbon black powders in water, the rheological behavior of steady SiC-B4C-C slurries, and solidification process of foamed suspensions were systematically investigated. Porosities as well as the microstructures of sintered bulks were tailored by varying the solid content of slurries and amount of foaming agent. The relationship between the compressive strength (41–151 MPa) and porosity (49–68%), and the fracture behaviors were discussed. For improving the connectivity, grain grading was adopted and its effects were revealed.

In order to attain high thermal conductivity, SiC was doped with ultra-low amounts of B and C as sintering additives using boric acid together with d-fructose as boron–carbon sources. The contents of in situ generated B and C were both tailored as low as 0.4 wt.%, which can significantly reduce the impurities induced phonon scattering effect. The SiC ceramics were pressureless densified at 2150 °C for 1 h, and some samples experienced subsequent annealing at 1950 °C for 4 h. High thermal conductivities of 180.94 W/(m K) for the as-sintered SiC ceramics and 192.17 W/(m K) for the annealed specimens at room temperature were achieved. The reasons for the high thermal conductivity in the polycrystalline SiC ceramics were specified, based on the close correlation with microstructure.

The combination of Al2O3 and CeO2 was testified as suitable sintering additive for liquid phase sintering of SiC ceramics, which has lower sintering temperature than that sintered with Al2O3 and Y2O3 as sintering aids. However, the mechanical properties including flexural strength, Vickers’ hardness and fracture toughness of this system were similar to those of the samples sintered with Al2O3 and Y2O3 as sintering aids. The good wettability of the eutectic liquid phase on SiC plate, the high solubility of SiC particles into the liquid phase and the penetration of the liquid phase along the SiC–SiC grain boundaries all confirmed the suitability of the combination of Al2O3 and CeO2 as liquid phase sintering additive for SiC.

Boron carbide (B4C)-based ceramics were pressureless sintered to a relative density of 96.1% at 2150 °C, with the co-incorporation of tungsten carbide and pyrolytic carbon. The as-batched boron carbide power was 7.89 m2 g−1 in surface area. A level of fracture toughness as high as 5.80 ± 0.12 MPa m1/2 was achieved in the BW-6C composite. Sintering aids of carbon and tungsten boride were formed by an in situ reaction. The toughness improvement was attributed to the presence of thermal residual stress as well as the W2B5 platelets. The thermal conductivity and thermal expansivity of the BW-6C composite as a function of temperature are also reported in this work. Our current study demonstrated that the B4C–W2B5 composites could be potential candidate materials for structural applications.

Mullite-zircon (Al2O3–SiO2–ZrSiO4), as a new liquid-phase sintering (LPS) additives system, was introduced into the silicon carbide (SiC) ceramics in order to improve the mechanical and thermal shock properties. The reaction mechanism of the SiC ceramics with additive system during the sintering process was analyzed by thermodynamic calculation and the mullite-zircon system was detected as expected in the final specimens. The ZrSiO4 phase was generated from the reactions of ZrB2 and SiO2 in air and distributed homogeneously in the SiC matrix. The mullite phase in the form of crystal whisker was completely crystallized at 1350 °C, and the size of the crystallite increased with the increase of the sintering temperature from 1350 °C to 1400 °C. This process was explained by vapor–liquid–solid (VLS) mechanism. The influences of sintering temperature and the content of ZrB2 content on their properties and the reaction mechanism were studied. It was found that the specimens were compact when sintered at 1400 °C, and the content of ZrB2 played an important role in the densification process.

Refractory metal diboride ceramics are promising materials for various high temperature applications. In this work, systematic studies were conducted on the mechanical properties, including the strength at elevated temperatures, of in-situ toughened ZrB2–TiB2 ceramic composites that were prepared via pressureless sintering at 2100 °C for 2 h. Linear shrinkage and open porosity measurements indicated that the composites were fully densified. Microstructural observations revealed intergranular cracking behaviour. Thermal residual stress along with elongated SiC platelets contributed to the improvement in toughness. The toughness was strongly affected by the ZrB2 grain size. The high temperature degradation of the flexural strength was attributed to the presence of oxide species.

Sintered silicon carbide with different microstructures: equiaxed grain (EQ) and elongated grain (EL), were prepared. The role of microstructure on surface and subsurface damage during grinding and polishing was investigated. At the same time, the correlation between microstructure and material removal mechanism was studied. The surface and subsurface damage were observed by atomic force microscope (AFM) and transmission electron microscopy (TEM). More grain pulling-out was found on the polished surface with EQ than that with EL. The degree of grain pulling-out is related to aspect ratio and grain diameter of individual grains. Lateral cracks appeared in subsurface for silicon carbide samples with EQ during grinding and polishing, while only plastic deformation could be observed with EL during polishing. Material removal mechanism was found to change from brittle fracture to plastic deformation for sintered silicon carbide ceramics with EL as machine procedure moved on from grinding to polishing. The research related surface and subsurface damage to the microstructure of SiC ceramics and will provide valuable insights into the material removal mechanism.Research highlights▶ More grain pulling-out in equiaxed grain (EQ) than in elongated grain (EL) of S-SiC. ▶ Grain pulling-out is related to aspect ratio and grain diameter. ▶ Only plastic deformation in EL during polishing. ▶ Brittle fracture during grinding while plastic deformation during polishing in EL.

Systematic studies were carried out on the sintering of ZrB2–SiC ceramics without any external pressure being applied. The densities, mechanical properties and microstructures with different contents of B4C were investigated. The results showed that B4C additions were beneficial not only for the sintering process but also for the oxidation resistance. The sintered ZrB2–SiC ceramics with 0.4 wt.% B4C addition had an average strength of 382 ± 24 MPa and a modulus of 405 ± 34 GPa. The hardness and toughness were 13.4 ± 0.4 GPa and 3.7 ± 0.2 MPa m1/2, respectively. The effect of B4C addition on the oxidation resistance of ZrB2–SiC was studied by observing the mass changes and the residual strengths of samples after oxidized treatments.

Dense Ni/Al2O3 composites with different microstructures were fabricated using the spark plasma sintering technique. The toughness of the composites is improved by 49% when a larger percentage of nickel particles are present within the alumina grains. Microcracks and dislocations caused by nickel were observed by transmission electron microscopy and crack deflection and ductile particle bridging by scanning electron microscopy. The thermal stress and critical grain size of the second phase were calculated, to explain the initiation of microcracks and the toughening mechanisms are discussed.

Polyacrylic acid (PAA) with three different molecular weights (PAA-3000, PAA-15000, PAA-100000) was used as a dispersant to investigate the aqueous dispersion behavior of commercially available TaC powder. TaC stabilized with PAA-3000 polymer showed the best dispersion behavior according to zeta potential and rheological measurements. In addition, the TaC slurry in the presence of PAA-3000 had a minimized average particle size of merely 1.08 µm when dispersed in alkaline aqueous solutions. This was effective to break down any original agglomerates. A characteristic carboxylic peak was detected after adding PAA-3000 to the aqueous TaC system, using XPS analysis. Our TEM results confirmed that the surface properties of TaC were modified by the PAA-3000 dispersant. The slip-cast green body exhibits enhanced homogeneity compared to its dry-pressed counterpart. A dense TaC monolithic ceramic (>99%) was obtained after sintering. This study contributes to the understanding of advanced wet-forming techniques for TaC ceramics.