Effects of SiC content on the densification,microstructure, and mechanical properties of HfB2–SiC composites

To investigate the effects of SiC on microstructure, hardness, and fracture toughness, 0, 10, 20, and 30 vol% SiC were added to HfB₂ and sintered by SPS. Upon adding SiC to 30 vol%, relative density increased about 4%; but HfB₂ grain growth had a minimum at 20 vol% SiC. This may be due to grain boundary silicate glass, responsible for surface oxide wash out, enriched in SiO₂ with higher fraction of SiC. By SiO₂ enrichment, the glass viscosity increased and higher HfO₂ remained unsolved which subsequently lead to higher grain growth. Hardness has increased from about 13 to 15 GPa by SiC introduction with no sensible variation with SiC increase. Residual stress measurements by Rietveld method indicated high levels of tensile residual stresses in the HfB₂ Matrix. Despite the peak residual stress value at 20 vol% SiC, fracture toughness of this sample was the highest (6.43 MPa m^0.5) which implied that fracture toughness is mainly a grain size function. Tracking crack trajectory showed a mainly trans-granular fracture, but grain boundaries imposed a partial deflection on the crack pathway. SiC had a higher percentage in fracture surface images than the cross-section which implied a weak crack deflection.     

Fabrication and mechanical properties of laminated HfC–SiC/BN ceramics

Laminated HfC–SiC/BN ceramics were successfully fabricated by tape casting and hot pressing. Fully dense HfC–SiC ultra-high temperature ceramics with homogeneous structure were obtained. The introduction of the weak BN layer resulted in a slight decrease of the flexural strength but significantly improved the fracture toughness compared with monolithic HfC–SiC ceramics. The fracture toughness of laminated HfC–SiC/BN ceramics in the parallel direction peaked at 8.06 ± 0.46 MPa m^1/2, which increased by 115% than that of monolithic HfC–SiC ceramics. The composites showed non-catastrophic fracture behaviors in both parallel and perpendicular directions. It indicates that laminated structure design is a promising approach to obtain full density HfC–SiC ceramics with high fracture toughness.     

Influence of annealing on the mechanical properties of SiC–Si composites with sub-micron SiC microstructures

The influence of annealing temperature (1000, 1100 and 1200°C) on the mechanical properties of SiC–Si composites has been evaluated. Three SiC powders with particle sizes in the range of 0.24 to 0.7 μm were used to produce the composites. Before application the SiC powders were treated with hydrofluoric acid to remove the extent of SiO₂. With this treatment a successful infiltration of green-bodies especially produced of SiC powder with a mean particle size of 0.24 μm was possible. The bending strength decreased with decreasing SiC starting particle size as well as with increasing annealing temperature. However, the fracture toughness was independent on SiC starting particle size and annealing temperature. XRD diffraction analysis showed that internal stress, expressed by broadening of XRD peaks, is low and had no effects on the mechanical properties of the composites.     

Oxidation behavior of hot pressed ZrB2–SiC and HfB2–SiC composites

Non-isothermal, isothermal and cyclic oxidation behavior of hot pressed ZrB₂–20 (vol.%) SiC (ZS) and HfB₂–20 SiC (HS) composites have been compared. Studies involving heating in thermogravimetric analyzer have shown sharp mass increases at 740 and 1180 °C for ZS, and mass gain till 1100 °C followed by loss for HS. Isothermal oxidation tests for 1, 24 and 100 h durations at 1200 or 1300 °C have shown formation of partially and completely stable oxide scales after ～24 h exposure for ZS and HS, respectively. X-ray diffraction, scanning electron microscopy and energy or wavelength dispersive spectroscopy has confirmed presence of ZrO₂ or HfO₂ in oxide scales of ZS or HS, respectively, besides B₂O₃–SiO₂. Degradation appears more severe in isothermally oxidized ZS due to phase transformations in ZrO₂; and is worse in HS on cyclic oxidation at 1300 °C with air cooling, because of higher thermal residual stresses in its oxide scale.     

Microstructure and mechanical properties of Al2O3–YSZ spherical polycrystalline composites

The mechanical behavior and microstructure of highly densified, spherically shaped, polycrystalline Al₂O₃–YSZ composites, processed from pseudoboehmite powders by sol–gel is reported here. Processing was carried out by combining nanometric sized α-Al₂O₃ (120 nm) seeds and YSZ particles of tetragonal structure. The YSZ particles were homogeneously distributed in a coarse-grained matrix of alumina, both inside grains and along grain boundaries. Fracture surfaces, achieved by impact tests showed toughening effects of the zirconia particles. The tetragonality of the YSZ phase stability even after fracture events and fracture toughness measurements by Vickers indentation, where the crack tip interacts with YSZ particles, are all provided and discussed. The local mechanical properties, such as elastic modulus, indentation hardness and the onset of plastic deformation or fracture contact pressure of both YSZ particles and the Al₂O₃ matrix were quantified by nanoindentation. Evidence of coercive contact pressure was observed in YSZ from indentation stress–strain curves.     

Microstructure and mechanical properties of in-situ hot pressed (TiB2 + SiC)/Ti3SiC2 composites with tunable TiB2 content

Without impurity phases detected, fully dense (TiB₂?+?SiC)/Ti₃SiC₂ composites have been successfully synthesised by in-situ reaction hot pressing. The effect of TiB₂ content on phase composite, sintering properties, microstructure, and mechanical properties of the composites were thoroughly investigated. With TiB₂ content increasing from 0 to 50?vol.-%, the flexural strength increases first and then decreases, whereas fracture toughness, hardness and modulus show a linear increase. The maximum strength of 826?MPa was obtained at 20?vol.-% TiB₂. On the whole, the (TiB₂?+?SiC)/Ti₃SiC₂ composites exhibit a superior comprehensive mechanical properties superior to other reported Ti₃SiC₂-based composites reinforced by singular reinforcement. The significant strengthening and toughening effect induced by the in-situ incorporated TiB₂ can be ascribed to the unique properties of TiB₂ and the synergistic action of many mechanisms including particle reinforcement, pulling out of grains, crack deflection and grain refinement strengthening.     

Microstructure and mechanical properties of (Zr,Ti)B2–SiC composites obtained by pressureless sintering of ZrB2–SiC–TiO2 powder mixtures

Multiphase ceramics have been highlighted due to the combination of different properties. This work proposes to obtain the multiphase composite of (Zr,Ti)B₂–SiC based on the mixture of ZrB₂, SiC, and TiO₂ sintered without pressure. The effect of TiO₂ addition on solid solution formation with ZrB₂, densification, microstructure, and mechanical properties was investigated. For this, 2.0 wt% TiO₂ was added to ZrB₂–SiC composites with 10–30 vol% SiC and processed by reactive pressureless sintering at 2050 °C with a 2 h holding time. Sinterability, crystalline phases, microstructure, Vickers hardness, and indentation fracture toughness of these composites were analyzed and compared to the non-doped ZrB₂–SiC samples. The XRD analysis and EDS elemental map images indicated the incorporation of Ti atoms into the ZrB₂ crystalline structure with solid solution generation of (Zr,Ti)B₂. The addition of TiO₂ resulted in matrix grain size refinement and a predominant intergranular fracture mode. The relative densities were not significantly modified with the TiO₂ addition, though a higher weight loss was detected after the sample sintering process. The composites doped with TiO₂ showed an increase in fracture toughness but exhibited a slightly lower Vickers hardness compared to composites without TiO₂ addition.     

Microstructure and mechanical properties of ZrB2–SiC porous ceramic by camphene-based freeze casting

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.     

Microstructure,mechanical properties and thermal conductivity of (Ti0.5Nb0.5)C–SiC composites

A series of (Ti_0.5Nb_0.5)C-x wt.% SiC (x = 0, 5, 10, 20) composites were prepared by spark plasma sintering. Dense microstructures with well‐dispersed SiC particles were obtained for all composites. With the increment of SiC content, the Vickers hardness, Young's modulus and fracture toughness increase monotonically. An optimized flexural strength of 706 MPa was achieved in (Ti_0.5Nb_0.5)C-5 wt.%SiC composite. (Ti_0.5Nb_0.5)C-20 wt%SiC composite exhibits the highest fracture toughness of 6.8 MPa m^1/2. The crack deflections and the suppression of grain growth were the main strengthening and toughening mechanisms. Besides, (Ti_0.5Nb_0.5)C-20 wt%SiC composite exhibit the highest thermal conductivity of 45 W/m·K at 800 °C.     

Effects of Sc2O3 sintering aid for the densification and mechanical properties of SiC–ZrB2 composites

This study examined the effects of a Sc₂O₃ sintering aid on the density, microstructure and mechanical properties of SiC–5 vol% ZrB₂ composites prepared by hot-pressing. Microstructural studies showed that the addition of Sc₂O₃ not only caused a decrease in the hot-pressing temperature from 1950 to 1750 °C by liquid phase sintering, but also resulted in the formation of crystalline Sc₄Zr₃O₁₂ at the grain boundaries via a reaction with ZrO₂ on the surface of the ZrB₂ powder. The addition of Sc₂O₃ produced a fine-grained microstructure with a 43% (430→615 MPa) and 20% (3.6→4.3 MPa m^1/2) increase in flexural strength and fracture toughness, respectively, compared to the SiC–ZrB₂ composite without Sc₂O₃.     

Microstructure and mechanical properties improvement of the Nextel™ 610 fiber reinforced alumina composite

The alumina slurry with high solid content was prepared, and a rapid lamination route for fabricate the Nextel? 610 fiber reinforced alumina composite was proposed in this work. The microstructure and mechanical properties of the as-received all-oxide composite were investigated by a series of techniques. The shrinkage cracks in matrix were reduced, while porous structure in composite was maintained. The N610/alumina composite has weak matrix and weak interface, as the Young’s modulus of the alumina matrix and the interfacial shear strength of the composite are 140.8±2.5GPa and 129.1±14.6MPa. The mechanical properties of the composite are much higher than lots of oxide/oxide composites, given its flexural strength, interlaminar shear strength and the fracture toughness are 398.4±5.7MPa、27.0±0.5MPa and 14.1±0.9MPa·m^1/2, respectively. The flexural strength of the virgin composite keep stable at 25–1050 °C, while dramatically decrease at 1100–1200 °C.     

Effect of heat treatment on the mechanical properties of PIP–SiC/SiC composites fabricated with a consolidation process

Continuous SiC fiber reinforced SiC matrix composites (SiC/SiC) have been considered as candidates for heat resistant and nuclear materials. Three-dimensional (3D) SiC/SiC composites were fabricated by the polymer impregnation and pyrolysis (PIP) method with a consolidation process, mechanical properties of the composites were found to be significantly improved by the consolidation process. The SiC/SiC composites were then heat treated at 1400 °C, 1600 °C and 1800 °C in an inert atmosphere for 1 h, respectively. The effect of heat treatment temperature on the mechanical properties of the composites was investigated, the mechanical properties of the SiC/SiC composites were improved after heat treatment at 1400 °C, and conversely decreased with increased heat treatment temperature. Furthermore, the effect of heat treatment duration on the properties of the SiC/SiC composites was studied, the composites exhibited excellent thermal stability after heat treatment at 1400 °C within 3 h.     

Influence of mechanical activation of reactive mixtures on the microstructure and properties of SHS-ceramics MoSi2–HfB2–MoB

In this work, the influence of modifications of SHS-process on the microstructure and performance characteristics of composite ceramics MoSi₂-HfB₂-MoB with two-level structure was studied. Partial texturing of MoSi₂ grains in samples obtained by force SHS pressing technology was revealed. The effect of preliminary mechanical activation on the macrokinetic parameters of combustion and on the microstructure of the synthesized ceramics was studied. A significant grinding of the synthesized ceramics grain and an increasing of physical-mechanical properties are achieved by increasing the velocity and lowering the combustion temperature of the activated mixtures. The sample obtained by hot pressing of SHS powder from MA reaction mixture showed the most optimal combination of hardness (19.5 GPa), porosity (0.4%) and oxidation resistance (1.82∙10^-6 mg/(cm²∙s)).     

Tensile creep properties and damage mechanisms of 2D-woven C/HfC–SiC composites in high temperature

2D-C/HfC–SiC composites were prepared by a combination of precursor infiltration and pyrolysis (PIP) and chemical vapor infiltration (CVI). Creep tests were performed at 1100°C in air under different stress conditions. Unlike most, C/SiC and SiC/SiC ceramic matrix composites only underwent primary and secondary creep stages, and the C/HfC–SiC composites underwent tertiary creep stage in the creep process. The reason was that the mechanical properties of C/HfC–SiC materials prepared by PIP + CVI methods were different from those prepared by traditional methods. The microscopic morphological analysis of the sample fracture showed that the oxidation products SiO₂ and Hf–Si–O glass phases of the HfC–SiC matrix played a crack filling role in the sample during creep. In turn, it provided effective protection to the internal fibers of the sample. The creep failure of C/HfC–SiC composites in a high-temperature oxidizing atmosphere was caused by the oxidation of the fibers. The total creep process was dominated by the oxidation of carbon fibers. It is noteworthy that there was the generation of Hf_xSi_yO_z nanowires in the samples after high-temperature creep. The analysis of the experimental data showed that the creep stress had a linear negative correlation with the creep life.     

Microstructure and mechanical properties of superplastically deformed silicon nitride–silicon oxynitride in situ composites

Silicon nitride–silicon oxynitride in situ composites were fabricated by plane-strain-compressing dense silicon nitrides, starting from 93 wt.% ultrafine β-Si₃N₄ and 7 wt.% cordierite, at 1600 °C under a constant load of 40 MPa and subsequent annealing at 1750 °C for 30 min. The resulting composites featured a microstructure of elongated Si₂N₂O grains (∼0.64 μm in diameter and ∼5.5 in aspect ratio) dispersed in a fine-grained β-Si₃N₄ matrix (∼ 0.30μm in diameter and ∼3.5 in aspect ratio), with the amount of Si₂N₂O, which had relatively strong textures, being strain-dependent. The mechanical properties were found to be improved due to the development of elongated Si₂N₂O grains, the texture formation, and the coarsening of β-Si₃N₄. Fracture toughness, however, was still low (∼5.2 MPa m^1/2) for these composites in comparison to self-reinforced silicon nitrides, resulted from the strong Si₂N₂O-matrix interfacial bond and nearly equiaxed β-Si₃N₄ with a small grain size. Anticipated property anisotropies were clearly observed as a result of the textured microstructure.     

Effects of SiC on the microstructures and mechanical properties of B4C–SiC–rGO composites prepared using spark plasma sintering

In this study, B₄C–SiC–rGO composites with different SiC contents were prepared by spark plasma sintering at 1800 °C for 5 min under a uniaxial pressure of 50 MPa. The effects of SiC on the microstructures and mechanical properties of the B₄C–SiC–rGO composites were investigated. The optimal values for flexural strength (545.25 ± 23 MPa) and fracture toughness (5.72 ± 0.13 MPa·m^1/2) were obtained simultaneously when 15 wt.% SiC was added to 5 wt.%–GO reinforced B₄C composites (BS15G5). It was found that SiC and rGO inhibited the grain growth of B₄C and improved the mechanical properties of the B₄C–SiC–rGO composites. The clear and narrow grain boundaries of rGO–B₄C and rGO–SiC, as well as the semi-coherent B₄C–SiC interface, indicated strong interface compatibility. The twin structures of SiC and B₄C observed in the composites improved their fracture toughness. Crack deflection and crack bridging caused by the SiC grains as well as rGO bridging and rGO pull-out were observed on the crack propagation path.     

Microstructure and mechanical properties of WC–C nanocomposite films

WC–C nanocomposite film was prepared by using a hybrid deposition system of r.f.-PACVD and DC magnetron sputtering. W concentration in the film was varied from 5.2 to 42 at.% by changing the CH₄ fraction of the mixture sputtering gas of Ar and CH₄. Hardness, residual compressive stress and electrical resistivity were characterized as a function of W concentration. Raman spectroscopy, XRD and high resolution TEM were employed to analyze the structural change in the film for various W concentrations. In the present W concentration range, the film was composed of nano-sized WC particles of diameter less than 5 nm and hydrogenated amorphous carbon matrix. Content of the WC particles increased with increasing W concentration. However, the mechanical properties of the film increased only when the W concentration was higher than 13 at.%. Structural analysis and electrical conductance measurements evidently showed that the increase in hardness and residual stress occurred as the WC particles were in contact with each other in the amorphous carbon matrix.     

Microstructure and mechanical properties of B4C–TiB2 ceramic composites prepared via a two-step method

B₄C–TiB₂ ceramic composites were fabricated by a two-step method. First, B₄C–TiB₂ composite powders were synthesized from TiC–B powder mixtures at 1400 ℃, then mixed with commercial B₄C powders by ball milling and the B₄C–TiB₂ ceramic composites were prepared by hot pressing at 1950 ℃. This two-step method not only effectively refined TiB₂ grains, but also allowed the composition of the composites to be freely designed. The microstructure and mechanical properties of the composites were investigated. The results showed that the B₄C–TiB₂ ceramic composite with a 10 wt% TiB₂ content obtained the ideal comprehensive performance, with a volume density, Vickers hardness, bending strength, and fracture toughness of 2.61 g/cm³, 35.3 GPa, 708 MPa, and 5.82 MPa m^1/2, respectively. The advantages of the in-situ reaction process were fully exerted by the two-step method, which made a remarkable contribution to the excellent properties of B₄C–TiB₂ ceramic composites.     

Mechanical and physical behaviors of short pitch-based carbon fiber-reinforced HfB2–SiC matrix composites

Short Pitch-based carbon fiber-reinforced HfB₂ matrix composites containing 20 vol% SiC, with fiber volume fractions in the range of 20–50%, were manufactured by hot-press process. Highly dense composite compacts were obtained at 2100 °C and 20 MPa for 60 min. The flexural strength of the composites was measured at room temperature and 1600 °C. The fracture toughness, thermal and electrical conductivities of the composites were evaluated at room temperature. The effects of fiber volume fractions on these properties were assessed. The flexural strength of the composites depended on the fiber volume fraction. In addition, the flexural strength was significantly greater at 1600 °C than at room temperature. The fracture toughness was improved due to the incorporation of fibers. The thermal and electrical conductivities decreased with the increase of fiber volume fraction, however.     

Processing and mechanical properties of hydroxyapatite–polysulfone laminated composites

Designing biocomposites that mimic bone with specific mechanical properties of toughness and elastic modulus is a long-standing challenge in the biomaterials field. Traditional biocomposites comprise polymer matrices reinforced with ceramic particles. Laminated composites are structures also found in nature that can offer improved mechanical properties such as strength, elastic modulus and toughness. Hydroxyapatite/polysulfone laminated composites were fabricated to develop biologically compatible, toughened composites that would match the elastic modulus of bone. Multilayered composites were successfully designed with improved toughness measured by the work of fracture. Toughness measurements were more than an order of magnitude greater than monolithic hydroxyapatite. The toughness and modulus values of hydroxyapatite/polysulfone were within the range of cortical bone.