Microstructure and properties of ZrB2–SiC composites prepared by spark plasma sintering using TaSi2 as sintering additive

ZrB₂–SiC composites were fabricated by spark plasma sintering (SPS) using TaSi₂ as sintering additive. The volume content of SiC was in a range of 10–30% and that of TaSi₂ was 10–20% in the initial compositions. The composites could be densified at 1600 °C and the core–shell structure with the core being ZrB₂ and the shell containing both Ta and Zr as (Zr,Ta)B₂ appeared in the samples. When the sintering temperature was increased up to 1800 °C, only (Zr,Ta)B₂ and SiC phases could be detected in the samples and the core–shell structure disappeared. Generally, the composites with core–shell structure and fine-grained microstructure showed the higher electrical conductivity and Vickers hardness. The completely solid soluted composites with coarse-grained microstructure had the higher thermal conductivity and Young's modulus.     

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.     

Effect of temperature on the structure and mechanical properties of SiC–TiB2 composite ceramics by solid-phase spark plasma sintering

SiC composite ceramics have good mechanical properties. In this study, the effect of temperature on the microstructure and mechanical properties of SiC–TiB₂ composite ceramics by solid-phase spark plasma sintering (SPS) was investigated. SiC–TiB₂ composite ceramics were prepared by SPS method with graphite powder as sintering additive and kept at 1700 °C, 1750 °C, 1800 °C and 50 MPa for 10min.The experimental results show that the proper TiB₂ addition can obviously increase the mechanical properties of SiC–TiB₂ composite ceramics. Higher sintering temperature results in the aggregation and growth of second-phase TiB₂ grains, which decreases the mechanical properties of SiC–TiB₂ composite ceramics. Good mechanical properties were obtained at 1750 °C, with a density of 97.3%, Vickers hardness of 26.68 GPa, bending strength of 380 MPa and fracture toughness of 5.16 MPa m^1/2.     

Densification and mechanical properties of cBN–TiN–TiB2 composites prepared by spark plasma sintering of SiO2-coated cBN powder

cBN–TiN–TiB₂ composites were fabricated by spark plasma sintering at 1773–1973 K using cubic boron nitride (cBN) and SiO₂-coated cBN (cBN(SiO₂)) powders. The effect of SiO₂ coating, cBN content and sintering temperature on the phase composition, densification and mechanical properties of the composites was investigated. SiO₂ coating on cBN powder retarded the phase transformation of cBN in the composites up to 1873 K and facilitated viscous sintering that promoted the densification of the composites. Sintering at 1873 K, without the SiO₂ coating, caused the relative density and Vickers hardness of the composite to linearly decrease from 96.2% to 79.8% and from 25.3 to 4.4 GPa, respectively, whereas the cBN(SiO₂)–TiN–TiB₂ composites maintained high relative density (91.0–96.2%) and Vickers hardness (17.9–21.0 GPa) up to 50 vol% cBN. The cBN(SiO₂)–TiN–TiB₂ composites had high thermal conductivity (60 W m⁻¹ K⁻¹ at room temperature) comparable to the TiN–TiB₂ binary composite.     

Effect of Co addition on microstructural and mechanical properties of WC–B4C–SiC composites

In this study, the effect of Co addition on microstructural and mechanical properties of WC-B₄C–SiC composites sintered by spark plasma sintering (SPS) method was investigated. For this purpose, three batches of WC-B₄C–SiC with different contents of Co (10 vol%, 15 vol%, and 20 Vol %) were sintered at 1400 °C. The results of X-ray diffraction (XRD) analysis of the samples indicated the formation of W₂B₅, W₃CoB₃ as well as the remained C phases and unreacted SiC phase. It was observed that by increasing the Co content, the amount of W₂B₅ phase reduces and W₃CoB₃ and C contents increase. Therefore, W₂B₅ peaks were not detected in the sample containing 20vol% Co. Relative density values above 97% were obtained for all the composites. However, a decrease was observed in relative density by increasing the Co content in the composites. The highest flexural strength (510 ± 42 MPa), fracture toughness (10.34 ± 0.82 MPa m^1/2), and hardness (20.63 ± 0.75 GPa) were also obtained for the sample containing 10vol% Co compared to the other samples. In addition, Transgranular fracture of SiC as well as pulling out of W₃CoB₃ and W₂B₅ particles were observed in the fracture surface micrographs of the samples. The presence of micro-cracks in the SiC grains, fracture of W₃CoB₃ grains, and crack deflection was reported as dominant toughening mechanisms.     

Densification behaviour and mechanical properties of B4C–SiC intergranular/intragranular nanocomposites fabricated through spark plasma sintering assisted by mechanochemistry

High-performance B₄C–SiC nanocomposites with intergranular/intragranular structure were fabricated through spark plasma sintering assisted by mechanochemistry with B₄C, Si and graphite powders as raw materials. Given their unique densification behaviour, two sudden shrinkages in the densification curve were observed at two very narrow temperature ranges (1000–1040 °C and 1600–1700 °C). The first sudden shrinkage was attributed to the volume change in SiC resulting from disorder–order transformation of the SiC crystal structure. The other sudden shrinkage was attributed to the accelerated densification rate resulting from the disorder–order transformation of the crystal structure. The high sintering activity of the synthesised powders could be utilised sufficiently because of the high heating rate, so dense B₄C–SiC nanocomposites were obtained at 1700 °C. In addition, the combination of high heating rate and the disordered feature of the synthesised powders prompted the formation of intergranular/intragranular structure (some SiC particles were homogeneously dispersed amongst B₄C grains and some nanosized B₄C and SiC particles were embedded into B₄C grains), which could effectively improve the fracture toughness of the composites. The relative density, Vickers hardness and fracture toughness of the samples sintered at 1800 °C reached 99.2±0.4%, 35.8±0.9 GPa and 6.8±0.2 MPa m^1/2, respectively. Spark plasma sintering assisted by mechanochemistry is a superior and reasonable route for preparing B₄C–SiC composites.     

Effects of density on the mechanical properties of spark plasma sintered β-SiC

The influence of density on the hardness value of β-SiC samples was studied based on Knoop and Vickers indentation tests. Hardness measurements were performed on additive-free spark plasma sintered SiC samples in the [80%–95%] density range and on highly dense samples (>99%) sintered with very low content of sintering aids. Results revealed that the density has a strong influence on the hardness value, which increases of about 7 GPa between samples presenting densities of 80% and 95% and even reaches 21 GPa under 2 kg with Knoop indenter for the densest samples sintered with very low content of sintering aids. These results allowed us to give a comprehensive model-supported analysis of the mechanical properties of spark plasma sintered β-SiC with controlled porosity that currently does not exist in the literature. The calculation of Young modulus and toughness further resulted in encouraging properties for our samples with regards to mechanical and ballistic performances.     

Microstructural and mechanical investigation of hydroxyapatite–zirconia nanocomposites prepared by spark plasma sintering

Nano hydroxyapatite (nHA)–zirconia (ZrO₂) composites have been produced by spark plasma sintering (SPS). During the SPS process low temperatures (600–950 °C) and short dwelling time (5 min) have been applied to avoid the decomposition of nHA as well as the reaction between nHA and ZrO₂. The grain size of the sintered composites was between 200 and 1000 nm. Carbon diffusion was induced from the graphite die and layered composite structure was formed. These observations might be related to the spark plasma sintering side effects. The microstructure and mechanical properties of high hydroxyapatite content zirconia composites have been found to be influenced and strongly correlated with the specialties of SPS method.     

Evaluating the role of uniformity on the properties of B4C–SiC composites

Fully dense boron carbide-silicon carbide composites were successfully produced by spark plasma sintering method at 1950 °C under 50 MPa applied pressure. The effect of dry and wet mixing methods on uniformity was observed. Density, elastic modulus, microstructure, Vickers hardness and fracture toughness were evaluated. The results showed that dry mixing did not provide uniformity on composites properties. On the other hand wet mixing provided uniformity in microstructure and consistency in material properties. The hardness of the sample containing 50 wt% B₄C was measured to be 30.34 GPa hardness value was found at 50 wt% B₄C content sample. The increase in the B₄C content of the composites decreased the Young's modulus, shear modulus, bulk modulus and fracture toughness. The highest values were found at 10 wt% B₄C sample which were 415 GPa (E), 177 GPa (G), 209 GPa (K), and 2.89 MPa m^1/2 fracture toughness (K_Ic).     

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.     

Microstructures and mechanical properties of B4C–SiC and B4C–SiC–TiB2 ceramic composites fabricated by hot pressing

Boron carbide (B₄C) ceramic composites with excellent mechanical properties were fabricated by hot-pressing using B₄C, silicon carbide (SiC), titanium boride (TiB₂), and magnesium aluminum silicate (MAS) as raw materials. The influences of SiC and TiB₂ content on the microstructural evolution and mechanical properties of the composites were systematically investigated. The mechanism by which MAS promotes the sintering process of composites was also investigated. MAS exists in composites in the form of amorphous phase. It can effectively remove the oxide layer from the surface of ceramic particles during the high temperature sintering process. The typical values of relative density, hardness, bending strength, and fracture toughness of B₄C–SiC–TiB₂ composites are 99.6%, 32.61 GPa, 434 MPa, and 6.20 MPa m^1/2, respectively. Based on the microstructure observations and finite element modeling, the operative toughening mechanism is mainly attributed to the crack deflection along the grain boundary, which results from the residual stress field generated by the thermal expansion mismatch between B₄C and TiB₂ phase.     

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.     

Effects of density and heat treatment of C/C preforms on microstructure and mechanical properties of C/C–SiC composites

Twill multidirectional carbon-fiber-reinforced carbon and silicon carbide composites (i.e., C/C–SiC) were prepared via chemical vapor infiltration combined with reactive melt infiltration process. The effect of heat treatment (HT) on the microstructure and mechanical properties of C/C–SiC composites obtained by C/C preforms with different densities was thoroughly investigated. The results show that as the bulk density of C/C preforms increases, the thickness of the pyrolytic carbon (PyC) layer increases and open pore size distribution narrows, making the bulk density and residual silicon content of C/C–SiC composites decrease. Moreover, the flexural strength and tensile strength of the C/C–SiC composites were improved, which can be attributed to the increased thickness of the PyC layer. The compressive strength reduces due to the decrease of the ceramic phase content. HT improves the graphitization degree of PyC, which reduces the silicon–carbon reaction rate and thereby the content of the SiC phase. HT induces microcracks and porosity but not obviously affects the mechanical properties of C/C–SiC composites. However, the negative impact of HT can be compensated by the increased density of the C/C preforms.     

Effects of 1600 °C annealing atmosphere on the microstructures and mechanical properties of C/SiC composites fabricated by precursor infiltration and pyrolysis

Effects of 1600 °C annealing atmosphere on microstructures and mechanical properties of the C/SiC composites fabricated by PIP route were remarkable. Due to carbothermic reductions, the ratios of weight loss of the C/SiC composites were all above 7 wt% in 1 h. Consequently, the mechanical properties all had a significant drop during the first hour of annealing because of the bonding between the fibers and matrix remarkably weaken by cracks and pores. And then the flexural strengths gradually decreased with the annealing time increasing, when the flexural moduli slightly changed within the range of 44.2–49.7 GPa. However, the fracture behaviors of the C/SiC composites annealed under Ar faster became brittle than the C/SiC composites annealed under vacuum. The C/SiC composites annealed under Ar for 5 h and under vacuum for 10 h both became brittle mainly due to the sensitive to annealing of the weak carbon interphase, while the C/SiC composites annealed under Ar for 7 h became brittle mainly due to the chemical bonding between the fibers and matrix. And these phenomena were confirmed by the post densification and the stress-releasing annealing.     

Effects of high-temperature annealing on the microstructure and properties of C/SiC–ZrC composites

C/SiC–ZrC composites were prepared by a combining slurry process with precursor infiltration and pyrolysis, and then annealed from 1200 °C to 1800 °C. With rising annealing temperature, their mass loss rate increased, and the flexural strength and modulus decreased from 227.9 MPa to 41.3 MPa and from 35.3 GPa to 22.7 GPa, respectively. High-temperature annealing, which elevated thermal stress and strengthened interface bonding, was harmful to the flexural properties. However, it improved the ablation properties by increasing the crystallization degree of SiC matrix. The mass loss rate and linear recession rate decreased with increasing annealing temperature and those of the samples annealed at 1800 °C were 0.0074 g/s and 0.0011 mm/s respectively. Taking mechanical and ablation properties into consideration simultaneously, the optimum annealing temperature was 1600 °C.     

Toughened carbon nanotube–iron–mullite composites prepared by spark plasma sintering

Carbon nanotube–iron–mullite nanocomposite powders were prepared by a direct method involving a reduction in H₂–CH₄ and without any mechanical mixing step. The carbon nanotubes are mostly double- and few-walled (3–6 walls). Some carbon nanofibers are also observed. The materials were consolidated by spark plasma sintering. Their electrical conductivity is 2.4 S/cm whereas pure mullite is insulating. There is no increase in fracture strength, but the SENB toughness is twice than the one for unreinforced mullite (3.3 vs. 1.6 MPa m^1/2). The mechanisms of carbon nanotube bundle pullout and large-scale crack-bridging have been evidenced.     

Microstructure and mechanical behaviour of transient liquid phase spark plasma sintered B4C–SiC–TiB2 composites from a B4C–TiSi2 system

Almost fully-dense B₄C–SiC–TiB₂ composites with a high combination of strength and toughness were prepared through in situ reactive spark plasma sintering using B₄C and TiSi₂ as raw materials. The densification, microstructure, mechanical properties, reaction, and toughening mechanisms were explored. TiSi₂ was confirmed as a reactive sintering additive to promote densification via transient liquid-phase sintering. Specifically, Si formed via the reaction between B₄C and TiSi₂ that served as a transient component contributed to densification when it melted and then reacted with C to yield more SiC. Toughening mechanisms, including crack deflection, branching and bridging, could be observed due to the residual stresses induced by the thermoelastic mismatches. Particularly, the introduced SiC–TiB₂ agglomerates composed of interlocked SiC and TiB₂ played a critical role in improving toughness. Accordingly, the B₄C–SiC–TiB₂ composite created with B₄C-16 wt% TiSi₂ achieved excellent mechanical performance, containing a Vickers hardness of 33.5 GPa, a flexural strength of 608.7 MPa and a fracture toughness of 6.43 MPa m^1/2.     

Synthesis and spark plasma sintering of Si3N4–ZrN self-healing composites

A Si₃N₄–ZrN wear-resistant self-healing composite material was developed. Si₃N₄–ZrN composite ultrafine powders were synthesized at a temperature of 1200 °С via solid-state reactions without milling and densified by spark plasma sintering at 1650 °C to a relative density of 97 ± 0.5%. Balls 13.494 mm in diameter for ball bearings manufactured by spark plasma sintering had a fine-grained structure with a grain size of 200–500 nm, Vickers hardness of 22.5 ± 1.8 GPa, and indentation fracture toughness of 6.2 ± 0.4 MPa. The tribological properties of the composite were investigated under static and dynamic loading. The self-healing capability of the Si₃N₄–ZrN composite was evaluated in the temperature range 500–550 °С. High-temperature three-point bending tests of notched specimens showed a bending strength of 383 ± 21 MPa at room temperature and 413 ± 30 MPa at 500 °С, which confirmed the self-healing of the composite.     

Microstructure and mechanical properties of carbon-precursor-added B4C and B4C−SiC ceramics subjected to pressureless sintering

Organic-carbon-precursor-added B₄C and B₄C–SiC ceramics were subjected to pressureless sintering at various temperatures. The carbon precursor increased the densification of the B₄C and B₄C–SiC ceramics sintered at 2200 °C to 95.6 % and 99.1 % theoretical density (T.D.), respectively. The pyrolytic carbon content of the B₄C–SiC composite decreased with increasing SiC content. The graphitization degree of pyrolytic carbon decreased slightly with the amount of carbon precursor and content of SiC. The 95 wt. % B₄C–5 wt. % SiC composite added with 7.5 wt. % carbon precursor and sintered at 2200 °C outperformed the other B₄C–SiC composites, and its sintered density, flexural strength, Young’s modulus, and microhardness were 98.6 % T.D., 879 MPa, 415 GPa, and 28.5 GPa, respectively. These values were higher than those of composites prepared via pressureless sintering and comparable to those of composites fabricated via hot pressing and/or using metal or oxide additives.     

Effects of oxide addition on the microstructure and mechanical properties of lamellar SiC scaffolds and Al–Si–Mg/SiC composites prepared by freeze casting and pressureless infiltration

Silicate-bonded porous SiC scaffolds with lamellar structures were prepared by freeze casting and liquid-phase sintering. It was found that the viscosity and solidification velocity of SiC water-based slurries with 30 vol% solid loading decreased with increasing Al₂O₃–MgO (AM) addition. As the AM content increased from 10 to 30 wt%, the lamellae of the sintered scaffolds became denser and the porosity decreased from 69±0.5% to 62±0.5%, while the compressive strength improved from 25±2 to 51±2 MPa. The dynamics of pressureless infiltration for an Al–12 Si–10 Mg alloy on the SiC porous scaffold was measured and the composites with lamellar-interpenetrated structures were successfully produced. Both the compressive strength and the elastic modulus of the composites increased with increasing AM content. The maximum strength reached 952±24 MPa and the highest elastic modulus about 156 GPa, respectively, in a longitudinal direction, increasing about 32% and 11% as compared with those of the composites without AM.