Microstructure and mechanical properties of TiB2–B4C ceramics fabricated by hot-pressing

In this study, TiB₂–B₄C composite ceramics were prepared using Y₂O₃ and Al₂O₃ as the sintering aids. Different contents of B₄C were added to seek promoted comprehensive mechanical properties of the composites. The mixed powders were sintered at 1850 °C under a uniaxial loading of 30 MPa for 2 h via hot-pressing. Through the measurement of XRD, SEM and related mechanical properties, the influence of B₄C content on the microstructure and mechanical properties of TiB₂–B₄C composites ceramics was discussed. The experimental results show that TiB₂–B₄C composite ceramics exhibit excellent mechanical properties, which can be attributed to the dense microstructure and fine grain size. In addition, TiB₂–B₄C composite ceramic shows a relatively high comprehensive properties when the addition amount of B₄C is 20 wt%. The relative density, Vickers hardness, fracture toughness and flexural strength are measured to be 99.61%, 27.63 ± 1.73 GPa, 4.77 ± 0.06 MPa m^1/2, 612.5 ± 28.78 MPa, respectively.     

Fracture and property relationships in the double diboride ceramic composites by spark plasma sintering of TiB2 and NbB2

Bulk titanium diboride–niobium diboride ceramic composites were consolidated by spark plasma sintering (SPS) at 1950°C. SPS resulted in dense specimens with a density exceeding 98% of the theoretical density and a multimodal grain size ranging from 1 to 10 μm. During the SPS consolidation, the pressure was applied and released at 1950 and 1250°C, respectively. This allowed obtaining a two-phase composite consisting of TiB₂ and NbB₂. For these ceramics composites, we evaluated the flexural strength and fracture toughness and room and elevated temperatures. Room-temperature strength of thus produced bulks was between 300 and 330 MPa, at 1200°C or 1600°C an increase in strength up to 400 MPa was observed. Microstructure after flexure at elevated temperatures revealed the appearance of the needle-shape subgrains of NbB₂, an evidence for ongoing plastic deformation. TiB₂–NbB₂ composites had elastic loading stress curves at 1600°C, and at 1800°C fractured in the plastic manner, and strength was ranged from 300 to 450 MPa. These data were compared with a specimen where a (Ti,Nb)B₂ solid solution was formed during SPS to explain the behavior of TiB₂–NbB₂ ceramic composites at elevated temperatures.     

Ultra-high elevated temperature strength of TiB2-based ceramics consolidated by spark plasma sintering

Spark plasma sintering of TiB₂–boron ceramics using commercially available raw powders is reported. The B₄C phase developed during reaction-driven consolidation at 1900 °C. The newly formed grains were located at the grain junctions and the triple point of TiB₂ grains, forming a covalent and stiff skeleton of B₄C. The flexural strength of the TiB₂–10 wt.% boron ceramic composites reached 910 MPa at room temperature and 1105 MPa at 1600 °С. Which is the highest strength reported for non-oxide ceramics at 1600 °C. This was followed by a rapid decrease at 1800 °C to 480–620 MPa, which was confirmed by increased number of cavitated titanium diboride grains observed after flexural strength tests.     

High‐Strength B4C–TaB2 Eutectic Composites Obtained via In Situ by Spark Plasma Sintering

The in situ synthesis/consolidation of B₄C–TaB₂ eutectic composites by spark plasma sintering (SPS) is reported. Samples for the evaluation of bending strength were cut from specimens with diameters of 30 mm. The sample prepared for the three‐point flexural strength test had fibers of tantalum diboride with diameter of 1.3 ± 0.4 μm distributed in the B₄C matrix, thereby reducing composites brittleness and yielding an indentation fracture toughness of up to 4.5 MPa·m^1/2. Furthermore, the Vickers hardness of B₄C–TaB₂ eutectics formed by SPS was as high as 26 GPa at an indentation load of 9.8 N. The flexural strength of the B₄C–TaB₂ system has been reported for the first time. Some steps were identified in the load–displacement curve, suggesting that micro‐ and macrocracking occurred during the flexural test. Ceramic composites with a eutectic structure exhibited a room‐temperature strength of 430 ± 25 MPa. Compared with other eutectic composites of boron carbide with transition‐metal diborides, room‐temperature strength the B₄C–TaB₂ was 40% higher than that of B₄C–TiB₂ ceramics, demonstrating advantage of the in situ synthesis/consolidation of eutectic composites by SPS.     

Rapid synthesis of Ti3AlC2/TiB2 composites by the spark plasma sintering (SPS) technique

Dense Ti₃AlC₂/TiB₂ composites were successfully fabricated from B₄C/TiC/Ti/Al powders by spark plasma sintering (SPS). The microstructure, flexural strength and fracture toughness of the composites were investigated. The experimental results indicate that the Vickers hardness increased with the increase in TiB₂ content. The maximum flexural strength (700 ± 10 MPa) and fracture toughness (7.0 ± 0.2 MPa m^1/2) were achieved through addition of 10 vol.% TiB₂, however, a slight decrease in the other mechanical properties was observed with TiB₂ addition higher than 10 vol.%, which is believed to be due to TiB₂ agglomeration.     

Spark plasma sintering of B4C and B4C-TiB2 composites: Deformation and failure mechanisms under quasistatic and dynamic loading

Spark plasma sintering (SPS) was employed to consolidate powder specimens consisting of B₄C and various B₄C-TiB₂ compositions. SPS allowed for consolidation of pure B₄C, B₄C-13 vol.%TiB₂, and B₄C-23 vol.%TiB₂ composites achieving ≥99 % theoretical density without sintering additives, residual phases (e.g., graphite), and excessive grain growth due to long sintering times. Electron and x-ray microscopies determined homogeneous microstructures along with excellent distribution of TiB₂ phase in both small and larger-scaled composites. An optimized B₄C-23 vol.%TiB₂ composite with a targeted low density of ～3.0 g/cm³ exhibited 30–35 % increased hardness, fracture toughness, and flexural bend strength compared to several commercial armor-grade ceramics, with the flexural strength being strain rate insensitive under quasistatic and dynamic loading. Mechanistic studies determined that the improvements are a result of a) no residual graphitic carbon in the composites, b) interfacial microcrack toughening due to thermal expansion coefficient differences placing the B₄C matrix in compression and TiB₂ phase in tension, and c) TiB₂ phase aids in crack deflection thereby increasing the amount of intergranular fracture. Collectively, the addition of TiB₂ serves as a toughening and strengthening phase, and scaling of SPS samples show promise for the manufacture of ceramic composites for body armor.     

Enhancement mechanical properties of in-situ preparated B4C-based composites with small amount of (Ti3SiC2+Si)

B₄C-based composites were synthesized by spark plasma sintering using B₄C、Ti₃SiC₂、Si as starting materials. The effects of sintering temperature and second phase content on mechanical performance and microstructure of composites were studied. Full dense B₄C-based composites were obtained at a low sintering temperature of 1800 °C. The B₄C-based composite with 10 wt% (TiB₂+SiC) shows excellent mechanical properties: the Vickers hardness, fracture toughness, and flexural strength are 33 GPa, 8 MPa m^1/2, 569 MPa, respectively. High hardness and flexural strength were attributed to the high relative density and grain refinement, the high fracture toughness was owing to the crack deflection and uniform distribution of the second phase.     

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.     

Synthesis and properties of conductive B4C ceramic composites with TiB2 grain network

High electrical resistance and low fracture toughness of B₄C ceramics are 2 of the primary challenges for further machining of B₄C ceramics. This report illustrates that these 2 challenges can be overcome simultaneously using core‐shell B₄C‐TiB₂&TiC powder composites, which were prepared by molten‐salt method using B₄C (10 ± 0.6 μm) and Ti powders as raw materials without co‐ball milling. Finally, the near completely dense (98%) B₄C‐TiB₂ interlayer ceramic composites were successfully fabricated by subsequent pulsed electric current sintering (PECS). The uniform conductive coating on the surface of B₄C particles improved the mass transport by electro‐migration in PECS and thus enhanced the sinterability of the composites at a comparatively low temperature of 1700°C. The mechanical, electrical and thermal properties of the ceramic composites were investigated. The interconnected conductive TiB₂ phase at the grain boundary of B₄C significantly improved the properties of B₄C‐TiB₂ ceramic composites: in the case of B₄C‐29.8 vol% TiB₂ composite, the fracture toughness of 4.38 MPa·m^1/2, the electrical conductivity of 4.06 × 10⁵ S/m, and a high thermal conductivity of 33 W/mK were achieved.     

Mechanical properties and pre-oxidation behavior of spark plasma sintered B4C ceramics using (Ti3SiC2+CeO2/La2O3) as sintering aid

B₄C ceramic with the addition of 5 wt % (Ti₃SiC₂+ CeO₂/La₂O₃) as sintering aids was fabricated by spark plasma sintering at a relatively low temperature of 1650 °C for 5 min at 80 MPa. The phase composition, microstructures, and comprehensive mechanical properties of the ceramics were studied in detail. The existence of reinforced second phase particles, the refinement of the matrix grains, the formation of residual stress along the grain boundaries and the appearance of the mixed fracture mode had a synergetic strengthening effect on the mechanical properties. The flexural strength, fracture toughness and Vickers hardness of B₄C ceramics reached 565.2 ± 21.8/551.0 ± 25.2 MPa, 6.28 ± 0.01/6.41 ± 0.12 MPa·m^0.5, and 28.51 ± 0.86/27.23 ± 1.08 GPa, respectively. In addition, to reduce the crack sensitivity of the ceramic, the ceramics were pre-oxidized at 800 °C for different durations. The flexural strength was increased by approximately 13.4% after the ceramic was oxidized at 800 °C for 45 min due to the crack-healing effect induced by the oxide glass B₂O₃ on the ceramic surface.     

Preparation of B4C composites toughened by TiB2-SiC agglomerates

High-performance B₄C composites toughened by TiB₂-SiC agglomerates were fabricated via reactive hot pressing with B₄C, TiC and Si as raw materials. The TiB₂-SiC composite serves as a composite toughening phase formed in the B₄C matrix through an in situ reaction; its agglomerates are composed of interlocked TiB₂ and SiC, which can remarkably improve the toughness of the B₄C composites. The Vickers hardness, flexural strength and fracture toughness of the B₄C-TiB₂-SiC composite reached 35.18 ± 0.45 GPa, 567 ± 14 MPa, and 6.38 ± 0.18 MPa m^1/2 respectively. The special toughening structure of the TiB₂-SiC composite introduced into B₄C ceramics was evaluated for the first time in this study.     

Microstructure and mechanical properties of WB2–B4C composites fabricated by boro/carbothermal reduction and SPS

The phase composition, microstructure, and mechanical properties of the WB₂–B₄C composites fabricated by a combination of boro/carbothermal reduction and spark plasma sintering (SPS) method with WO₃, B₄C, and graphite as raw materials were investigated in this study. The experimental results showed that the relative density of the as-sintered WB₂–B₄C composites was ∼93.1% and ∼99.5%, respectively, after being SPS sintered at 1600°C under the applied load of 30 MPa for 10 min. Scanning electron microscope analysis showed that a network structure with WB₂ grains surrounded by B₄C grains was observed after sintering. Analyses of high-resolution TEM showed semi-coherent interface and lattice distortion transition region between WB₂ and B₄C grains. The Vickers hardness of WB₂–B₄C composite increased to 22.3 ± 0.9 GPa at 9.8 N owing to the fully dense, solid solution of C, and three-dimensional network structure. Moreover, the fracture toughness and flexural strength of WB₂–B₄C composite reach 6.04 ± 0.81 MPa m^1/2 and 750 ± 80 MPa, respectively, which could be attributed to the semi-coherent interface between WB₂ and B₄C grains.     

Hard and easy sinterable B4C-TiB2-based composites doped with WC

B₄C-TiB₂ composites were contaminated with WC to study the effect on densification, microstructure and properties. WC was introduced through a mild or a high energy milling with WC-6?wt%Co spheres or directly as sintering aid to 50?vol% B₄C / 50?vol%TiB₂ mixtures. High energy milling was very effective in improving the densification thanks to the synergistic action of WC impurities, acting as sintering aid, and size reduction of the starting TiB₂-B₄C powders. As a result, the sintering temperature necessary for full densification decreased to 1860?°C and both strength and hardness benefited from the microstructure refinement, 860?±?40 MPa and 28.5?±?1.4?GPa respectively. High energy milling was then adopted for producing 75?vol% B₄C/25?vol% TiB₂ and 25?vol% B₄C/ 75vol%TiB₂ mixtures. The B₄C-rich composition showed the highest hardness, 32.2?±?1.8?GPa, whilst the TiB₂-rich composition showed the highest value of toughness, 5.1?±?0.1?MPa?m^0.5.     

Effect of short carbon fiber content on the mechanical properties of TiB2-based composites prepared by spark plasma sintering

In this study, TiB₂-30 vol% SiC composites containing 0, 5, 10, and 15 vol% short carbon fibers (C_f) were produced by spark plasma sintering (SPS). The effect of carbon fiber content on microstructure, density, and mechanical properties (micro-hardness and flexural strength) of the fabricated composites was studied. Scanning electron microscopy (SEM) results indicated that the fibers were uniformly dispersed in the TiB₂–SiC matrix using wet ball milling before SPS process. Fully dense TiB₂–SiC–C_f composites were achieved by SPS process at 1900°C for 10 min under 30 MPa. With the addition of fibers, the relative density of the composites did not change considerably. Mechanical tests revealed that microhardness was reduced about 19% by the incorporation of carbon fibers, whereas the flexural strength improved significantly. However, the flexural strength diminished by adding carbon fibers above to critical value (5 vol%) due to residual thermal stresses, nonhomogeneous structure and graphitization of carbon fibers. It was found that the composite with 5 vol% C_f had the highest flexural strength (482 MPa), which was enhanced by 20% compared with the TiB₂–SiC composite.     

B4C‒TiB2 composite with modified microstructure and enhanced properties from optimal size coupling of raw powders

B₄C‒15 vol% TiB₂ composites were fabricated by in situ reactive spark plasma sintering with B₄C, TiC, and amorphous B powders as the raw materials. The size coupling of initial B₄C and TiC particles was optimized based on the reaction mechanism to derive B₄C‒TiB₂ composites with enhanced microstructure and properties. During the reactive sintering, fine B₄C–TiB₂ particles were firstly formed by an in situ reaction between TiC and B. Then, large B₄C particles tended to grow at the cost of small B₄C particles. The in situ TiB₂ grains gradually grew up and interconnect, distributing around the large B₄C grains to form an intergranular TiB₂ network. The results showed that the B₄C‒15 vol% TiB₂ composite prepared from 3.12 μm B₄C powder and 0.80 μm TiC powder had the optimal comprehensive properties, with a relative density of 99.50%, a Vickers hardness of 31.84 GPa, a flexural strength of 780 MPa, a fracture toughness of 5.77 MPa·m^1/2, as well as an electrical resistivity of 3.01 × 10⁻² Ω·cm.     

Effects of discrete and simultaneous addition of SiC and Si3N4 on microstructural development of TiB2 ceramics

The impact of Si₃N₄ and SiC additives incorporation in the microstructure and sintering behavior of TiB₂-based composites were studied. Three ceramic composites including TiB₂–Si₃N₄, TiB₂–SiC, and TiB₂–SiC–Si₃N₄ were manufactured by spark plasma sintering (SPS) at 1950 °C for 8 min under 35 MPa. The acquired ceramics were analyzed by X-ray diffractometry and scanning electron microscopy. In addition, the sintering thermodynamic was investigated using the HSC Chemistry package. X-ray diffraction patterns of the prepared ceramics revealed the in-situ formation of graphite and boron nitride in the final composites initiated from SiC and Si₃N₄, respectively. The thermodynamic assessments proved the role of liquid phase sintering on the sinterability enhancement of all composite samples. Field emission scanning electron microscopy and energy-dispersive X-ray spectroscopy verified the in-situ formation of both BN and graphite components in the sample containing SiC and Si₃N₄ additives. Finally, the fractographical investigations clarified the transgranular breakage as the main fracture mode in the TiB₂-based ceramics.     

Multidimensional nano-graphite constructed TiB2–SiC–B4C composite ceramics with the integration of mechanical and electromagnetic properties

The goal of this study is to create structure-functional integrated ceramic matrix composites with high structural strength and electromagnetic absorbing properties. The multidimensional nano-graphite (1-Dimensional rod-like nano-graphite, 0-Dimensional dispersive nano-graphite, and 2-Dimensional lamellar nano-graphite) were employed to construct TiB₂–SiC–B₄C composites via high-energy ball milling, vacuum filtration, and reactive SPS sintering. The microstructure of multidimensional nano-graphite was investigated using XRD and HRTEM and determined to be a crystal-amorphous coexisting. Furthermore, solid solution reaction and interfacial evolution are confirmed as the primary influence on the microstructure of TiB₂–SiC–B₄C composite. A significant improvement occurs on the flexural strength (647.6 MPa) and bending toughness (5.1 MPa m^1/2). Meanwhile, the multi-dimensional nano-graphite gives the TiB₂–SiC–B₄C composite the loss ability of electromagnetic waves, and the matching thickness of the 10 vol% sample is 2.4 mm and the absorption range is 10.4–11.3 GHz.     

High‐temperature strength and plastic deformation behavior of niobium diboride consolidated by spark plasma sintering

Bulk niobium diboride ceramics were consolidated by spark plasma sintering (SPS) at 1900°C. SPS resulted in dense specimens with a density of 98% of the theoretical density and a mean grain size of 6 μm. During the SPS consolidation, the hexagonal boron nitride (h‐BN) was formed from B₂O₃ on the powder particle surface and residual adsorbed nitrogen in the raw diboride powder. The room‐temperature strength of these NbB₂ bulks was 420 MPa. The flexural strength of the NbB₂ ceramics remained unchanged up to 1600°C. At 1700°C an increase in strength to 450 MPa was observed, which was accompanied by the disappearance of the secondary h‐BN phase. Finally, at 1800°C signs of plastic deformation were observed. Fractographic analysis revealed a number of etching pits and steplike surfaces suggestive of high‐temperature deformation. The temperature dependence of the flexural strength of NbB₂ bulks prepared by SPS was compared with data for monolithic TiB₂, HfB₂ and ZrB₂. Our analysis suggested that the thermal stresses accumulated during SPS consolidation may lead to additional strengthening at elevated temperatures.     

Performance of B4C/CeB6 composites fabricated via hot pressing under different processes

In this work, CeO₂ sintering additive reinforced B₄C ceramic composites were prepared by hot-pressing reaction sintering under different processes of low temperature–long holding time (1980°C, 30 MPa, 3 h, 4 wt% CeO₂) and high temperature–short holding time (2050°C, 30 MPa, .5 h, 4 wt% and 6 wt% CeO₂). The effect of sintering process and CeO₂ content on the microstructure and mechanical properties of B₄C-CeB₆ composites were investigated. The existed impurities in the obtained composites were also analyzed. Results show that CeO₂ is an active sintering additive. CeB₆ is formed by the reaction between CeO₂, B₄C and C in sintering process. The densification of B₄C ceramics is enhanced, and the grains can be refined by the formed CeB₆, which promotes the strength. The thermal expansion coefficient mismatch, crack deflection, and fracture mode change caused by the in situ formed CeB₆ improve the toughness. The process of low temperature–long holding time is more suitable for playing the role of CeO₂ additive in sintering of B₄C, under which condition the relative density, flexural strength, fracture toughness, and hardness reach 99%, 417 MPa, 5.32 MPa·m^1/2, and 30.66 GPa, respectively. The impurities in the composites are the kinds of Ti-contained, C-O-Mg-Ca-contained, C-O-Ca-S-contained, and Si-contained impurities.     

Effect of catalyst on carbon nanotubes synthesis on titanium diboride via chemical vapor deposition

Carbon nanotubes (CNTs) were in situ synthesized on titanium diboride (TiB₂) substrates using different catalysts (Fe, Co, and Ni) via ethylene chemical vapor deposition (CVD). The effects of various catalysts on the quality and quantity of grown CNTs were investigated using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and Raman characterization techniques. Next, as-received powders were sintered by spark plasma sintering to obtain compact bulk composites. The effects of the metallic catalysts as sintering aids on the microstructural and mechanical properties of TiB₂-based composites were also studied. Results show that metal Ni not only plays a significant role in the catalyst to produce larger numbers of CNTs but also promotes the densification of TiB₂ ceramics in sintering and has been shown to provide good reinforcement. Maximum values of flexural strength (1093 ± 19 MPa) and toughness (13.9 ± 0.3 MPa ·m^1/2) were obtained using the Ni catalyst, which were significantly higher than the same values for Fe and Co catalysts.