Effects of boron content on the microstructures and mechanical properties of reactive hot-pressed BxC-TiB2-SiC composites

B_xC-TiB₂-SiC ceramic composites were fabricated via reactive hot pressing using TiC, B, and Si as the raw materials. The phase transition process was studied by heating powder mixtures to different temperatures in combination with X-ray diffraction analysis. The stoichiometric ratio between B and C in boron carbide is variable. A series of powder mixtures containing excess boron (0 wt%, 10 wt%, 20 wt%, or 30 wt% B) were sintered, and the microstructures and mechanical properties of the composites were investigated. The results showed that the B_6.1C-TiB₂-SiC composite prepared from the starting powders with 30 wt% excess boron had the best comprehensive mechanical properties, with a relative density, hardness, bending strength, and fracture toughness of 98.32%, 33.2 GPa, 840 MPa, and 5.22 MPa m^1/2, respectively. Excessive boron substitution may cause lattice distortion in boron carbide, and the boron carbide grains in this state may form a large number of twins under the compressive stress generated by the TiB₂ grains, which will affect the properties of the composites.     

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.     

Microstructure and mechanical property of (TiB2-SiC) agglomerate-toughened B4C-TiB2-SiC composites

B₄C-TiB₂-SiC composites toughened by (TiB₂-SiC) agglomerates were prepared via reactive hot pressing with B₄C and TiSi₂ as raw materials. Phase composition, microstructure, and mechanical properties of the fabricated composites were investigated. The function of (TiB₂-SiC) agglomerates was analyzed, and the strengthening and toughening mechanism were also discussed. Results indicated that some of the in situ formed TiB₂ and SiC were interlocked to form special (TiB₂-SiC) agglomerates in the matrix. The good comprehensive performances of 510 MPa flexural strength, 5.84 MPa·m^1/2 fracture toughness, and 31.93 GPa hardness were obtained in the composites fabricated with 30 wt% TiSi₂. The in situ introduced fine TiB₂ and SiC grains refined the grains of B₄C due to the pinning effect, which enhanced the strength. The special (TiB₂-SiC) agglomerates and the existing toughening phenomena such as crack deflection, branching, and microcrack regions induced by the mismatch of thermal expansion coefficients, had cumulative effects on improving the fracture toughness.     

Finite element simulation and experimental analysis of B4C-TiB2-SiC ceramic cutting tools

Here, cutting properties and wear mechanism of the home-made B₄C-TiB₂-SiC ceramic cutting tools in turning of AISI 4340 steel workpieces were studied through a combination of finite element simulation using Deform-3D software and turning experiments. Simulation results show that cutting parameters have significant effects on the main cutting force and tool temperature of the B₄C-TiB₂-SiC cutting tool. The optimal cutting parameters for the ceramic cutting tool are cutting speed of 300 m/min, depth of cut of .3 mm, and feed rate of .1 mm/r. Experimental results show the cutting length of the B₄C-TiB₂-SiC cutting tool is about 101 m, which is 21.0% and 32.9% larger than that of the home-made B₄C-TiB₂ ceramic cutting tool and commercially available tungsten carbide tool, indicating that the B₄C-TiB₂-SiC cutting tool has a desired service life. The surface roughness of the workpieces processed by the B₄C-TiB₂-SiC cutting tool is 2.43 µm, which is 29.4% lower than that of the workpieces processed by the B₄C-TiB₂ cutting tool, indicating that the B₄C-TiB₂-SiC cutting tool has a satisfying machining accuracy. Wear forms of the B₄C-TiB₂-SiC ceramic cutting tool involve craters, chipping, and flank wear, and the main wear mechanisms are abrasive, adhesive, oxidative, and diffusion wear.     

Pressureless sintering of B4C-TiB2 composites with Al additions

The effects of Al addition on pressureless-sintering of B₄C-TiB₂ composites were studied. Different amounts of Al from 0% to 5 wt.% were added to B₄C-TiB₂ mixtures (containing up to 30 wt.% TiB₂) and the samples were pressureless sintered at 2050 °C and 2150 °C under Ar atmosphere. Physical, microstructural and mechanical properties were analysed and correlated with TiB₂ and Al additions and sintering temperature. Addition of Al to B₄C-TiB₂ results in increased shrinkage upon sintering and final relative density and lower porosity, the effect is being more evident when both Al and TiB₂ are present. Fracture strength, elastic modulus and fracture toughness of 450 MPa, 500 GPa and 6.2 MPa.m^1/2, respectively were measured.     

Microstructure and anisotropic mechanical properties of B6.5C-TiB2-SiC-BN composites fabricated by reactive hot pressing

B_6.5C-TiB₂-SiC-BN composite ceramics were prepared by a novel solid-state reaction using TiCN, B, and Si as raw materials. The final products obtained by hot pressing at 1950 °C possessed a fine microstructure, homogeneous distribution, and excellent mechanical properties. The obtained bulk B_6.5C-TiB₂-SiC-BN composite ceramic shows a high relative density (98.8 %). The mechanical properties of the composites are anisotropic because of the orientation growth and structural characteristics of TiB₂ and h-BN grains. The values of hardness, bending strength, and fracture toughness measured along the hot-pressing direction were 19.6 GPa, 801 MPa, and 4.30 MPa m^1/2, respectively, which were higher than those measured perpendicular to the hot-pressing direction. The formation of twin structures in B_6.5C and SiC grains and the crack deflections induced by h-BN and TiB₂ grains are beneficial for improving the mechanical properties of these composites.     

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.     

In situ synthesis and densification of submicrometer-grained B4C-TiB2 composites by pulsed electric current sintering

B₄C composites with 15 and 30 vol% TiB₂ were pulsed electric current sintered from B₄C-TiO₂-carbon black mixtures in vacuum at 2000 °C. Full densification could be realised when applying an optimized loading cycle in which the maximum load is applied after completion of the B₄C-TiB₂ powder synthesis, allowing degassing of volatile species. The influence of the sintering temperature on the phase constitution and microstructure during synthesis and densification was assessed from interrupted sintering cycles. The in situ conversion of TiO₂ to TiB₂ was a complex process in which TiO₂ is initially converted to TiB₂ with B₂O₃ as intermediate product at 1400-1700 °C. At 1900-2000 °C, B₂O₃ reacted with C forming B₄C and CO. The B₄C and TiB₂ grain size in the fully densified 30 vol% TiB₂ composite was 0.97 and 0.63 μm, combining a Vickers hardness of 39.3 GPa, an excellent flexural strength of 865 MPa, and modest fracture toughness of 3.0 MPa m^1/2.     

Preparation of B4C composites toughened by MoB2/Mo2B5-SiC interlocking structure via reactive hot pressing

B₄C composites toughened by MoB₂/Mo₂B₅-SiC interlocking structure were prepared via reactive hot pressing with B₄C and MoSi₂ as raw materials. The phase composition, microstructure, and mechanical properties of the fabricated B₄C composites were studied. The crack propagation and fracture surface were observed, and the toughening mechanism was analyzed. The results indicate that the interlocking structure of MoB₂/Mo₂B₅-SiC is formed in the obtained B₄C composites. The relative density, flexural strength, and fracture toughness of the B₄C composites reach 99.3%, 480 MPa, and 5.2 MPa·m^1/2, respectively, when the MoSi₂ content is 30 wt%. The hardness is 33 GPa when the MoSi₂ content is 20 wt%. The special laminar fracture of the interlocking structure of MoB₂/Mo₂B₅-SiC elongates the crack extending path and thus consumes more energy of crack extension. The phenomena of crack bridging and branching and the special laminar fracture of the interlocking structure have a synergistic effect on promoting the overall fracture toughness.     

Microstructure and properties of B4C-SiC composites prepared by polycarbosilane-coating/B4C powder route

B₄C-SiC composites with fine grains were fabricated with hot-pressing pyrolyzed mixtures of polycarbosilane-coated B₄C powder without or with the addition of Si at 1950 °C for 1 h under the pressure of 30 MPa. SiC derived from PCS promoted the densification of B₄C effectively and enhanced the fracture toughness of the composites. The sinterability and mechanical properties of the composites could be further improved by the addition of Si due to the formation of liquid Si and the elimination of free carbon during sintering. The relative density, Vickers hardness and fracture toughness of the composites prepared with PCS and 8 wt% Si reached 99.1%, 33.5 GPa, and 5.57 MPa m^1/2, respectively. A number of layered structures and dislocations were observed in the B₄C-SiC composites. The complicated microstructure and crack bridging by homogeneously dispersed SiC grains as well as crack deflection by SiC nanoparticles may be responsible for the improvement in toughness.     

Fabricating toughened super-hard B4C composites at lower temperature by transient liquid-phase assisted spark plasma sintering with MoSi2 additives

Toughened, super-hard B₄C triplex-particulate composites were densified by spark plasma sintering with MoSi₂ additives (5, 10, and 15 vol.%) at temperatures in the range 1750–1850 °C at which the reference monolithic B₄C ceramics are porous. It is proved that MoSi₂ is a reactive sintering additive that promotes densification by transient liquid-phase sintering, thus yielding fully-dense B₄C-MoB₂-SiC composites at relatively lower temperatures. Specifically, the MoSi₂ first reacts at moderate temperatures (<1150 °C) with part of B₄C to form MoB₂, SiC, and Si. This last is a transient component that eventually melts (at ～1400 °C), contributing to densification by liquid-phase sintering, and then (at 1500–1700 °C) reacts with free C present in the B₄C starting powders to form more SiC, after which densification continues by solid-state sintering. It is found that these B₄C-MoB₂-SiC composites are super-hard (～30 GPa), tough (～3–4 MPa m^1/2), and fine-grained, a combination that renders them very appealing for structural applications. Finally, research opportunities are discussed for the future microstructural design of a novel family of toughened, ultra-hard/super-hard multi-particulate composites based on B₄C plus refractory borides and carbides.     

Fracture toughness in some hetero-modulus composite carbides: carbon inclusions and voids

Structure and mechanical characteristics of dense ceramic composites synthesised by reactive hot pressing of TiC–B₄C powder mixtures at 1800–1950°C under 30?MPa were investigated by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM and EDX). The results show that during hot pressing solid-phase chemical reaction 2TiC?+?B₄C?=?2TiB₂?+?3C has occurred with final products like TiB₂–TiC–C, TiB₂–C or TiB₂–B₄C–C hetero-modulus composite formation with around one micrometer size carbon precipitates. The fracture toughness depends on the amount of graphite precipitation and has a distinct maximum K1C?=?10?MPa?m^1/2 at nearly 7?vol.-% of carbon precipitate. The fracture toughness behaviour is explained by the developed model of crack propagation. Within the model, it is shown that pores (voids) and low-modulus carbon inclusions blunt the cracks and can increase ceramic toughness in some cases.     

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.     

Micromechanics-based simulation of B4C-TiB2 composite fracture under tensile load

Micromechanics modeling was performed to study the effects of thermal residual stress, weak interphases, TiB₂ volume fraction and particle size on the mechanical responses and fracture behaviors of B₄C-TiB₂ composites. Experimentally observed fracture behaviors including micro-cracking and crack deflection were successfully captured. The weak interphases at B₄C-TiB₂ boundaries and the thermal residual stress induced during cooling by the large CTE mismatch between B₄C and TiB₂ were identified as two major factors to promote micro-cracking that caused the enhanced progressive failure behavior. Micro-cracking was enhanced with higher TiB₂ volume fraction due to higher fraction of weak interphase and material affected by thermal residual stress. Meanwhile, micro-cracking behaviors exhibited limited change with varying TiB₂ particle sizes. This modeling study successfully captured the main fracture behaviors and their trends by varying micro-structures of B₄C-TiB₂ composites and can potentially aid microstructure design of tougher B₄C-TiB₂ composites in the future.     

Densification,mechanical, and tribological properties of ZrB2-ZrCx composites produced by reactive hot pressing

ZrB₂-ZrC_x composites were produced using Zr:B₄C powder mixtures in the molar ratios of 3:1, 3.5:1, 4:1, and 5:1 by reactive hot pressing (RHP) at 4-7 MPa, 1200°C for 60 minutes. X-ray diffraction analyses confirmed the formation of nonstoichiometric zirconium carbide (ZrC_x) with different lattice parameters and enhanced carbide formation by increasing the Zr mole fraction. An increase in applied pressure from 4 to 7 MPa was responsible for the improved relative density (RD) of 4Zr:B₄C composition from 86% to 99%. Microstructural studies on Zr-rich composites showed a reduction in unreacted B₄C particles and enriched elongated ZrB₂ platelets. Reaction and densification mechanism in 4Zr:B₄C composition were studied as a function of temperature increased from 600 to 1200°C at an applied constant pressure of 7 MPa. After 1000°C, <40 vol.% of unreacted Zr was observed during the densification process. Concurrently, low energies of carbon diffusion and carbon vacancy formation were found to enhance nonstoichiometric ZrC_x formation, which was found to be responsible for the completion of the reaction. The plastic deformation of unreacted Zr was responsible for the densification of the ZrB₂-ZrC_x composite. The results clearly showed that the applied pressure is five times lower than the reported values. Moreover, a temperature of 1200°C was sufficient to produce dense ZrB₂-ZrC_x composites. The improved microhardness, flexural strength, fracture toughness, and specific wear rate were 8.2-15 GPa, 265-590 MPa, 2.82-6.33 MPa.m^1/2, and 1.43-0.376 × 10⁻² mm²/N, respectively.     

Fabrication of Ti3SiC2-Ti4SiC3-SiC ceramic composites through carbosilicothermic reduction of TiO2

Dense Ti₃SiC₂-SiC, Ti₄SiC₃-SiC, and Ti₃SiC₂-Ti₄SiC₃-SiC ceramic composites were fabricated through carbosilicothermic reduction of TiO₂ under vacuum, followed by hot pressing of the as-synthesized products under 25 MPa at 1600°C. In the reduction step, SiC either alone or in combination with elemental Si was used as a reductant. A one-third excess of SiC was added in the reaction mixtures in order to ensure the presence of approximately 30 vol.% SiC in the products of synthesis. During the hot pressing step, the samples that contained Ti₃SiC₂ showed better densification compared to those containing Ti₄SiC₃. The obtained composites exhibited the strength properties typical of coarse-grained MAX-phase ceramics. The flexural strength values of 424 and 321 MPa were achieved in Ti₃SiC₂-SiC, and Ti₃SiC₂-Ti₄SiC₃-SiC composites, respectively. The fracture toughness values were 5.7 MPa·m^1/2.     

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.     

Microstructure and mechanical properties of B4C matrix composites prepared via hot-pressing sintering with Pr6O11 as additive

High-performance B₄C-PrB₆ composites were prepared via hot-pressing sintering with matrix phase B₄C and with 2–5 wt% Pr₆O₁₁ as additive. The effects of different sintering processes and Pr₆O₁₁ content on the microstructure and mechanical properties of the composites were studied in detail. It is found that increasing sintering temperature and pressure will contribute to the densification of B₄C-PrB₆ composites. Coarse grains are formed in B₄C without additives at high temperature conditions, resulting in the decrease of the densification. Pr₆O₁₁ can effectively hinder the formation of coarse grains and finally promote the densification of the composites. The main toughening mechanisms of composites was crack deflection. The composites with 4 wt% Pr₆O₁₁ prepared at 2050 °C and 25 MPa had the best comprehensive mechanical properties. The relative density, hardness, flexural strength and fracture toughness reached to 98.9%, 37.6 GPa, 339 MPa and 4.4 MP am^1/2, respectively.     

Fabrication and characterization of FAST sintered micro/nano boron carbide composites with enhanced fracture toughness

Toughening of boron carbide (B₄C) without hardness degradation, was achieved by hierarchical structures consisting of B₄C micro-grains, titanium diboride (TiB₂) grains, and graphitic phases along B₄C grain boundaries. Such hierarchical structures were uniquely achieved by co-sintering of B₄C micro-powder and carbon-rich B₄C nano-powder, in situ formation of TiB₂, and by utilizing the short sintering time of field-assisted sintering technology. Toughening mechanisms observed after micro-indentation include crack deflection and delamination of graphite platelets, micro-crack toughening and crack deflection/bridging by TiB₂ grains. Fracture toughness enhancement was achieved while maintaining hardness: 4.65 ± 0.49 MPa m^1/2 fracture toughness and 31.88 ± 1.85 GPa hardness for a micro/nano B₄C-TiB₂ composite (15 vol% TiB₂ and 15 vol% B₄C nano-powders) vs. 2.98 ± 0.24 MPa m^1/2 and 32.46 ± 1.67 GPa for a reference micro B₄C sample. In future, macro-scale mechanical testing will be conducted to further evaluate how these micro-scale hierarchical structures can be translated to macro-scale mechanical properties.     

B4C–TiB2 composites fabricated by hot pressing TiC–B mixtures: The effect of B excess

Previous research have reported that B₄C–TiB₂ composites could be prepared by the reactive sintering of TiC–B powder mixtures. However, due to spontaneous oxidation of raw powders, using TiC–B powder mixtures with a B/TiC molar ratio of 6: 1 introduced an intermediate phase of C during the sintering process, which deteriorated the hardness of the composites. In this report, the effects of B excess on the phase composition, microstructure, and mechanical properties of B₄C–TiB₂ composites fabricated by reactive hot pressing TiC–B powder mixtures were investigated. XRD and Raman spectra confirmed that lattice expansion occurred in B-rich boron carbide and B_xC–TiB₂ (x > 4) composites were obtained. The increasing B content improved the hardness and fracture toughness but decreased the flexural strength of B_xC–TiB₂ (x > 4) composites. When the molar ratio of B/TiC increased from 6.6:1 to 7.8:1, the Vickers hardness and the fracture toughness of the composites were enhanced from 26.7 GPa and 4.53 MPa m^1/2 to 30.4 GPa and 5.78 MPa m^1/2, respectively. The improved hardness was attributed to the microstructural improvement, while the toughening mechanism was crack deflection, crack bridging and crack branching.