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.     

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.     

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.     

High-strength TiB2-B4C composite ceramics sintered by spark plasma sintering

Spark plasma sintering (SPS) is an advanced sintering technique because of its fast sintering speed and short dwelling time. In this study, TiB₂, Y₂O₃, Al₂O₃, and different contents of B₄C were used as the raw materials to synthesize TiB₂-B₄C composites ceramics at 1850°C under a uniaxial loading of 48 MPa for 10 min via SPS in vacuum. The influence of different B₄C content on the microstructure and mechanical properties of TiB₂-B₄C composites ceramics are explored. The experimental results show that TiB₂-B₄C composite ceramic achieves relatively good comprehensive properties and exceptionally excellent flexural strength when the addition amount of B₄C reaches 10 wt.%. Its relative density, Vickers hardness, fracture toughness, and flexural strength reach to 99.20%, 24.65 ± .66 GPa, 3.16 MPa·m^1/2, 730.65 ± 74.11 MPa, respectively.     

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.     

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.     

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.     

Densification and mechanical properties of mullite/TiO2-coated B4C composites

Mullite/TiO₂-coated B₄C composites with up to 40 wt.% B₄C were fabricated by coating B₄C powders with Ti-chelate compounds via a sol-gel method prior to the hot-pressing of mullite/B₄C mixtures at 1600 °C for 1 h. The effects of TiO₂-coated B₄C on the densification and the mechanical properties of mullite/B₄C composites were investigated. TiO₂ reacts with B₄C at high temperatures to produce B₂O₃ and TiB₂, both seems to be favorable for the densification of mullite/B₄C composites. The formation of TiB₂, besides, may lead to the generation of thermal residual stresses, which is deemed to be beneficial to the fracture toughness but detrimental to the flexural strength. The mechanical properties of mullite/B₄C composites such as hardness, flexural strength and fracture toughness, are enhanced remarkably with the increasing addition of B₄C. However, with a B₄C addition more than 20 wt.%, the flexural strength tends to decrease gradually, probably due to the increasing residual stresses originated from the thermal mismatch of TiB₂ with mullite and B₄C matrix. B₄C contents above 30 wt.% easily cause the aggregation of B₄C particles in mullite matrix, which will lower the degree of improvement in fracture toughness.     

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.     

Microstructure and mechanical properties of B4C–TiB2–SiC composites toughened by composite structural toughening phases

B₄C–TiB₂–SiC composites toughened by composite structural toughening phases, which are the units of (TiB₂–SiC) composite, were fabricated through reactive hot pressing with B₄C, TiC, and Si as raw materials. The units of (TiB₂–SiC) composite with the size of 10‐20 μm are composed of interlocking TiB₂ and SiC with the size of 1‐5 μm. The addition of TiC and Si can effectively promote the sintering of B₄C ceramics. The relative densities of all the B₄C composites with different contents of TiB₂ and SiC are close to completely dense (98.9%‐99.4%), thereby resulting in superior hardness (33.1‐36.2 GPa). With the increase in the content of TiB₂ and SiC, the already improved fracture toughness of the B₄C composite continuously increases (5.3‐6.5 MPa·m^1/2), but the flexure strength initially increases and then decreases. When cracks cross the units of the (TiB₂–SiC) composite, the cracks deflect along the interior boundary of TiB₂ and SiC inside the units. As the crack growth path is lengthened, the crack propagation direction is changed, thereby consuming more crack extension energy. The cumulative contributions improve the fracture toughness of the B₄C composite. Therefore, the composite structural toughening units of the (TiB₂–SiC) composite play an important role in reinforcing the fracture toughness of the composites.     

Synergistic effects of TiB2 and graphene nanoplatelets on the mechanical and electrical properties of B4C ceramic

Monolithic B₄C, B₄C–TiB₂, and B₄C–TiB₂–graphene nanoplatelets (GNPs) were fabricated by hot pressing (HP) at 1900 °C for 1 h under an axial pressure of 30 MPa. The microstructures and mechanical and electrical properties of the B₄C composites were investigated. The results show that the GNPs are distributed homogeneously in B₄C-based ceramic composites. Compared with monolithic B₄C, the TiB₂–GNPs-containing B₄C composite exhibits an approximately 68 % increase in flexural strength and a 169 % increase in fracture toughness due to the synergistic effects of TiB₂ particles and GNPs. The toughening mechanisms mainly include TiB₂ crack deflection, crack branching, transgranular fracture and GNPs crack deflection, crack bridging, and GNPs pull-out. Additionally, the electrical conductivity of the B₄C composite reinforced with dual fillers is three orders of magnitude higher than that of monolithic B₄C due to the establishment of a conductive network. The addition of GNPs can efficiently connect the isolated conductive TiB₂ particles in the B₄C matrix and provides an additional channel for electron migration.     

High-performance B4C–TiB2–SiC composites with tuneable properties fabricated by reactive hot pressing

In this work, we systematically studied the effects of powder characteristics (B₄C, TiC and Si powders) on the existential form of toughening phases (SiC and TiB₂) as well as the overall microstructure and properties of B₄C–TiB₂–SiC composites fabricated by reactive hot pressing. The particle size of the TiC powder plays a largely determining role in the development of novel toughening phases, the TiB₂–SiC composite structure, that are formed in the B₄C matrix, while the Si particle size affects the agglomerate level of the SiC phase. The TiB₂–SiC composite structure and SiC agglomerates enhance the fracture toughness, but decrease the flexural strength. Both the microstructure and mechanical properties of B₄C–TiB₂–SiC composites can be effectively tuned by regulating the combinations of the particle sizes of the starting powders. The B₄C–TiB₂–SiC composites demonstrate flexural strength, fracture toughness and Vickers hardness in the respective range of 567–632 MPa, 5.11–6.38 MPa m^1/2, and 34.8–35.6 GPa.     

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.     

Dense and core-rim structured B4C-TiB2 ceramics with Mo-Co-WC additive

B₄C-TiB₂ ceramics (TiB₂ ranging 5~70 vol%) with Mo-Co-WC as the sintering additive were prepared by spark plasma sintering. In comparison with B₄C-TiB₂ without additive, the enhanced densification was evident in the sintered specimen with Mo-Co-WC additive. Core-rim structured grain was observed around TiB₂ grains. The interface of the rim between TiB₂ and B₄C phases demonstrated different feature: the inner borderline of the rim exhibited a smooth feature, whereas a sharp curved grain boundary was observed between the rim and the B₄C grain. The formation mechanism is discussed: the epitaxial growth of (Ti,Mo,W)B₂ rim around the TiB₂ core may occur as a result of the solid solution and dissolution-precipitation between TiB₂ phase and the sintering additive. It was revealed that the fracture toughness increased as the content of TiB₂ content increased, alongside the decreased hardness. B₄C-30 vol% TiB₂ specimen demonstrated the optimal combination of mechanical properties, reaching Vickers hardness of 24.3 GPa and fracture toughness of 3.33 MPa·m^1/2.     

(Ta,Nb)C composites formed with graphene nanoplatelets by spark plasma sintering

TaC-NbC with the addition of sintering additive (5 vol.% B₄C or 5 vol.% Si) and 3 vol.% Graphene Nanoplatelets (GNP) are consolidated by spark plasma sintering (SPS). GNP are aligned in a uniformly oriented direction perpendicular to the processing axis of the SPS equipment during consolidation. High load instrumented indentation is performed and the projected area of residual damage is compared to estimate relative fracture toughness. The projected residual damaged area after indentation in the surface direction (out-of-plane GNP orientation) was 89% greater in the TaC-NbC-5B₄C-3GNP sample, and 96% greater in the TaC-NbC-5Si-3GNP sample when compared to indentation in the orthogonal (in-plane GNP orientation) direction. The toughening mechanisms prevalent in the orthogonal indentation direction (crack bridging and crack deflection) result in greater energy dissipation than the prevalent mechanisms in the surface indentation orientation (sheet sliding, crack bridging, and crack arrest).     

Processing and properties of zirconium diboride-based composites

Two zirconium diboride-base composites were produced and characterised. The chosen starting compositions were: 55 wt.% ZrB₂+41 wt.%TiB₂+4 wt.% Ni and 83 wt.% ZrB₂+13 wt.% B₄C+4 wt.% Ni. The microstructure and properties of these composites were compared to those of a monolithic ZrB₂+4 wt.% Ni material. In all cases, metallic Ni as the sintering aid promoted the formation of the liquid phase which improved mass transfer mechanisms during sintering. From the powder mixture ZrB₂+TiB₂, two solid solutions of Zr–Ti–B were obtained. In the case of the other mixture, B₄C particles were dispersed in the ZrB₂ matrix. The composite materials have better mechanical properties than those of the monolithic ZrB₂ ceramic; in particular the fracture toughness and the flexural strength were almost doubled at room temperature. Long term oxidation tests indicated that the ZrB₂-based composites, particularly the composite containing B₄C as the second phase, were more resistant to oxidation than the monolithic ZrB₂ due to the formation of surface oxide products which were protective against the complete degradation by oxidation observed for the ZrB₂ matrix material.     

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.     

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.     

Microstructure and mechanical properties of hot-pressed B4C/TiC/Mo ceramic composites

Boron carbide (B₄C)/TiC/Mo ceramic composites with different content of TiC were produced by hot pressing. The effect of TiC content on the microstructure and mechanical properties of the composites has been studied. Results showed that chemical reaction took place for this system during hot pressing sintering, and resulted in a B₄C/TiB₂/Mo composite with high density and improved mechanical properties compared to monolithic B₄C ceramic. Densification rates of the B₄C/TiC/Mo composites were found to be affected by additions of TiC. Increasing TiC content led to increase in the densification rates of the composites. The sintering temperature was lowered from 2150 °C for monolithic B₄C to 1950 °C for the B₄C/TiC/Mo composites. The fracture toughness, flexural strength, and hardness of the composites increased with increasing TiC content up to 10 wt.%. The maximum values of fracture toughness, flexural strength, and hardness are 4.3 MPa m^1/2, 695 MPa, and 25.0 GPa, respectively.     

Investigation the effect of FeNiCoCrMo HEA addition on properties of B4C ceramic prepared by spark plasma sintering

In this study, monolithic B₄C and B₄C-based ceramics incorporating FeNiCoCrMo dual-phase (FCC and BCC) high entropy alloys (HEAs) were produced by spark plasma sintering (SPS). The effect of additives on the densification behavior, mechanical properties, microstructures, and phase evaluation of the samples were investigated. X-ray analysis confirmed the existence of FCC structured HEA and depletion of BCC structured HEA, after high-temperature reaction between B₄C-HEAs. The addition of HEAs enhanced the densification behavior by liquid phase sintering. Furthermore, hardness and fracture toughness values of the samples increased with increasing HEAs content. Fracture toughness and hardness values for all composites were higher than the monolithic B₄C. A combination of the highest density (∼99.22 %) and the best mechanical properties (32.3 GPa hardness and 4.53 MPa m^1/2 fracture toughness) was achieved with 2.00 vol.% HEA addition.