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.     

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.     

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.     

Toughening of SiC matrix with in-situ created TiB2 particles

SiC–TiB₂ composites with up to 50 vol% TiB₂ were fabricated by in-situ reaction between TiO₂, B₄C and C. The densification of the uniaxially pressed samples was done using pressureless sintering in the presence of sintering aids consisting of Al₂O₃ and Y₂O₃. The influence of the volume fraction of TiB₂ and sintering temperature on density and fracture toughness was examined. It was found that fracture toughness is strongly affected by the volume fraction of TiB₂. The presence of TiB₂ particles suppresses the grain growth of SiC and facilitates different toughening mechanisms to operate which, in turn, increases fracture toughness of the composite. The highest value for fracture toughness of 5.7 MPa m^1/2 was measured in samples with 30 vol% TiB₂ sintered at 1940 °C.     

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.     

Hot-pressing and characterization of TiB2–SiC composites with different amounts of BN additive

This research aimed to study the influence of different amounts of hBN additive on the mechanical properties and microstructure of TiB₂-15 vol% SiC samples. All ceramics, containing 0, 3.5, and 7 vol% hBN, were sintered at 2000 °C using a hot-pressing route and reached their near full densities. Thanks to two different chemical reactions among the SiC reinforcement and the TiB₂ surface oxides (B₂O₃ and TiO₂), the in-situ phases of SiO₂ and TiC were generated over the sintering process. The intergranular mode was identified as the predominant fracture type in all three composite samples. The hBN additive could contribute to grain refining of composites so that the sample containing 7 vol% hBN reached the finest microstructure. Finally, the highest Vickers hardness of 25.4 HV_0.5 kg and flexural strength of 776 MPa were attained for the TiB₂–SiC and TiB₂–SiC-7 vol% hBN samples, respectively.     

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.     

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 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 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.     

Strengthening and toughening of TiN-based and TiB2-based ceramic tool materials with HfC additive

Effects of HfC addition on the microstructures and mechanical properties of TiN-based and TiB₂-based ceramic tool materials have been investigated. Their pore number decreased gradually and relative densities increased progressively when the HfC content increased from 15 wt% to 25 wt%. The achieved high relative densities to some extent derived from the high sintering pressure and the metal phases. HfC grains of about 1 µm evenly dispersed in these materials. Both TiN and TiB₂ grains become smaller with increasing HfC content from 15 wt% to 25 wt%, which indicated that HfC additive can inhibit TiN grain and TiB₂ grain growth, leading to the formation of a fine microstructure advantageous to improve flexural strength. Especially, TiB₂-HfC ceramics exhibited the typical core-rim structure that can enhance flexural strength and fracture toughness. The toughening mechanisms of TiB₂-HfC ceramics mainly included the pullout of HfC grain, crack deflection, crack bridging, transgranular fracture and the core-rim structure, while the toughening mechanisms of TiN-HfC ceramics mainly included pullout of HfC grain, fine grain, crack deflection and crack bridging. Besides, HfC hardness had an important influence on the hardness of these materials. Higher HfC content increased Vickers hardness of TiN-HfC composite, but lowered Vickers hardness of TiB₂-HfC composite, being HfC hardness higher than for TiN while HfC hardness is lower than for TiB₂. The decrease of fracture toughness of TiN-HfC ceramic tool materials with the increase of HfC content was attributed to the formation of a weaker interface strength.     

Investigation of the density and microstructure homogeneity of square-shaped B4C-ZrB2 composites produced by spark plasma sintering method

Square-shaped monolithic B₄C and B₄C-ZrB₂ composites were produced by spark plasma sintering (SPS) method to investigate the effect of 5, 10, 15 vol% ZrB₂ addition on the densification, mechanical and microstructural properties of boron carbide. The relative density of B₄C increased with the increasing volume fraction of ZrB₂ and density differences in different regions of the sample narrowed down. Homogeneous density distribution and microstructure were accomplished with the increasing holding time from 7 to 20 min for the B₄C-15 vol% ZrB₂ composites, and the highest overall relative density was achieved as 99.23%. The hardness and fracture toughness of composites were enhanced with the addition of ZrB₂ compared to monolithic B₄C. The enhancement in fracture toughness was observed due to the crack deflection, crack bridging and crack branching mechanisms. The B₄C-15 vol% ZrB₂ composite exhibited the combination of superior properties (hardness of 33.08 GPa, Vickers indentation fracture toughness of 3.82 MPa.m^1/2).     

Study on the toughening mechanism of in-situ synthesis (TixZr1−x)B2 in solid-state sintered SiC composite ceramics

High-dense SiC-(Ti_xZr_1?x)B₂ composite ceramics were fabricated by in-situ synthesis of (Ti_xZr_1?x)B₂ solid solution using solid-state spark plasma sintering (SPS). 64 vol% SiC, 20 vol% ZrB₂, 15 vol% TiB₂, and 1 vol% graphite powders are chosen as raw materials. The composite ceramics has the relative density of 99.97 %, the Vickers hardness of 24.71 GPa, the flexure strength of 435 MPa and the fracture toughness of 8.05 MPa ? m^1/2. Compared with the single-phase SiC ceramics and SiC-TiB₂ composite ceramics, the fracture toughness of SiC-(Ti_xZr_1?x)B₂ composite ceramics increased by 242.6 % and 53.6 %, respectively. A shell-core structure is found in the SiC-(Ti_xZr_1?x)B₂ composite ceramics, in which (Ti_xZr_1?x)B₂ solid solution is the core and fine SiC grain is the shell. The results show that the toughening effect of solid-state sintered SiC-(Ti_xZr_1?x)B₂ composite ceramics is attributed to the shell-core structure.     

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.     

Influence of TiB2 content on the properties of TiC–SiCw composites

The impact of various volume percentages of TiB₂ additive (0, 10, 20, and 30) on the microstructure, relative density (RD), Vickers hardness, flexural strength, and thermal conductivity of as-sintered TiC-10 vol% SiC_w-based composite samples were scrutinized. All four samples were sintered using the SPS method under the following circumstances; sintering temperature of 1900 °C, dwell time of 7 min, and external pressure of 40 MPa. The best relative density of 98.73% was achieved for the sample with no TiB₂ additive, indicating the negative effect of TiB₂ additive on the RD and formation of porosity. The microstructural observations and XRD results confirmed the chemical interaction of TiO₂ and B₂O₃ oxide layers and SiC_w and in-situ formation of the TiSi brittle phase and TiC. The most significant values of flexural strength (511 MPa) and hardness (27.67 GPa) were related to TiC-10 vol% SiC_w and TiC-10 vol% SiC_w-30 vol% TiB₂ samples, respectively. On the contrary, the specimens with 30 vol% and 10 vol% TiB₂ as additive presented the poorest qualities of flexural strength (234 MPa) and Vickers hardness (22.12 GPa). Finally, the influence of the TiB₂ content on the thermal conductivity was evaluated, indicating the positive impact of this secondary phase on this characteristic, so with adding 30 vol% TiB₂ to TiC-10 vol% SiC_w, a thermal conductivity of 30.7 W/m.K was obtained.     

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.     

Microstructures and mechanical properties of B4C-TiB2-SiC composites fabricated by ball milling and hot pressing

B₄C-TiB₂-SiC composites were fabricated via hot pressing using ball milled B₄C, TiB₂, and SiC powder mixtures as the starting materials. The impact of ball milling on the densification behaviors, mechanical properties, and microstructures of the ceramic composites were investigated. The results showed that the refinement of the powder mixtures and the removal of the oxide impurities played an important role in the improvement of densification and properties. Moreover, the formation of the liquid phases during the sintering was deemed beneficial for densification. The typical values of relative density, hardness, bending strength, and fracture toughness of the composites reached 99.20%, 32.84?GPa, 858?MPa and 8.21?MPa?m^1/2, respectively. Crack deflection, crack bridging, crack branching, and microcracking were considered to be the potential toughening mechanisms in the composites. Furthermore, numerous nano-sized intergranular/intragranular phases and twin structures were observed in the B₄C-TiB₂-SiC composite.     

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.     

High hardness and high fracture toughness B4C-diamond ceramics obtained by high-pressure sintering

The B₄C-diamond composite with high hardness and toughness was first prepared by high-pressure sintering of B₄C and diamond powders at 5 GPa and 1600 °C. The effect of the diamond fraction on the densification, microstructure and mechanical properties of B₄C-diamond composite were investigated. The results indicated that the hardness of the as-prepared composite ceramics increased gradually with the increase in diamond content. The composite having 40 vol% diamond exhibited excellent comprehensive mechanical properties with a relative density of 98.3%, a density of 2.86 g/cm³, Vickers hardness of 39.8 GPa and fracture toughness of 8.1 MPa·m^1/2. The use of superhard diamond enhanced the fracture toughness of the B₄C while maintaining its lightweight and high hardness. The main toughening mechanisms were crack bridging, crack deflection and pull-out of homogeneously dispersed diamond grains. Superhard second phase dispersion high-pressure sintering provides a new technical route to improve the properties of advanced 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.