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.     

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.     

Microstructure evolution and performance of B4C–NdB6 composites fabricated by in situ hot pressing

B₄C–NdB₆ composites were fabricated by in situ hot pressing at different temperatures (1950–2150°C) with B₄C and Nd₂O₃ (2–4 wt%) as raw materials. The microstructure evolution of the composites with sintering temperature and Nd₂O₃ content was studied in detail, and the influence of pressure on the sintering of B₄C with different contents of Nd₂O₃ was also investigated. The performance of the fabricated composites was researched and the toughening mechanism was discussed. The results indicate that Nd₂O₃ can react with B₄C to form the thin-sheet intermediate products (Nd(BO₂)₃, Nd₂CO₅) first, which then transform to band-shaped NdB₆. Pressure can reduce the distance of B₄C and Nd₂O₃, accelerating the mass transfer and contributing to the formation of NdB₆. NdB₆ and intermediate products are first in agglomerate structure at 1950°C, and then the agglomerates are broken to form dispersive micron and submicron NdB₆ at 2000°C by the synergistic function of pressure, diffusion at high temperature, and liquid phase sintering. NdB₆ can enhance the densification owing to the bonding function. Excessive Nd₂O₃ content leads to residual pores, and excessive temperature (2150°C) results in the coarsening of phases. The coexistence of transgranular and intergranular fracture of NdB₆ promote the fracture toughness.     

High-performance B4C matrix composites fabricated from the B4C–Si–CeO2 system via reactive hot pressing

Boron carbide (B₄C) matrix composites had the advantages of high hardness, high melting point and low density. However, due to the low relative density and poor fracture toughness of B₄C, its comprehensive properties were limited in engineering applications. In this work, in order to improve the comprehensive properties of B₄C composites, B₄C–SiC–SiB₆–CeB₆ composites were designed and fabricated via reactive hot pressing at 2050 °C and 20 MPa with B₄C matrix and novel additives (Double doping of Si and CeO₂) as raw materials. The effects of additive CeO₂ content on the microstructures and mechanical properties of composite were investigated, and reaction mechanisms of B₄C, Si and CeO₂ at different temperatures were studied in detail. The work showed that liquid phase Si and SiB₆ greatly improved the densification of composites. CeB₆ played an indispensable role in the formation of SiC–SiB₆ agglomerate structure, increasing strength and supplementing toughness. When the content of CeO₂ was 6 wt%, the relative density, hardness, flexural strength and fracture toughness reached to 99.7%, 34.9 GPa, 461.46 MPa and 5.57 MPa m^1/2, respectively. Our strategy benefited from the formation of two liquid phases and SiC–SiB₆ agglomerate structure, showing great potential in promoting sintering and improving fracture toughness.     

Synthesis and performance of TiB2–B4C composites by high pressure of TiC–B mixture

TiB₂–B₄C composites were in situ synthesized and consolidated by high pressure synthesis method from a mixture of TiC and B powders at the pressure and temperature of 5.0 GPa and 1500℃-1900℃. The phase composition, microstructure, density, hardness, thermal conductivity, and electrical resistivity of TiB₂–B₄C composites were analyzed. As the increase in the synthesis temperature, the products were TiB₂ and B₄C phases and that crystallinity improved. TiB₂–B₄C composites were dense without obvious pores. TiB₂–B₄C composites synthesized at 1800℃ obtained the optimized performance, including the relative density of 98.2%, the Vickers hardness of 31.7 ± 1.2 GPa with the load of 9.8 N, the thermal conductivity of 30.3 ± 0.7 W/(m K), and the electrical resistivity of 3.3 × 10⁻³ Ω cm, respectively. The grain size of the TiB₂–B₄C composites changed with the increase in synthesis temperature, leading to the changes in hardness, thermal conductivity, and electrical resistivity.     

Effect of TiB2 particles on microstructure and mechanical properties of B4C–TiB2 ceramics prepared by hot pressing

B₄C-20 wt% TiB₂ ceramics were fabricated by hot pressing B₄C and ball-milled TiB₂ powder mixtures. The effects of the TiB₂ particle size on the microstructure and mechanical properties were investigated. The results showed that the TiB₂ particle size played an important role in the mechanical properties of the B₄C–TiB₂ ceramics. In addition, SiO₂ introduced by ball milling was beneficial for densification but detrimental to the mechanical properties of the B₄C–TiB₂ ceramics. The typical values of relative density, hardness, flexural strength, and fracture toughness of the ceramics were 99.20%, 35.22 GPa, 765 MPa, and 7.69 MPa m^1/2, respectively. The toughening mechanisms of the B₄C–TiB₂ ceramics were explained by crack deflection and crack branching. In this study, the effects of high pressure and temperature caused liquefying SiO₂ to migrate to the surface of B₄C–TiB₂ and react with diffused carbon source in the graphite foil to form a 30 μm thick SiC layered structure, which improved the high-temperature oxidation resistance of the material. The unique SiC layered structure overcame the insufficient oxidation resistance of B₄C and TiB₂, thereby improving the oxidation resistance of the ceramics under high-temperature service conditions.     

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.     

Mechanical properties and deformation mechanisms of B4C–TiB2 eutectic composites

Samples of B₄C–TiB₂ eutectic are laser processed to produce composites with varying microstructural scales. The eutectic materials exhibit both load dependent and load independent hardness regimes with a transition occurring between 4 and 5 N indentation load. The load-independent hardness of eutectics with a microstructural scale smaller than 1 μm is about 31 GPa, and the indentation fracture toughness (5–10 N indenter load) of the eutectics is 2.47–4.76 MPa m^1/2. Indentation-induced cracks are deflected by TiB₂ lamellae, and indentation-induced spallation is reduced in the B₄C–TiB₂ eutectic compared to monolithic B₄C. Indentation-induced amorphization in monolithic B₄C and the B₄C phase of the eutectic is detected using Raman spectroscopy. Sub-surface damage is observed using TEM, including microcracking and amorphization damage in B₄C and B₄C–TiB₂ eutectics. Dislocations are observed in the TiB₂ phase of eutectics with an interlamellar spacing of 1.9 μm.     

Densification of MCMB–SiC composites via two-step hot pressing

Mesocarbon microbead–SiC (MCMB–SiC) composites with 30 wt% MCMBs were densified using a two-step hot pressing method. Based on the pyrolysis of the initial MCMB powders, the effects of the pressing schedule on densification were investigated and the optimal first-step pressing temperature was determined. To reveal the influence of temperature on their microstructures, the raw MCMB powders were heat-treated at different temperatures in the range 400–1400 °C. The morphologies and degrees of carbonisation at different temperatures were additionally studied. The results showed that densification was mainly affected by the micro-gaps in the lamellar structure formed during the pyrolysis of the MCMBs. When the samples were first hot-pressed at a lower temperature and then at a higher temperature, the densification pressure required was effectively decreased. Furthermore, when the samples were first pressed at an appropriate temperature, the relative density of the composites was improved to a rather high value of 98.6%. The two-step hot pressing method was effective in fabricating dense C–SiC composites with high C content.     

Characteristics of quadruplet Ti–Mo–TiB2–TiC composites prepared by spark plasma sintering

The spark plasma sintering process was implemented to produce four different composites, namely Ti-10 wt% Mo-(0.5, 1, 2, and 4) wt% (TiB₂ + TiC). All samples were sintered at 1300 °C for 5 min under 50 MPa. A full study was carried out on the mechanical properties and the relative density of these SPSed composite samples. The best relative density of around 98.7% was related to the sample with 1 wt% (TiB₂ + TiC). The role of relative density was so predominant that the best values for all mechanical properties, i.e., bending strength, hardness, elongation, and ultimate tensile strength (UTS), were achieved for those with the highest relative density values. The formation of the in-situ TiB phase was proved by the XRD analysis. Besides, microscopical investigations (optical and SEM) showed that adding more ceramic additives led to an increased amount of porosity while Mo solubility decreased in the titanium matrix. Finally, different fracture modes on the surfaces of composite samples were studied using SEM images.     

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.     

Mechanisms of TiB2 and graphite nucleation during TiC–B4C high temperature interaction

Reactive hot pressing of TiC–B₄C precursors was undertaken at 1800 °C to produce TiB₂ with carbon inclusions. Atomic mechanisms of titanium diboride nucleation, as well as sponge-like carbon inclusions and submicron platelets of graphite precipitation have been investigated. Precursor grain size, green body composition and synthesis time were varied to analyze phase transformation. The carbon left after B₄C high temperature decomposition is shown remaining as graphite sponge-like inclusions. Ab-initio calculations confirm that the boron atoms accumulation on (111) TiC plains leads to tensile stress. The developed stress cleaves TiC grains and enhances further reaction. Most of carbon expelled from TiC during its transformation into TiB₂ forms graphite submicron platelets.     

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.     

Characterization of triplet Ti–TiB–TiC composites: Comparison of in-situ formation and ex-situ addition of TiC

Four hybrid titanium-based samples with different amounts of B₄C as reinforcement and the same numbers of specimens with TiB₂ + TiC additives were produced using the SPS method at 1200 °C. For a reliable comparison, stoichiometry relations of starting powders were applied to estimate the precise contents of mentioned reinforcements to reach the same vol% of final TiB and TiC phases. The microstructure, relative density (RD), and mechanical properties of both series of SPSed composite samples were studied to make a reliable comparison. The best value of the RD (99.9%) was achieved for the sample doped with 0.48 wt% B₄C. The in-situ TiB_w phase formation in both series of samples was confirmed using XRD and microscopical evaluations. Samples doped with B₄C exhibited better UTS, tensile elongation, and bending strength than the ones doped with TiB₂ + TiC. The sample with the least B₄C content presented the best value of mentioned mechanical properties.     

Mechanical properties of B4C–CrB2 binary eutectic composites prepared by arc-melting

B₄C–CrB₂ composites were prepared by arc-melting using B₄C and CrB₂ powders as raw materials. The eutectic composition of B₄C–CrB₂ system was 30B₄C–70CrB₂ (mol%) with a labyrinth-like irregularly layered eutectic microstructure, composed of B₄C phase about 1–2 μm in thickness dispersing in CrB₂ matrix, much smaller than raw powders. The interface of the eutectic composite was well bonded, and there were edge dislocations at the interface to alleviate the interface mismatch. The eutectic temperature of B₄C–CrB₂ composites was approximately 2200 K. At the eutectic composition, the B₄C–CrB₂ composites showed the maximum Vickers hardness (24.6 GPa) and fracture toughness (4.3 MPa m^1/2) at room temperature.     

Preparation of TiB2–SiC composites toughened with interlocking microstructure by self-assembled TiB2 plates

The spark plasma sintering (SPS) technique was found to effectively improve the mechanical properties of TiB₂–SiC ceramic by forming a unique interlocking structure. This study investigated the phase transition process of the hexagonal micro-platelets TiB₂ powders with self-assembled structure during the molten-salt-mediated carbothermal reduction and its effect on promoting the mechanical properties of TiB₂-based ceramics. It was found that the SPS approach ensured a highly densified TiB₂–SiC ceramics with enhanced Vickers hardness of 21.0 ± 1.3 GPa and fracture resistance of 7.8 ± 0.3 MPa m^1/2. The performance enhancement of the resultant TiB₂–SiC composite was attributed to the interlocking structure from the original anisotropic TiB₂ powders, which could effectively absorb the energy and facilitate the crack deflection.     

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.     

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.     

In-situ synthesis and low-temperature fabrication of B4C/TiB2-based multilayer graded composites with B4C and Ti–Al intermetallics mixtures through transient liquid phase sintering

The seven-layer B4C/TiB2-based graded composites was prepared with B4C and Ti–Al intermetallics through stepped laminating processing and transient liquid phase spark plasma sintering. The sintering strategy of the graded composites was proposed based on the sintering products of monolayer materials with different contents of Ti–Al intermetallics from 5 wt% to 60 wt%. The top three layers and bottom three layers were sintered respectively at 1650 °C and 1500 °C, and then the middle layer was used as the binder to joint the as-preserved two sections at 1550 °C. The apparent density of the as-prepared B4C/TiB2-based multilayer graded composites was 2.94 g/cm3, which was lower than that of most advanced ceramics. With the increase in the addition of Ti–Al intermetallics, the hardness of B4C/TiB2-based multilayer graded composites decreased from 31 GPa (B4C-riched) to 25 GPa (TiB2-riched), whereas the fracture toughness increased from 3.8 MPa·m0.5–6.02 MPa·m0.5. The compressive strength was up to 1100 MPa, displaying the jagged stress-strain curve. Crack propagation resistance mechanisms such as deflection and bridging enhanced the fracture toughness. The B4C/TiB2-based multilayer graded composites fabricated at low temperature possess high front hardness, high rear toughness, high overall strength and low density, and has promising applications in impact-resistant fields such as lightweight ceramic armor.