In situ fabrication and properties of 0.4MoB-0.1SiC-xMoSi2 composites by self-propagating synthesis and hot-press sintering

Mo, Si and B₄C powders were used to fabricate 0.4MoB-0.1SiC-xMoSi₂ composites by self-propagating high-temperature synthesis (SHS) and hot pressing (HP). The effects of MoSi₂ content (x = 1, 0.75, 0.5 and 0.25) on phase composition, microstructure and properties of the composites were investigated. The results showed that the 0.4MoB-0.1SiC-xMoSi₂ composite exhibited Vickers hardness of 10.7–15.2 GPa, bending strength of 337–827 MPa and fracture toughness of 4.9–7.0 MPa m^1/2. The fracture toughness increased with the increasing volume fraction of MoB and SiC particles which were promoted by the toughening mechanisms, such as crack bridging, cracks deflection and crack branching. Moreover, the electrical resistivity showed an increasing trend with decreasing volume fraction of MoSi₂.     

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.     

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.     

Titanium diboride–silicon carbide–boron carbide ceramics with super‐high hardness and strength

Triplex particulate composites composed of boride and carbide ceramics were found to have high strength, hardness, and fracture toughness values. Two compositions consisting of 70:15:15 and 1:1:1 volume ratios of TiB_2, SiC, and B₄C were produced from commercially available powders by hot‐pressing. The 70:15:15 ceramic exhibited a strength of ~1.3 GPa, while the 1:1:1 ceramic had a strength of ~0.9 GPa. These strengths are comparable to super‐strong Y₂O₃‐PSZ and β‐SiAlON based composites. The Vickers’ hardness values of these ceramics were ~32 GPa for indent loads of 9.8 N. Hardness increased as indentation load decreased. The 1:1:1 ceramic had a hardness of ~53 GPa at an indentation load of 0.49 N, higher than values reported for so‐called “super‐hard” ceramics, and comparable to c‐BN.     

Effect of graphite content on properties of B4C‐W2B5 ceramic composites by in situ reaction of B‐Gr‐WC

Strip‐shaped W₂B₅ reinforced B₄C ceramic composites were prepared via in situ reaction of boron(B)‐graphite(Gr)‐WC system by powder metallurgy (P/M). In order to study the effect of the graphite content on the properties of the as‐fabricated ceramic composites, the powder mixture of B‐Gr‐WC with various amounts of Gr powder were blended and consolidated by spark plasma sintering (SPS). The sintering parameters were shown as following: sintering pressure was set as 30 MPa; The three‐step sintering temperature was 1100‐1550‐1700°C and the duration time was set as 5‐5‐6 minutes, respectively. In situ formed strip‐shaped W₂B₅ particles were dispersed homogeneously in B₄C matrix, which resulted in a remarkable improvement on the fracture toughness and mechanical properties. Appropriate 5vol% residual Gr in the composite shows positive effect on the mechanical properties which achieved an optimal counter‐balance of fracture toughness and hardness, the relative density was 99.8%, the Vickers hardness can reach 30.2 GPa, and the fracture toughness was 11.9 MPa·m^1/2 when the sintering temperature was set at 1700°C.     

Microstructure and Mechanical Properties of (TiB2 + SiC) Reinforced Ti3SiC2 Composites Synthesized by In Situ Hot Pressing

Fully dense (TiB₂ + SiC) reinforced Ti₃SiC₂ composites with 15 vol% TiB₂ and 0–15 vol% SiC were designed and synthesized by in situ reaction hot pressing. The increase in SiC content promoted densification and significantly inhibited the growth of Ti₃SiC₂ grains. The in situ incorporated TiB₂ and SiC reinforcements showed columnar and equiaxed grains, respectively, providing a strengthening–toughening effect by the synergistic action of particulate reinforcement, grain's pulling out, “self‐reinforcement,” crack deflection, and grain refining. A maximum bending strength of 881 MPa and a fracture toughness of 9.24 MPam^1/2 were obtained at 10 vol% SiC. The Vickers hardness of the composites increased monotonously from 9.6 to 12.5 GPa.     

Effects of SiC on the microstructures and mechanical properties of B4C–SiC–rGO composites prepared using spark plasma sintering

In this study, B₄C–SiC–rGO composites with different SiC contents were prepared by spark plasma sintering at 1800 °C for 5 min under a uniaxial pressure of 50 MPa. The effects of SiC on the microstructures and mechanical properties of the B₄C–SiC–rGO composites were investigated. The optimal values for flexural strength (545.25 ± 23 MPa) and fracture toughness (5.72 ± 0.13 MPa·m^1/2) were obtained simultaneously when 15 wt.% SiC was added to 5 wt.%–GO reinforced B₄C composites (BS15G5). It was found that SiC and rGO inhibited the grain growth of B₄C and improved the mechanical properties of the B₄C–SiC–rGO composites. The clear and narrow grain boundaries of rGO–B₄C and rGO–SiC, as well as the semi-coherent B₄C–SiC interface, indicated strong interface compatibility. The twin structures of SiC and B₄C observed in the composites improved their fracture toughness. Crack deflection and crack bridging caused by the SiC grains as well as rGO bridging and rGO pull-out were observed on the crack propagation path.     

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.     

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.     

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.     

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.     

Microstructures and mechanical properties of B4C–SiC intergranular/intragranular nanocomposite ceramics fabricated from B4C,Si, and graphite powders

B₄C–SiC intergranular/intragranular nanocomposites with high hardness and high toughness were fabricated through mechanochemical processing with B₄C, Si, and graphite powders and subsequent hot pressing without any sintering aid. The milled powders are composed of stacking-disordered SiC and nanocrystalline B₄C. Most nano/micron-sized SiC particles are homogeneously dispersed in B₄C matrix, and some nano-sized SiC and B₄C particles are embedded into B₄C grains to form an intergranular/intragranular structure. The disordered characteristic of the milled powders is the essential factor for the formation of the intragranular structure, sudden densification within the narrow temperature range (1700–1900 °C), and the preparation of dense samples under a relatively low temperature (1900 °C). The relative density, Vickers hardness, and fracture toughness of the samples sintered at 1950 °C are 98.6%, 34.3 GPa, and 6.0 MPa m^1/2, respectively. The intergranular/intragranular structure plays an important role in improving fracture toughness and hardness of the composites.     

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.     

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.     

Mechanical properties and microstructure evolution of pressureless-sintered B4C–SiC ceramic composite with CeO2 additive

Boron carbide ceramic composites (B₄C)-silicon carbide (SiC) with the cerium oxide (CeO₂) additive, which was varied from 0 wt% to 9 wt%, were prepared by pressureless sintering at 2150 °C for 60 min. The effect of CeO₂ additive content on the microstructure and mechanical properties of the B₄C–SiC ceramic composites was investigated in detail. In-situ synthesised cerium hexaboride (CeB₆) was identified in the B₄C–SiC ceramic composites. B-rich transition zones (such as B_38.22C₆, B_51.02C_1.82) were formed between the B₄C and CeB₆ grains, which introduced local lattice distortion to increase the sintering driving force. The thermal conductivity coefficient of CeB₆ was higher than that of B₄C, which benefited the delivery of heat quantity and helped achieve a highly dense and uniform sintered body. When the CeO₂ additive was excessively increased (more than 5 wt%), the CeB₆ grains had a large grain size and exhibited an increase in the amount of generated carbon monoxide (CO) gas, which led to an increase in the porosity of the B₄C–SiC ceramic composites and decrease in the mechanical properties. The optimum values of the relative density, Vickers hardness, flexural strength, and fracture toughness of the B₄C–SiC ceramic composite with 5 wt% CeO₂ additive were 96.42%, 32.21 GPa, 380 MPa, and 4.32 MPa m^1/2, respectively.     

Microstructure and properties of a graphene platelets toughened boron carbide composite ceramic by spark plasma sintering

A kind of B₄C/SiC composite ceramic toughened by graphene platelets and Al was fabricated by spark plasma sintering. The effects of graphene platelets and Al on densification, microstructure and mechanical properties were studied. The sintering temperature was decreased about 125–300?°C with the addition of 3–10?wt% Al. Al can also improve fracture toughness but decrease hardness. The B₄C/SiC composite ceramic with 3?wt%Al and 1.5?wt% graphene platelets sintered at 1825?°C for 5?min had the optimal performances. It was fully densified, and the Vickers hardness and fracture toughness were 30.09?±?0.39?GPa and 5.88?±?0.49?MPa?m^1/2, respectively. The fracture toughness was 25.6% higher than that of the composite without graphene platelets. The toughening mechanism of graphene platelets was also studied. Pulling-out of graphene platelets, crack deflection, bridging and branching contributed to the toughness enhancement of the B₄C-based ceramic.     

Microstructure and mechanical properties of (Zr,Ti)B2–SiC composites obtained by pressureless sintering of ZrB2–SiC–TiO2 powder mixtures

Multiphase ceramics have been highlighted due to the combination of different properties. This work proposes to obtain the multiphase composite of (Zr,Ti)B₂–SiC based on the mixture of ZrB₂, SiC, and TiO₂ sintered without pressure. The effect of TiO₂ addition on solid solution formation with ZrB₂, densification, microstructure, and mechanical properties was investigated. For this, 2.0 wt% TiO₂ was added to ZrB₂–SiC composites with 10–30 vol% SiC and processed by reactive pressureless sintering at 2050 °C with a 2 h holding time. Sinterability, crystalline phases, microstructure, Vickers hardness, and indentation fracture toughness of these composites were analyzed and compared to the non-doped ZrB₂–SiC samples. The XRD analysis and EDS elemental map images indicated the incorporation of Ti atoms into the ZrB₂ crystalline structure with solid solution generation of (Zr,Ti)B₂. The addition of TiO₂ resulted in matrix grain size refinement and a predominant intergranular fracture mode. The relative densities were not significantly modified with the TiO₂ addition, though a higher weight loss was detected after the sample sintering process. The composites doped with TiO₂ showed an increase in fracture toughness but exhibited a slightly lower Vickers hardness compared to composites without TiO₂ addition.     

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).     

Effect of fiber type on mechanical properties of short carbon fiber reinforced B4C composites

In order to enhance the mechanical properties of B₄C without density increase, the short carbon fibers M40, M55J and T700 reinforced B₄C ceramic composites were fabricated by hot-pressing process. The addition of the carbon fibers accelerates the densification of the B₄C, decreases their densities, and improves their strength and toughness. The enhancement effects of the three kinds of carbon fibers were studied by investigating the density, Vickers hardness and the mechanical properties such as flexural strength, flexural modulus and fracture toughness of the composites. The fiber type has a great influence on the mechanical properties and enhancement of the short carbon fiber reinforced B₄C composites. The flexible carbon fiber with high strength and low modulus such as T700 is appropriate to reinforce the B₄C matrix ceramic composites.     

Synthesis and mechanical properties of (Mo0.94Nb0.06)(Si0.97Al0.03)2−x vol% SiC composites fabricated via SHS-HP

(Mo_0.94Nb_0.06)(Si_0.97Al_0.03)₂–SiC composites were consolidated via vacuum HP sintering from the mixture of (Mo_0.94Nb_0.06)(Si_0.97Al_0.03)₂ powder and SiC nanoparticles. The (Mo_0.94Nb_0.06)(Si_0.97Al_0.03)₂ powder was prepared by self propagating synthesis (SHS). The effects of SiC content on the ambient temperature mechanical properties and microstructure of the SHS-HP composites were investigated. The results show that the indentation fracture toughness, bending strength and Vickers hardness of (Mo_0.94Nb_0.06)(Si_0.97Al_0.03)₂−x vol% SiC composites increase gradually with the increase of SiC content. The grain size and relative density of the composites decrease gradually with increase of SiC content. (Mo_0.94Nb_0.06)(Si_0.97Al_0.03)₂−15 vol% SiC composite exhibits excellent mechanical properties: Vickers hardness 12.62 GPa, fracture toughness 5.01 MPa m^1/2, bending strength 472 MPa. With the addition of SiC, the fracture mode transforms from a mixed mode of transcrystalline and intercrystalline fracture to mainly transcrystalline fracture.