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 particle size,heating rate and CNT addition on densification,microstructure and mechanical properties of B4C ceramics

Monolithic B₄C ceramics and B₄C–CNT composites were prepared by spark plasma sintering (SPS). The influence of particle size, heating rate, and CNT addition on sintering behavior, microstructure and mechanical properties were studied. Two different B₄C powders were used to examine the effect of particle size. The effect of heating rate on monolithic B₄C was investigated by applying three different heating rates (75, 150 and 225 °C/min). Moreover, in order to evaluate the effect of CNT addition, B₄C–CNT (0.5–3 mass%) composites were also produced. Fully dense monolithic B₄C ceramics were obtained by using heating rate of 75 °C/min. Vickers hardness value increased with increasing CNT content, and B₄C–CNT composite with 3 mass% CNTs had the highest hardness value of 32.8 GPa. Addition of CNTs and increase in heating rate had a positive effect on the fracture toughness and the highest fracture toughness value, 5.9 MPa m^1/2, was achieved in composite with 3 mass% CNTs.     

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.     

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.     

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.     

Role of MgO on densification and mechanical properties in spark plasma sintered Os0.9Re0.1B2 ceramic

To promote the densification and therefore the mechanical properties of boride-based ceramics, MgO was added as sintering aid into Os_0.9Re_0.1B₂ powders for densification by using spark plasma sintering (SPS). The Os_0.9Re_0.1B₂ powders were synthesized by mechanochemical method from powder mixture of Os, Re and amorphous B. The role of MgO on densification, phase composition, microstructure and mechanical properties (hardness, fracture toughness and wear behavior) were studied by using X-ray diffraction (XRD), scanning electron microscope (SEM) with energy-dispersive spectroscopy (EDS), micro indentation and ball-on-disk tribometer. The results show that, with the introduction of MgO as sintering aid, the relative density of the Os_0.9Re_0.1B₂ ceramic samples increased. When the MgO content reached 9 wt%, the as-sintered sample is almost fully dense. No obvious regularity was found from the samples with the addition of different content of MgO. Vickers hardness values of the samples with 0, 3 wt% and 9 wt% MgO are found to be very close with each other within the experimental error (~30 GPa), while the sample with the addition of 6 wt% MgO exhibits the highest hardness of ~35 GPa. The fracture toughness of the samples is decreased slightly with the addition of MgO. The friction coefficient and wear rate of the sample with the addition of 6 wt% MgO was also found to be the lowest among all samples, which indicate best wear resistance. As a whole, with the addition content of 6 wt% MgO, the Os_0.9Re_0.1B₂ ceramic sample performs relatively excellent mechanical properties among four groups of samples.     

Hard and tough ultrafine-grained B4C-cBN composites prepared by high-pressure sintering

Tough and hard ultrafine-grained B₄C-cBN composites were firstly fabricated by high-pressure sintering mixed B₄C and cBN nanopowders at 6 GPa and 1700 °C. The phase transition from cBN to hBN is avoided by high pressure during the sintering process. The effects of the cBN content on the densification and mechanical properties of B₄C-cBN composites were evaluated. The results indicated that the hardness of the as-fabricated composites increased gradually with the increase of cBN content. The composite composed of 50 wt.% cBN exhibited excellent comprehensive mechanical properties with relative density of 98.6 %, density of 2.9 g/cm³, Vickers hardness of 36.2 GPa and fracture toughness of 6.7 MPa·m^1/2. The introduction of superhard cBN maintained the lightweight and high hardness while enhancing the fracture toughness of the B₄C. The main toughening mechanisms were crack bridging, crack deflection and pull-out of homogeneously dispersed cBN grains.     

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.     

Processing and characterization of TiB2‐TiNiFeCrCoAl high‐entropy alloy composite

A series of TiB₂‐TiNiFeCrCoAl high‐entropy alloy (HEA) composites with HEA contents of 5, 10, and 20 wt.% were fabricated to a relative density of above 95% without applied external pressure at 1550°C in flowing argon. Research indicates (i) the formation of liquid HEA is promoted with the assistance of an optimized ball mill process, which facilitates the sintering through a liquid phase sintering mechanism, and (ii) HEA can react with the oxide impurities in TiB₂ and can consequently enhance the densification. Continual observations on microstructures and properties indicate that the sintered composites have a submicronmeter grain size of 0.90±0.11, 0.79±0.09, and 0.74±0.07 μm; a high hardness of 23.3±0.9, 21.6±1.0, and 17.5±1.2 GPa; and a high indentation fracture toughness of 8.7±0.4, 11.1±0.6, and 12.8±0.6 MPa·m^1/2, respectively, for samples with 5, 10, and 20 wt.% HEA. The addition of HEA to TiB₂ can significantly enhance the densification, results in fine microstructures, and improve the mechanical performance.     

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.     

B4C-based hard and tough ceramics densified via spark plasma sintering using a novel Mg2Si sintering aid

Full-dense B₄C-based ceramics with excellent mechanical properties were fabricated using spark plasma sintering with Mg₂Si as a sintering aid at a low temperature of 1675 °C while applying a uniaxial pressure of 50 MPa. The effect of Mg₂Si addition on the densification behaviours, mechanical properties and microstructure of as-sintered ceramics were investigated. Not only did the formation of ultra-fine grained SiC using the in-situ reaction effectively inhibit the growth of B₄C grains, but it also contributed to the strength and toughness of the resultant ceramics. Additionally, microalloying Mg imparted more metal bonding characteristics to the B₄C matrix, thereby improving their ductility. The results indicate that the composite containing 7 wt% Mg₂Si had excellent mechanical properties, including a light weight of 2.54 g/cm³, Vickers hardness of 34.3 GPa, fracture toughness of 5.09 MPa m^1/2 and flexural strength of 574 MPa.     

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.     

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.     

Densification and mechanical properties of boron carbide prepared via spark plasma sintering with cubic boron nitride as an additive

Hexagonal boron nitride (h-BN) can reinforce boron carbide (B₄C) ceramics, but homogeneous dispersion of h-BN is difficult to achieve using conventional methods. Herein, B₄C/h-BN composites were manufactured via the transformation of cubic (c-) BN during spark plasma sintering at 1800 °C. The effects of the c-BN content on the microstructure, densification, and mechanical properties of B₄C/h-BN composites were evaluated. In situ synthesized h-BN platelets were homogeneously dispersed in the B₄C matrix and the growth of B₄C grains was effectively suppressed. Moreover, the c-BN to h-BN phase transformation improved the sinterability of B₄C. The sample with 5 vol.% c-BN exhibited excellent integrated mechanical properties (hardness of 30.5 GPa, bending strength of 470 MPa, and fracture toughness of 3.84 MPa⋅ m^1/2). Higher c-BN contents did not significantly affect the bending strength and fracture toughness but clearly decreased the hardness. The main toughening mechanisms were crack deflection, crack bridging, and pulling out of h-BN.     

Microstructure and mechanical properties of spark plasma sintered TiB2 ceramics combined with a high-entropy alloy sintering aid

A high-entropy alloy (HEA), CoCrFeNiMn_0.5Ti_0.5, is used as a sintering aid for the densification of TiB₂ sintered by spark plasma sintering. The HEA content in the starting TiB₂-HEA mixture is varied from 0 to 10?wt-%. The microstructure and mechanical properties of the sintered samples are analysed and the optimum HEA content of 10% is found for the preparation of the TiB₂-HEA ceramics, allowing combining high mechanical properties (Vickers hardness of 2174.64?HV and flexural strength of 427.69?MPa) and high relative density of 99.1%.     

Synthesis and microstructure evolution of WCoB based cermets during spark plasma sintering

WCoB based cermets were prepared by spark plasma sintering at sintering temperature among 600°C-1200 °C. The phase evolution was investigated by detecting density behavior, phase composition, microstructure and mechanical properties during sintering process. The sintering process can be divided into three stages: powder densification, solid phase reaction and liquid phase sintering. WCoB hard phase forms at 1000 °C during solid phase sintering, showing better mechanical properties than Co₂B, especially on Vicker's hardness. WCoB hard phase forms on the basis of Co₂B binary boride and its content increases in liquid phase sintering stage with high density. The Vicker's hardness and transverse rupture strength (TRS) reach the maximum value of 1262 Hv and 1212 MPa at 1200 °C and 1170 °C, respectively. The fracture toughness reaches the maximum value of 21.8 MPa m^1/2 at 1050 °C, and the inter-granular fracture is the main fracture mechanism.     

Improving the dry sliding-wear resistance of B4C ceramics by transient liquid-phase sintering

A critical comparison is made between the dry sliding-wear resistance of a B₄C composite fabricated by transient liquid-phase sintering with Ti-Al intermetallic additive and two reference monolithic B₄C ceramics fabricated by solid-state sintering. It is shown that, as a consequence of its full densification and super-hardness, the B₄C composite is, despite containing secondary phases, markedly more wear resistant (significantly lower coefficient of friction, specific wear rate, worn volume, and wear damage) than the reference monolithic B₄C ceramic fabricated under identical spark-plasma-sintering (SPS) conditions, and at least as wear resistant as the reference monolithic B₄C ceramic fabricated at much higher SPS temperature. In all materials, wear is nonetheless mild and occurred by two-body abrasion dominated by plastic deformation at the micro-contact level plus, in the porous reference monolithic B₄C ceramic, three-body abrasion dominated by fracture. Implications for the lower-cost manufacture of superhard B₄C tribocomponents are discussed.     

Effect of phase transformation of CoCrFeNiAl high-entropy alloy on mechanical properties of WC-CoCrFeNiAl composites

WC-CoCrFeNiAl composites were fabricated via SPS using commercial CoCrFeNiAl and WC powders, with the optimal addition content of CoCrFeNiAl determined. Furthermore, the influence of phase transformation in CoCrFeNiAl high-entropy alloy on the mechanical properties of WC-CoCrFeNiAl composites was investigated. The results indicate that the hysteresis diffusion effect of CoCrFeNiAl HEA can significantly impede the growth of WC grains. Moreover, during the sintering process, a BCC-to-FCC phase transformation occurs in CoCrFeNiAl HEA. The phase transition of HEA can be regulated by adjusting the sintering temperature, resulting in a decrease in hardness and an increase in fracture toughness of WC-CoCrFeNiAl composites as HEAs undergo phase transformation. The Vickers hardness and fracture toughness values of WC-10CoCrFeNiAl composites sintered at 1250 °C are comparable to those of WC-10Co hard alloy, with respective values of 17.64 GPa and 12.3 MPa m^1/2.     

Low temperature sintering of Hf0.95Nb0.05B2-based ceramics with submicron-scaled grains and enhanced mechanical properties

Hf_0.95Nb_0.05B₂ ceramics and their composites containing 20 vol% SiC were prepared via high-pressure spark plasma sintering in the study. The densification, microstructures, and mechanical properties of the prepared materials were then investigated. It is challenging to achieve full densification of HfB₂ ceramics, even with markedly refined Hf_0.95Nb_0.05B₂ solid solution powder under the sintering conditions of 2000 °C/30 MPa. However, under the sintering conditions of 1700 °C/200 MPa, a dense microstructure of Hf_0.95Nb_0.05B₂ ceramics was achieved. Moreover, the Hf_0.95Nb_0.05B₂-20 vol% SiC composite was densified at a lower temperature (1500 °C) and exhibited ultrafine grains (300 nm) and high-density defects, including stacking faults, Lomer-Cottrell locks, and twins, thus resulting in exceptional comprehensive mechanical properties, such as ultra-high hardness (32 GPa) and significantly improved fracture toughness (5.2 MPa.m^1/2).     

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.