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.     

Densification and strengthening mechanism in spark plasma sintering of B4C–VB2 ceramics via carbide boronizing

To reduce the negative effects of the long-time and B₂O₃ phase on the traditional sintering process for B₄C-based composite ceramics, nearly fully dense B₄C–VB₂ composite ceramics were prepared by reactive spark plasma sintering (SPS) technology at 2000 °C with B and V₈C₇ powders as raw materials in this paper. The effects of the degassing time during SPS on the microstructure and the mechanical properties of the final products were investigated in detail. The results revealed that the proper degassing time was beneficial for the vent of B₂O₃ during the sintering process, which refined the grain size, promoted densification and improved the mechanical properties of the composite ceramic. However, the redundant degassing time increased the holding time at high temperature, resulting in abnormal grain growth and mechanical performance deterioration. In the present work, the optimal degassing time was 6 min, and the final product prepared under the above conditions exhibited excellent comprehensive performance with a relative density of 99.2%, Vickers hardness of 31.2 GPa, bending strength of 654 MPa and fracture toughness of 5.7 MPa m^1/2. In addition, the strengthening and toughening mechanisms of the products were mainly attributed to the residual thermal stresses and bridging structure caused by the fine B₄C and VB₂ grains distributed uniformly.     

Microstructure and mechanical properties of B4C based ceramics with Fe3Al as sintering aid by spark plasma sintering

B₄C based ceramics were fabricated with different Fe₃Al contents as sintering aids by spark plasma sintering at relatively low temperature (1700 °C) in vacuum by applying 50 MPa pressure and held at 1700 °C for 5 min. The effect of Fe₃Al additions (from 0 to 9 wt%) on the microstructure and mechanical properties of B₄C has been studied. The composition and microstructure of as-prepared samples were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron probe microanalyzer (EPMA) equipped with WDS (wavelength dispersive spectrometry) and EDS. The mixtures of B₄C and Fe₃Al underwent a major reaction in which the metal borides and B₄C were encountered as major crystallographic phases. The sample with 7 wt% of Fe₃Al as a sintering aid was found to have 32.46 GPa Vickers hardness, 483.40 MPa flexural strength, and 4.1 MPa m^1/2 fracture toughness which is higher than that of pure B₄C.     

Microstructure and mechanical properties of B4C matrix composites prepared via hot-pressing sintering with Pr6O11 as additive

High-performance B₄C-PrB₆ composites were prepared via hot-pressing sintering with matrix phase B₄C and with 2–5 wt% Pr₆O₁₁ as additive. The effects of different sintering processes and Pr₆O₁₁ content on the microstructure and mechanical properties of the composites were studied in detail. It is found that increasing sintering temperature and pressure will contribute to the densification of B₄C-PrB₆ composites. Coarse grains are formed in B₄C without additives at high temperature conditions, resulting in the decrease of the densification. Pr₆O₁₁ can effectively hinder the formation of coarse grains and finally promote the densification of the composites. The main toughening mechanisms of composites was crack deflection. The composites with 4 wt% Pr₆O₁₁ prepared at 2050 °C and 25 MPa had the best comprehensive mechanical properties. The relative density, hardness, flexural strength and fracture toughness reached to 98.9%, 37.6 GPa, 339 MPa and 4.4 MP am^1/2, respectively.     

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.     

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.     

Properties and ballistic tests of strong B4C-TiB2 composites densified by gas pressure sintering

B₄C-TiB₂ composites with classical 75/25 vol ratio were sintered by pressureless sintering with and without gas pressure application in the final stage of densification, using a novel prototypal furnace. A small fraction of WC was introduced through high energy milling of the starting powders with WC-Co spheres. High energy milling facilitated the densification thanks to incorporation of WC impurities acting as sintering aid, and size reduction of the starting powders. Strength, stiffness and toughness of the ceramic densified at 2050 °C via gas pressure sintering were even better than hot pressed composites at 1900 °C. Depth of Penetration tests on plates with 3–5 mm thickness demonstrated that the gas pressure sintered material had a superior performance compared to the hot pressed one. This work also revealed that hardness was not the property spotting the best ballistic performance.     

Densification,microstructure and properties of ultra-high temperature HfB2 ceramics by the spark plasma sintering without any additives

Ultra-high temperature HfB₂ ceramic with nearly full densification is achieved by using gradient sintering process of SPS without any additives. The effect of the sintering temperature on the densification behavior, relative density, microstructure, mechanical and thermionic properties is systematically investigated. The results show that the fast densification of HfB₂ ceramic occurs at the heating stage, and the highest relative density of 96.75% is obtained at T =1950 °C, P = 60 MPa and t =10min. As the temperature is increased from 1800 to 1950 °C, the grain size of HfB₂ increases from 6.12 ±1.33 to 10.99 ± 2.25 μm, and refined microstructure gives the excellently mechanical properties. The highest hardness of 26.34 ±2.1GPa, fracture toughness of 7.12 ± 1.33 MPa m^1/2 and bending strength of 501 ±10MPa belong to the HfB₂ ceramic obtained at T =1950°C. Moreover, both the Vickers hardness and fracture toughness obey the normal indentation size effect. HfB₂ ceramic also exhibits the thermionic emission characterization with the highest current density of 6.12 A/cm² and the lowest work function of 2.92 eV.     

Pressureless sintering of boron carbide with Cr3C2 as sintering additive

In this study, chromium carbide (Cr₃C₂) was selected as the sintering additive for the densification of boron carbide (B₄C). Cr₃C₂ can react with B₄C and form graphite and CrB₂ in situ, which is considered to be effective for the sintering of B₄C composites. The sintering behavior, microstructure development and mechanical properties of B₄C composites were studied. The density of B₄C composite increased with the increase of Cr₃C₂ content and sintering temperature. The formation of liquid phase could effectively improve the densification of B₄C composites. The abnormal grains began to appear at 2080 °C. The bending strength could reach 440 MPa for the 25 wt% and 30 wt% Cr₃C₂ samples after sintering at 2070 °C.     

Microstructural evolution of ZnO via hybrid cold sintering/spark plasma sintering

In this work, we demonstrate a hybrid cold sintering/spark plasma sintering (CSP-SPS) process to densify ZnO ceramic with controlled grain growth. The densification of ZnO is initially activated at 85 °C, and high densities (>98%) are achieved at 200–300 °C in only 5 min with a low assisted pressure of 3.8–50 MPa. The microstructure of ZnO grains experiences a mild coarsening from ~205–680 nm during the CSP-SPS. In comparison, a much higher temperature (>770 °C) is required to sinter ZnO ceramic via SPS, and the grain size exhibits an obvious overgrowth to ~10 µm. The calculated apparent activation energy of grain growth using CSP-SPS is 69.3 ± 6 kJ/mol, which is much lower than that of SPS samples with 296.8 ± 59 kJ/mol. In addition, the conduction mechanism of the CSP-SPS and SPS samples is investigated using impedance spectroscopy. Overall, CSP-SPS is promising for the fabrication of fine ceramics with mild sintering conditions.     

Mechanism of improving the mechanical properties of Si3N4/TiC ceramic tool materials prepared by spark plasma sintering

Spark plasma sintering (SPS) is a new sintering method having shorter sintering time and higher densification speed than the traditional sintering methods. In this paper, the Si₃N₄/TiC ceramic tool material is sintered by SPS. The microstructure and mechanical properties of the material under different sintering parameters are compared. The sintering process of the material is then analyzed, and the best sintering parameters are obtained. Heat the material to 1600°C and keep the temperature for 15 min, then continue to heat to 1700°C and keep the temperature for 10 min, Si₃N₄/TiC ceramic tool material has high mechanical properties, its bending strength, fracture toughness, and Vickers hardness are 959 MPa, 8.61 MPa·m^1/2, and 15.21 GPa, respectively. The scanning electron microscope (SEM) analysis shows that under this condition, the sintering additives and Si₃N₄/TiC material form the liquid phase, which makes the Si₃N₄ particles rearrange, dissolve, precipitate, and transform into rod shape β-Si₃N₄. In addition, under the action of pulse current and external pressure, electric sparks are generated between TiC particles, which allows the material transfer and particle refinement. Therefore, the β-Si₃N₄ has uniform grain size, and it is vertically and horizontally arranged in the structure, which makes the material have excellent mechanical properties.     

On the effects of LiF on the synthesis and reactive sintering of gahnite (ZnAl2O4)

The effects of LiF on the synthesis and reactive sintering of polycrystalline gahnite (zinc aluminate spinel, ZnAl₂O₄) were studied using XRD, high-temperature simultaneous thermal analysis and a spark plasma sintering (SPS) apparatus. It was demonstrated that the LiF reduces the onset of synthesis by about 200 °C and plays an important role in the densification process. SPS consolidation of a LiF-doped ZnO-Al₂O₃ mixture under an applied pressure of 150 MPa and at a sintering temperature of 1100 °C for 20 min generated fully dense gahnite with adequate transparency and mechanical properties.     

Strengthening Hf0.95Ta0.05B2 ceramic with ultrafine grains and high-density dislocations

Full densification and fine microstructures are the two key optimization targets of ceramic materials. Although fine Hf_0.95Ta_0.05B₂ powder (∼ 0.36 µm) has been synthesized, it was still difficult to obtain densified Hf_0.95Ta_0.05B₂ ceramics with ultrafine grains (< 1 µm) using conventional high temperature sintering. Increasing sintering pressure could provided higher densification driving force, but it usually negatively promoted grain growth for nanoceramics. Our strategy was to gain the fully dense Hf_0.95Ta_0.05B₂ ceramic under a high pressure at a selected temperature with retarded grain growth. In this work, fully dense Hf_0.95Ta_0.05B₂ ceramic was prepared at 1700 °C under a high pressure of 200 MPa. The limited grain growth was achieved with the average grain size of 0.6 µm. Therefore, the mechanical properties were significantly improved, including Vickers hardness (24.8 GPa) and fracture toughness (4.2 MPa.m^1/2), which were ascribed to Hall-Petch and dislocation strengthening mechanism.     

Joining of C/SiC composites by spark plasma sintering technique

CVD–SiC coated C/SiC composites (C/SiC) were joined by spark plasma sintering (SPS) by direct bonding with and without the aid of joining materials. A calcia-alumina based glass–ceramic (CA), a SiC + 5 wt% B₄C mixture and pure Ti foils were used as joining materials in the non-direct bonding processes. Morphological and compositional analyses were performed on each joined sample. The shear strength of joined C/SiC was measured by a single lap test and found comparable to that of C/SiC.     

Spark plasma sintering of Sm2O3-doped aluminum nitride

The high sintering temperature required for aluminum nitride (AlN) at typically 1800 °C, is an impediment to its development as an engineering material. Spark plasma sintering (SPS) of AlN is carried out with samarium oxide (Sm₂O₃) as sintering additive at a sintering temperature as low as 1500–1600 °C. The effect of sintering temperature and SPS cycle on the microstructure and performance of AlN is studied. There appears to be a direct correlation between SPS temperature and number of repeated SPS sintering cycle per sample with the density of the final sintered sample. The addition of Sm₂O₃ as a sintering aid (1 and 3 wt.%) improves the properties and density of AlN noticeably. Thermal conductivity of AlN samples improves with increase in number of SPS cycle (maximum of 2) and sintering temperature (up to 1600 °C). Thermal conductivity is found to be greatly improved with the presence of Sm₂O₃ as sintering additive, with a thermal conductivity value about 118 W m⁻¹ K⁻¹) for the 3 wt.% Sm₂O₃-doped AlN sample SPS at 1500 °C for 3 min. Dielectric constant of the sintered AlN samples is dependent on the relative density of the samples. The number of repeated SPS cycle and sintering aid do not, however, cause significant elevation of the dielectric constant of the final sintered samples. Microstructures of the AlN samples show that, densification of AlN sample is effectively enhanced through increase in the operating SPS temperature and the employment of multiple SPS cycles. Addition of Sm₂O₃ greatly improves the densification of AlN sample while maintaining a fine grain structure. The Sm₂O₃ dopant modifies the microstructures to decidedly faceted AlN grains, resulting in the flattening of AlN–AlN grain contacts.     

In-situ synthesis and densification of HfB2 ceramics by the spark plasma sintering technique

Hafnium diboride (HfB₂) ceramics were in-situ synthesized and densified by the spark plasma sintering (SPS) method using HfO₂ and amorphous boron (B) as starting powders. Both synthesis and densification processes were succesfully accomplished in a single SPS cycle with one/two step heating schedules, which were designed by considering thermodynamic calculations made by Factsage software. In two step heating schedule, soaking at 1000 °C, which was supposed to be the synthesis temperature of HfB₂ particles, caused a creep like behaviour in final ceramic microstructures. A single step synthesis/densification schedule at 2050 °C with a 30 min hold time under 60 MPa uniaxial pressure leads to obtain monolithic HfB₂ ceramics up to 94% of it's theoretical density. Considering the literature, low hardness values (max. 12 GPa) were achieved, which were directly attributed to the low bonding between HfB₂ grains in terms of the residual stresses occurred during the synthesis and cooling steps. Samples produced by applying one step heating schedule showed transgranural fracture behaviour with a, fracture toughness of 3.12 MPa m^1/2. The fracture toughness of the samples produced by applying two step heating schedule was higher (5,06 MPa m^1/2) and the fracture mode changed from transgranular to mixed mode.     

Spark plasma sintering and pressureless sintering of SiC using aluminum borocarbide additives

Aluminum borocarbide powders (Al₃BC₃ and Al₈B₄C₇) were synthesized, and the ternary powders were used as a sintering additive of SiC. The densification of SiC was nearly completed at 1670 °C using spark plasma sintering (SPS) and pressureless sintering was possible at 1950 °C. The sintering behavior of SiC using the new additive systems was nearly identical with that using the conventional Al–B–C system, but grain growth was suppressed when adding the borocarbides. In addition, oxidation of the fine additive powders did not intensively occur in air, which has been a problem in the case of the Al–B–C system for industrial application. The hardness, Young's modulus and fracture toughness of a sintered SiC specimen were 21.6 GPa, 439 GPa and 4.6 MPa m^1/2, respectively. The ternary borocarbide powders are efficient sintering additives of SiC.     

An economic and environment friendly way of recycling boron carbide waste to prepare B4C/Al composite ceramic

Recently, with the rapid growth of sapphire wafer applications, boron carbide as abrasive, has shown an increasing demand. Great amounts of boron carbide waste (low purity and small grain size with D₅₀ ≈ 1 μm) are therefore produced during the production of boron carbide abrasives and barely recovered and utilized. This paper is aimed at developing an economic and environment friendly process to recycle the boron carbide waste through adding a certain amount of Al powder to prepare B₄C/Al composite ceramic. Prior to the sintering process, samples were firstly mixed with different Al powder and then pelleted and dehydrated. The effects of the pelletizing factors on performances of the pellets and the ceramics were optimized as binder hydroxypropyl methylcellulose addition 0.4%, pelleting pressure 30 MPa, Al addition 9 wt%, sintering time 90 minutes. Under these conditions, the apparent porosity, bulk density, compressive strength and flexural strength of the sintered B₄C/Al are 19.08%, 1.84 g/cm³, 246.88 MPa and 71.10 MPa respectively. Al addition can not only attribute to the low-temperature liquid sintering and densification of the product, but also generation of some stable phases including AlB₁₂, AlB₁₂C₂ and Al₃BC, which in turn increase the performance of the ceramic composite.     

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).     

Fracture and property relationships in the double diboride ceramic composites by spark plasma sintering of TiB2 and NbB2

Bulk titanium diboride–niobium diboride ceramic composites were consolidated by spark plasma sintering (SPS) at 1950°C. SPS resulted in dense specimens with a density exceeding 98% of the theoretical density and a multimodal grain size ranging from 1 to 10 μm. During the SPS consolidation, the pressure was applied and released at 1950 and 1250°C, respectively. This allowed obtaining a two-phase composite consisting of TiB₂ and NbB₂. For these ceramics composites, we evaluated the flexural strength and fracture toughness and room and elevated temperatures. Room-temperature strength of thus produced bulks was between 300 and 330 MPa, at 1200°C or 1600°C an increase in strength up to 400 MPa was observed. Microstructure after flexure at elevated temperatures revealed the appearance of the needle-shape subgrains of NbB₂, an evidence for ongoing plastic deformation. TiB₂–NbB₂ composites had elastic loading stress curves at 1600°C, and at 1800°C fractured in the plastic manner, and strength was ranged from 300 to 450 MPa. These data were compared with a specimen where a (Ti,Nb)B₂ solid solution was formed during SPS to explain the behavior of TiB₂–NbB₂ ceramic composites at elevated temperatures.