Ultra-high elevated temperature strength of TiB2-based ceramics consolidated by spark plasma sintering

Spark plasma sintering of TiB₂–boron ceramics using commercially available raw powders is reported. The B₄C phase developed during reaction-driven consolidation at 1900 °C. The newly formed grains were located at the grain junctions and the triple point of TiB₂ grains, forming a covalent and stiff skeleton of B₄C. The flexural strength of the TiB₂–10 wt.% boron ceramic composites reached 910 MPa at room temperature and 1105 MPa at 1600 °С. Which is the highest strength reported for non-oxide ceramics at 1600 °C. This was followed by a rapid decrease at 1800 °C to 480–620 MPa, which was confirmed by increased number of cavitated titanium diboride grains observed after flexural strength tests.     

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.     

Spark plasma sintering and high-temperature strength of B6O–TaB2 ceramics

Tantalum diboride – boron suboxide ceramic composites were densified by spark plasma sintering at 1900 °C. Strength and fracture toughness of these bulk composites at room temperature were 490 MPa and 4 MPa m^1/2, respectively. Flexural strength of B₆O–TaB₂ ceramics increased up to 800 °C and remained unchanged up to 1600 °C. At 1800 °C a rapid decrease in strength down to 300 MPa was observed and was accompanied by change in fracture mechanisms suggestive of decomposition of boron suboxide grains. Fracture toughness of B₆O–TaB₂ composites showed a minimum at 800 °C, suggestive a relaxation of thermal stresses generated from the mismatch in coefficients of thermal expansion.Flexural strength at elevated temperatures for bulk TaB₂ reference sample was also investigated.Results suggest that formation of composite provides additional strengthening/toughening as in all cases flexural strength and fracture toughness of the B₆O–TaB₂ ceramic composite was higher than that reported for B₆O monoliths.     

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.     

Microstructure,mechanical and thermal properties of in situ toughened boron carbide-based ceramic composites co-doped with tungsten carbide and pyrolytic carbon

Boron carbide (B₄C)-based ceramics were pressureless sintered to a relative density of 96.1% at 2150 °C, with the co-incorporation of tungsten carbide and pyrolytic carbon. The as-batched boron carbide power was 7.89 m² g^?1 in surface area. A level of fracture toughness as high as 5.80 ± 0.12 MPa m^1/2 was achieved in the BW-6C composite. Sintering aids of carbon and tungsten boride were formed by an in situ reaction. The toughness improvement was attributed to the presence of thermal residual stress as well as the W₂B₅ platelets. The thermal conductivity and thermal expansivity of the BW-6C composite as a function of temperature are also reported in this work. Our current study demonstrated that the B₄C–W₂B₅ composites could be potential candidate materials for structural applications.     

Bulk boron carbide nanostructured ceramics by reactive spark plasma sintering

Bulk boron carbide (B₄C) ceramics was fabricated from a boron and carbon mixture by use of one-step reactive spark plasma sintering (RSPS). It was also demonstrated that preliminary high-energy ball milling (HEBM) of the B+C powder mixture leads to the formation of B/C composite particles with enhanced reactivity. Using these reactive composites in RSPS permits tuning of synthesized B₄C ceramic microstructure. Optimization of HEBM + RSPS conditions allows rapid (less than 30 min of SPS) fabrication of B₄C ceramics with porosity less than 2%, hardness of ~35 GPa and fracture toughness of ~ 4.5 MPa m ^1/2     

Hard polycrystalline eutectic composite prepared by spark plasma sintering

A polycrystalline eutectic B₄C–TiB₂ composite was prepared by spark plasma sintering. The starting eutectic powder was obtained by mechanical grinding of the directionally solidified eutectic B₄C–TiB₂ alloy. The microstructure of the polycrystalline composite exhibited randomly oriented eutectic grains with an average size of about 50–100 μm. Eutectic grains consisted of boron carbide matrix reinforced by titanium diboride inclusions. The secondary eutectic structure in the grain boundary is formed at sintering temperature higher than 1700 °C. XRD analysis revealed that the eutectic B₄C–TiB₂ composite consist mainly of B₄C and TiB₂ phases. The measured Vickers hardness was in the range of 32.35–54.18 GPa and the average fracture toughness of the samples was as high as 4.81 MPa m^1/2. The bending strengths of the composite evaluated at room temperature and at 1600 °C were 230 and 190 MPa, respectively.     

Pressureless densification and mechanical properties of hafnium diboride doped with B4C: From solid state sintering to liquid phase sintering

A pressureless sintering process, using a small amount of boron carbide (≤2 wt%) as sintering aid, was developed for the densification of hafnium diboride. Hafnium diboride ceramics with high relative density were obtained when the sintering temperature changed from 2100 °C to 2350 °C. However, the sintering mechanism was varied from solid state sintering (SSS, below 2300 °C) to liquid phase sintering (LPS, above 2300 °C). Boron carbide addition improved densification by removing the oxide impurities during solid state sintering and by forming a liquid phase which was well wetting hafnium diboride grains during liquid phase sintering process. The different roles of B₄C on the microstructure development and mechanical properties of the sintered ceramics were investigated.     

Room and high temperature flexural failure of spark plasma sintered boron carbide

Dense (95–98.6%) bulk boron carbide prepared by Spark Plasma Sintering (SPS) in Ar or N₂ atmospheres were subject to three-point flexural tests at room and at 1600 °C. Eight different consolidation conditions were used via SPS of commercially available B₄C powder. Resulting specimens had similar grain size not exceeding 4 µm and room-temperature bending strength (σ_25 °C) of 300–600 MPa, suggesting that difference in σ_25 °C is due to development of secondary phases in monolithic boron carbide ceramics during SPS processing. To explain such difference the composition of boron carbide and secondary phases observed by XRD and Raman spectroscopy. The variation in intensity of the Raman peak at 490 cm⁻¹ of boron carbide suggests modification of the boron carbide composition and a higher intensity correlates with a higher room-temperature bending strength (σ_25 °C) and Vickers hardness (HV). Secondary phases can modify the level of mechanical characteristics within some general trends that are not dependent on additives (with some exceptions) or technologies. Namely, HV increases, σ_25 °C decreases, and the ratio σ_1600 °C/σ_25 °C (σ_1600 °C – bending strength at 1600 °C) is lower when fracture toughness (K_IC) is higher. The ratio σ_1600 °C/σ_25 °C shows two regions of low and high K_IC delimited by K_IC=4.1 MPa m^0.5: in the low K_IC region, boron carbide specimens are produced in nitrogen.     

Pressureless sintering of beta silicon carbide nanoparticles

This study reports the pressureless sintering of cubic phase silicon carbide nanoparticles (β-SiC). Green blended compounds made of SiC nano-sized powder, a fugitive binder and a sintering agent (boron carbide, B₄C), have been prepared. The binder is removed at low temperature (e.g. 800 °C) and the pressureless sintering studied between 1900 and 2100 °C. The nearly theoretical density (98% relative density) was obtained after 30 min at 2100 °C.The structural and microstructural evolutions during the heat treatment were characterised. The high temperatures needed for the sintering result in the β-SiC to α-SiC transformation which is revealed by the change of the composite microstructure. From 1900 °C, dense samples are composed of β-SiC grains surrounding α-SiC platelets in a well-defined orientation.TEM investigations and calculation of the activation energy of the sintering provided insight to the densification mechanism.     

Gel-cast hierarchical porous B4C/C preform and its role in fabricating reaction bonded boron carbide composites

The resorcinol-formaldehyde (RF) gel-casting system is employed for the first time to fabricate a hierarchical porous B₄C/C preform, which was subsequently used for the fabrication of reaction bonded boron carbide (RBBC) composites via a liquid silicon infiltration process. The effect of the carbon content and carbon structures of this perform on the microstructures and mechanical properties of B₄C/C preform and the resultant RBBC composites is reported. The B₄C/C preform (16 wt% carbon) exhibit a strength of 34±1 MPa. The obtained RBBC composites shown uniform microstructure is consisted of SiC particles bonded boron carbide scaffold and an interpenetrating residual silicon phase. The Vickers hardness, flexural strength and fracture toughness of the RBBC composites (16 wt% carbon) are 24 GPa, 452 MPa and 4.32 MPa m^1/2, respectively.     

Pressureless sintering of boron carbide

The effect of small Al addition on pressureless-sintering and mechanical properties of B₄C ceramic was analyzed. Different amounts of aluminium powder, from 0% to 5 wt%, were added to the base material and pressureless-sintering was conducted at 2050 and 2150 °C under argon atmosphere. Microstructure, crystalline phases, density evolution, fracture strength, elastic modulus, hardness and fracture toughness were analyzed and correlated to Al additions and firing temperature. Density and grain size of sintered samples increased significantly with Al load while the effect of sintering temperature was less evident; 94% dense material was obtained by adding 4 wt% Al. Bending strength, hardness and fracture toughness of sintered B₄C samples were shown to increase for Al content up to 4 wt% while further additions resulted in a decrease of the mechanical resistance. Conversely, elastic modulus showed an increase with Al load especially between 1 and 3 wt%.     

Microstructure and mechanical properties at room and elevated temperatures of reactively hot pressed TiB2–TiC–SiC composite ceramic tool materials

TiB₂-based composite ceramic tool materials with different amounts of TiC and SiC were fabricated via a reactive hot pressing process. The mechanical properties at room temperature and flexural strength at 800–1300 °C were tested in ambient air. The composition and microstructure before and after the high-temperature strength tests were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) equipped with an energy-dispersive spectrometer (EDS). The flexural strength increment/degradation mechanisms at elevated temperatures were investigated. In-situ synthesized TiC improved the sinterability and mechanical properties of the materials at both room and elevated temperatures. Comparing with TTS (TiB₂–15.9 wt%TiC–10.6 wt%SiC) and TS (TiB₂–22.4 wt%SiC), TTS3 (TiB₂–8.1 wt%TiC–16.4 wt%SiC) had the optimum room temperature mechanical properties, i.e., flexural strength of 862 MPa, fracture toughness of 6.4 MPa m^1/2, hardness of 22.8 GPa, and relative density of 99.3%. The improved mechanical properties were ascribed to the fine grain size. The flexural strength of the TTS composite at 800 °C was higher than that at room temperature. The improvement of the flexural strength was attributed to the healing of preexisting flaws and the relief of residual stress. Substantial strength degradation took place when the temperature exceeded 1000 °C, due to softening of the grain boundaries, surface oxidation and elastic modulus degradation.     

Effect of particle size on the mechanical properties of reaction bonded boron carbide ceramics

In the present communication, effect of boron carbide particle size on the mechanical properties such as hardness, fracture toughness and flexural strength of reaction bonded boron carbide (RBBC) ceramics were investigated. RBBC composites were produced by the reactive infiltration of molten silicon into porous preform containing boron carbide and free carbon. Boron carbide powders with mean particle size of 18.65 μm, 33.70 μm and 63.35 μm were chosen for the RBBC composites. The experimental results show that hardness increases from 1261.70±64.74 kg/mm² to 1674.90±100.00 kg/mm² and fracture toughness drops from 5.76±0.26 MPa m^1/2 to 3.4±0.37 MPa m^1/2. However, flexural strength decreases from 403.41±5.70 MPa to 256.15±25.05 MPa with the increase in particle size. Indentation induced cracks in RBBC are mainly median type and number of cracks increase with the increase of starting particle size.     

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.     

Microstructural characterization of spark plasma sintered boron carbide ceramics

Fully dense boron carbide specimens were fabricated by the spark plasma sintering (SPS) technology in the absence of any sintering additives. Densification starts at 1500 °C and the highest densification rate is reached at about 1900 °C. The microstructure of the ceramic sintered at 2200 °C, with heating rates in the 50–400 °C/min range, displays abnormal grain growth, while for a 600 °C/min heating rate a homogeneous distribution of finely equiaxed grains with 4.05 ± 1.62 μm average size was obtained. TEM analysis revealed the presence of W-based amorphous and of crystalline boron-rich B₅₀N₂ secondary phases at triple-junctions. No grain-boundary films were detected by HRTEM. The formation of a transient liquid alumino-silicate phase stands apparently behind the early stage of densification.     

Spark plasma sintering of TiC ceramic with tungsten carbide as a sintering additive

By adding a small amount of tungsten carbide (WC) as sintering aids, nearly fully dense TiC ceramics were obtained by spark plasma sintering at 1450–1600 °C. The results show that the densification temperature of TiC ceramic was significantly decreased with the addition of 3.5 wt% WC. Compared with the monolithic TiC, the densification temperature of TiC–3.5 wt% WC is lower by ～150 °C and no deterioration of mechanical properties is observed. The TiC composite sintered at 1600 °C exhibits full density, a Vickers hardness of 28.2 ± 1.2 MPa, a flexural strength of 599.5 ± 34.7 MPa and a fracture toughness of 6.3 ± 1.4 MPa m^1/2.     

Wear mechanisms of TiN–TiB2 ceramic in sliding against alumina from room temperature to 700 °C

TiN–TiB₂ ceramic was prepared by the reactive hot-pressing method using titanium and BN powders as raw materials. The friction and wear properties of TiN–TiB₂ ceramic were evaluated in sliding against alumina ball from room temperature to 700 °C in air. The TiN–TiB₂ ceramic has a relative density of 98.6%, a flexural strength of 731.9 MPa and a fracture toughness of 8.5 MPa m^1/2 at room temperature. The TiN–TiB₂ ceramic exhibits a distinct decrease in friction coefficient at 700 °C as contrasted with the friction data obtained at room temperature and 400 °C. Wear mechanisms of TiN–TiB₂ ceramic depend mainly upon testing temperature at identical applied loads. Lubricious oxidized products caused by thermal oxidation provide excellent lubrication effects and greatly reduce the friction coefficient of TiN–TiB₂ ceramic at 700 °C. However, abrasive wear and tribo-oxidation are the dominant wear mechanisms of TiN–TiB₂ ceramic at 400 °C. Mechanical polishing effect and removal of micro-fractured grains play important roles during room-temperature wear tests.     

Process-tolerant pressureless-sintered silicon carbide ceramics with alumina-yttria-calcia-strontia

Process-tolerant SiC ceramics were prepared by pressureless sintering at 1850–1950 °C for 2 h in an argon atmosphere with a new quaternary additive (Al₂O₃-Y₂O₃-CaO-SrO). The SiC ceramics can be sintered to a > 94% theoretical density at 1800–1950 °C by pressureless sintering. Toughened microstructures consisting of relatively large platelet grains and small equiaxed grains were obtained when SiC ceramics were sintered at 1850–1950 °C. The presently fabricated SiC ceramics showed little variability of the microstructure and mechanical properties with sintering within the temperature range of 1850–1950 °C, demonstrating process-tolerant behavior. The thermal conductivity of the SiC ceramics increased with increasing sintering temperature from 1800 °C to 1900 °C due to decreases of the lattice oxygen content of the SiC grains and residual porosity. The flexural strength, fracture toughness, and thermal conductivity of the SiC ceramics sintered at 1850–1950 °C were in the ranges of 444–457 MPa, 4.9–5.0 MPa m^1/2, and 76–82 Wm^?1 K^?1, respectively.     

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.