Effect of residual excess carbon on the densification of ultra-fine HfC powder

The residual carbon content of ultra-fine hafnium carbide (HfC) powder was controlled by the optimization of the synthesis process, and the effect of residual carbon on the densification of HfC powder was analyzed. The amount of residual carbon in the HfC powder could be reduced by the de-agglomeration of HfO₂ powder before the carbo-thermal reduction (CTR) process. The average particle size of HfO₂ powder decreased from 230 to 130 nm after the de-agglomeration treatment. Ultra-fine (d₅₀: 110 nm) and highly pure (metal basis purity: >99.9 % except for Zr) HfC powder was obtained after the CTR at 1600 °C for 1 h using the C/Hf mixing ratio of 3.3. In contrast, the C/Hf ratio increased to 3.6 without the de-agglomeration treatment, indicating that a large amount of excess carbon was required for the complete reduction of the agglomerated HfO₂ particles. HfC ceramics with high relative density (>98 %) were obtained after spark plasma sintering at 2000 °C under 80 MPa pressure when using the HfC powder with low excess carbon content. In contrast, the densification did not complete at a higher temperature (2300 °C) and pressure (100 MPa) when the HfC powder contained a large amount of residual carbon. The results clearly indicated that residual carbon suppressed the densification of HfC powder in case the carbide powder had low oxygen content, and the residual carbon content could be controlled by the optimization of the synthesis process. The average grain size and Vickers hardness of the sintered specimen were 6.7(±0.7) μm and 19.6 GPa, respectively.     

Shock-wave synthesis of diamonds from fullerenes C60−C100

Shock-wave synthesis of diamond from C₆₀–C₁₀₀-fullerene powder was first accomplished by using the explosive compaction technique with plane wave loading in the pressure range of 24–40 GPa. The compacts of various initial composition comprised diamond, FCC C₆₀-fullerite, graphite, and amorphous carbon. The largest diamonds of 0.1–1.0 m were obtained under shock loading of pellets consisting of copper powder with 5 wt. % fullerite at 24 and 38 GPa, and pellets consisting of copper powder with 10 wt. % fullerite at 40 GPa. The end product consists of diamond without intermediate diamondlike phases such as n-diamond and hexagonal diamond (lonsdaleite).Central Machine-Building Technology Research Institute, 109088 Moscow. Translated from Fizika Goreniya i Vzryva, Vol. 31, No. 2, pp. 131–138, March–April, 1995.     

Reaction‐Bonded B4C with High Hardness

Reaction‐bonded B₄C (RBBC) was fabricated through molten Si infiltrating porous B₄C preforms. A preform with a 75% relative green density was obtained by mixing two different sized B₄C powders. Carbon black added to the preform slightly reduces average pore size, but increases porosity. RBBC ceramics showed a dense and homogeneous microstructure. Vickers hardness was about 15 GPa for RBBC fabricated from a single type of B₄C powder and could reach 22–23 GPa for the carbon‐added samples after infiltration. Morphological evolution and the influence of the microstructure on the Vickers hardness were investigated and discussed.     

Cost-effective synthesis of AlMgB14–xTiB2

As an alternative to mechanical alloying, high temperature synthesis (HTS) of ultra-hard, super-abrasive AlMgB₁₄ was performed under normal pressure. The reaction mixture consisted of elemental aluminium and boron, whereas magnesium was added in the form of a Mg-precursor which liberates elemental magnesium approximately 400 °C above the melting point of magnesium, in this way reducing its evaporation during heating-up. The composition after the conversion was 95 wt.% of AlMgB₁₄ and 5 wt.% of MgAl₂O₄. The synthesized AlMgB₁₄ baseline powder, as well as mixtures of AlMgB₁₄ consisting of 30, 50 and 70 wt.% of TiB₂, were hot pressed to near theoretical density. The various samples produced were characterized for microstructure and hardness. A micro-hardness of 29.4 GPa in hot pressed AlMgB₁₄ and a maximum Vickers hardness of 30.2 GPa in hot pressed samples of AlMgB₁₄ reinforced with 70 wt.% of TiB₂ were achieved.     

Influence of carbon nanotubes as secondary phase addition on the mechanical properties and amorphization of boron carbide

The mechanical properties and amorphization response of a carbon nanotube (5 wt.%) boron carbide (CNT-B₄C) composite with 1 μm grain size are investigated, and compared to those of coarse-grained (10 μm grain size) and ultrafine-grained (0.3 μm grain size) monolithic boron carbides. The quasi-static and dynamic uniaxial compressive strengths for CNT-B₄C were statistically the same as those of the ultrafine-grained ceramic and higher than the coarse-grained material, contradicting the expected grain size hierarchy (Hall-Petch-type relationship). Addition of CNTs to B₄C resulted in decreased quasi-static hardness compared to the large grain size material; however, dynamic hardness was substantially improved compared to quasi-static values. CNT pullout and crack bridging were observed to be possible toughening mechanisms. Finally, Raman spectroscopy was used to quantify amorphization, and it was concluded that addition of CNTs to boron carbide does not alter the propensity for amorphization, but does improve mechanical properties by enhanced toughening.     

Synthesis,densification, microstructure,and mechanical properties of samarium hexaboride ceramic

Samarium hexaboride (SmB₆) powders were synthesized by boro/carbothermal reduction of Sm₂O₃ with B₄C. Nominally pure SmB₆ powder had a mean particle size of about 400 nm and an oxygen content of 0.12 wt%. SmBO₃ formed as an intermediate phase during the synthesis. The synthesized powder was hot pressed at 1950°C to produce SmB₆ ceramics with relative densities >99.6% and a mean grain size of 4.4 μm. Vickers’ hardness was 20.1 ± 0.7 GPa. Young's modulus measured by bending and ultrasonic methods was 271 and 244 GPa, respectively. The flexure strength was 253 ± 79 MPa and fracture toughness was 2.1 ± 0.1 MPa m^1/2. These are the first reported results of the microstructure and bulk mechanical behavior of SmB₆ ceramics.     

Densification of SiC by colloidal processing and SPS without sintering additives

Abstract

Silicon carbide is one of the most important ceramics used as structural and functional materials in a wide variety of applications. Many studies have reported the densification of SiC using oxide and nonoxide additives such as the Al₂O₃, B₄C and Al–B–C system. However, it is difficult to densify SiC at temperatures below 2000°C without sintering additives even if spark plasma sintering (SPS) is used. The authors attempted to densify SiC using colloidal processing and SPS without sintering additives. A commercially available SiC powder with the average particle size of 0·55 μm was used as the starting material. The densities of the green body prepared by slip casting and the sintered body by SPS were 65·5 and 98·7% respectively.     

Liquid-phase sintering of PZT ceramics

Lead zirconate titanate (abbreviated as PZT) ceramics are of considerable commercial importance for a host of piezoelectric and pyroelectric applications. Conventionally, many PZT ceramics are sintered at temperatures above 1250°C. Such extreme temperatures are undesirable due to the increased energy consumption, limitation of electrode material and evaporation of volatile components. A liquid-phase sintering aid incorporating Cu₂O and PbO is presented which demonstrates a reduction in the required sintering temperature of these ceramics. This new aid is described with particular reference to a commercial PZT, termed Pz26, used industrially for its optimised piezoelectric properties. Pz26 has a composition near the morphotropic phase boundary and possesses a tetragonal crystalline structure. Typically this material is sintered between 1260 and 1300°C for 1 h to achieve the required densification. With the inclusion of sintering aid, sintered densities comparable to those obtained by conventional sintering are achieved at only 800°C. The optimum weight percentage of sintering aid varies for different ceramic materials, particle sizes, morphology and the desired sintering temperature. However, with standard “mixed-oxide” produced Pz26 powder and with a median particle size in the range 1.6–1.7 μm, a value of 5 wt.% allows sintering at 800°C, according to densification, dielectric and piezoelectric measurements (ϵ=873, tan δ=1.13 %, k_p=43.1%). When finer grained powder is used (d_0.5=1.1 μm), improved properties (ϵ=960, tan δ=1.04%, k_p=51.7%) are obtained for an addition of 3 wt.% sintering aid and a sintering temperature of 850°C.     

Effect of Quasi-Hydrostatic Compression on the Mechanical Properties of Ceramics in the ZrO2 + 3 MOL.% Y2O3 System

The effect of quasi-hydrostatic compression on the strength of ZrO₂ + 3 mol.% Y₂O₃ ceramic specimens of two series was studied. The series 1 ceramic was a powder commercially available from TOSOH Co. (Japan), with a density of 6.1 g/cm³, and the series 2 ceramic was a powder with a density of 5.9 g/cm³ prepared under laboratory conditions at the IPM Research Institute (National Academy of Sciences, Ukraine). The pressure range was up to 1.2 GPa, and the pressure-transmitting medium was a coarse-grained corundum powder. In the series 1 specimens, the strength increases with pressure over the entire pressure range (from 670 MPa to 1098 MPa at 1.2 GPa); in the series 2 specimens, the strength increases only to a pressure of 0.8 GPa (from 695 MPa to 828 MPa) and then, with further increase in pressure drops sharply to nearly zero (30 MPa at 1.2 GPa). It was proposed that the observed effect might be associated with a martensite transformation in the zone of structural imperfections (discontinuities). On reaching a critical value determined by the strength of the matrix, the martensite transformation becomes a cause of failure of the material.     

Effect of Yttrium (20%) doping on mechanical properties of rare earth nano lanthanum phosphate (LaPO4) synthesized by aqueous sol-gel process

In this study, Nascent Lanthanum Phosphate (LaPO₄) and LaPO₄ with 20% reinforced Yttria (Y₂O₃) precursor ceramic powder was synthesized through aqueous Sol-gel route, where theLaPO₄ and LaPO₄/Y₂O₃ powder was sourced from Lanthanum chloride (LaCl₃) and orthophosphoric acid (H₃PO₄). By sintering the precursor powder at 1400 °C for 2 h, a relatively high-density composite material of LaPO₄ (97.4%) and LaPO₄/Y₂O₃ (98.4%) were fabricated. Through X-ray powder diffraction (XRD) experiment, the results revealed that LaPO₄ and LaPO₄/Y₂O₃ electrolyte samples sintered at 1400 °C for 2 h is impurity free and thereby elemental composition of the powders is also verified. Further, a small quantity of LaP₂ and La₂O₃elements formed during the synthesis of LaPO₄ was found to contribute more on corrosion resistance and high withstanding capacity of the material. 26% of YPO₄ formed due to a chemical reaction between Yttrium and Lanthanum made this composite a suitable candidate material for high-temperature applications. Various mechanical characterization tests such as micro-hardness, flexural strength, and compression capacity were studied, where the microscopic analysis provided comprehensive information about both transgranular and intergranular types of failure on the fractured regions of LaPO₄ and LaPO₄/Y₂O₃ material. The MicroVicker'shardness of LaPO₄ and composite was found to be 5 GPa and 5.2 GPa, respectively. When compared with the nascent sample, on an average, 22% reduction in flexural strength and a 1.05% increase in Young's modulus are observed for the high-density composite.     

Proton-conducting ceramics as electrode/electrolyte materials for SOFC's—part I: preparation,mechanical and thermal properties of sintered bodies

The aim of these investigations was to prepare and to examine compounds of a high temperature solid oxide fuel cell with a proton conducting electrolyte in view of the mechanical and thermal properties. The powders were made by the conventional solid reaction of carbonates and oxides. The stoichiometry of the electrolyte Ba,Ca niobate (BCN) was varied with x=0, x=0.12 and x=0.18. As potential cathode material SrCeO₃ and SrZrO₃ stabilised with 5% Yb was prepared, and as anode material cermet of BCN and Ni with 50:50 wt.% was synthesised. The mechanical properties like bending strength (room and high temperature), Young modulus (E), modulus of rigidity (G), Poison's ratio, micro hardness and fracture toughness were measured on sintered samples. The highest values for bending strength, E and G could be found for BCN12 (156 MPa, 160 GPa, 63 GPa) and the cerate (175 MPa, 145 GPa, 56 GPa), the lowest for the cermet BCN/Ni (72 MPa, 68 GPa, 29 GPa). The investigation of the thermal properties of the bulk material showed a thermal stability to a temperature of 1400 °C. The thermal expansion coefficient measured at 1000 °C was found to be in the range of 10–12×10⁻⁶/K. Further investigations with respect to the mechanical and thermal properties have to be made for the whole system of cathode–electrolyte–anode.     

On the manufacturing and electrical and mechanical properties of ultra-high wt.% fraction aligned MWCNT and randomly oriented CNT epoxy composites

The effect of CNT orientation on electrical and mechanical properties is presented on the example of an ultra-high filler loaded multi-walled carbon nanotube (68 wt.% MWCNTs) epoxy-based nanocomposite. A novel manufacturing method based on hot-press infiltration through a semi-permeable membrane allows to obtain both, nanocomposites with aligned and randomly oriented CNTs (APNCs and RPNCs) over a broad filler loading range of ≈10–68 wt.%. APNCs are based on low-defected, mm-long aligned MWCNT arrays grown in chemical vapour deposition (CVD) process. Electrical conductivity and mechanical properties were measured parallel and perpendicular to the direction of CNTs. RPNCs are based on both, aligned mm-long MWCNTs and randomly oriented commercial μm-long and entangled MWCNTs (Baytube C150P, and exemplarily Arkema Graphistrength C100). The piezoresistive strain sensing capability of these high-wt.% APNCs and RPNCs had been investigated towards the influence of CNT orientations. For the highest CNT fraction of 68 wt.% of unidirectional aligned CNTs a Young’s modulus of E_|| ≈ 36 GPa and maximum electrical conductivity of σ_|| ≈ 37·10⁴ S/m were achieved.     

Microstructure and mechanical properties of carbon-precursor-added B4C and B4C−SiC ceramics subjected to pressureless sintering

Organic-carbon-precursor-added B₄C and B₄C–SiC ceramics were subjected to pressureless sintering at various temperatures. The carbon precursor increased the densification of the B₄C and B₄C–SiC ceramics sintered at 2200 °C to 95.6 % and 99.1 % theoretical density (T.D.), respectively. The pyrolytic carbon content of the B₄C–SiC composite decreased with increasing SiC content. The graphitization degree of pyrolytic carbon decreased slightly with the amount of carbon precursor and content of SiC. The 95 wt. % B₄C–5 wt. % SiC composite added with 7.5 wt. % carbon precursor and sintered at 2200 °C outperformed the other B₄C–SiC composites, and its sintered density, flexural strength, Young’s modulus, and microhardness were 98.6 % T.D., 879 MPa, 415 GPa, and 28.5 GPa, respectively. These values were higher than those of composites prepared via pressureless sintering and comparable to those of composites fabricated via hot pressing and/or using metal or oxide additives.     

Optimization of a commercial brake pad formulation

A brake pad material used in a popular, commercially available vehicle that consisted of steel wool, iron powder, graphite, coke, styrene–butadiene rubber, MgO, BaSO₄, and phenolic resin was tested with the friction assessment and screening test. The average friction coefficient (0.357) and total wear (19.75 wt %) were measured. An alternative friction material formulated with identical constituents but optimized with the golden section principle and relational grade analysis was produced in a laboratory environment. This material exhibited an average friction coefficient of 0.419 and a low total wear of 6.25 wt %. An analysis of component costs indicated that the large volume price of the commercial material, $1.01/kg, was less than that of the laboratory material, $1.21/kg. However, the performance/cost ratio of the new material was appreciably greater. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 2498–2504, 2002     

Suppressing η-phase development in steel-cemented tungsten carbide: A spark plasma sintering study

We describe the phase stability of a cemented tungsten carbide prepared using a high-vanadium tool steel as the cementing/binder phase and confirm suppression of (Fe, W)₆C η-phase formation, attributed to the preferential formation of a V_0.78W_0.22C_1−x phase that exists as islands within the Fe-rich binder matrix. The samples were prepared using spark plasma sintering (SPS), starting from commercially available WC and A11-LVC tool steel powders. The starting powders were ball milled adding 10, 15, and 20 vol.% steel. An A11-LVC tool steel was chosen as a low-cost hard steel (49 HRC) that does not contain Ni or Co but has a high vanadium (~9 wt.%) and carbon (~1.75 wt.%) content. Our results show that sintering by SPS can produce high-density (>98%) WC-steel specimens in which the matrix wets the WC grain surfaces and formation of the brittle η-phase is avoided. The η phase is often regarded as embrittling and undesirable, and its presence can result in degradation of mechanical properties. Microhardness values for the WC-10 and WC-15 vol.% steel samples were 12.3 ± 1.2 and 13.0 ± 0.9 GPa, respectively, whereas the fracture toughness values were 8.83 ± 0.48 and 8.81 ± 0.61 MPa·m^1/2, respectively.     

Controlling micro-porous size in TiO2 pellets processed by sol-gel and rapid liquid phase sintering

A fast and lower electric energy consumption process to synthesize TiO₂ pellets with interconnected micropores, is proposed. Pellets were prepared by rapid liquid-phase sintering (RLPS) at different temperatures (900, 1000 and 1100 °C) and times (2, 5, 7 and 10 min). The density of these samples increases when temperature rises and decreases for longer sintering times; the highest density, of 2.78 g/cm³ was obtained when sintering at 1100 °C/2min. The addition of PEG and the annealing at 450 °C/2 min produced pores of 38.51 ± 27.51 μm and 48.98 ± 32.34 μm when PEG3350 and PEG8000 respectively, were used. An additional RLPS at 1100 °C/2 min gives rise to TiO₂ pellets in a rutile phase, with pores of 76.82 ± 34.23 μm and 173.04 ± 68.03 μm for PEG3350 and PEG8000, respectively. Interconnectivity of pores is obtained in all samples. The elastic module of these pellets was 39.22 ± 0.16 GPa, for the sample prepared with PEG3350; and 121.30 ± 0.04 GPa for the one made with PEG8000. The achieved pore size and interconnectivity at 1100 °C/2 min are a result of the optimized sintering conditions and the better control of PEG vapor pressure released when the intermediate annealing at 450 °C/2 min is introduced.     

Processing and room temperature mechanical properties of a zirconium carbide ceramic

Zirconium carbide (ZrC) powder, batched to a ratio of 0.98 C/Zr, was prepared by carbothermal reduction of ZrO₂ with carbon black. Nominally phase-pure ZrC powder had a mean particle size of 2.4 μm. The synthesized powder was hot-pressed at 2150°C to a relative density of > 95%. The mean grain size was 2.7 ± 1.4 μm with a maximum observed grain size of 17.5 μm. The final hot-pressed billets had a C/Zr ratio of 0.92, and oxygen content of 0.5 wt%, as determined by gas fusion analysis. The mechanical properties of ZrC_0.92O_0.03 were measured at room temperature. Vickers’ hardness decreased from 19.5 GPa at a load of 0.5 kgf to 17.0 GPa at a load of 1 kgf. Flexural strength was 362.3 ± 46 MPa, Young's modulus was 397 ± 13 MPa, and fracture toughness was 2.9 ± 0.1 MPa•m^1/2. Analysis of mechanical behavior revealed that the largest ZrC grains were the strength-limiting flaw in these ceramics.     

Preparation and characterization of polyurethane microcapsules containing n‐octadecane with styrene‐maleic anhydride as a surfactant by interfacial polycondensation

A series of polyurethane microcapsules containing a phase change material (PCM) of n‐octadecane was successfully synthesized by an interfacial polymerization in aqueous styrene‐maleic anhydride (SMA) dispersion with diethylene triamine (DETA) as a chain extender reacting with toluene‐2,4‐diisocyanate (TDI). The average diameter of microPCMs is in the range of 5–10 μm under the stirring speed of 3000–4000 rpm. Optical and SEM morphologies of microPCMs had ensured that the shell was regularly fabricated with the influence of SMA. FTIR results confirmed that the shell material was polyurethane and the SMA chains associated on core material reacted with TDI forming a part of shell material. The shell thickness was decreasing in the range of 0.31–0.55 μm with the molar ratio of DETA/TDI from 0.84 to 1.35 and the weight of core material increasing from 40 to 80% (wt %). By controlling the weight ratio of PCM as 40, 50, 60, 70, and 80% in microPCMs, it was found using DSC that the T_m and T_c of microPCMs were in the range of 29.8–31.0^oC and 21.1–22.0°C and an obvious phase change had been achieved nearly the same temperature range of that of PCM. The results from release curves of microPCM samples prepared by 1.4, 1.7, and 2.0 g of SMA indicated the release properties were affected by the amount of the dispersant, which attributed to the emulsion effect and shell polymerization structure. The above results suggest that the shell structure of microPCMs can be controlled and the properties of microPCMs determined by shell will perform proper practical usage. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 4996–5006, 2006     

Manufacturing B4C parts with Ti-Al intermetallics by aqueous colloidal processing

The aqueous colloidal processing of submicrometre B₄C powder (∼0.6 μm) with coarse Ti-Al powder (∼40 μm) as sintering additive was investigated. Firstly, by measuring the zeta potential, pHs were identified that promote the individual colloidal stability of the B₄C and Ti-Al particles as well as their co-dispersion in water with two different deflocculants (one anionic and the other cationic). It was found that the anionic and cationic deflocculants shift the isoelectric points of B₄C and Ti-Al to more acidic and more basic pHs, respectively, making their co-dispersion possible at neutral pH. And secondly, by means of rheological studies, conditions were identified (sonication time, deflocculant type, and deflocculant content) that at quasi-neutral pH yield B₄C + Ti-Al shear-thinning concentrated suspensions (30 vol.% total solids) with low viscosity and small hysteresis loop. Interestingly, those deflocculated with the cationic polyelectrolyte had better rheological behaviour, being also less viscous and almost non-thixotropic. These suspensions were freeze-dried, obtaining powder mixtures that were compacted by conventional spark plasma sintering (SPS), and also slip-cast, obtaining robust green pieces that were densified by pressureless SPS. The two B₄C composites thus obtained are superhard, with Vickers hardnesses greater than 30 GPa.     

Electrical discharge machining of boron carbide-graphene nanoplatelets composites

Boron carbide (B₄C) composites containing 0–10 wt.% graphene nanoplatelets (GNPs) were consolidated using hot pressing at 1950 °C for 60 min under 30 MPa in an argon atmosphere. Their electrical discharge machining (EDM) characteristics were evaluated for the first time. Additionally, the effects of GNPs on the microstructure, electrical conductivity, material removal rate (MRR), and surface roughness (R_a) of B₄C composites were investigated. The results show that the B₄C composite containing 10 wt.% GNPs exhibited the highest electrical conductivity of 4997 S·m^?1 because of the formation of GNPs conductive networks. Under a fine machining condition, the MRR of the B₄C composite was enhanced by 43.5 % (from 6.55 to 9.40 mm³ min^?1) compared with that of monolithic B₄C. Furthermore, the R_a decreased to 1.12 μm, which was significantly lower than that of pure B₄C (2.44 μm). Scanning electron microscope and energy disperse spectroscopy analysis of the EDM surfaces were used to determine that the main material removal mechanisms for B₄C composites with less than 2 wt.% GNPs were spalling and melting. As the GNPs content increased, B₄C grain fallout, melting, and evaporation became more dominant. The other mechanisms, including thermal shock and oxidation, are also discussed in detail.