Preparation and mechanical properties of ZrB<Subscript>2</Subscript>-based ceramics using MoSi<Subscript>2</Subscript> as sintering aids

ZrB₂ (zirconium diboride)-based ceramics reinforced by 15vol.% SiC whiskers with high density were successfully prepared using MoSi₂ as sintering aids. The effects of sintering condition and MoSi₂ content on densification behavior, phase composition, and mechanical properties of SiC_w/ZrB₂ composites were studied. Nearly, fully dense materials (relative density >99%) were obtained by hot-pressing (HP) at 1700°C–1800°C in flow argon atmosphere. The grain size of ZrB₂ phase in the samples sintered by HP at 1700°C–1800°C were very fine, with mean size below 5 μm. Mechanical properties (such as flexural strength, fracture toughness, and Vickers hardness) of the sintered samples were measured. The sample with 15vol.% MoSi₂ addition sintered by HP at 1750°C displayed the best mechanical properties.     

Preparation and characterization of ZrB2-SiC ultra-high temperature ceramics by microwave sintering

ZrB₂-SiC ultra-high temperature ceramic composites reinforced by nano-SiC whiskers and SiC particles were prepared by microwave sintering at 1850°C. XRD and SEM techniques were used to characterize the sintered samples. It was found that microwave sintering can promote the densification of the composites at lower temperatures. The addition of SiC also improved the densification of ZrB₂-SiC composites and almost fully dense ZrB₂-SiC composites were obtained when the amount of SiC increased up to 30vol.%. Flexural strength and fracture toughness of the ZrB₂-SiC composites were also enhanced; the maximum strength and toughness reached 625 MPa and 7.18 MPa·m^1/2, respectively.     

Synthesis and evaluation of Zr0.5Ti0.5B2 solid solution

Boride ceramics are useful materials because of their high strengths, hardness and melting points, which allow them to be used as high-temperature structural materials. In this study, sintered bodies of a solid solution of ZrB₂-TiB₂ system were prepared using hot pressing (HP) and spark plasma sintering (SPS). The sintering behavior was evaluated, and the effect of pulverization on sinterability and reactivity was examined using a Nanomizer. The combination of SPS processing at 2200 °C and pulverization yielded a nearly single-phase Zr_0.5Ti_0.5B₂ solid solution having a relative density of 95%.     

Effect of annealing treatment on mechanical properties of a ZrB2-SiC-graphite ceramic

The ZrB₂-SiC-graphite (ZSG) ceramic was annealed at 1600, 1700 and 1800 °C for different times, respectively. It was revealed that the annealing treatment is favorable to increase the interface bonding and relative density, and to decrease the thermal residual stress, to result in the crimp of the graphite flake. It was the optimal annealing treatment process conditions of 1700 °C and 90 min according to the best combination of the mechanical properties. To compare the mechanical properties of the specimen before the annealing treatment, the hardness, strength and toughness of the specimen annealed at 1700 °C for 90 min were enhanced by 20.6% and 20.2%, decreased by 8.2%, respectively.     

Preparation and characterization of ZrB<Subscript>2</Subscript>-SiC ultra-high temperature ceramics by microwave sintering

ZrB₂-SiC ultra-high temperature ceramic composites reinforced by nano-SiC whiskers and SiC particles were prepared by microwave sintering at 1850°C. XRD and SEM techniques were used to characterize the sintered samples. It was found that microwave sintering can promote the densification of the composites at lower temperatures. The addition of SiC also improved the densification of ZrB₂-SiC composites and almost fully dense ZrB₂-SiC composites were obtained when the amount of SiC increased up to 30vol.%. Flexural strength and fracture toughness of the ZrB₂-SiC composites were also enhanced; the maximum strength and toughness reached 625 MPa and 7.18 MPa·m^1/2, respectively.     

Spark plasma sintering of ultra refractory compounds

Spark plasma sintering experiments were conducted on Zr- and Hf-based borides and carbides with the addition of 1, 3, and 9 vol% MoSi₂ as sintering aid. For comparison, as-received ZrC, HfC, ZrB₂, HfB₂ powders were also sintered. The microstructural features were investigated by means of scanning electron microscop–energy dispersive spectroscopy technique. Silicon carbide was detected in all the doped compositions along with significant amounts of oxide species (Hf/ZrO₂, and SiO₂). The effect of the MoSi₂ content on densification, microstructure, and mechanical properties is analyzed.     

Spark plasma sintering of ZrB<Subscript>2</Subscript>–ZrC powder mixtures synthesized by MA-SHS in air

Mechanical activation-assisted self-propagating high-temperature synthesis (MA-SHS) in air was successfully applied to the synthesis of the powder mixtures of ZrB₂ and ZrC as a precursor of the ZrB₂–ZrC composite. When the powder mixtures of Zr/B/C = 4/2/3–6/10/1 in molar ratio were mechanically activated (MA) by ball milling for 45–60 min and then exposed to air, they self-ignited spontaneously and the self-propagating high-temperature synthesis (SHS) was occurred to form ZrB₂ and ZrC. The ZrB₂–ZrC composites were produced from these MA-SHS powders by spark plasma sintering (SPS) at 1800 °C for 5–10 min and showed the fine and homogeneous microstructure composed of the <5 μm-sized grains. The mechanical properties of the composites evaluated by Vickers indentation method showed the values of Vickers hardness of 13.6–17.8 GPa and fracture toughness of 2.9–5.1 MPa·m^1/2, depending on the molar ratio of ZrB₂/ZrC. Thus, the better microstructure and mechanical properties of the ZrB₂–ZrC composites were obtained from the MA-SHS powder mixtures, compared with those obtained from the MA powder, the mixing powder and the commercial powder mixtures.     

Spark plasma sintering of UHTC powders obtained by self-propagating high-temperature synthesis

Fully dense ZrB₂–SiC and HfB₂–SiC ultra-high-temperature ceramics (UHTCs) composites are fabricated by first synthesizing via self-propagating high-temperature synthesis (SHS) the composite powders from B₄C, Si, and Zr or Hf reactants, and subsequently consolidating the product by spark plasma sintering (SPS) without the addition of any sintering aid. It was found that the SHS technique leads to the complete conversion of reactants to the desired products and the SPS allows for the full consolidation (>99.5% relative density) under the optimal operating conditions of 1800 °C/20 min/20 MPa and 1800 °C/30 min/20 MPa, for the cases of ZrB₂–SiC and HfB₂–SiC, respectively. Based on the results reported in this work, it can be stated that the combination of SHS and SPS methods represents a particularly rapid and convenient preparation route (lower sintering temperature and processing time) for UHTCs as compared to the techniques available in the literature for the fabrication of analogous products.     

Sintering, crystallization, and properties of a low-viscosity SnO–MgO–P2O5 glass

A lead-free, low-viscosity SnO–MgO–P₂O₅ glass powder was fabricated. Sinterability, wetting, flowability, crystallization, and the resulting properties of the glass powder were investigated. It is shown that the powder compact can be fully densified above 362 °C and show good wet to the substrate above 417 °C. The properties (coefficient of thermal expansion and chemical durability) of the sintered glass depend on the sintering temperature and are discussed in terms of the development of crystalline phases during sintering.     

Influence of sintering temperature on piezoelectric properties of (K0.4425Na0.52Li0.0375)(Nb0.8925Sb0.07Ta0.0375)O3 lead-free piezoelectric ceramics

Microstructure characteristics, phase transition, and electrical properties of (K_0.4425Na_0.52Li_0.0375) (Nb_0.8925Sb_0.07Ta_0.0375)O₃ (KNLNST) lead-free piezoelectric ceramics prepared by normal sintering were investigated with an emphasis on the influence of sintering temperature. The microstructure and piezoelectric, ferroelectric, and dielectric properties were investigated, with a special emphasis on the influence of sintering temperature from 1,100 to 1,140 °C. Orthorhombic phases mainly exist in the ceramics sintered at 1,100–1,130 °C, whereas the tetragonal phase becomes dominant when sintering temperature is above 1,130 °C. Because of the existence of MPB-like transitional behavior, the piezoelectric coefficient (d ₃₃), electromechanical coupling coefficient (kp), and dielectric constant (ε) show peak values of 304pC/N, 0.48, and 1,909, respectively, which are obtained in the sample sintered at 1,120 °C, and its Curie temperature (T _C) is about 271 °C.     

Observation of Flux Jump in (MgB2)0.96Ni0.04 Superconductor Doped with Milled Ni powders

Bulk (MgB₂)_0.96Ni_0.04 samples doping with premilled Ni powders were sintered at 750 °C for 30 min. During sintering, liquid Mg–Ni phase prompts solid–solid reaction between Mg and B and the size of milled Ni powder determines the final distribution of the secondary MgNi_2.5B₂ phase in the sintered samples. A flux jump was observed in the (MgB₂)_0.96Ni_0.04 samples doping with Ni powders. Recognized from the measured superconductive properties, smaller-sized Ni powders can provide more effective flux pinning centers and thus improve the performance of the critical current density.     

Hydroxyapatite–TiO2 composites

This article reports the mechanical properties, the microstructure, and the crystallography of composite materials made of hydroxyapatite, obtained from natural bovine bone, and TiO₂ (5 and 10 wt.%), which were sintered at different temperatures between 1000 and 1300 °C. Higher sintering temperatures resulted in better densification. The samples sintered at 1300 °C had the highest microhardness. The best compressive strengths were obtained after sintering at 1300 °C for the samples containing 5% TiO₂, and at 1200 °C for the samples with 10% TiO₂.     

ZrB2-ceramic toughened by refractory metal Nb prepared by hot-pressing

ZrB₂–Nb (ZN) composites were prepared through hot-pressing at a temperature of 1800 °C. A contribution of Nb was believed a significant influence on the sinterability, microstructure and mechanical properties of ZN composites. The values of flexural strength of ZN composites rang from 395 to 773 MPa, who are dependent on Nb contents. The highest strength obtained for the ZN composite containing 25 vol.% Nb (773 MPa). A fracture toughness of 7.1 MPa m^1/2 of ZN was revealed, which was much higher than that of monolithic ZrB₂. The improvement in fracture toughness strongly depended on an introduction of Nb–ZrB₂ matrix. Crack deflection and branching were believed to be the toughening mechanism of ZN.     

Characteristics evaluation of SiC/Si nanocomposites produced by spark plasma sintering

SiC–Si composites are widely used either as a bulk material or as a matrix for fibre reinforced ceramics. In the current research, nanocomposites of SiC–Si with different volume fractions of Si were sintered by spark plasma sintering (SPS) for the first time. The effect of Si content and different sintering parameters on relative density, microstructure, hardness and fracture toughness of the sintered materials have been investigated. The relative density increased from about 83 to 99% by increasing the sintering temperature to 1700°C, sintering time to 10?min, and pressure to 70?MPa for composites containing >20?vol.-% Si. The results revealed that the full dense SiC–20?vol.-%Si composite can be obtained by SPS at 1700°C, 10?min and 70?MPa. Moreover, in this condition, the hardness and toughness of the composites reached the optimum values.     

Densification and mechanical properties of mullite-SiC nanocomposites synthesized through sol-gel coated precursors

Mullite-SiC nanocomposites are synthesized by introducing surface modified sol-gel mullite coated SiC particles in the matrix and densification and associated microstructural features of such precursor are reported. Nanosize SiC (average size 180 nm) surface was first provided with a mullite precursor coating which was characterized by the X-ray analysis and TEM. An average coating thickness of 120 nm was obtained on the SiC particles. The green compacts obtained by cold isostatic pressing were sintered in the range 1500–1700°C under pressureless sintering in the N₂ atmosphere. The percentage of the theoretical sintered density decreases with increase in SiC content. A maximum sintered density of 97% was achieved for mullite-5 vol.% SiC. The fractograph of the sintered composite showed a highly dense, fine grained microstructure with the SiC particles uniformly distributed along the grains as well as at the grain boundaries inside the mullite. The Vicker’s microhardness of mullite-5 vol.% SiC composite was measured as 1320 kg/mm² under an applied indentation load of 500 g. This value gradually decreased with an increase in SiC content.     

Effects of weak boundary phases (WBP) on the microstructure and mechanical properties of pressureless sintered Al2O3/h-BN machinable composites

Al₂O₃/h-BN machinable composites were fabricated by pressureless sintering at 1750 °C for 2 h in nitrogen atmosphere. The relative density of the sintered composites decreased, while the porosity increased with increasing h-BN content. By adding 20 vol.% h-BN to the composites, the porosity increased up to 16.7%. The effects of weak boundary phases (WBP), including h-BN and pores, on the microstructure, mechanical properties and machinability of the composites were investigated. The flexural strength, fracture toughness, Young's modulus and hardness of the composites decreased with increasing WBP content. When WBP content increased up to 18.8 vol.%, the composites can be machined easily by cemented carbide drills. Furthermore, the machining mechanisms of the composites were investigated using Hertzian contact tests.     

The preparation and characterization of dense,highly conductive Na5GdSi4O12 nasicon (NGS)

Na₅GdSi₄O₁₂ has been prepared via conventional ball-milling technique and through spray-freezing/freeze-drying. The ball-milled materials were calcined at 700°C or 925°C and sintered at 1050°C/3.5h to 89% dense multiphase ceramics. Spray-frozen/freeze-dried powders were calcined at 530°C and sintered at 1050°C/25 min to 99 ± 1% theoretical density. The latter material was single-phase NGS nasicon with a 300°C resistivity of 3.8 Ω·cm, an activation energy for Na⁺-conduction of 4.4 kcal/mol, a flexural strength of 105 MPa, a duplex grain structure with average sizes 0.4 μm and 3 μm and a unique linear thermal expansion coefficient (25–540°C; α = 12.6 × 10^?6/°C±4%).     

Room and high temperature tensile behaviour of a P/M 2124/MoSi2 composite at different heat treatment conditions

In the present work, a 2124/15 vol%MoSi₂ composite was obtained by powder metallurgy. Its microstructure and mechanical properties were investigated at room and at high temperature (up to 200°C) in conditions T351, T4 and after heat treatments at 495°C for up to 100 h. Up to 150°C, tensile properties of 2124/MoSi₂ in T351 resulted similar to those of a ceramic reinforced 2124/SiC composite. Yield stress of the 2124/MoSi₂ material, after heating at 495°C for up to 100 h, resulted higher than that of the monolith 2124 alloy heated for the same periods. No diffusion reaction phases were formed surrounding the MoSi₂ reinforcing particles during such long exposures to high temperature. Only at 100 h, large plate-like precipitates that contain Al, Cu, Mg and Si appeared. The high thermal stability of this 2124/MoSi₂ composite and its good mechanical properties at room and at elevated temperature makes MoSi₂ intermetallic a competitor of ceramic reinforcements.     

High velocity compaction of 0.9Al2O3/Cu composite powder

The 0.9Al₂O₃/Cu composite powder was compacted by high velocity compaction (HVC) technique and the effects of sintering temperature on density and mechanical properties such as tensile strength and hardness were studied. The results showed that with an increase in impact velocity the green density of the compacts significantly increased. At impact velocity of 9.40 m s⁻¹, the maximum green density of the compacts reached up to 8.460 g/cm³ (RD 96.8%). The green compacts were then sintered at different temperatures and it was found that with the increase in sintering temperature the sintered density and the mechanical properties also increased. At sintering temperature of 1080 °C, the compacts obtained the maximum relative sintered density of 98%, a tensile strength of 346 MPa and hardness of 71.1 HRB. Additionally with the increase in sintering temperature, the shrinkage along both axial and radial direction increased. The electrical conductivity of the samples was measured as 71% IACS.