Solid solution synthesis of tantalum carbide‐hafnium carbide by spark plasma sintering

Solid solutions of Tantalum carbide (TaC) and Hafnium carbide (HfC) were synthesized by spark plasma sintering. Five different compositions (pure HfC, HfC‐20 vol% TaC, HfC‐ 50 vol% TaC, HfC‐ 80 vol% TaC, and pure TaC) were sintered at 1850°C, 60 MPa pressure and a holding time of 10 min without any sintering aids. Near‐full density was achieved for all samples, especially in the HfC‐contained samples. The porosity in pure TaC samples was caused by the oxygen contamination (Ta₂O₅) on the starting powder surface. The addition of HfC increased the overall densification by transferring the oxygen contamination from TaC surface and forming ultrafine HfO₂ and Hf‐O‐C grains. With the increasing HfC concentration, the overall grain size was reduced by 50% from HfC‐ 80 vol% TaC to HfC‐20 vol% TaC sample. The solid solution formation required extra energy, which restricted the grain growth. The lattice parameters for the solid solution samples were obtained using X‐ray diffraction which had an excellent match with the theoretical values computed using Vegard's Law. The mechanical properties of the solid solution samples outperformed the pure TaC and HfC carbides samples due to the increased densification and smaller grain size.     

Kinetic Analysis of Solution-Precipitation During Liquid-Phase Sintering of Alumina

Densification controlled by solution-precipitation during liquid-phase sintering was analyzed for the aluminamagnesium aluminosilicate glass system. As a model system for liquid-phase sintering, narrowly sized alumina powders and up to 20 vol% magnesium aluminosilicate glass samples were isothermally sintered at 1550° to 1650°C. Densification rate increases with increasing liquid content and sintering temperature but decreases with increasing density. For samples with >15% grain growth, the densification rate during the solution-precipitation stage of sintering was proportional to (particle size)⁻² and thus interface reaction-controlled. Activation energies ranged from 270 to 500 kJ/mol over the relative density range of 66% to 96%, respectively. The low activation energy is attributed to densification by particle rearrangement, whereas the higher activation energy is due to densification controlled by interface-reaction-controlled solution-precipitation. Intermediate activation energies are attributed to simultaneous densification by the two mechanisms.     

Liquid Phase Sintering of Alumina, I. Microstructure Evolution and Densification

The microstructure evolution and densification of alumina containing 10 vol% calcium aluminosilicate glass and 0.5 wt% magnesium oxide sintered at 1600°C were quantified by measuring the evolution of pore-size distribution, the redistribution of liquid phase, and the fraction of closed and open pores. The densification stopped at a limiting relative density during the final stage of sintering, and the small and large pores were filled simultaneously by glass during sintering. In addition, the results indicate that the pressure build-up of the trapped gases in pores causes a significantly negative contribution to the driving force, and consequently the observed reduction in densification during the final stage of liquid phase sintering.     

Effect of Liquid-Phase Sintering on the Breakdown Strength of Barium Titanate

A glass used to liquid-phase sinter BaTiO₃ was developed and its properties were tested to determine the feasibility of using this glass to aid densification behavior. Glass was added to pure BaTiO₃ powder and pressed into dimpled pellets with glass contents varying from 4 to 20 vol%. The breakdown strength was measured and compared with pure BaTiO₃ and two commercially available dielectric materials. Liquid-phase sintering increased the breakdown strength of pure BaTiO₃ for all the compositions tested. The breakdown strength of the samples containing 20 vol% glass additions was increased by a factor of 2.8 compared with pure BaTiO₃.     

Freezing Dilute Ceramic/Camphene Slurry for Ultra-High Porosity Ceramics with Completely Interconnected Pore Networks

Highly porous alumina ceramics with completely interconnected pore channels were fabricated by freezing dilute alumina/camphene slurries with solid loadings ranging from 5 to 20 vol%. This method fundamentally made full use of the three-dimensional camphene dendritic network for producing interconnected pore channels and the concentrated alumina powder network for achieving dense alumina walls. Firstly, alumina/camphene slurries were prepared at 60°C using ball milling and then cast into molds at 20°C. After subliming the frozen camphene, the samples were sintered at 1400°C for 5 h. This method enabled us to freeze very dilute ceramic slurries with a low solid loading of ≤20 vol% without the collapse of the sample after sintering. As the initial solid loading decreased from 20 to 5 vol%, the porosity linearly increased from 66% to 90% with an increase in the pore size, while completely interconnected pore networks were obtained in all cases. In addition, the free surfaces of the alumina walls showed full densification after sintering even at a low temperature of 1400°C, while some pores were present in the inner regions of the alumina walls.     

Densification and Phase Transformation of β-SiAlON–Cubic Boron Nitride Composites Prepared by Spark Plasma Sintering

β-SiAlON–cubic boron nitride (cBN) composites were prepared from β-SiAlON and cBN powders at 1600°–1900°C under a pressure of 100 MPa by spark plasma sintering. The effects of cBN content and sintering temperature on densification and phase transformation of the β-SiAlON–cBN composites were studied. When 10–30 vol% cBN was added to β-SiAlON, the shrinkage rate of the compacts increased. The compacts of β-SiAlON–BN composites originally containing 10–30 vol% cBN ceased to shrink at a temperature lower than that of β-SiAlON and the density of the composites increased. The densification of β-SiAlON–BN composites originally containing >40 vol% cBN was suppressed. The phase transformation of cBN to hexagonal BN in the β-SiAlON–BN composite was inhibited to a greater degree than that in the cBN body.     

Pressureless Sintering of SiC-Whisker-Reinforced Al2O3 Composites: II, Effects of Sintering Additives and Green Body Infiltration

Pressureless sintering of SiC-whisker-reinforced Al₂O₃ composites was investigated. In Part II of the study, the effects of Y₂O₃/MgO sintering additives and green body infiltration on densification behavior and microstructure development are reported. Both sintering additives and green body infiltration resulted in enhanced densification. However, the infiltration approach was more effective for samples with high SiC whisker concentrations. Samples with 27 vol% whiskers could be pressureless sintered to ∼93% relative density and ∼3% open porosity. Fracture toughness values and microstructural features (e.g., grain size) for the infiltrated samples remained approximately the same as observed in the uninfiltrated samples.     

Pressureless Sintering of ZrB2–SiC Ceramics

A pressureless sintering process was developed for the densification of zirconium diboride ceramics containing 10–30 vol% silicon carbide particles. Initially, boron carbide was evaluated as a sintering aid. However, the formation of a borosilicate glass led to significant coarsening, which inhibited densification. Based on thermodynamic calculations, a combination of carbon and boron carbide was added, which enabled densification (relative density >98%) by solid-state sintering at temperatures as low as 1950°C. Varying the size of the starting silicon carbide particles allowed the final silicon carbide particle morphology to be controlled from equiaxed to whisker-like. The mechanical properties of sintered ceramics were comparable with hot-pressed materials with Vickers hardness of 22 GPa, elastic modulus of 460 GPa, and fracture toughness of ∼4 MPa·m^1/2. Flexure strength was ∼460 MPa, which is at the low end of the range reported for similar materials, due to the relatively large size (∼13 μm long) of the silicon carbide inclusions.     

Spark plasma sintering of TiN ceramics codoped with SiC and CNT

In this research, we investigated the effects of SiC and multi-walled carbon nanotube (MWCNTs) addition on the densification and microstructure of titanium nitride (TiN) ceramics. Four samples including monolithic TiN, TiN-5?wt% MWCNTs, TiN-20?vol% SiC and TiN-20?vol% SiC-5?wt% MWCNTs were prepared by spark plasma sintering at 1900?°C for 7?min under 40?MPa pressure. X-ray powder diffraction patterns and scanning electron microscope (SEM) micrographs of the prepared ceramics showed that no new phase was formed during the sintering process. The highest calculated relative density was related to the TiN ceramic doped with 20?vol% SiC, while the sample doped with 5?wt% MWCNTs presented the lowest density. In addition, the SEM investigations revealed that the addition of sintering aids e.g. SiC and MWCNTs leads to a finer microstructure ceramic. These additives generally remain within the spaces among the TiN particles and prohibit extensive grain growth in the fabricated ceramics.     

Graphene nanoplatelets for electrically conductive 3YTZP composites densified by pressureless sintering

3 mol% yttria tetragonal zirconia polycrystalline (3YTZP) ceramic composites with 2.5, 5 and 10 vol% graphene nanoplatelets (GNP) were pressureless sintered in argon atmosphere between 1350 and 1450 °C. The effects of the GNP content and the sintering temperature on the densification, microstructure and electrical properties of the composites were investigated. An isotropic distribution of GNP surrounding ceramic regions was exhibited regardless the GNP content and sintering temperature used. Electrical conductivity values comparable to the ones of fully dense composites prepared by more complex techniques were obtained, even though full densification was not achieved. While the composite with 5 vol% GNP exhibited electrical anisotropy with a semiconductor-type behaviour, the composite with 10 vol% GNP showed an electrically isotropic metallic-type behaviour.     

Processing of a Silicon-Carbide-Whisker-Reinforced Glass-Ceramic Composite by Microwave Heating

A calcium magnesium aluminosilicate-based glass that contained 10 wt% of silicon carbide whiskers (SiC _w ) as reinforcement was prepared by tape casting, followed by sintering either in a conventional furnace or in a microwave oven. The results were consistent with retardation of glass sintering through whisker bridging. The glass, by itself, was sintered to almost-full density at 750°C for 4 h by conventional furnace sintering; the best sintered composite, with an estimated density of ∼90%, was obtained at 800°C with a dwell time of 4 h. Sintering at a temperature of >800°C did not improve the densification but rather resulted in severe whisker oxidation. A reduced densification rate was observed for the samples that were sintered in nitrogen. By contrast, in the microwave oven, almost-full density for the glass and ∼95% of the theoretical density for the composite were obtainable at 850°C for 15 min, which represented a reduction of ∼10 h of the total processing time and a reduced SiC _w oxidation.     

Pressureless Sintering of SiC-Whisker-Reinforced Al2O3 Composites: I, Effect of Matrix Powder Surface Area

Pressureless sintering of SiC-whisker-reinforced Al₂O₃ composites was investigated. In Part I of the study, the effect of the matrix (Al₂O₃) powder surface area on densification behavior and microstructure development is reported. Compacts prepared with higher surface area Al₂O₃ powder showed enhanced densification at lower whisker concentrations (5 and 15 vol%). Samples with 15 vol% whiskers could be pressureless sintered to ∼97% relative density with zero open porosity and ∼1.6-μm matrix average grain intercept size.     

Characterization of a soldering system consisting of a glass from the system CaOAl2O3B2O3SiO2 and wollastonite

In this work, the characterization of a soldering system with wollastonite as filler added as powder to the powdered base glass is shown. A soldering system consisting of the base glass and a coexisting crystalline phase is created which is then pressed to pellets and sintered to maximum density.Wollastonite increases the thermal expansion coefficient of the base glass after sintering the samples. In the present paper, the sintering behavior of the soldering system and possible interactions between glass and filler are analyzed. Also, density and porosity after sintering and the resulting crystalline phases are characterized.The mixtures with wollastonite contents up to 30% showed suitable sintering behavior and good density (low porosity). From 40% wollastonite on, the sintering of the samples was not satisfying and porosity was still high after sintering. The main crystalline phases were anorthite, wollastonite and pseudowollastonite, which occurred in all samples. This shows that all three phases are coexisting phases of the glass composition.     

Combined role of SiC particles and SiC whiskers on the characteristics of spark plasma sintered ZrB2 ceramics

In this research work, the effects of silicon carbide (SiC) as the most important reinforcement phase on the densification percentage and mechanical characteristics of zirconium diboride (ZrB₂)-matrix composites were studied. In this way, a monolithic ZrB₂ ceramic (as the baseline) and three ZrB₂ matrix specimens each of which contains 25 vol% SiC as reinforcement in various morphologies (SiC particulates, SiC whiskers, and a mixture of SiC particulates/SiC whiskers), have been processed through spark plasma sintering (SPS) technology. The sintering parameters were 1900 °C as sintering temperature, 7 min as the dwell time, and 40 MPa as external pressure in vacuum conditions. After spark plasma sintering, a relative density of ~96% was obtained (using the Archimedes principles and mixture rule for evaluation of relative density) for the unreinforced ZrB₂ specimen, but the porosity of composites containing SiC approached zero. Also, the assessment of sintered materials mechanical properties has shown that the existence of silicon carbide in ZrB₂ matrix ceramics results in fracture toughness and microhardness improvement, compared to those measured for the monolithic one. The simultaneous addition of silicon carbide particulates (SiC_p) and whiskers (SiC_w) showed a synergistic effect on the enhancement of mechanical performance of ZrB₂-based composites.     

Investigation of hot pressed ZrB2–SiC–carbon black nanocomposite by scanning and transmission electron microscopy

Hybrid ZrB₂-based composite having 10 vol% nano-sized carbon black and 20 vol% SiC was fabricated by vacuum hot pressing at 1850 °C under 20 MPa for 60 min. The microstructure and sinterability of the as-sintered ceramic was studied by X-ray powder diffraction, scanning electron microscopy, X-ray spectroscopy, scanning transmission electron microscopy and transmission electron microscopy analyses. A fully-dense hybrid composite could be achieved by hot pressing method under the aforementioned conditions. No new in-situ phase formation was detected after sintering process. Although the densification progressed in a non-reactive manner, the addition of carbonaceous material assisted the sinterability acting as the surface oxides cleaner. The precise phase and nanostructural investigations of the prepared ceramic verified the partial graphitization of carbon black and conversion of amorphous nano-additive into crystalline graphite nano-flakes.     

Effect of the Heating Rate on the Relative Rates of Sintering and Crystallization in Glass

The sintering of glass can sometimes be impeded by crystallization. In the optimum process the glass should sinter to full density before the onset of crystallization. In an earlier paper we showed how sinter-forging experiments can be used to investigate the coupling between densification and crystallization. Here we demonstrate that increasing the heating rate can delay crystallization so that a certain glass can sinter to full density.     

Improved Energy Storage Density in Barium Strontium Titanate by Addition of BaO–SiO2–B2O3 Glass

The energy storage density of a Ba_0.4Sr_0.6TiO₃ ceramic with the addition of 5–20 vol% glass was investigated. The results show that the improvement of the energy density in glass-added Ba_0.4Sr_0.6TiO₃ samples arises due to two factors: one is that the breakdown strength is notably improved due to the decrease of the porosity and the reduction of the grain size and pore size in glass-added samples and the other is that the remnant polarization of glass-added samples is decreased. The energy density of the samples containing 5 vol% glass additive was improved by a factor of 2.4 compared with that of pure Ba_0.4Sr_0.6TiO₃.     

High-Strength Porous Alumina Ceramics by the Pulse Electric Current Sintering Technique

High-strength porous alumina has been fabricated with a microstructure control using the pulse electric current sintering (PECS) technique. During sintering the discharge, which is assumed to take place in the voids between the particles, is thought to promote the bridging of particles by neck growth in the initial stages of sintering, leaving high porosity. The effect of dopants (MgO, 200 ppm; TiO₂, 1000 ppm) and of secondary inclusions (3 vol% 3Y-TZP) on the constrained densification and the improvement in the mechanical behavior of porous alumina ceramics has been reported. The porosity of the fabricated porous alumina was controllable between 30% and 50% depending on the sintering temperature. The flexural strength of alumina having 30% and 42% porosity showed impressive values of 250 and 177 MPa, respectively. The dominance of the preferential neck growth of grains over densification significantly improved the mechanical properties of porous alumina, besides leaving high porosity.     

Anisotropic Sintering Shrinkage in Alumina Ceramics Containing Oriented Platelets

The effect of oriented α-Al₂O₃ platelets on densification and shrinkage behavior of tape-cast α-Al₂O₃ during solid-state sintering was studied using dilatometric sintering shrinkage experiments and microstructural characterization. Platelet-free alumina samples exhibit isotropic shrinkage with an ∼17% linear final shrinkage. The addition of platelets retards densification, and results in anisotropic shrinkage. Initially during sintering, shrinkage is isotropic for platelet-containing systems; however, it becomes highly anisotropic by inhibition of shrinkage in the platelet orientation direction. In the 10 vol% platelet-containing system, shrinkage was ∼12% in the casting and the transverse directions and ∼20% in the direction perpendicular to the casting plane.     

Spark plasma sintering of TiC–SiCw ceramics

Silicon carbide whiskers (SiC_w) in TiC had impressive impacts on the properties and made it possible for special applications which generally would not be conceivable with TiC alone. In the present work, SiC_w reinforced TiC based composites were prepared by spark plasma sintering (SPS) technique, at the temperature of 1900 °C under the pressure of 40 MPa for sintering time of 7 min. To test out the effects of different amount of SiC whisker (0, 10, 20 and 30 vol%) on the characteristics of TiC, the sintered samples were investigated about sinterability and physical-mechanical properties. Microstructure observations and density measurements confirmed that the composites were dense with uniformly distributed reinforcement, and the specimen doped with higher than 10 vol% SiC_w could attain higher relative density (>100%) than pure TiC and TiC–10 vol% SiC_w. Also, the highest values for hardness (29.04 GPa) and thermal conductivity (39.2 W/mK) were achieved in specimen containing 30 vol% SiC_w, whereas the optimum bending strength (644 MPa) was recorded in material containing 20 vol% SiC_w. It seems that one of the reasons which contributes to this trend of properties variation is the generation of near-stoichiometric TiC_x phase and new Ti₃SiC₂ compound.