Effect of low content sintering aids addition on β-SiC sintered by spark plasma sintering

The influence of low content of sintering aids addition on the properties of β-SiC sintered by spark plasma sintering was studied based on X-ray diffraction and SEM observations. Three dopants, respectively Al₂O₃, Al₂O₃-Y₂O₃ and Al₂O₃-AlN were chosen, and the investigation was undertaken in the [0.5–5 wt%] content range. The analyses revealed the possibility to increase the density of sintered pellets close to 100% of theoretical density (T.D.), even with the addition of very low sintering aids contents, as low as 0.5 wt%, for each dopant. The combination with high sintering pressure allowed keeping a fine microstructure and a pristine cubic crystalline structure while avoiding the formation of secondary phases.     

Pressureless sintering of PMN–PT ceramics

PMN–PT ceramics with PMN to PT ratios of 60:40, 65:35, and 70:30 were prepared from PMN–PT powders synthesized by the columbite precursor method, and their sintering and grain growth characteristics at temperatures less than 1000°C were investigated. Results indicate that the PMN–PT ceramics can be pressureless-sintered to a relative density of approximately 96% at 950°C. However, full densification was prevented by the onset of abnormal grain growth. The addition of 0·5 wt% PbO to 65:35 PMN–PT ceramics lowered their sintering temperature to 900°C, but caused abnormal grain growth at lower temperatures. Preliminary TEM analyses indicate the presence of submicron-sized MgO particles at some ceramic microstructure triple points. Further studies will be required to understand abnormal grain growth behavior and to devise means for full densification.     

Pixelated sintering of α-Al2O3

The sintering of α-alumina by a brand new and innovative technique, called pixelated sintering (PS), is here studied. Densification and grain growth by PS of perfectly controlled granular compacts are analysed and compared to results obtained using Spark Plasma Sintering (SPS) and Pressure-Less SPS (PL-SPS). Materials are exposed to the same temperature profiles whatever the sintering technique used in order to assess the potential of PS in terms of microstructure control. It is shown that PS can be used as an alternative technique to SPS for fast sintering with the advantages of a much simpler and cost-effective set-up, as well as a better control of the localised heat input. PS also appears to be a very modular technology in the way it controls the temperature gradients allowing its implementation for multi-step sintering approaches, as well as for the fabrication of large and complex parts.     

Two-step sintering of fine alumina–zirconia ceramics

This work investigates the feasibility to the fabrication of high density of fine alumina–5 wt.% zirconia ceramics by two-step sintering process. First step is carried out by constant-heating-rate (CHR) sintering in order to obtain an initial high density and a second step is held at a lower temperature by isothermal sintering aiming to increase the density without obvious grain growth. Experiments are conducted to determine the appropriate temperatures for each step. The temperature range between 1400 and 1450 °C is effective for the first step sintering (T₁) due to its highest densification rate. The isothermal sintering is then carried out at 1350–1400 °C (T₂) for various hours in order to avoid the surface diffusion and improve the density at the same time. The content of zirconia provides a pinning effect to the grain growth of alumina. A high ceramic density over 99% with small alumina size controlled in submicron level (0.62–0.88 μm) is achieved.     

Recycling of coal ash for production of dense β-Sialon/ZrN/ZrON-based ceramics without sintering aids via pressureless sintering

β-Sialon(z = 2, Si₄Al₂O₂N₆)/ZrN composite powders have been synthesized from coal ash, zircon, and active carbon at 1550°C for 6 hours, and β-Sialon(z = 2)/ZrN/ZrON-based composite ceramics (SZZCCS) have been prepared from as-synthesized β-Sialon/ZrN composite powders via pressureless sintering process. The effects of sintering temperatures (1450, 1500, 1550°C) on the phase compositions, microstructures, linear shrinkage ratio, bulk density, and oxidation characteristics of the SZZCCS were investigated in detail, and the oxidation process was also analyzed. It was found that the dense SZZCCS could be prepared at 1550°C for 1 hour, and they mainly consisted of β-Sialon(z = 2), ZrN, and ZrON. The ZrN and ZrON particles were uniformly distributed on the β-Sialon matrix. The sintering properties of the SZZCCS were greatly improved with increasing the sintering temperature. The SZZCCS sintered at 1550°C for 1 hour possessed excellent oxidation resistance at 900°C for 6 hours due to their dense microstructures.     

Spark plasma sintering of TiN–TiB2 composites

TiN–TiB₂ composites were fabricated by spark plasma sintering at 1773–2573 K. Effects of TiN and TiB₂ content on relative density, microstructure, and mechanical properties were investigated. Above 2373 K, TiN–TiB₂ composites exhibited relative densities over 95%. A high density of 99.7% was obtained at 2573 K with 20–30 vol% TiB₂. Shrinkage of the TiN–70 vol% TiB₂ composite was the highest at 1573–2473 K. For the TiN–70 vol% TiB₂ composite prepared at 1973–2373 K, TiN grains were small, while at 2573 K, TiB₂ became a continuous matrix, in which irregular-shaped TiN dispersed. hBN was formed in the TiN–TiB₂ composite containing 50–60 vol% TiB₂ above 2373 K. The maximum Vickers hardness and fracture toughness obtained for the TiN–80 vol% TiB₂ composite sintered at 2473 K was 26.3 GPa and 4.5 MPa m^1/2, respectively.     

Low-temperature sintering and microstructure control of mullite–Mo composites

Dense mullite–Mo (45 vol%) composites with homogeneous microstructure have been obtained by plasma activated sintering of a mixture of Mo and mullite precursors at a relatively low temperature (1350 °C) and a pressure of 30 MPa. The mullite precursor was synthesized by a sol–gel process followed by a heat-treatment at 1000 °C. The influence of different mullite precursors on the densification behavior and the microstructure of mullite–Mo composites has been studied. The precursor powder heat-treated at 1000 °C with only Si–Al spinel but no mullite phase shows an excellent sintering activity. Following the sintering shrinkage curves, a two-stage sintering process is designed to enhance the composite densification for further reducing the sintering temperature. The study reveals that viscous flow sintering of amorphous SiO₂ at low temperatures effectively enhances the densification. Moreover, microstructure of these composites can be controlled by selecting different precursors and sintering temperatures.     

Pressureless sintering of bioverit® iii/ti particle biocomposites

A new bioactive material, based on Bioverit® III glass-ceramic (B3), was synthesised: it is a composite material (named B3T) having a glass-ceramic matrix and titanium particles as toughening phase. The aim was to toughen the Bioverit® III glass-ceramic, already successfully tested by in vivo experiments. The B3T biocomposite, reinforced by 15 vol% Ti particles, was prepared by pressureless sintering, starting from Bioverit® III base glass powders. The sintering conditions were carefully optimised by using differential scanning calorimetry and hot stage microscopy. On the basis of this study a low temperature viscous flow sintering process was chosen, and high density glass-matrix composites were obtained. A further crystallisation, using the temperature and time conditions for bulk Bioverit® III glass ceramic, was performed to obtain a glass-ceramic matrix/Ti particle composite. A qualitative mechanical characterisation revealed toughening benefits in the Ti additions. An in vitro test (fibroblast growth) was performed on the composites in order to demonstrate their biocompatibility. ©     

Spark plasma sintering of ultra-porous γ-Al2O3

Nanocrystalline gamma alumina (γ-Al₂O₃) powder with a crystallite size of ~10 nm was synthesized by oxidation of high purity aluminium plate in a humid atmosphere followed by annealing in air. Spark plasma sintering (SPS) at different sintering parameters (temperature, dwell time, heating rate, pressure) were studied for this highly porous γ-Al₂O₃ in correlation with the evolution in microstructure and density of the ceramics. SPS sintering cycles using different heating rates were carried out at 1050–1550 °C with dwell times of 3 min and 20 min under uniaxial pressure of 80 MPa. Alumina sintered at 1550 °C for 20 min reached 99% of the theoretical density and average grain size of 8.5 µm. Significant grain growth was observed in ceramics sintered at temperatures above 1250 °C.     

Physical characteristics and sintering behavior of ultrafine zirconia–ceria powders

Ultrafine zirconia–12 mol% ceria powders have been prepared by the coprecipitation technique. The azeotropic distillation with n-butanol has been carried out to ensure complete elimination of the residual water in the precipitate. This procedure has proved to be quite effective in preventing the formation of agglomerates, which are responsible for inhomogeneities in the sintered microstructure, and for non-densification at low temperatures. The crystallization of the solid solution occurs at 430 °C as determined by thermal analyses. The specific surface of the calcined powder is 127.9 m² g⁻¹ and the pore size distribution exhibits only a maximum at approximately 9 nm. Total shrinkage of the compacted powder reached 30% at 1200 °C. Sintered specimens show six bands characteristics of the tetragonal phase in the Raman spectrum. Specimens with apparent densities >95% of the theoretical density and average grain size of 230–400 nm were obtained after sintering at 1200 °C.     

Thermally activated sintering–coarsening–coalescence-polymerization of amorphous silica nanoparticles

An onset sintering–coarsening–coalescence-polymerization (SCCP) event of amorphous SiO₂ nanoparticles (ca. 40–100 nm in size) by isothermal firing in the 1150–1300 °C range in air was characterized by an N₂ adsorption–desorption hysteresis isotherm coupled with X-ray diffraction and vibrational spectroscopy. The apparent activation energy of such a rapid SCCP process was estimated as 177±32 kJ/mol, based on 30% reduction of a specific surface area with an accompanied change of medium range orders, i.e. forming Si₂O₅ while retaining the Si–2ndO yet losing the Si–2ndSi without appreciable crystallization. The minimum temperature of the SCCP process, as of concern to industrial silica applications and sedimentary/metamorphosed sandstone formation, is 1120 °C based on the extrapolation of steady specific surface area reduction rates to null.     

Volume expansion during reaction sintering of γ-Bi12SiO20

Preparation of γ-Bi₁₂SiO₂₀ from a compacted mixture of α-Bi₂O₃ and SiO₂ powders is described. A very large volume expansion, related to the phase formation, is observed during heat treatment at 600°C. It is shown that the dedensification results from a preferential diffusion of bismuth and oxygen ions towards SiO₂, through the layer of γ-Bi₁₂SiO₂₀ which forms around a silica grain. The expansion begins when γ-Bi₁₂SiO₂₀ grains form a continuous skeleton. When bismuth oxide grains are isolated in the skeleton, expansion and reaction rates are proportional. A quantitative model is proposed to describe this situation assuming an isotropic matter transfer and no coalescence between the γ-Bi₁₂SiO₂₀ grains.     

Mullite–molybdenum composites fabricated by pulse electric current sintering technique

Pulse electric current sintering of monolithic mullite and mullite/0–100 vol.% Mo composites was performed in vacuum of 4.5×10⁻⁵ Torr at temperatures and pressures of 1500 °C and 20 MPa, respectively. No traces of additional phases were observed by SEM and XRD for these composites. Microstructural observations reveal that Mo (molybdenum) particles dispersed uniformly at lower Mo contents and exhibited flaky and elongated structure at higher content. Simultaneous increase of fracture strength and toughness occurred with increase in Mo content. It attained a maximum of 1.1 GPa and 9.2 MPa m^1/2, respectively for 90 vol.% Mo composites. The increase in flexural strength is due to smaller initial flaws in mullite/Mo composites for lower Mo contents and due to plastic deformation of Mo phase for higher Mo contents. Similarly, frontal process zone toughening and crack bridging are expected to be the responsible mechanisms for enhanced toughness in these composites. Partial debonding in the mullite–Mo interface, giving rise to plastic deformation of Mo phase also contributes in the increase of toughness values.     

Rationalizing the role of magnesia and titania on sintering of α-alumina

The rationalization of selection of sintering additives for α-alumina was investigated using two oxides (MgO and TiO₂) to discern their individual roles. Using both dynamic heating study in a thermomechanical analyzer and static heat treatment, the precise role of each oxide was established. Grain growth trajectory of different doped samples sintered at 1700 °C revealed that MgO neither significantly affected densification nor facilitated grain growth upto 1700 °C. MgO reacted with alumina to form spinel prior to the densification process. Thus it could not generate further extrinsic defects in corundum lattice during sintering, which usually facilitate densification. In contrast, TiO₂ significantly enhanced the densification and promoted grain growth in α-alumina. At 1700 °C, the average grain size of titania doped samples were 7.7x larger than undoped ones and 10x larger than magnesia dopes samples. The sintered grains developed higher aspect ratio when TiO₂ was used which may be ascribed to preferred growth of the 012 and 024 planes of corundum. The nearly perfect junction of grain boundaries meeting at ~120° indicates absence of liquid phase and that the entire sintering process most probably took place in solid state for both MgO and TiO₂ doped samples.     

Spark plasma sintering of β-SiAlON–BN composites from combustion-synthesized powders

Investigated was the spark plasma sintering (SPS) of β-SiAlON/0–30 wt % BN ceramic composites. The raw materials (β-Si₅AlON₇ and BN powders) were prepared by infiltration-mediated combustion synthesis (CS). Experimentally established were the following process parameters for SPS of composites with high relative density (>95%) and flexural strength of 250–300 MPa: (a) heating rate 50 deg/min, (b) maximum temperature 1650–1750°C, (c) and holding time 5 min.     

Spark plasma sintering of ZrC–SiC ceramics with LiYO2 additive

Spark plasma sintering (SPS) of ZrC–SiC composite powders in the presence of LiYO₂ sintering additive was studied. The starting powders were obtained by a carbothermal reduction (CTR) of natural mineral zircon (ZrSiO₄), which provided an intimate mixing of in-situ created ZrC and SiC powders. This composite powder and LiYO₂ as additive were densified by spark plasma sintering. Microstructural features of the composite were investigated by XRD, SEM/EDS and AFM analysis. The sintered composite material possesses promising mechanical properties and excellent cavitation resistance which was observed with a cavitation erosion test. The values of Vickers microhardness and fracture toughness of the composite material are 20.7 GPa and 5.07 MPam^1/2, respectively.     

Solid state synthesis and sintering of solid solutions of BNT–xBKT

Various compositions of the solid solution system (100 − x) Bi_0,5Na_0,5TiO₃–xBi_0,5K_0,5TiO₃ (x = 0, 10, 25, 50, 75, 90, 100) were prepared by the mixed oxide route. The formation reaction was analyzed by thermogravimetry coupled with mass spectroscopy and differential scanning calorimetry. In situ high temperature X-ray diffraction up to 770 °C indicated emerging and vanishing of phases during the calcination. Intermediate phases such as alkalipolytitanate (Na/K)₂Ti₆O₁₃ and bismuth titanate Bi₂Ti₂O₇ were identified as forming the perovskite phase. The formation reactions were proposed based on the data obtained. Furthermore the microstructure and the dielectric behavior of the sintered samples were observed by scanning electron microscopy, impedance spectrometry and polarization measurements.     

Solid-state pressureless sintering of silicon carbide below 2000 °C

To date, solid-state pressureless sintering of silicon carbide powder requires sintering aids and high sintering temperature (>2100 °C) in order to achieve high sintered density (>95% T.D.). Two-step sintering (TSS) method can allow to set sintering temperature lower than that conventionally required. So, pressureless two-step sintering process was successfully applied for solid-state sintering (boron carbide and carbon as sintering additives) of commercial SiC powder at 1980 °C. Microstructure and mechanical properties of TSS-SiC were evaluated and compared to those obtained with the conventional sintering (SSiC) process performed at 2130 °C. TSS-SiC showed finer microstructure and higher flexural strength than SSiC with very similar density (98.4% T.D. for TSS-SiC and 98.6% T.D. for SSiC).     

Spark-plasma sintering of boron carbide–silicon carbide composites at 1400 °C from B4C+Si: Densification and sintering/reaction mechanisms

The feasibility of fabricating novel boron carbide–silicon carbide composites by spark-plasma sintering (SPS) of B₄C+Si powder mixtures at only 1400 °C was investigated. First, it is shown that B₄C can be fully densified at 1400 °C if ~20 vol% Si aids are used, leading to bi-particulate composites constituted by boron carbide (major phase) and SiC (minor phase). The formation of these composites is due to the fact that Si acts as a reactive sintering additive during SPS. Lower and higher proportions of Si aids are not optimal, the former leading to porous bi-particulate composites and the latter to dense triplex-particulate composites with some residual free Si. Importantly, it is also shown that these novel boron carbide–SiC composites are fine-grained, nearly-ultrahard, moderately tough, and more affordable to fabricate, a combination that makes them very appealing for many engineering applications. Second, it is demonstrated that during the heating ramp of the SPS cycles a eutectic melt is formed that promotes full low-temperature densification by transient liquid-phase sintering if sufficient Si aids are used. Otherwise, a subsequent stage of solid-state sintering is required at higher temperatures once the eutectic liquid has been consumed in the in-situ formation of SiC. And third, it is demonstrated that during SPS the original B₄C undergoes a gradual isostructural crystallographic transition towards a Si-doped carbon-deficient boron carbide that is more relevant with increasing proportion of Si aids, and it is identified that the carbon source for the formation of SiC is almost exclusively the carbon exsoluted from the B₄C crystals themselves during their isostructural transition. Finally, implications of interest for the ceramic and hard-material communities are discussed.