Liquid phase sintering of Al2O3/SiC nanocomposites

Al₂O₃/SiC nanocomposites are usually prepared by hot pressing or using high sintering temperatures, viz. 1700°C. This is due to the strong inhibiting effect of the nano-sized SiC particles on the densification of the material. Liquid phase sintering (LPS) can be used to improve densification. This work explored two eutectic additive systems, namely MnO₂.SiO₂ (MS) and CaO.ZnO.SiO₂ (CZS). The additive content in Al₂O₃/5 wt% SiC nanocomposite material varied from 2 to 10 wt%. Densities of up to 99% of the theoretical value were achieved at temperatures as low as 1300°C. Characterisation of the materials by XRD, indicated the formation of secondary crystalline phases in addition to Al₂O₃ and SiC. SEM and TEM analysis showed the presence of a residual glassy phase in the grain boundaries, and an increase in the average grain size when compared to nanocomposites processed without LPS additives.     

Liquid phase sintering and characterization of SiC ceramics

The present study focuses on the sintering of silicon carbide-based ceramics (SiC) by liquid phase sintering (LPS) followed by characterization of the produced ceramics. AlN/Re₂O₃ mixtures were used as additives in the LPS process. In the first step, the LPS-SiC materials were produced in a graphite resistance furnace in the form of discs at different temperatures. The conditions with the best results regarding real density and relative density were taken as reference for sintering in the form of prismatic bars. In the second step, these samples were evaluated regarding fracture toughness (K_IC), by the Single Edge V Notch Beam – SEVNB – method, and flexural strength. K_IC behavior was evaluated according to the depth and curvature radius of the notches. Reliable K_IC values were presented when the ceramic displayed a small curvature radius at the notch tip. When the radius was large, it did not maintain the square root singularity of the notch tip. Tests were carried out to determine K_IC values in atmospheric air and water. K_IC results were lower in water than air, with a decrease ranging between 2.56% and 11.26%. The observations indicated a direct grain size correlation between K_IC values and fracture strength of the SiC ceramics.     

Enhanced particle rearrangement during liquid phase spark plasma sintering of silicon nitride-based ceramics

Densification evolution has been analysed in liquid phase assisted spark plasma sintering (SPS) of silicon nitride (Si₃N₄)-based materials. Monolithic ceramics with variable amounts of Al₂O₃/Y₂O₃ sintering additives, and carbon nanotubes (CNTs) containing Si₃N₄ composites have been considered. The shrinkage behaviour of SPSed monolithic Si₃N₄ showed a noticeable enhancement of the particle rearrangement stage, exhibiting a complete wetting of the grain boundary phase even for the lowest additive content (1.70 vol.%), unlike the corresponding hot pressed (HPed) material. An improvement of the liquid phase wetting by the presence of electromechanical forces is proposed to explain the enhanced densification occurring in the particle rearrangement stage of the SPS process. Furthermore, the addition of CNTs seems to increase the efficiency of this mechanism, decreasing the particle rearrangement temperature.     

Densification of liquid phase sintered silicon carbide

The densification and phase formation of liquid phase sintered silicon carbide (LPSSiC) with 10 wt.% additives were investigated. The ratio of the Al₂O₃/Y₂O₃-additives was changed between 4:1 and 1:2. Densification was carried out by hot pressing and gas pressure sintering. The different densification routes result in different major grain boundary phases—aluminates in gas pressure sintered materials and silicates in hot pressed samples. Thermodynamic calculations were carried out to determine the amount of liquid phase during densification and for the interpretation of the results.     

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.     

Effects of mechanically alloying Al2O3 and Y2O3 additives on the liquid phase sintering behavior and properties of SiC

In order to improve the sintering of SiC, mixtures of Al₂O₃ and Y₂O₃ powders are commonly included as sintering additives. The aim of this work was to use mechanically alloyed Al₂O₃–Y₂O₃ mixtures as sintering additives to promote liquid phase sintering of SiC using spark plasma sintering. The results showed that milling reduced the particle size of the powders and led to the formation of complex oxide phases (YAP, YAM, and YAG) at low temperatures. As the ball milling time increased, the mass loss of specimens sintered with mechanically alloyed Al₂O₃–Y₂O₃ mixtures decreased, and accordingly the relative density increased. However, the hardness and flexural strength of sintered SiC specimens first increased and then decreased. Because the specimens prepared with oxides milled for a long time contained too much YAG/YAP and accordingly too much liquid at sintering temperature. This negatively affected the mechanical properties of the SiC specimens because of the increased volume of the complex oxide phases, which have inferior mechanical properties to SiC, in the sintered specimens. When the ball milling time was 6 h, the hardness (24.02 GPa) and flexural strength (655.61 MPa) of the SiC specimens reached maximum values.     

High thermal conductivity silicon nitride ceramics prepared by pressureless sintering with ternary sintering additives

Silicon nitride ceramics were pressureless sintered at low temperature using ternary sintering additives (TiO₂, MgO and Y₂O₃), and the effects of sintering aids on thermal conductivity and mechanical properties were studied. TiO₂–Y₂O₃–MgO sintering additives will react with the surface silica present on the silicon nitride particles to form a low melting temperature liquid phase which allows liquid phase sintering to occur and densification of the Si₃N₄. The highest flexural strength was 791(±20) MPa with 12 wt% additives sintered at 1780°C for 2 hours, comparable to the samples prepared by gas pressure sintering. Fracture toughness of all the specimens was higher than 7.2 MPa·m^1/2 as the sintering temperature was increased to 1810°C. Thermal conductivity was improved by prolonging the dwelling time and adopting the annealing process. The highest thermal conductivity of 74 W/(m∙K) was achieved with 9 wt% sintering additives sintered at 1810°C with 4 hours holding followed by postannealing.     

Nano- versus macro-hardness of liquid phase sintered SiC

Silicon carbide polycrystalline materials were prepared by liquid phase sintering. Different rare-earth oxides (Y₂O₃, Yb₂O₃, Sm₂O₃) and AlN were used as sintering additives. The final microstructure consists of core–rim structure owing to the incorporation of AlN into the rim of SiC grains by solid solution. Nano- versus macro-hardness of polycrystalline SiC materials were investigated in more details. The nano-hardness of SiC grains was in the range of 32–34 GPa and it depends on the chemical compositions of grains. The harness followed the core–rim chemistry of grains, showing lower values for the rim consisting of SiC–AlN solid solution. The comparison of nano- and macro-hardness showed that nano-hardness is significantly higher, generally by 5–7 GPa. The macro-hardness of tested samples had a larger scatter due to the influence of several factors: hardness of grains (nano-hardness), indentation size effect (ISE), microstructure, porosity, and grain boundary phase. The influence of grain boundary phase on macro-hardness is also discussed.     

Kinetic field approach to study liquid phase sintering of ZnO based ceramics

Liquid phase sintering kinetics in the system ZnO–Bi₂O₃–Sb₂O₃ was studied using closed crucibles and an optical dilatometer. A modified kinetic field technique was applied for the first time to investigate the densification rates. The values obtained were assessed with existing liquid phase sintering models. Grain growth data were derived from the kinetic field diagram and compared to those obtained from microstructure analysis of quenched samples. Good agreement was obtained between both techniques. Values for both the activation energies (activation energies for grain growth and densification) were also reported for the ZnO–Bi₂O₃–Sb₂O₃ system for the first time. In the initial sintering stage mechanisms were identified which retard densification and are essentially unaffected by temperature. It was shown how the position and slope of the iso-strain lines in the modified kinetic field diagram can be used for a qualitative understanding of the interaction of coarsening, liquid redistribution and densification during sintering.     

Effect of low activation rare-earth oxides on sintering of β-SiC

The feasibility of sintering β-SiC at 1850 ℃ with using low-activation rare-earth oxides as sintering additives was studied. Thermodynamic calculation suggested that all five rare-earth oxides (Y₂O₃, Sc₂O₃, CeO₂, La₂O₃, and Pr₆O₁₁) were suitable sintering additives. Comprehensive microstructure characterization further revealed that Pr₆O₁₁ was the most effective sintering additive as it suffered the least influence from the content effect. A mechanism based on liquid phase sintering was proposed based on microstructure observations. The radiation stability of sintered SiC was investigated as well. The intergranular phases (rare-earth oxides and silicates) and SiC grains remained after being irradiated to 17 dpa at 750 ℃.     

Liquid phase flash sintering in magnesia silicate glass-containing alumina

Magnesia silicate glass-containing alumina was flash sintered using an E-Field in the 500–1500 V/cm range. The addition of glass allows to reduce the current needed for densification and improves the shrinkage obtained during field-assisted sintering process. This behaviour is related to the different sintering mechanisms involved in the two materials, i.e. solid state sintering for pure alumina and liquid phase sintering for glass-containing alumina.The estimated activation energy for conduction during flash sintering is compatible with ionic diffusion in silicate melt. Moreover, evidence of magnesium diffusion toward the cathode is recorded. The estimated sample temperature is in almost all cases lower than 1355 °C, which is the temperature at which the first liquid is formed in the ternary system MgO-SiO₂-Al₂O₃. Finally, it is shown that the application of an E-Field accounts for efficient liquid phase sintering at temperatures at which it cannot be reproduced conventionally.     

Hybrid microwave sintering of alumina and 3 mol% Y2O3-stabilized zirconia in a multimode cavity – Influence of the sintering cell

In microwave (MW) sintering, samples directly heat by absorption of the electromagnetic field, leading to a fast volumetric heating. Thus, the materials are heated thanks to their own dielectric properties. In multimode cavities, samples are always heated with a sintering cell in order to get an insulation of the samples and a heating homogeneity. However, to date, very limited research has been carried out to study the effects of materials used in the sintering cell. In this paper the microstructure and densification of Al₂O₃ and 3Y-TZP were investigated and compared between three different sintering cells. The used sintering cell contains three main elements: thermal insulators, a SiC susceptor and a protective mullite tube. Higher final densities (98.3 ± 0.6% and 98.6 ± 0.6% of T.D. for Al₂O₃ and 3Y-TZP respectively), lower densification temperatures and better microstructure homogeneity were obtained by using the sintering cell containing both mullite tube and SiC susceptor. But, sintering using the sintering cell without the SiC susceptor was also possible, especially for the lowest lossy material,i.e., Al₂O₃. This can be explained by a possible susceptor effect of the mullite tube. However, the microstructural observations showed a difference of homogeneity in the microstructure for Al₂O₃ and 3Y-TZP with some sintering cells. It can be linked to the difference of dielectric properties of the two materials. The influence of the sintering cell is less critical for 3Y-TZP, which couples better with MW than Al₂O₃. It was also observed that densification curves were switched to higher temperatures during sintering with the presence of SiC susceptor and without mullite tube. These results were justified by an error in the temperature measurement of the IR-pyrometer.     

Spark plasma sintering of multi-cation doped (Yb,Sm) α/β-SiAlON ceramic tool materials: Effects of cation type,composition, and sintering temperature

The multi-cation doped α/β-SiAlON composite ceramics tool materials were prepared via spark plasma sintering. The effects of cation type (Yb, Sm, Yb/Sm), composition, and sintering temperature on densification behavior, phase formation, microstructural evolution, and mechanical properties of α/β-SiAlON were studied. Results showed that the addition of Sm₂O₃ in Yb/Sm-SiAlON could decline the shrinkage temperature and accelerate the densification process due to the increased liquid phase volume and decreased viscosity. The Sm₂O₃ played the role of both sintering additives and stabilizing cation of α-SiAlON, which could promote the formation of α-SiAlON and the elongation of β-SiAlON, thus acquired refined α grains and large aspect ratio of β grains. The SPS-sintered multi-cation doped Yb/Sm-SiAlON with 2 wt% additional Sm₂O₃ possessed excellent comprehensive mechanical properties (3.40 g/cm³, 18.53 ± 0.18 GPa, and 6.13 ± 0.23 MPa m^1/2).     

Preparation of O′-Sialon-based ceramics by two-stage liquid phase sintering and study on the toughening mechanism of ultrafine-grained sintered clusters

Ultrafine-grained O′-Sialon-based ceramics were prepared by two-stage sintering at 1250 °C, with large particle GH4169 superalloy powder and nano Al₂O₃–Y₂O₃ as composite sintering aids. The effects of these aids on the densification, microstructure, and mechanical properties of O′-Sialon-based ceramics during two-stage sintering were also studied. Studies have shown that the densification process of O′-Sialon-based ceramics promoted by composite sintering additives, presents with the characteristics of two-stage liquid-phase sintering. In the first stage, GH4169 formed ultrafine-grained sintered clusters in the sintered material through liquid phase diffusion. In the second stage, the uniformly dispersed nano Al₂O₃–Y₂O₃ realized the uniform sintering of the material. In the fracture process, the ultrafine-grained sintered clusters hindered the crack propagation and promoted multiple deflections of the crack around the edge of the clusters, achieving the effect of crack deflection toughening. This effect, dominated by ultrafine-grained sintered clusters, significantly improved the fracture toughness of O′-Sialon-based ceramics up to 8.52 MPa m^1/2.     

Conduction and sintering mechanism of high electrical conductivity Magnéli phase Ti4O7

In this paper, the crystal structure and electronic structure of Ti₄O₇ were calculated based on density functional theory, and Magnéli phase Ti₄O₇ bulks were successfully prepared by spark plasma sintering (SPS). Results indicated that the contribution of Ti 3d to Fermi level increased due to the lack of oxygen atom in lattice, and the energy band gap of Ti₄O₇ was reduced compared with that of TiO₂. By calculating the relationship between the densification rate and effective stress in the process of SPS, it can be known that the densification mechanism of Ti₄O₇ powders was controlled by diffusion. Based on this, under the conditions of sintering temperature of 1000 °C, holding time of 10 min and sintering pressure of 30 MPa, Ti₄O₇ bulks with the optimal electrical conductivity (961.5 S cm⁻¹) could be obtained, which was more than 30% higher than the graphite material reported in literature. The results reveals that Ti₄O₇ will be one of the most promising electrode materials in the electrochemical field.     

Phase formation in porous liquid phase sintered silicon carbide: Part I:: Interaction between Al2O3 and SiC

The structure and phase formation of porous liquid phase sintered silicon carbide (porous LPS-SiC), containing yttria and alumina additives have been studied. The present paper is focused on the system Al–Si–C–O, which is part of the system describing the interactions with sintering additives.The influence of different sintering atmospheres, namely argon and Ar/CO, and different temperatures on structure and composition was investigated by XRD and SEM. Additionally, reaction products were calculated from thermodynamic data and correlated with experimentally determined reaction products. Alumina and SiC reacted at 1950 °C in an argon atmosphere, forming a metal melt of aluminium and silicon. No reduction of Al₂O₃ was observed in a CO-containing argon sintering atmosphere.In the second and third parts of this paper the interactions between Y₂O₃–SiC and Y₂O₃–Al₂O₃–SiC are analysed [J. Eur. Ceram. Soc. (in press), parts II and III].     

Sintering and characterization of aluminum titanate ceramics with enhanced thermomechanical properties

Aluminum titanate (Al₂TiO₅) was synthesized from Al₂O₃-TiO₂ system by solid-phase reaction with additives of light magnesium carbonate and SiO₂, incorporating with silicon carbide (SiC) particle. The influence of additives on the phase composition, microstructure configuration, sintering, and thermomechanical properties of the developed materials was investigated. The experimental results illustrated that the utilization of additives with applicable contents was conducive to the development of complex microstructures. The microstructures characterized by microcracks matrix together with interlocked grains were identified. The samples incorporating SiC and SiO₂ simultaneously displayed exceptional densification properties and microstructure as a result of a SiO₂ liquid phase accelerating SiC oxidation. Meanwhile, high flexural strength (41.51 MPa) and low thermal expansion coefficient of 0.485 × 10⁻⁶ K⁻¹ (RT-1000 °C) were detected, encouraging refractory applications with high thermomechanical solicitations.     

Role of different rare earth oxides on the reaction sintering of magnesium aluminate spinel

Role of three rare earth oxides, viz., La₂O₃, CeO₂ and Yb₂O₃ on reaction sintering of magnesium aluminate spinel having molar ratio of MgO:Al₂O₃?=?1:2 from its solid oxide precursors was investigated in static and dynamic heating conditions. Effect of these additives (3?wt%) on densification behavior, phase assemblage and microstructure development were studied in the temperatures of 1500–1700?°C. Yb₂O₃ enhanced the sintering of spinel, while La₂O₃ and CeO₂ negatively impacted the sintering of magnesium aluminate spinel which can be discerned from the shrinkage curve of TMA as well as from static firing regime. This is ascribed to the formation of secondary phases in La₂O₃ and CeO₂ containing samples which have different crystalline structures to that of spinel. This anisotropy due to different crystallinity hindered the pore shrinkage and pore removal and thereby retarded the densification. Whereas, the cubic structure of the secondary phase formed in Yb₂O₃ containing sample which is isotropic with the crystalline orientation of the parental spinel phase assisted the densification.     

Phase formation in porous liquid phase sintered silicon carbide:: Part II Interaction between Y2O3 and SiC

During the sintering of porous liquid phase sintered silicon carbide (porous LPS-SiC) a strong interaction with the atmosphere takes place, influencing the composition and stability of porous LPS-SiC components. The present paper is focused on the interaction of Y₂O₃ with SiC, which is part of the common used sintering additives for LPS-SiC (Y₂O₃–Al₂O₃–SiC). The interaction of Al₂O₃ and SiC has been studied in a previous paper [J. Eur. Ceram. Soc. (in press)].The reaction products of the interaction of Y₂O₃ with SiC and the resulting microstructures were analysed using model experiments. The effects of the influence of different sintering atmospheres, namely Argon and Ar/CO, as well as vacuum and different temperatures have been investigated. The phase formation was determined by X-ray diffraction (XRD) and can be explained on the basis of thermodynamic calculations. Depending on the sintering conditions, silicides or yttrium carbides can be formed in addition to stable oxides, which can result in the decomposition of the samples after sintering. Reactions between SiC and Y₂O₃ during sintering can be suppressed successfully if an Ar/CO atmosphere is used.     

Compositional identification of the intergranular phase in liquid phase sintered SiC

A model system consisting of coarse SiC (32–160 μm) as starting powder and Y₂O₃ and AlN as sintering additives was liquid phase sintered. Coarse-grained starting powder led to large intergranular phase regions which allowed an accurate determination of the chemical composition by wavelength-dispersive X-ray microanalysis (WDS). When N₂ was used as sintering atmosphere, a N-rich amorphous phase (about 44 at.% N) was identified by WDS to be the main triple-junction phase in the sintered SiC ceramics, while three further crystalline intergranular phases were AlN, Y₂SiN₄O₃ and an O-rich phase (Y₁₀Al₂Si₃O₁₈N₄). The overall O content was found to be reduced in comparison to the initial powder composition. The incorporation of N from the sintering atmosphere into the intergranular phase and a subsequent carbothermal reduction are believed to be responsible for the removal of O and the formation of the N-rich amorphous phase.