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.     

Micro-segregations in liquid phase sintered silicon carbide ceramics

The densification behaviour of LPSSiC ceramics with different amount of secondary phases was investigated during Field Assisted Sintering (FAST). In the densified materials micro-segregations were found with dimensions of several 100 μm. Sometimes such segregations were found in gas pressure sintered materials. The investigation of the state of crystallisation by EBSD and XRD revealed that these micro-segregations are connected with the formation of large YAG (Yttrium aluminium garnet) crystals. The mobility of yttrium in the grain boundaries was investigated by measuring concentration profiles in diffusion couples. The high diffusion coefficient determined at 1850 °C (10^?6 cm/s) indicates that the observed segregations are caused by the crystallisation kinetics of the secondary phases during cooling.     

Liquid phase formation in the system SiC,Al2O3, Y2O3

The liquid phase formation in the system SiC–Al₂O₃–Y₂O₃ was investigated via differential thermal analysis (DTA) combined with thermogravimetry (TG). For this purpose mixtures of various alumina and yttria mol ratios and 10 and 20 mol% silicon carbide were densified and heat treated at different temperatures. It was shown that silicon carbide in the examined amounts has low influence on the melting temperature of the oxide phase. The compositions and microstructures formed were studied by SEM, EDX and XRD. The results were compared to thermodynamic calculations.     

Highly resistive SiC ceramics sintered with Al2O3-AlN-Y2O3 additions

A polycrystalline SiC ceramic prepared by pressureless sintering of α-SiC powders with 3 vol% Al₂O₃-AlN-Y₂O₃ additives in an argon atmosphere exhibited a high electrical resistivity of ~10¹³ Ω cm at room temperature. X-ray diffraction revealed that the SiC ceramics consisted mainly of 6H- and 4H-SiC polytypes. Scanning electron microscopy and high resolution transmission electron microscopy investigations showed that the SiC specimen contained micron-sized grains surrounded by an amorphous Al-Y-Si-O-C-N film with a thickness of ~4.85 nm. The thick boundary film between the grains contributed to the high resistivity of the SiC ceramic.     

Pressureless sintered Al4SiC4 ceramics with Y2O3 addition

Highly densified Al₄SiC₄ ceramics with a relative density of 96.1% were prepared by pressureless sintering using 2 wt% Y₂O₃ as additives. The densification mechanism, phase composition, microstructures and mechanical properties of Al₄SiC₄ ceramics were investigated. Y₂O₃ in-situ reacted with the oxygen impurities in Al₄SiC₄ powder to form a yttrium aluminate liquid phase during sintering, which promoted the densification and anisotropic grain growth. The final Al₄SiC₄ ceramics were composed of equiaxed grains and columnar grains, and presented a bimodal grain distribution. The mechanical properties of the pressureless sintered Al₄SiC₄ ceramics were better than those reported for hot pressed Al₄SiC₄, including a flexural strength of 369 ± 24 MPa, fracture toughness of 4.8 ± 0.1 MPa m^1/2 and Vickers hardness of 11.3 ± 0.2 GPa. Pressureless sintering of Al₄SiC₄ ceramics is of great significance for the development and practical application of Al₄SiC₄ ceramic parts, especially with big size and complex shape.     

Electrochemical corrosion of solid and liquid phase sintered silicon carbide in acidic and alkaline environments

Solid and liquid phase sintered silicon carbide (SiC) ceramics are used in aggressive environments, e.g. as seals and linings in chemical plant equipments. There exist data concerning corrosion of solid phase sintered SiC (SSiC), but there are only few data concerning their electrochemical corrosion behaviour. The corrosion of liquid phase sintered SiC ceramics (LPS SiC) containing yttria aluminium oxide grain boundary phases has been investigated by standard methods that have shown the decisive influence of the oxide grain boundary on the corrosion stability of these materials. But no electrochemical investigations are known. In this study therefore, potentiodynamic polarisation measurements have been used to determine the corrosion mechanisms of SSiC and LPS SiC ceramics at room temperature in acidic and alkaline environments. The investigation has shown a pronounced electrochemical corrosion in acids and alkaline solutions for both types of materials. In HCl and HNO₃ pseudo-passivity features due to the formation of a thin layer of SiO₂ on the surface were observed, whereas in NaOH soluble silicate ions were observed resulting in more pronounced corrosion. Microstructural observations of initial and corroded samples revealed that the residual carbon found in the microstructure of SSiC did not dissolve preferentially. The corrosion current densities of the LPS SiC materials were caused by the dissolution of SiC and not by the corrosion of the oxide grain boundary phase. The corrosion current densities of the LPS SiC materials investigated were lower than those of the SSiC materials.     

Phase formation in Al2O3-C refractories with Al addition

Depending on the recipe and the firing conditions, several non-oxides can be formed in Al₂O₃-C refractories. In this paper, the effect of the purity of the recipe components on the phase formation in Al₂O₃-C refractories with Al addition was investigated. Two test series were sintered from 800 °C to 1600 °C under air embedded in coke breeze. One test series was with brown fused alumina, and the other was with tabular alumina. At temperatures of up to 1200 °C the phase formation was the same for both recipes. For temperatures greater than 1400 °C, the impurities of brown fused alumina enhanced the formation of a polytype, while Al₄O₄C and Al₂₈O₂₁C₆N₆ were formed in the other series. The findings explain the occurrence of several non-oxides in disequilibrium at the chosen temperatures. The occurrence of Al₄C₃ was of particular interest due to its low hydration resistance. It was formed at 1200 °C.     

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.     

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.     

Peculiarities of phase formation in Al2O3-CeO2 nanopowders

The results of the investigations on the influence of CeO₂ adding on the transformation of the metastable Al₂O₃ forms obtained by thermal treatment of xerogels are presented. It is established that in the presence of CeO₂ the process of the formation of a stable phase of α-Al₂O₃ (corundum) takes place in a wider temperature range and is accomplished at a higher temperature (1300°C). It is suggested that the revealed mismatches on the diffractograms between the intensity values of the main analytical peaks of the corundum and its content in the samples are associated with the effect of screening caused by the differences in the mass coefficients of absorption of (μ) CeO₂ and Al₂O₃.     

多孔Al2O3/SiC、MoSi2/SiC复合材料的制备及吸波性能

以Si、Al₂O₃、MoSi₂微粉和生物竹材为原料,采用包埋烧结法分别制备出SiC多孔材料、Al₂O₃/SiC、MoSi₂/SiC复合材料。采用XRD、SEM及波导法测试其物相组成、显微结构及吸波性能。结果表明：MoSi₂/SiC复合材料的厚度为2 mm时有明显的吸波特性,有效吸收带宽在X波段的9.65~12.4 GHz频率范围内达2.75 GHz,且最低反射损耗为-38.27 dB。Al₂O₃/SiC复合材料孔道内的Al₂O₃与SiC晶须交缠,形成大量电偶极矩,产生介电损耗;MoSi₂/SiC复合材料除介电损耗外还存在电阻损耗,使得复合材料电磁损耗增加,是较有前途的结构功能吸波材料。     

Direct-bonded Al/Al2O3 using a transient eutectic liquid phase

Directed bonding with Al and Al₂O₃ was achieved using a transient liquid phase (TLP) method after annealing at the low melting point of Al, which deposited Ni, Cu, Ge, and Si on the Al₂O₃ substrate. Al/Al₂O₃ microstructures were evaluated using a scanning electron microscopy and transmission electron microscopy. A reaction layer was absent at the Al/Al₂O₃ interface, and all deposited metal films dissolved into the Al foil and reacted with Al to form an eutectic liquid phase near the interface to wet and join with the Al₂O₃. Al₉Fe₂ and Al₃Fe intermetallic compounds were formed in the Al substrate because of Fe precipitation, which is an impurity of Al foil, and the reaction with Al at the grain boundaries of Al. The bonding area percentage, shear strength, and thermal conductivity for Al and Al₂O₃ were assessed using scanning acoustic tomography (SAT), the ISO 13 124 shear strength test, and the laser flash method, respectively. The Al/Al₂O₃ specimen deposited with the Ni film had the highest shear strength (33.74 MPa), thermal conductivity (42.3 W/mK), and bonding area percentage (96.78%). The Al/Al₂O₃ specimens deposited with Ge and Si exhibited relatively poor bonding because of the oxidation of Ge and Si at the surface of Al₂O₃ before bonding with Al.     

Low-temperature joining of SiC ceramics using NITE phase with Al2O3-Ho2O3 additive

In order to avoid the property degradation resulting from high-temperature joining process, nano-infiltrated transient eutectoid (NITE) phase with the Al₂O₃-Ho₂O₃ as the joining adhesives was adopted to join silicon carbide (SiC) ceramics with the attempts to lower down the joining temperature. The liquid-phase-sintered silicon carbide (LPS-SiC) specimens were joined at 1500-1800°C by spark plasma sintering (SPS) under the pressure of 20 MPa. The results of the shear test and microstructure observation showed that the joining process could be finished at a relatively lower temperature (1700°C) compared to other NITE-phase joining. In contrast to the shear strength of 186.4 MPa derived from the SiC substrate, the joint exhibited the shear strength of 157.8 MPa with the fully densified interlayer.     

Fabrication and microstructure of porous SiC ceramics with Al2O3 and CeO2 as sintering additives

In the present study, porous silicon carbide ceramics were prepared via spark plasma sintering at relatively low temperatures using Al₂O₃ and CeO₂ as sintering additives. Sacrificial template was selected as the pore forming mechanism, and gelcasting was used to fix the slurry in a short time. The evolution process of the microstructures during different steps was observed by SEM. The influence of the sintering temperature and sintering additives on the shrinkage and porosity of the samples was studied. The microstructures of different samples were characterized, and the mechanical properties were also evaluated.     

AC,DC conduction and dielectric behaviour of solid and liquid phase sintered Al2O3-15 mol% V2O5 pellets

Microstructural and dielectric characterization of porous Al₂O₃-V₂O₅ pellets sintered under both solid (600 °C) and liquid phase (850 °C) conditions are reported. The low temperature solid state sintering (SSS) leaves a very porous network of dense alumina particles surrounded by smaller facetted vanadia, whereas under liquid phase sintering (LPS), the V₂O₅ melts and recrystallizes over the relatively inert Al₂O₃, leading to a connected network structure. The ac conductivity and dielectric parameters of the pellets investigated from room temperature (RT) to 400 °C exhibited both universal dielectric (diffusive) behaviour at low frequencies and nearly constant loss (sub-diffusive) regimes at low temperatures. Activation energies calculated for dc/ac conduction at different frequencies suggests a composite conduction mechanism controlled by the Vanadia phase: at low frequencies, the calculated energies (E_a≈0.5 eV) compare with ionic diffusion of vanadium in V₂O₅, while at high frequencies and low temperatures an additional (E_a≤0.16 eV) polaronic hopping mechanism is seen. This cross-over frequency is significant for the LPS specimen indicating that the vanadia connected network assists conduction by providing a continuous (but disordered) diffusion route. Impedance and modulus spectral analysis show the presence of distributed time constants arising from electrodes, pore/matrix and phase interfaces and GBs.     

Effect of atmosphere and sintering time on the microstructure and mechanical properties at high temperatures of α-SiC sintered with liquid phase Y2O3–Al2O3

The influence that the atmosphere (N₂ or Ar) and sintering time have on microstructure evolution in liquid-phase-sintered α-SiC (LPS-α-SiC) and on its mechanical properties at high temperature was investigated. The microstructure of the samples sintered in N₂ was equiaxed with a grain size of 0.70 μm and a density of 98% of the theoretical value regardless of the sintering time. In contrast, samples sintered in Ar had an elongated-grain microstructure with a density decreasing from 99 to 95% and a grain size increasing from 0.64 to 1.61 μm as the sintering time increased from 1 to 7 h. The mechanical behaviour at 1450 °C showed the samples sintered in nitrogen to be brittle and fail at very low strains, with a fracture stress increasing from 400 to 800 MPa as the sintering time increased. In contrast, the samples sintered in Ar were quasi-ductile with increasing strain to failure as the sintering time increased, and a fracture stress strongly linked to the form and size of the grains. These differences in the mechanical properties of the two materials are discussed in the text. During mechanical tests, a loss of intergranular phase takes place in a region, between 50 and 150 μm thick, close to the surface of the samples—the effect being more important in the samples sintered in Ar.     

Low pressure plasma-sprayed Al2O3 and Al2O3/SiC nanocomposite coatings from different feedstock powders

This paper describes a preliminary investigation of a nanocomposite ceramic coating system, based on Al₂O₃/SiC. Feedstock Al₂O₃/SiC nanocomposite powder has been manufactured using sol-gel and conventional freeze-drying processing techniques and then low pressure plasma sprayed onto stainless steel substrates using a CoNiCrAlY bond coat. Coatings of a commercial Al₂O₃ powder have also been manufactured as a reference for phase transformations and microstructure. The different powder morphology and size distribution resulting from the different processing techniques and their effect on coating microstructure has been investigated. Phase analysis of the feedstock powders and of the as-sprayed coatings by X-ray diffractometry (XRD) and nuclear magnetic resonance (NMR) showed that the nano-scale SiC particles were retained in the composite coatings and that equilibrium α-Al₂O₃ transformed to metastable γ- and δ-Al₂O₃ phases during plasma spraying. Other minority phases in the sol-gel Al₂O₃/SiC nanocomposite powder such as silica and aluminosilicate were removed by the plasma-spraying process. Microstructure characterisation by scanning electron microscopy (SEM) of the as-sprayed surface, polished cross-section, and fracture surface of the coatings showed evidence of partially molten and unmolten particles incorporated into the predominantly lamella microstructure of the coating. The extent of feedstock particle melting and consequently the character of the coating microstructure were different in each coating because of the effects of particle morphology and particle size distribution on particle melting in the plasma.     

Microstructure and mechanical strength of transient liquid phase bonded Ti3SiC2 joints using Al interlayer

Based on the structure characteristic of Ti₃SiC₂ and the easy formation of Ti₃Si_1−xAl_xC₂ solid solution, a transient liquid phase (TLP) bonding method was used for bonding layered ternary Ti₃SiC₂ ceramic via Al interlayer. Joining was performed at 1100–1500 °C for 120 min under a 5 MPa load in Ar atmosphere. SEM and XRD analyses revealed that Ti₃Si(Al)C₂ solid solution rather than intermetallic compounds formed at the interface. The mechanism of bonding is attributed to aluminum diffusing into the Ti₃SiC₂. The strength of joints was evaluated by three point bending test. The maximum flexural strength reaches a value of 263 ± 16 MPa, which is about 65% of that of Ti₃SiC₂; for the sample prepared under the joining condition of 1500 °C for 120 min under 5 MPa. This flexural strength of the joint is sustained up to 1000 °C.