Sintering additives for SiC based on the reactivity: A review

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].

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

Alumina ceramics with different sintering temperatures in argon atmosphere were obtained using stereolithography-based 3D printing. The effects of sintering temperature on microstructure and physical and mechanical properties were investigated. The results show that the average particle size, shrinkage, bulk density, crystallite size, flexural strength, Vickers hardness, and nanoindentation hardness increased with the increase in sintering temperature, whereas the open porosity decreased with increasing sintering temperature. No change was observed in phase composition, chemical bond, atomic ratio, and surface roughness. For the sintered samples, the shrinkage in Z direction is much greater than that in X or Y direction. The optimum sintering temperature in argon atmosphere is 1350 °C with a shrinkage of 3.0%, 3.2%, and 5.5% in X, Y, and Z directions, respectively, flexural strength of 26.7 MPa, Vickers hardness of 198.5 HV, nanoindentation hardness of 33.1 GPa, bulk density of 2.5 g/cm3, and open porosity of 33.8%. The optimum sintering temperature was 70 °C higher than that sintering in air atmosphere when achieved the similar properties.

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Effect of sintering temperature in argon atmosphere on microstructure and properties of 3D printed alumina ceramic cores

An atmosphere-assisted heating process was utilized to transform mesostructured SiC−C nanocomposites in situ into ordered mesoporous SiOC and SiCN ceramics with relatively small structural shrinkage. The mesostructured SiC−C nanocomposites were fabricated using commercially available polycarbosilane (PCS) as a ceramic precursor and mesoporous carbon CMK-3 as a hard template that itself was prepared by a nanocasting procedure from mesoporous silica SBA-15. Reactive gases including air and ammonia were employed to simultaneously incorporate O or N into SiC ceramics and oxidize or reduce the carbon template and excess carbon deposits. The procedure was carried out at 500 °C for 10 h and at 1000 °C for 10 h for air- and ammonia-assisted in situ transformations, respectively. SAXS, XRD, N2 sorption, and TEM analyses revealed that the mesoporous SiOC and SiCN ceramics exhibit open, continuous frameworks similar to that of the primary template ordered mesoporous SBA-15. The ordered mesoporous SiOC and SiCN ceram...

Ordered Mesoporous SiOC and SiCN Ceramics from Atmosphere-Assisted in Situ Transformation

This study reports the pressureless sintering of cubic phase silicon carbide nanoparticles (β-SiC). Green blended compounds made of SiC nano-sized powder, a fugitive binder and a sintering agent (boron carbide, B 4 C), have been prepared. The binder is removed at low temperature ( e.g. 800 °C) and the pressureless sintering studied between 1900 and 2100 °C. The nearly theoretical density (98% relative density) was obtained after 30 min at 2100 °C. The structural and microstructural evolutions during the heat treatment were characterised. The high temperatures needed for the sintering result in the β-SiC to α-SiC transformation which is revealed by the change of the composite microstructure. From 1900 °C, dense samples are composed of β-SiC grains surrounding α-SiC platelets in a well-defined orientation. TEM investigations and calculation of the activation energy of the sintering provided insight to the densification mechanism.

Pressureless sintering of beta silicon carbide nanoparticles

Gas pressure sintering of SiC–AlN composites in nitrogen atmosphere

Decomposition reactions in the SiC-Al-Y-O system during gas pressure sintering

Oxidation of SiC is a major constraint during development of metal oxide–silicon carbide composites when processed in oxygen containing environment such as in air. In the present investigation, Mg+2, Al+3 and Zr+4 hydrogels were used as a source of respective oxide and oxidation of SiC in each system was studied. A three-stage mechanism was found to be operative in Al+3 and Zr+4 systems where oxidation at the initial stages was found to be controlled by the nature of the polynuclear complexes formed on the surfaces of SiC particles. At the intermediate stage a transition from polynuclear complex to metal silicate protective layer formation changes the oxidation characteristics. Finally the metal silicates provided the ultimate protection. Mg+2 was found to be ineffective. The extent of retention of SiC in the final composites could be premonitored by controlling the amount and the nature of complexing cations.

K K Dhargupta

Papers

Gas pressure sintering of SiC–AlN composites in nitrogen atmosphere

Decomposition reactions in the SiC-Al-Y-O system during gas pressure sintering

Retention of SiC during development of SiC-MxSiyOz composites [M = Al, Zr, Mg] by reaction bonding in air

Near net shape SiC–mullite composites from a powder precursor prepared through an intermediate Al-hydroxyhydrogel

SiC–YAG sintered composites from hydroxy hydrogel powder precursors