Mechanism of Liquid-Phase Sintering of Silicon Carbide and Nitride with Oxide Activating Additives

Abstract

The mechanism of sintering and the particulars of structure formation in liquid-phase-sintered silicon carbide and sintered silicon nitride were studied. The formation of the structure core (SiC or Si 3 Ni 4 grains) -boundary layer (SiC oxide or SiAlON) -intergrain phase (oxides) was recorded.Owing to their high strength and hardness, good stability against oxidation and resistance to thermal shock as well possibilities for retaining mechanical properties at high temperatures silicon carbide based ceramic materials are widely used as construction materials in many areas of technology. Silicon carbide belongs to a class of compounds with strong covalent bonds, which impedes mass transfer during sintering without the use of special additives. Ordinarily, dense SiC-ceramic is obtained by means of liquid-phase sintering with oxide additives. The most promising ones are Al 2 O 3 , Y 2 O 3 and MgO, which promote the formation of melt and sintering by the liquid-phase method during heat-treatment [1].A model of the formation of SiC-grain structure consisting of nuclei with a boundary layer on their surface is presented in [2]. The presence of the elements Y, Al and O in the boundary layer of silicon carbide grains was determined by metallographic methods. A detailed study of the mechanism of dissolution -recrystallization during liquid-phase sintering of silicon carbide with a sintering additive consisting of yttrium aluminum garnet is presented in [2,3].A similar model of the structure nucleus -boundary layer -intergrain phase is characteristic for many ceramic systems, which Si 3 N 4 /SiAlON, BaTiO 3 -based solid alloys and carbonitrides with Co-and Ni-binder are [4,5].The formation of an amorphous boundary phase on silicon nitride grains was discovered during sintering of the composition Si 3 N 4 -MgO [6,7]. The model developed for sintering of silicon nitride with oxide additives [8,9] showed the formation of an amorphous film of the order of 1 -2 nm thick on silicon nitride grains; this is a result of attractive (Van der Waals) forces acting between grains and repulsive (capillary) forces at the boundary of a grain and the oxide additive. Subsequently, it was shown that a SiAlON boundary layer forms on Si 3 N 4 nuclei during sintering of Si 3 N 4 with the addition of aluminum-nitrate garnet 3Y 2 O 3 × 5Al 2 O 3 [10,11].The present work is devoted to a more detailed study of the mechanism of liquid-phase sintering of the carbide LPSSiC and silicon nitride SSN with oxide sintering additives in the system Al 2 O 3 -Y 2 O 3 -MgO. MATERIALS AND METHODSThe following powders were used: SiC (Volzhsk Abrasives Plant, Volzhsk), comminuted in a jet mill to particle size d 0.5 = 0.85 mm; Si 3 N 4 , LS-12 grade (Berlin, Germany), obtained by furnace sintering, average particle diameter d 0.5 = 0.75 mm (Table 1). The oxides Al 2 O 3 , Y 2 O 3 and MgO were used as sintering additives; they were added in a strictly eutectic ratio in accordance with the phase diagram of the ternary system MgO-Y 2 O 3 -Al 2 O 3 along the garnet-spin...

Microstructures, mechanical properties and oxidation resistance of SiBCN ceramics with the addition of MgO, ZrO2 and SiO2 (MZS) as sintering additives

“…The addition of the two sintering aids can effectively lower the porosity in SiBCN ceramics and contribute to the densication of ceramics because of liquid phase sintering mechanisms. [20][21][22][23] These results reveal that only a small amount of MZS1 or MZS2 additive could promote the sintering ability of SiBCN powder effectively, resulting from the reduction of the sintering temperature and the increased density of the bulk materials. However, a few residual pores are still remaining in the SM2 sample and this seems to indicate a better effect on densication by the MZS1 additive than the MZS2 additive, as shown in Fig.…”

Thermodynamic assessment within the Zr–B–C–O quaternary system

“…At these elevated temperatures, ZrB 2 remains in equilibrium with the oxide phases as shown in the ternary domain (gas + ZrB 2 + t‐ZrO 2 ) and (hcp‐Zr + ZrB 2 + c‐ZrO 2 ). Refractory ceramic systems benefit from the presence and addition of oxides or other additives during processing to enhance densification 72 ; however, composite systems of UHTCs for high temperature applications avoid the use of these low melting point phases in order to maximize strength or oxidation resistance at elevated temperatures 2 . For this reason, the processing of ZrB 2 is combined with carbon‐based additives to react with the oxides 9,69,73 …”

High-Density Boron-Carbide Ceramics

“…RBBC-материал, спеченный по традиционной технологии реакционного спекания, характеризуется присутствием зерен В 4 С, фазы SiC II , частично растворенной в кремнии, и фазы твердого раствора кремния в карбиде бора B 12 (C, Si, B) 3 , кристаллизующейся на зернах карбида бора плотными кольцеобразными слоями. Подобная структура материала имеет название «ядро -оболочка» [19][20][21][22]. Роль «ядра» выполняют зерна исходного карбида бора, а «оболочки»фаза твердого раствора кремния в карбиде бора, соединенная между собой в 3-мерную матрицу.…”