Effect of Boron and Carbon on Sintering of SIC

Abstract

PURE covalent solids, e.g. B, Si, C, Sic, and BN, cannot be densified by either sintering or conventional hot-pressing . However, some of these covalent solids can be densified in the presence of liquid-forming additions. For example, appreciable densification has been obtained in S&N, sintered with MgO' and YzO, + SO,?In Sic, liquid sintering has been observed in the presence of a B,C-Sic eutectic3 and in a melt of a composition in the system Al-B-C-Si.4 With liquid-phase sintering, however, only limited densification (= 85 to 90%) was obtained.P m~h a z k a~.~ has reported sintering of Sic to densities near theoretical with the use of additions of several tenths of a percent boron to pure p-Sic powders. It was concluded that densification occurred by solid-state sintering. Boron, introduced as elemental B (amorphous or crystalline), B,C, or LiBH,, induces sintering in fine-grained S i c powders, and densities >95% of theoretical can be consistently obtained. Compacts of the same powders exposed to the same sintering conditions (2100°C in He) without B did not shrink significantly. Further experiments indicated that the presence of free C in the powder compacts is another necessary condition for achieving of high sintered densities. The present note describes sintering experiments in which the effects of both B and C were evaluated quantitatively and separately.Free C was removed from an SIC powder by exposing it to air at 650°C for 50 h; the expected oxidation products were removed from the surface of the S i c crystallites by leaching the powder with 20% HF and washing it with water and alcohol. Spectrographic analysis of the yellowish powder obtained revealed (in ppm) B 20, A1 10, Fe 70, W 300, Ca 50, and other metals (Cr, Mg, Ti, and Mn).Chemical analysis showed 0.17% 0,0.008% N, and 0.00% freeC. The surface area of the sample was 9.2 m2/g, whereas the mean average particle size was 0.13 pm. The S i c phase present was p-Sic.Carbon was reintroduced by mixing the powder with a carbonaceous organic compound in solution (polymethylphenylene in benzene), and B was added by mixing the sample with another powder which was heavily doped with B on preparation but otherwise had identical characteristics and in which B was assumed to be present partly in solid solution but mainly as B4C. Three-gram cylinders were pressed from each composition and sintered in flowing He at 1 atm (0.7 ppm 0,) in a carbon-element resistance furnace at 2100°C (established by preliminary experiments). The shrinkages and final densities measured by liquid displacement are given in Table I. A lower limit to the B and free C concentrations necessary to effect the maximum density of S i c exists. At any level of C, densification of Sic in the absence of B is virtually nil and reaches a maximum at -0.3% B. The optimum B concentration is probably related to the solubility limit of B in Sic, which must be exceeded to initiate densification. The solubility of B in a-Sic at 2100°C is =0.2%.7*8 However, much higher estimates are found in the literat...

Microstructure development in hot-pressed silicon carbide: effects of aluminum, boron, and carbon additives

“…proposed that boron and carbon could produce a driving force for diffusional mass transport necessary for sintering, by reducing the grain boundary energy [47]. This assumption was confirmed by other groups in the following years.…”

“…In addition, Rijswijk et al reported that carbon reduces the loss of boron in the SiC lattice [46]. Therefore, the boron and carbon appear to have similar promotional effects on the β-to-α phase transformations as observed in the present study as well as discussed in other publications [46,47], but the effect of boron is more pronounced. It can be concluded that boron and carbon are key elements in inducing β-to-α phase transformations in SiC.…”

“…It can be concluded that boron and carbon are key elements in inducing β-to-α phase transformations in SiC. This explains why a dominant volume fraction of α-phases is produced when sintering β-SiC powder with boron and carbon additives only, without Al [42,47]. Aluminum additions, however, will react with boron and carbon at elevated temperatures to form secondary phases, thereby indirectly weakening the effect of boron and carbon on the β-to-α phase transformations.…”

Aluminum-containing intergranular phases in hot-pressed silicon carbide

“…Later, it was found that hot pressing with boron and carbon additions could lead to a SiC with 96% of the theoretical density [2], or better [3][4]. While the densification mechanism was not clear, it was already known that boron and carbon did not form intergranular films [5][6] but rather dissolved into the SiC grains, not supporting a liquid phase densification mechanism.…”

“…Al-based oxycarbide grain boundary film between SiC grains were observed [2,7,[17][18][19][20][21][22], as well as other secondary phases at triple junctions [21][22][23]. The best of these SiC materials yet investigated exhibited unprecedented combinations of strength, fracture toughness, fatigue, high temperature creep resistance, and wear resistance [9,14,17,18,[24][25][26][27][28][29][30][31], resulting from the presence of favorable grain boundary structures [22].…”