Silicon-based non-oxide structural ceramics such as CH, result in ultra fine and pure B-Sic the production proce powder properties dependent on reactions of SiH, or SiCl4 with hydrocarbons Table 2. Typical SiC powders. Additionally, the gas phase decom Production process Acheson Modified Gas phase position of organosilanes CH3SiCl3,( CH3)2 process Acheson reaction SiCl2(CH3)3 SiCI and(CH3)4Si or polycarbosi lanes [-R2Si-CH2-I,(R=CH3, H) leads to nano- Composition(wt%) crystalline B-SiC. The characteristic data of , sio 97 a-SiC >98 B-SiC 97.2 B-SiC SiC-powders produced by the different methods are listed in Table 2 00 0006 00009 4 CONVENTIONAL PROCESSING Diameter(um) 045 0·27 175 MICROSTRUCTURING AND MECHANICAL Specific surface area 14 (m2g) PROPERTIES OF MONOLITHIC Si,, AND SiC CERAMICS covalent and strongly directional chemical Classical ceramic materials consist mostly of bondings in Si, N, and Sic cause very low self oxides which are predominantly ionic materials. diffusion coefficients. 77.76 Hence the conditions Additionally, the bondings are nondirectior for bulk diffusion are unfavourable and sinter and the densification of these ceramics takes ing of covalent substances in general is diffi place by volume or grain boundary diffusion cult.79 chanced by vacancy formation due to non-stoi- Several methods have been used to overcome chiometry. In contrast to that, the highly this low sinterability of covalent ceramics. One of them is to enhance the sintering by applying an external isostatic pressure at high tempera- tures(HIP)which is unfortunately limited to small parts and high cost applications. Another part of the mater is produced in-situ by reaction from its ele- ments. The third way described in the following paragraphs is to add sintering aids. This process includes deagglomeration and mixing of starting powders, drying and sieving of the resulting mixtures, moulding into green bodies and sub- sequent sintering 4.1 Sili 4.1.1 Si of si In order to achieve elongated grain structures in the final microstructure increasing the frac ture toughness as described later Si,- ceramics are mainly produced from a-Si3N4-powders Oxides like MgO, Al2O3, Y2O3, rare earth oxides and mixtures of them are used as addi tives to produce dense ceramics by liquid phase sintering, which stages:.(i) the particle rearrangement by the development of capillary forces among the par ticles 4 due to the formation of an eutectic melt consisting of the used additives and the Sio2 on he si3N4 surface, (ii)the solution of a-Si Fig. 5. Crystalline SiC as received from the Acheson the diffusion of Si and n through the liquid phase and the reprecipitation on B-Si3NSilicon-based non-oxide structural ceramics 19 reactions of Sill4 or SIC14 with hydrocarbons such as CH 4 result in ultra fine and pure fl-SiC powders? Additionally, the gas phase decom￾position of organosilanes CH~SiC13, (CH3)2 SiCI2 (CH3)3SiCI and (CH3)4Si or polycarbosi￾lanes [-R2Si-CH2-],, (R=CH3, H) leads to nano￾crystalline {/-SIC. The characteristic data of SiC-powders produced by the different methods are listed in Table 2. 4 CONVENTIONAL PROCESSING, MICROSTRUCTURING AND MECHANICAL PROPERTIES OF MONOLITHIC Si3N4 AND SiC CERAMICS Classical ceramic materials consist mostly of oxides which are predominantly ionic materials. Additionally, the bondings are nondirectional and the densification of these ceramics takes place by volume or grain boundary diffusion enhanced by vacancy formation due to non-stoi￾chiometry. In contrast to that, the highly Fig. 5. Crystalline SiC as received from the Acheson process. Table 2. Typical SiC-powder properties dependent on the production process Production process Acheson Modified Gas phase process Acheson reaction process Composition (wt%) SiC 97 a-SiC > 98 fl-SiC 97.2/~-SiC Free C l'4 0.4 1.0 Free SiO2 0.7 0.3 1.3 Fe 0.06 0.04 0.006 Ai 0.01 0-03 0.0017 Ca -- -- 0.0009 Diameter (/~m) 0.45 0-27 0'3 Specific surface area 14 17.5 -- (m 2 g ') covalent and strongly directional chemical bondings in Si3N, and SiC cause very low self diffusion coefficients. 77"76 Hence, the conditions for bulk diffusion are unfavourable and sinter￾ing of covalent substances in general is diffi￾cult. TM Several methods have been used to overcome this low sinterability of covalent ceramics. One of them is to enhance the sintering by applying an external isostatic pressure at high tempera￾tures (HIP) which is unfortunately limited to small parts and high cost applications. Another is reaction sintering, where part of the material is produced in-situ by reaction from its ele￾ments. The third way described in the following paragraphs is to add sintering aids. This process includes deagglomeration and mixing of starting powders, drying and sieving of the resulting mixtures, moulding into green bodies and sub￾sequent sintering. 4.1 Silicon nitride ceramics 4. l. 1 Sintering of Si,N~-ceramics In order to achieve elongated grain structures in the final microstructure increasing the frac￾ture toughness as described later Si~N4-ceramics are mainly produced from ct-Si~N4-powders. Oxides like MgO, A120~, Y203, rare earth oxides and mixtures of them are used as addi￾tives to produce dense ceramics by liquid phase sintering, which can be subdivided into three stages: 8'''~' (i) the particle rearrangement by the development of capillary forces among the par￾ticles 84 due to the formation of an eutectic melt consisting of the used additives and the Si02 on the Si3N4 surface, (ii) the solution of ~-Si~N~, the diffusion of Si and N through the liquid phase and the reprecipitation on /~-Si~N4