Silicon-based non-oxide structural ceramics and/or restricted to small parts and uncompli- described the densification process to proceed cated shapes. The pressureless densification of in three stages: (i) the previously from Lange SiC-ceramics can be performed either by adding postulated rearrangement of particles due to of boron or aluminium and carbon or by liquid the formation of an Al2O3 rich melt,(ii) the phase sintering using oxides and nitrides like dissolution of mainly B-SiC and the reprecipita Al2O3, Y2O3 and AIN. The sinterability of Sic tion of a-SiC leading to a phase transformation with B and C at temperatures between 2000 and and a microstructure containing platelet shaped 2100C to nearly full density (983% rel. den- a-SiC-crystals in agreement with Mulla, (ii) ity) has been discovered by Prochazka and grain coarsening of a-SiC. The dissolution re Charles in 1973. 1 They explained the densifi- precipitation process has been confirmed by Sigl cation mechanism to proceed by elimination of and Kleebe by TEM investigation on Al2O/ SiO 2 present on the SiC surface by carbon Y2O3-SiC ceramics. They detected a shell struc increasing the surface energy (,sv) between Sic ture of the final a-SiC crystals. The core and vapour phase and by boron segregating at (preexisting nuclei) was found to be pure a-SiC the grain boundaries reducing the SiC-Sic grain whereas the outer regions grown during sinter- boundary energy (zgb ). This decreases the 7g/ ing contained Al,Y and o from the liquid Ysy ratio below 3(dihedral angle over 60) formed during the heat treatment. Several which is required to attain pore shrinkage authors23-127 reported on the utilization of Other authors suggested liquid phase sintering Al_O3/Y203 mixtures as sintering aids. The to be the densification mechanism in boron/car- most effective sintering was achieved at 1950C bon doped SiC ceramics. 2, 1. However, no using either 60 mol% Al 2O3 and 40 mol% intergranular phase, possibly acting as a liquid Y2O3 or 77 mol% Al O3 and 23 mol% phase, has been found in several microscopic Y2O3. 2>Other authors 28. 129 published experi investigations. 4-17 On the one hand, the ments on sintering of SiC at 1900C with the reduction effect of carbon during sintering addition of AIN and Y20 seems to be clear. On the other hand the influ ence of boron is discussed controversly Gresko- 4.2.2 Microstructure and thermo-mechanical vic and Rosolowski reported on the properties of Sic-ceramics suppression of SiC grain growth(analyzed by The main factors controlling the final SiC pecific surface area measurements) at lower microstructure are the a/B-phase ratio of the temperatures (1300-1800C)and on the SiC sintering powder, the used additive system hanced coarsening at 2000oC by boron and and the densification method. Sic-ceramics si carbon additions. Motzfeld"concluded from tered with B or Al and c contain no secondary these results that the role of boron is not pri- phase in the final microstructure have therefore marily to promote sintering but to prevent grain strong grain boundaries resulting in mainly growth before sintering. However, despite transgranular fracture mode. Consequently, low numerous investigations no final sintering fracture toughness of about 3-4 MPa'm are model for boron/carbon doped SiC-ceramics is measured. This type of Sic-material is noted for presently available. its high hardness 0, high temperature resist Alternatively, liquid phase sintering of Sic- ance, good oxidation32-14 and creep.35 1.36 ceramics using alumina and other oxides and resistance up to above 1500"C, as well as high itrides is performed. In 1975 Lange showed resistance to abrasion and corrosion. The good hat Sic with the addition of Al, O3 can be den- thermal shock behavior. of Sic-ceramics is sified by hot pressing at 1950.C. It was based on their high thermal conductivity and explained that densification was a liquid phase low coefficient of thermal expansion. Typical sintering process including the reaction of the properties of pressureless sintered (SSiC), hot added Al2O with the surface SiO2 on the Sic pressed( HPSiC), hot isostatically pressed(HIP quent particle rearrangement and solution of ceramics are summarized in Table 4.16, 13>)Sic- powder forming an eutectic melt and subse- SiC) and liquid phase sintered(LPSSiC) small SiC particles accompanied by the growth he microstructural development of liquid of larger SiC crystals. In 1991 Suzuki pro- ph chase sintered SiC-ceramics has been investi- duced Sic-ceramics by pressureless sintering of gated by several authors. 4. 120, 121. 4. 42 It was O3 containing B-Sic powder mixtures. He found that a platelet like microstructureSilicon-based non-oxide structural ceramics 25 and/or restricted to small parts and uncompli￾cated shapes. The pressureless densification of SiC-ceramics can be performed either by adding of boron or aluminium and carbon or by liquid phase sintering using oxides and nitrides like A1203, Y203 and AIN. The sinterability of SiC with B and C at temperatures between 2000 and 2100°C to nearly full density (98.3% rel. den￾sity) has been discovered by Prochazka and Charles in 1973.' ,1 They explained the densifi￾cation mechanism to proceed by elimination of SiO~ present on the SiC surface by carbon increasing the surface energy (~sv) between SiC and vapour phase and by boron segregating at the grain boundaries reducing the SiC-SiC grain boundary energy (e~.gb). This decreases the 7gb/ 7sv ratio below j3 (dihedral angle over 60 °) which is required to attain pore shrinkage. Other authors suggested liquid phase sintering to be the densification mechanism in boron/car￾bon doped SiC ceramics. 112''13 However, no intergranular phase, possibly acting as a liquid phase, has been found in several microscopic investigations. 114-117 On the one hand, the reduction effect of carbon during sintering seems to be clear. On the other hand, the influ￾ence of boron is discussed controversly. Gresko￾vic and Rosolowski 1,8 reported on the suppression of SiC grain growth (analyzed by specific surface area measurements) at lower temperatures (1300-1800°C) and on the enhanced coarsening at 2000°C by boron and carbon additions. Motzfeld 72 concluded from these results that the role of boron is not pri￾marily to promote sintering but to prevent grain growth before sintering. However, despite numerous investigations no final sintering model for boron/carbon doped SiC-ceramics is presently available. Alternatively, liquid phase sintering of SiC￾ceramics using alumina and other oxides and nitrides is performed. In 1975 Lange' 19 showed that SiC with the addition of A1203 can be den￾sifted by hot pressing at 1950°C. It was explained that densification was a liquid phase sintering process including the reaction of the added A120~ with the surface SiO~ on the SiC powder forming an eutectic melt and subse￾quent particle rearrangement and solution of small SiC particles accompanied by the growth of larger SiC crystals. In 1991 Suzuki '~° pro￾duced SiC-ceramics by pressureless sintering of AI~O3 containing /3-SIC powder mixtures. He described the densification process to proceed in three stages: (i) the previously from Lange 119 postulated rearrangement of particles due to the formation of an A1203 rich melt, (ii) the dissolution of mainly/~-SiC and the reprecipita￾tion of a-SiC leading to a phase transformation and a microstructure containing platelet shaped a-SiC-crystals in agreement with Mulla, '2' (iii) grain coarsening of a-SiC. The dissolution re￾precipitation process has been confirmed by Sigl and Kleebe Iz2 by TEM investigation on A1203/ Y203-SiC ceramics. They detected a shell struc￾ture of the final a-SiC crystals. The core (preexisting nuclei) was found to be pure a-SiC whereas the outer regions grown during sinter￾ing contained AI, Y and O from the liquid formed during the heat treatment. Several authors 123-'27 reported on the utilization of AI203/Y203 mixtures as sintering aids. The most effective sintering was achieved at 1950°C using either 60m01% A1203 and 40m01% Y203 '~6 or 77m01% A1203 and 23m01% Y~03.125 Other authors128" 129 published experi￾ments on sintering of SiC at 1900°C with the addition of A1N and Y203. 4.2.2 Microstructure and thermo-mechanical properties of SiC-ceramics The main factors controlling the final SiC microstructure are the a//%phase ratio of the SiC sintering powder, the used additive system and the densification method. SiC-ceramics sin￾tered with B or A1 and C contain no secondary phase in the final microstructure have therefore strong grain boundaries resulting in mainly transgranular fracture mode. Consequently, low fracture toughness of about 3-4 MPa.m '/2 are measured. This type of SiC-material is noted for its high hardness,'3°", 3, high temperature resist￾ance, good oxidation 132-'34 and creep '35''36 resistance up to above 1500°C, as well as high resistance to abrasion and corrosion. The good thermal shock behavior '37 of SiC-ceramics is based on their high thermal conductivity and low coefficient of thermal expansion. Typical properties of pressureless sintered (SSiC), hot pressed (HPSiC), hot isostatically pressed (HIP￾SiC) and liquid phase sintered (LPSSiC) SiC￾ceramics are summarized in Table 4.'6" ,3~ ,4,, The microstructural development of liquid phase sintered SiC-ceramics has been investi￾gated by several authors. 14, I 21), 1 2 I. 14 1, 142 It was found that a platelet like microstructure