Silicon-based non-oxide structural ceramics mics. In contrast to this, the strength as well as 5.1 Processing the oxidation 140, 140 and creep behavior deter lorate due to the softening of the secondary The processing of Si, N,/SiC-composites can be phase at elevated temperatures performed by three different routes: (i) mixing of crystalline powders, platelets or whisk- ers (ii)synthesis and liquid In- 5 Si,N,SiC-COMPOSITES-PROCESSING tering of amorphous Si-C-N powders followed MICROSTRUCTURE AND PROPERTIES by in-situ crystallization to Si,N,/SiC-compo- Site and,(ii) coating of Sin Monolithic Si N, and SiC are presently used ders with Sic-forming precursors, polysilanes or and developed for structural applications due to polycarbosilanes, subsequent pyrolysis and their favourable thermo-mechanical properties crystallization of the polymeric surface layers as like high strength and fracture toughness as well well as liquid phase sintering of the resulting s good thermal shock resistance of Si, N, and powders 33.34, 37-39 In the case of routes(i)and excellent creep and oxidation resistance of Sic (iii) the amount of SiC produced during heat ceramics. The idea behind the development of treatment can be adjusted either by the carbon SisN./SiC-composites is to combine the advan- content of the amorphous Si-C-n powder or tageous properties of both materials generating by the chemical composition and thickness of a new type of ceramics with outstanding proper- the polymer coatings, respectively ties even at elevated temperatures(1000oC<T The densification of Si N,/SiC-composites is <1500C) achieved mainly by liquid phase sintering in N2 01MPaN2-1925-1 0.MPaN2-1925C-4h ▲0.MPaN200°ch 6-V 0IMPa N2-20000C-3h 5 A!. T!, 80 100 B-SiC volume fraction [vol % Fig. 12. Infiuence of B-SiC phase content in the Sic-starting powder on the fracture toughness of liquid phase sintered (10-42 wt%Y2O3+2. 98 wt% Al,O3Sic-ceramicsSilicon-based non-oxide structural ceramics 27 mics. In contrast to this, the strength as well as the oxidation 6" ,4,,. 146 and creep behavior deter￾iorate due to the softening of the secondary phase at elevated temperatures. 5 Si3NJSiC-COMPOSITES m PROCESSING, MICROSTRUCTURE AND PROPERTIES Monolithic Si3N4 and SiC are presently used and developed for structural applications due to their favourable thermo-mechanical properties like high strength and fracture toughness as well as good thermal shock resistance of Si3N4 and excellent creep and oxidation resistance of SiC ceramics. The idea behind the development of Si3N4/SiC-composites is to combine the advan￾tageous properties of both materials generating a new type of ceramics with outstanding proper￾ties even at elevated temperatures (1000°C<T < 1500°C). 5.1 Processing The processing of Si3Na/SiC-composites can be performed by three different routes: (i) mixing of crystalline powders, platelets or whisk￾ers,7.8.,~ 26 (ii) synthesis and liquid phase sin￾tering of amorphous Si-C-N powders followed by in-situ crystallization to Si3N4/SiC-compo￾sites 27-32"35'36 and, (iii) coating of Si3Nn-pOw￾ders with SiC-forming precursors, polysilanes or polycarbosilanes, subsequent pyrolysis and crystallization of the polymeric surface layers as well as liquid phase sintering of the resulting powders. 33"34"37-39 In the case of routes (ii) and (iii) the amount of SiC produced during heat treatment can be adjusted either by the carbon content of the amorphous Si-C-N powder or by the chemical composition and thickness of the polymer coatings, respectively. The densification of Si3Nn/SiC-composites is achieved mainly by liquid phase sintering in N2- ' ' ' ' ' I ' I ' I • 0.1MPa N2-1925oC-lh 7-- l 0.1MPaN2_1925oc.14h .............................. - A 0.1MPa N2-2000°C-2h 6 V 0.1MPa N2-2000°C-3h !, iii iiiiiiiiiiii iii ii 3 .......... -' .................... . ........................ t t I t O O e e o o e e e ! i ' I ' ' I ' I ' I 0 20 40 60 80 100 Fig. 12. [~-SiC volume fraction [vol %] Influence of fl-SiC phase content in the SiC-starting powder on the fracture toughness of liquid phase sintered (10-42 wt% Y~_O3 + 2-98 wt% A1203) SiC-ceramics