G. Ziegler et al./Composites: Part A 30(1999)411-417 400 Fig. 1. Density and porosity changes of the C/siCn composite upon pyrolysis at 1000.C after three, five and seven infiltration and pyrolysis cycles temperatures as low as 130C. This procedure is, compared polymer-derived matrix are discussed. Particularly, three to the usually applied processing techniques for ceramic topics are investigated: microstructural development and matrices, a low temperature, pressureless process, enabling stress-strain behavior of C and Sic/SiCn composites matrix formation of complex shaped 2D or 3D composites, respectively, the coefficient of thermal expansion depending without damaging the fibers, neither chemically nor on fiber orientation, and oxidation behavior and siliconiza- impregnation and pyrolysis steps are necessary to achieve relatively dense matrices, which can be reduced by synthe- sizing suitable precursors showing a high ceramic yield 2. Experimental procedure A variety of aspects has to be considered if the whole Synthesis of the polymeric silazanes followed standard potential of this approach is going to be used. The polymers procedures, as described elsewhere [7. Preparation of cera- have to be processable, and thus liquid or meltable. Ther- nic matrix composites was based on multiple infiltration mosetting is a requirement in order to retain the shape after and pyrolysis of UD- and 2D(0/90%)fiber prepregs(T300 the forming step, preferably crosslinking via an addition mechanism without the evolution of gaseous products. A 6k, Toray Japan and Hi-Nicalon, Nippon Carbon).Fiber high ceramic yield is favorable in general with respect to bundles were fixed in a metal mold of known volume and the number of necessary infiltration cycles, however, not infiltrated with the liquid thermosetting polymer (fiber only the ceramic yield but actually the residual volume, volume 50%). As a curing catalyst, I wt% of dicumylper which depends on the bulk density of the polymer-derived oxide was added to the polymer, enabling setting tempera- amorphous structures, has to be considered. The density of ares of 130C. After curing, the fiber-reinforced polymers the polymer-derived structures may vary with varying poly were pyrolyzed in a tube furnace up to 1000C in argon and mer density, as has been shown in experiments, which are ambient pressure. Samples were reinfiltrated and pyrolyzed currently conducted. Furthermore, pyrolysis should result in up to seven times(P1-P7). Subsequently,composites a ceramic material with the desired chemical composition characterized by helium pcynometry (AccuPyc 1630 and microstructure. Although the chemical composition can Micromeritics, Germany), scanning electron (JSM be adjusted to some degree in the precursor, it is changing Jeol, Japan) and optical(Leica/Reichert, Polyvar 2Met, upon pyrolysis and crystallization at temperatures Austria)microscopy on polished cros-sections Thermoana- >1000C, as has been shown elsewhere [6]. Therefore, lytical methods(TG/DTA, STA409 and DIL402E, Netzsch, Germany)were used to monitor the associated mass and processing temperatures should be higher than the perspec- dimensional changes. Mechanical properties were evaluated tive application temperature, otherwise properties of the composite will change accordingly. Furthermore, even the using the 3-pt bending method with a span width of 70 mm pyrolysis behavior may change with increasing number of and a crosshead speed of 0. 1 mm/min Specific composite cycles due to increasing density of the matrix and a subse- samples were infiltrated with liquid silicon at 1800 C by quent change in ceramic yield, being dependent on the local Sintec, Buching, Germany, whereby part of the matrix partial pressures of the evolved gaseous species. With was expected to be transformed to a Si/SiC matrix respect to the mechanical properties, residual compressive stresses can develop at the fiber/matrix interface, caused by 3. Results and discussion the large shrinkage (30% linear) of the polymer upon py lysis. In addition, stresses caused by the thermal expansion 3.1 Microstructural development and stress-strain mismatch between fiber and matrix can be induced with a b C/SiCN combination being more critical than a SiC/SiCN one. with the Sic fiber and Sicn matrix having similar Density, poros hermal expansion an physical properties properties of C/Sicn composites with a fiber content of In this work, processing, mechanical and thermo-physical 50 vol% were characterized after three, five and seven infil properties of C and SiC fiber-reinforced composites with tration and pyrolysis cycles(P3, P5, P7). Fig. I shows thetemperatures as low as 1308C. This procedure is, compared to the usually applied processing techniques for ceramic matrices, a low temperature, pressureless process, enabling matrix formation of complex shaped 2D or 3D composites, without damaging the fibers, neither chemically nor mechanically. Using this process, however, multiple impregnation and pyrolysis steps are necessary to achieve relatively dense matrices, which can be reduced by synthe￾sizing suitable precursors showing a high ceramic yield. A variety of aspects has to be considered if the whole potential of this approach is going to be used. The polymers have to be processable, and thus liquid or meltable. Ther￾mosetting is a requirement in order to retain the shape after the forming step, preferably crosslinking via an addition mechanism without the evolution of gaseous products. A high ceramic yield is favorable in general with respect to the number of necessary infiltration cycles, however, not only the ceramic yield but actually the residual volume, which depends on the bulk density of the polymer-derived amorphous structures, has to be considered. The density of the polymer-derived structures may vary with varying poly￾mer density, as has been shown in experiments, which are currently conducted. Furthermore, pyrolysis should result in a ceramic material with the desired chemical composition and microstructure. Although the chemical composition can be adjusted to some degree in the precursor, it is changing upon pyrolysis and crystallization at temperatures .10008C, as has been shown elsewhere [6]. Therefore, processing temperatures should be higher than the perspec￾tive application temperature, otherwise properties of the composite will change accordingly. Furthermore, even the pyrolysis behavior may change with increasing number of cycles due to increasing density of the matrix and a subse￾quent change in ceramic yield, being dependent on the local partial pressures of the evolved gaseous species. With respect to the mechanical properties, residual compressive stresses can develop at the fiber/matrix interface, caused by the large shrinkage (30% linear) of the polymer upon pyro￾lysis. In addition, stresses caused by the thermal expansion mismatch between fiber and matrix can be induced, with a C/SiCN combination being more critical than a SiC/SiCN one, with the SiC fiber and SiCN matrix having similar physical properties. In this work, processing, mechanical and thermo-physical properties of C and SiC fiber-reinforced composites with polymer-derived matrix are discussed. Particularly, three topics are investigated: microstructural development and stress–strain behavior of C and SiC/SiCN composites, respectively, the coefficient of thermal expansion depending on fiber orientation, and oxidation behavior and siliconiza￾tion of C/SiC composites. 2. Experimental procedure Synthesis of the polymeric silazanes followed standard procedures, as described elsewhere [7]. Preparation of cera￾mic matrix composites was based on multiple infiltration and pyrolysis of UD- and 2D (08/908) fiber prepregs (T300 6k, Toray Japan and Hi-Nicalon, Nippon Carbon). Fiber bundles were fixed in a metal mold of known volume and infiltrated with the liquid thermosetting polymer (fiber volume 50%). As a curing catalyst, 1 wt% of dicumylper￾oxide was added to the polymer, enabling setting tempera￾tures of 1308C. After curing, the fiber-reinforced polymers were pyrolyzed in a tube furnace up to 10008C in argon and ambient pressure. Samples were reinfiltrated and pyrolyzed up to seven times (P1–P7). Subsequently, composites were characterized by helium pcynometry (AccuPyc 1330, Micromeritics, Germany), scanning electron (JSM 6400, Jeol, Japan) and optical (Leica/Reichert, Polyvar 2Met, Austria) microscopy on polished cros-sections. Thermoana￾lytical methods (TG/DTA, STA409 and DIL402E, Netzsch, Germany) were used to monitor the associated mass and dimensional changes. Mechanical properties were evaluated using the 3-pt bending method with a span width of 70 mm and a crosshead speed of 0.1 mm/min. Specific composite samples were infiltrated with liquid silicon at 18008C by Sintec, Buching, Germany, whereby part of the matrix was expected to be transformed to a Si/SiC matrix. 3. Results and discussion 3.1. Microstructural development and stress–strain behavior Density, porosity, thermal expansion and mechanical properties of C/SiCN composites with a fiber content of 50 vol% were characterized after three, five and seven infil￾tration and pyrolysis cycles (P3, P5, P7). Fig. 1 shows the 412 G. Ziegler et al. / Composites: Part A 30 (1999) 411–417 Fig. 1. Density and porosity changes of the C/SiCN composite upon pyrolysis at 10008C after three, five and seven infiltration and pyrolysis cycles