Selected thermal properties of C/Si-C-O/SiC composites obtained from polymeric precursors
Teresa Gumuła, Stanisław Błażewicz Akademia Górniczo-Hutnicza, Wydział Inżynierii Materiałowej i Ceramiki, al. Mickiewicza 30, 30-059 Kraków
Quarterly No. 1, 2006 pages 68-73
DOI:
keywords: ceramic matrix composites, carbon fibres, silicon oxycarbide, silicon carbide, thermal properties, oxidation resistance
abstract The aim of this work was to investigate thermal conductivity and oxidation resistance of new ceramic matrix composites obtained by pyrolytic conversion of organosilicon polymer. Carbon fibres were used as reinforcement. As ceramic matrix substrates four types of cheap commercially available polysiloxane resins were used. The basic characteristic of the polysiloxane polymer precursors is presented in Table 1. Unidirectional reinforced composites (1D) were manufactured by liquid impregnation method. The composites were heat treated in an inert argon atmosphere up to 1000°C in order to obtain C/Si-C-O composites (carbon fibre/silicon oxycarbide) and up to 1700°C in order to obtain C/SiC composites (carbon fibre/silicon carbide). The characteristic of such obtained ceramic composites is shown in Table 2. Thermal conductivity of the composites were determined by comparison of heat flow by standard and investigated samples. The scheme of thermal conductivity testing set up is shown in Figure 1. Oxidation resistance was examined by mass losses of samples heated in air atmosphere at 600°C for 2 h. Microstructure of the composites was observed by scanning electron microscope JEOL 5400 (SEM) with EDS accessory (LINK AN 10000 microanalyzer). Figure 2 reveals the effect of curing pressure on thermal conductivity of investigated composites. The C/Si-C-O composites cured under higher pressure have higher values of thermal conductivity in comparison with C/Si-C-O composites cured under lower pressure - the highest curing pressure the better contact between fibres and matrix, and, as a consequence, higher thermal conductivity values. Similar dependence is observed for C/SiC composites (Fig. 2). C/Si-C-O composites cured under the same pressure as C/SiC composites demonstrate higher thermal conductivity (Fig. 2). The effect of porosity on thermal conductivity of various C/Si-C-O can be seen in Figure 3. This Figure confirms that thermal conductivity of composites decreases with increasing of porosity. From the point of view of thermal conductivity values, measured according to fibres axes, the C/Si-C-O and C/SiC composite samples demonstrate medium thermal properties as insulating materials. C/Si-C-O composites represent relatively low oxidation resistance (Fig. 4). High mass losses occurring during oxidation test of C/Si-C-O composites results from oxidation of carbon from carbon fibres and free carbon phase from composite matrices. C/SiC composites show distinctly higher oxidation resistance in comparison with C/Si-C-O composites (Fig. 4). The reason of higher oxidation resistance of C/SiC composites is the presence of silicon carbide protective layer on carbon fibres surface (Fig. 5), forming during heat treatment of composites. In spite of high porosity of C/SiC composites (up to 39% - Table 2), due to the presence of silicon carbide protective layer on carbon fibres, such composites exhibit relatively high oxidation resistance. Silicon carbide layer is formed during thermal decomposition of silicon oxycarbide matrix, occurring above 1000°C mainly by gas phase. An evidence, that the decomposition process occurs mainly by gas phase is crystallisation of silicon carbide in the form of nanofilaments (Fig. 5) in composite pores.