wwceramics. org/ACT matri I matrix pyc 02 LONGITUDINAL TENSILE STRAIN (o) Fig. 2. Typical stress-strain tensile curves of 2D-SiC(Nicalon )/PyC/SiC composites with weak FM-bonding(material D)and stronger aterial刀 Finally, they are less fatigue-prone than metals and al- fibers fabricated from mesophase pitch and heat treated loys with stress threshold below which no fatigue failure beyond 2500C(P55-P130 series)but low(10 W/m. K occurs, of the order of 75% the ultimate failure stress and less) for poorly organized fibers(ex-PAN T300 fib- ers).In a similar manner, nearly stoichiometric SiC The tensile stress-strain behavior of SiC-matrix fibers fabricated at high temperatures(e. g, Tyranno S. composites does not change markedly up to N 1100C. fibers, Ube Industrial, Japan) display a much better However, some change may be observed either at higher conductivity than the quasi-amorphous Si-C-O fibers temperatures if the fibers are limited in thermal stability prepared at low temperatures, typically 65 and 10 W/ (case of the unstable Si-C-O fibers) or even at lower m K at room temperature, respectively. Equally im- temperatures when an oxidizing atmosphere has access portant is the effect of the residual porosity, a composite to the fibers and the interphase(case of insufficiently produced by RMI or hot pressing(Vp s5%)exhibiting protected materials). Further, SiC-matrix composites a higher conductivity than a composite fabricated by creep at high temperatures with a creep rate depending PIP or CVI (VP N 10-15%). Hence, a SiC/SiC com on the nature of the fibers(stoichiometric microcrystal- posite is expected to show a thermal conductivity of the line SiC fibers prepared or treated at high temperatures order of 30 W/m K at 1000.C when prepared from being more creep-resistant than their Si-C-O nano- nearly stoichiometric SiC fibers with almost no residual rystalline counterparts)and that of the matrix and possibly higher if the reinforcement con sists of graphitized carbon fibers(with, however, in this Thermal Conductivity case a risk related to the occurrence of microcracking due to CTE-mismatch that will lower the conductivity) Thermal conductivity is a key property in many HT applications of CMCs. Generally speaking, SiC Oxidation resistance matrix composites are relatively good conductors of heat but their thermal conductivity depends on the crystal- In most thermostructural applications, SiC-matrix linity of their constituents, the FM-bonding, and resid- composites are exposed to oxidizing atmospheres. Since al porosity. The thermal conductivity of carbon fibers their constituents are intrinsically oxidation-prone, their can be very high (100 W/m K and more) for the behavior under such environments is of key importandFinally, they are less fatigue-prone than metals and al￾loys with stress threshold below which no fatigue failure occurs, of the order of 75% the ultimate failure stress under static loading.20 The tensile stress–strain behavior of SiC-matrix composites does not change markedly up to
11001C. However, some change may be observed either at higher temperatures if the fibers are limited in thermal stability (case of the unstable Si–C–O fibers) or even at lower temperatures when an oxidizing atmosphere has access to the fibers and the interphase (case of insufficiently protected materials). Further, SiC-matrix composites creep at high temperatures with a creep rate depending on the nature of the fibers (stoichiometric microcrystal￾line SiC fibers prepared or treated at high temperatures being more creep-resistant than their Si–C–O nano￾crystalline counterparts) and that of the matrix.21 Thermal Conductivity Thermal conductivity is a key property in many HT applications of CMCs. Generally speaking, SiC￾matrix composites are relatively good conductors of heat but their thermal conductivity depends on the crystal￾linity of their constituents, the FM-bonding, and resid￾ual porosity. The thermal conductivity of carbon fibers can be very high (100 W/m K and more) for those fibers fabricated from mesophase pitch and heat treated beyond 25001C (P55–P130 series) but low (10 W/m K and less) for poorly organized fibers (ex-PAN T300 fib￾ers).22 In a similar manner, nearly stoichiometric SiC fibers fabricated at high temperatures Q2 (e.g., Tyranno SA fibers, Ube Industrial, Japan) display a much better conductivity than the quasi-amorphous Si–C–O fibers prepared at low temperatures, typically 65 and 10 W/ m K at room temperature, respectively.23 Equally im￾portant is the effect of the residual porosity, a composite produced by RMI or hot pressing (Vpr5%) exhibiting a higher conductivity than a composite fabricated by PIP or CVI (Vp
10–15%). Hence, a SiC/SiC com￾posite is expected to show a thermal conductivity of the order of 30 W/m K at 10001C when prepared from nearly stoichiometric SiC fibers with almost no residual porosity, and possibly higher if the reinforcement con￾sists of graphitized carbon fibers (with, however, in this case a risk related to the occurrence of microcracking due to CTE-mismatch that will lower the conductivity). Oxidation Resistance In most thermostructural applications, SiC-matrix composites are exposed to oxidizing atmospheres. Since their constituents are intrinsically oxidation-prone, their behavior under such environments is of key importance Fig. 2. Typical stress–strain tensile curves of 2D-SiC(Nicalon)/PyC/SiC composites with weak FM-bonding (material I) and stronger FM-bonding (material J), corresponding to different matrix crack deflection schemes (according to Droillard15). www.ceramics.org/ACT SiC-Matrix Composites: Application 79