enational yournal of Applied Ceramic Technology-Naslain Vol.2,No.2,2005 barrier top coat B:B,C: SiB,: Si-B-c functional layer(s) Si or sic CMC-substrate (a) 017K0X1,79810ymWD35 Fig 3. Nonoxide CMCs with improued oxidation resistance through the use of multilayered seal-coating (a)and multilayered selfhealing nara (b). Adapted from C nd vandenbulckeg and lamouroux for durability. When a SiC/SiC (or a C/SiC)composite the behavior of C/SiC and SiC/SiC composites in ox- with a pyrocarbon(or BN) interphase is heated in an idizing atmospheres is usually better at relatively high oxidizing atmosphere, active or passive oxidation phe- temperatures(1200C) than at lower temperatures nomena are observed depending on whether all the re- However, this protection due to condensed oxides is action products are gaseous( CO or CO2 for carbon, insufficient for long exposures under load and even dis- CO and Sio for SiC)or at least one reaction product is appears in wet atmospheres(volatilization of silica) ondensed as a covering protective scale(silica for Sic Under such conditions, some protection against oxida- and boria for BN), respectively. Fortunately, in many tion is necessary cases the oxidation regime is passive. Under this as- There are two ways to improve the oxidation resist sumption, the effect of oxidation on the microstructure, ance of Sic-matrix composites, which are based on mechanical, and thermal properties depends on the multilayered seal-coatings or self-healing matrices. Ho- oxidation conditions(temperature, oxygen partial pres- mogeneous single-layer coatings, such as dense SiC sure)and material parameters(interphase thickness). coating, provide an insufficient oxidation protection At low temperatures, 500< T<900C, the kinetics of for C/SiC and SiC/SiC composites submitted to ther idation of the pyrocarbon interphase in a SiC/PyC/ mal shocks or/and mechanical cyclic loading. In both SiC composite is already fast whereas that of SiC is al- cases, microcracks are formed in the coating that favor most negligible. As a result, oxidation is an in-depth the in-depth diffusion of oxygen. A first strategy is to use Phenomenon that progressively consumes the inter- a multilayered seal-coating that usually consists of the hase, destroys the FM-bonding, degrades the mecha bond coat, such as a dens cal behavior, and alters the thermal conductivity. The (for SiC CVI-matrix) or silicon (for SiC+ Si RMI-ma- effect is still more significant if the composite is trix) deposited on the external surface of the composite reinforced with carbon fibers and heavily microcracked at the end of the fiber preform densification, (i) a func- as a result of an applied load or CTE-mismatch2> tional layer containing species(such as B, B4C, SiB,,or Conversely, at high temperatures(1000-1200oC)the Si-B-C mixture)that can form Auid oxides(B2O3 or kinetics of formation of silica (and boria when a B2O3-SiO2) when exposed to an oxidizing atmosphere BN-interphase is used) is fast and the condensed oxide in a given range of temperature, and (ii)a barrier top scale(which is covering for both B2O3 on BN and silica coat that can be a dense SiC-layer, the overall thickness on SiC) is protective and tends to seal or/and fill the of the coating being of the order of 150-200 um(Fig residual pores and microcracks, stopping(or at least 3a). When the coating goes microcrack slowing down) the in-depth diffusion of oxygen. Hence, cyclic loading, the microcracks are being filled by thefor durability. When a SiC/SiC (or a C/SiC) composite with a pyrocarbon (or BN) interphase is heated in an oxidizing atmosphere, active or passive oxidation phe￾nomena are observed depending on whether all the re￾action products are gaseous (CO or CO2 for carbon, CO and SiO for SiC) or at least one reaction product is condensed as a covering protective scale (silica for SiC and boria for BN), respectively. Fortunately, in many cases the oxidation regime is passive. Under this as￾sumption, the effect of oxidation on the microstructure, mechanical, and thermal properties depends on the oxidation conditions (temperature, oxygen partial pres￾sure) and material parameters (interphase thickness).24 At low temperatures, 500oTo9001C, the kinetics of oxidation of the pyrocarbon interphase in a SiC/PyC/ SiC composite is already fast whereas that of SiC is al￾most negligible. As a result, oxidation is an in-depth phenomenon that progressively consumes the inter￾phase, destroys the FM-bonding, degrades the mechan￾ical behavior, and alters the thermal conductivity. The effect is still more significant if the composite is reinforced with carbon fibers and heavily microcracked as a result of an applied load or CTE-mismatch.25 Conversely, at high temperatures (1000–12001C) the kinetics of formation of silica (and boria when a BN-interphase is used) is fast and the condensed oxide scale (which is covering for both B2O3 on BN and silica on SiC) is protective and tends to seal or/and fill the residual pores and microcracks, stopping (or at least slowing down) the in-depth diffusion of oxygen. Hence, the behavior of C/SiC and SiC/SiC composites in ox￾idizing atmospheres is usually better at relatively high temperatures (
12001C) than at lower temperatures. However, this protection due to condensed oxides is insufficient for long exposures under load and even dis￾appears in wet atmospheres (volatilization of silica).26 Under such conditions, some protection against oxida￾tion is necessary. There are two ways to improve the oxidation resist￾ance of SiC-matrix composites, which are based on multilayered seal-coatings or self-healing matrices. Ho￾mogeneous single-layer coatings, such as dense SiC￾coating, provide an insufficient oxidation protection for C/SiC and SiC/SiC composites submitted to ther￾mal shocks or/and mechanical cyclic loading. In both cases, microcracks are formed in the coating that favor the in-depth diffusion of oxygen. A first strategy is to use a multilayered seal-coating that usually consists of the following: (i) a bond coat, such as a dense layer of SiC (for SiC CVI-matrix) or silicon (for SiC1Si RMI-ma￾trix) deposited on the external surface of the composite at the end of the fiber preform densification, (ii) a func￾tional layer containing species (such as B, B4C, SiB6, or Si–B–C mixture) that can form fluid oxides (B2O3 or B2O3–SiO2) when exposed to an oxidizing atmosphere in a given range of temperature, and (iii) a barrier top￾coat that can be a dense SiC-layer, the overall thickness of the coating being of the order of 150–200 mm (Fig. 3a). When the coating undergoes microcracking upon cyclic loading, the microcracks are being filled by the Fig. 3. Nonoxide CMCs with improved oxidation resistance through the use of multilayered seal-coating (a) and multilayered self-healing matrix (b). Adapted from Goujard and Vandenbulcke19 and Lamouroux et al.,27 respectively. 80 International Journal of Applied Ceramic Technology—Naslain Vol. 2, No. 2, 2005