OXIDATION BEHAVIOUR OF POLYMER-DERIVED CERAMICS

Georges Chollon (1)

1 LCTS - Laboratoire des Composites Thermostructuraux

For all chemical systems, regular (parabolic) oxidation rates are observed provided PDC are inherently stable, i.e. (i) Tp is sufficiently high to avoid the H2/H2O release from the pre-ceramic and (ii) the system is thermochemically stable to prevent decomposition, e.g., of oxycarb(onitr)ides+free C into SiO, CO, (N2). For instance, oxygen-rich Si-C-O ceramics should not be used over about 1200°C. Also, whereas a passive oxidation regime is observed for Si-C-N up to 1400°C, the severe increase of the oxidation rate at 1500°C is likely due to the reaction between silicon nitride and free carbon. The thermal stability of the Si-B-C-N system is significantly higher. The bubble formation in the oxide observed at 1500°C is more likely related to the overpressure of oxidation products at the interface, than that of decomposition gases. A harmful oxidation regime of the ceramic may also be observed if the free carbon phase is too abundant, segregated (for high Tp), if the silicon oxycarb(onitr)ide reactivity is too low and the oxide not protective (i.e., at low temperature). Within the intrinsic thermal stability domain, the parabolic rate Kp depends essentially on the nature of the oxide. All Si-C-(O) PDC display similar oxidation behaviours (Ea~100kJmol-1). Conversely, the increase of the nitrogen concentration in Si-C-N-(O) ceramics gives rise to an increase of Ea (up to~300kJmol-1) and decrease of Kp at low temperatures, close to the values for Si3N4. The influence of further heteroelements is variable. Only a slight decrease of Kp was noticed at 1350°C after the addition of ZrO2 in Si-C-N PDC, assigned to a free carbon concentration effect. Conversely for T≥1000°C, the addition of aluminium in Si-C-N PDC leads to a remarkable decrease of the oxidation rate after a transient stage, which was explained by the modification of the SiO2 network. However, this peculiar high temperature transitory regime and particularly the high initial oxidation rates, close to those for SiC, still have to be fully elucidated. The role of boron in the oxidation rate of Si-B-C-N ceramics is particularly complex. The exceptionally low oxidation rates initially reported might have been somehow underestimated for several reasons, e.g., the low oxide/ceramic volume ratio, the borosilicate viscous flow and/or the B2O3 volatilization …). Furthermore, the dual B-N-O/SiO2 layer, though observed by few authors, was not clearly demonstrated to slow down the O2-inward diffusion. More recent studies reported Kp values close to those for SiC and Si3N4 at 1500°C, concluding to a common rate limiting regime, though significantly complicated by the combination of the above-mentioned effects (e.g., bubble formation). A further addition of aluminium in the Si-B-C-N ceramic was detrimental to the oxidation resistance at 1500°C, indicating no sign of B2O3 stabilization. Clearly there is a lack of data on the oxidation behaviour of Si-(X)-B-C-N PDC at low and intermediate temperatures (800-1000°C). This is regrettable since the use of these borosilicate formers may be valuable in crack healing within this particular temperature range. Several other features besides plain oxidation should be carefully examined to appraise a new PDC composition for a high temperature structural application. In real use, this component is likely to be associated with different materials, submitted to other corrosive species than O2 and often to stress. The reactivity between the oxide products and the other nearby materials, the corrosion under a H2O environment and the delayed fracture in these aggressive media appears therefore particularly worth considering.

Domaines: Matériaux

Georges Chollon. OXIDATION BEHAVIOUR OF POLYMER-DERIVED CERAMICS. P. Colombo, R. Riedel, G. D. Soraru and H.-J. Kleebe. Polymer Derived Ceramics: From Nano-structure to Applications, DEStech Publications, pp.292-308, 2010, 978-1-60595-000-6. ⟨hal-01374117⟩