OXIDATION BEHAVIOUR OF POLYMER-DERIVED CERAMICS

International audienceFor 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

Céramiques et carbones dans les composites : relations entre synthèse, composition/structure et propriétés

Les matériaux pyrolytiques issus de la décomposition thermique de polymères ou de gaz peuvent prendre des formes régulières, denses, dépourvues de défauts et acquérir ainsi de hautes performances mécaniques. C’est ainsi que sont élaborés la plupart des renforts et les revêtements d’interphase et de matrice des composites céramiques (CMC) ou carbone/carbone (C/C). Ces deux voies liquide et gazeuse sont des références en matière de contrôle de la fabrication et propriétés mécaniques des composites thermostructuraux.Dans ce travail, je me suis efforcé d’approfondir la connaissance et la maîtrise des processus d’élaboration et des caractéristiques physico-chimiques de matériaux supposés connus, et parfois même déjà utilisés à l’échelle industrielle. J’espère avoir apporté ici des résultats qui jettent un jour nouveau sur des matériaux « éprouvés » tels que les fibres Si-O-C et les revêtements CVD de pyrocarbone, SiC et (Si)-B-C. Fort de cette expérience, je propose deux projets de recherche sur les céramiques issues de la voie gazeuse mais qui s’écartent, de par leur nature, leur architecture et leurs applications, des CMC traditionnels

Mechanisms and kinetics during reactive infiltration of molten silicon in porous graphite

Liquid silicon Infiltration (LSI) is a fast and economical process to manufacture SiC-based ceramics. For a better understanding of reactive melt infiltration of liquid silicon, the wetting and infiltration of porous graphite by molten silicon were investigated at 1450, 1500 and 1550 °C for duration comprised between 10 s and 1 h. Infiltrations tests were performed in an argon atmosphere with an inductively heated furnace operating with heating and cooling rates of 300 °C.min−1. The formation and growth of SiC grains were investigated at the outer surface and within graphitic carbon substrates with 11% porosity and a narrow pore size distribution centered at 2 μm. The length of the infiltrated zone and the SiC crystals growth were determined from scanning electron microscopy. Rapid spreading and infiltration of molten silicon are observed from the first 60 s. The growth rate of the interfacial SiC layer obeys a fourth-power law with an activation energy of 260 ± 30 kJ mol−1. Pore filling by SiC is limited by volume diffusion with an activation energy equal to 320 ± 40 kJ mol−1

Contribution of X-ray CMT and image processing to the modelling of pyrocarbon Chemical Vapour Infiltration

International audienceThe Chemical Vapour Infiltration (CVI) process is used to fabricate the pyrocarbon matrices of C/C composites. This process involves complex physico-chemical phenomena such as the transport of precursor, carrier, and by-product gases in the reactor and inside a fibrous preform, heat transfer, chemical reactions (pyrolysis and deposition), and the structural evolution of the preform. It is able to provide high-quality materials because the processing conditions are rather mild with respect to the fibres; however it is expensive and sometimes difficult to optimize. This process has been the object of extensive modelling efforts, because of imperative optimization needs. The present work presents an approach suited to the exploitation of computerized microtomographs of C/C composites, which features image acquisition, computation of geometrical and transport properties, and infiltration modelling, as applied to the infiltration of needled carbon fibre fabrics. Another application to the reinforcement of carbon foams is also presented, as an example of inserting this approach in a global modelling strategy

Experimental and theoretical investigation of BCl_3 decomposition in H_2

International audienceA combined experimental and theoretical study of the homogeneous decomposition of BCl3 in a H2 carrier gas is presented. A detailed description of the B/Cl/H thermodynamic equilibrium is first obtained from ab-initio calculations from which a restricted low energy chemical mechanism is identified to model the decomposition of BCl3. Transition state theory is then invoked to obtain reaction rates and the resulting kinetic mechanism is incorporated in a 1D model of a CVD reactor. Comparison of calculated steady state concentrations with in-situ FT-IR measurements shows a good agreement at low temperatures, thus validating the kinetic model. The divergence observed at higher temperatures is attributed to boron deposition

Kinetic and gas-phase study of the chemical vapor deposition of silicon carbide from C2H3SiCl3/H2

The chemical vapor deposition (CVD) of silicon carbide from vinyltrichlorosilane (VTS) was studied to identify a range of conditions leading to pure crystalline SiC. The deposition rate was recorded to evidence the various deposition regimes. Gas phase, elemental analyses and infiltration tests were also performed. Three distinct chemical reaction regimes were identified. In CVD conditions, carbon is co-deposited at low temperature while VTS is only partially decomposed. In infiltration conditions, the composition switches to pure SiC inside the porous substrate because of a depletion of reactive hydrocarbon species. Competing heterogeneous reactions are responsible for a hysteresis versus temperature, in both deposition rate and composition of the deposit. The high temperature domain is the most suitable to deposit pure crystalline SiC in CVD conditions. Hydrogen dilution strongly accelerates the homogeneous decomposition of VTS as compared to argon. Assumptions on the reaction mechanism were proposed describing the chemistry of this precursor

Synthesis and properties of multiscale porosity TiC-SiC ceramics

A process combining the pyrolysis of a lignocellulosic structure and reactive gas treatments has been developed to prepare porous TiC-SiC ceramics for solar receivers. The natural micro-porosity of balsa was complemented by a high open macro-porosity by laser cutting a periodical arrangement of parallel channels. The lignocellulosic structure was first pyrolysed into carbon. This reactive carbon material was then converted into TiC by Reactive Chemical Vapor Deposition (RCVD) using TiCl4/H2. After controlling the absence of cracks due to volume changes, the TiC structure was finally infiltrated by the Chemical Vapor Infiltration (CVI) of SiC using CH3SiCl3/H2. The density, porous structure, elemental and phase compositions, oxidation behavior and crushing strength were assessed after pyrolysis, RCVD and CVI. The SiC CVI coating significantly improves the compressive strength, the oxidation resistance and the thermal properties. The SiC layer is no longer fully protective at high temperature but the mechanical properties remain reasonably high

Synthesis and properties of macroporous SiC ceramics synthesized by 3D printing and chemical vapor infiltration/deposition

Open porosity cellular SiC-based ceramics have a great potential for energy conversion, e.g. as solar receivers. In spite of their tolerance to damage, structural applications at high temperature remain limited due to high production costs or inappropriate properties. The objective of this work was to investigate an original route for the manufacturing of porous SiC ceramics based on 3D printing and chemical vapor infiltration/deposition (CVI/CVD). After binder jetting 3D-printing, the green α-SiC porous structures were reinforced by CVI/CVD of SiC using CH3SiCl3/H2. The multiscale structure of the SiC porous specimens was carefully examined as well as the elemental and phase content at the microscale. The oxidation and thermal shock resistance of the porous SiC structures and model specimens were also studied, as well as the thermal and mechanical properties. The pure and dense CVI/CVD-SiC coating considerably improves the mechanical strength, oxidation resistance and thermal diffusivity of the material

Synthesis and optimization of low-pressure chemical vapor deposition-silicon nitride coatings deposited from SiHCl3 and NH3

Stoichiometric silicon nitride films were deposited by low-pressure chemical vapor deposition from the SiHCl3-NH3-H2-Ar system in a hot wall reactor at pressures ranging from 0.3 to 2 kPa. The films are amorphous for deposition temperatures up to 1000 °C and crystalline, in the α-form, at 1200 °C and above. A method for evaluating the internal stresses based on the curvature of the silicon substrate wafer and the resulting silicon Raman peak shift was developed. Some amorphous films exhibit high internal tensile stresses that can lead to cracking during deposition depending on the mechanism and effective precursors involved. Residual stresses can thus be reduced and cracking avoided by, in descending order of importance, reducing the concentration of reactive gases through dilution, increasing the deposition temperature and decreasing the total pressure. The effects of these parameters on the intrinsic stresses were related to the amount of residual hydrogen successively incorporated and thermally released during the growth of the coating according to the Noskov's model