Ablation of carbon-based materials: Multiscale roughness modelling

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Ablation of carbon-based materials: Multiscale roughness modelling
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  ABLATION OF CARBON-BASED MATERIALS: MULTISCALE ROUGHNESS MODELLING Gerard L. Vignoles 1 , Jean Lachaud  1 , Yvan Aspa 1,2 , Jean-Marc Goyhénèche 1  1  Laboratoire des Composites ThermoStructuraux (LCTS) - UMR 5801 Univ. Bordeaux 1-CNRS-SNECMA-CEA - 3, Allée La Boëtie, Domaine Universitaire, Pessac, F33600 France 2  Institut de Mécanique des Fluides de Toulouse (IMFT) - UMR 5502 INPT-CNRS – 1, Allée du Professeur Camille Soula, Toulouse, F 31000 France ABSTRACT Because of their unique ablative properties, many carbon-based heterogeneous materials are used as com- ponents for thermal protection of systems used at extreme temperatures, such as atmospheric re-entry shields and rocket nozzles. Among other issues, the design of such systems relies critically on the knowl-edge of surface roughness evolution. This paper gives a synthetic view of an approach of ablation, either  by oxidation or by sublimation, which is started from the material point of view. First, the morphology is studied: various features and scales of roughness are presented, and a classification is proposed. Second, a modelling strategy is built. It is based on the competition between bulk and heterogeneous transfer, with  possible reactivity contrasts between the material constituents. Numerical results at various scales are given. Some predicted morphologies are in correct agreement with the experimental observations. A pa-rameter variation study shows that the morphological features are dictated by the reactivity contrast of the material components, and by diffusion/reaction competitions. This allows to identify physico-chemical  parameters from the roughness geometry, as in an inverse method. The approach is validated quantita-tively in the case of the oxidation of a 3D C/C composite. 1. INTRODUCTION Among the well-known materials for atmospheric re-entry Thermal Protection Systems (TPS), carbon/carbon (C/C) and carbon/phenolic resin (C/R) composites, which are ab-lative, are of common use [1,2], because of their excellent compromise between ther-mal, thermo-chemical and mechanical properties [3]. The principle of thermal protec-tion is that an appreciable amount of the received heat flux is converted into outwards mass flux through endothermic processes like sublimation and chemical etching, which induce surface recession [4]. Surface roughening then appears: this banal but uncon-trolled phenomenon has several consequences of importance in the case of atmospheric re-entry. First, it increases the chemically active surface area of the wall; and second, it  promotes the laminar-to-turbulent transition in the surrounding flow [5,6]. Both of these modifications to the physico-chemistry lead to an increase in heat transfer, resulting in  an acceleration of the surface recession [7]. The TPS thickness design has to take this rather strong effect into account. Another spatial application for the same class of materials is the fabrication of rocket nozzle throats, divergents and inner parts. In this case, the dominant factor is the influ-ence of roughness on the effective recession rate ; also, a rougher surface is more sensi-tive to mechanical erosion [8]. For both applications, if general phenomenological tendencies are predictable, the un-derstanding of the interaction between the surrounding flow and the reactive, receding material has to be improved. In this work, an effort is done to enhance this comprehen-sion through the observation, the study and the modelling of roughness evolution, fo-cusing on the primary cause, which is heterogeneous transfer. This document brings an overview on the proposed approach and features three parts: First, a description and a classification of multi-scale surface roughness features appearing on carbon-based ma-terials are proposed. The second part describes the physico-chemical models which are set up to explain the formation of the typical roughness patterns. The last part presents numerical and analytical, which are validated and discussed with respect to experimen-tal observations. 2. ROUGHNESS OBSERVATION AND CLASSIFICATION The studied materials are: (1) A 3D C/C composite, made from a 3D ex-PAN carbon fibre preform and a  pitch-based carbon matrix. It is a heterogeneous multi-scale material. The “skeleton” of the composite consists in unidirectional bundles made of several thousands of fibres which are linked together by a pitch-based matrix (mesostructure). The bundles are fit together into a 3D orthogonal pattern repeated by translation on a cubic lattice. This  macro-structure leads to a network of parallelepipedic macro-pores (located near each node of the lattice), which are partially filled with a pitch-based carbon matrix. (2) A pyrolysed C/phenolic resin composite, made of ex-cellulose fibres grouped in yarns, which are themselves woven in satin plies; the plies are stacked together and im- pregnated with phenolic resin. Under typical ablation conditions, the phenolic resin is not any more present as such, but has suffered pyrolysis and is transformed into a highly  porous carbon with an approximate density of 700 kg.m -3 . (3) Polycrystalline graphite samples, with grain size ranging between 3 and 5 µm. The grains are bonded together by a less organised carbon obtained by carbonisation and graphitisation of pitch.   Unfortunately, it is quite difficult to recover samples from real flight experiments; how-ever, as far as roughness is concerned, arc-jet ground tests are supposed to be represen-tative of real flight conditions [9]. The samples have been submitted to arc-jet tests in stagnation point configuration. In this case, the material temperature is high enough (3000 K) to enable both oxidation and sublimation. Other tests have been performed in an atmospheric pressure oxidation reactor, with temperatures ranging from 773 to 973 K. The sample surfaces have been observed by binocular magnifier (BM), optical micro-scope (OM), scanning electron microscopy (SEM), laser profilometry and X-ray Com- puted Micro-Tomography (CMT). The observed roughness features have been collected and classified as presented in Table 1. Several characteristic features are illustrated at figs. 1 to 6. The morphological patterns may be organised following two criteria: (1) the  presence or absence of an underlying material heterogeneity, and (2) the characteristic length scale. Ablation features srcinated from the material heterogeneity will be called “structural roughness”; in the converse case we will refer to “physical roughness”.  Structural roughness is easier to understand and describe. According to the characteris-tic length, it is possible to distinguish (the numbers refer to the corresponding cases in table 1): (1) Epi-macrostructural roughness takes place on the lattice. It seems to result from the difference of reactivity between bundles and extra-bundle pitch-based matrix. Mechani-cal erosion sporadically occurs through the detachment of an extra-bundle matrix lump. The vertical bundles acquire a pyramidal shape. The section of bundles tangent to the surface is slightly undulating. Indeed, edges of bundles with an initially square section are emerging, creating crenulations in the underlying horizontal bundles, which are smoothed out to a wavy form by ablation, as visible on an OM micrograph presented on Fig. 1. Note that the present macro-scale has been termed “meso-scale” in past works on the same class of materials [10]. (2) Epimesostructural roughness develops at the top of emerging bundles. The surface of bundles perpendicular to the average material surface look like ''needle clusters''; on the other hand, tangent bundles look like ''needle layers''. In the literature, many micro-graphs show similar roughness features on carbon-based composites during ablation by oxidation [11, 12] or both by oxidation and sublimation [13-15]. Fig. 2 suggests that, due to an important recession of the intra-bundle matrix, the fibres, which are less reac-tive, are partially stripped, become thinner, and acquire a needle shape. (3) Epimicrostructural roughness appears on the microstructure. Fibre tips are faceted (Fig. 3). Some materials also show holes on the top of the fibres. In the same category, one finds the grain-related roughness of polycrystalline graphite, as illustrated at Fig. 5.  Tab. 1.  Classification of roughness morphologies. The black boxes correspond to photographs of figs. 1-6. Thick,continuous line borders refer to structural roughness and dashed line borders refer to physical roughness.   Polycr. Graphite C/phen C/C MICRO-SCALE   MESO-SCALE microstructure   Material scale mesostructure   composite fibre / grain    bundle Weave pattern PliesREV micrometre   millimetrecentimetre Roughness Scale Scallops Emerging bundles Needle clusters Facets and hole   Inter-bundleIntra-bundle  Emerging plies   Scallops   Inter-plies SwivelPallets 1234 Emerging grains. 56
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