Francis Rebillat

The concept of a strong interface applied to SiC/SiC composites with a BN interphase

Abstract Strong interfaces have been shown to allow improvement of the mechanical properties of ceramic matrix composites (CMC). The concept of a strong interface has been established in SiC/SiC composites with pyrocarbon (PyC) or multilayered (PyC/SiC) fiber coatings (also referred to as interphases). The present paper reports an attempt directed at applying the concept of a strong interface to SiC/SiC composites with a BN coating (referred to as SiC/BN/SiC). Fiber bonding and frictional sliding were investigated by means of push-out tests performed on 2D-composites as well as on microcomposite samples, and tensile tests performed on microcomposites. The stress–strain behavior of the SiC/BN/SiC composites and microcomposites is discussed with respect to interface characteristics and location of debonding either in the coating or in the fiber/coating interface.

Investigation of the Young's modulus of TiB needles in situ produced in titanium matrix composite

In situ Ti/TiB composites with different volume fractions of discontinuous TiB reinforcements were produced by powder metallurgy. After compacting Ti� /TiB2 powders by hot unidirectional pressure, heat treatments led to the in situ formation of distinctive needles of TiB, randomly distributed in the titanium matrix. The Young’s modulus of TiB was evaluated using the ASW computation method and experimental Vickers micro-indentation. Three point bend tests were performed on Ti/TiB composites as a function of the TiB volume fraction in order to extract the Young’s modulus of TiB from the elastic properties of the composite. The different values obtained according to these three methods were discussed and compared with the literature. # 2002 Elsevier Science B.V. All rights reserved.

Oxidation resistance of SiC/SiC micro and minicomposites with a highly crystallised BN interphase

Abstract A three dimensionally ordered hex-BN is deposited by LPCVD from the BF3–NH3 system at relatively low temperature. This coating was studied in term of crystallisation by X-ray diffraction, and transmission electron microscopy. SiC/SiC microcomposites with such a BN interphase were produced in a first step. Their mechanical behaviour was determined under tensile loading at room temperature. They exhibited a wide non-linear stress–strain domain similar to that commonly observed during the damage of a ceramic matrix composite. Further, static fatigue tests at 700°C in air were carried out on a few microcomposites. The long lifetimes provide evidence that such a BN-based interphase brings a real improvement in the oxidation resistance. In a second step, from these latter results, minicomposites were prepared with a complex interphase consisting of a few layers of BN, which included a highly crystallised layer. Similar mechanical tests at room temperature and static fatigue tests at high temperature in air were performed. Despite the change of geometry (single fibre to a tow), damage capability and improved oxidation resistance are observed in comparison with minicomposites with a carbon interphase.

Chemical and mechanical alterations of SiC Nicalon fiber properties during the CVD/CVI process for boron nitride

Abstract To improve the interfacial properties in SiC/SiC composites, BN is an appropriate interphase material to control the fiber/matrix bond. Unfortunately, the gaseous phase (NH3,BF3,HF,Ar) used to deposit BN acts aggressively upon Si–C–O (ex-PCS) Nicalon fiber surfaces, and weakens that bond through the formation of a complex interfacial sequence (SiO2/C), which actually controls the localization of debonds. The reactions between each gas involved and the fiber surface have been studied. Further, if the fiber surface consists of SiC or any silicon-containing compound, the BF3 gas reacts through a substitution of the silicon by boron in the initial fiber composition. Then, the surface evolves from a (C,O,Si) composition to a (B,C,O,Si) glassy layer. Such a reaction occurs mainly before the BN nucleation, and it alters the reinforcing potential of fibers in composites. This boron-containing glass is shown to be very unstable in the presence of HF gas (the main reaction product).

Highly ordered pyrolytic BN obtained by LPCVD

Abstract The chemical and physical properties of boron nitride are improved as soon as its crystallisation degree in the hexagonal modification is high, the structure becoming somewhat similar to that of graphite. Under such conditions, it can be used (instead of pyrocarbon) as an interphase in ceramic matrix composites. Unfortunately, under commonly used CVD/CVI conditions, BN layers display only a poor turbostratic texture. Furthermore, when BN is deposited from the BF 3 -NH 3 system, the fibrous reinforcement can be chemically damaged. A wide range of BN coatings and especially a three-dimensionally ordered hex-BN have been deposited by LPCVD at relatively low temperature. Structural evolution in BN coatings was related to change in chemical rate control regimes. Finally, a compromise between conditions yielding anisotropic coatings and conditions resulting in low degradation of the fibres has been worked out.

ANALYSES OF FIBER PUSH-OUT TESTS PERFORMED ON NICALON/SiC COMPOSITES WITH TAILORED INTERFACES

Abstract The interfacial properties of Nicalon fiber-reinforced SiC composites with tailored (i.e. weak or strong) interfaces are characterized using single-fiber push-out tests. A simple analysis is proposed to include the surface roughness of the fiber in the push-out model. Specifically, the roughness effect is characterized by the difference between the fiber push-out stress and the fiber reseating stress, which is obtained from fiber push-back tests. A method is developed to analyze the loading stress vs fiber-end displacement relation (i.e. the push-out curve) during the push-out process to evaluate the interfacial properties. The push-out curve for composites with weak fiber bonding can readily be analyzed by the existing push-out model incorporated with the roughness effect. However, premature catastrophic debonding at the interface is required to interpret the push-out curves for composites with strong fiber bonding.

STIM tomography: A potential tool for the non-destructive characterisation of SiC microcomposite materials

Abstract Despite the fact that ion micro-tomography techniques were initiated several years ago, useful applications are still rare in material sciences. This work is a preliminary study on the non-destructive characterisation of a thin BN interphase in a ceramic matrix composite material by scanning transmission ion microscopy (STIM) tomography. The technique, initially developed at CENBG for applications in the biomedical field, was used here to investigate several model composites as processed and after oxidation tests at high temperature. The data reduction codes, based on analytic reconstruction methods, made it possible to determine the spatial extension of the BN interphase in transversal planes at different positions along the microcomposites (25 μm in diameter). This measurement was performed taking advantage of the density contrast between SiC and BN even for a layer thickness down to 3 μm. A first attempt was also made to separate the initial BN interphase from the B2O3 layer formed on its surface during oxidation.

Temperature induced transition from hexagonal to circular pits in graphite oxidation by O2

We report on an in-situ monitoring of graphite oxidation using a high temperature environmental scanning electron microscope. A morphological transition is clearly identified around 1040 K between hexagonal pits at low temperatures and circular pits at high temperatures, with apparently no change in the kinetic law. A kinetic Monte Carlo model allows rationalizing these findings in terms of the competitive oxidation of armchair and zig-zag edge sites and provides an estimate of the rate laws associated to these two events. Extended to three dimensions, the model also explains the “in-depth” transition between the stepwise hexagons and the hemispheres observed by atomic force microscopy.

Relation between Synthesis Process, Microstructure and Corrosion Resistance of Two Yttrium Silicates

Silicon-based ceramics are among the main candidates for high temperature structural components in aeronautic applications. One key drawback of silicon-based ceramics for these applications is the volatilization of the protective silica scale, in moisture and the resulting ceramic recession. Therefore, the further use of these ceramics components depends on the development of external protection against water vapour attack. Some of the most promising materials seem to be rare earth silicates. Based on much richer data in the bibliography, the purpose of this work is to investigate the influence of two yttrium silicates elaboration processes on both the capability to crystallize and the corrosion resistance in an oxidative moist atmosphere at high temperature. Taking into account the material’s morphology, composition and degree of crystallization, the composition (Y2SiO5 or Y2Si2O7) and the preferable synthesis process are discussed.

Influence of a multilayered matrix on the lifetime of SiC/BN/SiC minicomposites

The mechanical behaviour at room temperature and the lifetime in air at 700°C under static loading of SiC/BN/SiC minicomposites have been investigated. The minicomposites consisted of a single tow of Hi-Nicalon fibres coated with a BN interphase and a CVI-SiC matrix. For few of them, a BN layer was introduced within the matrix. All the minicomposites were heat treated at high temperature to improve the BN crystallinity. In some cases, the BN interphase was submitted to a specific treatment before the infiltration of the SiC matrix, to further improve its crystalline state. The differences in interfacial zone, as assessed by TEM, were correlated with those in mechanical properties. A significant improvement of the mechanical behaviour at room temperature and the lifetime of the minicomposites with a multilayered matrix was observed. The multilayered matrix is efficient when a silica layer on both sides of the BN layer within the matrix is present.