C. Vincent

Influence of carbide coatings on the oxidation behavior of carbon fibers

The oxidation behavior of reactive carbon fibers, such as ex-Pan T300, is studied in oxidizing environments by means of TGA and tensile strength measurements. Different carbide coatings are investigated with a view to fiber protection during an air exposition at high temperature. Thin, continuous, and uniform films of TiC, SiC, B4C, or B4CSiC multilayers are obtained by an RCVD process. Complementary experiments are performed on bulk graphite substrates coated by the same process. The carbide thicknesses do not exceed 60 nm for the T300 fibers and 60 μm for the bulk substrates. The presence of such coatings is found able to protect the carbonaceous materials against oxygen and air oxidation for extended periods to temperatures of at least 600 °C. However, SiC and TiC coatings that form silica and rutile films during oxidation offer only limited protection due to the diffusion of oxygen along microcracks and grain boundaries of the oxides. B4C and B4CSiC multilayers give better protection both in a dry oxygen atmosphere and in room air. Boron oxide and borosilicate have a glassy structure, and they act as a diffusion barrier, providing protection by inhibition of oxygen diffusion, thereby slowing down the carbon gasification. The mechanical characteristics of the fibers protected by a boron-silicon carbide coating remain constant after a half hour of oxidation at 600 °C.

Approche thermodynamique de l'interaction chimique entre l'aluminium et le carbure de titane

Phase equilibria and transformations occuring in the ternary system Al-C-Ti under a pressure of latm and at temperatures lower than 1273K have been investigated, using X-ray diffraction and differential thermal analysis. The results obtained provide a basis for describing the thermodynamics of the chemical interaction between titanium carbide and aluminum. For example, C-rich titanium carbide (TiCx with x < 0,9) is in equilibrium with a Al-rich liquid phase L in the temperature range 1273 to 1085K. At 1085 ± 15K, a quasi peritectic reaction has been shown to occur in the Al-C-Ti system. This reaction corresponds to the four-phase invariant equilibrium: L + TiCX = Al3Ti + Al4C3 As a consequence, C-rich TiCx can be decomposed by liquid or solid Al at any temperature lower than 1085K, the reaction products being Al3Ti and Al4C3.

Boron carbide formation from BCl3CH4H2 mixtures on carbon substrates and in a carbon-fibre reinforced Al composite

Abstract A coating of boron carbide on carbon fibres was found to be a suitable method for preventing the chemical reactions between the fibre carbon and an aluminium matrix during the fabrication of a composite material by a squeeze-casting process. Such a coating is also interesting since it has an excellent oxidation resistance. Unfortunately, the mechanical properties of composites are not always reproducible; they can be related rather to the microtexture and the physical properties of the coating than to a possible reaction between the two components of the composite. The interaction between boron trichloride and graphite or carbon fibres in the presence of hydrogen and/without methane have been studied in order to obtain a continuous layer of B y C without defects. A thermodynamic approach from the BCl 3 CH 4 H 2 C system is presented and solid phase diagrams calculated. The coatings were obtained on various substrates such as bulky graphite and some PAN-based and pitch-based carbon fibres. They were observed by scanning electron microscopy, transmission electronic microscopy, X-ray diffraction and electron probe analysis. Whatever the CH 4 content of the input gas phase, the coatings were polycrystalline and the grains contained numerous twins. For application as mechanical reinforcement components, strength measurements were performed on the coated filaments. A study by TEM showed the absence of reaction products between the carbide and an aluminium matrix and the presence of micropores at B 4 CAl interface in a composite, which suggests a poor wettability of the boron carbide by the liquid metal.

Thermodynamic and experimental conditions for the fabrication of a boron carbide layer on high-modulus carbon fiber surfaces by RCVD

Abstract A thin film of boron carbide is obtained by RCVD (reactive chemical vapor deposition) on the surface of carbon fibers at temperatures between 1200° and 1500°C. In this process, the fiber speed through the reactor is 25 m · h −1 and the pressure is one atmosphere. The treatment produces a continuous surface film. The initial gaseous mixture consists of boron trichloride and hydrogen. According to XPS analysis the coating is boron carbide, B 4 C. A theoretical study of the influence of different parameters on the species nature at equilibrium, and on the conversion rates of both carbon and boron trichloride is carried out by thermodynamic calculation of the free energy minimization of the BCl 3 -H 2 -C system. A continuous and uniform coating is obtained experimentally, which conserves the mechanical behavior of the fibers and improves their oxidation resistance at 600°C. We conclude that the optimum temperature is 1350°C, which decreases the oxidation rate of P55 fibers by a factor of 15.

Chemical interaction between carbon and titanium dissolved in liquid tin: crystal structure and reactivity of Ti2SnC with Al

Abstract Chemical interaction between carbon and titanium dissolved in liquid tin was studied at temperatures higher than 1000°C. The phase equilibria in the ternary system graphite–titanium–tin were investigated by thermal analysis, metallography, X-ray diffraction and electron microprobe analysis. The isothermal section at 1200°C is proposed. At this temperature, carbon can react to yield a binary phase, TiC, or a ternary phase, Ti 2 SnC. The structure of Ti 2 SnC is proposed on the basis of powder diffraction X-ray data (space group: P6 3 /m.m.c.; hexagonal axes a =0.31626 nm, c =1.36789 nm). Some experiments realized by immersion of T300 fibres in Sn–Ti alloys show that the chemical reaction leads to either a single layer of TiC or a double-layer, TiC–Ti 2 SnC. The results demonstrate that the ternary phase is not stable in liquid aluminium.

The effect of thermal exposure on the strength distribution of B4C-coated carbon fibers

Abstract The effect of thermal exposure on the strength distribution of B4C coated carbon fibers was evaluated using the loose bundle tensile test. These tests were performed on the as-received T300 carbon fibers and on the coated fibers before and after thermal exposure at 700°C and 750°C. The heating times ranged between 15 and 180 min in an industrial, and in a pure nitrogen atmosphere. The fiber strength distributions were analyzed using the S−e curve, in relation to the microstructural state of these fibers. It was shown that the local overthicknesses of the B4C coating make the fibers more brittle, leading to a premature failure of these carbon fibers. However, a significant improvement in the mechanical properties of the coated carbon fibers is observed for a heating time of 30 min at 700°C and 15 min at 750°C in an industrial nitrogen atmosphere. Using XPS analysis and microscopy observations, it was demonstrated that this effect is due to a reaction between the B4C coating and the environment which tends to form compliant protective layers, like BN and BxOy. Thermal treatments in controlled exposure conditions in a pure nitrogen atmosphere can lead to the formation of a compliant protective layer giving comparable mechanical properties to as-received T300 fibers.

Physico-chemistry of interfaces in inorganic-matrix composites

The performances of metal matrix composites (MMCs) or ceramic matrix composites (CMCs) are usually limited by the characteristics of the fibre/matrix interface or more generally those of the interfacial zone. Concerning MMCs, the optimization of this zone involves control of the chemical reactivity between the reinforcement and the matrix, which constitute usually an out-of-equilibrium system. In the case of CMCs, it is possible to obtain a non-brittle material by associating two brittle components and to exhibit a good resistance to oxidation. The physical chemist is able to offer a significant contribution for solving these problems by acting on the reinforcement surface, the matrix composition or the manufacturing conditions of the composite.

Texture, structure and chemistry of a boron nitride fibre studied by high resolution and analytical TEM

Abstract The present work is devoted to a TEM (transmission electron microscopy) study of the texture, structure and chemistry of a boron nitride (BN) fibre. The general structure of the fibre consists of two concentric parts; the near surface region is finely nano-crystallised, while the bulk of the fibre exhibits larger crystallites, although still of nanometric sizes. All grains appear to be randomly oriented with respect to the fibre axis, which makes the mechanical properties of this material remain modest. From a crystallographic point of view, both hexagonal and rhombohedral BN forms have clearly been identified by high resolution TEM (HRTEM). From a chemical point of view, EELS (electron energy loss spectroscopy) analysis shows that the N/B atomic ratio remains close to one, although it tends to decrease slightly from the outer surface to the ‘core’. No significant amount of impurities (e.g. carbon and oxygen) has been detected. The study of the B–K and N–K edges reveals a great similarity between the hexagonal and the rhombohedral forms.

Caractéristiques mécaniques et chimiques des fibres de carbone protégées par un revêtement de carbure formé par dépôt chimique réactif

Silicon, titanium and boron carbide layers were formed on carbon fibres by reactive chemical vapour deposition (RCVD) using mixtures of SiCl4-H2-Ar, TiCl4-H2 and BCl3-H2 gases. Two types of fibres were considered: fibres made using a polyacrylonitrile precursor (ex-pan) and fibres made using a tar precursor (ex-brai). The mechanical properties of the coated fibres were determined on monofilaments. The high temperatures involved in RCVD resulted in degradation of the fibres. In order to use these fibres as reinforced aluminium-matrix composites, the influence of thermal treatment in air or in the presence of metal was investigated. In both cases, the coating decreased the carbon reactivity. In air, at 450°C, the properties of the fibres increased after a few minutes of treatment, whereas at 600°C, the tensile strength decreased in Une with the microcrystallinity of the oxides obtained. In the presence of liquid aluminium, the silicon carbide protects the fibres as long as there is no chemical reaction between the two components. In particular, silicon oxide, present as an impurity in the coating, easily reacts with the metal to produce defects on the fibre surface.

Etude thermodynamique et expérimentale de la stoechiométrie du carbure de titane déposé par réaction entre un substrat de graphite et un mélange gazeux de tétrachlorure de titane et d'hydrogène

Abstract Titanium carbide may be deposited on a graphite substrate through the reaction of the gaseous mixture TiCl 4 -H 2 with solid carbon at temperatures higher than 1300 K and at atmospheric pressure. This chemical vapour deposition (CVD) process gives a uniform adherent coating. The process requires diffusion of carbon through an already deposited layer of carbide; the amount of carbon used and the stoichiometry of the carbide depending on the reaction time. Theoretical deposition diagrams have been drawn in order to predict the effects of temperature and of the amount of carbon on the carbide stoichiometry. This thermodynamic study shows that the rate of formation of TiC y increases with temperature. Experience verifies, in part, the validity of this thermodynamic model. MASE analyses, X-ray diffraction and microhardness measurements show the existence of a strong concentration gradient of carbon within the deposited layer and that agreement between thermodynamic prediction and experience occurs only near the TiC y -C interface.