LPCVD SiC COATINGS ON UNIDIRECTIONAL CARBON FIBRE-YARNS : APPLICATION TO ALUMINIUM MATRIX COMPOSITES

SiC has been chosen as a diffusion barrier in carbon/aluminium metal matrix composites. P55 carbon tows are coated with a thin SiC layer applied in a continuous LPCVD process from a Si(CH3)4-H2 mixture, using a hot wall reactor. Deposition parameters (temperature, pressure, flow rates of reactants, fibres passing rate) are determined in order to ensure a good compromise between a thorough infiltration of the yarn and the carbon amount in the SiC. The coatings on the fibres are characterized in two ways : direct observation and evaluation of the coating thickness on metallographic cross sections of a mounted tow ; determination of the composition and thickness of the layer by EPMA on isolated fibres. Coated fibres are included in an aluminium matrix and a composite is fabricated by liquid phase hot pressing : the effect of the coating is observed and discussed. An optimization of the CVD infiltration parameters is still necessary to reduce the variations in coating thickness between the core of the yarn and the external fibres

Interphases and mechanical properties in carbon fibres/Al matrix composites

The influence of the microstructure of the interfaces on the mechanical behaviour of unidirectional T800 carbon fibres /Al - 4.5 Mg matrix composites is investigated to optimize the composite performances. Three composites were selected. In two of them, a fibre coating was introduced by LPCVD. The coating was composed of pyrolytic carbon (Cp) or of a Cp/ Sic bilayer. As already known, a brittle Al4C3 interphase (≈ 300 nm large) is formed by fibre-matrix reaction at the T800/Al interface. The composite is weak (450 MPa) and brittle. The Cp coating (≈ 100 nm) exhibits a turbostratic structure similar to the fibre one. However, it does not react with the matrix. The composite is stronger (1400 MPa) and tough. The bilayered coating is made of a layer (≈ 100 nm) of turbostratic carbon next to the fibre and of a layer (50-200 nm) of small (20 nm) β-SiC grains. Some reaction occurs between the Sic coating and the matrix resulting in magnesium silicide (Mg2Si) and Al4C3. The composite exhibits an intermediate mechanical behaviour (740 MPa and very little pull out). As a result, it appears that brittle interphases (coating or reaction layer) are detrimental to the mechanical behaviour of the composites. Their influence depends on their composition but also of their granulometry. The Cp coating has a beneficent effect on the mechanical behaviour because it prevents the formation of a brittle interphase and decreases the failure resistance at the interface

Performance and Degradation Mechanisms of Thermal Barrier Coatings for Turbine Blades: a Review of ONERA Activities

International audienceThermal barrier coatings are used to protect blades and vanes in the hot sections of gas turbines. They consist of a thick porous ceramic layer deposited on an alumina forming metallic bond coat in contact with the nickel-based superalloy substrate. They are designed to prolong the components lifetimes or to increase gas temperature, and therefore efficiency. In service the structure and composition of the various layers evolve, due to sintering of the ceramic layer, oxidation of the bond coat, and interdiffusion phenomena with the substrate. As a result the properties of each layer are affected, as well as interfacial toughness. These evolutions, combined with applied external stresses may lead to bond coat rumpling, crack formation at the bond coat/ceramic interface and eventually the ceramic layer may spall off. In addition to these intrinsic degradation modes, interactions with environment can accelerate the system degradation. The present paper reviews the ageing phenomena occurring in thermal barrier coatings at high temperature and describe their degradation mechanisms, with illustrations taken from service experience and laboratory tests

10 Years-Activities at ONERA on Advanced Thermal Barrier Coatings

International audienceDeveloping thermal barrier coatings operating at higher temperature and/or for very long durations (commercial aircraft applications) is one of the technological and economical challenges for engine manufacturers. This includes the search for (i) low thermal conductivity, high thermal stability and CMAS resistant ceramic top coat, and (ii) alternative low cost bond coat with improved oxidation resistance and chemical compatibility with the substrate. This paper reviews the rationale sustaining the choice of new materials for each layer and presents some recommendations to develop more robust and more efficient systems with increased lifetime