Review ArticleDesign, preparation and properties of non-oxide CMCs for application in engines and nuclear reactors: an overview☆
Introduction
Non-oxide CMCs, i.e. mainly those consisting of a SiC-based matrix reinforced with either carbon or SiC-fibers and which will be referred to as C/SiC and SiC/SiC composites, have been extensively studied during the last two decades since their discovery in the mid seventies [1], [2], [3]. These tough ceramics have the potential for being used up to about 1500 °C, as structural materials, in different fields including advanced engines, gas turbines for power/steam co-generation, heat exchangers, heat treatment and materials growth furnaces, as well as nuclear reactors of the future.
The main advantage of CMCs with respect to their monolithic counterparts lies in the fact that they are tough although their constituents are intrinsically brittle. This key property is achieved through a proper design of the fiber/matrix (FM) interface arresting and deflecting cracks formed under load in the brittle matrix and preventing the early failure of the fibrous reinforcement [4]. In its classical treatment, crack deflection is controlled via the deposition of a thin layer of a compliant material with a low shear strength, on the fiber surface, referred to as the interphase and acting as a mechanical fuse (to protect the fiber). It has been postulated that the best interphase materials might be those with a layered crystal structure, such as pyrocarbon (PyC) or hexagonal boron nitride (hex-BN), or a layered microstructure, such as (PyC–SiC)n or (BN–SiC)n, the layers being deposited parallel to the fiber surface and the interphase strongly bonded to the fiber [5], [6]. From a mechanical standpoint, these CMCs are damageable elastic materials, i.e. when loaded at a high enough level, microcracking and FM-debonding occur, which are responsible for a stiffness lowering and non-linear stress–strain behavior. On the one hand, these damaging phenomena are beneficial since they are at the origin of the non-brittle character of these ceramics. On the other hand, they are detrimental since they favor the in-depth diffusion of oxygen towards the oxidation-prone interphase and fibers which in turn may embrittle the composites.
Hence, an important challenge has been to improve the oxidation resistance of these non-oxide CMCs, the lifetime under load at high temperatures and in oxidizing atmospheres (combustion gas, for example) which is required in some of the applications previously mentioned, being of several hundreds or thousands hours and even more. This problem has been addressed via the design of innovative self-healing interphases and matrices and through the use of specific coatings, with the result that the durability of these CMCs in severe environments is now good enough for applications in aeronautic engines [7], [8].
The use of SiC/SiC composites in high temperature nuclear reactors of the future, in place of monolithic SiC, such as the first wall, blanket and divertor of nuclear fusion reactor, is another very challenging potential application requiring among others a high thermal conductivity, an excellent hermeticity with respect to gases (cooling gas fluids or gaseous species formed by nuclear reactions) and low residual radioactivity. Preliminary research in this field appears to be very encouraging [9], [10].
The aim of the present contribution is to give an overview of the state of the art in the material design, processing and properties control of SiC-matrix during the last few years and underlying the weak points which still require a significant effort of research.
Section snippets
Processing
SiC-matrix composites are processed according to: (1) a gas phase route, also referred to as chemical vapor infiltration (CVI), (2) a liquid phase route including the polymer impregnation/pyrolysis (PIP) and liquid silicon infiltration (LSI) also called (reactive) melt infiltration (RMI or MI) processes, as well as (3) a ceramic route, i.e. a technique combining the impregnation of the reinforcement with a slurry and a sintering step at high temperature and high pressure. Each of these routes
Material design
In terms of material design, the objective is (1) to achieve the best mechanical behavior in static and cyclic loading, particularly at high temperatures and (2) to improve oxidation resistance (SiC/SiC being intrinsically oxidation-prone) and durability under load in corrosive environments, such as fuel combustion gas. The mechanical behavior of SiC-matrix composites is mostly controlled by the fibers and the interphase whereas the oxidation resistance and durability are depending upon the
Examples of potential application
Potential application of SiC-matrix composites in two different fields, namely aerojet engines and stationary gas turbines for electrical power/steam cogeneration, on the one hand, and nuclear fusion reactors, on the other hand, will be briefly discussed. These application fields correspond to very severe service conditions (high temperatures and corrosive environments in both cases, as well as radiation exposure in nuclear reactors) and long lifetime requirement (from a few hundreds to a few
Summary
SiC-matrix composites appear to be highly tailorable materials suitable to structural application at high temperatures. They are tough, although their constituents are intrinsically brittle, when the fiber-matrix bonding is properly optimized through the use of a thin interphase deposited on the fibers prior to the infiltration of the matrix. They display good mechanical properties at high temperatures when prepared from stable fibers, as well as a high thermal conductivity if their residual
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Presented at Multifunctional Materials and Structures: Present Status and Future Perspectives a Symposium in Honor of A.G. Evans on the occasion of his 60th birthday, Max-Planck Institute für Metallforschung, Stuttgart, 16–20 March 2003.