J. Lamon

CNRS/National Institute of Applied Science, Villeurbanne, France © 2012 Elsevier Ltd. All rights reserved.

SEP Societe Europeenne de Propulsion SiC/SiC SiC matrix composite reinforced by SiC fibers

2.12.1 Introduction

Silicon carbide is composed of tetrahedra of carbon and silicon atoms with strong bonds in the crystal lattice. This produces a very hard and strong ceramic with outstanding characteristics such as high thermal conductivity, low thermal expansion, and exceptional resistance to thermal shock and to corrosion in aggressive environments at high temperatures. How­ever, this implies a few inadequate characteristics for structural applications, such as low fracture tough­ness, high sensitivity to the presence of microstruc­tural flaws, brittle behavior, and lack of reliability. Reinforcing with continuous SiC-based fibers allows these weaknesses to be overcome. The composite SiC/SiC that is obtained is damage tolerant, tough, and strong, and it can be insensitive to flaws and notches. The concept of composite material is very powerful. Composites can be tailored to suit end — use applications through the sound selection and arrangement of the constituents. Ceramic matrix com­posites (CMCs) reinforced with continuous ceramic or carbon fibers are of interest in thermostructural applications.1-4 They are lightweight and damage tol­erant and exhibit a much greater resistance to high temperatures and aggressive environments than metals or other conventional engineering materials.

CMCs can be fabricated by different processing techniques, using either liquid or gaseous precursors. The chemical vapor infiltration (CVI) method can produce excellent SiC/SiC composites with a highly crystalline structure and excellent mechanical prop — erties.5 The quality of the material obtained by the polymer impregnation and pyrolysis (PIP) method is insufficient. A novel processing technique (nanopow­der infiltration and transient eutectic-phase proces­sing, NITE) was claimed to achieve good material quality.5-7

The SiC/SiC composites prepared using the CVI method and reinforced with the latest near­stoichiometric SiC fibers (such as Hi-Nicalon type S and Tyranno-SA3 fibers) appear to be promising candidates for nuclear applications7-12 because of their high crystallinity, high purity, near stoichiome­try and radiation resistance of the р-phase of SiC, as well as excellent resistance at high temperatures to fracture, creep, corrosion, and thermal shock. Studies on the р-phase properties suggest that CVI SiC/SiC composites have the potential for excellent radiation stability.3 CVI SiC/SiC is also considered for ap­plications as structural materials in fusion power reactors because of low neutron-induced activation characteristics coupled with excellent mechanical properties at high temperature.1

The CVI technique has been studied since the 1960s.13-19 It derives directly from chemical vapor deposition (CVD).13-15 In very simple terms, the SiC-based matrix is deposited from gaseous reactants on to a heated substrate of fibrous preforms (SiC).15 CVI is a slow process, and the obtained composite materials possess some residual porosity and density gradients. Despite these drawbacks, the CVI process presents a few advantages: (1) the strength of reinfor­cing fibers is not affected during the manufacture of the composite; (2) the nature of the deposited mate­rial can be changed easily, simply by introducing the appropriate gaseous precursors into the infiltration chamber; (3) a large number of components; and (4) large, complex shapes can be produced in a near-net shape.

Development of CVI SiC/SiC composites began in the 1980s when SEP (Societe Europeenne de Propul­sion), Amercorm, Refractory Composites, and others began to develop equipment and processes for produc­ing CVI components for aerospace, defense, and other applications. The development of CVI SiC/SiC com­posites has been inspired by the poor oxidation resis­tance of their predecessor CVI C/C composites. CVI SiC/SiC components have been produced and tested. SNECMA (formerly SEP) is at the forefront of this technology and has demonstrated satisfactory compo­nent performance in engine and flight tests.

The mechanical properties of SiC/SiC compo­sites depend on the fiber-matrix interface. Pyrocar — bon (PyC) has proved to be an efficient interphase to control fiber-matrix interactions and composite mechanical behavior.20 But PyC is sensitive to oxida­tion at temperatures above 450 °C. A few versions of high-temperature-resistant CVI SiC/SiC composites have been produced. In order to protect the PyC interphase against oxidation, multilayered inter­phases and matrices have been developed.3,21 Multi­layered matrices contain phases that produce sealants at high temperatures, preventing oxygen from reach­ing the interphase.22 This composite is referred to as CVI SiC/Si-B-C. Oxidation-resistant interphases such as BN or multilayered materials can also be coated on the fibers. An ‘oxygen getter’ can be added to the matrix to scavenge oxygen that might ingress into the matrix (enhanced CVI SiC/SiC).

The mechanical behavior of CMCs displays sev­eral typical features that differentiate them from the other composites (such as polymer matrix compo­sites, metal matrix composites, etc.) and from homo­geneous (monolithic) materials. These features are due to heterogeneous and multiscale composite microstructure and the respective properties of the constituents (interphases, fiber, and matrix). The main characteristics of CVD SiC, CVI SiC/SiC, and NITE-SiC/SiC are reviewed in this chapter. Features of mechanical behavior of SiC/SiC are dis­cussed with respect to microstructure, on the basis of the large amount of work done on CVI SiC/SiC.