Gaëlle Farizy

The creep mechanism of ceramic matrix composites at low temperature and stress, by a material science approach

Abstract This paper deals with the creep mechanism for ceramic matrix composites reinforced by long ceramic fibers in a ceramic or glass-ceramic matrix, tested at low stresses (

SiCf–SiBC composites: microstructural investigations of the as-received material and creep tested composites under an oxidative environment

SiCf–SiBC composites fabricated by Snecma Propulsion Solide (St Médard en Jalles, France) were investigated by SEM and HRTEM in the as‐received state and after creep tests performed in air, in a temperature range 1423–1573 K, under 170 and 200 MPa. These composites are reinforced by Hi‐Nicalon fibres (Nippon Carbon). A pyrocarbon interphase was first deposited on the fibres. The matrix was then deposited on the fibrous preform by several chemical vapour infiltrations (CVI). As a result the matrix is multilayered and based on the Si–B–C ternary system. This matrix is self‐sealing: this is due to the presence of boron inducing the formation of a sealant glass if the material is heated in an oxidative environment. This glass will protect fibres and fibre/matrix interphases against oxidation. Hi‐Nicalon fibres as well as the different matrix layers were studied by HRTEM and EDX. Some investigations were carried out on the creep‐tested specimens in order to characterize modifications observed in the different constituents of the composites, particularly at the interfaces between the matrix layers and at the fibre/matrix interface. It was shown that several matrix layers crystallized during the creep tests. Moreover, a thin silica layer was observed at the pyrocarbon/matrix interfaces. Differences between the behaviour of the same type of material creep tested under neutral atmosphere are discussed.

Microstructures of ceramic composites with glass-ceramic matrices reinforced by SiC-based fibres

Abstract A lot of studies have been carried out on the fibre/matrix interfaces in glass–ceramic matrix composites reinforced by SiC based fibres. Chemical and structural analyses at the nanometer scale have shown that the fibre/matrix interface has a very complex structure consisting of several sublayers. The most important point is the existence of a thin carbon layer which is often found textured close to the fibre. This carbon layer acts as a mechanical fuse with a low or extremely low interfacial debonding energy. The mechanism responsible of the carbon formation and of the complex interfacial microstructure is still mater of controversy. This review will be focused on the microstructure of the interfacial region and on the different techniques which have been used to obtain chemical and microstructural parameters of the fibre/matrix interfaces in a large variety of glass–ceramic composites. The approaches concerning the SiC/BN dual-coated Nicalon SiC fibre-reinforced BMAS matrix composites will be described as well as the thermomechanical properties of this class of glass–ceramic composites and future research.

Creep Behavior and Mechanism for CMCs with Continuous Ceramic Fibers

This paper gives an overview on the creep behavior and mechanism of some CMCs, with a SiC ceramic matrix, such as Cf-SiC, SiCf-SiC and SiCf-SiBC. Tensile creep tests were conducted under argon and air in order to have the influence of the environmental conditions on the macroscopical mechanical response. Nevertheless, multi-scale and multi-technique approaches were required to identify and quantify mechanism(s) which is (are) involved in the creep behavior. The initiation and propagation of damages which are occurring under high stress and temperature conditions were investigated at mesoscopic, microscopic and nanoscopic scales using SEM, TEM and HREM, in order to identify the mechanism(s) involved at each scale. Automatic image analysis was used in order to quantify the evolution of some damage morphological parameters. The macroscopical creep behavior has been investigated through a damage mechanics approach which seems to be the most promising route. A good correlation was found between the kinetics of the damage mechanisms and the creep behavior. For such ceramic matrix composites, the governing mechanism is a damage-creep one, with an additional delay effect due to formation of a glass when tests are performed under air.

Creep Investigation of SiCf-SiBC Composites

The concept of ceramic matrix composites was proposed in the eighties to be used in the aeronautical and space domains. Classical SiCf-SiC have changed into complex materials, such as SiCf-SiBC investigated in this work: that composite is reinforced with SiC Hi-Nicalon fibers and has a multi-layer and self-sealing matrix. This matrix has been fabricated by means of several chemical vapor infiltrations with different compositions. Prior to the matrix infiltration, a pyrocarbon interphase has been deposited on the fibrous preform. The self-sealing property of this composite is due to the presence of boron in certain matrix layers, which will react with the atmosphere and particularly oxygen, to create a sealant glass, which protects pyrocarbon and SiC fibers.