G. Boitier

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 (

AlN dispersed reinforced aluminum composite

Abstract In this paper the first results are presented on nitriding of aluminium powder and the fabrication of Al-AlN composites after milling and hot-pressing. Nitriding appears to follow a complex process. High energetic milling of these powders is an important factor in obtaining homogeneous materials with AlN nanometric grains. TEM and EDX nanoanalyses have shown that Al grains are surrounded by AlN nanocrystals, with some A12O3 needles and AlON crystals. Physical properties — thermal expansion, thermal conductivity, electrical conductivity, hardness, Youngs modulus, fracture strength — of these composites change with the AlN content, and the values for 0 vol.% AlN (process powders) always correspond to higher or lower values than for pure Al (unprocessed powders), reflecting the fact that processing introduces impurities. A comparison of composites fabricated from composite powders and from a mixture of Al-AlN commercially available powders is interesting. Generally these new composites exhibit better properties than those for Al-SiC or Al-Al2O3 composites with an apparently similar reinforcement content.

Understanding the creep behavior of a 2.5D Cf-SiC composite. II. Experimental specifications and macroscopic mechanical creep responses

Abstract Macroscopic results for a 2.5D C f –SiC composite creep tested in tension are presented. After the development and the optimization of a new accurate high temperature tensile device, tests were conducted in argon, under a reduced pressure, for stresses ranging from 110 to 220 MPa and temperatures between 1273 and 1673 K. The macroscopic mechanical creep responses of the composite were analyzed and interpreted. Since ceramic matrix composites (CMCs) contain constituents of a different nature, with an influence of a structural aspect, it is not possible to apply the hypotheses of homogeneity and isotropy as described in Dorn’s theory. Consequently, the physical meaning of the mechanical parameters, obtained by such a classical treatment, is limited. It is then necessary to discuss the global creep responses using an approach based on damage mechanics, which is more consistent with the specific features of the CMCs. This new approach adopted here reveals less classical parameters to be more accurate indicators of the creep behavior and the strain mechanisms of the 2.5D C f –SiC composite.

Understanding the creep behavior of a 2.5D Cf–SiC composite-I. Morphology and microstructure of the as-received material

Abstract A carbon fiber-reinforced silicon carbide matrix composite (2.5D C f –SiC composite) is characterized from both a morphological and a microstructural point of view, as a prerequisite to an investigation of its creep behavior. Using automatic image analysis, various morphological parameters have been characterized: the texture of the woven fibrous preform, the fiber size distribution and the volume fraction of macropores. An original study of the pre-existing microcracks has also been conducted to enable a quantitative estimate of the damage in the as-received composite to be made. A microstructural investigation down to the nanometric scale, via transmission and high resolution electron microscopies, has revealed the intimate structure of each constituent. Thus, the classical structural elements of the carbon fibers (i.e. basic structural units and areas of local molecular orientation) have been identified and measured. The texture of the pyrocarbon interphase has been clearly established especially at the fiber/pyrocarbon and pyrocarbon/matrix interfaces. Finally, the matrix presents the common features of chemically vapor deposited matrices.

Microstructural investigation of interfaces in CMCs

This paper is focused on the importance of pyrocarbon interfaces in two types of ceramic matrix composites (C f -SiC and Sic f -SiBC), during creep tests under argon. The development of micromechanisms which consume energy and then allow a damage tolerance, depends on the morphology of the fiber/matrix interphase, which has been investigated by TEM and HRTEM.

Understanding the creep behavior of a 2.5D Cf–SiC composite. III. From mesoscale to nanoscale microstructural and morphological investigation towards creep mechanism

Abstract A multiscale microstructural and morphological investigation of the creep tested 2.5 D C f –SiC specimens has been conducted via scanning, transmission and high resolution electron microscopies (SEM, TEM and HREM) and automatic image analysis. Five modes of matrix microcracking together with two types of interfacial sliding have been identified. The combination of these seven elementary mechanisms leads to the macroscopic creep strain according to a time-dependent mechanism, which can be assimilated to slow crack growth. In HREM, the so-called ‘nanocreep’ of the carbon fibers has been evidenced, but its contribution to the macroscopic creep strain appears negligible. The major role of the pyrocarbon interphase has been clearly demonstrated through the two types of interfacial sliding. ‘Step-creeping’ tests were performed in order to identify the sequence of the elementary mechanisms in the global creep mechanism of the composite. In parallel, a promising approach of damage quantification has been achieved by automatic image analysis. This study stands as an illustration of the damage-creep concept, which corresponds to the mechanism that governs the creep behavior of the 2.5 D C f –SiC composite.

TENSILE CREEP RESULTS ON A Cf-SiC COMPOSITE

Considering the needs of mainly aeronautics and space industry for high-tech materials that can stand high levels of stress and temperature, ceramic matrix composites (CMCs) appear to be the most interesting candidates. For long term thermomechanical applications, the creep behavior of such materials is of crucial importance in order to predict their useful lifetime and to provide information for the design engineer. Moreover, creep characterization can lead to the identification of many useful parameters for later modeling. The aim of this paper is to partly fill the lack of high temperature mechanical results on C{sub f}-SiC composites available in the literature. The first creep results are presented here. Tensile creep tests were performed under argon between 1,273 and 1,673 K for stress levels ranging from 110 to 220 MPa. Optical microscopy as well as scanning electron microscopy and transmission electron microscopy (classical and high resolution) were used to characterize the structural changes in the composites upon tests and to bring information on the mechanisms.

Multiscale investigation of the creep behaviour of a 2.5D Cf-SiC composition

This paper deals with some results on the creep behaviour of a 2.5D Cf-SiC composite. This material fabricated by CVI was tested in tension under an argon partial pressure for temperatures ranging from 1273 to 1673 K and stresses between 110 and 220 MPa. Results regarding creep curves (strain-time) and strain rate-time curves tend to confirm the existence of a secondary stage. Damage-stress and damage-time curves are also presented. The limits of the Dorn′s formalism are evidenced as well as the occurrence of a damage process leading to a so-called damage-creep mechanism. In order to explain this macroscopic creep behaviour of the composite, investigations at the mesoscopic, microscopic and nanoscopic scales were necessary. Five modes of matrix microcracking are observed together with different pull-out features regarding the extracted fibre surface. The damage accumulation via matrix microcracking appears to be a time dependent mechanism. Two modes of interfacial sliding are evidenced: at 1473 K and 220 MPa, the pyrocarbon (PyC) interphase is fractured leading to debonding between carbon layers, while at 1673 K, there is a loss of anisotropy of the PyC layer close to the matrix and, thus, an interfacial sliding appearing as a viscous flow. To elucidate the role of the carbon fibres, a nanoscale study via HREM has been conducted. An increase of the mean diameters of the basic structural units (BSUs) and of the areas of local molecular orientation (LMOs) within the fibres has been observed when increasing temperature under 220 MPa. In fact, these changes do not contribute to the macroscopic strain. Therefore, this restructuration effect has been called “nanocreep” of the carbon fibre as it appears to have a negligible contribution to the macroscopic creep behaviour of the 2.5D Cf-SiC composite.

The importance of damage and slow crack growth in the creep behavior of ceramic matrix composites

Three ceramic matrix composites (CMCs) were investigated after creep tests: a 2D SiCf-MLAS, a 2D SiCf-SiC and a 2.5D Cf-SiC. The damage features of each one are identified, using optical and scanning electron microscopies (SEM), and the complete damage sequences are presented. As a result of its time-dependence, damage in CMCs may be considered as slow crack growth in the temperature and stress fields investigated. But the quantification of damage, through the classical damage mechanics, appears as a complex issue, due to the architecture effects in composite materials.

Bridging at the Nanometric Scale in 2.5D Cf–SiC Composites

This short paper presents the matrix microcrack bridging at a nanometric scale which was evidenced during creep tests of 2.5D Cf–SiC composites. It also shows the importance of the investigations of mechanical behavior of composite materials at all the different scales: from the macroscopic down to the nanoscopic one.