J. M. Quenisset

Wetting improvement of carbon or silicon carbide by aluminium alloys based on a K2ZrF6 surface treatment: application to composite material casting

A surface treatment with aqueous solutions of K2ZrF6 has been carried out to improve, in dramatic manner, the wetting of carbon (or SiC)-base ceramics by liquid light alloys at low temperatures (i.e. within the 700 to 900°C range). The mechanism which is thought to be responsible for the wetting improvement involves two steps: (i) K2ZrF6 reacts with aluminium with the formation of K3AlF6, other complex fluoride species and intermetallics, (ii) K3AlF6 dissolves the alumina thin layer, coating the liquid light alloy and enables the wetting of the ceramics. The mechanism has been worked out from sessile drop experiments, solid state chemistry experiments and composite casting. The K2ZrF6 surface treatment appears to be particularly suitable for processing composite materials made of carbon (or SiC) fibrous preforms and aluminium-base matrices according to techniques directly derived from the light alloy foundry.

Composite materials made from a porous 2D-carbon-carbon preform densified with boron nitride by chemical vapour infiltration

Abstract2D-C-C/BN composites (with a BN volume fraction (VBN) up to 0.60) have been obtained by chemical vapour infiltration of hexagonal-BN from a BF3-NH3 mixture, within the pores of a 2D-C-C preform made of a stacking of carbon fabric layers which has been weakly consolidated with pyrocarbon. They were tested in compression on samples with the carbon fabric layers directed either parallel or perpendicular to the load axis. In the first case (p-direction), the compression behaviour is mainly elastic (at least for high enoughVBN). Bothσ∥R andE∥ increase regularly with risingVBN, as the BN-layer deposited within the pores of the preform (mainly located between adjacent carbon layers) becomes thicker,ε∥R is weak and progressively decreases with increasingVBN. In the second case (o-direction), the stress-strain curve exhibits both elastic and pseudoplastic domains whose respective extension depends onVBN (or onVp). The pseudoplastic behaviour is related to an irreversible microstructural damage of the interlayer C/BN filling.σ⊥R,σ⊥E andE⊥ increase with increasingVBN (or decreasingVp) according to parabolic laws,ε⊥R is much higher thanε∥R. The materials remain anisotropic even at highVBN. Oxidation tests in air (or oxygen/argon mixtures) have shown that 2D-C-C/BN, incompletely densified (Vp ∼ 0.10) by BN, exhibit a weight loss (oxidation of the carbon skeleton) at low temperatures and a weight increase (oxidation of BN) above 900° C. Oxidation resistance is enhanced by increasingVBN (which results in aVp decrease) and decreasing oxygen partial pressure. It is predicted that a good oxidation protection of the carbon skeleton requires a full densification by BN (Vp ∼ 0) and will be effective at medium temperatures. The results of the present study could be easily extended to 2D-BN-BN composites.

Tensile and creep properties of ultra high molecular weight PE fibres

Abstract In order to design multimaterial structures made of ultra high molecular weight PE fibres, their main mechanical properties were characterised from tensile and creep tests performed on single filaments or bundles, with various conditions of temperature and loading speed. After having described the experimental procedures, comparisons and deviations between the reported measures and data given by the manufacturer or published elsewhere, have led to interpretations and modelling. Thus, these tests showed the small dependence of the Young’s modulus and tensile strength on the strain rate, and gave rise to the description of the drop in mechanical properties near the melting temperature. Also, a constitutive law related to creep behaviour was determined through tensile tests at constant loading for various temperatures.

Effect of the filament nature on fatigue crack growth in titanium based composites reinforced by boron, B(B4C) and SiC filaments

Crack propagation testing has been applied to synthetic metal matrix composites (MMC) in order to compare failure mechanisms in Ti-6Al-4V alloy reinforced by uncoated boron, B(B4C) and chemical vapour deposition (CVD) SiC filaments. The impeding effect of the fibres leads to low crack growth rates, compared to those reported for the unreinforced Ti-6Al-4V alloy and to higher toughness despite the presence of the reinforcing brittle phases. After long isothermal exposures at 850° C, the MMC crack growth resistance is reduced mainly due to fibre degradation, fibre-matrix debonding and an increase in matrix brittleness. However, for short-time isothermal exposures (up to about 10 h for B/Ti-6Al-4V, 30 h for B (B4C)/Ti-6Al-4V and 60 h for SiC/Ti-6Al-4V) the crack growth resistance is significantly increased. This improvement is related to the build up of an energy-dissipating mechanism by fibre microcracking in the vicinity of the crack tip. This damaging mechanism allowing matrix plastic deformation is already effective for boron and B(B4C) in the as-fabricated state, but occurs only after 10 h of thermal exposure at 850° C in the case of SiC/Ti-6Al-4V composites.

Segregation of magnesium in squeeze-cast aluminium matrix composites reinforced with alumina fibres

Abstract Alumina preforms (Saffil fibres) were infiltrated into two foundry aluminium alloys (Al-7Si-0.3 Mg and Al7Si2.2Mg where the compositions are in approximate weight per cent) using the squeeze-casting technique. In order to establish the segregation and solidification mechanisms, small samples were cut from different areas in the infiltrated preform. The matrix was then dissolved in an HClHNO 3 solution and the amount of free magnesium was measured by atomic absorption spectroscopy. A noticeable loss of magnesium over the whole infiltrated preform has been observed depending on the amount of magnesium in the initial alloy. Whatever the alloy, this effect also occurs during a T6 thermal treatment. The alloying element segregations must be taken into account as they influence the age-hardening response of the matrix.

Mechanical behaviour in compression loading of 2D-composite materials made of carbon fabrics and a ceramic matrix

Abstract2D-carbon-carbon/ceramic composites, made from a 2D-carbon-carbon (2D-C-C) porous preform infiltrated with BN, SiC, TiC or B4C, are mechanically characterized under compression loading in directions parallel or orthogonal to the carbon fabric layers. Three types of behaviour are observed: non-linear and time dependent behaviour, a quasi-linear domain and a “pseudo-plastic” behaviour related to damaging mechanisms. Underp-compression, the variations of the Young modulus as a function of compacity obey a parabolic or linear law depending on whether the material is weakly or highly densified. Undero-compression, an exponential law is observed whatever the densification degree. The variations of failure strength compacity follow similar laws. Phenomenological models are given which depict quite well the mechanical behaviours of the composites. Undero-compression, failure occurs as the result of damaging mechanisms taking place within the inter-layer ceramic bridges binding the fabric layers together. Underp-compression, a transition is observed, from interlayer delamination to intralayer failure, for a critical compacity of about 0.85 provided the infiltrated ceramic is strong enough (i.e. for SiC and TiC). Such a transition is assumed to also occur for 2D-C-C/B4−C composites. On the contrary, for weak ceramic matrices (e.g. BN), failure inp-compression always occurs by delamination. The results suggest that the composite toughness could be increased by an optimization of the composite microstructure.

A comparative study of the impact behavior of ceramic matrix composites

Abstract Dynamic toughness of ceramic matrix composites is evaluated by instrumented Charpy impact testing. Assuming ceramic composites to be damageable elastic materials and utilizing a procedure based on a dynamic analysis of the notched bending specimen responses, various fracture parameters are studied. Dynamic toughness KId, derived from fracture initiation and dynamic crack growth resistance curves are used to characterize the dynamic toughness of various 2D and 3D ceramic matrix composites. Significant differences in R curves are correlated with the fibrous structure and the component nature of the composites. The usefulness of impact testing at moderate deformation speed is established.

Design and behaviour numerical simulation of multi-materials for an energetic matter container

Abstract The transportation of dangerous energetic matter needs the use of containers with specific properties, such as complete deconfinement of the matter for a given temperature increase. The mechanisms pointed out as possible responses to the structure and function requirements led to specific multi-materials design. This study shows how the use of suitable models, derived from analysis of the load supported by each component of the multi-material, is helpful for simulating the thermo-mechanical behaviour of such a complex structure. The validity of the deconfinement mechanisms was checked by evaluating the time required to achieve complete deconfinement of the energetic matter contained. The related simulation was based on the successive use of models taking into account only the polyethylene fibrous grid, then the moulded polyethylene layer, and finally all the combined components.

Correlation between microstructures of SiC-reinforced titanium matrix composite and liquid route processing parameters

A new procedure for filamentary metal matrix composite processing is described here. It consists in running carbon-coated SiC filaments through a liquid titanium bath in levitation. The liquid metal/carbon interaction must be significant enough to enable filament wetting and sufficiently low to avoid composite embrittlement. To insure both requirements, different configurations of the initial ceramic filament can be used: (1) SiC(C) filament free of any other coating, (2) SiC(C) filament coated with a carbide obtained by reactive chemical vapour deposition (R-CVD), or (3) SiC(C) previously coated by a first metal layer. In order to choose the best conditions for developing the process, the different processing configurations were studied through modelling and numerical simulations of the filament/matrix interaction. The microstructure of the interfacial zone between filament and matrix was investigated through SEM and Auger electron spectroscopy (AES) analyses. The microstructure of the interfacial zone between filament and matrix was investigated through SEM and AES analyses. The results show that in comparison to the first processing configuration, the best way to obtain filamentary composite semi-products without excessive fibre/matrix interaction is to use the second configuration. However, the latter requires preliminary R-CVD operations, while the third configuration leads to moderate carbon embrittlement effect without requiring additional equipment.

Heat and Mass Transfer Modeling and Simulation during Liquid Route Processing of SiC/Ti Filamentary Composites

To process SiC/Ti filamentary composites using a liquid route method, it is first necessary to overcome various major difficulties such as, high speed filament/matrix coupling, liquid titanium wetting of filament surfaces, and reduction of filament/matrix interaction. All of these requirements depend mainly on the heat and mass transfer, which occurs as the filament runs through a liquid titanium bath. Consequently, these transfers were modeled and simulated numerically during the different processing steps, particularly the cooling step. The results describe the physical phenomena which occur during the process: the carbon transfer from the carbon coated SiC filament to the liquid titanium, heat exchanges, formation of the TiC interphase at the filament surface, and, finally, the solidification of the titanium coating. Numerical simulation has shown the strong influence of running speed which governs the wettability of the filament by the liquid metal. Furthermore, the effects of an additional specific cooling device have been highlighted.