The powder metallurgy process are mainly hot press for 3D design or thick substrate (3 mm of thickness) and tape casting process for thin film (from 100 to 500 microns of thickness). Tape casting process offers an original solution for the elaboration of thin sheets of metal matrix composites [Corbin et al., 1997] or, in our case, carbon fibre reinforced copper matrix composites. Tape casting process is currently used on the large scale to produce thin ceramic or multilayer structures of various materials for different applications, [Roosen, 1999] such as Al2O3 and AlN substrates for electronic devices, BaTiO3 for multilayer capacitors, solid electrolytes for sensors or energy conversion, piezoelectric ceramics for actuators or transducers, etc.
The main characteristics of this process are low cost, accurate control of the thickness from 25 to 1000 pm, good surface finish and high quality of laminated materials [Geffroy et al., 2007], (fig. 9).
Fig. 9. Picture of discontinuous tape caster in SPCTS lab of Limoges, France.
One of the key points of tape casting process for the elaboration of the copper/carbon composite sheets is the elaboration of stable slurry with copper particles and short carbon fibres. Then, tape casting process (fig. 10) consists of the preparation of a suspension of the inorganic powder(s) in an aqueous or non-aqueous system. This slurry is a complex multicomponent system typically containing the following components: powder (ceramic and/or metallic), solvent, dispersant, binder, and plasticizer. The suspension must be stable, homogeneous and with a suitable rheological behaviour according to the tape casting process. The slurry is spread onto a support by means of a moving doctor blade. After evaporation of the solvent, the obtained dried tape, or green tape, was cut to the desired shape.
Fig. 10. Processing flow sheet of tape casting process
The main key points of tape casting process are discussed in the following sections. a. Choice of organic components for tape casting
The solvent must wet the carbon fibres and the copper powder and should have a low temperature of vaporization. The solvent is the azeotropic mixture of ethanol and 2-butanone (40/60). Indeed, it offers a low boiling point (74.8°C) and a low dielectric constant that is favourable for a good wettability of carbon fibre [Johnson et al., 1987]. The nature of the solvent and of the powder determines the choice of organic additives, such as dispersant, binder and plasticizer [Moreno, 1992]. According to sedimentation tests in the azeotropic mixture of ethanol/2-butanone (40/60), one phosphate ester (CP213, Cerampilot, France) corresponds to the best desagglomeration and dispersion of copper and carbon particles. The optimum concentration of dispersing agent can be determined by rheological measurements or sedimentation tests. The minimum viscosity, then the best state of dispersion, is usually obtained for a dispersant concentration of 0.5 wt% on the dry powders basis.
The binder ensures the cohesion of the green sheet to avoid cracking during drying and for handling. After evaporation, the binder molecules form organic bridges between copper particles and carbon fibres, resulting in high mechanical properties of the green tape [Bohnlein-Maufi et al., 1992]. The binder must be easily removed at low temperature without residues. The binder is commonly a PolyMethyl MethAcrylate (PMMA) with a molecular weight ranging between 100 000 and 200 000 g. mol-1.
The plasticizer, which confers the flexibility to the green tape for easy handling, induces a decrease of the Tg of the organic phase. An efficient plasticizer of the PMMA is usually the dibutyl phtalate or polyethylene glycol with a low molecular weight close to 300 g. moh1. A good compromise between the flexibility and the mechanical strength of the green tape was obtained for a binder/plasticizer ratio close from 1 to 1.5.
b. Slurry preparation and tape casting
The tape casting suspension is prepared in two steps. The first one consists in dispersing, by planetary milling, the copper powder and carbon chopped fibres in 2-butanone/ethanol solvent with phosphate ester as dispersant. The binder and the plasticizer are added to the suspension in a second step and the complete slurry was homogenised, also by planetary milling, but at a lower rotating velocity. After homogenization, the slurry is degassed and directly casted onto a siliconed Mylar carrier film with a doctor blade. The doctor blade speed is fixed at 0.5 m. min-1 with a gap of 0.5 mm. The solvent evaporation is carried out at room temperature under air.
c. Green tape
The geometrical density of the green tapes varying from 2 to 2.5 g. cm-3 corresponds to a relative density ranging from 0.55 to 0.65 of TD (theoretical density), that suggests a rather good arrangement of particles, whereas the particle shapes, like short fibres or dendrite copper particles, are not favourable to a good compaction. However, the carbon fibres are oriented in the casting plan due to shear imposed during tape casting.
d. Thermal treatments
The organic components are removed during a thermal treatment at low temperature, i. e. debinding. The composites are then pre-sintering at higher temperature. The resulting composite structures have sufficient strength and flexibility for handling, but still present an important porosity. The thin sheets are then subsequently fully sintered by hot pressing (fig. 11?). In this densification step, 5 or 10 sheets (with thickness equal to 200 pm) can be pressed together for the elaboration of thick systems.
e. Debinding and pre-sintering
In the first step of firing, the organic additives, i. e. the binder, plastizicer and dispersant are burned out carefully. The removal of plasticizer and binder occurs, under air, between 120°C and 350°C, whereas the extraction, under nitrogen, is performed between 140 and 400°C.
Due to copper oxidation starting at 150°C under air, debinding of copper/carbon composite green sheets are performed at 350°C under nitrogen. In the second step of firing, the copper matrix is pre-sintered at 750°C during 30 mn in nitrogen. The resulting composite has a sufficient strength and flexibility for handling, but still presents an important porosity (20-30 % in volume), which results from the presence of carbon fibres. The densification of the copper/carbon composites starts at about 650°C.
f. Densification by hot pressing
The pre-sintered individual tapes or multilayers are introduced between the two pistons of the steel mould which were heated, under vacuum (0.66 Pa), by induction system and regulation monitor. The usual conditions of hot pressing correspond to 650°C with a heating rate of 25°C. min-1 under 50 MPa during 1 min. This last step of densification by hot pressing is presented in more details in the next section.