Hot pressing process consists firstly in the preparation of powders mixing. This mixing, which contains ceramic, metallic powders and lubricant agents, must be stable and homogeneous.
The bad wettability of the carbon fibres by copper and the non reactivity between carbon and copper led to the formation of a stable porosity between the copper matrix and the carbon fibres during the densification stage. Thus, the full densification of copper/carbon composites is not possible by conventional sintering. Hot pressing technique, [Evans and McColvin, 1976] [Jha and Kumar, 1997], under controlled atmosphere, is finally used in order to obtain fully dense materials.
Fig. 11. Processing flow sheet of hot pressing
In a first step, powders are desagglomerated and mixed in ball milling with lubricant agent during 6 hours. Elaboration conditions, such as mixing conditions of the short carbon fibres and the copper powder, dimension and shape of the two powders must be optimized. The powders mixing is pressed to desired shape under 200 MPa, and the green sample obtained have finally sufficient strength and flexibility for handling but still shows important porosity close to 15-25%. Up to 200 MPa, the pressure can lead to the rupture of carbon fibres in green sample and the deterioration of final properties of composites.
In a second step, the green sample is heated and forged up to 600°C under 50 MPa. Figure 12 shows the hot press apparatus with the inducting heating system. The mould can be in steel or in tungsten carbide for higher temperature and pressure. To prevent copper oxidation or steel mould, the atmosphere of hot pressing chamber is usually inert atmosphere (nitrogen or argon) or primary vacuum. The temperature can be controlled with a thermocouple placed into the steel mould.
The temperature and pressure of hot pressing is adjusted in order to obtain full dense composite and to avoid the deterioration of carbon fibres. In the figure 13, the density of hot pressed sample increases with the pressure and the temperature of hot pressing. However, a high pressure can lead to breaks the carbon fibres if the copper is not enough soft. Then, the high temperature leads to a softening of copper matrix and to improving the copper flow around the carbon fibres during the hot pressing step. Finally, the optimal conditions correspond to a moderate temperature, 600-700°C, (which is a low sintering temperature of copper particles) with pressure between 50-100 MPa. The pressure must be higher than 20 MPa, which corresponds likely to the limit of elasticity deformation of copper particles before ductile deformation and copper flow step in composite materials.
Besides, a high temperature of hot pressure involves the important technological cost, and it can lead the pre-sinterisation of copper particles, which is prejudicial to suitable softening of copper matrix during the hot pressing. Then, the optimal conditions of hot pressing correspond usually to the best compromise between optimal properties of Cu/C composites and process cost.
Finally, the main parameters of hot pressing, such as temperature, time and atmosphere in the thermal properties of the Cu/C composite materials must be adjusted in relation with the optimal mechanical and thermal properties of Cu/C composites.