Level 2

Section 5.2: Introduction to laser machining of composites (II)


4. Carbon-Carbon Composites As the name implies, in this class of fiber-reinforced composites both the matrix and the fibers are fabricated from carbon. These materials offer a unique combination of properties, including the ability to withstand extremely high service temperatures (>3000C), high specific strength, excellent resistance to wear (they can be self-lubricating), good resistance to thermal shock, and reasonable machinability. However, the maximum use temperature for these composites is limited by oxidation problems. Typical applications include brake components, heat shields, and rocket nozzles. Carbon-carbon composites can be fabricated using the CVD methods by impregnating graphite fibers with a carbon-based polymer that is then pyrolyzed to "burn off " the noncarbon atoms in the polymer, or combinations of the two.

The poor surface quality is also due to the damage produced by the excessive heating of the sub-surface layer of the material. Fig. 5(a) shows an electron microscope image (back scattered) of the heat-affected sub-surface layers observed following machining of the PRMMC. It is evident from Fig. 9 that the microstructure of the matrix material is changed by the heat generated by the machining process. It can be seen that the matrix material shows white lines in its structure which are more prominent in Fig. 5(b) . An extensive X-ray (energy dispersive spectroscopy X-ray/EDS XR) analysis was undertaken and Fig. 5(c) shows the aluminium, silicon, copper and zinc content and their locations within the thermally affected sub-surface layer. The results shown in Fig. 5(b) and Fig. 5(c) indicate that the heat generated during laser machining results in the deposition of copper and zinc along the aluminium grain boundaries. It can be seen from Table 1 that the thickness of this deposition layer decreases with increased feed rate.

Fig. 5(a) Sub-surface layer of PRMMC after Laser cutting. Fig. 5 (b) Microstructure of deposition layer

Fig. 5(c) Sub-surface layer of PRMMC after Laser cutting (F. Müller and J. Monaghan 2000).

Laser machining offers significant productivity advantages for rough cut-off applications. It is apparent that a laser is very suitable for high feed rates (up to v=3000 mm/min) and can produce a cut with a narrow kerf width (wk0.4 mm). Reinforcing the aluminium matrix with SiC ceramic particles improves the machinability of the composite, due to the reduction in the optical reflectively of the material. However quality of the laser cut surface is relatively poor. Striation patterns on the cut surface and burrs at the exit of the laser (dross attachment) were observed. Significant thermal induced microstructural changes were also observed within the PRMMC(F. Müller and J. Monaghan 2000).

For introduction to laser machining of composites systems ( I ), please see Level 1. Section 5.3