the carbon-fullerene coating. When molten aluminum is poured into a die, it penetrates and removes the carbon nanofiber coating. By repeating the process with a carbon-fullerene-coated mold, consideration was given to the reasons for the carbonizing of the mold surface when the molten metal used for diecasting was 1,184F (640C), which is lower than ordinary carbonizing temperature. In aluminum diecasting, molten metal immediately absorbs oxygen as it seals and heats the cav- ity. As a result, it leaves no space for sublimed fullerene to escape, and the fullerene penetrates into the ferrous mold and diffuses. Tis is fullerene’s low temperature carburization effect when it is sealed and sublimated. To study the changes in carbon
crystal structure, the Raman spectros- copy analysis for the carbon-fullerene coating was performed before and after casting. Te black area of the surface indicated in Figs. 7a and 7d were an amorphous carbon state. Analysis of the gray area in the same images showed a different structure. Before casting, the main components in the area were C60 fullerene and C70 fullerene, but fuller- ene could not be found after use. Te peak wave numbers of G band were 1,339cm-1
and 1,599cm-1 , meaning they
were in the state of amorphous carbon. Te fullerene might have disappeared, but, as shown in Fig. 8, the carbon-
Fig. 7. The images show typical SEM micrographs of coating surface (a and d), cross sections (b and e) and C Kα image of carbon-fullerene-coating cross-section. Images (a), (b) and (c) are taken from carbon-fullerene coating before casting, and (d), (e) and (f) are taken after 100 shots.
fullerene coating after use was denser, which means it became dense amorphous carbon after repeated heating. To study the heat transfer
Fig. 6. The two diagrams show the TG scans for the three coatings in (a) an air atmosphere and (b) a nitrogen atmosphere.
properties of the coatings, the temperature of the mold and molten metal in a cavity with and without a carbon-fuller- ene coating was measured. Figure 9 shows the schematic diagram of the model cavity for the heat transfer measure- ments. Te metal temperature and the mold temperature were each measured at two spots. Te temperature of molten metal was
Fig. 9. The model cavity has four measuring points, two in the cavity and two in the mold.
Fig. 8. The graphs show typical Raman spectrums at the grey area in carbon-fullerene coating surface from Fig. 7.
February 2014 MODERN CASTING | 59
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