Carbon and solid carbides

The formation of a surface orientated perpendicularly to the basal plane needs the rupture of chemical carbon-carbon bonds. The surface energy for this prismatic orientation has not been determined experimentally, but is expected to be an order of magnitude higher than that of the basal plane (150 mJ / m2). 

In the case of vitreous carbon, the component Ogv of csv was measured by the technique used for graphite (Donnet et al. 1982) and found to be 32 mJ/m2. This is five times lower than that of the graphite basal plane and may be explained by the low density of vitreous carbon (1.5 x 103 kg/m3) compared to that of graphite (2.26 x 103 kg/m3) (see also Section 8.1). Because the atomic structure of polished surfaces of vitreous carbon is not known, it is not possible to evaluate the percentage of rSv that is due to r 3v. 

Carbides can be covalent or metal-like, the most important of the covalent carbides being SiC which like carbon crystallizes in both hexagonal and cubic structures. However, contrary to carbon, the basal planes in the hexagonal structure and the (111) faces of the cubic variant are linked by chemical bonds, so the corresponding surface energies should be about 103 rather than 102 mJ/m2. Estimated values for the surface energy of both faces are close to 1500 mJ/m2 (Appendix F). 

For other metal-like carbides (TiC, ZrC, VC, T aC), an estimation of the minimum value of ffsv can be made using contact angle data of ferrous metals (see Table 7.12) and assuming that tsl = 0- This leads to values close to 1500 mJ/m2. However, the calculations of Warren (1980) suggest that rSL in ferrous metal/metallic carbide systems is close to 500 mJ / m2, so a more plausible value for rsv is 2000 mJ/m2. 

Measurements of the surface energy of pure liquid metals performed in the last decades by different investigators and different techniques have led to values of 

---