Diamond slip systems

Silicon will serve as the paradigmatic example of slip in covalent materials. Recall that Si adopts the diamond cubic crystal structure, and like in the case of fee materials, the relevant slip system in Si is associated with 111 planes and 110> slip directions. However, because of the fact that the diamond cubic structure is an fee lattice with a basis (or it may be thought of as two interpenetrating fee lattices), the geometric character of such slip is more complex just as we found that, in the case of intermetallics, the presence of more than one atom per unit cell enriches the sequence of possible slip mechanisms. 

Ceramics with cubic F structures and lllKHO) slip systems exhibit the same hardness anisotropy as fluorite structure solids with 001 (110) slip systems in the sense that the hardest directions on 100 are (100) and the softest are (110). Thus in order to determine which system is operative, a combination of the analysis given in Section 3.6.1 and other techniques, such as slip line analysis, is necessary. Ceramics with the diamond cubic structure have this slip system, and the parallel of their hardness anisotropy with that of fluorites can be seen by comparing the results for cubic boron nitride, BN, with the InP data in Figure 3.7. 

Because this is not a commonly encountered indenter and because of the restricted type of plane that it can usefully investigate, there are very few results from which to draw conclusions. However, for cubic crystals the difference between those having 001 (011) slip systems and the other types is evident in the symmetry of the hardness anisotropy curves as it was in the case of the Knoop indenter and the Vickers diamond. 

Figure 3.24. Reciprocal mean effective resolved shear stress curves for Knoop indentation calculated for (001) planes of diamond cubic crystals for the lll (lT0) slip systems.

The most striking feature of the collected data in the tables in this chapter and in Chapter 6 on anisotropic indentation hardness values for crystalline ceramics is its dependence on the relevant active slip systems. This has been extended by observation to encompass materials beyond ceramics. Thus, the nature of anisotropy for a soft, face-centered cubic metal may be the same as for hard, covalent cubic crystals like diamond, since they both have lll (lTo) slip systems. Consequently it is natural that, in order to develop a universal model, we should first look for explanations based on mechanisms of plastic deformation. 

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