Crystal faces, varying hardnesses

Vickers Hardness (GPa) 24.5 - 28.2 (varies with crystal face) Modulus of Elasticity (GPa) 475 293K... 

Molybdenum disulphide alone can be used as the reservoir material, either in the form of single crystal or as a compact. It is difficult to define the structural strength of single-crystal molybdenum disulphide. Because of its anisotropic nature, the ultimate stress in shear, tension or compression varies critically with the direction of the applied stress in relation to the crystal orientation, as discussed in Chapter 4, but some indication is given by the hardness values on the crystal faces and edges of 1.5 and 8 Mohs respectively. 

Hardness characteristics—especially in calcite crystals— may vary as a property dependent upon the direction of the scratch (i.e., able show evidence of a particular Mohs number if scratched along one face or direction as opposed to a different hardness number if scratched in a different direction. 

WC is present in hardmetal in the form of single crystals. These have a hexagonal structure and are highly anisotropic [4], Table 1 lists the Vickers hardness of three WC crystal faces and shows that the hardness can vary from face to face by up to 100% [5]. On account of the superior hardness of the (0001) faces, attempts have been made to produce WC-Co parts with a (0001) texture [6],... 

As for the metals most often used in electrochemistry (namely silver, copper or zinc) the electron work function values are 4 to 5 eV. For single-crystals this value is influenced by the particular type of crystalline face. This is hardly surprising given that the metal s surface properties affect the electric surface voltage x- The differences that can be observed between different types of crystalline faces vary by about 0.1 eV. 

The hardness of single-crystal diamond varies as a function of the crystal orientation by aimost a factor of two. This is also true of CVD diamond, and the hardness vaiues depend on which crystal face is in contact with the indenter of the testing device. 

Crystals vary in hardness not only from substance to substance but also from face to face on a given crystal (Brookes, O Neill and Redfem, 1971). One of the standard tests for hardness in non-metallic compounds and minerals is the scratch test, which gave rise to the Mohs scale. Ten degrees of hardness are designated by common minerals in such an order that a given mineral will scratch the surface of any of the preceding members of the scale (see Table 2.2). 

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