Hardness, of solids

Although not one of the most frequently discussed properties of solids, hardness is an important consideration in many instances, especially in the area of mineralogy. In essence, hardness is a measure of the ability of a solid to resist deformation or scratching. It is a difficult property to measure accurately, and for some materials a range of values is reported. Because of the nature of hardness, it is necessary to have some sort of reference so that comparisons can be made. The hardness scale most often used is that developed by Austrian mineralogist F. Mohs in 1824. The scale is appropriately known as the Mohs scale. Table 7.11 gives the fixed points on which the scale is based. 

It has long been recognized that the Mohs scale is not totally satisfactory for several reasons. One reason is that some minerals have different resistance to scratching and deformation on different surfaces 

Mineral Mohs Hardness Modified Value Mineral Mohs Hardness Modified Value 

The data show that when compounds such as LiF and MgO are considered, even though the inter-nuclear distances are similar, there is a great difference in the hardness of the materials. Of course, there is also a great difference between the melting points and lattice energies of the compounds as 

In this chapter, a survey of the structure and properties of solids has been presented. Solid-state chemistry has emerged as an important area of the science, and although it is not exclusively so, much of the work deals with inorganic substances. For more information on this important area, the references at the end of the chapter should be consulted. 

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Example Approximate calculation of the hardness of solids. This concept of shear yielding - where we ignore the details of the grains in our polycrystal and treat the material as a continuum - is useful in many respects. For example, we can use it to calculate the loads that would make our material yield for all sorts of quite complicated geometries. 

A. P. Gerk, The Effect of Work-hardening Upon the Hardness of Solids Minimium Hardness , Jour. Mater. Science, 12, 735 (1977). 

The entropies of a few substances are given in Table 6-3. Notice how the entropy increases with increasing complexity of structure, with transitions from solid to liquid to gas, and with decreasing hardness of solid substances. 

Shinozaki K., Ishikura Y., Uematsu K., Mizutani N., Kato M., 1980, Vickers micro-hardness of solid solution in the system Cr203-Al203, J. Mater. Set., 15, 1314-1316. 

Johnson, K.L. 1996. Modeling the indentation hardness of solids. In Solid-Solid Interactions (M.J. Adams, S.K. Biswas, B.J. Briscoe, eds.), pp. 16-28, Imperial College Press, London. 

The values of H listed in Table 6.1 for the physical hardness of solids raise an interesting question. Should there not be a corresponding number, H for the physical hardness of molecules After all, there are force constants in molecules as well as in solids. Equation (6.13) might serve for a diatomic molecule, if n were simply set equal to one. 

Hardness of solid solutions, 75, 78 Homopolar bond, 25, 91 crystal, 27, 28, 91 Hull, 33... 

Mohs hardness scale Empirical scale by which the hardness of solids can be determined by comparison with 10 reference minerals ranked from 1 to 10 1, talc 2, gypsum 3, calcite 4, fluorite 5, apatite 6, orthoclase 7, quartz 8, topaz 9, corundum and 10, diamond. 

Rehbinder and co-workers were pioneers in the study of environmental effects on the strength of solids [144], As discussed by Frumkin and others [143-145], the measured hardness of a metal immersed in an electrolyte solution varies with applied potential in the manner of an electrocapillary curve (see Section V-7). A dramatic demonstration of this so-called Rehbinder effect is the easy deformation of single crystals of tin and of zinc if the surface is coated with an oleic acid monolayer [144]. 

The entropically driven disorder-order transition in hard-sphere fluids was originally discovered in computer simulations [58, 59]. The development of colloidal suspensions behaving as hard spheres (i.e., having negligible Hamaker constants, see Section VI-3) provided the means to experimentally verify the transition. Experimental data on the nucleation of hard-sphere colloidal crystals [60] allows one to extract the hard-sphere solid-liquid interfacial tension, 7 = 0.55 0.02k T/o, where a is the hard-sphere diameter [61]. This value agrees well with that found from density functional theory, 7 = 0.6 0.02k r/a 2 [21] (Section IX-2A). 

There are two classes of solids that are not crystalline, that is, p(r) is not periodic. The more familiar one is a glass, for which there are again two models, which may be called the random network and tlie random packing of hard spheres. An example of the first is silica glass or fiised quartz. It consists of tetrahedral SiO groups that are linked at their vertices by Si-O-Si bonds, but, unlike the various crystalline phases of Si02, there is no systematic relation between... 

Silcones are important products of silicon. They may be prepared by hydrolyzing a silicon organic chloride, such as dimethyl silicon chloride. Hydrolysis and condensation of various substituted chlorosilanes can be used to produce a very great number of polymeric products, or silicones, ranging from liquids to hard, glasslike solids with many useful properties. 

Substrate Properties. It is clear from equation 5 that higher hardness of the substrate lowers friction. Wear rate of the film also is generally lower. Phosphate undercoats on steel considerably improve wear life of bonded coatings by providing a porous surface which holds reserve lubricant. The same is tme for surfaces that are vapor- or sandblasted prior to appHcation of the soHd-film lubricant. A number of typical surface pretreatments are given in Table 13 to prepare a surface for solid-film bonding (61). 

Particle surface characteristics Type of solid (in terms of internal liquid content) gel, flocculated, hard particle Strength of particle (resistance to deformation under pressure) compressibility over time expressed cake... 

Figure 11.9 shows that the hardness of martensite increases rapidly with carbon content. This, again, is what we would expect. We saw in Chapter 8 that martensite is a supersaturated solid solution of C in Fe. Pure iron at room temperature would be b.c.c., but the supersaturated carbon distorts the lattice. 

Colbalt Hydrogenations of solid fuels and fuel oils Manufacture of terephthalic acid High pressure production of aldehydes Lung irritation (hard metal disease) respiratory sensitization... 

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