Layered structures, deformation

It can be seen that ceramic multilayer structures have been produced with increments of the hardness of up to 60 GPa, increasing the hardness by up to a factor of almost 3. Initial work in this area has developed a number of ideas, such as the effect of modulus mismatch, which in some cases give good agreement with the models suggested but in many others do not. It is suggested that at least some of this discrepancy can be accounted for by differences in the microstructure and residual stress-state of the film, both of which are often poorly characterized. Furthermore there is very little direct evidence about how these structures deform and in particular about how different layers must be strained in order to accommodate the indenter when it is pressed into the sample. Further advances in this area will require the greater use of numerical techniques to analyse the complex stress and strain behaviour under the indentation, coupled with the use of recently developed techniques that allow the localized deformation behaviour to be observed in detail. 

The physical properties of the silicates correlate closely with their structures. Talc, Mg3(Si40io)(OH)2, is an example of an infinite layered structure (see Fig. 22.If). In talc, all of the bonding interactions among the atoms occur in a single layer. Layers of talc sheets are attracted to one another only by van der Waals interactions, which (being weak) permit one layer to slip easily across another. This accounts for the slippery feel of talc (called talcum powder). When all four vertices of each tetrahedron are linked to other tetrahedra, three-dimensional network structures such as cristobalite (see Fig. 22.Ig) or quartz (Fig. 22.2) result. Note that the quartz network carries no charge consequently, there are no cations in its structure. Three-dimensional network silicates such as quartz are much stiffer and harder than the linear and layered silicates, and they resist deformation well. 

The structure of GeS and SnS is a layer structure similar to that of black P (see pp. 674 and 912) which may also be described as a very deformed version of the NaCl structure in which Sn(ii) has 3 pyramidal neighbours compare the environments of a metal atom in GeS, SnS, and PbS ... 

The deformation is given in terms of axial strain and bending strain which are the mean value of the strains in loading direction and in proportion to the curvature of the layered structure, respectively. 

The layered structure cannot be refined indefinitely by deformation due to the increasing hardness with decreasing crystallite size (for details see Sect. 3.3.2). The main alloying, therefore, occurs by an interdiffusion reaction at the created clean interfaces, if a thermodynamic driving force (negative free enthalpy of mixing) exists for this diffusion couple. The required temperature rise is provided by the heat released during the ball collisions (Sect. 3.3.2). 

Solid lubricants, which are greasy to the touch, are highly anisotropic solids with a low shear strength in at least one dimension. Solid lubricants fall into three main classes - inorganic solids with a lamellar (layer-Uke) crystal structure, solids that suffer plastic deformation easily and polymers in which the constituent chains can slip past each other in an unrestricted way. The categories of most importance are layer structures and soft inorganic compounds. 

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