Lattice friction

The construction, meaning and uses of such maps has been explained with great clarity in a monograph by Frost and Ashby (1982). The various mechanisms and rate-limiting factors (such as lattice friction or dislocation climb combined... 

From a mechanistic perspective, what transpires in the context of all of these strengthening mechanisms when viewed from the microstructural level is the creation of obstacles to dislocation motion. These obstacles provide an additional resisting force above and beyond the intrinsic lattice friction (i.e. Peierls stress) and are revealed macroscopically through a larger flow stress than would be observed in the absence of such mechanisms. Our aim in this section is to examine how such disorder offers obstacles to the motion of dislocations, to review the phenomenology of particular mechanisms, and then to uncover the ways in which they can be understood on the basis of dislocation theory. 

Tj is the lattice-friction stress in the slip plane. In this case, the obstacle is grain boundary, d, which is taken into account by Stroh [52], as seen in Eq. (8.45a) ... 

The crystalline nature of the lattice itself inhibits the dislocation motion. This is called lattice friction or Peierls stress. The Peierls stress for FCC metal is lO GPa, for HCP metal it is 5 x 10 GPa, and for BCC metal it is approximately 0.1 — 1 GPa. This also indicates why some types of nanoparticles are getting more attention compared with others in the MMCs (Cao et al., 2007). 

It may come as a surprise to some that two commensurate surfaces withstand finite shear forces even if they are separated by a fluid.31 But one has to keep in mind that breaking translational invariance automatically induces a potential of mean force T. From the symmetry breaking, commensurate walls can be pinned even by an ideal gas embedded between them.32 The reason is that T scales linearly with the area of contact. In the thermodynamic limit, the energy barrier for the slider to move by one lattice constant becomes infinitely high so that the motion cannot be thermally activated, and hence, static friction becomes finite. No such argument applies when the surfaces do not share a common period. 

Simulations of incommensurate surfaces showed a similar dependence on Vi, with first-order instabilities occurring if Vi < Vj, where Vj is some positive, critical value that depends on the degree of mismatch between the lattice constants of the top and bottom surfaces. This process leads to nonvanishing Fk as l o goes to zero. In the case where Vi < V, the atoms are dragged with the wall that exerts the maximum lateral force. It, in turn, leads to friction that scales linearly with the sliding velocity. As a result, the friction force will go to zero with vq. 

This phenomenon is not fully understood, but may be due to the effect of radiant heating and frictional heating on the redistribution and reformation of all wax into a loosely knit wax crystal lattice. The new wax lattice yields an oil with a higher pour point. 

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