Boundary between two ideal layered crystals

Title: Friction between incommensurate crystals

Author: J. Friedel and P -G de Gennes

Bibliographic details: Philosophical Magazine, Volume 87, Number 1, 1 January 2007, pp. 39-49(11)

Abstract: We present an overview of friction processes expected between two ideal crystals of strong layers (graphite, MoS₂, etc.) when one crystal is rotated with respect to the other by a certain angle θ. We assume perfect conditions: no impurities; no preexisting dislocations in the bulk of the crystals; slow gliding velocities. Two regimes show up: (a) Weak coupling when , where are typical intra- (inter-) layer interactions. Here we expect weak friction, controlled by two-phonon processes, and analyzed by Sokoloff et al. However, we point out that surface waves at the interface also play a role. (b) Strong coupling where two orthogonal sets of screw disclinations should build up in the contact plane, as shown long ago by F.C. Frank. Here (to a first approximation) the dislocations are arranged in ladders, and we expect solid friction with a Peierls-Nabarro threshold stress.

Notes: A paper that needs careful reading (which, I haven’t done yet).

Note added on August 8, 2007:  A follow-up paper in Philosophical Magazine Letters.

Grain boundary migration in bcc metals

Title: Collective motion of atoms in grain boundary migration of a  bcc metal

Authors: L Zhou, N Zhou, and G Song

Bibliographic details: Philosophical Magazine, Volume 86, Number 36, 21 December 2006, pp. 5885-5895(11)

Abstract:

Molecular dynamics simulations of grain boundary (GB) migration of a bcc metal, tungsten, have been carried out. The GB is of asymmetrical 〈 110〉 tilt type. Detailed examinations of atomic processes in the migration, show that the GB migration consists mainly of GB dislocation glides. Furthermore, each motion of a GB dislocation involves a cooperative motion of about three atoms on each of the atomic planes perpendicular to the tilt axis, leading to their realignment from the receding grain to the advancing grain. This collective motion is not synchronized in all of the atomic planes, but appears to be in two or three adjacent planes, suggesting a kink mechanism for glides of the GB dislocations.

Notes: In this paper, the authors try to answer two specific questions regarding grain boundary migration in bcc metals using molecular dynamics simulations:

1. The number of atoms involved in each collective motion (and what determines the number); and,
2. The relationship between the collective mechanism and grain boundary dislocation mechanism.

There also seems to be indications of grain rotations at around 1.7 nm or so (which is not pursued in this paper). Finally, to answer the questions above–the grain boundary migration is via dislocation glide; each glide is associated with a collective motion of three atoms; further, the glide might also be associated with a kink motion.