Beilby layers

The amorphous Beilby layer (as it is often called) has properties markedly different from the rest of the solid. It is much harder, and is usually more soluble and electrolytically more anodic, a fact of considerable importance in the corrosion of metals, as it is often found that corrosion starts at those points (such as the neighbourhood of a punched hole) where some degree of surface flow, or damage to the crystalline structure, has taken place in the metal. It has, apparently, powers of dissolving other metals, not possessed by a crystalline surface. Thus Finch, Quarrell, and Roebuck1 found that if small amounts of metals were deposited by condensation from vapour on to a polished surface of another metal, patterns indicative of the crystalline structure of the deposited metal were obtained temporarily, but disappeared after a few minutes or even seconds. Permanent patterns of zinc on copper could only be obtained by very many successive depositions. If, however, metals were similarly deposited on crystalline surfaces of other metals, one deposition was always sufficient to give the pattern of the deposited metal. 

The rate of travel up the surface of the cylinder depended on the state of the surface, being much slower up a cylinder with the surface as left by turning, than on a ground or polished surface. The authors ascribe the difference to roughness actually lengthening the path but it would appear possible that the rate of diffusion is intrinsically more rapid on a surface which has been worked or polished, and therefore has a Beilby layer. The course of the worked portion of the surface, up the turned cylinder, would be a helix with a very slow rate of ascent, i.e. a very long path. 

TWO SEPARATE ALTERED or damaged layers classically have been recognized on metal surfaces formed by cutting- or polishing-type processes namely, an amorphous-like "Beilby" layer and a plastically deformed layer. Modern work indicates that the Beiiby layer is not, in fact, formed by the common important methods of surface preparation but that a deformed layer always is. The detailed structure of this layer is reviewed. Some consideration is also given to residual elastic stresses, surface topography, and embedded abrasive. 

The subject is complicated by the very wide diversity of processes that may conceivably be used for surface preparation. Attention will be confined here to processes in which the new surface is machined by cutting, particularly by operations such as grinding and abrading which involve the use of abrasives, or is polished by methods in which the surface is worked against a fine abrasive. It has been accepted in the past that the possibility of the presence of two physically distinct layers must be recognized on surfaces produced by these processes namely, an outer layer known as the "Beilby Layer" and a layer of material which differs from the unaffected substrate only in that it has been plastically deformed. R will be necessary to consider these two layers separately. 

The existence of the Beilby layer has been reconsidered in a recent series of investigations (7-10) and it has been established with reasonable certainty that it is not formed during polishing by the standard metallographic and industrial methods which are under consideration here. The new view, the evidence for which has been reviewed fully elsewhere (11-13), will... 

These parameters can be checked independently by their consistency with macro-scopically determined ones on mechanically polished samples. Mechanical polishing yields an amorphous and therefore isotropic Beilby layer resulting in an averaged isotropic complex refractive index niso. The correlation between niso and the anisotropic parameters is given by the equation ... 

An increase in the melt temperature may complicate the crystallization process because of the interaction between the deposited components and the material matrix (Figure 4.9.10). For metals forming alloys with the deposited components, crystallization overvoltage is observed for a surface oxide film (Figure 4.9.11). After mechanical treatment of the surfaces of the working electrodes, they were electrochemically polished with simultaneous control of the substrate state in a microscope. The time of electrical polishing was determined with allowance made for the dissolution time of the Beilby layer [21]. 

Figure 8.6. Light (high reflection) and dark (low reflection) regions on the surface of etched specimen when viewed with vertical illumination, (a) Polished surface showing a structureless, highly reflecting layer that forms upon polishing (the so-called Beilby layer) and (b) surface etched by dilute acid showing a variable inclination of surface crystals due to different rates of etching for different lattice inclinations in adjacent crystals.

Stober, W., and M. Arnold, 1960. The Beilby layer on the surfaces of quartz powder. Beitr, Silikose-Forsch 4 73. 

---