Surface damage elastic deformation

Substances in this category include Krypton, sodium chloride, and diamond, as examples, and it is not surprising that differences in detail as to frictional behavior do occur. The softer solids tend to obey Amontons law with /i values in the normal range of 0.5-1.0, provided they are not too near their melting points. Ionic crystals, such as sodium chloride, tend to show irreversible surface damage, in the form of cracks, owing to their brittleness, but still tend to obey Amontons law. This suggests that the area of contact is mainly determined by plastic flow rather than by elastic deformation. 

Polystyrene and other materials have been reported to be plastically deformed at temperature even below Tg (compare, e.g., Sect. 3.2 in Chap. 3) thus, such experiments are most likely carried out beyond the elastic contact limit and surface damage may prevail. 

Diamond and sapphire differ in that they have lower than normal ju.f values in the area of 0.1, and its value depends on the load, as might be expected for materials that deform elastically rather than plastically. Such materials also begin to show surface damage beyond a certain load. Under very clean conditions, m for diamond has been found to rise to 0.6, suggesting that some mechanism such as the adsorption of a monomolecular water layer or slight surface oxide formation may act to lubricate the diamond surface naturally. 

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 traditional microindentation of the surface of ionic and covalent crystals allows one to study the effect of adsorption on the movement of the screw components of the dislocation half-loops formed, but only outside the contact zone. The capabilities are broadened with the use of the micro-sclerometric and ultramicrosclerometric (scratching) methods developed by Savenko and coworkers [46,68,70]. A step-by-step increase in the load applied to the indenter allows one to observe a transition from the reversible elastic contact to the appearance of the very first damage, that is, nearsurface dislocations, and further to the development of plastic deformations, and then to microcrack nucleation (Figure 7.42). The adsorption taking place from the active medium can both facilitate damageability and retard it. 

Stresses Caused by the Support System. After dropping below the transformation temperature, the glass is in a visco-elastic or a brittle-elastic condition. It must then be observed that the 8 m blank has a limited stiffness because of its low thickness. This means that, because of the deformation of the 8 m blank under its own weight, tensile stresses of > 5 N/mm occur at the surface with a maximum deformation of 0.8 mm with a static three-point support. In particular, blanks with damaged surfaces can break due to tensile stresses of this magnitude. 

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