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CAM MATERIALS AND LUBRICATION 261

(a) Surface topography. (b) Surface layers.

(c) Surface contact.
FIGURE 9.6. Surface topography and contact. (Adapted from Buckley, D.H., “Tribology Funda-
mentals,” Tribology for Aerospace Applications, E.V. Zaretsky, Ed., copyrighted by the Soc. of Trib.
and Lub. Eng., Park Ridge, Ill., with permission.)

the asperities that the adhesion between two solids occurs. Today, surface profiles can be
established on an atomic level by use of a scanning electron tunneling microscope. In this
way, a surface can be examined tribologically to study the structural, physical, and chem-
ical change in the surface.

9.4.2 Adhesive Wear
Adhesive wear, often called galling, scuffing, or scoring wear, occurs between two sliding
solid surfaces. It exists as an attraction between the surface atoms of contacting materi-
als, leading to a transfer of material from one surface to the other and eventually to the
formation of loose fragments.
Strong adhesive forces develop at the interface formed by the solids. The magnitude
of the forces relates to the surface condition (cleanliness, etc.) and fundamental properties
of the two solids. The surface contact is established in deep areas. High local pressure
combined with the relative sliding motion along crystallographic planes causes roughen-
ing of the interface of contact. When a pair of firmly interlocked high spots is forced apart,
breakage will occur where the section is weakest.