Adhesion initial frictional behavior

In functioning machinery the contacting parts repeatedly rub one another many times. The interaction of two surfaces on reiterated contact will in part depend on the condition in which the previous iteration left them. Under ordinary circumstances, with the machinery operating satisfactorily, each iteration is much like the one before and an analysis of steady-state wear or friction can be made on the basis of one cycle of surface interaction. Generally in such cases, but not necessarily always, asperity deformation is elastic rather than plastic. Whether an adhesive junction forms depends on the condition of the asperity surface. If the materials f>e.n. 4e are easily adhesive but the surfaces are covered by a film which inhibits adhesion, then to initiate adhesion obviously the film must first be removed, broken up or penetrated. The subsequent course of adhesive contact will then be governed by such factors as the size of the contact, the shape of the asperity, the impressed load, the strength of the material, etc., in accordance with the fundamental modes of behavior. 

Adhesion and agglomeration are vitally important in the gas-solid transport area although they have not received much attention in that respect. There have been numerous studies on these subjects relating the solid particles and surfaces to static conditions. As particles flow in a pipe, a certain number of the particles adhere to the soUd surface while others find their way into small crevices in the system. In the flow of a fine powder in a pipe, after the initial deposition of the particles to the pipe wall, the particles in suspension will interact with those adhering to the wall, causing different frictional and electrostatic behavior than would be anticipated from interactions of the particles with the pipe wall material alone, whether it is metal, plastic, or glass. 

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