Deformation component friction

Research on friction has concentrated on the quantitative description of the areas of actual contact, the strength of adhesion between surfaces, the shear strength of the interface, interactions between A and S, the deformation component P, and interaction between the deformation and adhesion. 

Polyimide friction. The friction of polymers consists of an adhesion component and a deformation component. The adhesion component arises from the shearing of adhered junctions and is usually modeled as the product of the real area of contact and the shear strength of the polymer. The deformation component arises from the frictional work required to balance the energy dissipated in plastic deformation. Some Investigators have developed friction models in which the adhesive bonds at the junctions Increase the amount of plastic deformation over that which would exist in the absence of these bonds (13). 

It can be seen that, in order to understand friction, the important unknowns include the real area of contact between surfaces. A, the shear strength of the points of contact, s, and the deformation component, P. If the various unknowns do not operate independently, their mutual interrelationships obviously becomes important. 

Following my presentation. Dr. Savkoor will discuss the adhesion mechanism, specifically for rubbers. Rubbers, unlike rigid polymers, have high deformation loss therefore, the loss tangent enters both the adhesion and the deformation components of the friction force. Since rubber was the first material to attract the attention of tribologists, we seem to know a lot more about rubber friction than about polymer friction. 

Polymer sliding friction force may consist of two components Fg, the adhesion component and Fj, the deformation component... 

The shear strength calculated from the friction test was found approximately equal to the bulk shear strength. However, In those cases, the shearing takes place within the bulk of the polymer rather than at the Interface because the whole of the polymer material around the contact area deforms as demonstrated by Tanaka. The shearing resulting from mere application of pressure affects the deformation component of the friction as well as the adhesion component. 

Equation (5) indicates that the internal friction within the bulk of a polymer is an integral part of the deformation component and the total friction force. 

Fig. la. Relation of the Deformation Component of the Friction Force to the Width of Friction Path. 

ADHESION AND DEFORMATION COMPONENTS OF FRICTION FORCE FOR POLYMERS (Data of A. D. Kuritsyna and P. G. Melsner (Ref. 19))... 

They found F3/F. for certain polymers to be PTFE, 0.025 polycaprolactam resin and fiber, O.O6 polyethylene, 0.032. Their results agree, in principle, with those obtained by Tanaka. However, without knowing the roughness of the counterfaces, it can not be concluded from their data that the deformation component is the major factor in determining polymer friction. 

The deformation component can be determined by the lubricated sliding friction. In the presence of a lubricant, the shearing of the interfacial junctions or contacts can be prevented therefore, the total frictional force essentially equals the deformation component. Under the lubricated conditions, surface energetics and shear strength of the lubricant control the friction instead (see Section ill. C.). 

Similar to the lubricated sliding friction, rubber rollinq friction generally involves only the deformation component. Flom later found that the rolling friction theory can be extended to thermoplastics. Flom showed that the coefficient of rolling friction X can be expressed ... 

If adhesion mechanism is indeed the major mechanism for polymer unlubricated sliding friction, we should be able to detect various effects of surface energetics upon the friction process. On the contrary, it has been very difficult to find any clear-cut correlation. One of the reasons is that the adhesion component in itself contains more complicated viscoelastic elements of a polymer than those involved in the deformation component, as discussed in the preceding section. Another reason is that the total friction force sometimes contains a measurable deformation component especially when the counterface is rough. 

Without knowing the extent of the deformation component, if any, in Tanaka s total friction force data , we find the correlation in Fig. k to be rather encouraging for polymers within a similar range of modulus. In other words, the coefficient of friction y appears to increase with the increase in surface energy or critical surface tension. This is especially interesting because the same data led Tanaka to believe that deformation is the friction mechanism, especially in the case of steel slider. 

Ideally, the comparison between friction force and surface energetics should be carried out for the adhesion component Fg alone because the deformation component Fj is not directly affected by surface energetics. In reality, there is not much data available which can be clearly identified for each component. For what was available, we obtained a linear correlation between Fg and in Fig. 5. 

Most published evidence on polymer friction led one to believe that the adhesion mechanism can explain many unrelated phenomena provided that the relaxation aspect is taken into account. It appears that as long as polymers are concerned, relaxation controls both adhesion and friction. Thus, a clear demarcation between adhesion and deformation components is unattainable, and many unnecessary arguments about mechanisms can be avoided if relaxation is considered to be the intrinsic property of polymer. 

In particulate-filled thermoplastics, the matrix is the load-bearing component and all deformation processes take place in the matrix. Particulate fillers are, in most cases, not capable of carrying any substantial portion of the load due to the absence of interfacial friction as the means of stress transfer. This is evidenced by the lack of broken particles on the surfaces of fractured filled thermoplastics. Hence, it seems appropriate to start this volume with a brief overview of the basic structural levels and manifestation of these levels in governing the mechanical properties of semicrystaUine thermoplastics used in compounding. 

The main disadvantages of the solvent method are that the treatment is effective only for about 48 hours, and also that it can be destroyed by friction on the surface. In addition, the immersion of the components in hot solvent may result in their deformation. 

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