Friction-zone temperature, dependence

Figure 2. Dependence of friction zone temperature (T) (curve 2) and the coefficient of friction (f) (curve l) upon PV.

The wear ratio of the materials tested that corresponds to the wear rate of 0.25 yUm/h is characterized by a wide range of values from 0.2 up to 2.1, which depend to a large extent on the temperature in the friction zone. Thus by using the dependence of the wear ratio on the temperature it is possible to forecast the performance of a true friction unit as it had been similarly done elsewhere (18, 2l). 

The method of reduced variables as outlined in Section B above, including fitting the function flr(r) to the WLF equation, is appropriate for data in the transition, plateau, and terminal zones of time scale, with the provision that all contributions to the measured viscoelastic properties involve the same friction coefficient ftfand moreover that the internal structure of the system does not change with changing temperature. The latter restrictions are inherent in the development of Section A. In glassy and crystalline polymers, and in situations where two or more different classes of molecular motions with different temperature dependences are involved, some modifications in the treatment are necessary. 

The effects of a diluent or plasticizer on viscoelastic properties in the transition zone can be analyzed in terms of its influence on the temperature dependence function, arit), and, separately, on the absolute values of friction coefficients and the shapes of viscoelastic spectra. 

Thus at a given temperature, the location of the transition zone of E(t)e on the time scale is determined by the monomeric friction factor, the height of the entanglement plateau by Af, and the width of the plateau by (Ml Mf)3. The time dependence of entanglement slippage. (0 nn describes the rate at which the entanglement plateau will drop to the equilibrium... 

Friction commonly is described with two coupled terms, interfacial (chemical) forces and ploughing (mechanical loss) forces. It is important to consider both when interpreting results to assign a mechanism of fnction. Figure 7 shows no pull-off (adhesive) dependence on temperature except when the temperature is sufficiently into the rubbery regime. Above Tg the contact zone will increase due to a decrease in polymer modulus. In the case of PMMA this increase in contact zone did not... 

Though the time-dependent processes are very slow, they are affected by temperature in the same manner as the friction coefficient which controls short-range molecular motions many decades removed in the time scale, since temperature reduction can be made with the same WLF coefficients as in the transition zone. Moreover, they are affected in the same manner by the introduction of diluent, which reduces the friction coefficient in a manner attributable to an increase in fractional free volume (Chapter 17). 

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