Friction measurements

In a typical friction measurement, a slider is pressed against a stationary block, and the force required to move it is measured. This force generally comprises two terms the first is the force F required to shear the junctions at the points of actual contact given by... 

Kumacheva E 1998 Interfacial friction measurements in surface force apparatus Prog. Surf. Sc/. 58 75... 

Overney R M, Meyer E, Frommer J, Brodbeck D, Luthi R, Flowald L, Guntherodt Fl-J, Fu]ihara M, Takano FI and Gotoh Y 1992 Friction measurements of phase separated thin films with a modified atomic force microscope Nature... 

One of the most important extensions is the measurement of lateral forces (friction). Friction measurements have accompanied tlie SFA teclmique since its early begiimings in the Cavendish laboratory in Cambridge [37] and a variety of different lateral force measurements are practised throughout tlie SFA conmumity. 

The atomic force microscope (ATM) provides one approach to the measurement of friction in well defined systems. The ATM allows measurement of friction between a surface and a tip with a radius of the order of 5-10 nm figure C2.9.3 a)). It is the tme realization of a single asperity contact with a flat surface which, in its ultimate fonn, would measure friction between a single atom and a surface. The ATM allows friction measurements on surfaces that are well defined in tenns of both composition and stmcture. It is limited by the fact that the characteristics of the tip itself are often poorly understood. It is very difficult to detennine the radius, stmcture and composition of the tip however, these limitations are being resolved. The AFM has already allowed the spatial resolution of friction forces that exlribit atomic periodicity and chemical specificity [3, K), 13]. 

The shape of AFM cantilevers (much thinner than the width) results in torsional deflection when forces push the tip laterally as in friction measurements (when the tip is sliding) or lateral stiffness measurements (when the tip is stuck). 

Fig. 17—Floating device for friction measurement [48]. 1—Carrier of strain gage 2—Strain gage 3—Beam 4—Plank 5—Steel ball 6—Oil cup 7—Mandril.

Alsten, J. V., and Granick, S., "Friction Measured with a Surface Forces Apparatus, Tribol. Trans., Vol. 32,1989,pp. 246-250. 

Another remarkable feature of thin film rheology to be discussed here is the quantized" property of molecularly thin films. It has been reported [8,24] that measured normal forces between two mica surfaces across molecularly thin films exhibit oscillations between attraction and repulsion with an amplitude in exponential growth and a periodicity approximately equal to the dimension of the confined molecules. Thus, the normal force is quantized, depending on the thickness of the confined films. The quantized property in normal force results from an ordering structure of the confined liquid, known as the layering, that molecules are packed in thin films layer by layer, as revealed by computer simulations (see Fig. 12 in Section 3.4). The quantized property appears also in friction measurements. Friction forces between smooth mica surfaces separated by three layers of the liquid octamethylcyclotetrasiloxane (OMCTS), for example, were measured as a function of time [24]. Results show that friction increased to higher values in a quantized way when the number of layers falls from n = 3 to n = 2 and then to M = 1. 

Though most experiments get similar results, some special results are also obtained in the measurements, e.g., the result shown in Fig. 31. The surface roughness of the point in the experiments of Fig. 31 is a little larger than for most other parts of the film. This shows that the topography of the L-B film plays an important role in the friction properties obtained by using FFM measurement. Therefore having the well-prepared and highly ordered L-B film is one important factor for friction measurements. However, results like the one shown in Fig. 31 can still be accepted. Because there are so many factors that can affect the experiment results at the nanometer scale, more experiments need to be done. 

Ruan, J. and Bhushan, B., Atomic-Scale Friction Measurements Using Friction Force Microscopy Part 1. General Principles and New Measurement Techniques, ASME J. Tribol., Vol. 116,1994, pp. 378-388. 

Suk, M. and GUIs, D. R., Comparison of Friction Measurement Between Load/Unload Ramps and Suspension Lift Tabs Using Strain Gage and Actuator Current, IEEE Trans. Magn., Vol. 36,2000, pp. 2721-2723. 

Choi, J., Ishida, T, Kato, T, and Fujisawa, S., Self-Assembled Monolayer on Diamond-Like Carbon Surface Formation and Friction Measurements, Tribol. Int., Vol. 36, 2003, pp. 285-290. 

Practically, aU data of friction measurements on wet tracks in the speed range of hydrodynamic lubrication exist as tire skid measurements. Figure 26.10 shows the results of a braking test on wet, finely structured concrete using a smooth tire and measuring the friction coefficient as function of... 

It is quite clear from these data that intrinsic resistance to dislocation motion in these metals does not determine their indentation hardnesses. Internal friction measurements have yielded similar results. Therefore, extrinsic factors need to be considered. 

Critical phenomena of gels have been studied mainly by dynamic light scattering technique, which is one of the most well-established methods to study these phenomena [18-20]. Recently, the critical phenomena of gels were also studied by friction measurement [85, 86] and by calorimetry [55, 56]. In the case of these methods, the divergence of the specific heat or dissipation of the friction coefficient could be monitored as a function of an external intensive variable, such as temperature. These phenomena might be more plausible to some readers than the divergence of the scattered intensity since they can observe the critical phenomena in terms of a macroscopic physical parameter. 

In the fatigue tests, the friction measured in the initial period of sliding during which no wear occurred was not affected significantly by the addition of siloxane or ATBN and CTBN modifiers to the epoxy as shown in Fig. 15 for ION load. At lower loads, the friction coefficients are higher but still show no significant change with the additions of the modifiers. 

The results of the friction measurements are shown in Fig. 8. The friction coefficient of the poly(acrylamide) gel decreases with an increase of the crosslinker concentration in the major part of the concentration range studied, except that it sharply decreases at a mole fraction of 0.2% of the cross-linker concentration. The measurements cannot be performed below this concentration because the gel becomes too soft to be used in the present experimental setup. It suggests that the system is dose to the gelation threshold. The sharp decrease of the friction observed in the gel with a mole fraction of 0.2% may be due to the effect of the sol-gel transition. It is also found from visual inspection that the gels becomes opaque above a cross-linker concentration of 3 mole-% fraction. 

During the friction measurement, a test tube containing a small piece of gel is placed in the same water bath and the appearance of the gel is continuously monitored. The decrease of the diameter of the free gel from the isochore diameter is less than 10% at the highest temperature of the present friction experiment. This observation confirms that the gel swelling or shrinking as a whole should not have a significant effect on the friction measurement. A slight opacity of the gel develops in the gel in the vicinity of the temperature at which the friction of the gel diminishes and is presumably due to the dynamic density fluctuations. 

An important part of designing a friction measurement apparatus is choosing the probe size, shape, and material. Because friction is an interaction between two surfaces, the probe geometry and material will affect the values calculated for the friction coefficient of the other surface. Also, results will be more accurate when the probe s normal force is maintained at a constant value or continuously monitored previous methods used to maintain the normal force include spring mechanisms or static weights to weigh down the probe. These parameters are revisited critically later in this article. 

Much effort has been spent in understanding how skin friction changes with differing biological conditions and upon the application of various products to the skin surface. These studies have been of interest to various industries that manufacture products meant as skin topical agents because friction measurements can provide clues regarding the effectiveness of their products. 

Friction measurements can offer quantitative insight into changes on the skin surface and the UMT offers technical advances over existing friction measurements. The control of the probe speed and the real-time monitoring of the normal load allow for real-time calculation of the friction coefficient. As seen in Figure 32.1, the control of the load is important because the friction coefficient does not adhere to Amonton s Law. Wolfram18 theoretically deduced that the friction coefficient would relate to the normal load as follows ... 

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