Testing friction, coefficient

Boundary friction coefficients were measured at temperatures between 40 °C and 140 C using a High Frequency Reciprocating Rig (HFRR), as shown schematically in Figure 5. The test comprises a steel ball reciprocating against a steel disc at 20 Hz with a stroke length of 1 mm. A 4 N load was used in all tests. Friction coefficient and temperature are measured as a function of time. 

Methods for determining fiber-to-fiber friction have been developed (29—31). The friction coefficient can also be measured in terms of the force required to pull entwined fibers apart (32—34) or the force necessary to remove a single fiber from a mass of fibers under pressure (35). Another test involves an apparatus wherein one or a series of parallel fibers are mounted across a small bridge similar to a violin bridge. This is then pressed against a surface that may be another fiber or some other material, and the fibers alternately sHp and stick as they sHde across each other (36,37). 

Friction and Adhesion. The coefficient of friction p. is the constant of proportionality between the normal force P between two materials in contact and the perpendicular force F required to move one of the materials relative to the other. Macroscopic friction occurs from the contact of asperities on opposing surfaces as they sHde past each other. On the atomic level friction occurs from the formation of bonds between adjacent atoms as they sHde past one another. Friction coefficients are usually measured using a sliding pin on a disk arrangement. Friction coefficients for ceramic fibers in a matrix have been measured using fiber pushout tests (53). For various material combinations (43) ... 

Friction coefficients will vary for a particular material from the value just as motion starts to the value it attains in motion. The coefficient depends on the surface of the material, whether rough or smooth, as well as the composition of the material. Frequently the surface of a particular plastics will exhibit significantly different friction characteristics from that of a cut surface of the same smoothness. These variations and others that are reviewed make it necessary to do careful testing for an application which relies on the friction characteristics of plastics. Once the friction characteristics are defined, however, they are stable for a particular material fabricated in a stated manner. 

The concentration of UDP also affects the friction coefficient as shown in Fig. 39. It is discovered that the friction coefficient of pure PEG also decreases gradually and reaches a somewhat reduced value due to a time effect of the film thickness [16,18]. At the speed of 2 mm/s and pressure of 174 GPa, the friction coefficient of pure PEG is the highest. That for PEG + 0.5 % UDP ranks second. Those for PEG + 0.1 % UDP and PEG+ 0.3 %UDP are almost the same and have the lowest friction coefficient among all tested oils. Therefore, there is a good concentration extent of UDP in the basic oil. If the concentration is out of such extent, the effect... 

Table 2 lists the results of the friction coefficient derived by different researchers under different test conditions [71,73,75-80]. From the table, it can be seen that the CNx films prepared by different techniques did not demonstrate surprising frictional behavior. 

The friction coefficient can be easily obtained in traditional tribological test. However, in the microscale friction test, the relative moving status of the tip is greatly affected by the surface morphology, and the friction coefficient should be obtained by theoretical analysis according to the experimental condition. A model shown in Fig. 4 is set up to analyze the relative moving status of the tip. 

The same nano scratch tester was used to carry out the friction tests. The Rockwell diamond tip (radius 2 /u.m) was used to draw at a constant speed 3 mm/min across the sample surface under a constant load of 20 mN for which no scratches occurred for all the samples. Feedback circuitry in the tester ensures the applied load is kept constant over the sample surface. The sliding distance is 3 mm. The friction coefficient is defined normally as the ratio of the friction force and the applied load. 

The constant applied load is only 20 mN so there were no crack formations occurring during the friction tests of all the samples though there is penetration depth on the sample surface. The friction coefficient reflects the surface feature of H DLC samples. Figure 37 shows the friction coefficient of all samples. Sample 2 has the lowest friction coefficient compared to other samples which have the friction coefficient in a range of 0.049-0.065. 

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... 

Dekempeneer et al. [96] studied the wear behavior of a-C(N) H films deposited by RFPECVD in CH4-N2 atmospheres, with up to 13-at.% N. The wear tests were done in a ball-on-disk tribometer, under air, at fixed 50% relative humidity. The initial friction coefficient was about 0.2 for all samples, and the wear life of the... 

The nontreated rubber samples began losmg the initial friction coefficient value during this test after 0.5 h... 

Result A friction coefficient less than or equal to 0.175 indicates the fuel passes the lubricity evaluation. A friction coefficient greater than 0.175 indicates the fuel fails the lubricity evaluation. Wear scar diameter on the test ball is determined. A wear scar diameter of 440pm or less is considered acceptable for distillate fuel. 

For all materials, the adhesive mechanism and the plastic deformation should be the main processes. During testing, the variation in friction coefficient values could be influenced by the specific AM behavior. Since AM corresponds to the crosslink density of a composite, for a qualitative assessment it can be concluded that the crosslink density decreases with increasing absorbed dose. PTFE500kGy-EPDM showed much lower AM and f90 values. It can be inferred that the state of cure is strongly dependent on the irradiation dose absorbed by the PTFE powder. 

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. 

In the case of internal flows extensive experimental data are available for turbulent pipe flow. The study of turbulent-friction coefficients in pipe flow has brought forth a number of effects displayed by flowing polymer solutions. Furthermore, many hydro-dynamic investigations in pipe flow have been made to elucidate the flow behavior (laminar and turbulent) of Newtonian fluids. Thus, the pipe is one of the most investigated and traditional pieces of test apparatus and one can easily compare the flow behavior of Newtonian fluids and polymer solutions under constant boundary conditions. 

Plane strain compression tests (Fig. 12.2b) may be also used. One principal dimension remains constant thus a = vo2, where a2 ar d 03 are the principal stresses in the three directions x, y and z. This test has to be performed with care, due to different breadth dies, and friction coefficients, but it is nevertheless used to complete the yielding criteria curves (see below). 

To understand the wear mechanism in valve train wear tests, samples of the worn tappet surface were analyzed for surface elements by electron probe microanalysis (EPMA) and X-ray photo electron spectroscopy (XPS). Results of EPMA analysis of the worn surface in terms of concentration of phosphorus and sulfur atoms for oil with primary ZnDDP without MoDTC, showed an increase of zinc and sulfur intensity after 100 hrs of test time, in spite of decreasing phosphorus intensity. Examination of the worn surface by XPS with primary and secondary ZDDP with addition of MoDTC showed the presence of MoS2 in the tribofilm. Using mixtures of ZDDP and MoDTC, the friction coefficient is reduced, and wear is comparable to that of using ZDDP alone (Kasrai et ah, 1997). 

This dynamometer engine test is milder than the in-vehicle-use testing and modified Sequence VIA testing. This is indicated by the fact that 75% of the MoDTC and 79 % of the antioxidant capacity remained at the end of the sequence VIA compared to 30%, 48% and 10%, 26%, respectively in the vehicle and modified sequence VIA tests. Also, ZDDP was not completely depleted as it was in other tests and the friction test showed a low friction coefficient (pi 0.04). The Sequence VIA aging cycle to stabilize the tested oil is milder than running 6,672 km of vehicle aging. 

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