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Tribo-material Properties
Published in Ahmed Abdelbary, Extreme Tribology, 2020
In a dry sliding, it is demonstrated that the interfacial shear strength inherently depends on the competition between the processes of detachment and re-attachment of the asperity junctions (micro-contacts) on the interface. The detachment and re-attachment of the asperities during sliding friction usually occurs in a very short time and it is highly influenced by the sliding velocity (Tian et al., 2016). At a low sliding velocity, the dynamics of the micro-contacts are creep-dominated if the stick time is long enough for the aging of asperities to play a leading role. On the other hand, when the sliding velocity is high enough, the asperities have no time to creep before slip occurs; the dynamics of stick-slip turns to be inertia-dominated. This may also give an explanation to the fact that the static coefficient of friction is higher than the dynamic coefficient. This is because the two surfaces in contact under a load tend to creep and comply with each other and increase the true contact area between them. The coefficient of friction is proportional to contact area, so more time in contact gives higher values.
Effect of Dust Contamination on Electrical Contacts
Published in Paul G. Slade, Electrical Contacts, 2017
As it is shown by J.B.P. Williamson [43], a pair of metallic surfaces in contact actually is two micro worlds in contact. The real contact area depends on the deformation of the micro peaks of two surfaces in connection; Influencing factors are the normal force, micro peaks height and their distribution, depth of micro valleys as well as the surface hardness of the metals. Therefore, from a micro point of view, there is a large gap or space between the two contact surfaces which can accommodate small particles. Figure 4.34, illustrates a new contact pair with average roughness Ra = 0.26 μm, 29 micro peaks are spread on a length of 149 μm (for simplicity, only x profile is considered). The average distance between peaks is 5.15 μm. Maximum peak height is 0.65 μm; maximum depth of micro valley is 0.82 μm. Assume the particle is a sphere, thus the largest particle size accepted by the contact pair can not exceed 1.6 μm. Practically the size of particles should be much smaller. When the contact surface has been worn out to make the surface rougher, accepted particle size can be larger.
Lubrication of Distribution Electrical Equipment
Published in Bella H. Chudnovsky, Electrical Power Transmission and Distribution, 2017
Lubricants reduce wear and heat between contacting surfaces in relative motion. While wear and heat cannot be completely eliminated, they can be reduced to negligible or acceptable levels, especially on sliding electrical contacts which see repetitive cycling or arc damage, two common causes of failures. Although evidence suggests that lubricants change or reduce arc patterns, the lubricant’s real job on sliding contacts is to separate the surfaces during operation and keep debris out of the contact area. Otherwise, the microscopic wear particles oxidize quickly, turning into insulators. Build-up of oxide particles also accelerates wear. In general, hydrocarbon lubricants work best at wear prevention because their molecular structure is more rigid than other base oils. Proper lubricants strike a balance between preventing wear and maintaining electrical continuity.
Effect of V-Groove Surface Pattern on the Tribological Properties of Epoxy
Published in Tribology Transactions, 2021
Byung Kook Kim, Kyeong-Hee Kang, Ming-Yu Gao, Jinseok Kim, Dae-Eun Kim
Figure 9 shows the variation in friction coefficient with respect to the contact area for both macro- and microscale contact conditions. In lubricated conditions, the friction coefficients for both macro- and microscale conditions decreased as the contact area increased. As explained earlier, this was because for PTFE particles to properly demonstrate the solid lubrication effect, a sufficient contact area was needed. In the case of dry sliding conditions for the microscale tests, the friction coefficient increased with increasing contact area, which generally followed the typical trend of frictional behavior of materials with respect to contact area. Overall, the lowest friction coefficient of 0.03 for the epoxy was obtained with the flat specimen under lubricated conditions at the macroscale.
Tribological properties of biodiesel: a review of recent advances
Published in Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2020
However, this rule appeared not to apply to tribopairs with coatings. When the Si3N4 material was coated with a single MCD layer, the wear coefficient reduced in the presence of a soybean biodiesel lubricant in conjunction with an increase in the applied load (Almeida et al. 2013).This result might be ascribed to the surface flaws on the materials with single-layered coatings. When the load is low, the contact area is fairly rough during the initiation period, and most of the load was carried by a small number of diamond asperities, resulting in a relatively high local contact pressure, as well as an elevated wear coefficient (Barros et al. 2000). As the load increases, continuous friction polishes the contact area, decreasing the irregularity of the surface and causing a dramatic reduction in the contact pressure, as well as the wear coefficient. Compared to Si3N4 coated with a single MCD layer, the Si3N4 materials with multilayer coatings display a more constant wear coefficient, which may be explained by the more even surface of the multilayer coatings.
A numerical-analytical approach to determining the real contact area of rough surface contact
Published in Tribology - Materials, Surfaces & Interfaces, 2020
This phenomenon is caused by the different ways SAM calculates the contact pressure and real contact area. The contact pressures are calculated directly by SAM, as described in Section 2, while the errors come mainly from the inadequate input information. This issue can be resolved by increasing the resolution of input. The calculation of the real contact area is, however, indirect: the algorithm first determines the contact pressures on each grid, and the grids with non-zero contact pressures will be marked as ‘in contact’. Then the contact area will be calculated by the total area of the grids in contact. However, using rectangle grid size, the grids will usually overly cover the contact boundaries. This ‘over covering’ effect leads to larger errors in calculating the real contact area values.