Asperity wear mechanism

The dominant role of asperity contact is also apparent from analysis of the texture of polished surfaces. Figure 9 illustrates a typical post CMP surface as examined via atomic force microscopy (AFM). Surface texture is composed of innumerable randomly oriented nanogrooves of a width and depth consistent with traveling Hertzian loaded contact [12] particle bombardment during turbulent liquid flow produces profoundly different texture. All classes of semiconductor materials examined show similar textures, indicating the general nature of the process. From the data to date, it appears that asperity contact is the dominant wear mechanism in CMP. 

Abrasive wear occurs when asperities of a rough, hard surface or hard particles slide on a softer surface, and damage the interface by plastic deformation or fracture in the case of ductile and brittle materials, respectively. In many cases, the wear mechanism at the start is adhesive, which generates wear particles that get trapped at the interface, resulting in three-body abrasive wear. In most abrasive wear situations, scratching is observed with a series of grooves parallel to the direction of sliding.75... 

Percussion is a repetitive solid body impact, such as experienced by print hammers in high-speed electromechanical applications and high asperities of the surfaces in a gas bearing. Repeated impacts result in progressive loss of solid material. Percussive wear occurs by hybrid wear mechanisms, which combine several of the following mechanisms adhesive, abrasive, surface fatigue, fracture, and tribochemical wear.75... 

The governing principle of pad conditioning is to introduce friction between the polishing pad and the diamond disc, which characterizes a two-body abrasive wear mechanism. As illustrated in Figure 13.3, the diamond abrasives embedded on the disc create microscopic cuts or furrows on the pad surface to continually regenerate new pad surface and asperities. At the same time, they remove the glazed or accumulated particles on the polishing pad surface. 

For the discussion of the wear mechanisms, it is worth to consider the contact and stress states developed between a counterpart asperity (e.g., a diamond... 

This chapter will also describe new methods for exploring the micro- and nanomechanical processes of wear. Here, a surface deformation mechanics analysis will be shown where the surface strains associated with single asperity wear behavior can be determined and related to both pol5mier structural changes (e.g., orientation of crystallites) and to changes in mechanical properties of the surface deformed UHMWPE (orientation softening) when... 

This method of wear asperity deformation measurement can be combined with other measurements to more fully describe local interactions. For example, DSI testing can be performed over the span of the asperity wear track, and the local mechanical properties can be directly correlated with the measured strain. Finally, surface structural characterization can be performed by a combination of methods from etching, FTIR, and other techniques to relate surface strains with reorientation of the crystals, for example. 

It can be seen that the peak in the wear-based strain arises partway between the outer contact point and the center of the asperity wear track (i.e., where the gradient in the displacement field is a maximum). The surface strain arising from the asperity-wear interaction results in a decrease in the hardness (see Figure 33.15) as well as the modulus and EDF (data not shown). That is, there is a strain softening process whereby the mechanical properties of modulus, hardness, and EDF decrease substantially (upwards of 35% in some UHMWPEs) as a function of the strain imparted by the asperity. 

This combination of single asperity wear testing, surface deformation mapping of the resultant deformation field, and the subsequent indentation test measurements of the changes in mechanical properties due to the wear process will provide detailed and significant information of the interrelationships between wear, structure, and property evolutions. These observations may also help discern the fundamental deformation mechanisms that result in wear particle formation and how different UHMWPE starting materials evolve with wear deformation. 

Wernle J, Gilbert JL. Three dimensional strain mapping of single asperity wear in UHMWPE effects of load and material on surface mechanical properties. Biomaterials 2008. In Press. 

Wong BKP, et al. Nano-wear mechanism for ultra-high molecular weight polyethylene (UHMWPE) sliding against a model hard asperity. Tribal Lett 200A n(3) 6l3-22. 

Figure 8 summarises where and how the four types of wear particles are produced. Types 2, 3 and 4 correspond to three modes of abrasive wear processes, i.e. ploughing, wedge formation, and cutting [9]. l pe 1 is distinctive of repeated contact of two asperities. Because practical sliding systems inherently suffer from multi-asperity contact, mechanisms involved in this wear mode may be very important. 

As the roughened ball would provide multiple asperity contacts, it was expected that parallel grooves would be formed on the aluminium surface and Type 1 wear would occur at ridges between the grooves. However, the particle analysis suggests that Type 1 is not the dominant wear mechanism. 

If the surfaces are damaged during sliding so that wear debris and multi-asperity contacts are involved in the process, the mechanism of friction will be substantially different from what we discussed for wearless friction. 

Mechanism. Basically, fretting is a form of adhesive or abrasive wear, where the normal load causes adhesion between asperities and oscillatory movement causes ruptures, resulting in wear debris. Most commonly, fretting is combined with corrosion, in which case the wear mode is known as fretting corrosion. For example, in the case of steel particles, the freshly worn nascent surfaces oxidize (corrode) to FejO, and the... 

Modeling fretting corrosion. An equation has been used for steel to evaluate the loss of weight W caused by fretting corrosion based on a model that combines the chemical and mechanical effect of the corrosion by fretting. The chemical factor concerns the oxidation that occurs at the time of wear, corresponding to adsorption of oxygen to form the oxide. The mechanical factor concerns the loss of particles, at the asperities on the opposite surface. 

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