Wear modes

Fig. 1. Schematic showing typical wear modes on a cutting tool.

The two predominant wear modes of cBN tools are DCL notching and microchipping. PolycrystaUine cBN tools exhibit flank wear where alumina... 

Sample frequency is a function of the mean time to failure from the onset of an abnormal wear mode to catastrophic failure. For machines in critical service, sampling every 25 hours of operation is appropriate. However, for most industrial equipment in continuous service, monthly sampling is adequate. The exceptions to monthly sampling are machines with extreme loads. In this instance, weekly sampling is recommended. 

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... 

Testing. The measurement of corrosion, wear, and corrosion-wear interactions as well as erosion-corrosion interactions is a multistep process. Each component of the interaction must be measured separately. The results may then be combined to identify the synergistic effects and create a complete picture of the damage process. Measurement of the interaction between corrosion and wear modes or damage is more difficult. The standard (ASTM, G119)4 applies to systems in liquid solutions or slurries and some aspects of it can be adapted to dry corrosion and wear interactions as well. (Tylczak and Adler)5... 

The relevance of bulk fracture properties has therefore been considered essentially within the context of cohesive wear modes such as abrasive and fatigue wear. During abrasive wear, the initial stage is considered to be the process of contact and scratch between the polymer surface and a sharp asperity. The accumulation of the associated microscopic failure events eventually generates wear particles and gives rise to weight loss. Early approaches initiated by Ratner and co-workers [15] and Lancaster [16] attempted to correlate the abrasive wear rate with some estimate of the work to failure of the... 

Whether CMP occurs as Hertzian indentation or fluid-based wear is not clear and has been the subject of some debate. The difference between the two wear modes is in the slurry fluid layer between the pad and wafer. As discussed in Section 4.2, if the fluid layer is not continuous, then pad-wafer contact occurs. Note, however, that the pad does not contact the wafer surface directly, but rather the pad presses abrasive particles against the surface. In such instances, the pad will drag the abrasives across the surface, resulting in Hertzian indentation. 

As will be discussed in Chapter 5, scuffing is one of the abrasive wear modes that often happens in metals. This equation is known as Blok s critical contact... 

Pad wear occurs during pre-CMP conditioning and during CMP. Pad life is also related to the chemical attack of water molecules, transferring wafer elements. Pad wear is mainly dominated by mixed wear modes. 

When polymers slide on machined metal surfaces, it is quite possible that steady-state wear Involves a combination of abrasive, fatigue, and adhesive wear mechanisms. To study fatigue wear, it would be desirable to minimize the contributions of the abrasive and adhesive wear modes. In this paper, the following polymers polycarbonate, polyvinyl chloride, ultra-high molecular weight polyethylene, siloxane modified epoxies, and polylmldes are tested in experiments in which the fatigue wear mode is predominant. 

The wear characteristics of polytetrafluoroethylene (PTFE) have been widely studied it is an important commercial polymer. This special attention has sometimes created a thesis that this polymer has very unusual or special wear characteristics when compared with the response of other polymers. This review compares the wear behaviour of PTFE with that of a range of pol37mers and examines the basis of this belief. The experimental evidence indicates that it is only in the area of transfer wear that a major contrast in characteristics is seen. Even in this restricted wear mode the differences are arguably ones of extent and not kind. 

Despite the fact that the peculiarities of metal wearing in contact with polymers were determined more than 30 years ago [35], the serviceability of metal-polymer friction joints has resisted estimates for a long time. The origin of this corrosion wear mode was considered only for plastics processing equipment, in which metals are in contact with the moving polymer melt [36]. 

Chemical reactions on the friction surfaces may in fact appear in any kind of media, although their effect on the friction behavior and wear of the friction pair is often far from apparent. Chemical wear in its broadest sense could mean abrasive wear of the tribocomponents in a corrosive liquid [1,4]. Nevertheless, it would be unfair to relate chemical wear to an individual wear mode since it is just one of the contributing processes exerting a significant effect on other wear modes. 

In his early classical work Welsh [10] reported the friction coefficient and wear rate variations for the mild-severe-mild wear transition as a function of the load growth (Fig. 4.1). The rise in local temperatures on loading P2 leads to transformation of the surface layer material into a martensite that forms an oxide layer. This reduces the friction coefficient and abruptly returns the tribosystem into the mild wear mode. 

Experimental Result and Discussion 15.3.1 Wear Mode of Abrasive-Grinding Wheel... 

When surface of the grinding wheel was not covered with an oxide film, either fine powder-like wear particles are formed by the propagation of microcracks (Hokkirigawa, 1991) or no t) ical wear particle is formed. Such a wear mode is referred to as plowing or powder formation. 

When the surface of grinding wheel is covered with an oxide film, wear intensity is caused by a brittle fracture, during which large flake-like wear particles are formed by a surface crack propagation (Kitsubnai et al., 1991). We call this wear mode as flake formation. 

Engel, P.A. 1993. Failure models for mechanical wear modes mechanisms. IEEE Trans, on Reliability 42(2) 262-267. 

Unpredictable interactions can result between wear and corrosion when the surfaces in contact have complex, multiple-phase microstructures that can lead to microgalvanic activity and selective phase corrosion (a localized attack), as well as three-body wear modes. Examples of such surfaces include composites or surfaces that undergo compositional changes induced by tribological interactions. For instance, the presence of carbides in a metallic surface, typically formed for improved wear resistance, establishes a microgalvanic corrosion cell as the carbide is likely to be cathodic with respect to the surrounding metallic matrix [4]. This can result in a preferential anodic dissolution of the metallic matrix close to or at the matrix/carbide interface, and thereby accelerate carbide removal from surfaces and reduce the antiwear properties of the surface. 

Erosion is one of several wear modes involved in tribocorrosion. Solid particle erosion is a process by which discrete small solid particles, with inertia, strike the surface of a material, causing damage or material loss to its surface. This is often accompanied by corrosion due to the environment. A major environmental factor with significant influence on erosion-corrosion rates is that of flow velocity, but this should be set in the context of the overall flow field as other parameters such as wall shear stress, wall surface roughness, turbulent flow intensity and mass transport coefficient (this determines the rate of movement of reactant species to reaction sites and thus can relate to corrosion wall wastage rates). For example, a single value of flow velocity, referred to as the critical velocity, is often quoted to represent a transition from flow-induced corrosion to enhanced mechanical-corrosion interactive erosion-corrosion processes. It is also used to indicate the resistance of the passive and protective films to mechanical breakdown [5]. 

Amplitude Fretting wear volume increases with amplitude. Above about 100 pm the rate increases owing to a change in the wear mode. Wear rate can go to zero at veiy low amplitudes if motion is purely elastic. 

The above test methods do not exactly simulate the actual fretting conditions. The surface damage produced by the test should be compared to the damage on parts damaged in service. If there is significant difference between scar morphologies, the wear mode may not be correctly simulated in the test. Corrections can be made in contact geometry, contact stress, and amplitude and velocities of movement. 

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