In order to understand the acting wear mechanism in a given tribological system, with the aim of validating the laboratory tests or in the failure analysis of real components, the observation of the worn surfaces (or wear tracks) is recom- mended. The observation of the wear fragments, if available, can be also very useful. Subsequently, the subsurface regions can be possibly observed, on carefully prepared metallographic cross-sections. The latter operation can be easily carried out in laboratory investigations on relatively small specimens, whereas it may be more difficult in the failure analysis of real components. Table4.2lists some of the most used techniques for the characterization of the worn surfaces, the wear frag- ments and the subsurface damaged regions.
In the case of adhesive wear, very useful information is obtained from the observations carried out using an Optical Microscope (OM), or in a Scanning Electron Microscope (SEM), especially in the Back-Scattered Electron (BSE) mode. As an example, Fig.2.20a shows the wear surface of a steel after dry sliding against a bronze. The occurrence of adhesive wear is clearly demonstrated by the
presence of transferred fragments that have a plate-like shape. The occurrence of transfer is also well evidenced by SEM observations in BSE mode or using the EDXS analysis (see, for example, Fig.2.29).
Visual inspection and OM observations are very fruitful also for detecting wear by tribo-oxidation. As an example, Fig. 4.21 shows the OM planar view of the surface of a steel that underwent tribo-oxidative wear at low sliding speed. The presence of dark scales of compacted oxides can be clearly appreciated. In some cases such dark scales or fragments are simply detected by naked eye.
The presence of grooves on the wear surface of specimens or components is a clear indication of abrasive wear. Such grooves can be detected by OM, as shown in Fig.4.10b. Such an operation can be easy accomplished when the grooves are all aligned along the same direction. It is much more difficult when they are produced by particles moving in different directions.
Figure4.14 shows a steel surface damaged by contact fatigue, observed in a SEM. This technique allows observing surfaces with a high depth of focus, and it is
Table 4.2 Most used techniques to characterize the wear damage Worn surfaces (or, wear tracks) Visual inspection Optical microscopy Electronic microscopy (SEM) equipped with micro- analysis (EDS) Special techniques (such as XPS) Wear fragments (o, debris) Visual inspection Optical microscopy Electronic microscopy (SEM) equipped with micro- analysis (EDS) X-ray diffraction (XRD). Transmission electron microscopy (TEM). Special techniques Sub-surface regions Optical microscopy Microhardness profiles Special techniques (such as SIMS) Fig. 4.21 OM observation of the wear track of a steel after tribo-oxidative wear [27]
therefore very useful in detecting spalled layers and pits produced by contact fatigue. An evidence of the occurrence of this kind of damage is also given by the limited, if any, presence of plastic deformations in the damaged areas. Indeed, in all other cases of wear damage, extensive plastic deformation is present.
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Wear Processes
In the study of the wear failures, it is usual to consider the wear processes defined with reference to the type and geometry of relative motion between two mating surfaces (Fig. 4.1). In general, each wear process is due to one (or more) wear mechanisms. Table5.1lists some examples of tribological systems with the main wear mechanism. In the next paragraphs, the characteristics of the main wear processes will be outlined, including the methods to control them. The role of materials will be also indicated, although more detailed information on the mate- rials’ selection and surface engineering in tribology will be given in the next two chapters.