Phenomenological Wear

By phenomenological wear we mean wear as it is actually observed and measured. The behavior encountered ranges from the crude qualitative observations of ordinary experience to the highly sensitive measurements of rigorously controlled laboratory experiments. Since our major interest in this discussion is in the fundamental and scientific aspects of wear, the treatment will be focused on quantitative behavior. 

It makes no essential difference, as far as the basic behavior is concerned, whether the change is fast or slow, desired or undesired, connected with a service function or unconnected. 

Type of motion Engineering example Laboratory example 

Pure sliding Journal in bearing Piston/cylinder Cam/fixed follower Pin on disk Pin on ring Block on ring 

Pure rolling Wheel on rail Cam on rolling follower Ring on ring (same diameters) 

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Most instances of phenomenological wear involve the interaction of two or more of the mechanistic processes described above, sometimes in orderly sequence, sometimes in competition. Although the quantitative outcome of a particular instance of wear is strongly influenced by the individualistic features of that particular case, nevertheless a number of useful generalities can be formulated. 

As an example of how wear behavior with a changing rate can be treated, let us examine a model for the competition of metal removal from a slider by direct adhesive transfer and by combined oxidation and ablation of the oxide. The total rate of phenomenological wear is given by... 

The rate expressions of Section 14.2.1 are for the most part not particularly well suited for direct application to phenomenological wear and require further treatment to adapt them to specific experimental procedures for measuring wear. In the case of Eqn 14-22a, which was originally derived on the basis of unit apparent conjunction area [9], q is the volume of material removed per unit apparent area in unit time and the expression has the dimension 1/i. therefore is... 

Ion Interaction. Ion-interaction theory has been the single most noteworthy modification to the computational scheme of chemical models over the past decade this option uses a virial coefficient expansion of the Debye-Huckel equation to compute activities of species in high ionic strength solutions. This phenomenological approach was initially presented by Pitzer ( ) followed by numerous papers with co-workers, and was developed primarily for laboratory systems it was first applied to natural systems by Harvie, Weare and co-workers (45-47). Several contributors to the symposium discussed the ion interaction approach, which is available in at least three of the more commonly used codes SOLMNEQ.88, PHRQPITZ, and EQ 3/6 (Figure 1). 

This is our definition of mechanical wear. It is essentially phenomenological, and in contrast to a purely conceptual model based on a pKlon. assumptions and inexorable logical reasoning, the governing conditions and conclusions of this definition are in terms of observable behavior. Nevertheless the definition is fundamental enough to fit all... 

The preceding chapter was devoted to the development of the fundamental concepts of mechanical wear and to the examination of generalized wear phenomenology in relation to these concepts. The same basic mechanistic processes govern both unlubricated and lubricated wear. The role of the lubricant in lubricated wear is essentially to ameliorate wear by modifying the extent and the rates of those basic processes that exert the critical influences in whatever particular case is under examination. Unlubricated wear is not necessarily simpler or more elementary than lubricated wear in fact, unlubricated wear is more likely to be so destructive under severe conditions that it may be effectively impossible to specify what occurred at a given stage of the wear process. 

Such being the case, further inferences about the nature of the wear process follow. A disrupted fluid film allows localized contacts at the rubbing surfaces, and it is the mechanistic processes at these contacts that determine the course of lubricated wear. When the wear process is abrasive, it is most likely influenced directly by fluid film thickness and surface roughness, whereas processes such as adhesion, transfer, oxidation, additive reaction and the like are responsive to surface conditions at the contacts as well as to the number of contacts. These are the aspects of lubricated wear that are emphasized in this chapter, from the viewpoint of phenomenology, mechanisms and modeling. 

Unfortunately surface wear of an implant results from its use, and therefore, cannot be avoided or eliminated. Because wear is a limiting factor in the successful outcome and lifetime of an implant, it is of the utmost importance to characterize the wear resistance of materials used in implant design, and the effect of the design on wear. The volume of material removed from surfaces in specific tribosystems as a result of wear processes has been described phenomenologically and estimated by different models (Table 7.2). Several experimental wear studies have been... 

A phenomenological description for tribocorrosion is developed by including electrochemical corrosion aspects into a third body approach. The developed third body approach stresses the relevance of electrochemical phenomena such as dissolution or formation of stable oxides for the overall wear process. Examples of electrochemical control of wear in engineering and laboratory systems are presented and discussed. 

Corrosion is an irreversible surface modification of a material due to chemical reaction with the environment that results in the formation of metal ions dissolved in the liquid (material loss) and, in the case of passive metals, of surface oxide films. A preliminary attempt to include particle flow in tribocorrosion was already proposed by Stemp [11] and Mischler et al [9] to explain the discrepancy mentioned above between first body degradation and mechanical wear. This paper is aimed at developing a phenomenological model of tribocorrosion by combining electrochemical corrosion effects with the third body concept of wear. The approach is applied to three electrochemically controlled wear situations, i.e. wear under cathodic protection (absence of corrosion), wear in presence of passive films and wear combined with metal dissolution. The proposed concepts are compared to already published results concerning carbon steel and stainless steels and their merits are discussed. 

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