Hisham A. Abdel-Aal

On the Influence of Thermal Properties on Wear Resistance of Rubbing Metals at Elevated Temperatures

This paper studies the effects of the heat dissipation capacity of a rubbing material on wear resistance at high temperatures. These effects are studied with a special focus on the dilatation of the thermal energy in sliding. These results suggest a connection between wear transition and the change in the heat dissipation capacity of a rubbing material. The nature of change in the thermal properties before and after the transition influences the thermal environment within the contacting layers. This controls the kinetics of oxide formation and thereby controls wear. Thermal dilatation is a super position of three functions that represent the competing effects of the room temperature thermal properties and their respective variation with temperature. Whence, the change in the thermal properties bears on dilatation. The possible relation between thermal dilatation and protective oxide formation is studied by examining the fretting wear data for two alloys-a Ni-based alloy and a Co-based alloy. The results indicate a strong correlation between the formation of protective oxides and the change in the thermal dilatation of the examined alloys with temperature. Moreover, examination of the wear data suggests that a critical ratio between the effects of the conductivity and those of the effusivity has to be established for favorable wear resistance. This ratio reflects on the intrinsic ability of the material to sustain an oxidative reaction of a controlled rate. So that, the protective glaze oxide layers are formed in a rate that is approximately equal to the rate of oxide layer breakdown. Whence, continuous compensation for the removed oxide layer (self-repairing oxides) is established.

Characterization of load bearing metrological parameters in reptilian exuviae in comparison to precision-finished cylinder liner surfaces

Design of precise functional surfaces is essential for many future applications. In the technological realm, the accumulated experience with construction of such surfaces is not sufficient. Nature provides many examples of dynamic surfaces worthy of study and adoption, at least in concept, within human engineering. This work probes the load-bearing metrological features of the ventral skin in snakes. We examine the structure of two snake species that mainly move by rectilinear locomotion. These are Python regius (Pythonidae) and Bitis gabonica (Vipridae). To this end, we focus on the load-bearing characteristics of the ventral skin surface (i.e., the Sk family of parameters). Therefore, detailed comparison is drawn between the metrological structure of the reptilian surfaces and two sets of technological data. The first set pertains to an actual commercial cylinder liner, whereas the second set is a summary of recommended surface finish metrological values for several commercial cylinder liner manufacturers. The results highlight several similarities between the two types of surfaces. In particular, it is shown that there is a striking correspondence between the sense of texture morphology within both surfaces (although their construction evolved along entirely different paths). It is also shown that reptilian surfaces manifest a high degree of specialization with respect to habitat constraints on wear resistance and adhesive effects. In particular, their surface displays a high degree of pre-conditioning to functional requirements, which eliminates the need for a running-in period.

Surface structure and tribology of legless squamate reptiles

Squamate reptiles (around 10,000 species of snakes and lizards) comprise a myriad of distinct terrestrial vertebrates. The diversity within this biological group offers a great opportunity for customized bio-inspired solutions that address a variety of current technological problems especially within the realm of surface engineering and tribology. One subgroup within squamata is of interest in that context, namely the legless reptiles (mainly snakes and few lizards). The promise of that group lies within their functional adaptation as manifested in optimized surface designs and locomotion that is distinguished by economy of effort even when functioning within hostile tribological environments. Legless reptiles are spread over a wide range in the planet, this geographical diversity demands customized response to local habitats. Customization, in turn, is facilitated through specialized surface design features. In legless reptiles, micro elements of texture, their geometry and topological layout advance mitigation of frictional effects both in locomotion and in general function. Lately, the synergy between functional traits and intrinsic surface features has emerged as focus of research across disciplines. Many investigations have sought to characterize the structural as well as the tribological response of legless species from an engineering point of view. Despite the sizable amount of data that have accumulated in the literature over the past two decades or so, no effort to review the available information, whence this review. This manuscript, therefore, endeavors to assess available data on surface metrology and tribological behavior of legless reptiles and to define aspects of that performance necessary to formulate an advanced paradigm for bio-inspired surface engineering.

Discussion: “An Application of Dimensional Analysis to Entropy-Wear Relationship,” (Amiri, M., Khonsari, M. M., Brahmeshwarkar, S., 2012, J. Tribol., 134, P. 011604)

The authors present an interesting thermodynamic interpretation of the Archard wear coefficient. However, their interpretation is confined to the steady state. The purpose of this discussion is, first, to extend the domain of the authors’ derivation to the entire regime of rubbing (running-in to steady state), and, second, to point out a possible thermodynamic functional interpretation of wear rate resulting from the current derivation. The starting point is to represent the hardness of the material as a linear function of the melting temperature Tm, viz: