FRICTION OF VISCOELASTIC GRADED MATERIALS ON ADHESIVE ROUGH SURFACES

The aim of this thesis is to highlight the potential of viscoelastic materials to enhance the friction coefficient through the tuning of their rheological properties during sliding against a rough substrate. This research was carried out under the supervision of Prof. Carbone, Prof. Menga, Prof. Putignano, and Dr. Mandriota. The work begins with a detailed overview of the relevance of viscoelastic materials, emphasizing the reasons why their mechanical behaviour is of considerable interest and outlining their principal fields of application. A general introduction to contact mechanics is also provided, with particular attention to the role of adhesion in governing interfacial phenomena. The second chapter is devoted to the analysis of homogeneous viscoelastic materials, discussing their current importance and presenting the fundamental methodologies adopted for their characterization. The third chapter focuses on the derivation of the Green’s function for both elastic and viscoelastic materials, considering homogeneous and bi-layered configurations, and highlights its relevance in predicting the mechanical response of such systems to prescribed inputs, in the form of forces or displacements, under both adhesive and non-adhesive contact conditions. Building on this theoretical framework, a series of simulations was performed using Mathematica to evaluate the system response as a function of selected parameters and to identify the material configurations required to achieve specific target outcomes. High attention was devoted to the possibility of significantly increasing the friction coefficient by appropriately tuning the relaxation times of the viscoelastic layers composing the bulk of the material in contact with a rough substrate. The fourth chapter presents the results of the numerical simulations carried out for the bi-layer case, which represents the most innovative contribution of this thesis. In agreement with the findings of Mandriota et al., the combined action of small-scale and bulk-scale effects is shown to play a crucial role, as both contribute to a local enhancement of the friction coefficient over different speed ranges. The first main outcome of this study concerns the role of the contact area in generating a local friction peak at low sliding speeds: as the characteristic size of the contact decreases, the corresponding peak becomes more pronounced. The second major result is the demonstration that viscoelastic graded materials can achieve higher friction coefficients than their homogeneous counterparts. This effect is particularly clear in bilayer systems, where, through a careful tuning of the relaxation times of the individual layers, the friction coefficient can be increased up to approximately twice that of an equivalent homogeneous material. Finally, the thesis concludes with a discussion of the obtained results and outlines open questions and potential directions for future studies.