Scientific Report: Micro/Nanoscale Models for Tribology

M. Ciavarella, A. Fasolino, L. Nicola, J. Scheibert, A. Vakis, V. Yastrebov

Forty leading tribologists and 20 junior researchers participated in the Lorentz Center @Oort workshop on “Micro/Nanoscale Models for Tribology” held in Leiden between 30 January and 3 February 2017. Participants came from 14 different counties (13 countries in Europe plus Japan) with a 9:1 ratio of academia to industry and 85% being (predominantly) modelers and theoreticians versus 15% being (predominantly) experimentalists. The makeup of this select group comprising applied physicists, materials scientists and engineers, resulted in lively discussions over a broad range of topics relevant to tribology, facilitated via 7 keynote lectures and 16 round-table discussion sessions. The expected outcomes of the workshop are: a review of the state-of-the-art in tribological research, a proposed research roadmap for the next 5-10 years, the establishment of an interdisciplinary network of scientists and engineers working on tribology in Europe to be facilitated via a collaborative platform, and the possibility of submitting collaborative proposals via COST Action or Marie-Curie ITN calls.

The workshop was organized thematically across five days: multiscale and multiphysics aspects, rough surface representations, analytical models, microscale models, and nanoscale models. The breakdown of continuum theories was discussed especially with reference to the nano- and microscale. Experimental aspects were addressed each day in dedicated sessions to provide grounding for the discussions via empirical observations. Each of the invited participants co-chaired a round-table discussion session during which the state-of-the-art was briefly presented and research questions were raised and discussed interactively, for example, motivated by brief presentations. By design, all participants could attend all sessions (there were no sessions held concurrently) to facilitate the aim of the workshop to bridge the gap between the applied physics, materials science and engineering fields represented by the participants. Furthermore, poster sessions gave the opportunity to junior participants to present their work, and engage in discussion with more senior scientists, while industrial participants presented the tribological problems encountered by their companies.

Critical topics in tribology were not confined to a specific session but addressed recurrently during the week. One of the main outcomes of the workshop was the realization that, despite the modeling community’s ability to address elastic problems of great complexity at various scales, significant effort is still required to account for the effects of plasticity, adhesion, friction, wear, lubrication and surface chemistry in tribological models. Weak, strong and monolithic coupling schemes between different physical phenomena at small scales, which are critical for tribological behavior at the system scale, were considered during respective round-table and key-note lectures. Numerical methods were also discussed relative to their inherent length scales: finite element and boundary element methods versus discrete dislocation dynamics for the microscale, and classical versus ab initio molecular dynamics and density functional theory for the nanoscale. Emphasis was placed on the advantages and disadvantages of employing concurrent or hierarchical multiscale schemes coupling these methods, while the need to develop improved tools requiring less computational effort and time was put forth by industrial participants who would like to use such tools to optimize and control their processes and products. All these issues constitute future areas for tribological research and have been made visible to participants who expressed their interest in pursuing this further in an inter-disciplinary manner.