Integrated Computational Materials Engineering Research Articles

Informatics and integrated computational materials engineering: Part II

This issue of JOM presents the second in a twopart series on materials informatics. In the previous issue (March 2008), some of the basic concepts of what constitutes informatics were introduced. In this issue, we continue that theme with papers that serve to emphasize the diversity of topics that encompass the fi eld of informatics. The timing of the publication of these JOM topics on materials informatics coincides with the issuing of a report of the National Materials Advisory Board on Integrated Computational Materials Engineering (ICME). This report defi nes that information as including “curated data sets, structure-property models, processing-structure relationships, physical properties and thermodynamic, kinetic and structural information.” It provides a good summary of the needs, status, and challenges Informatics and Integrated Computational Materials Engineering: Part II

Integrated Computational Materials Engineering (ICME): A “model” for the future?

Integrated Computational Materials Engineering (ICME): A “model” for the future?

Scale effects in friction of single–asperity contacts. II. Multiple–dislocation–cooperated slip

In this paper we explore the second transition in the mechanism of frictional slip of single–asperity contacts, which takes place at large contact sizes. This is a transition from single–dislocation–assisted (SDA) slip to multiple–dislocation–cooperated (MDC) slip. It is found that the friction stress is controlled by dislocation nucleation for SDA slip, and by dislocation mobility for MDC slip. A model of concentric dislocation loops is introduced to analyse dislocation pile–up processes and their relationship to the friction stress. Dislocations are stabilized to be piled up as a result of the non–zero effective Peierls stress of the interface. The analysis shows that slip occurs when the condition for the nucleation of a new dislocation and the condition for destabilizing the leading dislocation of the pile–up are simultaneously satisfied. It is also shown that, as the contact size increases, the friction stress approaches asymptotically a constant value equal to the effective Peierls stress of the interface. This result is in agreement with reported experimental results in the surface force apparatus (SFA). The case of a large number of dislocations in the pile–up is studied via an asymptotic analysis, a key concept of which is the existence of a dislocation–free zone that controls the dislocation nucleation process. The analysis provides the connection between the discrete dislocation model and the continuous cohesive zone model of single–asperity friction.

Scale effects in friction of single–asperity contacts. I. From concurrent slip to single–dislocation–assisted slip

A micromechanical dislocation model of frictional slip between two asperities is presented. The model suggests that when the contact radius is smaller than a critical value, the friction stress is constant, of the order of the theoretical shear strength, in agreement with reported atomic force microscope (AFM) friction experiments. However, at the critical value there is a transition beyond which the friction stress decreases with increasing area, until it reaches the second transition where the friction stress gradually becomes independent of the contact size. This is in contrast to previous theories, which assume that the friction stress is always independent of the size. The present model also predicts that the mechanisms of slip are size dependent. Before the first transition, the constant friction stress is associated with concurrent slip without the aid of dislocation motion. The first transition corresponds to the minimum contact size at which a single dislocation loop is nucleated and sweeps through the whole contact interface, resulting in a single–dislocation–assisted (SDA) slip. This mechanism is predicted to prevail for a wide range of contact sizes, from 10 nm to 10 µm in radius for typical dry adhesive contacts; however, there are no available experimental data in this size range. The second transition is found to be caused by the effective Peierls stress which stabilizes the dislocation loop within the contact region, resulting in dislocation pile–ups. Beyond the second transition, slip is assisted by cooperative glide of dislocations in the pile–up. For sufficiently large contacts the mechanism of cooperative glide induces a size–independent friction stress, in agreement with observations in surface force apparatus (SFA) friction experiments. This paper (Part I) addresses the first transition: from concurrent slip to SDA slip. The second transition is analysed in a companion paper (Part II).