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Title: Micro-plasticity of surface steps under adhesive contact. Part I. Surface yielding controlled by single-dislocation nucleation

Abstract

Mechanics of nano- and meso-scale contacts of rough surfaces is of fundamental importance in understanding deformation and failure mechanisms of a solid surface, and in engineering fabrication and reliability of small surface structures. We present a micro-mechanical dislocation model of contact-induced deformation of a surface step or ledge, as a unit process model to construct a meso-scale model of plastic deformations near and at a rough surface. This paper (Part I) considers onset of contact-induced surface yielding controlled by single-dislocation nucleation from a surface step. The Stroh formalism of anisotropic elasticity and conservation integrals are used to evaluate the driving force on the dislocation. The driving force together with a dislocation nucleation criterion is used to construct a contact-strength map of a surface step in terms of contact pressure, step height, surface adhesion and lattice resistance. Atomistic simulations of atomic surface-step indentation on a gold (1 0 0) surface have been also carried out with the embedded atom method. As predicted by the continuum dislocation model, the atomistic simulations also indicate that surface adhesion plays a significant role in dislocation nucleation processes. Instabilities due to adhesion and dislocation nucleation are evident. The atomistic simulation is used to calibrate the continuummore » dislocation nucleation criterion, while the continuum dislocation modeling captures the dislocation energetics in the inhomogeneous stress field of the surface-step under contact loading. Results show that dislocations in certain slip planes can be easily nucleated but will stay in equilibrium positions very close to the surface step, while dislocations in some other slip planes easily move away from the surface into the bulk. This phenomenon is called contact-induced near-surface dislocation segregation. As a consequence, we predict the existence of a thin tensile-stress sub-layer adjacent to the surface within the boundary layer of near-surface plastic deformation. In the companion paper (Part II), we analyze the surface hardening behavior caused by multiple dislocations.« less

Authors:
 [1];  [2];  [3];  [4]
  1. City College of New York
  2. Iowa State University
  3. ORNL
  4. Brown University
Publication Date:
Research Org.:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States); Center for Computational Sciences
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
941019
DOE Contract Number:
DE-AC05-00OR22725
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of the Mechanics and Physics of Solids; Journal Volume: 55; Journal Issue: 3
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; ADHESION; ADHESIVES; BOUNDARY LAYERS; DISLOCATIONS; ELASTICITY; FABRICATION; GOLD; NUCLEATION; PLASTICS; MATHEMATICAL MODELS

Citation Formats

Yu, H. H., Shrotriya, P., Gao, Yanfei, and Kim, Kyung Suk. Micro-plasticity of surface steps under adhesive contact. Part I. Surface yielding controlled by single-dislocation nucleation. United States: N. p., 2007. Web. doi:10.1016/j.jmps.2006.09.003.
Yu, H. H., Shrotriya, P., Gao, Yanfei, & Kim, Kyung Suk. Micro-plasticity of surface steps under adhesive contact. Part I. Surface yielding controlled by single-dislocation nucleation. United States. doi:10.1016/j.jmps.2006.09.003.
Yu, H. H., Shrotriya, P., Gao, Yanfei, and Kim, Kyung Suk. Mon . "Micro-plasticity of surface steps under adhesive contact. Part I. Surface yielding controlled by single-dislocation nucleation". United States. doi:10.1016/j.jmps.2006.09.003.
@article{osti_941019,
title = {Micro-plasticity of surface steps under adhesive contact. Part I. Surface yielding controlled by single-dislocation nucleation},
author = {Yu, H. H. and Shrotriya, P. and Gao, Yanfei and Kim, Kyung Suk},
abstractNote = {Mechanics of nano- and meso-scale contacts of rough surfaces is of fundamental importance in understanding deformation and failure mechanisms of a solid surface, and in engineering fabrication and reliability of small surface structures. We present a micro-mechanical dislocation model of contact-induced deformation of a surface step or ledge, as a unit process model to construct a meso-scale model of plastic deformations near and at a rough surface. This paper (Part I) considers onset of contact-induced surface yielding controlled by single-dislocation nucleation from a surface step. The Stroh formalism of anisotropic elasticity and conservation integrals are used to evaluate the driving force on the dislocation. The driving force together with a dislocation nucleation criterion is used to construct a contact-strength map of a surface step in terms of contact pressure, step height, surface adhesion and lattice resistance. Atomistic simulations of atomic surface-step indentation on a gold (1 0 0) surface have been also carried out with the embedded atom method. As predicted by the continuum dislocation model, the atomistic simulations also indicate that surface adhesion plays a significant role in dislocation nucleation processes. Instabilities due to adhesion and dislocation nucleation are evident. The atomistic simulation is used to calibrate the continuum dislocation nucleation criterion, while the continuum dislocation modeling captures the dislocation energetics in the inhomogeneous stress field of the surface-step under contact loading. Results show that dislocations in certain slip planes can be easily nucleated but will stay in equilibrium positions very close to the surface step, while dislocations in some other slip planes easily move away from the surface into the bulk. This phenomenon is called contact-induced near-surface dislocation segregation. As a consequence, we predict the existence of a thin tensile-stress sub-layer adjacent to the surface within the boundary layer of near-surface plastic deformation. In the companion paper (Part II), we analyze the surface hardening behavior caused by multiple dislocations.},
doi = {10.1016/j.jmps.2006.09.003},
journal = {Journal of the Mechanics and Physics of Solids},
number = 3,
volume = 55,
place = {United States},
year = {Mon Jan 01 00:00:00 EST 2007},
month = {Mon Jan 01 00:00:00 EST 2007}
}
  • The study of micro-plasticity behavior near and at the rough surface is the critical link towards a fundamental understanding of contact, surface failures, friction and adhesion at small length scales. In the companion paper (Part I), we have studied the onset of surface yielding due to single-dislocation nucleation from a stepped surface under adhesive contact. Here we analyze the contact hardening behavior due to multiple dislocations in a two-dimensional dislocation model. Continuum micromechanical analyses are used to derive the configurational force on the dislocation, while a modified Rice-Thomson model is used to describe the dislocation nucleation. Dislocations nucleated from amore » surface source are stabilized and pile up as a result of the balance between the resolved driving force and the non-zero lattice resistance in the solid. The pileup dislocations will exert a strong back stress to prevent further dislocation nucleation and thus lead to the surface contact hardening, the degree of which depends on the slip-plane orientation. Particularly, we find that the dislocation interactions between two slip planes can make the contact loading order-of-magnitude easy to nucleate multiple dislocations, which is thus named "latent softening". A mechanistic explanation shows that the latent softening is closely related to the mode mixity of the stress concentration at the surface step. Dislocation nucleation will modify the geometric characteristics of the surface step, so that the contact-induced stress state near the step, as described by the mode mixity, changes. The altered stress state affects subsequent dislocation nucleation. Our calculations show that dislocation pileup on one slip plane can even cause spontaneous dislocation nucleation on the other slip plane without further increase of contact pressure. Furthermore, it is found that the rough surface contact at small length scale can lead to the formation of a surface tensile sub-layer caused by segregated dislocation pileups. The discrete-dislocation model provides novel insights to understanding of surface step deformation at a small length scale, which bridge atomistic simulations and continuum plastic flow analysis of surface asperity contact. Implications of the theoretical predictions are also discussed.« less
  • We investigate deformation in high quality Au nanowires under both tension and bending using in-situ transmission electron microscopy. Defect evolution is investigated during: (1) tensile deformation of 〈110〉 oriented, initially defect-free, single crystal nanowires with cross-sectional widths between 30 and 300 nm, (2) bending deformation of the same wires, and (3) tensile deformation of wires containing coherent twin boundaries along their lengths. We observe the formation of twins and stacking faults in the single crystal wires under tension, and storage of full dislocations after bending of single crystal wires and after tension of twinned wires. The stress state dependence of themore » deformation morphology and the formation of stacking faults and twins are not features of bulk Au, where deformation is controlled by dislocation interactions. Instead, we attribute the deformation morphologies to the surface nucleation of either leading or trailing partial dislocations, depending on the Schmid factors, which move through and exit the wires producing stacking faults or full dislocation slip. The presence of obstacles such as neutral planes or twin boundaries hinder the egress of the freshly nucleated dislocations and allow trailing and leading partial dislocations to combine and to be stored as full dislocations in the wires. We infer that the twins and stacking faults often observed in nanoscale Au specimens are not a direct size effect but the result of a size and obstacle dependent transition from dislocation interaction controlled to dislocation nucleation controlled deformation.« less
  • The first objective of this note is to investigate a lack of a second transition region within the framework of the hot-spot model. The second objective of this note is to test the hot-spot hypothesis further for the lower contact angle to confirm the functional variation of the critical rewetting temperature with the contact angle. We use a two-dimensional cylindrical-coordinate transient conduction model to study the saturated pool nucleate boiling phenomenon in the second transition region on a horizontal surface. We have examined the lack of a second transition boiling region in the experiments of Wang and Dhir. In additionmore » to the original requirements of a thick heater with high thermal conductivity for observing the second transition boiling region presented by Unal et al. (1922a), an additional criterion determined here is that the active nucleation site density must not be too high. This new requirement should be included in the experimental efforts originally suggested and aimed at investigating the hot-spot hypothesis. It also indicates that the lack of existence of the second transition region on the boiling curve does not necessarily indicate the lack of significant heater surface dryout as we had suggested earlier. Calculations based on the data of Wang and Dhir (1991) showed that the surface temperatures on the wet regions are very low due to their high nucleation site density. This suggests that dry patches must exist on their heater, so that the surface-averaged temperatures would be as measured in their experiments. 8 refs., 2 figs., 2 tabs.« less
  • The study of cyclic stress strain response of Cu-single crystals has been the subject of numerous investigations during the past decade. It was pointed out that the cyclic loading under certain conditions leads to strain localization zones of highly deformable material layers called Persistent Slip Bands (PSB). These PSB`s have been connected with the evolution of a characteristic dislocation substructure (ladder-like structure) and the formation of extrusion/intrusion slip markings on the specimen surface. Under the most frequently applied plastic-strain controlled cycling, a saturation plateau stress in the CSS-curve was observed. The resolved shear stress of this plateau was reported tomore » be 28 MPa for single crystals of Cu oriented for single slip when cycled at room temperature. Kettunen reported results on Cu single crystals cycled under stress control with the desired stress amplitude reached in the first half-cycle. He found that PSB`s formed already at a shear stress amplitude as low as 17 MPa. Recent experiments with Cu-polycrystals showed that an increase in the length of the stress loading ramp resulted in an increase of the nucleating stress for PSB`s. For the case of direct loading a nucleation shear stress was calculated for values between 16 and 21 MPa depending on the constraint factor used to convert the stress values. Based on this knowledge the authors designed experiments on single crystal Cu specimens oriented for single slip under various loading ramp length (1--1,000 cycles) to study the nucleation conditions of PSB`s and the related saturation behavior.« less
  • By isolating the process of dislocation emission from a crack tip under an applied tensile stress, we extract from a molecular dynamics simulation the atomic-level displacement and stress fields on the activated slip plane before and after the nucleation event. The stress-displacement relations so obtained provide a direct link with recent continuum descriptions of brittle versus ductile behavior in crack propagation. Crack-tip shielding by the emitted dislocations is demonstrated, as is the role of surface steps in dislocation nucleation and crack-tip blunting. {copyright} {ital 1997} {ital The American Physical Society}