Session P32: Focus Session: Tribophysics -- Adhesion and Friction

Spreading of Nanoparticles on a Substrate

Using combination of the molecular dynamics simulations and theoretical analysis we have studied the deformation of crosslinked polymeric nanoparticles on solid substrate. Our simulations show that the deformation of the adsorbed nanoparticles is a function of the dimensionless parameter GR/W, where G is the particle shear modulus, R particle size and W is the work of adhesion. For small particle deformations, which is usually the case for the highly crosslinked nanoparticles, the particle deformation can be described in the framework of the classical Johnson, Kendall, and Roberts (JKR) theory. However, we observed a deviation from the classical JKR theory in the case of the weakly crosslinked nanoparticles which experience large deformations upon particle adsorption. We propose a theoretical model which accounts for large particle deformations.   

Directionally Sensitive Adhesion of Textured Surfaces

Understanding the mechanism of adhesive failure at textured interfaces is critically important to the design of advanced functional surfaces. Many naturally occurring adhesives rely on surface texture or mechanical property gradients to increase fracture toughness without manipulating surface chemistry. By avoiding chemical mechanisms, textured adhesives are able to exhibit reversible, directionally-dependant adhesion. We measured fracture toughness of interfaces with controlled adhesive heterogeneity induced through surface patterning and manipulation of sub-surface structure, thus revealing critical design elements for advanced adhesives. By creating mechanical property discontinuities and gradients at a microstructural level, it is possible to alter the interfacial stress profile prior to crack initiation, thus altering the energy cost for delamination. Geometric shape and orientation of surface texture were also varied to create anisotropic fracture behavior. Our initial designed surfaces have displayed up to 40{\%} reduction in load required to initiate delamination based on solely on changing the direction of crack propagation.   

Modification of Substrates for Improved Chemical Bonding at Epoxy Interfaces

Near-edge X-ray absorption fine structure (NEXAFS) is exploited to investigate the interface between epoxy resins and a solid substrate, which is critical for many industrial and defense related composite structures. The goal is to link the interfacial chemistry, structure and resulting properties to the adhesive properties, enabling the design of interfaces with tunable strength, energy dissipation, and impact performance. Silicon surfaces were chosen as a model substrate and were coated sequentially through reactions with glycidyl propyl trimethoxysilane (GPS), various polyoxyalkyleneamines (POAAs), and finally diglycidyl ether of bisphenol-A (DGEBA). By controlling the molecular ratios of the POAAs, which have varying functionality and molecular weight the reactivity of these surfaces toward epoxies can be controlled. In addition to NEXAFS which provides insight regarding the molecular organization and chemistry at the interface, mechanical testing will also be explored to determine the impact on interfacial properties. Asymmetric dual cantilever beam testing was chosen for these samples because it drives the crack to the interface and allows for ease of characterization with surface science techniques after failure. A combination of NEXAFS and XPS will be utilized to interrogate the fracture surfaces and provide insight into the adhesive failure mechanisms and location.   

Making and Breaking of Atomic Bonds in Carbon Tribocontacts

Friction and wear are particularly illustrating examples of the multitude of length and time scales that are relevant in the mechanics of materials. Any modelling has to span from the atomic scale to the size and lifetime of engineering components. Atomistic bond breaking processes and picosecond time scales are obviously crucial for the wear processes which in the long run determine the lifetime of machining components. Partly due to the difficulty in bridging all these length and time scale, the physics-based modelling and simulation of friction and wear processes is still in its infancy. I will describe our first atomistic approaches to the simulation of tribocontacts between diamond-like carbon (DLC) films or diamond. Different levels of approximations are required to assess the evolution of the friction contacts. Considerable attention must be paid to extracting relevant information from large scale atomistic simulations, which in turn first requires an atomistic model for the hydrocarbons that can describe well the making and breaking of the atomic bonds. I will present results for the evolution of an atomistically determined friction coefficient during running-in of such a contact and will later turn to the question of how wear processes and the polishing of diamond proceed.   

Study of Scratching Resistance Mechanism of Ultra-thin Diamond-Like-Carbon (DLC) Coatings Deposited on Glass

Depositing an ultra-thin diamond-like-carbon (DLC) coating of 4.5 nm thick on the surface of soda-lime glass, using the linear ion beam deposition technique, significantly improved the scratch resistance of the glass. To understand the mechanism, a nano-indenter with a conical-shaped diamond tip was used to carry out the scratch tests on the coated and uncoated glasses, and a scanning probe microscope was used to examine the damage morphologies along the scratches. The same indenter with a Berkovich diamond tip was also used to carry out the indentation tests, measuring the elastic modulus and hardness of the coated and uncoated glasses. It was found that while the coating increased the elastic modulus and hardness a little, the critical contributor of the improved scratch resistance is the compressive stress residing in the coating and the top layer of the glass, which was yielded during the deposition process. It counters the tensile stress of scratching and suppresses the cracking, thus the coated glass possesses a significantly better scratch resistance.   

Electromigration-Driven Surface Morphological Stabilization of a Strained Thin Film on a Substrate

This study investigates the surface morphological stability of a coherently strained, elastic and electrically conductive epitaxial thin film on a semi-infinite and on a finite deformable substrate. The film is subjected to an external electric field that drives surface electromigration but does not affect its elastic constants. Due to its lattice mismatch with the substrate, the film is in a state of biaxial strain that can cause surface morphological instabilities. We develop a three-dimensional, fully nonlinear continuum model that describes the surface morphological evolution of the film and carry out a linear stability analysis using a local approximation for the electric field. The resulting dispersion relation shows that, in certain cases, surface electromigration can inhibit the stress-induced instabilities. We report results of a systematic analysis that maps the surface morphological stability domain boundaries as a function of material properties, lattice-mismatch strain, film and substrate thicknesses, and electric-field strength.   

Characterize cohesive failure in simple shear of entangled polymer solutions on micron scales

We apply laser confocal microscopy in conjunction with a commercial rheometer to study the yielding behavior of entangled polymer solutions on length scales as low as 40 microns. The goal is to illustrate how the sample thickness dictates the overall mechanical response to sudden deformation and why the same behavior observed at conventional gap sizes (ca. 1 mm) must show up under proper conditions. Contrary to the claim of one recent study from Cornell, we show that for well-entangled polymeric liquids cohesive breakup of the entanglement network does occur for gap distance as low as 40 microns. In other words, interfacial slip alone is insufficient to take up the imposed displacement of the sample boundary.   

Fundamental deformation processes controlling nanoscale friction and wear

Thermally activated processes are often responsible for the kinetics of deformation and can control tribological performance. In this contribution two such processes are discussed in combination with nanoscale tribology experiments using atomic force microscopy (AFM). The first process describes single asperity wear as an atom-by-atom loss process driven by frictional shear stresses an interface. The wear rate is described by a thermally activated bond breaking process in which the energy barrier is reduced by the frictional shear stress. This leads to dramatic deviations from Archard's wear law which is commonly used to described macroscopic wear. Experimental confirmation of an atom-by-atom wear process is given by AFM wear experiments using different material combinations of tips sliding on surfaces [1]. The second process relates fundamental rearranging processes in polymers to friction. As an example, data of sliding friction between a silicon tip and a highly cross-linked polyaryletherketone film using friction force microscopy are presented. Energy dissipation into so-called molecular relaxations (alpha and beta relaxations) is identified as distinctive maxima of the friction force as a function of temperature between 150 and 500 K. A strong shift of such peak temperatures as a function of applied load is observed. Again, a model with an Arrhenius activation modulated by the applied shear stress describes experimental results quantitatively. The effect of the stress-shifted relaxation on friction-versus-load experiments is discussed [2]. Both processes will be discussed in the context of technological applications. \\[4pt] [1] B. Gotsmann and M. A. Lantz, Phys. Rev. Lett. 101, 125501 (2008) \\[0pt] [2] L. Jansen et al. Phys. Rev. Lett. 102, 236101 (2009)   

Formation of a One-Atom Contact on Pb: Adhesion and Energy Dissipation

Quartz Tuning Fork (TF) has been recently successfully implemented in force detection schemes for scanning probe microscopy applications [1]. Here we report its use as a nanotribometer for studying energy dissipation and the forces in the formation of a single atom contact. The idea behind such a noncontact mono-atomic energy dissipation detector, is to take advantage of the mechanical resonance of a TF which has a large Q-factor ($Q_{4.2K}\sim150000$) which in our set-up is a function of the tip-sample dissipative forces. Finally, we report measurements of the shift in the resonance frequency and the Q-factor degradation of the oscillating TF, either as a function of the tip-sample voltage or distance. These experiments have been done on Pb close to the superconducting transition temperature in high vacuum. \\[4pt] [1] S. Morita, \textit{et. al.} \textit{Noncontact Atomic Force Microscopy}, Nanoscience and Technology (Springer-Verlag, Berlin 2002)   

Effect of trace moisture on friction

Although the effects of humidity on friction have been widely studied, much less is known about friction behavior when only trace level moisture is present on a surface. Trace moisture is particularly important for hydrophilic surfaces because such surfaces will be often terminated with dissociated water even if they are transferred to nominally dry conditions. Here we demonstrate that friction of hydrophilic surfaces increases in the presence of trace moisture due to formation of hydrogen bonds across the interface. Using first principle calculations we discover that frictional instability during sliding is related to rotation of hydroxyl groups, which are bonded across the interface via hydrogen bonding. Using theoretical analysis based on Prandtl-Tomlinson's model we quantify free energy released during such instabilities and we predict that velocity dependence of trace moisture friction follows the same trend as dry friction.   

Investigation of Aqueous Lubricants on Polymer Surfaces by Nanoindenter-based Scratch Tests

Nanoindenter-based scratch (nanoscratch) tests were successfully used to study lubrication at the microscale in the presence of a fluid film. The influence of aqueous lubricants on both hydrophobic (polypropylene and polyethylene) and hydrophilic (cellulose) surfaces was investigated. The lubricants consisted of aqueous solutions of amphiphilic block copolymers of ethylene oxide (EO) and propylene oxide (PO). The coefficients of friction were measured in the presence of lubricant solution on the solid surfaces. An improved lubricity ($i.e$., coefficient of friction decreased) was observed to occur as the adsorption excess increased.