Thesis (Ph. D.)--Mechanical Engineering, Georgia Institute of Technology, 2005. / J. Carson Meredith, Committee Member ; J. Carlos Santamarina, Committee Member ; G. Paul Neitzel, Committee Member ; Richard F. Salant, Committee Member ; Jeffrey L. Streator, Committee Chair. Includes bibliographical references.

Effects of biomimetic micro-patterning of polymeric materials on their interfacial properties were studied experimentally. Micropillars of PDMS and SU-8 epoxy were fabricated through soft lithography and UV lithography techniques, respectively. PDMS pillars were topped by thin terminal films of the same material through dipping method with different thicknesses and viscosities. Adhesion and frictional properties of biomimetic microstructures were examined in two modes of contact, i.e. laid and conformal contact. In the first mode of contact, i.e. laid contact, the contact between adhesive and adherent is laid on top of the micro-protrusions or is in contact with side wall of micropillars. Adhesion properties of the smooth and patterned PDMS were characterized through micro-indentation test. Moreover, the friction properties of the smooth PDMS sample and PDMS micropillars with different aspect ratios were examined in unidirectional friction testing. JKR theory of continuum contact mechanics was utilized to interpret the obtained data. To study the effect of second mode of contact, peeling behaviour of a conformal contact between solidified liquid PDMS and SU-8 micropillars was monitored. Kendall’s model of elastic peeling was used to interpret the peeling data. It was found that patterning of the materials would decrease the real area of contact and accordingly adhesion and friction to the mating surface. Termination of the micropillars with a thin layer of the same material result in increment of adhesion as reduction of the real contact area could be compensated and the compliance of the near surface increases. Elastic energy dissipation as a result of enhanced compliance and crack trapping and crack propagation instabilities are the main reasons behind increment of adhesion of thin film terminated structures. Viscoelasticity of the terminal thin film remarkably increased the adhesion as a result of coupling mentioned mechanisms and viscoelastic loss on the surface. Decline of the overall friction could be tailored through use of different aspect ratios. Higher aspect ratios pillars show higher friction comparing to lower aspect ratio pillars. 550 folds enhancement of adhesion was observed for peeling of the PDMS tape from rigid micropillars with aspect ratio ranging from 0 to 6. It is concluded that for the lower aspect ratio micropillars, the elastic energy dissipation is playing the key role in adhesion enhancement. This role shifts toward side-wall friction during separation by increase in aspect ratio. These all give in hand a versatile tool to control and fine tune the interfacial properties of materials, whether they are concerned with adhesion or friction.

Effects of biomimetic micro-patterning of polymeric materials on their interfacial properties were studied experimentally. Micropillars of PDMS and SU-8 epoxy were fabricated through soft lithography and UV lithography techniques, respectively. PDMS pillars were topped by thin terminal films of the same material through dipping method with different thicknesses and viscosities. Adhesion and frictional properties of biomimetic microstructures were examined in two modes of contact, i.e. laid and conformal contact. In the first mode of contact, i.e. laid contact, the contact between adhesive and adherent is laid on top of the micro-protrusions or is in contact with side wall of micropillars. Adhesion properties of the smooth and patterned PDMS were characterized through micro-indentation test. Moreover, the friction properties of the smooth PDMS sample and PDMS micropillars with different aspect ratios were examined in unidirectional friction testing. JKR theory of continuum contact mechanics was utilized to interpret the obtained data. To study the effect of second mode of contact, peeling behaviour of a conformal contact between solidified liquid PDMS and SU-8 micropillars was monitored. Kendall’s model of elastic peeling was used to interpret the peeling data. It was found that patterning of the materials would decrease the real area of contact and accordingly adhesion and friction to the mating surface. Termination of the micropillars with a thin layer of the same material result in increment of adhesion as reduction of the real contact area could be compensated and the compliance of the near surface increases. Elastic energy dissipation as a result of enhanced compliance and crack trapping and crack propagation instabilities are the main reasons behind increment of adhesion of thin film terminated structures. Viscoelasticity of the terminal thin film remarkably increased the adhesion as a result of coupling mentioned mechanisms and viscoelastic loss on the surface. Decline of the overall friction could be tailored through use of different aspect ratios. Higher aspect ratios pillars show higher friction comparing to lower aspect ratio pillars. 550 folds enhancement of adhesion was observed for peeling of the PDMS tape from rigid micropillars with aspect ratio ranging from 0 to 6. It is concluded that for the lower aspect ratio micropillars, the elastic energy dissipation is playing the key role in adhesion enhancement. This role shifts toward side-wall friction during separation by increase in aspect ratio. These all give in hand a versatile tool to control and fine tune the interfacial properties of materials, whether they are concerned with adhesion or friction.

The relationship between the amount and conformation of apolymer at the solid-liquid interface, and the resultinginteraction forces between two such surfaces has beeninvestigated. With a degree of control of the polymerconformation, by varying the temperature, solvent quality,polymer charge density etc, it has been possible to measure andinterpret the resulting changes in the surface interactions.The recurring themes of dynamics and hydrodynamics have beencontinually considered due to the large range and viscoelasticnature of the polymeric systems. The polymeric systems investigated in this thesis are, poly(N-isopropylacrylamide), poly (12-hydroxystearate) and a seriesof AM-MAPTAC polyelectrolytes with variable chargedensities. Adsorption and conformation of polymers have beeninvestigated by the novel QCM instrument. By comparison tosimultaneously measured energy loss information, a greaterunderstanding of the conformation of the polymer has beengained, both as a function of layer build-up during initialadsorption, and as a result of induced conformational changes.Comparing the results toin situsurface plasmon resonance and subsequent x-rayphotoelectron spectroscopy measurements, the relativeconcentration of polymer within the layer is determined. Inaddition, efforts have been made to extend the scope of thetechnique, in such ways as measuring with QCM as a function oftemperature and deriving viscoelastic properties. The later isstill to be achieved in absolute terms for polymer layers inliquid environments, yet both the principle and experimentalcapabilities have been shown. Normal interaction forces have been measured as a functionof solvation of the polymer layer, for both adsorbed andgrafted polymer layers. For fully solvated (steric) polymerlayers, which can act as colloidal stabilisers, the dynamics ofthe repulsive force, including hydrodynamics have beeninvestigated. The same has been achieved for collapsed polymerlayers, in which the dynamic adhesion has also beeninvestigated. The effect on the adhesion of three differentdynamic mechanisms has been determined (which, like the surfaceforces, depend on the polymer conformation andviscoelasticity). These dynamic mechanisms are based onbridging forces, polymer entanglement and a viscoelastic‘bulk’response from the surface layers. Lateral or friction measurements have also been completed.The effect of load and rate have been investigated as afunction of both the polymer charge density and the underlyingsubstrate, which result in a variable conformation and bindingstrength to the substrate. This has resulted in a complexaddition of numerous mechanisms, the dominant mechanism beingdetermined by the binding strength to the surface, polymerconformation and viscoelasticity. The results have shown thatadsorbed polymer layers can be used to both increase anddecrease friction, and to change the direction of the ratedependence.

<p>The relationship between the amount and conformation of apolymer at the solid-liquid interface, and the resultinginteraction forces between two such surfaces has beeninvestigated. With a degree of control of the polymerconformation, by varying the temperature, solvent quality,polymer charge density etc, it has been possible to measure andinterpret the resulting changes in the surface interactions.The recurring themes of dynamics and hydrodynamics have beencontinually considered due to the large range and viscoelasticnature of the polymeric systems.</p><p>The polymeric systems investigated in this thesis are, poly(N-isopropylacrylamide), poly (12-hydroxystearate) and a seriesof AM-MAPTAC polyelectrolytes with variable chargedensities.</p><p>Adsorption and conformation of polymers have beeninvestigated by the novel QCM instrument. By comparison tosimultaneously measured energy loss information, a greaterunderstanding of the conformation of the polymer has beengained, both as a function of layer build-up during initialadsorption, and as a result of induced conformational changes.Comparing the results to<i>in situ</i>surface plasmon resonance and subsequent x-rayphotoelectron spectroscopy measurements, the relativeconcentration of polymer within the layer is determined. Inaddition, efforts have been made to extend the scope of thetechnique, in such ways as measuring with QCM as a function oftemperature and deriving viscoelastic properties. The later isstill to be achieved in absolute terms for polymer layers inliquid environments, yet both the principle and experimentalcapabilities have been shown.</p><p>Normal interaction forces have been measured as a functionof solvation of the polymer layer, for both adsorbed andgrafted polymer layers. For fully solvated (steric) polymerlayers, which can act as colloidal stabilisers, the dynamics ofthe repulsive force, including hydrodynamics have beeninvestigated. The same has been achieved for collapsed polymerlayers, in which the dynamic adhesion has also beeninvestigated. The effect on the adhesion of three differentdynamic mechanisms has been determined (which, like the surfaceforces, depend on the polymer conformation andviscoelasticity). These dynamic mechanisms are based onbridging forces, polymer entanglement and a viscoelastic‘bulk’response from the surface layers.</p><p>Lateral or friction measurements have also been completed.The effect of load and rate have been investigated as afunction of both the polymer charge density and the underlyingsubstrate, which result in a variable conformation and bindingstrength to the substrate. This has resulted in a complexaddition of numerous mechanisms, the dominant mechanism beingdetermined by the binding strength to the surface, polymerconformation and viscoelasticity. The results have shown thatadsorbed polymer layers can be used to both increase anddecrease friction, and to change the direction of the ratedependence.</p>

Friction modification within the wheel-rail interface is an important way of achieving ecologically friendly transportation of both persons and goods. This thesis aims to develop a new TOR lubricant, which will be able to maintain suitable frictional conditions while securing minimal adhesion required for traction. All measurements were carried out on tribometer MTM in the ball-on-disc configuration. In the first step, individual components were examined. More complex compositions were measured after that and the three best of them were compared with commercial TOR lubricants. The results show a good ability of developed compositions to maintain required adhesion, reduce wear, and all of them had resistivity against over-lubrication. Lastly, the process of lubricant verification before its application in real traffic was discussed.

Rubber friction is an important aspect to tire engineers, material developers and pavement engineers because of its importance in the estimation of forces generated at the contact, which further helps in optimizing tire and vehicle performances, and to estimate tire wear. It mainly depends on the material properties, contact mechanics and operating condition. There are two major contributions to rubber friction, due to repeated viscoelastic deformation from undulations of surface called hysteresis and due to Vander Waals interaction of the molecules called adhesion. The study focuses on analytical modeling of friction for stationary sliding of rubber block on rough surfaces. Two novel approaches are discussed and compared. Frictional shear stress is obtained from the energy dissipated at the contact interface due to the elastic deformations of rubber block at different length scales. Contact mechanics theories based on continuity approach combined with stochastic processes to estimate the real contact area, mean penetration depth and true stresses at contact depending on operating conditions. Rubber properties are highly temperature dependent. Temperature model developed based on heat diffusion relation is integrated to consider the effects of temperature rise due to frictional heating. Model results are validated with theoretical results of literature. Simulation results of friction model is obtained for Compound A sliding on rough surface. Material properties are obtained using Dynamic Mechanical Analysis and Time temperature superposition. Influence of the friction models under different conditions are discussed. Model results are validated with experimental data from Dynamic friction tester on a 120-grit surface followed by future works. / Master of Science

Tire, or in its primitive form, Wheel, an important invention for the transportation sector, has evolved from a circular block of hard and durable material to one of the most complex and influential components of an automobile. It is the only means of contact between the vehicle and the road and is responsible for generating forces and moments that impact vehicle performance, stability, and control. Tire tribology is the study of interacting surfaces in relative motion which includes friction and wear. Tire friction is an essential concept for estimating the tractive effort/ traction at the tire-road interface that further helps to determine the control and stability of the vehicle. In contrary, it also results in rolling resistance and wear. Tire and vehicle engineers are henceforth interested in a robust and efficient approach towards estimation of friction and wear. Past experimental observations using tread compound samples have revealed the different factors influencing the friction at the contacting interface. In addition, different mechanisms or components resulting in frictional losses, being Hysteretic, Adhesive and Viscous, and wear being abrasive, fatigue, adhesive and corrosive were also observed. Although experimental and empirical observations have provided us with an accurate estimation of friction and wear parameters, it is very tedious and expensive approach. Recent developments in the computational power encouraged researchers and engineers towards evolution of analytical and numerical models considering the underlying physical mechanisms at the contact interface. Past research studies developed multiscale techniques for estimation of friction coefficient due to hysteretic losses from internal damping of the rubber material because of oscillation from surface undulations. Later, contact mechanics models developed using Hertzian technique or stochastic approach were considered in conjunction with frictional losses to obtain the hysteretic component of friction to consider the effect of surface roughness. Previous studies at CenTiRe focused on surface characterization techniques and estimation of friction for dry surfaces using Persson and Klüppel's approach. Comparative studies unveiled the importance of considering pressure/ normal load towards friction estimation. In addition, it was found that effect of adhesion for estimation of contact mechanics parameters must be considered. The present work focusses on obtaining a conceptual framework to model a comprehensive friction model considering the effect of surface roughness, substrate condition and asperity interaction. A finite element simulation of rubber block sliding on a rough substrate is performed using a multiscale technique for estimation of friction and contact mechanics under dry condition. The estimated contact mechanics and friction is compared with analytical models and experimental measurements obtained using Linear sliding friction tester developed in collaboration with other members of the group. In addition, a FE model is developed to measure the wear properties of rubber material based on continuum damage mechanics and further obtain the wear profile of a rubber block sliding on a rough substrate. / Doctor of Philosophy / Tribology, a recent terminology for an age-old concept of friction, wear, and lubrication. the study of interacting surfaces in relative motion which includes friction and wear. Friction is the resisting force at the contact interface leading to heat build-up and material loss at the contact interface which is known as flash temperature and wear respectively. Tire is one of the most complex and influential components of a vehicle that helps in optimizing its performance for better stability and control. Knowledge of tire friction and wear is important for tire engineering and vehicle dynamics engineers as it helps in characterizing the handling characteristics of the vehicle, characterizing the tire material compounds to understand the tire durability. Rubber is a viscoelastic material, the friction and wear in rubber is intricate as opposed to other elastic materials. Based on experimental observations in the past, friction and wear are influenced by factors like material properties, normal load/ pressure, sliding velocity, temperature, surface characteristics, and environmental conditions. In addition, the frictional losses at the contact interface are considered to compose of adhesion, hysteresis and viscous components and wear is categorized as – adhesive, abrasive, fatigue, corrosive and erosive. Recent developments in computational power encouraged researchers and engineers in developing analytical and computational models that consider the physical mechanisms occurring at the contact interface. The present research focuses on obtaining a comprehensive friction and contact mechanics model considering the effect of surface roughness at different length scales, surface condition (dry/ wet) and asperity interaction. In addition, the developed model in conjunction with a brush model is considered for estimating the tire traction characteristics such as the forces and moments. A finite element simulation of rubber block sliding on a rough substrate is performed using a multiscale technique for estimation of friction, contact mechanics and abrasion parameters under dry condition. The results thus obtained are compared with the analytical model that is developed for wet conditions. Experimental validation of the friction estimated using the analytical and numerical methods will be performed using a linear sliding friction tester developed in collaboration with other members of the group.