Controlling tribological interactions, such as friction and adhesion between contacting interfaces is critical for the advancement of technologies such as microelectromechanical systems (MEMS) devices. The challenge in MEMS device lubrication lies in the inherent nature of the material’s surface at the nanoscale as well as the nature of the surfaces typically used during experimentation. Device surfaces often display nanoscale roughness with surface asperities dictating the tribological properties between interfaces, yet the vast majority of past research has focused predominately on nanotribological studies of thin films on flat silicon substrates to model the behavior of these self-assembled wear-reducing coatings. New model surfaces have been manufactured and integrated into experiments in which surfaces with controlled asperity sizes act as more realistic models of MEMS surfaces. As friction and adhesion between real surfaces in sliding contact are dominated by the interactions of nanoscaled surface asperities, this research is an extension of previous work, moving beyond smooth surfaces by manufacturing and implementing new experimental platforms possessing controlled asperity sizes. The influence of asperity size on the tribological properties of these contacts is being studied for both native oxide and organosilane derivatized surfaces. These studies more readily mimic the conditions found at true asperity-asperity contacts. This research has aimed to develop new lubricant thin films that can effectively protect MEMS device surfaces during use with the long term goal of bringing MEMS devices out of the laboratory and into wide scale commercial use. This work investigates how self-assembled monolayers (SAMs) on curved surfaces can be utilized in manners that their analogs on flat surfaces cannot. SAMs on curved asperities can be used to trap short chain alcohols, which during contact may be released to function as an additional lubricant layer on the surface. Both atomic force microscopy and Fourier transform infrared spectroscopy have been employed to evaluate how chain disorder influences the protective function of these molecular lubricant layers on asperities. It was found that functionalized surfaces resisted wear and were able to operate under continuous scanning for longer time frames than unfunctionalized surfaces and that multicomponent films improved upon the performance of their base, single component analogs.

No description available.

Neste trabalho, a relação entre a adesão e o grau de desordem de superfícies em contato é estudada por meio de simulações por dinâmica molecular. O sistema em estudo é composto por um indentador cilíndrico rígido e um bloco deformável. Um reservatório térmico é colocado logo abaixo do bloco, de forma a manter o sistema a temperatura ambiente. Os sólidos são feitos do mesmo material e são modelados por intermédio do potencial genérico de Lennard-Jones. A adesão entre as superfícies é variada gradualmente por meio da variação do raio de corte do potencial que descreve a interação entre as superfícies indentador-bloco. Cada simulação se inicia com um recozimento, após o qual são realizadas as simulações de contato propriamente ditas, até que ocorra penetração de 1,7 raios atômicos. A força normal, a energia potencial, a temperatura e a energia cinética são acompanhadas ao longo do processo. Os resultados reforçam a importância da adesão no jump-to-contact, estando este fenômeno relacionado à geração de defeitos cristalinos. Há indícios de que a distribuição da carga, bem como a dissipação de energia, seja influenciada pela adesão na interface. Mostra-se que a desordem apresentada pelos átomos do bloco é proporcional à contribuição da adesão. / In this work, the relationship between adhesion and the degree of disorder of surfaces in contact is studied by means of molecular dynamics simulations. The system consists of a rigid cylindrical indenter and a deformable block. A heat reservoir is placed just below the block, in order to maintain the system at room temperature. Both solids are made of the same material and are modeled through the generic Lennard-Jones potential. Adhesion between the surfaces is gradually varied by varying the cut-off radius of the potential describing the interaction between the indenter surface and the block. Each contact simulation is preceeded by an annealing step, and is conducted until the penetration reaches 1.7 atomic radii. The normal force, the potential energy, kinetic energy and temperature are monitored throughout the process. The results reinforce the importance of adhesion in the phenomenon of jump-to-contact, which is also related to the generation of crystalline defects. The results indicate that the load distribution and power dissipation is influenced by the adhesion at the interface. It is shown that the disorder presented by the atoms of the block is proportional to the contribution of adhesion.

Le but de cette thèse est de faire évoluer les connaissances et la compréhension des systèmes lubrifiants en milieux aqueux, synthétiques comme biologiques. Pour cela, nous élaborons des systèmes de monocouches auto-assemblées de microgels thermosensibles de pNIPAM cationiques afin d’en étudier les propriétés mécaniques et nanotribologiques. Nous mettons au point plusieurs synthèses de microgels afin d’étudier l’effet de l’élasticité sur le comportement tribologique. Nous regardons également l’effet de la nature du greffage des microgels en sur-face, en élaborant une méthode de couplage chimique novatrice, pour comparer les propriétés de monocouches physi- et chimisorbées. Nous étudions les propriétés mécaniques en mi-lieux aqueux des couches des différents microgels en fonction de la température, de la nature du greffage et du sel en présence, à l’aide d’une Microbalance à Cristal de Quartz avec mesure de Dissipation (QCM-D). Le coeur de notre étude est réalisé à l’aide d’un Appareil de Forces de Surface (SFA) modifié pour permettre des mesures tribologiques, dont les résultats seront traités en deux parties. La première consiste à caractériser les forces normales de surface lors-que l’on comprime deux surfaces décorées de microgels. La seconde est constituée de l’ana-lyse de ces surfaces sous compression et cisaillement. Nous explorons les propriétés lubrifiantes du système et observons l’apparition une force de normale dépendant de la vitesse de cisaillement, et dont nous cherchons l’origine. Nous avons ainsi découvert un mécanisme propre au substrat souple, décoré de particules discrètes avec un contact répulsif sans friction à longue portée. / The aim of this project is to advance the knowledge and understanding of lubricating systems, whether synthetic or biological, in aqueous media. For this purpose, we develop self-assem-bled monolayer 2D-arrays of cationic pNIPAM thermosensitive microgels in order to study their mechanical and nanotribological properties. We establish several synthetic routes to modulate the microgel rigidity and study its effect on the tribological behaviour. We also look at the effect of the grafting nature of microgels on the substrate, by developing an innovative chemical coupling method, to compare the properties of physisorbed and chemisorbed mon-olayers. We probe the mechanical properties of the microgel layers in aqueous environment while varying the temperature, the nature of the grafting and the salts added to the system, primarily by using a Quartz Crystal Microbalance with Dissipation monitoring (QCM-D). The core of our study is performed using a modified Surface Forces Apparatus (SFA) which allows for tribological measurements, the results of which will be treated in two parts. First, we char-acterise the normal surface forces when compressing two surfaces decorated with the micro-gel layers. Second, we study the behaviour of these surfaces under compression and shear. We explore their lubricant properties and observe the appearance of a shear-induced velocity-dependent lift force, whose origin we seek to determine. We thus discovered a mechanism specific to a compliant substrate, decorated with discrete particles presenting a repulsive con-tact without friction at long range.

Este trabalho apresenta uma interpretação química quantitativa do atrito nas camadas mais externas do aço AISI 1045 nitretado e pós-oxidado com diferentes tempos de oxidação (0, 1, 5, 10, 20 e 30 min). A caracterização da morfologia, microestrutura, espessura e composição química qualitativa das camadas nitretadas e pós-oxidadas foi realizada por microscopia eletrônica de varredura (MEV) e espectroscopia de emissão óptica por descarga luminescente (GD-OES). A estrutura cristalina presente nas camadas mais superficiais das amostras foi identificada por difração de raios X (DRX), empregando ângulo rasante. Na camada nitretada, as amostras apresentam as fases de nitretos g’-Fe4N e e-Fe2-3N, enquanto que na camada oxidada há uma combinação das fases µ-Fe2O3 e Fe3O4 ou somente a fase Fe3O4 (amostra pós-oxidada por 1 min). A nanodureza foi obtida através de indentações de baixa penetração (até 200 nm). O módulo de elasticidade reduzido, a rugosidade superficial e o coeficiente de atrito (CoF) foram obtidos por ensaios de deslizamento unidirecional, utilizando o equipamento nanoindentador Nanotest-600 da MicroMaterials. O regime de deformação provocado por esses ensaios foi identificado através do cálculo do índice de plasticidade (Y). As amostras apenas nitretada e a pósoxidada durante 1 min apresentaram resultados constantes, dentro do erro experimental e da faixa de profundidade analisada (entre 100 e 200 nm), das propriedades tais como dureza, módulo elástico reduzido, rugosidade e índice de plasticidade, porém com coeficientes de atrito (CoF) diferentes. Desta forma, a evolução do CoF em função da química da superfície pode ser avaliada sem a influência de mudanças nas propriedades mecânicas mais importantes, que determinam o CoF. Esses resultados experimentais podem ser explicados usando um modelo baseado na origem fonônica do atrito, que depende da frequência vibracional característica das camadas mais externas do material. O modelo apresenta boa concordância com os resultados experimentais, com uma diferença de 3% entre teoria e prática. / Submitted by Marcelo Teixeira (mvteixeira@ucs.br) on 2014-06-23T11:22:55Z No. of bitstreams: 1 Dissertacao Márcia Freislebem.pdf: 7918824 bytes, checksum: d6e46c5d7b5a22f4ff1607cb915fa302 (MD5) / Made available in DSpace on 2014-06-23T11:22:55Z (GMT). No. of bitstreams: 1 Dissertacao Márcia Freislebem.pdf: 7918824 bytes, checksum: d6e46c5d7b5a22f4ff1607cb915fa302 (MD5) / This work presents a quantitative chemical interpretation of friction for the most external layers of AISI 1045 steel previously nitrided and post-oxidized during 0, 1, 5, 10, 20 and 30 min. The characterization of morphology, microstructure, thickness and qualitative chemical composition of nitrided and post-oxidized layers was performed by scanning electronic microscopy (SEM) and glow discharge optical emission spectroscopy (GD-OES) analysis. The superficial crystalline structure was identified by grazing incident X-ray diffraction. The nitrided layer presented g’-Fe4N and e-Fe2-3N phases, while the postoxidized layer presented µ-Fe2O3 and Fe3O4 phases or just Fe3O4 phase (1 min postoxidized sample). The nanohardness was determined by low depth indentations (until 200 nm). The reduced elastic modulus, the surface roughness and the coefficient of friction (CoF) were determined by unidirectional sliding tests, using a MicroMaterials Nanotest- 600 nanoindenter. The deformation regime was identified through the plasticity index (Y). Nitrided only and 1 min post-oxidized samples showed constant results, considering the experimental error and the analyzed depth range (from 100 to 200 nm), for hardness, reduced elastic modulus, surface roughness and plasticity index, but different CoF. Therefore, the evolution of CoF as a function of the surface chemistry can be assessed without any influence from the change of the most important mechanical properties that determine the CoF. These experimental results can be explained using a phononic model relying on the characteristic vibrational frequency of the most external layers of the material. The model shows a reasonable agreement with the experimental results, with a difference of 3% between theoretical and practical results.

Este trabalho apresenta uma interpretação química quantitativa do atrito nas camadas mais externas do aço AISI 1045 nitretado e pós-oxidado com diferentes tempos de oxidação (0, 1, 5, 10, 20 e 30 min). A caracterização da morfologia, microestrutura, espessura e composição química qualitativa das camadas nitretadas e pós-oxidadas foi realizada por microscopia eletrônica de varredura (MEV) e espectroscopia de emissão óptica por descarga luminescente (GD-OES). A estrutura cristalina presente nas camadas mais superficiais das amostras foi identificada por difração de raios X (DRX), empregando ângulo rasante. Na camada nitretada, as amostras apresentam as fases de nitretos g’-Fe4N e e-Fe2-3N, enquanto que na camada oxidada há uma combinação das fases µ-Fe2O3 e Fe3O4 ou somente a fase Fe3O4 (amostra pós-oxidada por 1 min). A nanodureza foi obtida através de indentações de baixa penetração (até 200 nm). O módulo de elasticidade reduzido, a rugosidade superficial e o coeficiente de atrito (CoF) foram obtidos por ensaios de deslizamento unidirecional, utilizando o equipamento nanoindentador Nanotest-600 da MicroMaterials. O regime de deformação provocado por esses ensaios foi identificado através do cálculo do índice de plasticidade (Y). As amostras apenas nitretada e a pósoxidada durante 1 min apresentaram resultados constantes, dentro do erro experimental e da faixa de profundidade analisada (entre 100 e 200 nm), das propriedades tais como dureza, módulo elástico reduzido, rugosidade e índice de plasticidade, porém com coeficientes de atrito (CoF) diferentes. Desta forma, a evolução do CoF em função da química da superfície pode ser avaliada sem a influência de mudanças nas propriedades mecânicas mais importantes, que determinam o CoF. Esses resultados experimentais podem ser explicados usando um modelo baseado na origem fonônica do atrito, que depende da frequência vibracional característica das camadas mais externas do material. O modelo apresenta boa concordância com os resultados experimentais, com uma diferença de 3% entre teoria e prática. / This work presents a quantitative chemical interpretation of friction for the most external layers of AISI 1045 steel previously nitrided and post-oxidized during 0, 1, 5, 10, 20 and 30 min. The characterization of morphology, microstructure, thickness and qualitative chemical composition of nitrided and post-oxidized layers was performed by scanning electronic microscopy (SEM) and glow discharge optical emission spectroscopy (GD-OES) analysis. The superficial crystalline structure was identified by grazing incident X-ray diffraction. The nitrided layer presented g’-Fe4N and e-Fe2-3N phases, while the postoxidized layer presented µ-Fe2O3 and Fe3O4 phases or just Fe3O4 phase (1 min postoxidized sample). The nanohardness was determined by low depth indentations (until 200 nm). The reduced elastic modulus, the surface roughness and the coefficient of friction (CoF) were determined by unidirectional sliding tests, using a MicroMaterials Nanotest- 600 nanoindenter. The deformation regime was identified through the plasticity index (Y). Nitrided only and 1 min post-oxidized samples showed constant results, considering the experimental error and the analyzed depth range (from 100 to 200 nm), for hardness, reduced elastic modulus, surface roughness and plasticity index, but different CoF. Therefore, the evolution of CoF as a function of the surface chemistry can be assessed without any influence from the change of the most important mechanical properties that determine the CoF. These experimental results can be explained using a phononic model relying on the characteristic vibrational frequency of the most external layers of the material. The model shows a reasonable agreement with the experimental results, with a difference of 3% between theoretical and practical results.

Neste trabalho, a relação entre a adesão e o grau de desordem de superfícies em contato é estudada por meio de simulações por dinâmica molecular. O sistema em estudo é composto por um indentador cilíndrico rígido e um bloco deformável. Um reservatório térmico é colocado logo abaixo do bloco, de forma a manter o sistema a temperatura ambiente. Os sólidos são feitos do mesmo material e são modelados por intermédio do potencial genérico de Lennard-Jones. A adesão entre as superfícies é variada gradualmente por meio da variação do raio de corte do potencial que descreve a interação entre as superfícies indentador-bloco. Cada simulação se inicia com um recozimento, após o qual são realizadas as simulações de contato propriamente ditas, até que ocorra penetração de 1,7 raios atômicos. A força normal, a energia potencial, a temperatura e a energia cinética são acompanhadas ao longo do processo. Os resultados reforçam a importância da adesão no jump-to-contact, estando este fenômeno relacionado à geração de defeitos cristalinos. Há indícios de que a distribuição da carga, bem como a dissipação de energia, seja influenciada pela adesão na interface. Mostra-se que a desordem apresentada pelos átomos do bloco é proporcional à contribuição da adesão. / In this work, the relationship between adhesion and the degree of disorder of surfaces in contact is studied by means of molecular dynamics simulations. The system consists of a rigid cylindrical indenter and a deformable block. A heat reservoir is placed just below the block, in order to maintain the system at room temperature. Both solids are made of the same material and are modeled through the generic Lennard-Jones potential. Adhesion between the surfaces is gradually varied by varying the cut-off radius of the potential describing the interaction between the indenter surface and the block. Each contact simulation is preceeded by an annealing step, and is conducted until the penetration reaches 1.7 atomic radii. The normal force, the potential energy, kinetic energy and temperature are monitored throughout the process. The results reinforce the importance of adhesion in the phenomenon of jump-to-contact, which is also related to the generation of crystalline defects. The results indicate that the load distribution and power dissipation is influenced by the adhesion at the interface. It is shown that the disorder presented by the atoms of the block is proportional to the contribution of adhesion.

No description available.

[pt] Nesta tese, apresentamos um procedimento utilizado para a calibração do fotodetector e dos cantileveres utilizados em nosso AFM para a medida de força lateral. Desenvolvemos um código em Matlab para o controle do microscópio que permitiu a realização do estudo da influência da força normal na fricção. Também foi desenvolvido um segundo código em Matlab para a medida automatizada da adesão. Apresentamos e discutimos a influência da energia livre superficial na fricção e adesão de várias superfícies. Neste trabalho um estudo da influência da condensação capilar na forca lateral foi estudado para superfícies hidrofílicas, e hidrofóbicas. Encontramos que as nano asperezas podem realizar contatos singulares descritos pelo modelo de Hertz ou múltiplos contatos de acordo com o modelo de Greenwood. O tipo de contato entre as nano asperezas pode ser controlado através da hidrofobicidade e da umidade relativa no ambiente de medida. É verificado que os meniscos formados entre ponta e superfície influenciam a força lateral, através do aumento da força normal e também através da energia gasta pela ponta para arrastar ou deformar o capilar durante seu deslocamento sobre a superfície. O efeito da cinética de condensação capilar da água sobre a fricção foi também estudado. É mostrado que a molhabilidade é determinante para a definição dos mecanismos da dissipação de energia entre as nanoasperezas. Apresentamos também a influência da hidrofobicidade superficial no coeficiente de atrito. A correlação observada entre o ângulo de contato e o coeficiente de atrito reforça a importância da cinética da condensação capilar nos processos de fricção que ocorre na escala de nanômetros. / [en] In this work, the procedures developed to the calibration of the AFM photodetector and cantilevers for lateral force measurements in our AFM is presented. A Matlab code that controls the microscope allows the study of the influence of the normal force on the lateral one. A second Matlab code was developed in order to study the adhesion forces in an automated way. We present and discuss the influence of the surface free energy on the friction and adhesion forces. In this work, the lateral forces were measured at hydrophilic and hydrophobic surfaces. It was observed that the nano asperities may form single asperity contacts described by the Hertz model as well as multi-asperity type of contacts described by the Greenwood model. The nanoasperity contact may be controlled by the wettability and ambient relative humidity. It is seen that the capillar formed between the tip and the surface influences the tip-surface normal force and the friction forces due to the dissipation of energy caused by the drag or brake of the capillar meniscous. The effect of capillary condensation kinetics was studied as well. It is shown that the surface wettability is determinant to the energy dissipation mechanism in nanoscale. The influence of the surface wettability on the friction coefficient is presented. The observed correlation between the friction coefficient and contact angle enhances the influence of the surface wettability and its kinetics in the friction forces at nanoscale.

Polymer brushes provide the responsive smart surfaces which can be used for fabrication of various devices. In this thesis work, adhesion, friction, and wear of polystyrene (PS) - poly(2-vinyl pyridine) (P2VP) and polystyrene - poly(acrylic acid) (PAA) binary brushes and corresponding monobrushes were investigated in dried state under controlled environment. Spin-coated films were also investigated for comparison. The aim was to explore possibilities to control/tune adhesion, friction, and wear between inorganic or polymeric surfaces by use of polymer brushes. Atomic force microscopy (AFM) with sharp silicon nitride tip and colloidal probes was employed to investigate the nanoscale adhesion and friction forces between different inorganic and polymeric surfaces. Adhesion and friction on the polymer brushes were comparable to that on the spin-coated films. Adhesion and friction force values were correlated, and were in accordance with the wettability of the brush surfaces for most of the samples. Switching in the adhesion and friction forces was observed for the PS+P2VP and PS+PAA binary brushes on treatment with selective solvents. Maximum switching in adhesion force and friction coefficient was by a factor of 2.7 and 5.4, respectively. Furthermore, switching of friction for mixed brush surface was observed during macroscale friction measurements using nanoindenter. Friction coefficients at macroscale were higher than those at the nanoscale. Moreover, adhesion and friction forces between the surfaces were significantly influenced by the humidity, grafting density of polymer brushes, chemical composition of top of the binary brush surface, and tip scan velocity. Nanowear studies were carried out with AFM using sharp silicon nitride tip while macrowear studies were carried out using nanoindenter. Nanowear on the surfaces was affected by molecular entanglements, adhesion and friction forces as well as shape and status of the tip. It was observed that the typical wear mode for PS brushes (treated with toluene) was ripple formation. In case of P2VP brushes (treated with ethanol) and PAA brushes (treated with pH 10 water), wear occurred via removal of the polymeric material. Wear mechanism observed for the monobrushes was similar to that observed for the spin-coated thick films of the same polymeric material. However, extent of the wear on the brush surfaces significantly differed from that on the spin-coated films. In case of PS+P2VP and PS+PAA binary brush samples, change in the wear mode was observed on treatment with the different selective solvents. On treatment with toluene (PS on the top), both of these binary brushes showed the wear by formation of the ripples. On the other hand, when these binary brushes were treated with selective solvent for P2VP or PAA, wear occurred mainly via removal of the polymeric material. The amount of wear increased with the number of scans for all the polymer brush samples. Moreover, wear on the polymer brush surfaces was also increased on increase in the applied load and decrease in the scan speed. Wear behavior on macroscale was averaged due to contact between surfaces at large number of asperities. Our results show that adhesion, friction, and wear of polymer surfaces can be controlled/tuned by the use of binary polymer brushes.