Task-Specific Ionic Liquids and Deep Eutectic Solvents for Lubrication Technology

It is estimated that around 23% of the world’s total energy consumption comes from tribological contacts, namely due to energy losses during the mechanical movements, which accentuated the need for more efficient lubrication. Lubricants have been used to lower friction and wear by separating the surfaces sliding in relative motion, increasing the energy efficiency and lifetime of mechanical components. It is of extreme importance that the used lubricants comply with the need for higher sustainability and, with that in mind, researchers all over the world are continuously in the search for new and improved ways of reducing friction and wear. Ionic Liquids (ILs) are organic salts with low melting point (generally lower than 100 °C) that have attracted the research community due to their very interesting properties such as high chemical and thermal stability, high ionic conductivity, non-flammability, ease in dissolving organic, inorganic and polymeric materials and their tunability towards several applications through the combination of different cations and anions. Besides that, they have very low vapor pressures which makes them environmentally friendly materials. Ionic Liquids often have good performance in friction and wear reduction, by enhancing the tribofilm formation between different tribopairs, making them very promising alternative lubricants. However, they are very expensive to be used as pure lubricants. Viable alternatives are the use of ionic liquids as additives to a base oil or the use of Deep Eutectic Solvents (DESs), which are mixtures possessing a significant decrease on the melting temperature comparing to the original individual components. DESs have similar properties to ILs but can be less toxic, cheaper and easier to prepare, which makes them very interesting and promising lubricant candidates. In this thesis, the use of different ionic liquids as additives and deep eutectic solvents as lubricants is proposed, with the goal of reducing friction and wear between moving parts, towards a more sustainable world. The most promising fluids can be applied in Nano and Micro electromechanical devices (NEMS/MEMS) which are made of silicon, and also in bearings of steel, which is one of the main materials used in industry.Estima-se que ~23% da energia total consumida no mundo provem de contactos tribológicos, nomeadamente devido a perdas de energia durante movimentos mecânicos, o que acentua a necessidade de uma lubrificação mais eficiente. Os lubrificantes têm vindo a ser utilizados para reduzir o atrito e desgaste através da separação das superfícies deslizantes em movimento relativo, aumentando a eficiência energética e o tempo de vida dos componentes mecânicos. É de extrema importância que o uso de lubrificantes cumpra os critérios de uma maior sustentabilidade e, com isso em mente, os investigadores estão continuamente à procura de formas novas e melhoradas de reduzir o atrito e desgaste. Os Líquidos Iónicos são sais orgânicos com baixo ponto de fusão (geralmente abaixo de 100 °C) que têm vindo a atrair a atenção da comunidade científica devido às suas propriedades interessantes tais como elevada estabilidade química e térmica, elevada condutividade iónica, não inflamabilidade, facilidade em dissolver materiais orgânicos, inorgânicos e poliméricos e a sua capacidade de adequação para diferentes aplicações através da combinação de catiões e aniões diferentes. Além disso, apresentam uma pressão de vapor muito baixa, o que os torna materiais amigos do ambiente. Os líquidos iónicos têm frequentemente bom desempenho em termos de redução de atrito e desgaste, originando a formação de tribofilmes entre diferentes tribopares, o que os torna lubrificantes alternativos muito promissores. No entanto, o seu preço é demasiado alto para serem utilizados como lubrificantes puros. Algumas alternativas viáveis são o uso de líquidos iónicos como aditivos a um óleo base ou o uso de Solventes Eutéticos Profundos (em inglês Deep Eutectic Solvents, DESs), que são misturas cuja temperatura de fusão é muito menor do que as dos componentes individuais que lhes deram origem. Os DESs apresentam propriedades semelhantes às dos líquidos iónicos mas são menos tóxicos, mais baratos e mais fáceis de preparar, o que os torna muito interessantes como lubrificantes. Nesta tese, propõe-se a utilização de diferentes líquidos iónicos como aditivos e solventes eutéticos profundos como lubrificantes, com o objetivo de diminuir o atrito e desgaste entre superfícies com partes móveis, tendo em vista um mundo mais sustentável. Os fluídos mais promissores poderão ser utilizados em dispositivos Nano e Micro eletromecânicos (NEMS/MEMS) que são constituídos por silício, e também em rolamentos de aço, que é um dos principais materiais utilizados na indústria

Advance in Tribology Study of Polyelectrolyte Multilayers

This review introduced the preparation and structural characterization of polyelectrolyte multilayers in recent years and also summarized the tribology research progress of the polyelectrolyte multilayers, including tribological properties, surface adhesion characteristics, and wear resistance properties. Statistics analysis indicated that nanoparticles‐doped polyelectrolyte multilayers present better friction and wear performance than pristine polyelectrolyte multilayers. Furthermore, the in situ growth method resulted in improved structural order of nanoparticles composite molecular deposition film. In situ nanoparticles not only reduced the molecular deposition film surface adhesion force and friction force but also significantly improved the life of wear resistance. That was due to the nanoparticles that possessed a good load‐carrying capacity and reduced the mobility of the polymer‐chain segments, which can undergo reversible shear deformation. Based on this, further research direction of in situ nanoparticles molecular deposition film was proposed

Chameleon Coatings: Adaptive Surfaces to Reduce Friction and Wear in Extreme Environments

Adaptive nanocomposite coating materials that automatically and reversibly adjust their surface composition and morphology via multiple mechanisms are a promising development for the reduction of friction and wear over broad ranges of ambient conditions encountered in aerospace applications, such as cycling of temperature and atmospheric composition. Materials selection for these composites is based on extensive study of interactions occurring between solid lubricants and their surroundings, especially with novel in situ surface characterization techniques used to identify adaptive behavior on size scales ranging from 10−10 to 10−4 m. Recent insights on operative solid-lubricant mechanisms and their dependency upon the ambient environment are reviewed as a basis for a discussion of the state of the art in solid-lubricant materials

Book of Abstracts of 11th Iberian Conference on Tribology

info:eu-repo/semantics/publishedVersio

Tribology of self-lubricating SU-8 composites for micro-electro mechanical systems (MEMS) applications

Ph.DDOCTOR OF PHILOSOPH

Lubrication of High Sliding Silicon Micromachines

A major challenge in silicon devices based on micro-electro-mechanical systems (MEMS) is the provision of effective lubrication for sliding parts. This greatly limits the development and exploitation of MEMS devices, with current designs avoiding sliding contacts where possible. This thesis describes research aimed at lubricating high sliding MEMS devices. A micro-scaled tribometer has been constructed to obtain measurements of friction between two sliding silicon surfaces. This work focuses on lubricating MEMS with liquids, a self-replenishing lubrication method which had been dismissed previously as they were assumed to carry too much viscous drag. The major finding is that ferromagnetic fluids make excellent lubricants for sliding MEMS surfaces. These fluids provide low friction at high speeds, and reduce the boundary friction at low speeds when the hydrodynamic film is absent. The properties of such fluids allow the liquid to be contained in the presence of a magnetic field, meaning that only a small, localised amount is required. Low viscosity liquids were also shown to provide acceptably low friction at high speeds. These results agreed reasonably well with theory. Friction modifier (FM) additives were added to low viscosity liquids in order to reduce boundary friction by forming boundary films, when no hydrodynamic film is generated at low speeds. Drag has also been shown to be insignificant. A study of the wear of silicon surfaces under prolonged sliding was conducted. Previous studies have focussed on dry coatings and apparently untreated surfaces. In this thesis, the effects of different surface preparations, the use of low viscosity liquids and vapour phase lubrication on wear have been studied. This thesis concludes that it is feasible to use liquids to lubricate sliding MEMS. High sliding MEMS is possible and practicable in future if self-replenishing methods, such as those studied in this work, are employed in real devices

Tribological Behavior of Functional Surface: Models and Methods

Material loss due to wear and corrosion and high resistance to motion generate high costs. Therefore, minimizing friction and wear is a problem of great importance. This book is focused on the tribological behavior of functional surfaces. It contains information regarding the improvement of tribological properties of sliding elements via changes in surface topography. Tribological impacts of surface texturing depending on the creation of dimples on co-acting surfaces are also discussed. The effects of various coatings on the minimization of friction and wear and corrosion resistance are also studied. Friction can be also reduced by introducing a new oil

Exploration of Graphene-like 2D Materials for Energy Management and Interface Enhancement Applications

Ever since the discovery of graphene in 2004, graphene-like 2D materials and their derivatives have attracted extensive investigations because of their exceptional physical and chemical properties. At present, the study of graphene-like 2D materials is at a stage where most of their outstanding physical and chemical properties have been discovered, but the technology for incorporating them into practical commercial products is rarely revealed. For the potential practical industrial applications of graphene-like 2D materials, energy management and interface enhancement are two of the most promising areas. So far, the behavior of the commercialized graphene-like 2D material products is far from their theoretical performance and expectations as a result of defects and π-π agglomeration, etc. In this regard, there is plenty of research room at the bottom for exploring their practical industrial applications. At present, surface modification is the most widely used strategy to cope with agglomerations. While to be widespread in market, developing low-cost, uniform, and high-quality preparation technology, and encountering the intrinsic agglomeration issues of graphene-like 2D materials are two of the main challenges. To focus on the above two issues, we developed the functionalization method for graphene-like 2D materials, including graphene and hexagonal boron nitride, and explored their potential industrial applications in energy management and interface enhancement. Further, mass production technology and industrial demonstration for graphene and hexagonal boron nitride were explored in some chapters. The main scientific conclusions and innovations of this thesis are listed as below:At first, Chapter 2 presents the experimental research study on using graphene-like 2D materials for energy management, especially in heat dissipation. With the rapid development of microelectronics and 5G communications, efficient heat dissipation is severely demanded for future electronics. To improve heat dissipation efficiency of electronics, based on the ultrahigh thermal conductivity of graphene-like 2D materials, this chapter explored two experimental works, including lightweight and high-performance graphene enhanced heat pipe and hexagonal boron nitride enhanced thermally conductive and electrically insulation heat spreader. (1) Graphene Enhanced Heat PipeIn this work, a unique lightweight and high thermal performance graphene heat pipe were firstly designed and developed. At first, the inner structures of graphene enhanced heat pipe were optimized, including the wicker structures, the filling volume of working fluids and the preparation of high thermal conductivity graphene film. Compared to the conventional copper-based heat pipe, our graphene enhanced heat pipe improves the specific cooling capacity more than 3 times. Further, COMSOL Multiphysics was used to establish the cooling model for graphene enhanced heat pipe. And the equation for quantifying the contribution factor from container and phase change was established. Finally, a graphene/copper composite heat pipe was studied to further improve reliability and mechanical strength. (2) Hexagonal Boron Nitride Enhanced Heat SpreaderIn this work, a hexagonal boron nitride based heat spreader was prepared by electrospinning with polyvinylpyrrolidone. After electrospun, the hexagonal boron nitride nanosheets are aligned along the fiber, and thus increasing the thermal conductivity. At first, the exfoliation technology was investigated. The result shows that a mixture of water and isopropanol (Vwater:VIPA=1:3) shows the highest exfoliation efficiency. With the optimized hexagonal boron nitride particle geometry and loading, the in-plane thermal conductivity of hexagonal boron nitride based heat spreader reaches 22 W m-1 K-1, this value is comparable to most of the reported work. Particularly, such electrospinning process is constant and scalable, showing high potential for mass-production.Chapter 3 still focuses on the application of utilizing graphene-like 2D materials for energy management but specifically in energy storage. Based on the ultrahigh electric mobility, large surface area, flexible, lightweight properties, graphene is an attractive option for energy storage. Therefore, graphene was investigated for electrical double layer capacitors and in-plane micro-supercapacitors in this chapter.(1) Graphene Enhanced Electric Double-layer CapacitorIn this work, a scalable soft template strategy was developed to prepare graphene foam with high electrochemical performance as electrode for supercapacitors. The specific surface areas and wettability of graphene foam is tailored by doping. Further, density functional theory simulation reveals why increasing the polarity of graphene largely improves its wettability. Afterwards, the unique porous structure, low ohm resistance, and high electrical conductivity largely improve the electrochemical performance of graphene foam electrodes and thus achieve ultrahigh specific ca pa city (550 F g-1), cycling sta bility ( 96.1% ca pa city retention after 10 000 cycles at a high current density of 10 A g-1), and outstanding rate capability (308 \ua0\ua0F \ua0\ua0g-1 a \ua0t 100 \ua0\ua0A \ua0\ua0g-1). (2) Graphene Based In-plane Micro-supercapacitorIn this work, graphene assembled film was used to replace the conventional silicon wafer for fabricating flexible and high thermal performance micro-supercapacitors. The result shows that such replacement decreases the surface temperature of micro-supercapacitors by 4 \ub0C, and the graphene based micro-supercapacitor present a similar electrochemical behavior with the referenced silicon based micro-supercapacitor. In addition, the graphene assembled film substrate can work as heat spreader for micro-supercapacitor, thus saving spaces and optimizing the following packaging procedures. This work paves the way for utilizing graphene assembled film in semiconductors.Chapter 4 presents the application of using functional graphene-like 2D materials for interface enhancements due to their high Young’s module, large surface area, anti-friction, etc. Graphene-like 2D materials enhanced composites and bio-application are two of the main categories for the commercialization of interface enhancement. However, the graphene-like 2D materials suffer from π-π agglomeration, which leads to poor dispersibility in solvents and matrix. As a result, graphene-like 2D materials enhanced composites exhibit lower property than their theoretical expectations. At present, surface functionalization is the most effective strategy to encounter the π-π agglomeration. Therefore, this chapter explored the application of using functional graphene-like 2D materials in composites, including graphene enhanced water-borne epoxy coatings and hexagonal boron nitride enhanced cement repair materials.(1) Graphene Enhanced Water-borne Epoxy CoatingGraphene was used to lower the coefficient of friction and extend the lifetime of the water-borne epoxy coating in this work. To improve the dispersibility and the compatibility with epoxy, p-hydroxybenzene diazonium salt was prepared to functional graphene. With the optimized geometry and loading, 30 times less coefficient of friction than graphene-free coatings were achieved. And the wear-out time is more than 2 times longer than the three commercial graphene oxide enhanced coatings. This result is confirmed by Applied Nanosurface AB, Sweden. Besides, mass production technology up to 300 g per batch was developed for the functional graphene. The geometry of graphene was optimized, and the result shows that with the same functional groups, the larger graphene sheets show higher tribological performance than their smaller encounters. Finally, this functionalization strategy was further developed to improve the dispersibility of carbon nanotubes too. (2) Hexagonal Boron Nitride Enhanced Cement Repair MaterialThis work explored the application of using hexagonal boron nitride to enhance cement repair materials. To improve the dispersibility in cement repair materials and the adhesion with substrates, hexagonal boron nitride was functionalized by carboxymethyl cellulose. After functionalization, the surface zeta potential of hexagonal boron nitride decrease from -5.61 mV to -55.07 mV, and thus largely improves its dispersibility. Results show the incorporation of hexagonal boron nitride improve mechanical strength of cement repair materials by contributing to forming alite. Besides, for the repair material containing h-BN, most of the failure happened at the interface repair material/concrete, while the failure is mainly happening in the concrete for the sample containing FBN. Cooperated with a local cement company (Lanark AB), this work has demonstrated the commercial application as repair materials for walls.Besides, we studied the functional graphene quantum dots for mRNA based drug delivery platform. After complexed with mRNA, the transfection efficiency of the graphene quantum dots based drug delivery platform is 25% with a formation concentration as 4000 ng mL-1. A comparable transfection efficiency could be achieved at much lower doses if the ratio between the carrier and the cargo is optimized. This graphene quantum dots based drug delivery platform exhibits excellent processability. This work describes a potentially strategy for prepare stable and effective mRNA delivery systems

Surface Properties of Nanopore-Structured Metals and Oxides

The importance of understanding the properties of textured surfaces is growing with their potential wide engineering applications. In this thesis research, nanopore structures of metals and oxides were examined to determine the interactions between environmental object and the textured surfaces. The major applications of nanopore structures are micro/nanoelectromechanical systems (MEMS/NEMS), energy devices, sensors, and environmental devices. In order to achieve better performance in each, it needs to consider three critical surface properties such as surface forces, electrochemical performances, and wettability. In this research, the surface properties of nanopore structures have been explored with understanding the essence of contact. This research uses experimental approach combined with basic analysis in physical principles. Experiments include fabrication of nanopore structures, investigation of surface force, electrochemical evaluation, and wetting/electrowetting studies of nanopore structures. Metallic nanopore structures (MNSs) of nickel were characterized by using an atomic force microscope (AFM) and a triboscope. The mechanisms of bacteria desorption were examined by alumina nanopore structures (ANSs) with various pore sizes. The kinetics of ion-transfer on MNSs was studied using Electrochemical Impedance Spectroscopy (EIS) and Cyclic Voltametry (CV). The (electro-) wetting behavior of MNSs were examined using a droplet shape measurement system. A physics based analysis was conducted in order to understand the principles of the nanopore effects on environments suitable for various applications. Results lead to the successful identification of critical geometrical factors. A contact model has been established to understand properties of textured surfaces. Specific design factors, which are related to the geometry of the textured surfaces has been identified. This research revealed fundamental mechanisms of contact and establish a relationship between morphology/geometry and surface properties. The findings in this thesis research afford new approach to optimize applications of textured surface. The proposed contact models are beneficial to the surface design and application of sustainable micro/nanodevices. This thesis includes eight chapters. The first chapter introduces the background and fundamental knowledge related to current research in order to understand the basics. Followed by the chapter two of motivation and objectives, chapter three discusses materials and experimental details, chapter four and five cover the surface forces, chapter six studies the electrochemical performances, chapter seven investigates the (electro-)wettability, and the conclusions and future recommendations are presented in chapter eight