Future perspectives on sustainable tribology

AbstractThis paper highlights the future perspectives of sustainable tribology by examining the economic, environmental and social impact of three tribological case studies. One case study examines the sustainability and durability of micro-CHP systems looking the tribological phenomena generated within a scroll expander system. The scroll is the main part of a specific micro-CHP system and experiences wear and cavitation damage. The tribological optimization of the scroll expander improves the sustainability of the micro-CHP unit while it has a serious economic and environmental impact to the consumers and to the society in general. Another case study is focused on friction and wear performance of lifeboat launch slipways. The causes of high friction and wear during the RNLI's lifeboat launches along an inclined slipway are investigated with a view to reducing the environmental impact due to slipway panel wear and lubricant release into the marine environment. The project encompasses the sustainable design of slipway panels using design modifications based on tribological investigations to double their lifespan, while environmental and economic impact was significantly reduced by the use of biodegradable greases and water as lubricants. The final case study involves an investigation of recycled plastic materials to replace polyurethane used on skateboard wheels, scooters and similar applications. Polyurethane (PU) is difficult to recycle. With the dwindling resources and environmental problems facing the world today, recycling for both waste reduction and resource preservation has become an increasingly important aspect of sustainability. The tribological results showed that recycled polycarbonate plastic can effectively act as a substitute to polyurethane wheels. Moreover, sustainability considerations showing the environmental benefits of the use of recycled plastics over PU include reducing the CO2 footprint by 50% and the energy consumed by 60%, among other benefits. These case studies emphasise the importance of sustainable tribology in our epoch showing that increased sustainability performance can be achieved through tribology to a significant extent in many cases, providing stability to our world and more viable long term growth to our societies

30th International Conference on Condition Monitoring and Diagnostic Engineering Management (COMADEM 2017)

Proceedings of COMADEM 201

Nanoindentation Analysis of Evolved Bearing Steel under Rolling Contact Fatigue (RCF)

The bearing material operated under RCF is subjected to the triaxial stress state where work hardening followed by softening has been reported under the contact track. Such nonconformities (hardening/softening along with microstructural alterations) create complexities to model the cyclic hardening of bearing material under RCF. Current study presents a semi-empirical approach to evaluate the evolved subsurface response of bearing material with the help of a three-faced pyramidal Berkovich nanoindenter employing expanding cavity model for strain hardening materials. The expanding cavity model converts the localized measured hardness change to flow stresses which have been evolved during strain-hardening and microstructural phase changes of the bearing material. Moreover, to evaluate the representative stress-strain curve of the altered microstructure, a 5um spherical indenter was employed in a cyclic loading manner. The use of the spherical indenter with the integration of Field and Swain numerical model enables to extract the representative flow curve of the material at highly localized areas which cannot be possible even with miniature uniaxial tension/compression test

Solution-processed stretchable Ag2S semiconductor thin films for wearable self-powered nonvolatile memory

Department of Materials Science and EngineeringStretchable thin films have facilitated the developments in the field of flexible and stretchable electronics such as deformable sensors, displays, memories and energy devices which have received significant attention due to their potential use in smart wearable devices. Potential candidates for stretchable materials include polymer-based elastomers and their composites with functional fillers but they intrinsically suffer from low electrical properties. Compared to large plastic deformations observed in elastomeric organic materials, inorganic semiconductors have low plasticity (< 0.2%) due to their unique bonding properties which restrict applications in stretchable electronics. ??-Ag2S bulk crystals, a member of the metal chalcogenide, is considered as a promising candidate to produce deformable semiconductor layers in flexible and stretchable electronics due to its outstanding mechanical and electrical properties. An extremely low slippage energy and high cleavage energy between the crystal planes of ??-Ag2S bulk crystals exhibit both semiconducting behavior and ductility similar to metals. Such unusual characteristics have abundant potential for energy and electronic applications. Although several studies reported the use of Ag2S bulk ingots in energy devices, the fabrication of Ag2S thin films based stretchable electronics have never been realized due to highly complicated synthetic procedures. Typical procedures for producing ??-Ag2S crystals require energy intensive process, such as SPS sintering method and complicated deposition equipment set-up, limiting both the available substrates and processing temperatures for the fabrication of flexible and stretchable electronics. The solution process of metal chalcogenides thin films has been of great interest since it provides the ability to fabricate high quality and scalable thin films in a low-cost manner. Recent discoveries of alkahest solvent, a mixture of amine and thiol solutions allowed soluble inorganic to be utilized as ink processing for thin films. Furthermore, the metal chalcogenide inks can achieve highly crystalline and uniform thin films using a simple solution process and a subsequent heat treatment. I herein report solution-process synthesis of ductile ??-Ag2S thin films and the manufacturing process of all inorganic, self-powered and stretchable memory devices. The molecular Ag2S complex solution was synthesized by chemical reduction of bulk Ag2S powder to produce a wafer-scale, highly crystalline Ag2S thin film. Thin films exhibit elasticity through inherent ductility and maintain structural integrity with 14.9 % tensile strain. In addition, Ag2S-based resistive memory manufactured has excellent bipolar switching characteristics (Ion/Ioff ratio of ~105, operational endurance of 100 cycles, and retention time > 106 s) and excellent mechanical elasticity (no degradation to elasticity). On the other hand, the device is very durable in a variety of chemical environments and temperatures between -196 and 300 ??C, especially at 85% relative humidity and 85 ??C for 168 hours. Finally, I demonstrate self-powered memory device combined with motion sensors for a wearable health monitoring system, which potentially offers to design high-performance wearable electronics for everyday use in real-world environments.ope

COMPUTATIONAL STUDY OF LIQUID-SOLID INTERFACE

The solid interface is of fundamental importance to numerous scientific and technological fields, such as heterogeneous catalysis, water splitting, electrochemistry, corrosion, drug delivery, tribology, and wetting. However, there is still limited knowledge of the structures and properties of the first adsorbed monolayer at distinct solid interfaces. We perform theoretical calculations to study the interactions between the solid surfaces and the first adsorbed monolayer at the interface, to understand from the electronic/atomic levels how the properties of solid surface influence the adsorption of the first monolayer at the solid interfaces. In Chapter 3, we developed the force field parameter for the thiolate/defective Au(111) interface. A molecular-level understanding of the interplay between self-assembled monolayers (SAMs) of thiolates and gold surface is of great importance to a wide range of applications in surface science and nanotechnology. Despite theoretical research progress of the past decade, an atomistic model, capable of describing key features of SAMs at reconstructed gold surfaces, is still missing. We carried out the periodic ab initio density functional theory (DFT) calculations to develop a new atomistic force field model for alkanethiolate SAMs on a reconstructed Au(111) surface. Based on the newly-developed force field parameters, the molecular dynamics (MD) simulations showed that the geometrical features of the investigated Au−S interface models and structural properties of the C10S SAMs are in good agreement with the ab initio MD studies. In Chapter 4, we investigate the wettability transition of the first adsorbed water layer (FAWL) on metal surfaces under a compressive lattice strain. A molecular-level description of a near-surface water structure and a handy manipulation of its properties are relevant to a broad range of scientific and technological phenomena. Through a series of MD simulations, we report the observation and characterization of a low-mobility FAWL and its tunable wetting transition at three metal surface models. The results reveal that (i) there is a formation of the FAWL, resulting from competitive water−water hydrogen bonding and water−solid interactions, which in turn dictates the wettability at water−metal interfaces, (ii) applying compressive lattice strain to metal substrates can induce interfacial wettability transition, and (iii) by adjusting the lattice strains, the bimetallic junction can host a switchable wettability transition. In Chapter 5, we study the structures and dynamics of the FAWL at distinct titanium dioxide (TiO2) surfaces. The behavior of the FAWL at TiO2 surfaces is critical to the fundamental understanding of TiO2-based applications. Using classical MD simulations, we study the properties of FAWL at four TiO2 surfaces, including the density profile, the angular orientation distribution, the HB structural and dynamic properties, and the vibrational spectra of water molecules in the FAWL. The calculation results demonstrate that the water molecules show distinct adsorption structures and HB properties at the studied TiO2 surfaces, leading to completely different vibrational signatures for the OH groups. In Chapter 6, we explore the role of interfacial potassium on the surface-enhanced Raman spectroscopy for single-crystal TiO2 nanowhisker by combining experiments and theoretical calculations. For TiO2-based surface-enhanced Raman spectroscopy (SERS) substrates, maintaining a good crystallinity is critical to achieving excellent Raman scattering. we report the successful synthesis of TiO2 nanowhiskers with excellent SERS properties. The enhancement factor, an index of SERS performance, is 4.96×106 for methylene blue molecule detecting, with a detection sensitivity around 10−7 mol·L−1. The DFT calculations reveal that interfacial potassium can form a monolayer structure on the TiO2 surface, resulting in a negatively charged TiO2 nanowhisker surface. Such structures would promote the adsorption of methylene blue molecules and thereby significantly improves SERS performance via the electrostatic adsorption effect. In Chapter 7, we investigate the friction of ionic liquid (IL)–glycol ether mixtures by combining AFM experiments and nonequilibrium MD (NEMD) simulations. We have measured the negative “friction–load dependence” of IL/oil mixtures at Ti interfaces. Such a negative phenomenon was also confirmed by our NEMD simulations, in which the friction force declines as the normal load increases. NEMD simulations revealed a structural reorientation of the studied IL as the normal load increases, i.e., the cation alkyl chains of ILs change the orientation to preferentially stay parallel to the tip scanning path, similar to the “blooming lotus leaf.” This reoriented IL structures produce a new sliding interface and reduce the friction force

Optimisation of Triboelectric Nanogenerator performance in vertical contact-separation mode

Triboelectric nanogenerator (TENG) is one of the most promising energy harvesters – a technology that uses repeated or reciprocating contact of suitably chosen materials to generate charge via the triboelectric effect (TE) and utilizes this as usable voltage and current. TENGs are attractive as they can continuously generate charge over a wide range of operating conditions and have several valuable advantages such as light weight, simple structure, low cost and high efficiency. Therefore, TENGs have been explored in a wide range of applications, including self-powered wearable electronics, powering electronics and even for harvesting ocean wave/wind energy. One of the major limitations of TENGs is their low power output (usually <500 W/m2). This thesis focuses of a few specific approaches to optimising TENG output performance. This thesis begins by presenting a solution to this challenge by optimizing a low permittivity substrate beneath the tribo-contact layer. The open circuit voltage is found to increase by a factor of 1.3 in moving from PET to the lower permittivity PTFE. TENG performance is also believed to depend on contact force, but the origin of the dependence had not previously been explored. Herein, we show that this behaviour results from a contact force dependent real contact area Ar as governed by surface roughness. The open circuit voltage Voc, short circuit current Isc and Ar for a TENG were found to increase with contact force/pressure. Critically, Voc and Isc saturate at the same contact pressure as Ar suggesting that electrical output follows the same evolution as Ar. Assuming that tribo charges can only transfer across the interface at areas of real contact, it follows that an increasing Ar with contact pressure should produce a corresponding increase in the electrical output. These results underline the importance of accounting for real contact area in TENG design, as well as the distinction between real and nominal contact area in tribo-charge density definition. High-performance ferroelectricassisted TENGs (Fe-TENGs) are developed using electrospun fibrous surfaces based on P(VDFTrFE) with dispersed BaTiO3 (BTO) nanofillers in either cubic (CBTO) or tetragonal (TBTO) form in this thesis. TENGs with three types of tribo-negative surface were investigated and output increased progressively. Critically, P(VDF-TrFE)/TBTO produced higher output than P(VDFTrFE)/ CBTO even though permittivity is nearly identical. Thus, it is shown that BTO fillers boost output, not just by increasing permittivity, but also by enhancing the crystallinity and amount of the β-phase (as TBTO produced a more crystalline β-phase present in greater amounts)

Tribology and the Triboelectric Effect: A study into the influence of tribological factors on frictional electrification

The triboelectric effect has long been understood as the phenomenon of electric charge transfer resulting from mechanical contact. Although this has long been observed in a multitude of circumstances, ranging from daily life to specific engineering applications, a significant knowledge gap remains regarding the mechanisms that describe triboelectric charge transfer. Research into the triboelectric effect has however seen a recent surge in the previous decade, owing to the conception of the triboelectric nanogenerator (TENG) and its subsequent applications in energy recycling and self-powered sensing. Existing research now entails various models for predicting the output of TENG devices from an electrical engineering standpoint, in addition to qualitatively describing the mechanisms that drive triboelectric charging. However, many of these models fall short with regards to describing the role of tribological factors in these physical mechanisms. Similarly, the field of tribology has only recently begun to expand its interests into triboelectric phenomena. This research plays a key part in discerning the influence of tribology on triboelectric mechanisms and applications. Factors such as surface composition, contact topography, normal and tangential forces, and relative motion are determined, as well as the effects of material wear and the presence of contaminating media. This is achieved primarily through the modification of a mechanical testing apparatus with the integration of a high-impedance electronic measurement circuit for the correlation of mechanical and electrical measurements. Ultimately, a semi-analytical model is also constructed using these data as a way of providing a stepping stone towards a fundamental understanding of all aspects of the triboelectric effect