Elon J. Terrell

Two-dimensional nanostructured Y2O3 particles for viscosity modification

Nanoparticle additives have been shown to improve the mechanical and transport phenomena of various liquids; however, little has been done to try and explain the rheological modifications provided from such modifications from a theoretical standpoint. Here, we report a non-Einstein-like reduction of viscosity of mineral oil with the utilization of yttrium oxide nanosheet additives. Experimental results, coupled with generalized smoothed-particle hydrodynamics simulations, provide insight into the mechanism behind this reduction of fluid shear stress. The ordered inclination of these two-dimensional nanoparticle additives markedly improves the lubricating properties of the mineral oil, ultimately reducing the friction, and providing a way in designing and understanding next generation of lubricants.

Wind Turbine Tribology

Wind power is of increasing interest in society due to its prospects as an environmentally friendly source of renewable energy. The use of wind turbines to extract electrical energy from wind can be dated back to the late-1800s, with the 12 kW windmill generator by Charles Brush, as well as the mid-1900s, with the 1250 kW Smith-Putnam wind turbine. Developments in the wind industry were encouraged by the oil crisis in 1973.

Effect of Reciprocation Frequency on Friction and Wear of Vibrating Contacts Lubricated with Soybean-Based B100 Biodiesel

The use of renewable, bio-based fuels has become increasingly widespread in recent years, with a major example being biodiesel, a bio-derived alternative to Number 2 diesel fuel. The increased usage of biodiesel gives rise to an augmented need to understand its tribological effects on critical engine components. This study focused on determining the tribological performance of soybean-based B100 (i.e., pure) biodiesel within a fuel injector with varying oscillating frequency by performing a series of linear reciprocating tribological tests of biodiesel-lubricated interfaces with varying reciprocating frequency. Comparison of friction coefficient variation with reciprocating frequency indicated a transition from boundary lubrication to hydrodynamic lubrication as the frequency increased, while hysteresis loop and energy loss observations showed a transition between full stick and partial slip contact with increasing frequency. However, observations of induced wear showed the wear to increase with increasing frequency, most likely due to the augmented number of sliding cycles as well as an increased degree of interfacial slip.

Reducing contact resistance of macro-scale separable electrical contacts with single-layer graphene coatings

Systems in applications ranging from aerospace, automotive, to power distribution are all crucially comprised of electrical connectors. Every system with removable electrical components will have electrical connectors, and likewise these systems will be susceptible to the contact losses and failure modes of these connections. Corrosion is one of the largest causes of failure for electrical connectors and costs the electrical power distribution system in the United States hundreds of millions of dollars annually. This study demonstrates the viability graphene as a protective coating for copper connectors to mitigate corrosion and improve electrical contact performance. Graphene, which is constituted of single layers of hexagonally-oriented carbon atoms, is inherently ultra-thin, chemically and thermally stable, gas impermeable (a property which can inhibit corrosion), highly electrically conductive, and made from an abundant and low-cost components, making it an ideal candidate for implementation as a protective coating on electrical contacts. In this work, single-layer graphene was deposited on copper used in commercially-available electrical connectors. In static tests the electrical contact resistance of the graphene-coated copper contact was two orders of magnitude lower than the contact resistance of the bare copper, and performed similarly to a benchmark gold/nickel coating that is a widely-used anticorrosive coating, demonstrating graphenes protective utility.

Contact stress analysis of lightly compressed thin films: Modeling and experimentation

Although a number of models have been developed to describe the contact between nominally smooth surfaces, very few of these models have been validated with experiment. Therefore, in this study, an asperity-scale experimental contact measurement was conducted and compared to the predictions of two contact models. The experimental component of this study involved a flat diamond punch tip on a nanoindenter, which was used to compress a thin film that was lithographically patterned into isolated raised squares. This experimental method was developed in order to measure the predominantly elastic load response of the surface asperities on one of the isolated raised topography islands. The experimental measurements were compared to the predicted load responses of an existing analytical contact model as well as a finite element contact model that incorporated the topography of the raised island into its formulation. The predictions of both the models were shown to have reasonable agreement with the experimental ...

Current and Future Tribological Challenges in Wind Turbine Power Systems

The drive for an increase in the amount of sustainable energy sources has given rise to increasing interest in the development of wind turbine technology. Although wind turbines are being increasingly installed around the world, several of the components in a wind turbine have challenges related to tribology that can drastically reduce their expected lifetimes.Copyright

Influence of Coating Thickness and Substrate Elasticity on the Tribological Performance of PEEK Coatings

When a material is subjected to repeated sliding contact, a surface fatigue and crack nucleation may occur on its surface. This damage weakens the material and can lead to debris formation. In many practical engineering assemblies, a thin PEEK (poly-ether-ether-ketone) coating is applied to reduce the damage, since PEEK exhibits wear resistance, corrosion resistance self-lubricating capacity and is lightweight. However, little is known about the effect of coating thickness on the plastic deformation, residual stresses and energy dissipation of PEEK when placed under sliding load. Moreover, the effect of substrate rigidity on coating stresses and deformation under sliding load are also under-researched. Having such knowledge is of significant importance in order to reduce damage of engineering parts and extend their lifetime.In this study, the effects of PEEK coating thickness and substrate elasticity were analyzed using a 3D ball-on-flat finite element model as well as experimental analysis using a linear reciprocating tribometer. The experimental tests were performed with samples that incorporated PEEK coatings of various thicknesses on alumina and steel substrates. It was found that under a constant normal load, stresses, strain and energy dissipation were sensitive to both substrate material and coating thickness. It is shown from both simulation and experiment, the optimum combination, within the range the experiments were conducted, for minimizing residual stress and possible fatigue damage was an alumina substrate with 35μm PEEK coating thickness.Copyright

Squeeze Film Flow Analysis Using Moving Particle Semi-Implicit Calculation

We have presented an application of the modified Moving Particle Semi-implicit (MPS) method for squeeze film flows. In addition to calculating the flow field of a squeeze film using the full Navier-Stokes equations, this method has the advantage of being meshless, which gives it the capability of analyzing dynamic and/or highly transient squeeze films by discretizing the domain as a series of particles and numerically analyzing inter-particle interactions. Although past literature has indicated the MPS method in its original form to be unstable in terms of its calculation of pressure, a modified algorithm was implemented to provide agreement between the numerical results and the analytical solutions.Copyright

The Wear Characteristics of Graphene as an Atomically-Thin Protective Coating

Lamellar atomically-thin sheets such as graphene (and its bulk equivalent graphite) and molybdenum disulfide have emerged as excellent solid lubricants at the macro scale and show great promise as protective coatings for nanoscopic applications. In this study, the failure mechanisms of graphene under sliding are examined using atomistic simulations. An atomic tip is slid over a graphene membrane that is adhered to a semi-infinite substrate. The impact of sliding velocity and substrate rigidity on the wear and frictional behavior of graphene is studied. In addition, the interplay of adhesive and abrasive wear on the graphene coating is also examined. The preliminary results indicate that graphene has excellent potential as a nanoscale due to its atomically-thin configuration and high load carrying capacity.Copyright

Smoothed Particle Methods and Their Applicability to Tribology

An in-house solver was created to simulate contact mechanics and hydrodynamic lubrication using smoothed particle hydrodynamics (SPH), SPH is a meshfree, particle-based method that can be used to solve continuum problems in solid and fluid mechanics. In this approach, the problem domain is represented by particles that move according to prescribed governing equations. Because smoothed particle methods have been shown to have the advantage of being able to model large deformations without concern of degradation in numerical accuracy, they have received recent interest for application to complex problems in tribology—the exploration of which is the focus of this study.Copyright