W. Gregory Sawyer

Super-compressible foamlike carbon nanotube films.

We report that freestanding films of vertically aligned carbon nanotubes exhibit super-compressible foamlike behavior. Under compression, the nanotubes collectively form zigzag buckles that can fully unfold to their original length upon load release. Compared with conventional low-density flexible foams, the nanotube films show much higher compressive strength, recovery rate, and sag factor, and the open-cell nature of the nanotube arrays gives excellent breathability. The nanotube films present a class of open-cell foam structures, consisting of well-arranged one-dimensional units (nanotube struts). The lightweight, highly resilient nanotube films may be useful as compliant and energy-absorbing coatings.

The difficulty of measuring low friction : Uncertainty analysis for friction coefficient measurements

The experimental evaluation of friction coefficient is a common laboratory procedure; however, the corresponding measurement uncertainty is not widely discussed. This manuscript examines the experimental uncertainty associated with friction measurements by following the guidelines prescribed in international standards. The uncertainty contributors identified in this analysis include load cell calibration, load cell voltage measurement, and instrument geometry. A series of 20 tests, carried out under nominally identical conditions, was performed using a reciprocating pin-on-disk tribometer. A comparison between the experimental standard deviation and uncertainty analysis results is provided.

Writing in the granular gel medium

The reversible fluid-solid transition in granular gels enables the three-dimensional writing of soft, delicate, macroscopic structures with microscopic detail. Gels made from soft microscale particles smoothly transition between the fluid and solid states, making them an ideal medium in which to create macroscopic structures with microscopic precision. While tracing out spatial paths with an injection tip, the granular gel fluidizes at the point of injection and then rapidly solidifies, trapping injected material in place. This physical approach to creating three-dimensional (3D) structures negates the effects of surface tension, gravity, and particle diffusion, allowing a limitless breadth of materials to be written. With this method, we used silicones, hydrogels, colloids, and living cells to create complex large aspect ratio 3D objects, thin closed shells, and hierarchically branched tubular networks. We crosslinked polymeric materials and removed them from the granular gel, whereas uncrosslinked particulate systems were left supported within the medium for long times. This approach can be immediately used in diverse areas, contributing to tissue engineering, flexible electronics, particle engineering, smart materials, and encapsulation technologies.

Tribological Sensitivity of PTFE/Alumina Nanocomposites to a Range of Traditional Surface Finishes

Wear tests were performed with polytetrafluoroethylene (PTFE) + Al 2 O 3 nanocomposites on various manufactured surfaces to determine whether or not the wear resistance of these nanocomposites is a strong function of surface preparation. Four different surface finishes of grade 304 stainless steel counterfaces were used: electropolished (R q = 88 nm), lapped (R q = 161 nm), wet-sanded (R q = 390 nm), and dry-sanded (R q = 578 nm). PTFE + Al 2 O 3 nanocomposites made from powders of roughly 2-20 μm PTFE (matrix) and ∼44 nm Al 2 O 3 (filler) were prepared at filler weight percentages of 0, 1, 5, and 10% and tested on each surface finish. Additionally, 5 wt% 44-nm nanocomposites were compared to identically prepared 5 wt% 80- and 500-nm Al 2 O 3 filled PTFE composites on each surface. Friction coefficients were between 0.12 and 0.19 and wear rates decreased from K = 810 × 10− 6 mm 3 /(Nm) for the 5 wt% 500-nm alumina-filled PTFE on the dry-sanded surface to K = 0.8 × 10− 6 mm 3 /(Nm) for the 5 wt% 80-nm filled composite on the lapped surface. It was found that the minimum wear rate occurred on the lapped counterface for every composite, and the wear rate is a strong function of the transfer film thickness and morphology.

Effect of Particle Size on the Wear Resistance of Alumina-Filled PTFE Micro- and Nanocomposites

It was long supposed that the ability of hard particle fillers to reduce the wear rate of unfilled PTFE (typically ∼ 10− 3 mm 3 /Nm) by an order of magnitude or more was limited to fillers of microscale or greater, as nano-fillers would likely be encapsulated within the large microscale PTFE wear debris rather than disrupting the wear mechanism. Recent studies have demonstrated that nano-fillers can be more effective than microscale fillers in reducing wear rate while maintaining a low coefficient of friction. This study attempts to further elucidate the mechanisms leading to improved wear resistance via a thorough study of the effects of particle size. When filled to a 5% mass fraction, 40- and 80-nm alumina particles reduced the PTFE wear rate to a ∼ 10−7 mm 3 /Nm level, two orders of magnitude better than the ∼ 10−5 mm 3 /Nm level with alumina micro-fillers at sizes ranging from 0.5 to 20 μm. Composites with alumina filler in the form of nanoparticles were less abrasive to the mating steel (stainless 304) countersurfaces than those with microparticles, despite the filler being of the same material. In PTFE containing a mixture of both nano- and micro-fillers, the higher wear rate microcomposite behavior predominated, likely the result of the continued presence of micro-fillers and their abrasion of the countersurface as well as any overlying beneficial transfer films. Despite demonstrating such a large effect on the wear rate, the variation of alumina filler size did not demonstrate any significant effect on the friction coefficient, with values for all composites tested additionally falling near the μ = 0.18 measured for unfilled PTFE at this studys 0.01 m/s sliding speed.

Wear-Rate Uncertainty Analysis

Wear due to relative motion between component surfaces is one of the primary modes of failure for many engineered systems. Unfortunately, it is difficult to accurately predict component life due to wear as reported wear rates generally exhibit large scatter. This paper analyzes a reciprocating tribometer in an attempt to understand the instrument-related sources of the scatter in measured wear rates. To accomplish this, an uncertainty analysis is completed for wear-rate testing of a commercially available virgin polytetrafluoroethylene pin on 347 stainless steel counterface. It is found that, for the conditions selected in this study, the variance in the experimental data can be traced primarily to the experimental apparatus and procedure. Namely, the principal uncertainty sources were found to be associated with the sample mass measurement and volume determination.

Complex Dental Structure and Wear Biomechanics in Hadrosaurid Dinosaurs

A Toothy Problem Large mammalian herbivores such as horses and bison are well known to possess a complex, grinding dentition that facilitates processing of their tough, cellulose-rich plant diet. Hadrosaurid, or duck-billed, dinosaurs also possessed complex teeth, but how this was achieved has been unknown because reptiles typically possess simple teeth. Erickson et al. (p. 98) show how Hadrosaurs evolved teeth composed of six tissues, which allowed for the development of tooth complexity rivaling, or exceeding, that of modern herbivorous mammals. The teeth in duck-billed dinosaurs were as functionally refined as those of present-day mammals. Mammalian grinding dentitions are composed of four major tissues that wear differentially, creating coarse surfaces for pulverizing tough plants and liberating nutrients. Although such dentition evolved repeatedly in mammals (such as horses, bison, and elephants), a similar innovation occurred much earlier (~85 million years ago) within the duck-billed dinosaur group Hadrosauridae, fueling their 35-million-year occupation of Laurasian megaherbivorous niches. How this complexity was achieved is unknown, as reptilian teeth are generally two-tissue structures presumably lacking biomechanical attributes for grinding. Here we show that hadrosaurids broke from the primitive reptilian archetype and evolved a six-tissue dental composition that is among the most sophisticated known. Three-dimensional wear models incorporating fossilized wear properties reveal how these tissues interacted for grinding and ecological specialization.

Meeting the Contact-Mechanics Challenge

This paper summarizes the submissions to a recently announced contact-mechanics modeling challenge. The task was to solve a typical, albeit mathematically fully defined problem on the adhesion between nominally flat surfaces. The surface topography of the rough, rigid substrate, the elastic properties of the indenter, as well as the short-range adhesion between indenter and substrate, were specified so that diverse quantities of interest, e.g., the distribution of interfacial stresses at a given load or the mean gap as a function of load, could be computed and compared to a reference solution. Many different solution strategies were pursued, ranging from traditional asperity-based models via Persson theory and brute-force computational approaches, to real-laboratory experiments and all-atom molecular dynamics simulations of a model, in which the original assignment was scaled down to the atomistic scale. While each submission contained satisfying answers for at least a subset of the posed questions, efficiency, versatility, and accuracy differed between methods, the more precise methods being, in general, computationally more complex. The aim of this paper is to provide both theorists and experimentalists with benchmarks to decide which method is the most appropriate for a particular application and to gauge the errors associated with each one.

Sliding orientation effects on the tribological properties of polytetrafluoroethylene

The chemical inertness, high melting point, and intrinsic lubricity of polytetrafluoroethylene (PTFE) have been used to develop solid lubricating parts for operation in extreme environments, from frying pans to satellites. The atomic-level mechanisms associated with friction and wear at PTFE surfaces are elucidated here by systematic investigations of the frictional anisotropy measured with respect to chain orientation. In particular, a combination of atomic-scale simulations, nanometer-scale atomic force microscopy experiments, micrometer-scale microtribometers experiments, and macroscale pin-on-disk experiments are used. Data across these length scales, from both the computational and experimental approaches, provide a consistent view of the mechanisms by which the structural orientation of PTFE contributes to its unique tribological properties.

In Situ Lubrication with Boric Acid: Powder Delivery of an Environmentally Benign Solid Lubricant

In situ deposition of boric acid in dry powder form is investigated as a potential environmentally benign solid lubricant for sliding metal contacts. Boric acid is widely used in industrial processes and agriculture, is not classified as a pollutant by EPA, and produces no serious illnesses or carcinogenic effects from exposure to solutions or aerosols. In this study, boric acid powder is aerosolized and entrained in a low-velocity jet of nitrogen gas, which is directed at a self-mated 302 SS sliding contact in a rotating pin-on-disc tribometer. The effects of powder flow rate, sliding speed, normal load, and track diameter on coefficient of friction and wear rate are investigated. Friction coefficients below μ = 0.1 can be consistently reached and maintained as long as the powder flow continues. Wear rates are reduced over 2 orders of magnitude. Review led by Paul Bessette