Wear Mechanisms, Composition and Thickness of Antiwear Tribofilms Formed from Multi-Component Lubricants.

The antiwear properties of tribofilms formed on steel surfaces lubricated with various multi-component lubricants were investigated at an elevated temperature and under load-speed conditions conducive to sliding in the boundary lubrication regime. The lubricants contained base oil, reduced-level (secondary) zinc dialkyl dithiophosphate (ZDDP), and nitrogenous dispersant. The wear resistance of the tribofilms produced from different oil blends was evaluated in the context of the rate of change in the sliding track volume (wear rate for material loss) and the load-bearing capacity, chemical composition, and thickness of the tribofilms.

Microstructure Evolution and Fretting Wear Mechanisms of Steels Undergoing Oscillatory Sliding Contact in Dry Atmosphere.

Variations in the microstructure and the dominant fretting wear mechanisms of carbon steel alloy in oscillatory sliding contact against stainless steel in a dry atmosphere were evaluated by various mechanical testing and microanalytical methods. These included scanning electron microscopy and energy dispersive spectrometry with corresponding elemental maps of the wear tracks, in conjunction with cross-sectional transmission electron microscopy of samples prepared by focused ion beam machining to assess subsurface and through-thickness changes in microstructure, all as a function of applied load and sliding time. Heavily dislocated layered microstructures were observed below the wear tracks to vary with both the load and sliding time.

Dynamics of Tribofilm Formation in Boundary Lubrication Investigated Using In Situ Measurements of the Friction Force and Contact Voltage.

The complex dynamics of tribofilm formation on boundary-lubricated steel surfaces were investigated in real time by combining in situ measurements of the temporal variation of the coefficient of friction and contact voltage. Sliding experiments were performed with various blends consisting of base oil, zinc dialkyl dithiophosphate (ZDDP) additive, and two different dispersants at an elevated oil temperature for a wide range of normal load and fixed sliding speed. The evolution of the transient and steady-state coefficient of friction, contact voltage, and critical sliding distance (time) for stable tribofilm formation were used to evaluate the tribological performance of the tribofilms.

In vitro degradation of fibrous bilayer poly-L-lactic acid scaffolds.

Polymer degradation and mechanical properties are of paramount importance in tissue engineering. The degradation rate of polymeric scaffolds is influenced by several important material and environmental factors. In particular, the mechanical support provided by the scaffold to the surrounding tissue during tissue regeneration is critical for that it directly impacts the cell behavior through mechanical signals sensed by mechanoreceptors on the cell surface.

Mechanical Behavior of Bamboo-Like Structures under Transversal Compressive Loading.

Inspired by many biological structures in nature, biomimetic structures demonstrate significantly better mechanical performance than traditional engineering structures. The exceptional mechanical properties of natural materials are attributed to the hierarchical architecture of their structure. Consequently, the implementation of biomimetic structures in the design of lightweight structures with tailored mechanical properties has been constantly increasing in many fields of science and engineering.

Novel Electrospun Polycaprolactone/Calcium Alginate Scaffolds for Skin Tissue Engineering.

After decades of research, fully functional skin regeneration is still a challenge. Skin is a multilayered complex organ exhibiting a cascading healing process affected by various mechanisms. Specifically, nutrients, oxygen, and biochemical signals can lead to specific cell behavior, ultimately conducive to the formation of high-quality tissue.

Design Challenges in Polymeric Scaffolds for Tissue Engineering.

Numerous surgical procedures are daily performed worldwide to replace and repair damaged tissue. Tissue engineering is the field devoted to the regeneration of damaged tissue through the incorporation of cells in biocompatible and biodegradable porous constructs, known as scaffolds. The scaffolds act as host biomaterials of the incubating cells, guiding their attachment, growth, differentiation, proliferation, phenotype, and migration for the development of new tissue.

Thermal stability and diffusion characteristics of ultrathin amorphous carbon films grown on crystalline and nitrogenated silicon substrates by filtered cathodic vacuum arc deposition.

Amorphous carbon (a-C) films are widely used as protective overcoats in many technology sectors, principally due to their excellent thermophysical properties and chemical inertness. The growth and thermal stability of sub-5-nm-thick a-C films synthesized by filtered cathodic vacuum arc on pure (crystalline) and nitrogenated (amorphous) silicon substrate surfaces were investigated in this study. Samples of a-C/Si and a-C/SiN/Si stacks were thermally annealed for various durations and subsequently characterized by high-resolution transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS).

A molecular dynamics study of the oxidation mechanism, nanostructure evolution, and friction characteristics of ultrathin amorphous carbon films in vacuum and oxygen atmosphere.

Amorphous carbon (a-C) films are characterized by extraordinary chemical inertness and unique thermophysical properties that are critical to applications requiring oxidation-resistant, low-friction, and durable overcoats. However, the increasing demands for ultrathin (a few nanometers thick) a-C films in various emerging technologies, such as computer storage devices, microelectronics, microdynamic systems, and photonics, make experimental evaluation of the structural stability and tribomechanical properties at the atomic level cumbersome and expensive. Consequently, the central objective of this study was to develop comprehensive MD models that can provide insight into the oxidation behavior and friction characteristics of ultrathin a-C films exhibiting layered through-thickness structure.

Structure evolution during deposition and thermal annealing of amorphous carbon ultrathin films investigated by molecular dynamics simulations.

The evolution of the structure of amorphous carbon (a-C) films during deposition and thermal annealing is of significant interest from both the materials science and application perspectives. However, despite the voluminous literature of studies dealing with the deposition and physical properties of a-C films, basic understanding of the structure evolution due to phase change during film growth and heating is fairly sparse and empirical, presumably due to the lack of high-resolution instruments that can probe structural changes at the atomic and molecular levels in real time. Molecular dynamics (MD) is a powerful computational method for studying atomic/molecular-scale movement and interactions.

Highly flexible, foldable, and rollable microsupercapacitors on an ultrathin polyimide substrate with high power density.

The design and functionality of extremely flexible, foldable, and rollable microsupercapacitors (MSCs) with in-plane interdigital electrodes that consist of single-walled carbon nanotube (SWCNT) networks on an ultrathin polyimide substrate are demonstrated through experiments and finite element simulations. The all-solid-state MSCs can be reversibly bent, folded, and rolled purely elastically without degradation of their electrical performance. The simulation results confirm that the deformation in bent, folded, and rolled MSCs is purely elastic.

Highly Stretchable Microsupercapacitor Arrays with Honeycomb Structures for Integrated Wearable Electronic Systems.

The rapid development of portable and wearable electronics has greatly increased the demand for energy storage devices with similar physical properties and integration capability. This paper introduces a honeycomb polydimethylsiloxane substrate for stretchable microsupercapacitor (MSC) arrays, which enables facile integration with other electronics. The honeycomb structure can accommodate a large deformation without producing excessive strain in the MSCs and interconnects.

Failure mechanisms of single-crystal silicon electrodes in lithium-ion batteries.

Long-term durability is a major obstacle limiting the widespread use of lithium-ion batteries in heavy-duty applications and others demanding extended lifetime. As one of the root causes of the degradation of battery performance, the electrode failure mechanisms are still unknown. In this paper, we reveal the fundamental fracture mechanisms of single-crystal silicon electrodes over extended lithiation/delithiation cycles, using electrochemical testing, microstructure characterization, fracture mechanics and finite element analysis.

High-energy-density, all-solid-state microsupercapacitors with three-dimensional interdigital electrodes of carbon/polymer electrolyte composite.

Novel all-solid-state microsupercapacitors (MSCs) with three-dimensional (3D) electrodes consisting of active materials (i.e., graphene or activated carbon (AC) particles) and polymer electrolyte (PE) designed for high-energy-density storage applications were fabricated and tested in this work.

A catalytic path for electrolyte reduction in lithium-ion cells revealed by in situ attenuated total reflection-Fourier transform infrared spectroscopy.

Although controlling the interfacial chemistry of electrodes in Li-ion batteries (LIBs) is crucial for maintaining the reversibility, electrolyte decomposition has not been fully understood. In this study, electrolyte decomposition on model electrode surfaces (Au and Sn) was investigated by in situ attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy. Simultaneously obtained ATR-FTIR spectra and cyclic voltammetry measurements show that lithium ethylene dicarbonate and lithium propionate form on the Au electrode at 0.

Electrospun bilayer fibrous scaffolds for enhanced cell infiltration and vascularization in vivo.

Bilayer poly(L-lactic acid) fibrous scaffolds consisting of a thin aligned-fiber layer (AFL) and a relatively thick random-fiber layer (RFL) were fabricated by an electrospinning technique, which uses two slowly rotating parallel disks as the collector. The morphology and structure of the bilayer scaffolds were examined by high-magnification scanning electron microscopy and confocal microscopy. The bilayer scaffolds demonstrated a gradual variation in through-thickness porosity and fiber alignment and an average porosity much higher than that of conventionally electrospun scaffolds (controls) with randomly distributed fibers.

Sum frequency generation vibrational spectroscopy of 1,3-butadiene hydrogenation on 4 nm Pt@SiO2, Pd@SiO2, and Rh@SiO2 core-shell catalysts.

1,3-Butadiene (1,3-BD) hydrogenation was performed on 4 nm Pt, Pd, and Rh nanoparticles (NPs) encapsulated in SiO2 shells at 20, 60, and 100 °C. The core-shells were grown around polyvinylpyrrolidone (PVP) coated NPs (Stöber encapsulation) prepared by colloidal synthesis. Sum frequency generation (SFG) vibrational spectroscopy was performed to correlate surface intermediates observed in situ with reaction selectivity.

Mechanical properties of electrospun bilayer fibrous membranes as potential scaffolds for tissue engineering.

Bilayer fibrous membranes of poly(l-lactic acid) (PLLA) were fabricated by electrospinning, using a parallel-disk mandrel configuration that resulted in the sequential deposition of a layer with fibers aligned across the two parallel disks and a layer with randomly oriented fibers, both layers deposited in a single process step. Membrane structure and fiber alignment were characterized by scanning electron microscopy and two-dimensional fast Fourier transform. Because of the intricacies of the generated electric field, bilayer membranes exhibited higher porosity than single-layer membranes consisting of randomly oriented fibers fabricated with a solid-drum collector.

In vitro investigation of skin damage due to microscale shearing.

Despite several studies dealing with the mechanical and tribological properties of skin, the majority of these investigations have been performed at macroscopic levels. However, because of the multilayer structure of skin, it is necessary to perform studies at microscopic scales to reveal the effect of individual layer constituents on the overall skin response to mechanical stimuli. To bridge the gap in knowledge of the micromechanical behavior of skin, a custom-made mechanical tester, optical microscopy, and cross-sectional histology were used to examine the deformation and tribological behavior of porcine skin subjected to various normal and shear loadings.

Tuning the electronic structure of titanium oxide support to enhance the electrochemical activity of platinum nanoparticles.

Two times higher activity and three times higher stability in methanol oxidation reaction, a 0.12 V negative shift of the CO oxidation peak potential, and a 0.07 V positive shift of the oxygen reaction potential compared to Pt nanoparticles on pristine TiO2 support were achieved by tuning the electronic structure of the titanium oxide support of Pt nanoparticle catalysts.

Engineering the microstructure of electrospun fibrous scaffolds by microtopography.

Controlling the structure and organization of electrospun fibers is desirable for fabricating scaffolds and materials with defined microstructures. However, the effects of microtopography on the deposition and, in turn, the organization of the electrospun fibers are not well understood. In this study, conductive polydimethylsiloxane (PDMS) templates with different micropatterns were fabricated by combining photolithography, silicon wet etching, and PDMS molding techniques.

Plasma surface chemical treatment of electrospun poly(L-lactide) microfibrous scaffolds for enhanced cell adhesion, growth, and infiltration.

Poly(l-lactide) (PLLA) microfibrous scaffolds produced by electrospinning were treated with mild Ar or Ar-NH3/H2 plasmas to enhance cell attachment, growth, and infiltration. Goniometry, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS) measurements were used to evaluate the modification of the scaffold surface chemistry by plasma treatment. AFM and XPS measurements showed that both plasma treatments increased the hydrophilicity without affecting the integrity of the fibrous structure and the fiber roughness, whereas Ar-NH3/H2 plasma treatment also resulted in surface functionalization with amine groups.

Surface chemical patterning for long-term single-cell culture.

Surface chemical patterning of polystyrene (PS) dishes for long-term single-cell culture was accomplished by oxygen plasma treatment through the windows of a polydimethylsiloxane membrane mask that produced hydrophilic areas of different shapes and sizes, followed by overnight incubation with either Pluronic F108 solution or a mixture of Pluronic F108 solution and fibronectin. Selective cell attachment on pattern areas of PS dishes was investigated in light of cell seeding experiments and X-ray photoelectron spectroscopy measurements. Activation of the hydrophilic areas of patterned PS surfaces by serum proteins in the culture medium was conducive to cell attachment on the pattern areas of dishes incubated with only Pluronic solution.

Furan hydrogenation over Pt(111) and Pt(100) single-crystal surfaces and Pt nanoparticles from 1 to 7 nm: a kinetic and sum frequency generation vibrational spectroscopy study.

Sum frequency generation surface vibrational spectroscopy and kinetic measurements using gas chromatography have been used to systematically study the adsorption and hydrogenation of furan over Pt(111) and Pt(100) single-crystal surfaces and size-controlled 1.0-nm, 3.5-nm and 7.

Plasma-assisted surface chemical patterning for single-cell culture.

Polyethylene glycol (PEG)-like films were grafted onto various substrates by plasma polymerization of diethylene glycol dimethyl ether (diglyme). Low-power diglyme plasma conditions yielded PEG-like films exhibiting nonfouling behavior, as evidenced by the significant decrease in protein adsorption and cell attachment on substrates coated by these films. Different chemical patterns were produced on substrates with grafted PEG-like films using silicon or polydimethylsiloxane membrane shadow masks fabricated by photolithography and a simple one-step plasma etch process.