Search NASA⌕ Search

SEARCH · Search NASA

Results for “bonded and sliding”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Using irradiation-induced defects as pinning sites to minimize self-alignment in twisted bilayer graphene

Preparing bi-layer graphene under a magic twisting angle of ~1.1° has been challenging due to its strong tendency for self-alignment. We propose a method to pin graphene layers and minimize their self-rotation when positioned close to each other. The feasibility is demonstrated by the present study using molecular dynamics simulations. C 60 clusters are used to bombard two individual graphene layers, creating damage on both layers. When two irradiated layers are moving closer to each other, defects from irradiation damaged zones can interact with each other, hence acting as pinning sites to immobilize graphene and minimize rotation or gliding. Dangling bonds from defective regions of each plane induce the formation of sp bonds. Upon sliding, the bond is strong enough to induce the formation of one-dimensional carbon single chain, acting as a thread to constrain the relative movements.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Microscale mechanical modeling of deformable geomaterials with dynamic contacts based on the numerical manifold method

Abstract Micromechanical modeling of geomaterials is challenging because of the complex geometry of discontinuities and potentially large number of deformable material bodies that contact each other dynamically. In this study, we have developed a numerical approach for micromechanical analysis of deformable geomaterials with dynamic contacts. In our approach, we detect contacts among multiple blocks with arbitrary shapes, enforce different contact constraints for three different contact states of separated, bonded, and sliding, and iterate within each time step to ensure convergence of contact states. With these features, we are able to simulate the dynamic contact evolution at the microscale for realistic geomaterials having arbitrary shapes of grains and interfaces. We demonstrate the capability with several examples, including a rough fracture with different geometric surface asperity characteristics, settling of clay aggregates, compaction of a loosely packed sand, and failure of an intact marble sample. With our model, we are able to accurately analyze (1) large displacements and/or deformation, (2) the process of high stress accumulated at contact areas, (3) the failure of a mineral cemented rock samples under high stress, and (4) post-failure fragmentation. The analysis highlights the importance of accurately capturing (1) the sequential evolution of geomaterials responding to stress as motion, deformation, and high stress; (2) large geometric features outside the norms (such as large asperities and sharp corners) as such features can dominate the micromechanical behavior; and (3) different mechanical behavior between loosely packed and tightly packed granular systems.

58 GEOSCIENCES↗
DOE OSTI2021

Conductive and anti-freezing hydrogels constructed by pseudo-slide-ring networks

In this work, stretchable, tough, and anti-freezing hydrogels were prepared using partially carboxymethylated polyrotaxanes and polyacrylamides. The carboxylic acid groups of α-cyclodextrins in the polyrotaxane and the amide groups in polyacrylamide are hydrogen-bonded, affording a pseudo-slide-ring network, greatly enhancing the hydrogels’ macroscale mechanical properties, anti-freezing features, and electrical conductivity for the fabrication of a cold-temperature strain sensor.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Structurally Driven Environmental Degradation of Friction in MoS2 Films

Abstract We report an investigation of the friction mechanisms of MoS 2 thin films under changing environments and contact conditions using a variety of computational and experimental techniques. Molecular dynamics simulations were used to study the effects of water and molecular oxygen on friction and bonding of MoS 2 lamellae during initial sliding. Characterization via photoelectron emission microscopy (PEEM) and Kelvin probe force microscopy (KPFM) were used to determine work function changes in shear modified material within the top few nanometers of MoS 2 wear scars. The work function was shown to change with contact conditions and environment, and shown by density functional theory (DFT) calculations and literature reports to be correlated with lamellae size and thickness of the basally oriented surface layer. Results from nanoscale simulations and macroscale experiments suggest that the evolution of the friction behavior of MoS 2 is linked primarily to the formation or inhibition of a basally oriented, molecularly thin surface film with long-range order.

Curry, John F.↗
DOE OSTI2022

What is Retrogradation? [Slides]

Glucose is a monomer building block for complex polymers. Starch is made by covalently bonding together a chain of glucose monomers.Starch absorbs water and forms an amorphous gel upon heating – amorphous starch granules are an important component of baked bread.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Traction-separation response of bilayer graphene interfaces: The role of an intercalated single atomic layer of water molecules and hydroxyl groups

Molecular dynamics simulations of the normal and shear traction-separation response of dry and wet bilayer graphene interfaces were performed. The influence of a single, atomic layer of water molecules sandwiched between graphene layers and hydroxyl functionalization on one or both layers were investigated. Hydroxyl groups provided a shielding effect during normal separation while favoring stick-slip friction under sliding conditions. The single, atomic layer of water molecules confined between hydrophilic and asymmetric hydrophobic/hydrophilic surfaces was found to ease sliding friction. The mechanisms arose from the balance between the attractive and repulsive graphene/water interactions through bridges of single water molecules between the two surfaces and the structure of the water monolayer under confinement for which an out-of-plane water/water hydrogen bond network could not be developed. The single, atomic layer of water molecules reduced the sliding friction the most when both surfaces were hydrophilic and lowered the sliding friction of the asymmetric hydrophobic/hydrophilic interfaces to that when both surfaces were hydrophobic. An optimal hydroxyl surface coverage of less than 10% may exist for efficient water lubrication, with diminishing effect with increasing number of hydroxyl groups. Finally, the findings offer a design strategy for assembling bilayer graphene interfaces with tailored friction and sliding contact properties.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Friction-based riveting technique for AZ31 magnesium alloy

Rivet materials used in the aerospace and automotive industries are typically alloys of aluminum (Al) or steel. Magnesium (Mg) is an attractive alternative because it is 33% and 75% lighter than Al and steel, respectively. However, a primary barrier for using Mg as rivet material is its poor formability at room temperature, resulting in brittle fracture of the rivet head during cold riveting. Hot riveting is possible but suffers from high cycle time due to the need for pre-heating. A new friction-based riveting technique, Rotating Hammer Riveting (RHR), is reported to fully form AZ31 Mg rivet heads in a mere 0.23 seconds cycle time. Heat and pressure generated through severe plastic deformation during the process was sufficient to form the Mg rivet head without the need for a separate pre-heating operation. Due to dynamic recrystallization, AZ31 Mg grains in the rivet head were significantly refined from about 15 to 2.6 µm during RHR, which favors the formability of Mg rivet by triggering grain boundary sliding and reducing plastic anisotropy of Mg. In addition, RHR joints showed a metallurgical bond between the rivet head and top sheet, which eliminates a significant pathway for corrosion.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Active microrheology and dynamic phases for pattern-forming systems with competing interactions

We consider the driven dynamics of a probe particle moving through an assembly of particles with competing long-range repulsive and short-range attractive interactions, which form crystal, stripe, labyrinth, and bubble states as the ratio of attraction to repulsion is varied. We show that the probe particle exhibits a depinning-like threshold from an elastic regime, where the probe particle is trapped by interactions with the other particles, to a plastic flow regime, where the probe particle can break bonds in the surrounding medium. For a fixed particle density, the depinning threshold and sliding velocity of the probe particle vary nonmonotonically as the attraction term is increased. A velocity minimum appears near the crystal to stripe crossover, and there is a significant increase in the depinning threshold in the bubble regime when the probe particle is strongly confined inside the bubbles. For fixed attractive interaction but increasing particle density, the behavior is also nonmonotonic, and there are jumps and drops in the velocity and depinning threshold corresponding to points at which the system transitions between different structures. There are also several distinct flow states that can be characterized by the amount of plastic deformation induced by the probe particle in the surrounding medium. Each flow state generates a different amount of effective drag on the probe particle, and there can be jumps in the velocity-force curve at transitions between the states. In conclusion, we also find that when oriented stripes are present, the probe particle can move along the stripe in an edge transport state that has a finite Hall angle.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Contrasting structural reversibility and magnetic correlations in isostructural honeycomb magnets CrCl3 and 𝛼−RuCl3

We report a comparative neutron single crystal diffraction study of the structural and magnetic properties of layered halides CrCl3 and 𝛼−RuCl3. They host a honeycomb arrangement of transition metal ions with distinct electronic configurations and undergo a first-order structural transition between high-temperature 𝐶⁢2/𝑚 and low-temperature 𝑅⁢‾‾‾3. Both compounds show a step-like change in the 𝑐-lattice parameter across the structure transition. In contrast, the in-plane lattice response is quite different: 𝛼−RuCl3 exhibits an abrupt hysteretic change across the transition accompanied by progressive crystalline degradation upon thermal cycling, whereas CrCl3 shows a smooth in-plane lattice evolution and remains structurally robust. Magnetically, CrCl3 orders into an A-type antiferromagnetic structure at 𝑇𝑁=14 K and exhibits pronounced diffuse magnetic scattering extending up to about 40 K. 𝛼−RuCl3 shows no observable magnetic diffuse scattering above its zigzag antiferromagnetic ordering temperature 𝑇𝑁=7.6 K. These results suggest that the contrasting structural responses arise from an interplay between interlayer sliding energetics, stacking-strain coupling, and elastic accommodation of the stacking transition. The distinct chemical bonding and electronic configurations of the two compounds provide a microscopic basis for their different lattice responses to the structure transition and magnetic correlations.

Morgan, Zachary [ORNL] (ORCID:0000000243625911)↗
DOE OSTI2023

Using Ultrasonics to Optimize the Processing Parameters of Thermoset Polymers [Slides]

Ultrasonics is used as a non-destructive evaluation method to improve our understanding of polymers. Coupling ultrasonics and FTIR can help determine which bonds are more relevant to the mechanical properties of the polymer. Modeling the cure kinetics can determine if the curing process of a sample deviates from the baseline. Cure kinetics influence the final chemical structure of the polymer. This method opens a new pathway to design and adapt polymers for high demanding applications.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Ultrafast formation of a transient two-dimensional diamondlike structure in twisted bilayer graphene

Due to the absence of matching carbon atoms at honeycomb centers with carbon atoms in adjacent graphene sheets, theorists predicted that a sliding process is needed to form AA, AB', or ABC stacking when directly converting graphite into sp 3 bonded diamond. Here, using twisted bilayer graphene, which naturally provides AA and AB' stacking configurations, we report the ultrafast formation of a transient two-dimensional diamondlike structure (which is not observed in aligned graphene) under femtosecond laser irradiation. This photoinduced phase transition is evidenced by the appearance of bond lengths of 1.94 and 3.14 Å in the time-dependent differential pair distribution function using MeV ultrafast electron diffraction. Molecular dynamics and first-principles calculation indicate that sp 3 bonds nucleate at AA and AB' stacked areas in a moiré pattern. This work sheds light on the direct graphite-to-diamond transformation mechanism, which has not been fully understood for more than 60 years.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

MXene/graphene oxide nanocomposites for friction and wear reduction of rough steel surfaces

Development of solid lubricant materials that render reliable performance in ambient conditions, are amenable to industrial size and design complexities, and work on engineered surfaces is reported. These coatings are composed of Ti 3 C 2 T x -Graphene Oxide blends, spray-coated onto bearing steel surfaces. The tribological assessment was carried out in ambient environmental conditions and high contact pressures in a ball-on-disc experimental set-up. The evaluation yielded that the use of Ti 3 C 2 T x -Graphene-Oxide coatings led to substantial reduction in friction down to 0.065 (at 1 GPa contact pressure and 100 mm/s) in comparison to the uncoated of single-component-coated surfaces, surpassing the state-of-the-art. The coatings also provided excellent protection against wear loss of the substrate and counter-face. The results were explained based on the observations from Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, and nanoindentation measurements. In operando formation of a dense, hard and stiff, dangling-bond-saturated tribolayer was observed to be the reason for the sustained lubricity even at high test loads and sliding speeds. This report presents the holistic exploration and correlation of structure-property-processing pertaining to the advancement of solid lubrication science.

36 MATERIALS SCIENCE↗