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At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

In situ friction and wear behavior of rubber materials incorporating various fillers and/or a plasticizer in high-pressure hydrogen

Polymers are used routinely for equipment and infrastructure in hydrogen vehicle refueling stations, but significant gaps remain in understanding their hydrogen compatibility. The tribological properties of these materials in a high-pressure hydrogen environment is important in preventing component failure and the need for frequent replacement. We present in situ tribological studies on model rubbers, which include common fillers and plasticizer, using an in situ tribometer developed previously. Results suggest a clear, yet complicated, combined effect of the high-pressure hydrogen and the additives on the tribological performance of the chosen materials, as compared to matching experiments performed in ambient air. We find that the additives improved wear resistance in EPDM but deteriorated that in NBR due to disparate additive-polymer interactions.

Kuang, Wenbin↗
DOE OSTI2018

Energy dissipation in polymer-bonded explosives with various levels of constituent plasticity and internal friction

The ignition of energetic materials (EM) under dynamic loading is mainly controlled by localized temperature spikes known as hotspots. Hotspots occur due to several dissipation mechanisms, including viscoplasticity, viscoelasticity, and internal friction along crack surfaces. In this work, to analyze the contributions of these mechanisms, we quantify the ignition probability, energy dissipation, damage evolution, and hotspot characteristics of polymer-bonded explosives (PBXs) with various levels of constituent plasticity of the energetic phase and internal crack face friction. Using PBX9501 consisting of HMX (Octahydro-1,3,5,7-Tetranitro-1,2,3,5-Tetrazocine) and Estane as a reference material, we analyze variants of this material with several values of the yield stress of the energetic phase and coefficients of internal crack face friction, while other parameters are kept unchanged. The impact loading involves piston velocities between 200 and 1200 m/s. The analysis uses a Lagrangian cohesive finite element framework that explicitly accounts for finite-strain elastic-viscoplastic deformation of the grains, viscoelastic deformation of the binder, arbitrary crack initiation and propagation in the grains and the binder, debonding between the grains and the binder, contact between internal surfaces, friction and frictional heating along internal surfaces, heat generation resulting from inelastic bulk deformation, and heat conduction. To determine the ignition status of the material or “go” or “no-go” state, we use a criterion based on a criticality threshold obtained from chemical kinetics calculations. For PBX with various levels of HMX plasticity and friction, the probability of ignition, the evolution of dissipation caused by plasticity and friction, the density of cracks, and the locations of cracks are quantified. Results show that samples with higher levels of constituent plasticity (lower yield strengths) or lower levels of internal friction are less likely to ignite. The relative importance of plasticity and friction depends on load intensity, with frictional heating decreasing as load intensity increases. Although the overall viscoplastic heating outweighs the overall frictional heating, friction plays a very important role in hotspot development at all load intensities analyzed, owing to the fact that frictional heating is more localized than viscoplastic heating. Finally, the predicted thresholds and ignition probabilities are expressed in a load intensity-load duration relation for PBX with different constituent properties.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Temperature-Dependent Friction and Wear Behavior of PTFE and MoS 2

We present an investigation of the temperature-dependent friction behavior of PTFE, MoS 2 , and PTFE-on- MoS 2 . Friction behavior was measured while continuously varying contact temperature in the range -150 to 175°C while sliding in dry nitrogen, as well as for self-mated PTFE immersed in liquid nitrogen. These results contrast with previous reports of monotonic inverse temperature dependent friction behavior, as well as reported high-friction transitions and plateaus at temperatures below about -20°C that were not observed, providing new insights about the molecular mechanisms of macro-scale friction. The temperature-dependent friction behavior characteristic of self-mated PTFE was found also on the PTFE-on-MoS 2 sliding contact, suggesting that PTFE friction was defined by sub-surface deformation mechanisms and internal friction even when sliding against a lamellar lubricant with extremely low friction coefficient (μ ~ 0.02). The various relaxation temperatures of PTFE were found in the temperature-dependent friction behavior, showing excellent agreement with reported values acquired using torsional techniques measuring internal friction. Additionally, hysteresis in friction behavior suggests an increase in near-surface crystallinity at upon exceeding the high temperature relaxation, T α ~ 116°C.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Solvent friction effects propagate over the entire protein molecule through low-frequency collective modes

Protein solvation dynamics has been investigated using atom-dependent Langevin friction coefficients derived directly from molecular dynamics (MD) simulations. To determine the effect of solvation on the atomic friction coefficients, solution and vacuum MD simulations were performed for lysozyme and staphylococcal nuclease and analyzed by Langevin mode analysis. The coefficients thus derived are roughly correlated with the atomic solvent-accessible surface area (ASA), as expected from the fact that friction occurs as the result of collisions with solvent molecules. However, a considerable number of atoms with higher friction coefficients are found inside the core region. Hence, the influence of solvent friction propagates into the protein core. The internal coefficients have large contributions from the low-frequency modes, yielding a simple picture of the surface-to-core long-range damping via solvation governed by collective low-frequency modes. To make use of these findings in implicit-solvent modeling, we compare the all-atom friction results with those obtained using Langevin dynamics (LD) with two empirical representations: the constant-friction and the ASA-dependent (Pastor Karplus) friction models. The constant-friction model overestimates the core and underestimates the surface damping whereas the ASA-dependent friction model, which damps protein atoms only on the solvent-accessible surface, reproduces well the friction coefficients for both the surface and core regions observed in the explicit-solvent MD simulations. Furthermore, in LD simulation, the solvent friction coefficients should be imposed only on the protein surface.

59 BASIC BIOLOGICAL SCIENCES↗