Yip-Wah Chung

To enhance the poor wear resistance of thermoplastic low-density polyethylene (LDPE), precursor LDPE powders are prepared via solid-state shear pulverization (SSSP) containing 0-7 wt% as-received graphite and 0-3 wt% dicumyl peroxide (DCP) as radical initiator, then consolidated and cured by compression molding. X-ray diffraction confirmed substantial exfoliation of graphite by SSSP. Relative to neat LDPE, LDPE thermosets exhibit reduced crystallization onset temperature, crystallinity, and elongation at break with increasing cross-link density. Incorporating well-exfoliated graphite increases crystallization onset temperature, Young’s modulus, and ultimate strength relative to the respective neat LDPE and thermoset controls, with crystallinity unaffected within uncertainty. Cross-links and well-exfoliated graphite act synergistically to yield thermoset nanocomposites with enhanced wear resistance. Relative to neat LDPE, thermosets made with 3 wt% DCP and thermoset nanocomposites made with 3 wt% DCP and 3 wt% graphite exhibit wear volumes reduced sixfold and eightfold, respectively. Such thermosets and thermoset nanocomposites provide simple approaches to improve the wear performance of LDPE. Furthermore, the powder outputs from SSSP have known melt processability that could include extrusion, injection molding, powder coating, and rotational molding. Lastly, improved wear resistance should reduce polymer debris emissions; we discuss design principles as potential pathways to mitigate wear-induced microplastic generation.

Magnetic smart materials (MSMs) offer an alternative to the typical piezo-electric actuators currently used to control X-ray optics on beamlines. MSMs, combined with an overcoating of a magnetic hard material, create a deformable mirror that can operate in a power-off mode. The non-reflective side of the mirror is coated with an MSM and the magnetic hard overcoat. The process works by using an electromagnet (EM) to impose a magnetic field in the bilayer of the MSM and the magnetic hard overcoat, causing the mirror to deflect. Once the EM is turned off, the mirror settles into a new shape within minutes, which can remain intact for days. Since the EM is not fixed to the mirror, the exact placement of the magnetic field can be adjusted by relocating the EM. This feature allows for fine-scale adjustments and avoids the “dead pixel” replacement problem common with piezo patches attached to the mirror. Here, we provide a progress report based on laboratory-produced data.

This article reports a research study on sliding electrical contacts of copper–diamond materials, involving modeling and validation, contact status simulations, and parametric studies. The simulation model includes the analyses of both mechanical and electrical heat sources, evaluations of heat partition and temperature, and the capture of material thermal softening, plastic deformation, and material removal. The simulation model is validated through result comparison with experimental average temperature and measured wear. With this model, temperature variation and surface wear accumulation were numerically predicted under the influences of material property variations. Factors influencing wear of the copper surfaces were explored, and parameter sensitivity was investigated by means of the Taguchi L18 matrix. The results reveal that thermally related parameters of the contact interface strongly affect the wear of the copper pin, and thermal conductivity of its mating surface material is of critical importance. The research also suggests that the electromagnetic field change due to switching the polarity of the power supply does not affect the friction and wear results; however, the polarity change influences material electrochemistry, which results in a difference in friction and wear.

The performance and durability of high-pressure fuel systems in combustion engines are critical for consistent operation under extreme conditions. High-pressure fuel systems are traditionally lubricated with fuel that is compressed and delivered to the combustion chamber. However, lubrication with fuel presents significant challenges in these systems when used with low-viscosity fuels, leading to increased wear rates, especially in reciprocating contacts. This study delved into the tribological performance of steels of varying alloy content (annealed and hardened variants of AISI-52100, CF2, and D2) against alumina and hard 52100 counterbody materials in ethanol and decane environments. Friction and wear behaviors were evaluated, highlighting the influence of material interactions and environmental factors. Elastohydrodynamic lubrication analysis of the tested systems indicated that ethanol and decane form lubricating films of nanometer-scale thickness, confirming the boundary lubrication regimes of the performed tests. In summary, the tribological behavior trends were similar for alumina and 52100 counterbodies. Even though soft 52100 steel demonstrated low friction, its wear was the largest for both tested environments and counterface materials. Among all the tested materials, hard D2 experienced the lowest wear. 52100 and D2 steels showed opposite friction change behavior when comparing hard and soft samples, with lower friction observed for softer 52100 steel and harder D2 steel. Meanwhile, the wear was lower for harder candidates than for softer ones independent of the environment and counterbody material. Raman spectroscopy analysis of the formed wear tracks indicated the formation of carbon films with larger intensities of characteristic carbon peaks observed for more wear-resistant materials. These results suggest the synergistic effect of hardness and tribochemical activity in reducing the wear of materials.

In a previous paper, we presented the concept of using shape memory alloy sheets as the substrate for normal incidence multi-layer coated mirrors. A shape set sheet has been shown to deploy a figure good to 1 micron. This shape is good enough for a “light bucket.” However, if imaging is desired, then post-deployment corrections can be applied. We provide an update on the improvements we have made to our study. These improvements include ray tracing, polishing, adhesion, and fine (nm) surface adjustments.

Molecular Dynamics Simulations for Beginners: Key Topics in Materials Science and Engineering introduces readers to the concepts of particle-based materials modeling and the insight it can provide. A significant portion of the book consists of calculation examples, where readers learn how to set up and run material simulations using an open-source simulator, LAMMPS, developed at Sandia National Labs. The hands-on exercises cover a wide range of materials, properties, and behaviors and include step-by-step instructions along with input code, output plots, and explanations.