Energy coupled to matter for field–assisted processing

Energy Coupled to Matter (ECM) is an emerging technology area that goes beyond the traditional limits of materials research by exploring the use of applied physics-based fields and their influence over material structures, phase development, processing, properties, and responses over multiple length scales (i.e. from atomic to macroscale). The U.S. Army Research Laboratory (ARL) defines ECM as fundamental research to discover, explore, and exploit interactions between materials and intense energy fields in order to enable significant property enhancements and unique property combinations that overcome traditional engineering tradeoffs, and allow for responsive on-demand structure-property modifications. These field-material interactions can produce outcomes that are otherwise unattainable, expanding materials-by-design and processing/manufacturing science capabilities beyond the current state-of-the-art. The use of ECM for field-assisted processing has led to advances in materials development. The application of external electromagnetic (EM) fields to materials using techniques such as electric field-assisted sintering, flash sintering, microwave sintering, and high magnetic field processing has led to rapid production and improved properties. These methods have a few things in common, in that they use EM fields to contribute energy to heat treatment of materials, enabling full densification at lower temperatures (i.e. by hundreds of oC) and shorter times (i.e. from hours to second) as compared to conventional methods. The ability to rapidly densify materials under less extreme temperature conditions exemplifies one of the major advantages of ECM, as a smaller final grain size (i.e. nanoscale) can be preserved, resulting in enhancement of a number of physical and mechanical properties that are relevant to Army applications (i.e. strength, hardness, fracture toughness, etc.). As one example of successful ECM research at ARL, field-assisted processing has been used to investigate the development of transparent ceramics for laser host and protective system applications. In particular, single-mode microwave sintering has been explored, using a specially designed cavity as a waveguide to separate the electric and magnetic field components, enabling exposure of the material to pure electric or magnetic fields at microwave frequencies. By comparing conventional pressure-less sintering to two different types of microwave sintering, multi-mode and single-mode, the effect of different types of microwave fields on the sintering behavior of erbium-doped Al2O3 was studied. For single-mode microwave sintering, the percentages of electric and magnetic fields that the sample was exposed to during sintering were varied by adjusting the position of the sample along the chamber. Results shown in Table I suggest that the microwave sintering parameters had a profound influence on densification and rare earth migration/phase stability. Sintering in single-mode with a 30%E:70%H mixed field produced samples with significantly higher density than other samples sintered at 1400°C (and equal to samples conventionally sintered at 1700°C). These samples also contained the least amount of unwanted second phase, indicating that more Er formed a solid solution with Al2O3. Our findings suggested that the magnetic component may play a critical role in the processing of weakly magnetic materials such as Al2O3, and that the dopant material (rare earth in this case) may play an important role in the material response to EM fields

Surface Chemistry, Friction, and Wear Properties of Untreated and Laser-Annealed Surfaces of Pulsed-Laser-Deposited WS(sub 2) Coatings

An investigation was conducted to examine the surface chemistry, friction, and wear behavior of untreated and annealed tungsten disulfide (WS2) coatings in sliding contact with a 6-mm-diameter 440C stainless-steel ball. The WS2 coatings and annealing were performed using the pulsed-laser-deposition technique. All sliding friction experiments were conducted with a load of 0.98 N (100 g), an average Hertzian contact pressure of 0.44 GPa, and a constant rotating speed of 120 rpm. The sliding velocity ranged from 31 to 107 mm/s because of the range of wear track radii involved in the experiments. The experiment was performed at room temperature in three environments: ultrahigh vacuum (vacuum pressure, 7X(exp -10) Pa), dry nitrogen (relative humidity, less than 1 percent), and humid air (relative humidity, 15 to 40 percent). Analytical techniques, including scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDX), x-ray photo electron spectroscopy (XPS), surface profilometry, and Vickers hardness testing, were used to characterize the tribological surfaces of WS2 coatings. The results of the investigation indicate that the laser annealing decreased the wear of a WS2 coating in an ultrahigh vacuum. The wear rate was reduced by a factor of 30. Thus, the laser annealing increased the wear life and resistance of the WS2 coating. The annealed WS 2 coating had a low coefficient of friction (less than O.1) and a low wear rate ((10(exp -7) mm(exp 3)/N-m)) both of which are favorable in an ultrahigh vacuum

Wear Behavior of Low-Cost, Lightweight TiC/Ti-6Al-4V Composite Under Fretting: Effectiveness of Solid-Film Lubricant Counterparts

The wear behavior of low-cost, lightweight 10-wt% TiC-particulate-reinforced Ti-6Al-4V matrix composite (TiC/Ti- 6Al-4V) was examined under fretting at 296, 423, and 523 K in air. Bare 10-wt% TiC/Ti-6Al-4V hemispherical pins were used in contact with dispersed multiwalled carbon nanotubes (MWNTs), magnetron-sputtered diamondlike carbon/chromium (DLC/Cr), magnetron-sputtered graphite-like carbon/chromium (GLC/Cr), and magnetron-sputtered molybdenum disulphide/titanium (MoS2/Ti) deposited on Ti-6Al-4V, Ti-48Al-2Cr-2Nb, and nickel-based superalloy 718. When TiC/Ti-6Al-4V was brought into contact with bare Ti-6Al-4V, bare Ti-48Al-2Cr-2Nb, and bare nickel-based superalloy 718, strong adhesion, severe galling, and severe wear occurred. However, when TiC/Ti-6Al-4V was brought into contact with MWNT, DLC/Cr, GLC/Cr, and MoS2/Ti coatings, no galling occurred in the contact, and relatively minor wear was observed regardless of the coating. All the solid-film lubricants were effective from 296 to 523 K, but the effectiveness of the MWNT, DLC/Cr, GLC/Cr, and MoS2/Ti coatings decreased as temperature increased

Search for dark matter produced in association with bottom or top quarks in √s = 13 TeV pp collisions with the ATLAS detector

A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements