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Electrons injected into the build envelope during powder bed electron-beam additive manufacturing can accumulate on the irradiated particles and cause them to repel each other. Under certain conditions, these electrostatic forces can grow so large that they drive the particles out of the build envelope in a process known as “smoking”. In the present work, we investigate the causes of powder bed charging and smoking during electron-beam additive manufacturing. In the first part of the paper, we characterize the surface chemistry of a common feedstock material—gas-atomized Ti-6Al-4V powder—and find that a thick, electrically insulating oxide overlayer encapsulates the particles. Based on these experimental results, we then formulate an analytical model of powder bed charging in which each particle is approximated as a capacitor, where the particle and its substrate are the electrodes and the oxide overlayer is the dielectric. Using this model, we estimate the charge distribution in the powder bed, the electrostatic forces acting on the particles, and the conditions under which the powder bed will smoke. It is found that the electrical resistivity of the oxide overlayer strongly influences the charging behavior of the powder bed and that a high resistivity promotes charge accumulation and consequent smoking. This analysis suggests new quality control and process design measures that can help suppress smoking. [ABSTRACT FROM AUTHOR]

Lubricant anti-wear additives are known to chemically interact with metallic surfaces to form a self-healing, wear-protection tribofilm. Their interactions with non-metallic surfaces are however less understood. Here we report recent findings on whether and how a zinc dialkyldithiophosphate (ZDDP) and a phosphonium–organophosphate ionic liquid (IL) form tribofilms on three hard coatings, AlMgB 14 –TiB 2 , TiB 2 , and diamond like carbon (a-C:H DLC), when sliding against a steel ball. Systematic characterization was conducted on the coating wear scars including top surface morphology imaging and elemental mapping, layer-by-layer chemical analysis, and cross section nanostructural examination. The ZDDP and IL tribofilms on the boride coatings are up to 50–70 nm thick with 75–80% surface coverage while the tribofilms on DLC were <25 nm thick and only covered 20–30% of the contact area. The presence of iron compounds in the tribofilms suggests a critical role for wear debris in tribofilm formation. Oxidation products of TiB 2 were detected in the tribofilms, while no involvement of the DLC surface in tribofilm formation was observed. Results suggest that wear debris digestion and contact surface reaction both are critical in tribofilm formation: the former process is responsible in forming the bulk of the tribofilm and the latter provides strong bonding of the tribofilm to the contact surface. [ABSTRACT FROM AUTHOR]

Tribological evaluations were conducted on lubricating base oils of different viscosity grades with and without an anti-wear (AW) additive in lubricating steel-steel and ceramic-steel contacts. Two AW additives were applied: a conventional secondary zinc dialkyldithiophosphate (ZDDP) and an oil-miscible phosphonium-phosphate ionic liquid (IL). Tests were carried out using reciprocating ball-on-flat sliding at room temperature. The flat material was AISI A2 tool steel, and the ball material was either AISI 52100 bearing steel or silicon nitride. Four lubricants were tested: Chevron SAE 15W40 and 0W30 base oils, and the 0W30 base oil plus 1 wt% ZDDP or IL. For the steel–steel contact, the lower-viscosity 0W30 base oil resulted in a higher wear rate than did the 15W40 base oil, as expected. Both the ZDDP and the IL substantially reduced wear, and the IL-additized 0W30 base oil was the best performer. For the ceramic–steel contact, the ZDDP provided moderate wear protection for both ball and flat. In contrast, the IL reduced the steel flat wear more effectively but increased the ceramic ball wear rate. Cross-sectional transmission electron microscopy (TEM) examination and x-ray photoelectron spectroscopy (XPS) chemical analysis were used to reveal the thickness, nanostructure, and composition of the tribofilms formed by ZDDP and IL on the steel and silicon nitride surfaces. [ABSTRACT FROM AUTHOR]

A series of semiconducting zinc sulfide (ZnS) nanoparticles were scalably, reproducibly, controllably and economically synthesized with anaerobic metal-reducing Thermoanaerobacter species. These bacteria reduced partially oxidized sulfur sources to sulfides that extracellularly and thermodynamically incorporated with zinc ions to produce sparingly soluble ZnS nanoparticles with ∼5 nm crystallites at yields of ∼5 g l −1 month −1 . A predominant sphalerite formation was facilitated by rapid precipitation kinetics, a low cation/anion ratio and a higher zinc concentration compared to background to produce a naturally occurring hexagonal form at the low temperature, and/or water adsorption in aqueous conditions. The sphalerite ZnS nanoparticles exhibited narrow size distribution, high emission intensity and few native defects. Scale-up and emission tunability using copper doping were confirmed spectroscopically. Surface characterization was determined using Fourier transform infrared and X-ray photoelectron spectroscopies, which confirmed amino acid as proteins and bacterial fermentation end products not only maintaining a nano-dimensional average crystallite size, but also increasing aggregation. The application of ZnS nanoparticle ink to a functional thin film was successfully tested for potential future applications. [ABSTRACT FROM AUTHOR]

Zirconium diboride (ZrB 2 ) has many potentially beneficial properties for fusion plasma-facing component application, but almost no data exist on the response of ZrB 2 to ion irradiation. In this work, ZrB 2 samples were irradiated with 30 keV He + to fluences of 8.4 × 10 21 and 5.0 × 10 22 He/m 2 at temperatures of 920, 1020, and 1120 K in the Materials Irradiation Experiment (MITE-E) at the University of Wisconsin Inertial Electrostatic Confinement (UW-IEC) Laboratory to simulate some of the conditions of plasma-facing components in fusion reactors. The samples irradiated to the higher fluence developed surface morphology changes in the ion irradiated zone including rough, porous, and ripple structures. The ZrB 2 had similar mass loss as W irradiated to similar conditions. Additionally, the ZrB 2 samples did not exhibit flaking as did the SiC samples previously irradiated to similar conditions. This first look at ZrB 2 behavior under ion irradiation is promising and justifies further testing of this emerging ultra-high temperature ceramic material for fusion applications. [ABSTRACT FROM AUTHOR]

A potentiostatic polarization method is used to evaluate the corrosion behavior of SS316L in simulated anode and cathode environments of polymer electrolyte fuel cells. A passive barrier oxide film is observed to form and reach steady state within ∼10 h of polarization, after which time the total ion release rates are low and nearly constant at ∼0.4 μg cm −2 h −1 for all potentials investigated. The equilibrium film thickness, however, is a function of the applied potential. The main ionic species dissolved in the liquid are predominately Fe followed by Ni, that account for >90% of the steady-state corrosion current. The dissolution rate of Cr is low but increases systematically at potentials higher than 0.8 V. The experimental ion release rates can be correlated with a point defect model using a single set of parameters over a broad range of potentials (0.2–1 V) on the cathode side. The interfacial contact resistance measured after 48 h of polarization is observed to increase with increase in applied potential and can be empirically correlated with applied load and oxide film thickness. The oxide film is substantially thicker at 1.5 V possibly because of alteration in film composition to Fe-rich as indicated by XPS data. [ABSTRACT FROM AUTHOR]

Proton exchanges membrane (PEM) regenerative fuel cell electrolysis of water is of great recent interest as a hydrogen generation technology. Anode side titanium current collectors and separator plates used in these applications typically employ coatings of platinum group metals to achieve durability and performance requirements in the high voltage, oxidizing environment. The present work assessed the potential for lower cost surface modified titanium by both thermal (gas) nitridation and plasma nitridation approaches. The nitrided Ti was found to result in far less hydrogen uptake in coupon testing than did Pt-plated Ti. Short-term (48 h) single-cell performance at 25 °C was approximately 13% better (lower voltage) at 1.2 A cm −2 for thermal and plasma nitrided plates vs. untreated Ti. However, at 50 °C and 1.5 A cm −2 , the thermally nitrided plate exhibited only on the order of 3% better behavior (lower voltage) compared to the untreated Ti and plasma nitrided Ti. Durability testing for 500 h resulted in only a minor degradation in cell performance, on the order of 1–2% voltage increase, with the best behavior exhibited by the thermally nitrided Ti plate. Despite their relatively stable cell performance, extensive local oxidation of the thermally nitrided and plasma nitrided flow field regions was observed. [ABSTRACT FROM AUTHOR]

Magnesium alloys are susceptible to corrosion because of their high reactivity and low electrode potential. The present work introduces a conversion coating using a protic ammonium-phosphate ionic liquid (IL). Initial results on the AZ31B Mg alloy have demonstrated substantially improved corrosion resistance for the IL treatment at 300 °C (IL_300C) compared to the treatment at room temperature. Potentiodynamic polarization analysis of the IL_300C treated Mg surface in a NaCl solution exhibited a strong passivation behavior. No pretreatment is needed and the treated surface morphology is well preserved. Cross-sectional nanostructure examination using transmission electron microscopy and element mapping using energy-dispersive X-ray spectroscopy have revealed the IL_300C conversion coating to be a 70–80 nm thick with a two-layer structure. Further surface chemical analysis using X-ray photoelectron spectroscopy suggested such an IL conversion coating possibly composed of metal oxides, metal phosphates, and carbonaceous compounds. [ABSTRACT FROM AUTHOR]