Low Erosion Rates Research Articles

The phosphorus cycle and biological pump in Earth’s middle age: Reappraisal of the “Boring Billion”

<p indent="0mm">The Earth’s middle age (1.8–0.8 billion years ago, Ga) is collectively coiled the “Boring Billion”, referring to the invariant carbonate carbon isotope curve, persistently low atmospheric O₂ level, predominant oceanic anoxia and sluggish evolution of eukaryotes. The “Boring Billion” also witnessed the quiescence of orogenesis and a long-lived supercontinent Columbia (~1.7–1.3 Ga). It is widely accepted that the terrestrial phosphorus (P) input ultimately controls the long-term ocean primary productivity and organic carbon burial, and accordingly the weakened mountain-building might have reduced terrestrial P input, in turn limiting organic matter production in the surface ocean and leading to rather inactive biogeochemical cycles in Earth’s middle age. However, this scenario overlooks different behaviors of complex P speciation in continental weathering and P cycle in the ocean. Not all P from continental weathering is bio-available, and not all seawater P is eventually buried with organic matter. Therefore, it is essential to revisit P speciation and P cycle in the “Boring Billion”. On the one hand, apatite (the primary insoluble P minerals) is not completely dissolved in continental weathering, and dissolved P could be scavenged by Fe-oxides or precipitate as authigenic phosphate minerals, further reducing the bio-availability of terrestrial P input. On the other hand, seawater P is variably removed from the ocean inventory via inorganic P burial associated with Fe redox cycles or organic P burial coupled with the preservation of organic carbon. In detail, seawater P would be transported to sediment with FeOOH precipitation and sinking of particulate organic matter (POM). Further reduction of FeOOH by iron-reducing microbes (IRM) and organic matter decomposition in sediments release P into porewater, which either precipitates as authigenic carbonate-fluorapatite or diffuses back to seawater. The intensity of Fe-redox cycle controls the inorganic P sink and determines the availability of P in seawater. The marine P cycle is recorded in the P speciation of sediments/sedimentary rocks, including organic P, Fe bounded P, authigenic P in the form of carbonate-fluorapatite and detrital P in the form of apatite. The fraction of detrital P with respect to total P is determined by continental weathering, while the fraction of organic P relative to total active P (i.e., organic P + Fe bounded P + authigenic P) is related to the marine P cycle. We speculate that low primary productivity in the “Boring Billion” could be attributed to: (1) Low erosion rate in continents due to the quiescence of mountain-building, (2) low degree of P activation in the weathering process due to the absence of land plants and biological weathering, and (3) high inorganic P burial in the ocean as the consequence of active Fe redox cycle. The “Boring Billion” was ended by the reactivation of tectonics that elevated terrestrial P input and/or ocean oxygenation that reduced inorganic burial of seawater P. This interpretation is supported by the available P speciation data showing high fraction of detrital P in early Neoproterozoic sedimentary rocks. In addition, the marine P cycle is also controlled by the nature of biological pump. To sustain a P-C cycle balance, the prokaryote-dominated biology pump in the “Boring Billion” was characterized by the high instantaneous primary productivity and fast decomposition in the water column, favoring the development of ferruginous (anoxic and Fe²⁺-rich) ocean that promoted inorganic P burial. In contrast, the low C burial efficiency and high rate of organic matter decomposition of prokaryote-dominated biological pump would make microbial carbon pump (MCP) play a more important role. Thus, the ocean of the “Boring Billion” was featured by the high production of resistant dissolved organic carbon (RDOC) via active MCP, which might have played a key role in modulating the global biogeochemical cycles and the redox landscape in Earth’s middle age.

Inventory of large landslides along the Central Western Andes (ca. 15°–20° S): Landslide distribution patterns and insights on controlling factors

The western flank of the Central Andes hosts some of the largest terrestrial landslides (v > km3), which morphologies are particularly well-preserved due to low erosion rates related to the hyper-arid climate prevailing in this region since the Miocene. First-order questions are pending about the factors controlling the development and the triggering of those large-scale slope failures. Previous studies provided some geomorphological analysis and dating on individual study cases, but a regional-scale vision of landslide processes long the Central Western Andes is missing.Here we report an original inventory of large landslides (areas from 0.1 to 180 km2) established along the western flank of the Central Andes between latitudes ca. 15 and 20° S, and from the Pacific coast to the Altiplano. Based on manual mapping (using satellite images analysis, Google Earth and DEMs analysis) and a compilation of previous works, we inventoried more than a thousand large landslides in this region. We then statistically explored the database according to the landslides typology, size, abundance and relation to geologic, tectonic and climatic settings of the Central Western Andes in order to provide a first insight on their controlling factors. Landslide size-frequency distribution follows a power-law with an exponent of 2.31 ± 0.16 and a cut-off of 4.0 ± 1.9 km2 showing a strong contribution of the largest landslides to the cumulated landslide area. We revealed a dominance of rockslide typology (86%) characterized by in-mass slides, the rest being rock-avalanche type (14%) marked by typical granular-flow morphologies. Combination of specific lithology and great local relief emerge as favorable conditioning factor for large landslide initiation, in particular in the case of river incisions though ignimbrites of the Paleogene-Neogene (Huaylillas Formation), concentrating >30% of the landslides. Moreover, landslide clusters tend to follow crustal faults networks suggesting a long-term control of tectonic activity. Most of the identified landslides are paleo events. We tentatively argue that their triggering could not have been possible in the current hyper-arid conditions of the Atacama Desert and its periphery. Future research providing dating on some of the landslide clusters identified in this study is needed to explore possible temporal correlations between periods of landslide activity and external seismic and/or climatic cycles.

Influence of the upstream axial magnetic mirror field on the plume characteristics in the full cylindrical Hall thruster

Full cylindrical Hall thruster holds considerable promise in terms of its low wall erosion rate and long-lifetime performance advantages. As far as we know, its upstream magnetic mirror field plays a fundamental role in the determination of plasma behavior and hence the thruster performance. In this paper, a series of upstream axial magnetic mirror fields with different axial gradient values were generated by adjusting the inverse-electromagnetic coil currents near the upstream channel and its effect on the plume characteristics were investigated. The experiment results indicate that under a low magnetic mirror field with 44.8 G/mm gradient value, a bulb-like plasma discharge region emitting a visible high-intensity light is discovered near the thruster exhaust, its plume manifests a hollow profile with a plume angle of 77°. In contrast, as the axial gradient value of magnetic mirror field increases from 44.8 G/mm to 72.5 G/mm, this bulb-like plasma region near exhaust shrink gradually to disappear along with the plume angle decreasing to 65°. Meanwhile, the anode efficiency enhances significantly (from 16% to 28%) correspondingly. According to the analysis of ion current, electron current and ion energy distribution, the observed strong plasma discharge near the thruster exhaust should be intimately related with the extension of the ionization region towards exit due to the weak electron confinement under the low magnetic mirror field. However, the larger upstream magnetic mirror field could diminish the leaking of local electrons towards the anode efficiently, which could not only intensify the ionization, but also induce the shrink of the ionization region axially towards the upstream channel and the clearer separation of acceleration region from the ionization region. Ultimately, the enhanced propellant utilization and current utilization, higher ion energy and the collimated plume are achieved. This work provides a feasible way to recognize the ionization and acceleration process under the mirror-like magnetic field of the miniaturized full cylindrical Hall thruster.

Quantifying Late Pleistocene to Holocene Erosion Rates in the Hami Basin, China: Insights Into Pleistocene Dust Dynamics of an East Asian Stony Desert

AbstractWind is one of the major processes modifying the land surface in the Hami Basin, as evidenced by wind deflation gravel lags, yardangs, and gravel mantled eolian ripples. We report erosion rates for the Hami Basin using cosmogenic 10Be measurements. Bedrock erosion rates average 0.121 ± 0.0293 mm yr−1, which is similar to those of other wind‐eroded arid basins in East Asia, but is anomalously low when modern near‐surface wind speeds are considered. We posit that interglacial periods experienced lower erosion rates than glacial periods, and that the Hami Basin was likely once a larger dust producer than at present, with Pleistocene dust emissions driven by the eolian deflation of alluvial terrace deposits as well as bedrock weathering and abrasion. Wind erosion and dust production in the Hami Basin was largely regulated by the synoptically controlled spatial‐temporal distribution of precipitation and subsequent landscape evolution since at least the late Pleistocene.

Studies on Mechanical Attrition and Surface Analysis on Heat-Treated Nickel Alloy Developed through Additive Manufacturing

In this paper, the nickel-based superalloy SU718 is developed through the Direct Metal Laser Sintering (DMLS), an additive manufacturing process. Further, the material has been focused to study the effect of heat treatment and abrasive particle erosion. Two different heat treatment (HT) cycles are planed with ageing and annealing to enrich the metallurgical quality of the DMLS processed SU718 alloy. The heat treatment is performed with two different combinations of temperatures for annealing/solutionizing followed by ageing to improve the metallurgical properties. The influence of heat treatment on additively manufactured IN718 is imparting variations in the hardness, microstructure, and erosion resistance. Vickers hardness for as built, HT 1, and HT 2 of DMLS alloy is 264.15, 385.55, and 352.43 Hv; which has been increased for 45% for HT 1 and 33% for HT 2 from the as built DMLS alloy. After solutionizing, the grains are refined within the track boundary and the majority of the grains are homogenized. The air jet erosion test arrangement is used to conduct the study at a velocity of 250 m/s and impact angle of 90° at room temperature. The hardness of the treated samples has taken vital role to resist the erosion. The rate of erosion is higher for bare DMLS alloy whereas HT 1 has low erosion rate when compared with HT 2 and bare DMLS alloy. The erosion morphology of the samples was carried out by SEM images, and erosion mechanism is discussed. The ploughing and microcutting were found in all the impact angles, whereas erodent impingement is found in the bare DMLS alloy in additional. The good erosion resistance is observed for HT 1 DMLS alloy in all the impact angles.

Fuzz Growth under Fusion Reaction and Estimation of Blanket’s lifespan

Tungsten has been regarded as a candidate of plasma facing material (PFM) in magnetic fusion reactor (MFR) due to its high strength, high thermal conductivity, and low erosion rates. Alpha particles, as the by-product of the D-T fusion reaction, will collide onto the Tungsten divertor and form nanotendril structures called “fuzz” with an estimation of 3.4 MeV energy. In this paper, we simulated by Stopping and Range of Ions in Matter (SRIM) to investigate the diffusing time and damage rate of the alpha particles on Tungsten. To describe the fuzz generation, the concepts in the diffusion model are utilized to describe the fuzz generation with corresponding techniques (e.g., Fick’s Migration Law). Through this diffusion model, we can derive an estimated lifespan for a divertor blanket that matches the design of general fusion reactors. These results pave a path to investigate the irradiation damage for Tungsten in divertor blankets.

Alpine rockwall erosion patterns follow elevation-dependent climate trajectories

Mountainous topography reflects an interplay between tectonic uplift, crustal strength, and climate-conditioned erosion cycles. During glaciations, glacial erosion increases bedrock relief, whereas during interglacials relief is lowered by rockwall erosion. Here, we show that paraglacial, frost cracking and permafrost processes jointly drive postglacial rockwall erosion in our research area. Field observations and modelling experiments demonstrate that all three processes are strongly conditioned by elevation. Our findings on catchment scale provide a potential multi-process explanation for the increase of rockwall erosion rates with elevation across the European Alps. As alpine basins warm during deglaciation, changing intensities and elevation-dependent interactions between periglacial and paraglacial processes result in elevational shifts in rockwall erosion patterns. Future climate warming will shift the intensity and elevation distribution of these processes, resulting in overall lower erosion rates across the Alps, but with more intensified erosion at the highest topography most sensitive to climate change.

The effect of the SiC content on the high duration erosion behavior of SiC/ZrB2– SiC/ZrB2 functionally gradient coating produced by shielding shrouded plasma spray method

In this research, the effect of the ZrB 2 middle layer and SiC Weight percentage on the erosion behavior of SiC/ZrB 2 – SiC/ZrB 2 functionally gradient coating were investigated. For this purpose, SiC gradient coating was prepared via the reactive melt infiltration method (RMI). Afterwards ZrB 2 –SiC layers containing 10, 20 and 30 wt% SiC and, ZrB 2 as the outer layer were applied on SiC coated graphite via solid shielding shrouded plasma spraying (SSPS). To investigate the erosion resistance of the coating, the specimens were subjected to oxy-acetylene and propane flame. The results showed that by applying the ZrB 2 –SiC layer between SiC and ZrB 2 coating, due to the gradual change of the coefficient of thermal expansion mismatch and reduction of thermal stresses, erosion resistance improves, so that the coating with 20 wt% SiC with mass and linear erosion rate, −0.072 × 10 −4 g.cm −2 .s −1 , 0.0166 μm s −1 respectively had the best erosion resistance under oxy propane flame. In the oxyacetylene flame test, a similar result was obtained to the oxy propane test and the SiC/ZrB 2 -20% wt. SiC/ZrB 2 coating had the lowest erosion rate.

Study of hanging valley in loess-paleosoil sediments with soil erosion assessment using nuclear and erosion potential methods

This paper is dealing with soil erosion assessment using two different approaches: nuclear model and erosion potential method, also known as Gavrilovic's method. Complex valley systems on Titel Loess Plateau were selected for investigation. Radiocaesium is favored in many studies as an optimal erosion tracer due to its relatively long half-life, negligible renewal in the environment and strongly binding ability onto soil particles. The use of gamma-spectrometry in environmental testing laboratories acts as a precise and fast measurement technique for determination of 137Cs activity concentrations, without the need for complicate preparation of samples. Annual erosion and deposition rates were estimated according to three conversion models for uncultivated land: the profile distribution model with two years of dominant fallout of 137Cs (1963 and 1986) and the diffusion and migration model using WALLING software. The applied nuclear models were validated by comparison with erosion potential model which is the most relevant empirical model for erosion processes in torrent valleys. The obtained results indicate a good agreement with overall low values of average annual soil erosion rates on all soil profiles in the investigated area. Correlation analysis has determined the different influence of slope, terrain curvature, and vegetation index on the erosion models.

Endoscopic Removal of an Eroded LINX® Device Using a Mechanical Biliary Lithotriptor

Background: Magnetic sphincter augmentation (MSA) via the surgical placement of a LINX® device (LINX® Reflux Management System, Torax Medical, Shoreview, MN, USA) is an increasingly performed minimally invasive outpatient anti-reflux procedure with a low erosion rate. The most common initial approach to eroded LINX® devices is endoscopic removal. Often endoscopy centers do not have specialized devices to cut through the newer, more durable LINX® systems. In this paper we describe a unique approach for removal of a LINX® with intraluminal erosion using a commonly stocked mechanical biliary lithotripsy device. Case description: A 63-year-old male with a history of GERD and symptomatic type III paraesophageal hernia (PEH) underwent a robotic PEH repair with magnetic sphincter augmentation (1.5T, 17 bead) at an outside hospital. He developed an acute recurrence of his PEH, and subsequent upper endoscopy and contrast esophagram four weeks postoperatively revealed a gastric erosion of the LINX device, which had migrated 6 cm onto the stomach. Attempted endoscopic LINX® removal using the OVESCO remove DC Cutter device was unsuccessful. Using principles of prior endoscopic bariatric lap band foreign body removal, the entire LINX® device was successfully removed with the described biliary lithotriptor technique. Conclusion: Using a common biliary mechanical lithotriptor device and a guidewire to transect the newer 1.5T LINX® Reflux Management System is a safe, effective and familiar technique for endoscopic removal of an eroded MSA device.

The Comparison of Erosion Rates in Grassland, Teak Forest, Crops Land, and Gliricidia Forest Plantation in Wanagama Edu Forest

<p class='Els-Abstract-text'>Wanagama forest is an integral part of a nearby village. This forest is left unprotected, so local people can easily enter the forest area to take non-timber forest products, including cropland and grass. The increasing demand for grass for feeding the cattle in Wanagama resulted in a larger area of grassland. Therefore, it is highly important to review and measure the erosion rate of the grassland compared to other land uses. This research aims to measure the erosion rates of four land uses and determine the correlation between rainfall and erosion at those four land uses: grassland, teak forest, crops land, and Gliricidia forest plantation. In measuring the erosion rates, this research uses four plots of 22 x 4 meters established at those four land uses. Each plot has two drums at the outlet. We took the erosion samples at every single rainfall event. To investigate the rainfall, there were two ombrometers installed in the field. The results show that the grassland has the lowest erosion rate (0.45 tons/ha) compared to a teak forest (0.5 tons/ha), crops land (0.84 tons/ha), and Gliricidia plantation forest (1.66 tons/ha). The rainfall has a positive coefficient correlation to erosion; the lowest was in grassland (r = 0.723), followed by teak forest (r = 0.828), Gliricidia (r = 0.830), and Crops land (0.873). The higher the rainfall depth will result in the higher erosion rate at the four land uses.

Bedrock gorge incision via anthropogenic meander cutoff

Abstract Bedrock river-gorge incision represents a fundamental landscape-shaping process, but a dearth of observational data at &amp;gt;10 yr timescales impedes understanding of gorge formation. I quantify 102 yr rates and processes of gorge incision using historical records, field observations, and topographic and image analysis of a human-caused bedrock meander cutoff along the North Fork Fortymile River in Alaska (USA). Miners cut off the meander in 1900 CE, abruptly lowering local base level by 6 m and forcing narrowing and steepening of the channel across a knickpoint that rapidly incised upstream. Tectonic quiescence, consistent rock erosivity, and low millennial erosion rates provide ideal boundary conditions for this 102 yr gorge-formation experiment. Initial fast knickpoint propagation (23 m/yr; 1900–1903 CE) slowed (4 m/yr; 1903–1981 CE) to diffusion (1981–2019 CE) as knickpoint slope decreased, yielding an ~350-m-long, 6-m-deep gorge within the pre–1900 CE channel. Today, diffusion dominates incision of a 500-m-long knickzone upstream of the gorge, where sediment transport likely limits ongoing adjustments to the anthropogenic cutoff. Results elucidate channel width, slope, discharge, and sediment dynamics consistent with a gradual transition from detachment- to transport-limited incision in fluvial adjustment to local base-level lowering.

Sedimentary architecture of a Late Cretaceous under‐filled rift basin, Canterbury Basin, New Zealand

AbstractThe Canterbury Basin in southeastern Zealandia was initiated during the late Albian (ca. 105 Ma) as a rift system, prior to the onset of seafloor spreading between Zealandia and eastern Gondwana at ca. 85 Ma. Basin‐fill architecture and sediment types have been determined from interpretation of 2D and 3D seismic‐reflection lines tied to five wells and compared to outcrop data from the literature. These data show that Cretaceous syn‐rift basin‐fill architecture was controlled by normal faulting, which produced basin and range topography that persisted for more that ca. 30 Myr after the cessation of faulting. Initial sedimentation was dominated by short drainage systems sourced from within the basin to produce alluvial fans along fault scarps, which inter‐fingered with axial‐flowing braided river conglomerates, coal measures and mudstone‐rich lake deposits in more central portions of the basins. Marine incursion of the basin from the east commenced during rifting and onset of Gondwana breakup, with maximum water depths achieved in the Oligocene. Post‐rifting, detrital sediments were mainly sourced locally from structural highs and augmented by pelagic sedimentation, which collectively draped and eventually buried most of the earlier‐formed horsts by the Early Eocene. The temporal persistence of basin and range topography reflected the low rates of erosion of horst blocks compared to the rates of fault displacement. The lack of substantial sediment input from outside of the rift basin was a key factor in the under‐filling of the Canterbury Basin. This research emphasises the key role played by sediment supply in rift basin filling. Despite an abundance of active faulting at initiation, some rift basins may fill slowly over tens of million years after rift cessation.

Tracing erosion patterns in South Tibet: Balancing sediment supply to the Yarlung Tsangpo from the Himalaya versus Lhasa Block

AbstractThe Yarlung Tsangpo, draining the Himalayan‐Tibetan orogen along the Indus‐Yarlung suture zone, receives detritus from the deformed remnants of both Indian margin to the south and Asian margin to the north. High resolution petrographic, heavy‐mineral and geochemical datasets, combined with published geochronological data, allow us to monitor compositional changes, estimate erosion rates and investigate lithologic, climatic and anthropic controls on sediment fluxes along this large sediment‐routing system entirely developed within high mountain areas. Sediment generated from the Lhasa Block along the southern margin of Asia is characterized by abundant feldspar and volcanic rock fragments, amphibole‐dominated transparent heavy mineral suite and high concentrations of K, Rb, Be, Th and Pb. Himalayan‐derived sand, instead, is characterized by sedimentary to low‐rank metasedimentary rock fragments, poor transparent heavy mineral suite dominated by durable recycled (zircon, tourmaline) or metamorphic minerals (chloritoid, garnet) and high Ca concentration. Sand from the ophiolitic suture is distinguished by serpentinite grains and mafic volcanic or metavolcanic detritus, transparent heavy mineral suite including olivine, Cr‐spinel, enstatite and clinopyroxene, and high Mg, Cr, Ni and low rare earth elements. Himalayan detritus is prominent in Yarlung Tsangpo upper reaches, whereas detritus from the Lhasa Block becomes progressively predominant in the middle and lower reaches. Provenance budgets based on integrated petrographic, heavy‐mineral and geochemical datasets indicate that ca. 83% of the detritus is supplied by the Lhasa Block and the rest mostly from the Himalaya (ca. 12%) and subordinately from the ophiolitic suture (5%). A low average erosion rate of ca. 0.06 mm/a was estimated for the Yarlung Tsangpo catchment upstream of the Namche Barwa syntaxis, which resulted from dry climate, relatively mild average relief and sediment storage in wide valley tracts of the middle and lower reaches. The decrease in sediment flux recorded in recent decades is mainly ascribed to growing human activities, which are becoming a prominent control on sediment generation and transportation even in the high‐mountain area of South Tibet.

The impact of erosion on fault segmentation in thrust belts: Insights from thermochronology and fluvial shear stress analysis (southern Longmen Shan, eastern Tibet)

Along the southern Longmen Shan thrust belt in eastern Tibet, the Dayi subsegment stands out as it remained unruptured in both the 2008 Wenchuan and the 2013 Lushan earthquakes. Whether localized weak materials or differential erosion caused this fault segmentation is unclear. According to the critical-taper wedge theory, these two mechanisms would generate distinct morphotectonic features and erosional distributions, such as a recess vs. a salient structure at the range front, and higher vs. lower erosion rate than that of the adjacent areas along strike in the hinterland, respectively. We integrated two independent methods to evaluate these mechanisms. We obtained 10 apatite fission track and two apatite (U-Th)/He dates from the Dayi subsegment, which yield ages of ~3–44 Ma, and deduced an average erosion rate of 0.5–0.6 mm/yr and 0.2–0.4 mm/yr since ~6–8 Ma in the hanging-wall and footwall of the Shuangshi-Dachuan Fault, respectively. We also calculated the fluvial shear stress and erosion rate at 708 sites along five rivers that flow across the faults in the southern Longmen Shan. These two methods yield consistent mean erosion rates, implying an exhumational steady state since at least 5 Ma. Intriguingly, a systematic, heterogeneous pattern of erosion is found on a 3-by-3 grid along and across the Dayi subsegment. In the hinterland, the erosion rate in the Dayi subsegment is lower than that of the adjacent areas along strike, whereas at the range front, the erosion rate in the Dayi subsegment is greater. The spatial patterns of erosion and topography suggest that differential erosion plays a major, but not exclusive, role in regulating the fault segmentation and earthquakes in the Longmen Shan. Moreover, the Dayi subsegment could have acted as a barrier during the recent great earthquakes, posing substantial seismic potential to this region.

Hybrid TiN-CCPN coating of AISI-201 stainless steel by physical vapor deposition combined with cathodic cage plasma nitriding for improved tribological properties

The poor mechanical properties of AISI 201 stainless steel under harsh environment limit its uses in modern building structures, automobiles and aerospace industry. Various technologies, such as physical vapor deposition (PVD) and cathodic cage plasma nitriding (CCPN) at different temperatures, are being used to reduce wear and tear of metals but the optimal method with acceptable results is still under development. In this paper, a novel duplex surface coating method, coupled with DC pulsed CCPN, was used to develop anti-wear and anti-corrosive TiN-coating over AISI 201 stainless steel. The CCPN, TiN coatings over AISI 201 stainless steel were performed in alternative manner and characterized for surface hardness, surface morphology, wear rate, chemical composition and erosion rate. XRD analysis revealed the formation of iron nitride and titanium nitride planes in the coated samples when compared with base materials. The TiN coated coupons exhibited reasonably good tensile strength, which increased manifold on CCPN treatment. The CCPN treated TiN coated coupons showed very low wear rate, wear volume and erosion rate among all the tested samples. Scanning probe microscopy was used to study the erosion rate and topology of the base and coating coupons under different water jet velocities. The erosion rate of base coupon at a jet velocity of 5 m/s was measured about 1.27 mm/year, which fell to 0.25 mm/year after duplex coating. The results show that duplex coatings, when applied in the proper order, can improve mechanical qualities by reducing wear rate, erosion rate, and wear volume.

Wear trends of the 12.5 kW HERMeS Hall thruster

This work presents the results of over 6500 h of wear testing completed during the maturation of the NASA 12.5 kW Hall Effect Rocket with Magnetic Shielding. Erosion of the thruster front pole covers was found to be the primary life-limiting mechanism and exhibits strong dependencies on thruster operating condition and material properties. Specifically, average pole cover erosion rates increased by 76% as discharge voltage decreased from 600 to 300 V and 42%–96% as the magnetic field increased from 0.75 to 1.25 times the nominal value. The cause of both trends is hypothesized to be ion heating from a modified two-stream instability that becomes dominant for 300 V operation and grows with magnetic field strength. Rougher pole covers were observed to have 33% lower average erosion rates than those that were polished due to local surface features that lower the effective angle of attack of eroding ions and the concomitant sputter yields. Alumina pole covers were shown to erode over 250% faster due to the higher sputter yield of alumina relative to graphite. Shifting the cathode upstream of the pole covers reduced average keeper erosion rates by 84% by reducing the view factor to high-energy beam ions. Cathode keeper erosion was also shown to exhibit azimuthal nonuniformities, which resemble the azimuthal oscillations observed in the cathode region. Taken together, these results provide in-depth insights into the life-limiting mechanisms impacting magnetically shielded Hall thrusters.

Investigation of machining and modified surface features of 316L steel through novel hybrid of HA/CNT added-EDM process

Hydroxyapatite powder (HAp) mixed-EDM process causes surface defects including a high roughness, crack and a thick recast layer attributing a low wettability and amorphous alloys as well as a low material erosion rate (MER) while processing the metallic biomaterials. To mitigate these occurrences, many measures have been conducted. This research aims to investigate the feasibility of hybrid HA and carbon nanotubes (CNT) powders mixed EDM process for processing 316L steel. This study also aims to explore the impacts of CNT and the associated variables on machining and surface characteristics of 316L steel. To ameliorate the machining and surface responses, a high electrically and thermally conductive CNT particles have been added in the HAp suspended mineral oil. To address the set objectives, several characterizations techniques including profilometer, goniometer, energy dispersive X-ray (EDX), X-ray diffraction (XRD), and scanning-electron microscope (SEM) have been carried out. In this study, the multiple additives (HAp and CNT) mixed-EDM process has been successfully introduced. Addition of the CNT in the HAp mixed mineral oil improves MER to 26.47 mg/min and declines SR to 3.16 μm. A thin uniform recast layer of 12.4 μm attributing cracks free surface and a high hydrophilic in nature having contact angle of 35° has been obtained in this study. Elemental composition analysis reveals a high content of Ca, P, O and C in the recast layer formed. The oxides, carbides, and CNT reinforced Ca-P based oxides with both crystalline and amorphous phases have been achieved. In this research work, a novel and effective method for processing the biomedical devices made of 316L steel has been proposed.

Sediment Exchange Between the Created Saltmarshes of Living Shorelines and Adjacent Submersed Aquatic Vegetation in the Chesapeake Bay

Rising sea levels and the increased frequency of extreme events put coastal communities at serious risk. In response, shoreline armoring for stabilization has been widespread. However, this solution does not take the ecological aspects of the coasts into account. The “living shoreline” technique includes coastal ecology by incorporating natural habitat features, such as saltmarshes, into shoreline stabilization. However, the impacts of living shorelines on adjacent benthic communities, such as submersed aquatic vegetation (SAV), are not yet clear. In particular, while both marshes and SAV trap the sediment necessary for their resilience to environmental change, the synergies between the communities are not well-understood. To help quantify the ecological and protective (shoreline stabilization) aspects of living shorelines, we presented modeling results using the Delft3D-SWAN system on sediment transport between the created saltmarshes of the living shorelines and adjacent SAV in a subestuary of Chesapeake Bay. We used a double numerical approach to primarily validate deposition measurements made in the field and to further quantify the sediment balance between the two vegetation communities using an idealized model. This model used the same numerical domain with different wave heights, periods, and basin slopes and includes the presence of rip-rap, which is often used together with marsh plantings in living shorelines, to look at the influences of artificial structures on the sediment exchange between the plant communities. The results of this study indicated lower shear stress, lower erosion rates, and higher deposition rates within the SAV bed compared with the scenario with the marsh only, which helped stabilize bottom sediments by making the sediment balance positive in case of moderate wave climate (deposition within the two vegetations higher than the sediment loss). The presence of rip-rap resulted in a positive sediment balance, especially in the case of extreme events, where sediment balance was magnified. Overall, this study concluded that SAV helps stabilize bed level and shoreline, and rip-rap works better with extreme conditions, demonstrating how the right combination of natural and built solutions can work well in terms of ecology and coastal protection.

Analysis of hybrid HA/CNT suspended-EDM process and multiple-objectives optimization to improve machining responses of 316L steel

A low material erosion rate (MER) and a high roughness are challenges encountered when machining the biomaterials using the hydroxyapatite powder (HAp) suspended-electric discharge machining (EDM) process. Thus, a comprehensive analysis is needed as the required machining outputs are highly dependent on EDM process factors due to its stochastic behavior during the operation. In this context, the research aims at a detailed investigation of the EDM process factors on the machining responses and the feasibility of multiple additives mixed-EDM process needed for processing 316L steel to realize its full potential. To improve MER and reduce both surface roughness (SR) and tool erosion rate (TER), a hybrid of carbon nanotubes (CNT) and HAp supported EDM technique is employed. SEM and EDX analyses have validated the machined surface morphology and migration of the elements. The EDM process parametric optimization has been carried out utilizing Taguchi-based orthogonal design assisting to get the lowest both TER and SR of 0.245 mg/min and 1.85 μm, respectively. The highest MER of 26.47 mg/min as a machinability metric is achieved. Peak current, pulse-on time and HA/CNT quantity are found to be significant determinants in MER, SR and TER using the ANOVA method. The predicted best 21 solution sets using multi-objective spotted hyena optimizer (MOSHO) and following the set objective functions are tabulated. The estimated experimental evaluation errors are lower than 10%, indicating that the predicted solution sets are highly accurate. This research offers a novel approach to process the 316L steel for biomedical applications with enhanced machining performances.