Microstructure,mechanical, tribological,and oxidizing properties of AlCrSiN/AlCrVN/AlCrNbN multilayer coatings with different modulated thicknesses

Multilayer structure design is one of the most promising methods for improving the comprehensive performance of AlCrN-based hard coatings applied to cutting tools. In this study, four types of AlCrSiN/AlCrVN/AlCrNbN multilayer coatings, with different modulated thicknesses, were deposited to investigate their microstructure, mechanical, tribological, and oxidizing properties. All multilayer coatings exhibited grain growth along the crystallographic plane of (200) with a NaCl-type face-centered cubic (FCC) structure. The results show that, as the modulation thickness decreases from ～35 nm to ～10 nm, (1) the grain refinement effect is increasingly evident; (2) all multilayer coatings show a hardness of >30 GPa and an elastic modulus of >300 GPa. Both the ability to resist elastic strain to failure and the plastic deformation of multilayer coatings increase. In addition, their resistance to cracking reduces; (3) the wear rates of these multilayer coatings reduce successively from 1.78 × 10^?16 m³ N^?1 m^?1 to 7.7 × 10^?17 m³ N^?1 m^?1. This is attributed to an increase in self-lubricating VO_x and a decrease in adhesives from the counterparts; (4) the best high-temperature oxidation resistance was obtained for the multilayer coating with a modulated thickness of ～15 nm.     

New horizon in mechanochemistry—high-temperature,high-pressure mechanical synthesis in a planetary ball mill—with magnesium hydride synthesis as an example

A new route of materials synthesis, namely, high-temperature, high-pressure reactive planetary ball milling (HTPRM), is presented. HTPRM allows for the mechanosynthesis of materials at fully controlled temperatures of up to 450 °C and pressures of up to 100 bar of hydrogen. As an example of this application, a successful synthesis of magnesium hydride is presented. The synthesis was performed at controlled temperatures (room temperature (RT), 100, 150, 200, 250, 300, and 325 °C) while milling in a planetary ball mill under hydrogen pressure ($>$50 bar). Very mild milling conditions (250 rpm) were applied for a total milling time of 2 h, and a milling vial with a relatively small diameter (φ = 53 mm, V = ～0.06 dm³) was used. The effect of different temperatures on the synthesis kinetics and outcome were examined. The particle morphology, phase composition, reaction yield, and particle size were measured and analysed by scanning electron microscopy, X-ray diffraction, differential scanning calorimetry (DSC) techniques. The obtained results showed that increasing the temperature of the process significantly improved the reaction rate, which suggested the great potential of this technique for the mechanochemical synthesis of materials.     

Investigation of stress fields for non-standard sized glass plates loaded by ring-on-ring

A ring-on-ring (ROR) test is a prevailing test method for evaluating the equi-biaxial strength of glass materials. However, current ROR test standards limit the strength and size of glass to prevent a nonlinear behavior. In this study, the feasibility of ROR testing for non-standard, high-strength glass, such as tempered or ion-exchanged rectangular glass is investigated. To this end, ROR simulation based on theory and experiment is conducted for thirty non-standard glasses with widths of 100–300 mm and aspect ratios of 1.0–2.0. As a result, the maximum measurable stress was about 215.6 MPa for 100 × 200 mm glass and 481.3 MPa for 300 × 600 mm glass with a 3% deviation, which is well above the strength of regular tempered glass. The main purpose of this work is to understand the range of aspect ratio of horizontal and vertical widths of a glass plate that can be evaluated by the standard ROR test.     

Applications of sketches in network traffic measurement: A survey

Accurate and timely network traffic measurement is essential for network status monitoring, network fault analysis, network intrusion detection, and network security management. With the rapid development of the network, massive network traffic brings severe challenges to network traffic measurement. However, existing measurement methods suffer from many limitations for effectively recording and accurately analyzing big-volume traffic. Recently, sketches, a family of probabilistic data structures that employ hashing technology for summarizing traffic data, have been widely used to solve these problems. However, current literature still lacks a thorough review on sketch-based traffic measurement methods to offer a comprehensive insight on how to apply sketches for fulfilling various traffic measurement tasks. In this paper, we provide a detailed and comprehensive review on the applications of sketches in network traffic measurement. To this end, we classify the network traffic measurement tasks into four categories based on the target of traffic measurement, namely cardinality estimation, flow size estimation, change anomaly detection, and persistent spreader identification. First, we briefly introduce these four types of traffic measurement tasks and discuss the advantages of applying sketches. Then, we propose a series of requirements with regard to the applications of sketches in network traffic measurement. After that, we perform a fine-grained classification for each sketch-based measurement category according to the technologies applied on sketches. During the review, we evaluate the performance, advantages and disadvantages of current sketch-based traffic measurement methods based on the proposed requirements. Through the thorough review, we gain a number of valuable implications that can guide us to choose and design proper traffic measurement methods based on sketches. We also review a number of general sketches that are highly expected in modern network systems to simultaneously perform multiple traffic measurement tasks and discuss their performance based on the proposed requirements. Finally, through our serious review, we summarize a number of open issues and identify several promising research directions.     

Generation,validation and application of dynamic load profiles in flow measurement using cavitating Herschel–Venturi nozzles

Today, utility meters for water are tested for measurement behavior at stable operating conditions at specified flow rates as part of the approval process. The measurement error that occurs during start and stop or when changing between flow rates may not be taken into account. In addition, there are new technologies whose measuring behavior under real-world conditions is only known to a limited extend. To take these facts into account, a new method has been developed and tested to determine the measurement behavior of water meters under dynamic load profiles as they occur in the real application. For this purpose, a test rig for flow rate measurement was extended by a cavitation nozzle apparatus and the generation of dynamic load profiles was validated. For the cavitation nozzles used, possible factors influencing the flow rate, such as temperature and purity of the water as well as the upstream pressure were investigated. Using different types of domestic water meters, the applicability of the dynamic test procedure was demonstrated and the measurement behavior of the meters was characterised.     

B4C–TiB2 composites fabricated by hot pressing TiC–B mixtures: The effect of B excess

Previous research have reported that B₄C–TiB₂ composites could be prepared by the reactive sintering of TiC–B powder mixtures. However, due to spontaneous oxidation of raw powders, using TiC–B powder mixtures with a B/TiC molar ratio of 6: 1 introduced an intermediate phase of C during the sintering process, which deteriorated the hardness of the composites. In this report, the effects of B excess on the phase composition, microstructure, and mechanical properties of B₄C–TiB₂ composites fabricated by reactive hot pressing TiC–B powder mixtures were investigated. XRD and Raman spectra confirmed that lattice expansion occurred in B-rich boron carbide and B_xC–TiB₂ (x > 4) composites were obtained. The increasing B content improved the hardness and fracture toughness but decreased the flexural strength of B_xC–TiB₂ (x > 4) composites. When the molar ratio of B/TiC increased from 6.6:1 to 7.8:1, the Vickers hardness and the fracture toughness of the composites were enhanced from 26.7 GPa and 4.53 MPa m^1/2 to 30.4 GPa and 5.78 MPa m^1/2, respectively. The improved hardness was attributed to the microstructural improvement, while the toughening mechanism was crack deflection, crack bridging and crack branching.     

Fabrication of alumina ceramics with functional gradient structures by digital light processing 3D printing technology

Alumina ceramics with different unit numbers and gradient modes were prepared by digital light processing (DLP) 3D printing technology. The side length of each functional gradient structure was 10 mm, the porosity ratio was controlled to 70%, and the number of units were (1 × 1 × 1 unit) and (2 × 2 × 2 unit) respectively. The different gradient modes were named FCC, GFCC-1, GFCC-2 and GFCC-3. SEM, XRD, and other characterization methods proved that these gradient structures of alumina ceramics had only α-Al2O3 phase and good surface morphology. The mechanical properties and energy absorption properties of alumina ceramics with different functional gradient structures were studied by compression test. The results show that the gradient structure with 1 × 1 × 1 unit has better mechanical properties and energy absorption properties when the number of units is different. When the number of units is the same, GFCC-2 and GFCC-3 gradient structures have better compressive performance and energy absorption potential than FCC structures. The GFCC-2 gradient structure with 1 × 1 × 1 unit has a maximum compressive strength of 19.62 MPa and a maximum energy absorption value of 2.72 × 105 J/m3. The good performance of such functional gradient structures can provide new ideas for the design of lightweight and compressive energy absorption structures in the future.     

Interface regulation effects on mechanical behavior of heterostructure Ti6Al4V/TiAl-based laminated composite sheets

Design of architectured composites with layered-ordered structure can solve the strength-toughness mismatch problem of structural materials. In the present study, heterostructure Ti6Al4V/TiAl laminated composite sheets with different thicknesses of interface layer and TiAl composite layer were successfully produced by hot-pressing technology. The effects of interface regulation and laminated structure on their mechanical properties, crack propagation, and fracture behavior were studied. The results indicated that compressive strength of the sheets increased with the decrease in interface thickness. Compressive strength of TiAl composite sheet with thicker composite layer reached 1481.55 MPa at the arrester orientation with sintering holding time of 40 min, which was 25.96% higher than that of the sheet obtained at 120 min. Analysis indicated that the interface area transferred stress through slip bands and through-interface cracks. Compressive strength at the divider orientation reached 1443.06 MPa, which was 45.78% higher than that of the sheet obtained at 120 min. In this case, the interface area transferred stress through slip bands and along-interface cracks. For TiAl composite sheets with thinner composite layer, compressive strength was further improved to 1631.01 MPa and 1594.66 MPa at the arrester and divider orientations with sintering holding time of 40 min, respectively. The ductile metal layer exerted a significant toughening effect. Both interface regulation and laminated structure transformation could enhance the hetero-deformation induced (HDI) strengthening and improve the comprehensive mechanical properties of the composite sheets.     

Analysis and proposition of limiting current density measurement protocol

Limiting current density at different temperatures, backpressures, and balance gases can be used to separate molecular diffusion resistance, Knudsen diffusion resistance and local transport resistance of membrane electrode assembly (MEA). However, the measurement of limiting current density has no unified protocol. The diverse choices in the literature, either in the control of current or voltage or in the atmosphere like relative humidity and O₂ concentrations, make it difficult to compare the results and identify the true bottleneck hindering the mass transport. In this work, the current-voltage curves obtained by current scanning/stepping and voltage scanning/stepping methods under dilute O₂ of different concentrations and a wide range of relative humidity were measured and analyzed systematically. It is found that the voltage stepping method is superior to the other three ways of control for the reliable determination of the limiting current density. Aided with simultaneous electrochemical impedance spectroscopy measurement, the limiting current density can be determined with pinpoint accuracy. When the limiting current density is just used to qualitatively evaluate different MEA, the voltage scanning method can be used instead for its high time efficiency. The selection of the atmosphere also plays an important role in suppressing the distortion from excessive water and reducing the spurious contribution from proton conduction resistance. It is found that O₂ concentrations at 0.5 vol% and relative humidity at 90% can give the best estimation of O₂ transport resistance in membrane electrode assembly.