A super wear-resistant coating for Mg alloys achieved by plasma electrolytic oxidation and discontinuous deposition

Magnesium alloys are lightweight materials with great potential,and plasma electrolytic oxidation(PEO)is effective surface treatment for necessary improvement of corrosion resistance of magnesium alloys.However,the∼14µm thick and rough PEO protection layer has inferior wear resistance,which limits magnesium alloys as sliding or reciprocating parts,where magnesium alloys have special advantages by their inherent damping and denoising properties and attractive light-weighting.Here a novel super wear-resistant coating for magnesium alloys was achieved,via the discontinuous sealing(DCS)of a 1.3µm thick polytetrafluoroethylene(PTFE)polymer layer with an initial area fraction(A_(f))of 70%on the necessary PEO protection layer by selective spraying,and the wear resistance was exceptionally enhanced by∼5500 times in comparison with the base PEO coating.The initial surface roughness(Sa)under PEO+DCS(1.54µm)was imperfectly 59%higher than that under PEO and conventional continuous sealing(CS).Interestingly,DCS was surprisingly 20 times superior for enhancing wear resistance in contrast to CS.DCS induced nano-cracks that splitted DCS layer into multilayer nano-blocks,and DCS also provided extra space for the movement of nano-blocks,which resulted in rolling friction and nano lubrication.Further,DCS promoted mixed wear of the PTFE polymer layer and the PEO coating,and the PTFE layer(HV:6 Kg·mm^(−2),A_(f):92.2%)and the PEO coating(HV:310 Kg·mm^(−2),A_(f):7.8%)served as the soft matrix and the hard point,respectively.Moreover,the dynamic decrease of Sa by 29%during wear also contributed to the super wear resistance.The strategy of depositing a low-frictional discontinuous layer on a rough and hard layer or matrix also opens a window for achieving super wear-resistant coatings in other materials.

Dynamic behavior of outburst two-phase flow in a coal mine T-shaped roadway:The formation of impact airflow and its disaster-causing effect

The study of the dynamic disaster mechanism of coal and gas outburst two-phase flow is crucial for improving disaster reduction and rescue ability of coal mine outburst accidents.An outburst test in a T-shaped roadway was conducted using a self-developed large-scale outburst dynamic disaster test system.We investigated the release characteristics of main energy sources in coal seam,and obtained the dynamic characteristics of outburst two-phase flow in a roadway.Additionally,we established a formation model for outburst impact flow and a model for its flow in a bifurcated structure.The results indicate that the outburst process exhibits pulse characteristics,and the rapid destruction process of coal seam and the blocking state of gas flow are the main causes of the pulse phenomenon.The outburst energy is released in stages,and the elastic potential energy is released in the vertical direction before the horizontal direction.In a straight roadway,the impact force oscillates along the roadway.With an increase in the solid–gas ratio,the two-phase flow impact force gradually increases,and the disaster range extends from the middle of the roadway to the coal seam.In the area near the coal seam,the disaster caused by the two-phase flow impact is characterized by intermittent recovery.In a bifurcated roadway,the effect of impact airflow on impact dynamic disaster is much higher than that of two-phase flow,and the impact force tends to weaken with increasing solid-gas ratio.The impact force is asymmetrically distributed;it is higher on the left of the bifurcated roadway.With an increase in the solid-gas ratio,the static pressure rapidly decreases,and the bifurcated structure accelerates the attenuation of static pressure.Moreover,secondary acceleration is observed when the shock wave moves along the T-shaped roadway,indicating that the bifurcated structure increases the shock wave velocity.

A review on the flow instability of nanofluids

Nanofluid flow occurs in extensive applications, and hence has received widespread attention. The transition of nanofluids from laminar to turbulent flow is an important issue because of the differences in pressure drop and heat transfer between laminar and turbulent flow. Nanofluids will become unstable when they depart from the thermal equilibrium or dynamic equilibrium state. This paper conducts a brief review of research on the flow instability of nanofluids, including hydrodynamic instability and thermal instability. Some open questions on the subject are also identified.

Distribution of non-spherical nanoparticles in turbulent flow of ventilation chamber considering fluctuating particle number density

The Reynolds-averaged general dynamic equation(RAGDE)for the nanoparticle size distribution function is derived,including the contribution to particle coagulation resulting from the fluctuating concentration.The equation together with that of a turbulent gas flow is solved numerically in the turbulent flow of a ventilation chamber with a jet on the wall based on the proposed model relating the fluctuating coagulation to the gradient of mean concentration.Some results are compared with the experimental data.The results show that the proposed model relating the fluctuating coagulation to the gradient of mean concentration is reasonable,and it is necessary to consider the contribution to coagulation resulting from the fluctuating concentration in such a flow.The changes of the particle number concentration M_(0) and the geometric mean diameter dg are more obvious in the core area of the jet,but less obvious in other areas.With the increase in the initial particle number concentration m00,the values of M_(0) and the standard deviation of the particle sizeσdecrease,but the value of d_(g) increases.The decrease in the initial particle diameter leads to the reduction of M_(0) andσand the increase in d_(g).With the increase in the Reynolds number,particles have few chances of collision,and hence the coagulation rate is reduced,leading to the increase in M_(0) andσand the decrease in d_(g).

Mixture flow of particles and power-law fluid in round peristaltic tube

The erythrocyte and blood flowing in the blood vessel can be treated as the two-phase flow of the mixture of particles and a power-law fluid in a peristaltic tube.In the present work, the peristaltic transport of a power-law fluid and the suspension of particles in a tube is investigated by a perturbation method using the long wavelength approximation. The influence of different parameters on the velocity profile and streamlines is explored. Results show that there is a deflection of the flow field when the power-law index n = 0.5 or 1.5 compared with the Newtonian fluid where the trapping zone is symmetric to a certain cross section. The flux rate and reflux of the material are identified,and the conditions under which the reflux appears are determined. Moreover, a reflux phenomenon occurs near the wall. The trapping zone is related to not only the tube geometry and the flow flux but also the fluid properties. Both the length and width of the trapping zone increase with an increase in θ or φ. The trapping zone is more difficult to produce in the shear-thinning fluid than the shear-thickening fluid.

原位集成光浮栅的多层SnS_(2)/少层MoS_(2)异质结用于高性能光电探测器与光学成像

自单层MoS_(2)光电晶体管问世以来,二维层状材料一直被认为是实现下一代新型光电器件与系统的最引人瞩目的候选材料之一.然而,大多数报道的二维层状材料光电探测器都存在一定的缺点,如响应率低、暗电流大、比探测率低、开关比低、响应速率慢等.在本研究中,通过堆叠由大气压化学气相沉积技术所生长的MoS_(2)和SnS_(2)纳米片,制备出了多层SnS_(2)/少层MoS_(2)范德华异质结.相应的SnS_(2)/MoS_(2)异质结光电探测器展示出了具有竞争力的综合性能:大开关比(171)、高响应率(28.3 A W^(-1)),以及出色的比探测率(1.2×10^(13)Jones).此外,该器件还实现了响应/恢复时间低至1.38 ms/600μs的超快响应速率.其优异的性能与SnS_(2)/MoS_(2)异质结的Ⅱ型能带排列以及原位形成的无缝光浮栅的协同作用相关,这有助于分离光激发的电子-空穴对,并延长非平衡载流子的寿命.得益于出色的光敏性,该SnS_(2)/MoS_(2)器件实现了概念验证的光学成像应用.总体而言,本研究为实现具有优异综合性能的先进光电探测器提供了独特视角.

Ultrastrong and ductile(CoCrNi)_(94)Ti_(3)Al_(3)medium-entropy alloys via introducing multi-scale heterogeneous structures

The coarsening-grained single-phase face-centered cubic(fcc)medium-entropy alloys(MEAs)normally exhibit insufficient strength for some engineering applications.Here,superior mechanical properties with ultimate tensile strength of 1.6 GPa and fracture strain of 13.1%at ambient temperature have been achieved in a(CoCrNi)_(94)Ti_(3)Al_(3)MEA by carefully architecting the multi-scale heterogeneous structures.Electron microscopy characterization indicates that the superior mechanical properties mainly originated from the favorable heterogeneous fcc matrix(1-40μm)and the coherent sphericalγ’precipitates(10-100 nm),together with a high number density of crystalline defects(2-10 nm),including dislocations,small stacking faults,Lomer-Cottrell locks,and ultrafine deformation twins.

Antimony smelting process generating solid wastes and dust:Characterization and leaching behaviors

A large amount of solid waste has been produced by the antimony smelting process in the＂World Capital of Antimony＂, Xikuangshan area in China. This study comprehensively investigated the physical and chemical characteristics of the various solid wastes, as well as the leaching behavior of the solid wastes, which included water-quenched slag,arsenic-alkali residue, desulfurized slag and blast furnace dust. These four types of waste were enriched in a variety of heavy metals and metalloids and more specifically with As and Sb levels up to 8.6 × 104 and 3.16 × 105mg/kg, respectively, in arsenic-alkali residue. For desulfurized slag and water-quenched slag, the leaching concentration of Sb significantly exceeded the acceptable limits during the leaching tests using the toxicity characteristic leaching procedure and the synthetic precipitation leaching procedure. In addition, As leaching in arsenic-alkali residue was extraordinarily hazardous, being three orders of magnitude higher than the regulatory level of As. According to the results of the extraction tests, all the tested wastes were classified as hazardous waste.

Additive manufacturing of a high strength Al-5Mg_(2)Si-2Mg alloy:Microstructure and mechanical properties

A novel Al-5 Mg2 Si-2 Mg alloy was processed by selective laser melting(SLM) to understand its representative features of microstructural evolution and mechanical properties during additive manufacturing(AM). The as-SLM fabricated Al-5 Mg_(2) Si-2 Mg alloy is found to deliver much less hot cracks and other defects. Meanwhile, excellent mechanical property is also achieved, i.e. 452 ± 11 MPa for ultimate tensile strength, 295 ± 14 MPa for yield strength, and 9.3 ± 2.5% for elongation. Clearly, these mechanical properties are better than that obtained by high pressure die casting(HPDC), and better than some other alloys obtained by SLM. The as-SLM fabricated Al-5 Mg_(2) Si-2 Mg alloy is featured by the significantly refined microstructures in the compact primary α-Al, the divorced Mg_(2) Si eutectic networks distributing atα-Al grain boundaries, and some α-Al Fe Mn Si phase in association with eutectic Mg_(2) Si phase. The high strength and ductility of the alloy are attributed to its unique features including(a) the reduced solidification range,(b) the possible increase of the eutectic level in the microstructure, and(c) the shift of eutectic point and the maximum solubility point of Mg2 Si in Al matrix.

Tribological behavior and microstructural evolution of lubricating film of silver matrix self-lubricating nanocomposite

The aim of this study is to fabricate the nanocomposite with low friction and high wear resistance using binary solid lubricant particles.The microstructure and tribological performance of the nanocomposite are evaluated,and the composition and film thickness of the lubricating film are observed and analyzed by scanning electron microscopy(SEM)and X‐ray photoelectron spectroscopy(XPS).The nanocomposite exhibited improved tribological properties with a friction coefficient as low as 0.12 and a low wear rate of 2.17×10^(-6) mm^(3)/(N∙m)in high‐purity nitrogen atmosphere.Decreasing sliding speed can increase lubricating film thickness,and the thickest lubricating film is approximately 125 nm.As the film thickness of the lubricating film exceeded 90 nm,the friction coefficient curves became smooth.In compared with WS_(2),MoS_(2) can be more effective in forming the transfer layer on the worn surfaces at the initial stage of the tribological process.

Exceptional strength-ductility synergy of additively manufactured CoCrNi medium-entropy alloy achieved by lattice defects in heterogeneous microstructures

The selective laser melting(SLM)with subsequent cold rolling and annealing is used to produce high-density lattice defects and grain refinement in the CoCrNi medium-entropy alloys(MEAs).The superior comprehensive mechanical properties have been achieved in the as-SLMed CoCrNi alloy after rolling and annealing.The as-SLMed alloys delivered the yield strength of 693.4 MPa,the ultimate tensile strength of 912.7 MPa and the fracture strain of 54.4%.After rolling with 70%reduction in thickness and annealing at 700℃for 2 h.the yield strength,ultimate tensile strength and fracture strain reached 1161.6 MPa,1390.8 MPa and 31.5%,respectively.The exceptional strength-ductility synergy is mainly attributed to the refined hierarchical microstructures with coarsening grains at a level of 30μm and ultrafine grains at a level of 1μm,and the heritage of dislocation-formed sub-grains and other lattice defects.This investigation demonstrates that the SLM with subsequent rolling and annealing is beneficial to fabricate high strength and ductile MEAs with single face-centered cubic(fcc)structure.