表面工程系统接触力学模型

在任何严峻条件下现代机械运行中综合性能的迫切需求促进了许多先进表面工程技术的迅速发展。因而,设计者在结构设计时将有许多表面工程技术可以选择。但是,怎样为一种特定的应用选择一种最优化的表面工程方法对设计者来说又是一种挑战。为了这个目标,在现代多层表面接触理论的基础上建立起一种接触力学模型,考虑到了多层结构、真实表面粗糙度和摩擦影响。这种力学模型可以成功地预测表面工程结构的性能。文中首先回顾了赫兹接触理论的发展过程,然后讨论了伯明翰多层真实粗糙表面接触力学模型,并用实例示范了一系列关于动载荷下表面工程结构设计的主要步骤。

Study of the Active Screen Plasma Nitriding

Active screen plasma nitriding (ASPN) is a novel nitriding process, which overcomes many of the practical problems associated with the conventional DC plasma nitriding (DCPN). Experimental results showed that the metallurgical characteristics and hardening effect of 722M24 steel nitrided by ASPN at both floating potential and anodic (zero) potential were similar to those nitrided by DCPN. XRD and high-resolution SEM analysis indicated that iron nitride particles with sizes in sub-micron scale were deposited on the specimen surface in AS plasma nitriding. These indicate that the neutral iron nitride particles, which are sputtered from the active screen and transferred through plasma to specimen surface, are considered to be the dominant nitrogen carrier in ASPN. The OES results show that NH could not be a critical species in plasma nitriding.

Surface modification of ultra-high molecular weight polyethylene for joint prosthesis and sports applications

The recent progresses in the surfaee modification of ultra high molecular weight polyethylene (UHMWPE) using such advanced surface modification technologies as conventional ion implantation (CⅡ), new plasma immersion ion implantation (PⅢ) and novel active screen plasma (ASP), were all reported. Significantly improved wear resistance was achieved, which has great potential for extending the life-span of joint replacement prostheses and enhancing the performance of such sports equipment as skis and snowboards.

塔里木盆地北部下寒武统底部黑色页岩的Re-Os和Nd同位素特征及其与扬子地台的对比

报道了塔里木盆地北部下寒武统底部黑色页岩的Re-Os和Nd同位素地球化学特征,并通过其Re-Os和Nd同位素特征探讨了该黑色页岩的物源性质.塔里木盆地北部下寒武统底部黑色页岩的Re/Os比值为7.18;187Re/188Os为5.6438;187Os/188Os为1.9616.这些特征反映了该黑色页岩具有壳源性质.其εNd(0)值为-13.17,εNd(540Ma)值为-7.32;Nd模式年龄TDM集中在1.535Ga.黑色页岩中Nd同位素的这些特征与盆地基底岩石的Nd同位素参数十分吻合.综合Re-Os和Nd同位素特征,说明塔里木盆地北部下寒武统底部的这套黑色页岩的物源中应包括以盆地基底为主要组成的大陆地壳.同时,通过与扬子地台下寒武统底部黑色页岩Re-Os同位素的对比,发现两者之间存在着较大的不同,可能反映了两者在沉积背景上的差异.这为正确分析塔里木地台与扬子地台的成因关系提供了Re-0s同位素方面的信息.

Mechanisms of eutectic lamellar destabilization upon rapid solidification of an undercooled Ag-39.9 at.% Cu eutectic alloy

The eutectic Ag-Cu alloys exhibiting fine Ag-Cu lamellar eutectic structure formed upon rapid solidification have great potentials being used in various engineering fields.However,the desired fine primary lamellar eutectic structure(PLES)is usually replaced by a coarse anomalous eutectic structure(AES)when the undercooling prior to solidification exceeds a certain value.The forming mechanism of AES in the undercooled eutectic Ag-Cu alloy has been a controversial issue.In this work,the undercooled Ag-39.9 at.% Cu eutectic alloy is solidified under different cooling conditions by using techniques of melt fluxing and copper mold casting.The results show that the coupled eutectic growth of this alloy undergoes a transition from a slow eutectic-cellular growth(ECG)to a rapid eutectic-dendritic growth(EDG)above a undercooling of 72 K,accompanying with an abrupt change of the distribution and amount of AES in as-solidified microstructures.Two kinds of primary lamellar eutectic structures are formed by ECG and EDG during recalescence,respectively.The destabilization of PLES that causes the formation of AES is ascribed to two different mechanisms based on the microstructural examination and theoretical calculations.Below 72 K,the destabilization of PLES formed by slow ECG is caused by the mechanism of"termination migration"driven by interfacial energy.While above 72 K,the destabilization of PLES formed by rapid EDG is attributed to the unstable perturbation of interface driven by interfacial energy and solute supersaturation.

Influence of Al_2O_3 particle pinning on thermal stability of nanocrystalline Fe

Second-phase particle pinning has been well known as a mechanism impeding grain boundary （GB） migration, and thus, is documented as an efficient approach for stabilizing nanocrystalline （NC） materials at elevated temperatures. The pinning force exerted by interaction between small dispersed particles and GBs strongly depends on size and volume fraction of the particles. Since metallic oxides, e.g. Al2O3, exhibit great structural stability and high resistance against coarsening at high temperatures, they are expected as effective stabilizers for NC materials. In this work, NC composites consisting of NC Fe and Al2O3 nanoparticIes with different amounts and sizes were prepared by high energy ball milling and annealed at various temperatures （Tann） for different time periods （tann）. Microstructures of the ball milled and annealed samples were examined by X-ray diffraction and transmission electron microscopy. The results show that the addition of Al2O3 nanoparticles not only enhances the thermal stability of NC Fe grains but also reduces their coarsening rate at elevated temperatures, and reducing the particle size and/or increasing its amount enhance the stabilizing effect of the Al2O3 particles on the NC Fe grains.

Effects of dealloying and heat treatment parameters on microstructures of nanoporous Pd

Microstructures of nanoporous Pd are essentially important for its physical and chemical properties.In this work,we show that the microstructures of nanoporous Pd can be tuned by adjusting compositions of the precursor alloys,and dealloying and heat treatment parameters.Both the ligament and pore sizes decrease with increasing the electrochemical potential upon dealloying and the concentration of noble component in the precursor alloys.Heat treatment causes coarsening of the nanoporous structure.Above a critical temperature,the nanoporous structures are subjected to significant coarsening.Below the critical temperature,surface diffusion is believed to dominate the coarsening process.Above the critical temperature,the nanoporous structure coarsens remarkably at a rather high rate,which is ascribed to a multiple-mechanism controlled process.

On the mechanism of Si-promoted destabilization of TiC_(x)particles in Al alloys

TiC_(x)is an excellent composite strengthening particle and grain refiner for Al alloys.However,the stability of TiC_(x)is poor when solute Si exists in Al alloy melts,which significantly depresses its strengthening and grain refining effects.In this work,the destabilization mechanisms of the TiC_(x)particles in Al-Si alloy melt with a composition of Al-7Si-7.5TiC were explored via experiments,first-principles calculations and thermodynamic calculations.The experimental results show that Si atoms diffuse into TiC_(x)and Ti atoms are released into the Al melt to form a Ti-rich transition zone during the insulation of TiC_(x)in Al-Si melt,and the TiAlySiz and Al_(4)C_(3)phases are solidified in the Ti-rich zone and at Ti-rich zone/TiC_(x)interface,respectively.The first principles calculations show that the low formation energy of C vacancies facilitates the rapid diffusion of Si atoms in TiC_(x),while the doping of Si atoms reduces the energy barrier of diffusion of Ti atoms in TiC_(x)and promotes the formation of Ti-rich zones.The thermodynamic calculations show that the wide crystallization temperature range of the destabilized product TiAlySiz phase is the key to continuous decomposition of TiC_(x)particles.In addition,the driving force of the main destabilization reaction of TiC_(x)in the Al-Si alloys is about 44 times higher than that in the Al alloys without Si addition.This indicates that the presence of solute Si remarkably promotes the subsequent decomposition process of TiC_(x)in the Al-Si alloy melts.

Direct acceleration of an annular attosecond electron slice driven by near-infrared Laguerre-Gaussian laser

A new near-infrared direct acceleration mechanism driven by Laguerre-Gaussian laser is proposed to stably accelerate and concentrate electron slice both in longitudinal and transversal directions in vacuum.Three-dimensional simulations show that a 2-μm circularly polarized LG_(p)^(l)(p=0,l=1,σ_(2)=-1)laser can directly manipulate attosecond electron slices in additional dimensions(angular directions)and give them annular structures and angular momentums.These annular vortex attosecond electron slices are expected to have some novel applications such as in the collimation of antiprotons in conventional linear accelerators,edge-enhancement electron imaging,structured X-ray generation,and analysis and manipulation of nanomaterials.