电沉积Ni-W梯度镀层及耐磨性研究

通过电沉积技术并调节镀液中镍钨离子配比浓度在铝合金基体表面制备了Ni-W梯度镀层,研究了镀层中W质量分数的变化对梯度镀层性能的影响.结果表明:所制备梯度镀层表面均匀平整,与基体结合良好,沿镀层生长方向W质量分数逐层增加;相比于Ni-W均质合金,Ni-W梯度镀层具有较低的摩擦系数,磨损形貌较为平整,表面剥落少,表现出较好的耐摩擦磨损性能.     

CrN和CrAlN涂层海水环境摩擦学性能研究

采用多弧离子镀在316L不锈钢上沉积CrN和CrAlN涂层.采用X射线衍射仪(XRD)、X射线光电子能谱仪(XPS)和扫描型电子显微镜(SEM)表征涂层的成分和结构,并用纳米压痕和划痕仪测试其硬度和结合力.采用UMT-3往复式摩擦磨损试验机对涂层在海水环境中的摩擦磨损性能进行测试.结果表明:CrN和CrAlN涂层在海水中摩擦系数相差不大,而316L摩擦系数明显大于涂层,且摩擦系数震荡剧烈,表明316L在海水中润滑性较差.涂层在海水中磨损率远小于316L,且CrAlN涂层比CrN涂层在海水环境中具有更优的耐磨性.CrN涂层的磨痕表面出现大量剥落坑,这是由于CrN涂层表面的大颗粒剥落形成的.而CrAlN涂层致密的结构、较为优越的耐蚀性以及摩擦时产生的具有自润滑效果的Al2O3保护层,使其在硬度值较低的情况下仍具有优异的耐磨性.因此海水环境中摩擦性能需综合考虑材料的机械性能、结构、耐蚀性以及耐磨性.     

微弧氧化一步制备石墨-PTFE共添加的自润滑膜层

在Na2SiO3-KOH基础电解液中添加不同的固体润滑微粒,利用微弧氧化技术在LY12铝合金上制备了含有固体润滑剂的微弧氧化膜层.利用傅里叶变换红外光谱(FTIR)、X射线衍射(XRD)和扫描电子显微镜(SEM)研究了未添加、添加聚四氟乙烯(PTFE)和石墨-PTFE共添加微弧氧化膜层的组成和微结构.采用往复球-盘试验机评价了膜层的摩擦学性能.结果表明:加入到电解液中的固体润滑剂在微弧氧化过程中成功进入到膜层中,从而得到含有固体润滑剂的复合膜层.添加PTFE的微弧氧化膜层相较于未添加的膜层具有更小的摩擦系数,而石墨-PTFE共添加能够进一步降低膜层的摩擦系数.     

不同摩擦工况下纯铁试样表面磁记忆现象研究

采用纯铁圆盘试样与标准碳钢钢球(45钢)组成球-盘摩擦副进行旋转摩擦试验,研究在干摩擦条件下,不同的滑动速度及法向载荷下摩擦表面的磁记忆现象.试验结果表明:磨损前后磁记忆信号存在较大差异,磨损后的磁记忆信号在磨痕处出现信号突变,此突变信号可以准确地表征磨痕的位置及磨痕的宽度.对磁记忆信号进行梯度处理,可以发现不同工况下磨痕处的信号峰面积S、交点距n及梯度峰峰值Vpp三者均不相同,且与磨痕的特征存在一定的关系.此外,通过获取不同干摩擦条件下磨痕处的梯度信号特征值,发现信号峰面积S、交点距n及梯度峰峰值Vpp三者均随滑动速度及法向载荷的增大而增大.     

TA2合金激光熔覆自润滑复合涂层组织与摩擦学性能

以三种不同质量分数配比为40%Ti–19.5%Ti C–40.5%WS_2、40%Ti–25.2%Ti C–34.8%WS_2、40%Ti–29.4%Ti C–30.6%WS_2的复合粉末为预置原料,采用激光熔覆技术在钛合金TA2表面原位合成自润滑耐磨复合涂层.系统地分析了涂层的物相、组织、显微硬度及其摩擦学性能与机理.结果表明:三种涂层的显微硬度分别为HV_(0.5)927.1、HV_(0.5)1007.5和HV_(0.5)1052.3,相对于基体(HV_(0.5)180)有极大的提高;三种涂层的摩擦系数和磨损率分别为0.41和30.98×10~(–5) mm~3/(N·m);0.30和18.92×10~(–5) mm~3/(N·m)以及0.34和15.98×10~(–5) mm~3/(N·m).WS_2质量分数为34.8%和30.6%的预置粉末制备的涂层表现出较好的耐磨减摩性能,其磨损机理为轻微的塑性变形和黏着磨损.     

偏压及测试环境对新型MoS2-Ti复合膜摩擦学性能的影响

采用新型高功率脉冲复合磁控溅射技术制备MoS2-Ti复合膜,并研究基体偏压和测试环境对复合膜摩擦学性能的影响.结果表明:制备的MoS2-Ti复合膜表面呈现颗粒状结构,Ti在薄膜表层与O反应形成氧化物有效抑制MoS2的氧化.随着基体负偏压从OV增大到-400 V,复合膜的S/Mo原子比逐渐减小.在-300 V偏压下,颗粒堆积最为紧密,薄膜硬度和弹性模量达到最大值,分别为9.7和137.1GPa,并具有最低的平均摩擦系数值(0.04)和磨损率[(10-7mm3/(N·m)].多种测试环境下的摩擦研究显示:在室温大气环境下复合膜的摩擦学性能与其结构的致密性紧密相关,而在N2以及不同湿度环境下薄膜表现出的优异摩擦学性能则归因于在摩擦过程中有效形成的转移膜贡献.     

Steady and transient sliding under rate-and-state friction

The physics of dry friction is often modelled by assuming that static and kinetic frictional forces can be represented by a pair of coefficients usually referred to as μ_s and μ_k, respectively. In this paper we re-examine this discontinuous dichotomy and relate it quantitatively to the more general, and smooth, framework of rate-and-state friction. This is important because it enables us to link the ideas behind the widely used static and dynamic coefficients to the more complex concepts that lie behind the rate-and-state framework. Further, we introduce a generic framework for rate-and-state friction that unifies different approaches found in the literature.We consider specific dynamical models for the motion of a rigid block sliding on an inclined surface. In the Coulomb model with constant dynamic friction coefficient, sliding at constant velocity is not possible. In the rate-and-state formalism steady sliding states exist, and analysing their existence and stability enables us to show that the static friction coefficient μ_s should be interpreted as the local maximum at very small slip rates of the steady state rate-and-state friction law.Next, we revisit the often-cited experiments of Rabinowicz (J. Appl. Phys., 22:1373–1379, 1951). Rabinowicz further developed the idea of static and kinetic friction by proposing that the friction coefficient maintains its higher and static value μ_s over a persistence length before dropping to the value μ_k. We show that there is a natural identification of the persistence length with the distance that the block slips as measured along the stable manifold of the saddle point equilibrium in the phase space of the rate-and-state dynamics. This enables us explicitly to define μ_s in terms of the rate-and-state variables and hence link Rabinowicz's ideas to rate-and-state friction laws.This stable manifold naturally separates two basins of attraction in the phase space: initial conditions in the first one lead to the block eventually stopping, while in the second basin of attraction the sliding motion continues indefinitely. We show that a second definition of μ_s is possible, compatible with the first one, as the weighted average of the rate-and-state friction coefficient over the time the block is in motion.     

Polymer Induced Drag Reduction in a Turbulent Pipe Flow Subjected to a Coriolis Force

The skin friction factor f in a turbulent wall-bounded flow can be greatly reduced by using polymer solutions. In this paper we discuss experimental results on the effect of the Coriolis force on turbulent drag reduction. To study this, a horizontal smooth-walled pipe with internal diameter 25?mm is placed on a horizontal table rotating about its vertical axis. The rotation is made non-dimensional with friction velocity and pipe diameter, to form the Rotation number Ro. For a range of bulk Rotation number (Ro _b ) between 0 and 0.6 for two different Reynolds numbers (Re _b = 15 & 30 × 10³), the pressure drop is measured, from which the average friction factor f is obtained. Additionally the effect of four different polymer concentrations has been investigated. The single-phase results show that the friction factor increases monotonic but gradual with Rotation. With polymer additives a drag reduction is found that increases with concentration, but which is not affected by the rotation.     

Multiaxial yield behaviour of Al replicated foam

Multiaxial experiments are performed on replicated aluminium foam using a custom-built apparatus. The foam structure is isotropic, and features open monomodal pores in average diameter. Plane stress (σ₁, σ₂, σ₃=0) and axisymmetric (σ₁, σ₂=σ₃) yield envelopes are measured using cubical specimens, supplemented by tests on hollow cylindrical and uniaxial samples. In addition to the three stress components at 0.2% offset strain, the computer-controlled testing apparatus also measures the three instantaneous displacement vectors.Results show that the shape of the yield surface is independent of the relative density of the foam in the explored range (13-28%). Strain increment vectors lie, within error, roughly normal to the line traced through data points in stress space. Replicated foams feature asymmetric yield behaviour between tension and compression. The data additionally show an influence on the yield surface of the third stress tensor invariant (i.e., of the Lode angle). Simple general expressions for the yield surface are fitted to the data, leading to conclude that their behaviour is slightly better captured by parabolic rather than elliptic expressions dependent on all three stress invariants.     

A complete mechanical-property characterization of a single composite specimen

For a unidirectional lamina, the in-plane mechanical properties of interest areE ₁₁,E ₂₂, ν₁₂, ν₁₂ andG ₁₂. Only three of the first four listed are independent. The fourth may be calculated from the reciprocal relationshipE ₁₁ ν₂₁ =E ₂₂ ν₁₂, which results from stiffness-tensor symmetry. Measurement of any four of the five properties listed will completely characterize the in-plane mechanical properties. A longitudinal uniaxial tension test was used to evaluate longitudinal modulus and Poisson's ratio. A modified rail-shear apparatus was designed to measure shear modulus in a rail-shear orientation, as well as a totally constrained transverse modulus in a uniaxial tension test with an orientation transverse to the longest dimension of the specimen. A flexure test was also used to estimate transverse moduli, since the achievement of the totally constrained transverse-modulus measurement was impaired by the variability of load distribution and constraint across the breadth (10-in. length) of the specimen. An apparatus was designed that applied a line loading to the specimen, simply supported on two opposite sides, with the two remaining sides free. The feasibility of this technique was documented by comparing measured values of the transverse modulus and out-of-plane displacement for a [0°]₄ specimen to values obtained from the Levy solution for bending of an orthotropic plate.     

A note on the Mohr-Coulomb and Drucker-Prager strength criteria

This paper presents different expressions of the Mohr-Coulomb (M-C) criterion as well as the interrelationships between them, which lays a foundation for the definition of the equivalent M-C friction angle φ_mc. The characteristics of four types of Drucker-Prager cones matched with the M-C surface are compared as the friction angle φ varies from 0° to 90°. The minimum and maximum value of φ for them is given and the influence of the intermediate principal stress σ₂ to the major principal stress σ₁ is demonstrated using their φ_mc.     

偏压对CrN涂层结构与海水环境摩擦学行为的影响

利用多弧离子镀技术在M2高速钢和p(100)单晶硅片上用不同偏压条件分别制备了4种CrN涂层,考察了涂层的显微结构、力学性能、海水环境中的电化学特性与摩擦学行为,分析了涂层的裂纹形貌与断裂机制.结果显示:交替偏压下制备的多层CrN涂层内部结构致密且硬度与择优取向梯度变化,具有高的膜基结合力(Lc大于150 N),较小的平均晶粒尺寸(70 nm),较高的自腐蚀电位(-0.234 V)和较低的自腐蚀电流(3.052×10-8A).在海水环境中与直径为3 mm的YG-6(94%WC+6%Co)硬质合金球配副,Hertzian接触应力达到3.47 GPa时,平均摩擦系数低于0.15,磨损率低于1.26×10-15m3/(Nm),磨痕内没有明显的涂层崩裂失效,耐磨损性能明显优于其余3种CrN涂层.     

Neutron Diffraction Residual Strain Measurements of Molybdenum Carbide-Based Solid Oxide Fuel Cell Anode Layers with Metal Oxides on Hastelloy X

Thermal spray deposition processes impart residual stress in layered Solid Oxide Fuel Cells (SOFC) materials and hence influence the durability and efficiency of the cell. The current study which is the first of its kind in published literature, reports results on using a neutron diffraction technique, to non-destructively evaluate the through thickness strain measurement in plasma sprayed (as-sprayed) anode layer coatings on a Hastelloy®X substrate. Through thickness neutron diffraction residual strain measurements were done on three different anode coatings (Mo-Mo₂C/Al₂O₃, Mo-Mo₂C/ZrO₂ and Mo-Mo₂C/TiO₂) using the vertical scan mode. The three anode coatings (developed through optimised process parameters) investigated had porosities as high as 20%, with thicknesses between 200 μm to 300 μm deposited on 4.76 mm thick Hastelloy®X substrate discs of 20 mm diameter. The results showed that while the through thickness residual strain in all three anodes was dissimilar for the investigated crystallographic planes, on average it was tensile. Other measurements include X-ray diffraction, nanoindentation and SEM microscopy. As the anode layer microstructures are complex (includes bi-layer alternate phases), non-destructive characterisation of residual strain, e.g. using neutron diffraction, provides a useful measure of through thickness strain profile without altering the stress field in the SOFC electrode assembly.     

Ermittlung von Fließkurven in verschiedenen Ausführungsformen des Stauchversuches

Several modifications of the upsetting test at room temperature are discussed under the aspect of suppressing friction and other possible sources of error.In the upsetting test according to Rastegaev, friction is reduced, and the testpiece remains cylindrical up to high strains, however the specimen's height is determined with a large error. Therefore it is proposed to measure the increase of the specimen's diameter instead of the reduction of height.Upsetting under sticking friction conditions can formally be treated like upsetting with lubrication for calculating the stress-strain curve. However, the applicability and the limits of this method have not yet been studied sufficiently.In the plane-strain upsetting test, friction is not suppressed completely, but the error of height measurement propagates only weakly into the calculated stress-strain curve.

     

碳钢表面微弧氧化膜的制备及摩擦磨损性能研究

在碳钢表面利用微弧氧化技术分别在以铝酸盐和硅酸盐为主的电解液体系中制备了氧化膜.用扫描电子显微镜(SEM)、能谱仪(EDS)和X射线衍射仪(XRD)研究了两种氧化膜的结构、元素含量及分布和相组成;用往复摩擦试验机评价了氧化膜的摩擦磨损性能,并对膜层和对偶表面的磨痕进行了表征.结果表明:铝酸盐体系中制得的微弧氧化膜主要由Fe3O4和铁铝尖晶石(Fe Al2O4)组成,而硅酸盐体系制得的微弧氧化膜成膜元素为Fe、Si和O,并且以非晶态存在.干摩擦条件下,铝酸盐体系中制备的微弧氧化膜具有比硅酸盐体系中制备的微弧氧化膜更低的摩擦系数和磨损率,这是由于铝酸盐中制备的氧化膜含有的Fe3O4在摩擦过程中向对偶发生了一定程度的转移,起到了减摩抗磨的作用.     

The effect of suction or injection on the boundary layer flows induced by continuous surfaces stretched with prescribed skin friction

The boundary layer flow and heat transfer on a stretched surface moving with prescribed skin friction is studied for permeable surface. Three major cases are studied for isothermal surface (n=0) stretched corresponding to different dimensional skin friction boundary conditions namely; skin friction at the surface scales as (x ^?1/2) at m=0, constant skin friction at m=1/3 and skin friction scales as (x) at m=1. The constants m and n are the indices of the power law velocity and temperature exponent respectively. Similarity solutions are obtained for the boundary layer equations subject to power law temperature and velocity variation. The effect of various governing parameters, such as Prandtl number Pr, suction/injection parameter f _w , m and n are studied. The results show that for isothermal surface increasing m enhances the dimensionless heat transfer coefficient for fixed f _w at the suction case and the reverse is true at the injection case. Furthermore, for fixed m, as f _w increases the dimensionless heat transfer coefficient increases. Large enhancements are observed in the heat transfer coefficient as the temperature boundary condition along the surface changes from uniform to linear where the dimensional skin friction is of order (x) at m=1. This enhancement decreases as the suction increases.     

Effective property models for homogeneous two-phase flows

Using an analogy between thermal conductivity of porous media and viscosity in two-phase flow, new definitions for two-phase viscosity are proposed. These new definitions satisfy the following two conditions: namely (i) the two-phase viscosity is equal to the liquid viscosity at the mass quality = 0% and (ii) the two-phase viscosity is equal to the gas viscosity at the mass quality = 100%. These new definitions can be used to compute the two-phase frictional pressure gradient using the homogeneous modeling approach. These new models are assessed using published experimental data of two-phase frictional pressure gradient in circular pipes, minichannels and microchannels in the form of Fanning friction factor (f_m) versus Reynolds number (Re_m). The published data include different working fluids such as R-12, R-22, argon (R740), R717, R134a, R410A and propane (R290) at different diameters and different saturation temperatures. Models are assessed on the basis minimizing the root mean square error (eRMS). It is shown that these new definitions of two-phase viscosity can be used to analyze the experimental data of two-phase frictional pressure gradient in circular pipes, minichannels and microchannels using simple friction models.     

On the skin friction coefficient in viscoelastic wall-bounded flows

Analysis of the skin friction coefficient for wall bounded viscoelastic flows is performed by utilizing available direct numerical simulation (DNS) results for viscoelastic turbulent channel flow. The Oldroyd-B, FENE-P and Giesekus constitutive models are used. First, we analyze the friction coefficient in viscous, viscoelastic and inertial stress contributions, as these arise from suitable momentum balances, for the flow in channels and pipes. Following Fukagata et al. (Phys. Fluids, 14, p. L73, 2002) and Yu et al. (Int. J. Heat. Fluid Flow, 25, p. 961, 2004) these three contributions are evaluated averaging available numerical results, and presented for selected values of flow and rheological parameters. Second, based on DNS results, we develop a universal function for the relative drag reduction as a function of the friction Weissenberg number. This leads to a closed-form approximate expression for the inverse of the square root of the skin friction coefficient for viscoelastic turbulent pipe flow as a function of the friction Reynolds number involving two primary material parameters, and a secondary one which also depends on the flow. The primary parameters are the zero shear-rate elasticity number, El₀, and the limiting value for the drag reduction at high Weissenberg number, LDR, while the secondary one is the relative wall viscosity, μ_w. The predictions reproduce both types A and B of drag reduction, as first introduced by Virk (Nature, 253, p. 109, 1975), corresponding to partially and fully extended polymer molecules, respectively. Comparison of the results for the skin friction coefficient against experimental data shows good agreement for low and moderate drag reduction which is the region covered by the simulations.     

Influence of the powder mixture composition on the deposition coefficient and the properties of NI+B<Subscript>4</Subscript>C CGDS coatings

The cold gas dynamic spray (CGDS) method used to form composite Ni+B₄C coatings from mechanical powder mixture with various content of abrasive components is investigated, and the surface and microstructure of these coatings are considered. An experimental dependence of the deposition coefficient on the abrasive content in the mechanical powder mixture is obtained. The coatings are studied by interference profilometry, optical microscopy, and microindentation methods. The dependence of the bulk and mass B₄C content in the sprayed material on the abrasive content in the sprayed powder mixture is obtained. The bulk B₄C content in the coating c _V ≈ 0.27 is attained. The dependence of the microhardness of composite CGDS coatings on the boron carbide content in them is investigated. The results of this paper demonstrate that the powder mixture composition significantly affects the CGDS coating growth and the properties of these coatings and can be used to control the properties of the CGDS cermet materials.     

Numerical solutions to regularized long wave equation based on mixed covolume method

The mixed covolume method for the regularized long wave equation is developed and studied. By introducing a transfer operator γh , which maps the trial function space into the test function space, and combining the mixed finite element with the finite volume method, the nonlinear and linear Euler fully discrete mixed covolume schemes are constructed, and the existence and uniqueness of the solutions are proved. The optimal error estimates for these schemes are obtained. Finally, a numerical example is provided to examine the efficiency of the proposed schemes.