Comparison of tribological properties of CrN, TiCN and TiAlN coatings sliding against SiC balls in water

CrN, TiCN and TiAlN coatings were deposited on WC cemented carbide disks using enhanced cathodic arc magnetron sputtering and their topographies and structures were observed and analyzed using scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The friction and wear properties of CrN, TiCN and TiAlN coatings sliding against SiC balls in water were investigated and compared synthetically using ball-on-disk tribometer. The results showed that the CrN/SiC tribopairs showed the lowest friction coefficient of 0.076, while the TiCN/SiC tribopairs displayed the highest friction coefficient of 0.264. For the CrN/SiC tribopairs the specific wear rate of CrN coatings was lowest while that of SiC balls became highest. But for the TiAlN(TiCN)/SiC tribopairs, the specific wear rate of TiAlN coatings was highest while that of SiC balls for the TiCN/SiC tribopairs became lowest. This indicated that the friction and wear behaviors of nitride coatings/SiC balls tribo-systems in water were more strongly influenced by the anti-oxidative ability of tribomaterials in water than by their mechanical properties.     

Investigation of the running-in process and friction coefficient under the lubrication of ionic liquid/water mixture

The tribological properties of three different films commonly used in microelectromechanical systems (MEMS) under the lubrication of ionic liquid (IL)/water mixtures with various concentrations in the running-in process have been investigated. Results show that coefficients of friction (COFs) and wear rates for low temperature silicon oxide (LTO)/Si₃N₄ vary in a similar way to the ones for poly-Si/Si₃N₄ under the lubrications of different IL/water mixtures. In contrast, the differences in COFs and wear rates are more significant in that the COFs and wear rates increase dramatically with the decrease in IL/water concentration in the case of self-mated Si₃N₄, while the differences in COFs and wear rates for the two other tribopairs are relatively small when the concentration is changed. The period of the running-in process reduces with the increase in IL/water concentration for all the tribopairs. Effective hydrodynamic lubrication can be found in the case of Si₃N₄/Si₃N₄ tribopair at higher IL/water concentrations without an evident running-in process, however, such a phenomenon cannot be observed for the other two tribopairs. Different wear mechanisms will also be analyzed in this paper.     

Significance of stacking fault energy on microstructural evolution in Cu and Cu–Al alloys processed by high-pressure torsion

Disks of pure Cu and several Cu–Al alloys were processed by high-pressure torsion (HPT) at room temperature through different numbers of turns to systematically investigate the influence of the stacking fault energy (SFE) on the evolution of microstructural homogeneity. The results show there is initially an inhomogeneous microhardness distribution but this inhomogneity decreases with increasing numbers of turns and the saturation microhardness increases with increasing Al concentration. Uniform microstructures are more readily achieved in materials with high or low SFE than in materials with medium SFE, because there are different mechanisms governing the microstructural evolution. Specifically, recovery processes are dominant in high or medium SFE materials, whereas twin fragmentation is dominant in materials having low SFE. The limiting minimum grain size (d _min) of metals processed by HPT decreases with decreasing SFE and there is additional evidence suggesting that the dependence of d _min on the SFE decreases when the severity of the external loading conditions is increased.     

Langmuir-Blodgett膜摩擦分子动力学模拟和机理研究

采用分子动力学方法,模拟了在两块石英基板上由脂肪酸(C₁₅H₃₁COOH)组成的单层Langmuir-Blodgett (LB)膜间的摩擦特性,探究了超薄膜在滑动过程中的摩擦和结构机理.得出对于单层LB膜在滑动过程中,在速度小于60m/s时,随着速度的增大,其剪切压增大;在速度大于60 m/s时,剪切压随速度的增加而减小.其链的倾斜角随着滑动速度的增加而减小.单层膜内的分子之间以氢键方式形成了较大的分子簇,由此导致了剪切压呈现较长的周期性,但在单层膜之间无氢键形 关键词： 分子动力学模拟 朗缪尔布洛杰特膜 纳米摩擦 氢键     

Room temperature precipitation in quenched Al–Cu–Mg alloys: a model for the reaction kinetics and yield strength development

The microstructural evolution during low temperature ageing of two commercial purity alloys (Al–1.2Cu–1.2Mg–0.2Mn and Al–1.9Cu–1.6Mg–0.2Mn?at.%) was investigated. The initial stage of hardening in these alloys is very rapid, with the alloys nearly doubling in hardness during 20?h ageing at room temperature. The microstructural evolution during this stage of hardening was investigated using differential scanning calorimetry (DSC), isothermal calorimetry and three–dimensional atom probe analysis (3DAP). It is found that, during the hardening, a substantial exothermic heat evolution occurs and that the only microstructural change involves the formation of Cu–Mg co–clusters. The kinetics of cluster formation is analysed and the magnitude of the hardening is discussed on the basis of a model incorporating solid solution hardening and modulus hardening originating from the difference in modulus between Al and clusters.     

Dynamics of a creep-slip model of earthquake faults

Starting off from the relationship between time-dependent friction and velocity softening we present a generalization of the continuous, one-dimensional homogeneous Burridge–Knopoff (BK) model by allowing for displacements by plastic creep and rigid sliding. The evolution equations describe the coupled dynamics of an order parameter-like field variable (the sliding rate) and a control parameter field (the driving force). In addition to the velocity-softening instability and deterministic chaos known from the BK model, the model exhibits a velocity-strengthening regime at low displacement rates which is characterized by anomalous diffusion and which may be interpreted as a continuum analogue of self-organized criticality (SOC). The governing evolution equations for both regimes (a generalized time-dependent Ginzburg–Landau equation and a non-linear diffusion equation, respectively) are derived and implications with regard to fault dynamics and power-law scaling of event-size distributions are discussed. Since the model accounts for memory friction and since it combines features of deterministic chaos and SOC it displays interesting implications as to (i) material aspects of fault friction, (ii) the origin of scaling, (iii) questions related to precursor events, aftershocks and afterslip, and (iv) the problem of earthquake predictability. Moreover, by appropriate re-interpretation of the dynamical variables the model applies to other SOC systems, e.g. sandpiles.     

摩擦导致的聚合物表层微观结构改变

应用大规模分子动力学方法, 模拟了锥形探头在非晶态聚合物薄膜表面的滑动摩擦过程, 研究了摩擦导致的聚合物薄膜表层微观结构改变, 以及探头与基体间黏着作用、滑动速度和分子链长度对基体表层微观结构改变的影响. 当探头与基体之间为黏着作用时, 摩擦导致基体表面滑痕区域的键取向沿滑动方向重新取向, 导致表层分子链回转半径沿滑动方向伸长, 并且这些表层微观结构的改变程度随滑动速度的减小而增大. 在摩擦导致结构改变的过程中, 链端单体和链中单体的贡献作用不同, 形成了不同的分子链拉伸变形机制. 当样本缠结度较大或探头滑动速度较小时, 相比于链中单体, 探头对链端单体的拖曳作用使更多分子链发生拉伸变形. 研究还发现, 在探头与聚合物薄膜系统中, 使薄膜表层微观结构发生改变是摩擦能量耗散的重要途径.     

Influence of velocity in nanoscale friction processes

Force-microscopy images of boric acid crystals were obtained experimentally and simulated with the use of a two-dimensional mechanical model. An analysis of the stick and slip movement of the microscope tip shows that the energy-dissipation mechanism is strongly influenced by the non-linear dynamics of the sliding system. The contributions of stick and viscous forces on the energy dissipation (or friction forces) are studied as a function of the relative scanning velocity. At low relative velocities, the stick forces are shown to be responsible for the energy dissipation. This energy is velocity-dependent, due to the coupling between the two degrees of freedom of the sliding system. As the scanning velocity increases the stick forces are damped; the viscous force is then predominant in the energy-dissipation process. Received: 30 October 2001 / Accepted: 17 May 2002 / Published online: 22 November 2002 RID="*" ID="*"Corresponding author. Fax: +55-21/2295-9397, E-mail: prioli@vdg.fis.puc-rio.br     

Microstructure and wear properties of laser clad Cuss/Cr5Si3 metal silicide composite coatings

Wear resistant Cu-based solid solution (Cu_ss) toughened Cr₅Si₃ metal silicide composite coatings were fabricated on austenitic stainless steel AISI321 by laser cladding process. Due to the rapidly solidified microstructural characteristics and the excellent toughening effect of Cu_ss on Cr₅Si₃, the Cu_ss/Cr₅Si₃ coatings have outstanding wear resistance and low coefficient of friction under room temperature dry sliding wear test conditions coupling with hardened 0.45% C steel.     

Synthesis and tribological properties of AA5052-base insitu composites

Abstract

It is important to optimize the properties of a material for a particular application, hence, to find the suitable material for tribological applications, the wear and friction behaviour of AA5052 in situ composites with different kind of reinforcements have been investigated. For present study, three in situ formed composites have been produced with different reinforcements namely Al₃Zr, ZrB₂ and combination of both (Al₃Zr + ZrB₂) by direct melt reaction (DMR) technique. The as-cast composites and base alloy have been characterized by X-ray diffraction (XRD), optical microscopy, electron microscopy, tensile testing, hardness and dry sliding wear and friction tests. XRD results indicate the successful formation of second phase reinforcement particles in all composites. Wear test results indicate that the cumulative volume loss increases with an increase in sliding distance while coefficient of friction shows a fluctuating tendency, whereas with increasing applied load, wear rate shows an increasing trend while coefficient of friction shows decreasing trend. The variation of wear rate with composites indicates that the composite with multiple reinforcement (Al₃Zr + ZrB₂) has lowest wear rate among all as-cast composites and base alloy, while coefficient of friction is higher. The responsible mechanisms concerned with wear and friction results have been discussed in detail with the help of the observation on worn surface analysis by scanning electron microscope (SEM) and 3D-profilometer. All tribological results have been correlated with the microstructural properties, strength parameters and bulk hardness of the composites.     

Third-order nonlinear optical response of Cu/Ag nanoclusters by ion implantation under 1064 nm laser excitation

The evolution of nanoclusters in sequentially ion-implanted Cu/Ag into silica glasses has been studied. The doses for implantation (×10¹⁶ ions/cm²) were 5Cu/5Ag, 5Cu/10Ag and 5Cu/15Ag, respectively. The microstructural properties of the nanoclusters are characterized by optical absorption spectra and transmission electron microscopy (TEM). Fast nonlinear optical refraction and nonlinear optical absorption coefficients were measured at 1064 nm of wavelength using Z-scan technique. Results in this project indicate that different optical nonlinearities could be selectively obtained at the near-infrared region of 1064 nm of wavelength by changing the metal ingredient percentage in silica.     

基于非连续能量耗散的滑动摩擦系数计算模型

分析了界面摩擦状态下能量非连续耗散过程,建立了简化条件下晶体材料界面摩擦滑动摩擦系数计算模型.结果表明:在弹性接触状态下,滑动摩擦系数与载荷及实际接触面积无关,当实际接触面积接近名义接触面积时,滑动摩擦系数随载荷增加而减小.在缓慢滑动时,滑动摩擦系数随滑动速度的增高而缓慢增大,相对滑动速度愈高,滑动摩擦系数增大趋势愈显著.滑动摩擦系数随晶格常数的增加而降低,而当晶格常数较大时,其变化对滑动摩擦系数影响较小.同时,滑动摩擦系数随原子的可能温升增加而增大.研究结论对工程应用及相关的理论研究具有一定的参考意义. 关键词： 滑动摩擦系数 非连续能量耗散 界面摩擦     

Ageing of a microscopic sliding gold contact at low temperatures

Nanometer-scale friction measurements on a Au(111) surface have been performed at temperatures between 30 and 300?K by means of atomic force microscopy. Stable stick slip with atomic periodicity is observed at all temperatures, showing only weak dependence on temperature between 300 and 170?K. Below 170?K, friction increases with time and a distortion of the stick-slip characteristic is observed. Low friction and periodic stick slip can be reestablished by pulling the tip out of contact and subsequently restoring the contact. A comparison with molecular dynamics simulations indicates that plastic deformation within a growing gold junction leads to the observed frictional behavior at low temperatures. The regular stick slip with atomic periodicity observed at room temperature is the result of a dynamic equilibrium shape of the contact, as microscopic wear damage is observed to heal in the sliding contact.     

3 GPa熔融盐固体介质三轴高温压力容器的轴压摩擦力标定

 在温度标定和围压标定的基础上,采用轴压循环方法,对3 GPa固体介质三轴高温高压实验系统的轴压摩擦力进行了标定,分析了围压、温度、轴向位移速率、装样方式（盐套类型）等实验条件对轴压摩擦力的影响。结果表明：静摩擦力、挤压摩擦力和滑动摩擦力3种轴压摩擦力对轴向应力的影响不同,其中静摩擦力和挤压摩擦力对轴向应力的影响很小,影响应力精度的主要是滑动摩擦力。静摩擦力及滑动摩擦力与围压正相关;静摩擦力与轴向位移速率正相关,但受其影响较小,滑动摩擦力不受其影响;静摩擦力和滑动摩擦力与温度负相关,并且受其影响较显著;盐套类型对轴压摩擦力的影响较大,当实验条件接近盐套熔点时,轴压摩擦力显著降低,当样品周围的盐套处于熔融状态时,轴压摩擦力最小。基于此,确定了标定轴压摩擦力的具体步骤,并对角闪岩的应力-应变曲线进行了轴压摩擦力标定。对比轴压摩擦力校正前、后的应力-应变曲线发现,经过轴压摩擦力校正的应力-应变曲线能更好地反映样品的实际变形情况。     

An investigation of nanoscale tribological characteristics under different interaction forces

The present study employs a Finite Element Method (FEM) atomic approach to investigate the nanoscale mechanisms of sliding friction. The current investigation chooses diamond-like carbon as the hard material, and copper as the soft material. The atomic configurations following sliding under non-interactive, attractive, and repulsive interaction forces are observed for soft-to-soft, hard-to-soft, and hard-to-hard sliding systems. The relationships between the normal force, the friction force, and the sliding distance are discussed. The current simulation results exhibit a similar trend with the findings of previous studies using molecular dynamics approach.     

Dynamic vortex defects in deformed material

The paper studies the generation and evolution of dynamic vortex structures in a material on different structural scales. It is shown that the generation of dynamic vortex structures can be the main accommodation mechanism in a material under external mechanical loading. On the microscale, these structures can provide inter-granular sliding and grain boundary migration with anomalously high rates. On higher structural scales, their evolution can be the main process responsible for nucleation and propagation of cracks, fragmentation of material, formation of a “quasiliquid” layer in friction pairs, etc. The data and conclusions derived from the study are confirmed by numerical calculations for different types of materials in the framework of molecular dynamics and movable cellular automaton methods.     

Velocity plateaus and jumps in carbon nanotube sliding

The friction between concentric carbon nanotubes sliding one inside the other has been widely studied and simulated, but not so far using external force as the driving variable. Our molecular dynamics (MD) simulations show that as the pulling force grows, the sliding velocity increases by jumps and plateaus rather than continuously as expected. Dramatic friction peaks [similar to that recently noted by P. Tangney, M.L. Cohen, S.G. Louie, Phys. Rev. Lett. 97 (2006) 195901] which develop around some preferential sliding velocities, are at the origin of this phenomenon. The (stable) rising edge of the peak produces a velocity plateau; the (unstable) dropping edge produces a jump to the nearest stable branch. The outcome is reminiscent of conduction in ionized gases, the plateau corresponding to a current stabilization against voltage variations, the jump corresponding to a discharge or breakdown.     

Effect of correlated temperature fluctuations on the phase dynamics in an ultrathin lubricant film

The melting of an ultrathin lubricant film at friction between atomically smooth surfaces is studied with allowance for fluctuations of its temperature, which are described by the Ornstein-Uhlenbeck process. The behavior of the most probable types of shear stresses arising in the lubricant is considered, and phase diagrams for second-and first-order phase transformations (the melting of an amorphous lubricant and that of a crystalline lubricant, respectively) are constructed. It is shown that, in the former case, lubricant temperature fluctuations lead to the formation of a stick-slip friction domain separating the domains of dry and sliding friction, which is typical of first-order transitions. In the latter case, three domains of stick-slip friction arise, which mark the transitions between dry friction and metastable and stable sliding friction. As the time of correlation of lubricant temperature fluctuations gets longer, the temperature of rubbing surfaces rises to the point where sliding friction sets in.     

Friction between α-Al2O3(0 0 0 1) surfaces and the effects of surface hydroxylation

Molecular dynamics simulations were performed to study the friction between hydroxylated α-Al₂O₃(0 0 0 1) surfaces at the temperature of 300 K. Effects of the degree of surface hydroxylation and sliding velocity have been discussed. Results indicate that the friction coefficient decreases with increased degrees of hydroxylation. For all degrees of surface hydroxylation, the friction law crosses over from thermal activation to viscous damping at sliding velocity of 80 m/s.     

Analysis of flow stress and deformation mechanism under hot working of ZK60 magnesium alloy by a new strain-dependent constitutive equation

The present study investigates the variation of flow stress and microstructural evolution with strain for ZK60 magnesium alloy. A new constitutive equation was used to model the flow stress with excellent results. This constitutive analysis and the microstructural studies carried out on strained samples revealed the existence of two different regimes. At temperatures above 300 °C, moderate grain growth and intragranular dislocation activity. Yet, the calculated value of the activation energy and the marked increase in the equivalent strain to fracture indicated grain boundary sliding as a dominant mechanism in this regime of strain rate and temperature, with dislocation motion playing an ancillary role. At lower temperatures, deformation was exclusively governed by dislocation motion, with the extensive occurrence of dynamic recrystallization, which started at low strains, and absence of grain growth.