In Situ Analysis of Third Body Contributions to Sliding Friction of a Pb–Mo–S Coating in Dry and Humid Air

Reciprocating sliding tests of ion-beam deposited (IBD) Pb–Mo–S coatings were performed with an in situ tribometer that allows real-time visualization and Raman analysis of the sliding contact through a transparent hemisphere. Experiments were performed in dry air, ambient air (∼50% RH) and mixtures of dry and humid air cycled between low and high humidity. Third bodies formed in the sliding contact were monitored through an optical microscope and analyzed by Raman Spectroscopy. Third body velocity accommodation modes were identified and correlated with friction behavior in dry and ambient air. The dominant velocity accommodation mode in both dry and humid air was interfacial sliding between the outer surface of the transfer film and the wear track; this interface, based on present and earlier studies, is crystalline MoS₂. Therefore, the friction coefficient was controlled by the interfacial shear strength of MoS₂ sliding against MoS₂. Humid air sliding was accompanied by a rise in the friction coefficient and a small but observable second velocity accommodation mode: shear/extrusion of the transfer film. It is concluded that the friction rise in humid air was due to an increase in the interfacial shear strength, and that the rise in friction caused the third body to deform rather than the deformation causing the friction to rise.     

薄膜-半无限大基体复合材料界面剪应力分析

利用影响系数法，将基体看作半无限大体，对薄膜作平面处理，从而对膜－基复合材料的界面剪应力计算进行分析研究。在基体受单向拉伸时，薄膜－基体界面产生剪应力，分别利用有限元法和解析法对薄膜和基体的影响系数进行计算，根据薄膜－基体界面位移协调条件，建立以界面剪应力为未知量的积分方程，最后得出界面剪应力。克服了完全采用有限元法在网格划分时遇到的困难，并且减少计算量。数值计算结果表明，本文对薄膜和基体的处理是有效的，能够较好地反映薄膜－基体界面剪应力的实际变化规律，为膜－基复合材料强度分析提供新的途径。     

Effects of condensation on the interfacial shear stress in laminar film condensation on a flat plate

Laminar film condensation of a saturated vapor in forced flow over a flat plate is analysed. The problem is formulated as an exact boundary-layer solution. From numerical solutions of the governing equations for the ordinary water vapor, Freon-11 and mercury at normal boiling point, it is found that nondimensional values of the film thickness, interfacial shear stress, interfacial velocity and condensation rate are directly affected by the surface temperature of the wall. For the strong condensation case (large C_pΔT/P_r·h_fg), it is found that the magnitude of the interfacial shear stress at the liquid-vapor interphase boundary is approximately equal to the momentum transferred by condensation, i.e., \(\tau _1 \approx \dot m''(U_0 - U_1 )\) .     

Friction and Shear Strength of Polymers

The authors have measured the shear strength and friction of a number of polymers. The shear strength measurements were made on thin films of the polymers deposited on hard, smooth substrates. The values of the coefficients of friction obtained from these shear strength measurements have been compared with the values obtained when bulk polymer slides either on itself or on a hard, smooth surface. The results support the view that in the sliding of bulk polymer energy is dissipated within a thin region close to the sliding interface and it is the pressure in the contact which largely governs the magnitude of the interfacial shear strength.     

Molecular tribology of lubricants and additives

Knowledge of the bulk viscosity provides little guidance to predict accurately the interfacial shear strength and effective viscosity of a fluid in a lubricated contact. To quantify these differences between bulk and thin-film viscosity, an instrument was developed to measure the shear of parallel single crystal solids separated by molecularly-thin lubricant films. The effective shear viscosity is enhanced compared to the bulk, relaxation times are prolonged, and nonlinear responses set in at lower shear rates. These effects are more prominent, the thinner the liquid film. Studies with lubricant additives cast doubt on the usefulness of the concept of a friction coefficient for lubricated sliding.     

Investigation of the effect of diffusion bonding parameters on microstructure and mechanical properties of 7075 aluminium alloy

In the present study, diffusion bonding of aluminium alloy (AA7075) sheet materials which are used especially in the automobile and aerospace industry has been investigated at temperatures of 425 and 450 °C and pressures of 2 and 3 MPa for 180 min in argon atmosphere. The microstructural and mechanical properties of bonding have been characterized with different welding parameters such as bonding temperature and pressure. The microstructure was characterized by light optical microscope, scanning electron microscope and energy dispersive spectroscopy, while the mechanical properties were determined by tensile-shear tests and microhardness tests. The results obtained are discussed from both the microstructural and mechanical points of view. It was observed in the microstructural investigations that the interfacial oxide layer decreased with increasing of the bonding temperature and pressure. The maximum shear strength was found to be 131 MPa for the Al 7075 sample bonded at 450 °C and 3 MPa for 180 min. It is shown that in certain extent, the bonding temperature and bonding pressure have great effect on the joint shear strength. With the increasing of bonding temperature and pressure, the shear strength of the joints increases due to diffusion of atoms in the interface. The strength achieved after bonding were dependent on interface grain boundary migration and on grain growth during the bonding process. The maximum hardness value of the Al 7075 sample bonded at 450 °C, 3 MPa for 180 min is 92.5 HV_0.2. Increasing hardness with increasing temperature can be attributed to the formation of metallic bond at high temperatures and pressures.     

碳纳米管增强聚合物复合材料的界面脱黏及应力分布

碳纳米管和复合材料基体间的界面力学行为是影响复合材料宏观力学性能的重要因素,为此本文利用有限单元法对单壁碳纳米管增强聚合物复合材料的界面脱黏、切应力分布及拔出载荷进行了数值模拟。建立了一个轴对称三圆柱壳模型,引入ABAQUS中的Cohesive单元模拟了单壁碳纳米管和聚合物基体之间的界面层,分析了单壁碳纳米管的长细比、界面强度以及热残余应力等因素对碳纳米管与聚合物基体间的界面切应力以及拔出载荷的影响。模拟结果表明：当单壁碳纳米管的长度变化为50~100 nm、与基体之间的界面强度为50~100 MPa、环境温度变化为100℃ 时,碳纳米管的长细比、界面强度以及由于热失配所引起的残余应力对单壁碳纳米管与聚合物基体间的界面切应力以及拔出载荷有着显著的影响。     

Formation of Cylindrical Sliding-Wear Debris on Silicon in Humid Conditions and Elevated Temperatures

The present study investigates the mechanics of roll formation between sliding bodies at elevated temperatures and humid conditions. Silicon is used as the model material for reciprocating linear sliding tests. The evolution of tribological rolls initially involves the rapid oxidation of silicon wear debris by water, the deformation of SiO₂ particles into platelets, and then the compaction of these particles into a film deposited on the wear surface. The formation of compacted silica film requires minimum adsorption of water which enhances the adhesion between silica platelets. The stress cycle imposed on the film leads to the delamination of platelets near the sliding surface. The delaminated debris cluster into multiple aggregates that are subsequently rolled into dense cylindrical particles so as to relieve the interfacial shear stress. When the film and rolls are formed, the friction and wear rate is maintained at low steady state values.     

Tribological Properties of Alkoxyl Monolayers on Oxide Terminated Silicon

The tribological properties of alkoxyl monolayers on oxide terminated silicon surfaces have been investigated using interfacial force microscopy. For a C18 alkoxyl monolayer, both adhesion and frictional properties are similar to those of a self-assembled monolayer of octadecanethiol on gold. Friction is shown to increase as the alkyl chain length of the molecules decreases. Analysis using contact mechanics models has been carried out to estimate reduced modulus, adhesion energy, and friction shear strength. These interfacial mechanical properties are correlated to molecular structures at the interface.     

Experimental Investigation of Viscoelastic Rolling Contacts: A Comparison with Theory

To address the issue of a pin sliding against a boundary film, we calculate the pressure-dependent shear strength of a bilayer of potassium chloride sandwiched between tungsten carbide (WC) slabs using first-principles, density functional theory (DFT) calculations. It has been shown experimentally that the shear strength S of a KCl film on metal substrates varies with pressure P as S = S ₀ + αP, and S ₀ = 65 ± 5 MPa and α = 0.14 ± 0.02. Calculations are performed for KCl in contact with the (1 $ \bar{1} $ 00) and (10 $ \bar{1} $ 0) faces of WC which have almost square surface unit cells. The effect of pressure is mimicked by varying the distance between the outermost layers of the WC slabs. The DFT calculations confirm that the shear strength depends on pressure and yield average values of S ₀ of 70 ± 10 MPa for the WC(1 $ \bar{1} $ 00) and 51 ± 13 MPa for the WC(10 $ \bar{1} $ 0) faces, in reasonable agreement with experiment. Since the calculations were performed for a KCl slab in registry with the WC slabs, the agreement with experiment suggests that the atoms at the interface between the tip and film are also in registry. In addition, the calculated and experimental shear strengths are much lower than the shear modulus of KCl, indicating that shear occurs between the tip and film surface without forming a transfer film, in agreement with previous experimental measurements.     

Modeling Bearing and Shear Forces in Molecularly Thin Lubricants

Under the effects of high shear rate and confinement between solid surfaces, the behavior of a thin lubricant film deviates from that of the bulk, resulting in significant increases of lubricant viscosity and interfacial slip. A semi-empirical model accounting for the breakdown of continuum theory at the nanoscale is proposed—based on film morphology and chemistry from available experimental and molecular dynamics simulation data—to describe lubricant behavior under shear. Viscosity stiffening and interfacial slip models are introduced into the formulations of the normal (bearing) and shear forces acting on a sphere that moves within a thin lubricant film parallel to a rigid plane. The experimentally measured ‘apparent’ viscosity confounding the effects of both stiffening and slip is used to predict the hydrodynamic forces acting on a fully or partially submerged sphere for the purposes of describing lubricant contact in magnetic storage. The proposed sphere-on-flat model forms the basis of a future, dynamic contact with friction model that will account for lubricant contact in the context of molecularly thin lubricated rough surface contact.     

Estimation of lubricant interlayer shear strength under conditions of imperfect boundary lubrication

This paper considers the use of contact electrical conductivity for investigations into molecular tribology. The shear strength of thin lubricant films has been studied at nominal point contact between two crossed cylindrical steel probes. The interface was simultaneously monitored using electrical contact resistance. It is shown that the binomial law of friction holds for the sliding path portions where electrical measurements point to a continuous lubricant film. The experimental data can be used to evaluate the molecular friction parameters. An algorithm and software with which to evaluate shear strength and its components under conditions of imperfect boundary lubrication have been formulated.     

Two-dimensional two-fluid model for air-oil wavy flow in horizontal tube

This study concerns the development of a two-dimensional two-fluid model for wavy flows in horizontal tubes. To deal with the curved walls of the liquid and gas phases and the gas-liquid interface simultaneously, the bipolar coordinate system was used. Experiments on air-oil mixture flow in horizontal tubes with diameters of 20 and 40 mm were conducted to observe wavy flow patterns accompanying the two-dimensional (2D) and Kelvin-Helmholtz (KH) waves and to measure the pressure gradient under different flow conditions. Two different previous correlations for the interfacial friction factor were employed in the model for predicting the wavy flows with 2D and KH waves. Predictions of the model of the liquid film height, the average values of wall shear stresses of each phase, and the average interfacial shear stress were compared for different diameters and different superficial gas and liquid Reynolds numbers. Also presented are detailed predictions of the model for four different flow conditions, including the local values of interfacial shear stress, wall shear stress of the liquid phase, interfacial friction factor, liquid film height, and two-dimensional velocity distribution in the liquid phase at the cross-section of the tube.

     

Interfacial effect on thermal conductivity of diamond-like carbon films

Diamond-like carbon (DLC) has been of interest as a promising coating for protection and insulating layer in micro-electromechanical systems due to high hardness, wear resistance, transparency in IR range, chemical inertness and biocompatibility. The interfacial effect on thermal transport is studied for DLC films deposited on Al₂O₃ substrates with an ion gun method. Thermal conductivity of DLC thin films is measured with a 3ω method. DLC films show the thickness-dependent thermal conductivity, which is understood with the interfacial thermal resistance between DLC thin film and Al₂O₃ substrate. The interfacial thermal resistance and thermal conductivity of bulk DLC are determined with the measured thickness-dependent thermal conductivity of DLC films.     

Super-low friction of MoS2 coatings in various environments

The ultra-low friction coefficient (typically in the 10⁻² range) of MoS₂-based coatings is generally associated with the friction-induced orientation of ‘easy-shear’ planes of the lamellar structure parallel to the sliding direction, particularly in the absence of environmental reactive gases and with moderate normal loads. We used and AES/XPS ultra-high vacuum tribometer coupled to a preparation chamber, thus allowing the deposition of oxygen-free MoS₂ PVD coatings and the performance of friction tests in various controlled atmospheres. Friction of oxygen-free stoichiometric MoS₂ coatings deposited on AISI 52100 steel was studied in ultra-high vacuum (UHV: 5 × 10⁻⁸ Pa), high vacuum (HV: 10⁻³ Pa), dry nitrogen (10⁵ Pa) and ambient air (10⁵ Pa). ‘Super-low’ friction coefficients below 0.004 were recorded in UHV and dry nitrogen, corresponding to a calculated interfacial shear strength in the range of 1 MPa, about ten times lower than for standard coatings. Low friction coefficients of about 0.013–0.015 were recorded in HV, with interfacial shear strength in the range of 5 MPa. Friction in ambient air leads to higher friction coefficients in the range of 0.2. Surface analysis performed inside the wear scars by Auger electron spectroscopy shows no trace of contaminant, except after friction in ambient air where oxygen and carbon contaminants are observed. In the light of already published results, the ‘super-low’ friction behaviour (10⁻³ range) can be attributed to superlubricity, obtained for a particular combination of cystallographic orientation and the absence of contaminants, leading to a considerable decrease in the interfacial shear strength.     

The Frictional Response of VC(100) Surfaces: Influence of 1-Octanol and 2,2,2-Trifluoroethanol Adsorption

In this report, we present ultrahigh vacuum (UHV) atomic-scale measurements of the frictional response of the VC(100) surface and the influence on friction through the adsorption of 1-octanol (CH₃(CH₂)₇OH) and 2,2,2-trifluoroethanol (CF₃CH₂OH). Atomic force microscopy (AFM) has been used to determine the changes in interfacial friction and adhesion, while scanning tunneling microscopy (STM) has revealed changes in surface morphology upon adsorption. X-ray photoelectron spectroscopy (XPS) has been utilized to determine the composition of the surface formed through the reaction of these adsorbates with VC. Adsorption of 1-octanol on the VC(100) surface at room temperature causes a 15% reduction in the friction measured between a clean VC surface and a silicon nitride AFM tip. STM images, combined with XPS results, reveal that 1-octanol does not completely cover the surface and that saturation occurs approximately at a 500L exposure. Adsorption of 2,2,2-trifluoroethanol on the VC(100) surface at room temperature produces a significant increase in friction while at the same time producing a decrease in adhesion. These contrasting results are interpreted in terms of differences in interfacial shear strength, chemical composition, and the molecular details of the adsorbed layer.     

基于Evans-Johnson流变模型弹流润滑膜厚公式

回归出重载工况下基于牛顿流变模型等温线接触弹流润滑膜厚计算公式,在此基础上回归出基于Evans-Johnson流变模型润滑膜厚公式。结果表明:高速、重载和大滑滚比的工况下,弹流润滑膜厚的降低不仅与材料参数、速度参数和载荷参数有关,还与润滑剂剪切强度有关,在其它工况参数不变的条件下,润滑剂的剪切强度越低,弹流油膜厚度越小。     

The Frictional Properties of Thin Inorganic Halide Films on Iron Measured in Ultrahigh Vacuum

Friction coefficients are measured in ultrahigh vacuum using a tungsten carbide tribopin against thin films of sodium chloride and potassium iodide deposited onto clean iron. It is found, in accordance with previous measurements for potassium chloride on iron, that the friction coefficient falls from an initial value of 2 for the clean iron surface to a minimum value after a few tens of Ångstroms of the halide have been deposited onto the surface, and remains constant for thicker films. The minimum friction coefficient is independent of applied load and therefore obeys Amontons' law. Simple theories for the effect of a low-shear-strength film suggest that the friction coefficient should depend on the shear strength of the film. This idea is tested by plotting the minimum friction coefficient versus the hardness of the film material, which is proportional to its shear strength, where a linear correlation was found. The lack of dependence of friction coefficient with film thickness for thicker films implies that ploughing forces do not contribute significantly to the friction coefficient.     

内燃机曲轴-轴承系统曲轴变形引起的轴承润滑状态变化对曲轴强度的影响

在计及曲轴变形的轴承润滑分析的基础上,应用得到的轴承油膜压力分布作为载荷边界条件计算曲轴应力和强度,以分析目前曲轴强度计算中作用在轴颈上的载荷普遍采用假设的轴承油膜压力分布形式对结果精度的影响。计算中采用整体曲轴梁单元法计算曲轴变形和轴承负荷,采用动力学法进行曲轴轴承的润滑分析,应用有限元法计算曲轴应力。结果表明,计及曲轴受载变形的影响时,轴承油膜压力产生偏布且最大油膜压力明显增加,导致曲轴轴颈过渡圆角表面局部区域的应力数值明显增大,曲轴安全系数减小。因此为使曲轴设计更加合理、更接近实际,曲轴强度计算时应取曲轴—轴承系统为研究对象,根据轴承润滑分析实际计算结果,确定作用在曲轴轴颈上的载荷分布。     

Spatially resolved electron energy-loss studies of metal–ceramic interfaces in transition metal/alumina cermets

Composites consisting of an alumina matrix and 20 vol.% transition metal (Ni or Fe) particles, prepared by hot pressing powder blends, have been studied using spatially resolved transmission electron energy-loss spectroscopy (EELS), and, to a lesser extent, by high-resolution electron microscopy (HREM). Particular attention was paid to the elucidation of the chemical bonding mechanisms at the metal-ceramic interface; EELS spectra from interfacial regions being obtained via a spatial difference technique. From both qualitative and quantitative interpretation of EELS near-edge structures, as well as observed HREM images, the data appear to be consistent with the presence of an Al-terminated alumina at the interface and the formation of direct transition metal – aluminium bonds in Al(O₃M) (M = Ni or Fe) tetrahedral units, possibly as a result of the dissolution and interfacial reprecipitation of Al during processing. These results correlate well with similar model studies on diffusion-bonded Nb/Al₂O₃ interfaces and may, in the light of recent theoretical electronic structure calculations, have implications for the resultant interfacial bond strength in such materials.