The Microstructure and Wear Characteristics of Cu–Fe Matrix Friction Material with Addition of SiC

The Cu–Fe matrix continuous braking friction materials using SiC as abrasive were fabricated by powder metallurgy technique, and the effect of content and size of SiC were investigated. The tribological properties of friction materials sliding against AISI 1045 steel ring were carried out on a block-on-ring tester at different loads and sliding speeds. The strengthening effect of nano-SiC (55 nm) was superior to that of micro-SiC (70 μm) of the tribological properties for friction materials. The friction coefficients of friction materials increased with increasing nano-SiC content. However, the wear rates decreased with increasing nano-SiC content and then increased when the content of nano-SiC particle exceeded 10 wt%. The specimen contained 10% nano-SiC had the best tribological properties at different testing conditions.     

Effects of Surface Roughness and Wood Grain on the Friction Coefficient of Wooden Materials for Wood–Wood Frictional Pair

In order to investigate the effects of the surface roughness and wood grain on the friction coefficient of wooden materials, the friction coefficients of solid wood, medium-density fiberboard (MDF), and particle board (PB) with varying surface roughness were tested by a friction coefficient tester. The friction coefficients of solid wood for a wood–wood frictional pair were measured under varying wood grain (the orientation of fibrils). The results showed that the friction coefficients of the solid wood increased linearly with the arithmetic mean deviation of the surface profile (R_a). The friction coefficients of MDF and PB increased sharply at first and then stabilized with increasing R_a. The friction coefficient of solid wood was respectively maximized and minimized when the grain directions of two wood specimens were both perpendicular to the sliding direction and perpendicular to each other.     

The Influence of Surface Modification on Friction and Lubrication Mechanism Under a Bovine Serum–Lubricated Condition

Diamond-like carbon (DLC) and microdimples are two potential surface modification techniques that are extensively studied to be utilized in biotribological interfaces in order to reduce the friction coefficient and wear rate. However, in situ observation of bovine serum–lubricated DLC and microdimpled surface contacts are not well understood. In this study, a DLC-coated and a microdimpled steel ball rubbing against a Cr-coated glass disk, where 25% bovine serum was used as a lubricant and the temperature was maintained at 37°C, were investigated. The behaviors of ithe nterface were ca`ptured using optical interferometry and the friction coefficients were simultaneously measured using a torque sensor. The experimental results reveal that DLC/glass sliding is scuffing-free, with a lower friction coefficient; however, the formation of a lubricating film is insignificant. On the other hand, the dimples retained lubrication and, as a result, the wear of the glass disk was minimized; however, the friction coefficient was not reduced. Therefore, DLC and microdimples individually have few improved tribological features, but their combination should be considered to maximize performance.     

Frictional Properties and Mechanisms of an Organic–Metal Brake Pair Braking Repeatedly in Magnetic Field

With the aim to investigate repeated braking of organic–metal brake pairs, tribological and scanning electron microscopy (SEM) experiments were performed to reveal the influence of a magnetic field on the tribological performance of brakes. A nonasbestos copper-based brake pad and gray cast iron brake disc were selected as the brake pair. The X-DM pad-on-disc friction tester was improved to set up a tribological tester under a magnetic field. The worn surfaces were observed by SEM to reveal the friction mechanisms. It was found that a magnetic field can ameliorate the dynamic friction and wear. In addition, the global mean friction coefficient increases and the wear resistance of brake materials improves. A magnetic field promotes surface oxidation and aggravates the surface heat emission condition. As a result, the mean temperature on the friction surface increases obviously. An appropriate magnetic field can improve the dynamic temperature rise and decrease the global temperature rise on the friction surface. It is considered that a magnetic field has important influences on tribological performance in repeated braking. Therefore, this research could provide theoretical references for studying the tribological performance in repeated braking and/or under a magnetic field.     

Investigation of Die–Workpiece Interface Friction with Lubrication During the Upsetting Process

A thermo-elastic plastic model of a large deformation combined with a hydrodynamic lubrication model is developed in this paper. The former formulation solves the interaction between mechanical load and thermal load using the finite-element method and the finite-difference method in which the flow stress is taken as a function of strain, strain rate and temperature. While the latter one accounts for the different lubrication regions which may occur at different areas of the die–work-piece interface or at different times, and in which the full film lubrication and the mixed and boundary lubrication conditions are included. Thus, a realistic deformation behaviour can properly be modelled. The calculated forging load and the deformed shape of the workpiece are in good agreement with the results obtained from the upsetting experiment.     

The Study of Arc Rate,Friction, and Wear Performance of C/C Composites in Pantograph–Catenary System

A series of tests on arc rate, friction coefficient, and wear rate of electrical current collectors sliding against overhead contact wires under different conditions was carried out on a high-speed friction and wear testing machine with a pin-on-disc configuration. The worn surface morphology and composition were examined using a scanning electron microscope and energy dispersion spectrum analyzer, respectively. The effects of current, velocity, and load on the arc rate, friction coefficient, and wear rate of C/C composites/QCr0.5 couples were investigated, and the influence mechanism of test parameters on C/C composites was explained. It is concluded that the wear rate increases with an increase in current and velocity and has a decreasing trend with the increase in load. The friction coefficient increases with an increase in velocity and load. The arc rate of C/C composites/QCr0.5 couples increases with an increase in current and velocity. Under the condition of the same current and velocity, when the load is 70 N, the arc rate is the lowest.     

Pre-Polishing the Metal Counterface of Metal–UHMWPE Wear Pair with Filler-Filled UHMWPE Composites to Generate Counterface Changes for an Effective Reduction in Pure UHMWPE Wear

Ultra-high molecular weight polyethylene (UHMWPE) is well known for high-wear-resistance applications. Its long-chained easy sliding molecules and semi-crystalline structures enable the polymer’s great wear resistance. UHMWPE composites made for higher wear resistance study have been analyzed in this paper. Pure UHMWPE, 1 wt% CNT UHMWPE, 1 wt% PEEK UHMWPE, 1 wt% alumina (nano)–UHMWPE composites were made to be tested against metal disk on pin-on-disk tribometer. The metal disk surface conditions were found to have significant influence on the UHMWPE–polymer wear than the composite itself. This result indicates a simple and industrial applicable method that involves transfer film on the counterface to reduce polymer wear for metal–polymer wear pair applications.     

The Friction and Wear Properties of Ti–Al–Nb Intermetallics by Plasma Surface Alloying

Both plasma chromizing and carburization following plasma chromizing (duplex treatment) for Ti–Al–Nb alloy were performed, respectively, and the microstructure, dynamic ultra-microhardness, and elastic modulus of the alloying layer were determined. Using silicon nitride (Si₃N₄) balls as the counterface materials, dry sliding friction tests on the substrate, the chromized layer, and the duplex-treated layer were completed by ball-on-disk tribometer at room temperature. The results indicated that the duplex-treated layer was mainly composed of Cr₂₃C₆, Cr₂Nb, pure chromium, and carbon phases, while the chromized layer consisted of Al₈Cr₅ and Cr₂Nb phases. The ultra-microhardness of the duplex-treated layer was higher than that of the chromized layer, whereas the elastic modulus of the duplex-treated layer was lower than that of the chromized layer. The friction coefficient of the duplex-treated layer was about three times lower than that of the chromized layer, while the wear rate was one order of magnitude lower than that of the chromized layer.     

The Analysis of the Lubrication Condition and Friction Losses of a Single Acting Cross Head Guide Shoe of a Low Speed Cross Head Diesel Engine: PART III — Friction and Its Minimization

This paper describes a model which can be used to analyze the friction losses in the guide shoe of the cross head bearing of a large two stroke diesel engine. It describes further methods by which the friction losses in this component can be significantly reduced.     

Glass–Metal Seals: The Role and Conditions of Metal Annealing before Sealing to Glass

It is generally recommended that the chromium-containing alloy blanks used to produce vacuum-tight glass–metal seals be previously annealed in humid hydrogen. Using the 47 alloy sealed to 95-2 chemical glass as an example, it is shown that annealing of the alloy in humid hydrogen at a temperature of 1050–1100°C leads to formation of a durable oxide film on the surface of the blank. The film is highly wettable with melted glass. Our experiments showed that preliminary annealing can also be performed in an atmosphere of humid argon or helium at a temperature of 1050–1100°C for 1 h. Highly explosive hydrogen can thus be excluded from the production process. The seals obtained by this method are on a par with those produced by the conventional procedure.     

Determination of a Mobile-Ion Concentration in the Dielectric Films of Metal–Insulator–Semiconductor Structures

An experimental technique for determining the surface concentration N _Sof mobile ions in dielectric films of metal–insulator–semiconductor (MIS) structures is described. The technique is based on synchronous recording of the dynamic volt–ampere and low-frequency capacity–voltage characteristics of a sample under investigation. These experimental dependences are shown to ensure accurate extraction of the ion current peaks whose areas are proportional to N _S. These characteristics also allow the relaxation of the surface semiconductor potential to be found, which is needed for reconstructing the dependence of the convection ion current on the voltage drop across the insulation gap of the MIS capacitor. A comparative analysis with other known methods for determining N _Sis carried out. The proposed technique helps find a mobile-ion concentration from a 5 × 10⁹to 10¹³-cm^–2range, including the case when ion current peaks do not appear on the current–voltage characteristics.     

The Effect of Spherical Silica Powder on the Tribological Behavior of Phenolic Resin–Based Friction Materials

The friction and wear behavior of friction materials filled with irregular silica, spherical silica and surface-treated spherical silica particles is discussed in this paper. Compared to irregular silica, spherical silica powders improve the wear resistance, but decrease the friction coefficient. Surface-treated spherical silica powder is more effective in the improvement in the wear resistance, but with the similar friction coefficient of irregular silica-filled materials. This makes it possible to be used as friction-improving fillers in brake materials. Mechanisms for the improvement are also discussed in this paper.     

Analysis of the Contribution of Adhesion and Hysteresis to Shoe–Floor Lubricated Friction in the Boundary Lubrication Regime

Slip and fall accidents cause frequent occupational injuries. Despite recent evidence that boundary lubrication is relevant to slipping, few studies have examined the mechanisms that contribute to shoe?Cfloor friction in this lubrication regime. This study aims to identify the contributions of adhesion and hysteresis to friction in boundary lubrication. Three shoe materials (40 Shore A hardness polyurethane, 60 Shore A hardness rubber, and 70 Shore A hardness rubber), two floor materials (vinyl and marble), and six lubricants (water, 1.5?% detergent, 25?% glycerol?C75?% water, 50?% glycerol?C50?% water, 75?% glycerol?C25?% water, and canola oil) were tested at a single sliding speed (0.01?m?s^?1). Dry adhesion and hysteresis were quantified for each of the shoe?Cfloor combinations and lubricated adhesion was quantified for all shoe?Cfloor-fluid combinations. The contribution of adhesion and hysteresis to shoe?Cfloor-lubricant friction was affected by both the shoe and floor material due to differences in hardness and roughness. Lubricated adhesion was complex and multifactorial with contributions from the shoe, fluid, shoe?Cfloor interaction, floor-lubricant interaction, and shoe-lubricant interactions. A simple regression model including two fluid coefficients and the dry adhesion friction force was able to predict 49?% of the lubricated adhesion friction variability.     

Friction–wear behavior and tribochemical reactions of different ceramics in HFC-134a gas

The friction and wear properties of the ionic ceramics Al₂O₃ and ZrO₂, and the covalent ceramics Si₃N₄ and SiC rubbing against an Al₂O₃ ball in vacuum (10⁻⁵ Pa) and in CF₃CH₂F (HFC-134a) gas at 10⁴ Pa were investigated using a ball-on-disk type tribometer. Without exposure to air, the surface composition and chemical state of the wear tracks and debris on the disks were determined with X-ray photoelectron spectroscopy (XPS). It is found that HFC-134a gas significantly reduces the friction and wear of all the ceramic couples, and that the ionic ceramic pairs show lower friction and wear. On the other hand, metal fluorides and/or fluorine-containing organic compounds are detected on the sliding surfaces. The differences in the friction–wear behavior of the ceramics rubbing in HFC-134a gas may be due to the products of tribochemical reactions, which are dependent on the bond type of the ceramics.     

The Effectiveness of the ZDDP Tribofilms in Lubrication of Sol–Gel Titania in Technical Dry Friction Conditions

The objective of this study was the investigation of the tribological properties of the sol–gel derived titania modified by physically deposited zinc dialkyldithiophosphate (ZDDP) films. Titania coatings were prepared on silicon wafers Si(100) using sol–gel dip-coating method. Amorphous, anatase, and rutile titania were obtained in the post-preparation annealing process conducted at 100, 500, and 1000 °C, respectively. Deposition of ZDDP having butyl- (C4) or dodecyl- (C12) alkyl chain was performed by means of dip-coating (DC), self-assembly (SA), and Langmuir–Blodgett (LB) methods. The effectiveness of the modification was monitored by the wetting contact angle measurement. An increase of the surface hydrophobicity was observed upon modification. The surface topography, imaged with the use of Atomic Force Microscopy (AFM), revealed the presence of island-like agglomerates having different size of ZDDP films deposited using DC and SA method. Smooth and compact C12ZDDP films were observed when LB deposition was applied. The tribological performance of the ZDDP films on titania coatings was tested with the use of microtribometer operating in the normal loads range of 30–100 mN in the technical dry friction conditions. It was found that ZDDP tribofilms effectively decrease the coefficient of friction and effectively reduce the wear of titania coatings.     

Ultra-Low Friction with a Protic Ionic Liquid Boundary Film at the Water-Lubricated Sapphire–Stainless Steel Interface

The sapphire/AISI 316L contact has been lubricated with water, with the protic ionic liquid (PIL) bis (2-hydroxyethylammonium) succinate (MSu), and with a 1 wt% solution of MSu in water. Neat water evaporates after the running-in period to give a transition to dry contact. Neat MSu reduces the running-in period, reducing the friction coefficient to 0.09, an 88 % lower than water. Water + 1 wt% MSu reduces the running-in period, and after water evaporation, a PIL boundary film is formed, with an ultralow minimum friction coefficient of 0.0001, and a mean friction value of 0.02, a 97 % lower than that of water, and a 78 % lower than that of neat MSu. Surface interactions have been studied by 3D profilometry, FTIR, SEM-EDX, and XPS analysis.     

Application of Computational Fluid Dynamics and Fluid–Structure Interaction Method to the Lubrication Study of a Rotor–Bearing System

Computational methods were used to analyse the elasto-hydrodynamic lubrication of a complex rotor–bearing system. The methodology employed computational fluid dynamics (CFD), based on the Navier–Stokes equation and a fluid–structure interaction (FSI) technique. A series of models representing the system were built using the CFD–FSI methodology to investigate the interaction between the lubrication of the fluid film, and elastic dynamics of the rotor and journal bearing. All models followed an assumption of isothermal behaviour. The FSI methodology was implemented by setting nodal forces and displacements to equilibrium at the fluid–structure interface, therefore allowing the lubrication of the fluid and the elastic deformation of structures to be solved simultaneously. This is significantly different to the more common techniques—such as the Reynolds equation method—that use an iterative solution to balance the imposed load and the force resulting from the pressure of the fluid film to within a set tolerance. Predictions using the CFD–FSI method were compared with the results of an experimental study and the predictions from an ‘in-house’ lubrication code based on the Reynolds equation. The dynamic response of the system was investigated with both rigid and flexible bodies for a range of different bearing materials and dynamic unbalanced loads. Cavitation within the fluid film was represented in the CFD–FSI method using a simplified phase change boundary condition. This allowed the transition between the liquid and vapour phases to be derived from the lubricant’s properties as a function of pressure. The combination of CFD and FSI was shown to be a useful tool for the investigation of the hydrodynamic and elasto-hydrodynamic lubrications of a rotor–bearing system. The elastic deformation of the bearing and dynamic unbalanced loading of the rotor had significant effects on the position of its locus.     

Friction and wear of aluminium–steel contacts lubricated with ordered fluids-neutral and ionic liquid crystals as oil additives

Friction and wear of ASTM B211 aluminium–AISI 52100 steel contacts have been determined using pin-on-disk tests under variable conditions of normal applied load, sliding speed and temperature, in the presence of a lubricating base oil modified with a 1 wt.% proportion of three different liquid crystalline additives.The tribological behavior of the ionic liquid crystal n-dodecylammonium chloride (LC3) has been compared with that of two neutral liquid crystals: a non-polar species, 4,4′-dibutylazobenzene (LC1) which had previously shown its ability to lower friction and wear of metallic pairs as compared to the base oil, and a cholesterol derivative, cholesteryl linoleate (LC2).At low temperature and low sliding speed values, the friction coefficients obtained for LC1 are lower than those of LC3. As the severity of the contact conditions increases, this tendency reverses and the ionic species LC2 gives rise to lower friction values than LC1.Wear volume losses under increasing normal loads, between 2.45 and 5.89 N, are always lower in the presence of the ionic additive LC3.Lubrication and wear mechanisms are discussed from optical microscopy and SEM observation of the wear scars and wear debris morphology.     

The Effect of Dwell Time on the Static Friction in Creeping Elastic–Plastic Polymer Spherical Contact

A theoretical model is developed to study the effect of dwell time on the junction growth and static friction of a creeping polymer sphere in contact with a rigid flat under full stick contact condition. A rapid normal loading into the elastic–plastic contact regime is followed by a rest period during which creep takes place causing contact area growth, and stress relaxation that can completely eliminate the plastic zone in the sphere. At the end of this rest time, an increasing tangential loading is applied to the flat till sliding inception occurs. During this loading step, further increase of the contact area and reappearing of a plastic zone in the sphere take place. An increase in static friction resulting from the dwell time during the creep stage is clearly demonstrated and explained.