Friction and wear at high sliding speeds

The wear of rotating band materials and projectile steel at very high sliding speeds is of great importance in the development of improved cannon. This is especially true for large caliber, high muzzle velocity weapons where excess wear on the bore near the muzzle can limit their useful lives. An extensive experimental study was supported by the U. S. Army from about 1946 to 1956 and a great deal of data was collected at sliding speeds up to 1800 ft s^?1 using a sophisticated high speed pin-on-disk test device. These data indicate that the mechanism of wear at high sliding speeds is surface melting followed by subsequent removal of a portion of the melted surface layer. This means that a rotating band material must be high melting if it is to have good wear resistance at high sliding speeds. It does not mean, however, that compatibility, crystal structure, hardness etc. have no effect because, for a short distance down the bore, the surface of the rotating bands is not completely melted.     

Friction and wear of blended polyoxymethylene sliding against coated steel plates

Measurements were made of the dynamic friction coefficients and specific wear rates of several thermoplastics rubbing against relatively soft coatings on steel plates. Polyoxymethylene (POM)-based composites were investigated using reciprocating, line contact tests against two types of corrosion-protected steel plates (electro-deposited cathodic epoxy layers, called “E-coatings”, and galvanised plates). In addition to virgin POM, composites containing glass fibres, polytetrafluoroethylene (PTFE) fibres, PTFE micro-powder, and high-viscosity silicon oil were investigated. Sliding speeds ranged from 0.05 to 0.3 m/s, and normal loads ranged from 5 to 30 N. The E-coating failed at high loads and velocities. The beneficial effects of lubricating additives in tests with uncoated steel counterfaces were also observed with the coated steel surfaces. POM with glass fibre additives was found to be more abrasive than the base material. The considered non-conformal contact produced similar friction and wear trends than those obtained for the conformal contact.     

Friction, wear and material transfer of sintered polyimides sliding against various steel and diamond-like carbon coated surfaces

Polyimide cylinders are slid under 50 N normal load and 0.3 m/s sliding velocity against carbon steel (R_a=0.2 and 0.05 μm), high-alloy steel (R_a=0.05 μm), diamond-like carbon (DLC, R_a=0.05 μm) and diamond-like nanocomposite (DLN, R_a=0.05 μm). Only for a limited range of test parameters, the friction of polyimide/DLN is lower than for polyimide/steel, while polyimide shows higher wear rates after sliding against DLN compared to steel counterfaces. The DLN coating shows slight wear scratches, although less severe than on DLC-coatings that are worn through thermal degradation. Therefore, also friction against DLC-coatings is high and unstable. Calculated bulk temperatures for steel and DLN under mild sliding conditions remain below the polyimide transition temperature of 180 °C so that other surface characteristics explain low friction on DLN counterfaces, as surface energy, structural compatibility and transfer behaviour. Friction is initially determined through adhesion and it is demonstrated that higher surface energy provides higher friction. After certain sliding time, different polyimide transfer on each counterface governs the tribological performance. Polyimide and amorphous DLC structures are characterised by C–C bonds, showing high structural compatibility and easy adherence of wear debris on the coating. However, it consists of plate-like transfer particles that act as abrasives and deteriorate the polyimide wear resistance. In sliding experiments with high-alloy steel, wear debris is washed out of the contact zone without formation of a transfer film. Transfer consists of island-like particles for smooth carbon steel and it forms a more homogeneous transfer film on rough carbon steel. The latter thick and protective film is favourable for low wear rates; however, it causes higher friction than smooth counterfaces.     

Effect of arc discharge on the wear rate and wear mode transition of a copper-impregnated metallized carbon contact strip sliding against a copper disk

Wear of the contact strip on the pantograph of electric railway vehicles is governed mainly by arc discharge occurring simultaneously with break of contact between the strip and trolley wire. As a step to clarify the wear mechanism of metallized carbon contact strips under the occurrence of contact break arc discharge, a detailed sliding wear test of 30 min duration was carried out for the combination of a copper-impregnated carbon strip and a copper disk at a sliding speed of 100 km/h. The worn surfaces of the strip and disk were observed every 5 min. The voltage drop and electric current were measured throughout the test, and the occurrence and energy of the arc discharge were evaluated for each revolution of the disk. The wear process is considered in terms of the wear mode transition, and the effect of arc discharge on the mode transition and wear rate of the strip is discussed.     

Sliding wear behavior of copper alloy contact wire against copper-based strip for high-speed electrified railways

This paper reports the study of the sliding wear behavior of the Cu–Ag–Cr wire. Cu–Ag–Cr alloy is a promising contact wire material for high-speed electrified railways, which has an excellent combination of mechanical strength and electrical conductivity. Wear tests were conducted under laboratory with a special sliding wear apparatus, which simulated the tribological conditions of sliding current collectors on overhead wires in the railway system. The Cu–Ag–Cr alloy wire was slid against a copper-based powder metallurgy strip under unlubricated conditions. The same strip as those in the train systems were used. Worn surfaces of the Cu–Ag–Cr alloy wire were analyzed by scanning electron microscopy (SEM) and energy dispersive X-ray spectrum (EDS). Within the studied range of electrical current, normal pressure and sliding speed, the wear rate increased with the increasing electrical current and the sliding distance. Compared with a Cu–Ag contact wire under the same conditions, the Cu–Ag–Cr alloy wire had much better wear resistance. Adhesive wear, abrasive wear and arc erosion were the dominant mechanisms during the electrical sliding processes.     

Friction and wear behaviour of stainless steel rubbing against copper-impregnated metallized carbon

A series of experimental tests were carried out using stainless steel rubbing against copper-impregnated metallized carbon under electrical current on a pin-on-disc test rig. The test parameters include the sliding speed of 60-100 km/h, normal force of 40-80 N and electrical current of 0-50 A. During testing, the friction coefficient and wear volume were recorded. The topography of worn surfaces was also observed with SEM. The cross sectional profiles of worn surfaces of stainless steel were measured with Ambios profiler. The result displays that electrical current, normal load and sliding speed have a distinct effect on the friction and wear behaviour of stainless steel rubbing against copper-impregnated metallized carbon. Without electric current, the friction coefficient is largest but the wear volume of copper-impregnated metallized carbon is lowest. With increasing electric current, the friction coefficient decreases while the wear volume of copper-impregnated metallized carbon increases. Through the whole test, it is found that the wear loss of stainless steel was light. The wear of copper-impregnated metallized carbon becomes severe when electrical current or sliding speed is high. When the electrical current or sliding speed is high, arc ablation is a dominant wear mechanism of copper-impregnated metallized carbon.     

Friction and wear characteristics of powder metallurgy copper-graphite brushes at high sliding speeds

Friction experiments using several commercial powder metallurgy copper-graphite brushes against an AISI 4340 steel rotor were conducted at sliding velocities ranging from 20 to 235 m s^?1. The measured wear rates ranged from a minimum of 4.3 × 10^?5 cm km^?1 at a sliding velocity of 100 m s^?1 for a brush with high graphite content to a maximum of 8.4 × 10^?3 cm km^?1 at a sliding velocity of 230 m s^?1 for a brush with high metal content. The coefficients of friction ranged from a minimum of about 0.08 to a maximum of about 0.47 and were greatly affected by the presence of oxide layers at the sliding interface. Almost all the brushes exhibited some degree of edge breaking. The velocity at which edge breaking occurred was dependent on the powder grain size. Brushes with a large grain size seem to exhibit edge breaking at a lower speed than brushes with a fine grain size. High interface temperatures which occur at high sliding speeds result in melting of the lead-tin binders used in most powder copper-graphite brushes.     

Friction and wear thresholds of alumina-chromium steel pairs sliding at high speeds under dry and wet conditions

Friction and wear of alumina sliding against two chromium steels and against itself under dry and wet conditions are reported. Tests were performed using a pin-on-disc device. Loads and speeds, respectively, ranged from 5 to 300 N and 3 to 10.7 m/s. The evolution of the friction coefficient and wear rates point to the existence of a load threshold correlated with the sudden degradation of the ceramic under both wet and dry conditions. Below the threshold, the ceramic remains undamaged while steels exhibit a moderate wear rate. Worn surfaces show thickening metallic transfer on the ceramic and abrasion grooves on steels. Above the threshold, the ceramic damage is induced by grain boundaries failure which leads to a coarse roughness and the release of abrasive particles. The circulation of the latter increases the steel wear and reduces the metallic transfer thickness on the ceramic. Water in the contact zone significantly lowers the threshold value and the friction coefficient value. The mechanical, thermal and chemical effects of load, speed and water are discussed with regard to damage undergone by the sliding bodies. A particular emphasis is focused on the calculation of average and maximum temperatures occuring in the contact area in relation to ceramic grain boundary damage.     

Friction and wear performances of brake material dry sliding against a composite with a semi-interpenetrating network structure of ceramics and Al-alloy

The friction and wear performances of brake material dry sliding against semi-interpenetrating network ceramics/Al-alloy composites were determined using a SRV testing machine. For applied loads from 40-160 N, the friction decreased at 100 and 250 °C. The former friction was superior to the latter. Wear increased at 100 °C but decreased at 250 °C, and converged gradually in both cases. Friction fade took place at high temperatures, followed by overrecovery upon cooling. Higher temperatures increased wear. The proposed friction models incorporated with scanning electron microscopy and energy dispersive X-ray analysis explain the test results better.     

Friction and dry sliding wear of bismaleimide filled with carbon nanotubes

Three types of bismaleimide–carbon nanotubes (CNTs) nanocomposites were fabricated using two types of original multiwalled CNTs with different diameters and one amide functionalized CNTs. The influence of diameter, content and functionalization of CNTs on the flexural and dry sliding wear behaviour were measured with universal testing machine and pin-on-disc wear apparatus. The experimental results indicated that at 1.5 wt-%, the bismaleimide-functionalized MWCNTs exhibited highest flexural strength of 156 MPa which is increased by 164% as compared to the neat matrix, and lowest specific wear rate of 1.8 × 10^?4 mm³ N^?1 m^?1 which is decreased by 90% as compared to the neat matrix. This was attributed to the dispersion of CNTs in the matrix and the filler-matrix adhesion and internal strength of the composite.     

Dry sliding wear characteristics of some industrial polymers against steel counterface

In this investigation, the influence of test speed and applied pressure values on the friction and wear behaviour of polyamide 66 (PA 66), polyoxymethylene (POM), ultrahigh molecular weight polyethylene (UHMWPE), 30% glass fibre reinforced polyphenylene-sulfide (PPS+30%GFR) and aliphatic polyketone (APK) polymers were studied. Friction and wear tests of PA 66, POM, UHMWPE, PPS+30%GFR and APK versus AISI D2 steel were carried out at dry condition on a pin-on-disc arrangement. Tribological tests were performed at room temperature at different pressures (0.35–1.05 MPa) and sliding speeds (0.5–2.0 m/s). The results showed that, for all polymers used in this investigation, the coefficient of friction decreases linearly with the increase in pressure. The specific wear rate for UHMWPE, PPS+30%GFR and APK were in the order of 10⁻⁵ mm³/N m, while the wear rate value for PA 66 was in the order of 10⁻⁶ mm³/N m. In addition to this, the wear rate value for POM was in the order of 10⁻³ mm³/N m. Furthermore, as the results of this investigation, the wear rate showed very little sensitivity to the applied pressures and test speed.     

Friction and wear of copper samples in the steady friction state

The effect of sliding velocity and load on the friction and wear of Cu-steel pairs was studied. Elasto-hydrodynamic (EHL), mixed (ML) and boundary lubrication (BL) regions were analyzed using the Stribeck curve. The lubrication number of Schipper, Z, was used in the analysis of the Stribeck curve. Steady friction states were observed in the mixed EHL and BL regions, however two types of the ML region are revealed. The first type is the stable ML range. The second one is the range of unstable friction and wear when a decrease of the lubricant film leads to abrupt change of all controlled parameters. It was found that a transition to the unstable ML region occurs within a narrow range of Z parameter. Wear modes in the lubrication regions were studied. Deformation hardening in the lubricant regimes is discussed.     

Friction and dry sliding wear behavior of carbon and glass fabric reinforced vinyl ester composites

Friction and dry sliding wear behavior of glass and carbon fabric reinforced vinyl ester composites have been presented. The results show that the coefficient of friction and wear rate increased with increase in load/sliding velocity and depends on type of fabric reinforcement and temperature at the interphase. The excellent tribological characteristics were obtained with carbon fiber in vinyl ester. It is believed that a thin film formed on counterface was seems to be effective in improving the tribological characteristics. The worn surfaces examined through SEM, showed higher levels of broken glass fiber in vinyl ester compared to carbon-vinyl ester composites.     

Friction and wear behaviors of nanocrystalline surface layer of medium carbon steel

A nanocrystalline (nc) surface layer, which is about 30 μm thick, was fabricated on a medium carbon steel plate using high-energy shot peening (HESP). The tribological behavior of the nanocrystalline surface layer was investigated under the unlubricated slide using a ring-on-disc tester. The observation of the surface topography shows that the nanocrystalline surface layer may reduce the effect of fatigue wear and improve the friction and wear properties of medium carbon steel. Experimental results show that the friction coefficient and wear weight loss decrease and the wear resistance increases with the surface nanocrystallized samples under lower loads.     

Friction, wear and N2-lubrication of carbon nitride coatings: a review

Recent results of tribological properties of carbon nitride (CN_x) coatings are reviewed. CN_x coatings of 100 nm thickness were formed on Si-wafer and Si₃N₄ disks by the ion beam mixing method. Friction and wear tests were carried out against Si₃N₄ balls in the environments of vacuum, Ar, N₂, CO₂, O₂ or air by a ball-on-disk tribo-tester in the load range of 80-750 mN and in the velocity range of 4-400 mm/s.It was found that friction coefficient μ is high (μ=0.2-0.4) in air and O₂, and low (μ=0.01-0.1) in N₂, CO₂ and vacuum. The lowest friction coefficient (μ<0.01) was obtained in N₂. It was also found that N₂ gas blown to the sliding surfaces in air effectively reduced the friction coefficient down to μ≈0.017. Wear rate of CN_x coatings varied in the range 10⁻⁹-10⁻⁵ mm³/N m depending on the environment.The wear mechanisms of CN_x in the nanometer scale were studied by abrasive sliding of an AFM diamond pin in air. It was confirmed that the major wear mechanism of CN_x in abrasive friction was low-cycle fatigue which generated thin flaky wear particles of nanometre size.     

Friction and wear characteristics of ZDDP in the sliding of steel against aluminum alloy

The tribological properties of zinc dialkyldithiophosphate (ZDDP) antiwear additives in the sliding of steel against aluminum alloy were investigated by an oscillating friction and wear apparatus, an Optimol SRV tester. It was found that ZDDP produced larger wear of aluminum alloy than base stock, especially at high concentration. The acting mechanism of ZDDP in the lubricated aluminum-on-steel system was proposed.     

Thermal wear and electrical sliding wear behaviors of the polyimide modified polymer-matrix pantograph contact strip

In the present study, the polyimide resin (PI)/cashew-modified resin (YM) polymer-matrix pantograph contact strip (PMPCS) was prepared by using hot repressing, hydro-solidification and dipping treatment processes. The thermal properties of cured resins were studied by thermogravimetry analyzer and differential scanning calorimetry. The thermal wear and electrical sliding wear behaviors of PMPCS against copper were evaluated by a ring block wear tester at elevated temperature under dry sliding conditions and a wear tester which simulated the train motion under laboratory conditions, respectively. Worn surfaces and wear debris of PMPCS were analyzed by scanning electron microscopy and energy dispersive spectrometer, and the wear mechanism was discussed. It has been found that the thermal stability of the PI/YM is superior to that of the YM under the same testing conditions. The results also showed that PI/YM-PMPCS had superior wear resistance than that of YM-PMPCS at elevated temperature and with electrical current. At elevated temperature, the wear mechanism of tribological pair evolved from adhesive wear to oxidative wear with mild delamination wear. Arc erosion wear, oxidative wear, and adhesive wear were the dominant mechanisms of tribological pair during the electrical wearing process.     

Friction and wear behavior of human teeth under various wear conditions

Previous reports have described the differences in the friction and wear behavior between different zones of human teeth. The objective of this research was to study the friction and wear behavior of human teeth under different wear conditions to extend the understanding of the tooth wear process, as well as to provide a more rational explanation for wear mechanism of teeth. Two typical wear tests, namely two- and three-body wear, were conducted on human tooth enamel using a reciprocating apparatus. The effect of food particles was of particular interest. Three loads, 10, 20 and 40 N, were used. Wear was assessed by sample wear volume. The results show that human tooth enamel exhibits lower friction and smaller wear volume under three-body wear conditions than under two-body wear conditions. Under three-body wear conditions, although increasing normal load results in a progressive increase in the wear volume of enamel, the increasing rate is lower at high load than that under two-body wear conditions. Further analysis of wear surfaces indicates that human tooth enamel experiences different wear mechanisms under different wear conditions.     

Friction coefficient and wear rate of sputter-deposited thin films and plasma-sprayed thick coatings at high temperatures in room air

Silver, calcium fluoride (CaF_x with x = 1.85) and chromium-carbon (Cr₃C₂) thin films were deposited onto various tribological test specimens by sputtering. The friction properties of sputter-deposited Ag and CaF_x single layers as well as Ag/CaF_x multilayer films were determined by ball-on-disk tribological tests conducted in room air under various experimental conditions. The tribological properties (friction coefficient and wear rate) of sputter-deposited CaF_x films were also determined at 500°C by pin-on-disk tribological tests performed with pin specimens made of cobalt-based alloy (alacrite). Chromium-carbon films sputter-deposited onto alacrite disk and counterfaces were found to be of interest for reducing the formation of alacrite wear debris in the wear tracks; thus reduced friction coefficient and wear rate values were obtained. The friction behavior of sputter-deposited CaF_x/Cr₃C₂ thin bilayer structures and plasma-sprayed (PS) chromium carbide/Ag/BaF₂-CaF₂ eutectic composite coatings (PS-212 type coatings) was investigated by plane-on-plane tribological tests conducted in room air at 500°C and 700°C. The friction performance of solid lubricant thin bilayer films was compared with that of thick PS-212 type coatings similar to coatings developed by NASA.     

Friction and wear behavior of Ni-P coated Si3N4 reinforced Al6061 composites

Al6061 matrix composite reinforced with nickel coated silicon nitride particles were manufactured by liquid metallurgy route. Microstructure and tribological properties of both matrix alloy and developed composites have been evaluated. Dry sliding friction and wear tests were carried out using pin on disk type machine over a load range of 20-100 N and sliding velocities of range 0.31-1.57 m/s. Results revealed that, nickel coated silicon nitride particles are uniformly distributed through out the matrix alloy. Al6061-Ni-P-Si₃N₄ composite exhibited lower coefficient of friction and wear rate compared to matrix alloy. The coefficient of friction of both matrix alloy and developed composite decreased with increase in load up to 80 N. Beyond this, with further increase in the load, the coefficient of friction increased slightly. However, with increase in sliding velocity coefficient of friction of both matrix alloy and developed composite increases continuously. Wear rates of both matrix alloy and developed composites increased with increase in both load and sliding velocity. Worn surfaces and wear debris was examined using scanning electron microscopy (SEM) for possible wear mechanisms. Energy dispersive spectroscope (EDS), X-ray diffraction (XRD) and X-ray photoelectron spectroscope (XPS) techniques were used to identify the oxides formed on the worn surfaces and wear debris.