Friction coefficient and wear mode transition in micro-scale abrasion tests

The purpose of this work was to conduct ball-cratering wear tests to monitor both normal and tangential forces. Balls of 52100 steel, a specimen of H10 tool steel and an abrasive slurry prepared with silicon carbide particles and distilled water were used. Optical microscopy analysis of the worn craters revealed the presence of only grooving abrasion. However, a more detailed analysis conducted by SEM has indicated that different degrees of rolling abrasion have also occurred along the grooves. The results also showed that the normal force plays an important role in the scattering of the values of the friction coefficient.     

Friction and wear of diamond-containing polyimide composites in water and air

Polyimide-based composites containing fine diamond powder were fabricated using spark plasma sintering. The based material was polyimide (PI) containing a small amount of polytetrafluoroethylene (PTFE). Two types of diamond powder were used: one synthesized by statically high pressure, i.e., high-pressure diamond (HD), and the other synthesized by shock compression, i.e., shock-compression diamond (SD). We evaluated their tribological properties using a reciprocating friction tester in water and air using an Al₂O₃ mating ball. Adding HD to the polyimide-PTFE-based material decreased the composite's friction in water, but the effect of this addition in air was negligible. The specific wear rate of composites with different HD content was similar to that of the based material alone in water, while the wear of composites decreased with the addition of diamond in air. The effect of diamond powder size on friction and wear of composites was generally low in both water and air. The addition of SD decreased the friction coefficient of composites, but SD content only negligibly affected the friction in water and air. The specific wear rate was minimal at SD content of 5 vol.%, when diamond content was varied. Wear was almost independent of diamond powder size. SD reduced composite friction and wear better than HD; regardless of environment, its friction coefficient was less than 0.1 and the specific wear rate was in the level of 10⁻⁷ mm³/N m in both water and air.     

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.     

RSM base study of the effect of deposition temperature and hydrogen flow on the wear behavior of DLC films

A statistical study of the effects of deposition temperature and hydrogen flow on the wear behavior of DLC films was examined using the RSM method based on a central composite design. DLC films were deposited on the nitrocarburized AISI 4140 steel by the pulsed DC PACVD method at temperature range of 60–120 °C and in an atmosphere of hydrogen range of 0–40 sccm. Results indicated that a combination of relatively high deposition temperature and low hydrogen flow or low deposition temperature and high hydrogen flow produce DLC film with low wear rate and low friction coefficient.     

Friction and wear behavior of pure carbon strip sliding against copper contact wire under AC passage at high speeds

A series of tests on the friction and wear behaviour of pure carbon strip/copper contact wire with alternating current were conducted on a ring-on-block sliding tester at a high speed. The electric current, normal force and sliding velocity have distinct effects on the test results. The worn scar has the smallest size without electric current. The worn scar becomes larger with increasing electric current. Arc ablation pits, dark stream-lines of arc ablation, slipping marks, spalling blocks and the copper-like layer are found on the worn surfaces. Arc erosion, abrasive wear and adhesive wear are main wear mechanisms.     

Modelling of friction and wear for designing cryogenic valves

From a tribological point of view, the selection of materials for seals for valves of cryogenic rocket engines is a critical issue for the designer. Due to the lack of comprehensive information on this topic, data have been obtained in the framework of research programmes.In the first step, two polymers potentially usable for sealing cryogenic fluids were identified. They were submitted to general test conditions to gain some fundamental understanding of their tribological behaviour when immersed in a cryogenic fluid. This fundamental understanding proved to be helpful for an efficient technical approach.This approach with the test conditions as close as possible to those met in real valves has been used in the second step. In order to obtain the best information with the minimum number of tests, the statistical method of Doehlert has been adopted. The use of such a method led to the construction of surface response for modelling friction and wear. These surfaces and their associated equation are directly and easily usable by the designer. This way of treating technical tribological problem is illustrated by an example.     

Friction and wear behaviour of AISI 5140 steel under rectangular wave loading conditions

Abstract

The tribological characteristics and wear mechanisms of AISI 5140 steel at ambient temperature were investigated using a home built ball on disc tribometer under constant normal loading and rectangular wave loading respectively. The worn surface and wear debris collected from the disc were studied using scanning electron microscope. Results show that the friction coefficients under the constant normal force correspond to the traditional theory. The coefficients all exhibited increased normal loads, whereas under the rectangular wave condition, the highest coefficient appeared when the peak value of the periodically alternative load was 90?N. The different underlying wear mechanism under different loading conditions was explored. It was found that abrasive wear was the main mechanism in the constant loading, whereas severe plastic deformation and adhesive wear were the main wear mechanism in the rectangular wave loading cases. This phenomenon can be attributed to the role of debris in the ‘lubrication’ process under the rectangular wave loading conditions.     

Friction and wear properties of duplex MAO/CrN coatings sliding against Si3N4 ceramic balls in air, water and oil

It is evident that the micro-arc oxidation (MAO) ceramic coatings often exhibit relatively high friction coefficients as sliding against many mating materials. To reduce the friction coefficient for the MAO coatings, the duplex MAO/CrN coatings were deposited on 2024Al alloy using combined micro-arc oxidation and reactive radio frequency magnetron sputtering. The microstructure and phase of the duplex coatings were observed and determined using scanning electron microscope (SEM) and X-ray diffraction (XRD), respectively. The friction and wear behaviors of the duplex coatings sliding against Si₃N₄ balls in air, water and oil were investigated using a ball-on-disk tribometer. The wear rate of the duplex coating was determined by non-contact optical profilometer and the wear tracks on the duplex coatings were observed by SEM. The results showed the CrN coatings mainly consisted of Cr, CrN and Cr₂N phases. The duplex coatings/Si₃N₄ tribopair exhibited the highest friction coefficient in air, while displayed the lowest friction coefficient in oil. When the normal load and the sliding speed increased, the friction coefficient in air increased from 0.65 to 0.72, whereas decreased from 0.58 to 0.36 in water and 0.20 to 0.08 in oil. The specific wear rates for the duplex coatings in air were higher than those in oil. In comparison to the MAO coatings, the duplex MAO/CrN coatings displayed excellent tribological properties under the same conditions.     

Friction, wear and structure of Cu samples in the lubricated steady friction state

Friction and wear of copper rubbed with lubrication in wide range of loads and sliding velocities were studied. The results of friction and wear experiments are presented as the Stribeck curve where the boundary lubrication (BL), mixed (ML) and elasto-hydrodynamic lubrication (EHL) regions are considered. The structural state of subsurface layers in different lubricant regions is studied by X-ray photoelectron spectroscopy, optical, transmission and scanning microscopy analysis. Dislocation density of dislocations in EHL and BL lubricant regimes was determined. Nanohardness at thin surface layers rubbed under different lubricant regimes is compared. The dominant friction and wear mechanisms in different lubrications regions are discussed.     

Friction and wear behaviour of hard and superhard coatings at cryogenic temperatures

The work presents data on friction and wear behaviour of pin-on-disc pairs with superhard diamond-like carbon (DLC) coatings and hard coatings of zirconium nitride (ZrN) and titanium nitride (TiN) in liquid nitrogen with loads of 2.5 and 10 N and sliding speed of 0.06 m/s. It is shown that at cryogenic temperatures the friction coefficients of pairs of two types of DLC coatings obtained by vacuum-arc deposition of filtered high-speed carbon plasma fluxes depend to a great deal on the mechanical properties of the coatings defined by predominant sp² or sp³ hybridization of valence electrons. A friction coefficient of 0.76 was observed for friction pairs of superhard (90 GPa) DLC coatings having properties similar to those of diamond. For “softer” DLC coatings of 40 GPa and properties similar to those of graphite the friction coefficient shows lower values (0.24–0.48) dependent on normal load and counterbody material. The DLC coatings obtained by the filtered arc technology exhibit good wear resistance and have strong adhesion to the substrate under friction in liquid nitrogen. With a normal load of 10 N under cryogenic temperature a low wear rate (of the order of 7.2×10⁻⁴ nm/cycle) was found for superhard DLC coatings. The friction coefficient of pairs with hard ZrN and superhard DLC coatings on steel discs was revealed to be linearly dependent on the counterbody material hardness between 20 and 100 GPa. The hardness of the pin was varied by means of depositing TiN or DLC coatings and also by using high-hardness compounds (boron nitride and synthetic diamond). Proceeding this way can be promising since it offers the possibility of creating low-temperature junctions of required friction properties.     

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.     

Triboelectrochemical investigation of the friction and wear behaviour of TiN coatings in a neutral solution

Triboelectrochemical techniques use an electrochemical set-up (mainly of the three-electrode type) for controlling the potential of the surface of a conducting material subjected to rubbing in a tribometer. In this way it is possible to carry out friction and wear tests in electrolytic solutions under well-defined chemical conditions determined by the applied electrode potential. In addition, triboelectrochemical techniques offer the possibility of following in-situ and in real time the kinetics of electrochemical oxidation reactions (corrosion) by the simple measure of an electrical current. In the present study triboelectrochemical experiments were carried out on sputter deposited TiN coatings sliding against alumina in a borate solution (pH 8.4). The surface of selected worn coatings was characterised by X-ray photoelectron spectroscopy (XPS) and the topography by scanning electron microscopy (SEM). Results show that the rate of wear critically depends on the prevailing (electro)chemical conditions which determine the chemical surface state of the TiN coating. The behaviour is attributed to the lubricating properties of surface oxide films having a thickness in the nanometre range.     

Tribological characteristics of low-pressure plasma-sprayed A12O3 coating from room temperature to 800 °C

The tribological characteristics of low-pressure plasma-sprayed (LPPS) Al₂O₃ coating sliding against alumina ball have been investigated from room temperature to 800 °C. These friction and wear data have been compared quantitatively with those of bulk sintered alumina to obtain a better understanding of wear mechanisms at elevated temperatures. The friction and wear of Al₂O₃ coating show a strong dependence on temperature, changing from a mild to a severe wear regime with the increase of temperature. The coefficient of friction at room temperature is approximately 0.17 to 0.42, depending on applied load. The tribochemical reaction between the coating surface and water vapor in the environment and the presence of the hydroxide film on the Al₂O₃ coating reduce the friction and wear at room temperature as contrasted to those of bulk sintered alumina. At intermediate temperatures, from 400 to 600 °C, the friction and wear behavior of Al₂O₃ coating depends on the inter-granular fracture and pull-out of Al₂O₃ grains. At above 700 °C, formation and deformation of fine grain layer, and abrasive wear in the form of removal of fine alumina grains further facilitate the friction and wear process of Al₂O₃ coating.     

Effect of sliding speed on friction and wear behaviour of nanocrystalline nickel tested in an argon atmosphere

The sliding speed dependence of the coefficient of friction (COF) and wear rate (W) of a nanocrystalline (nc) Ni with a grain size of 15 ± 3 nm and a hardness of 5.09 ± 0.11 GPa was compared to that of a microcrystalline (mc) Ni with a grain size of 20 ± 5 μm and a hardness of 1.20 ± 0.05 GPa. The sliding wear tests were performed in an argon environment under a constant normal load of 2 N using three different sliding speeds of 0.2 × 10⁻², 0.8 × 10⁻² and 3.0 × 10⁻² m/s. The lesser wear damage in the nc Ni at any given speed was attributed to its higher hardness and its greater elastic depth recovery ratio compared to the mc Ni. The mc Ni's COFs and wear rates were independent of the sliding speed over the relatively small range used. However, the same small increase in sliding speed caused an 86% reduction in the nc Ni's wear rate, from 3.44 × 10⁻³ to 0.47 × 10⁻³ mm³/m, and a 31% increase in its COF, from 0.49 ± 0.05 to 0.64 ± 0.06. A modified Archard equation was proposed to predict wear rates of Ni as a function of grain size and sliding speed. Increasing the sliding speed made it increasingly difficult for surface damage by plastic deformation to occur in the nc Ni, because the grain-boundary-induced deformation mechanisms are more difficult to operate at higher strain rates. At the highest speed, the smallest amount of debris was generated, which was not sufficient to form protective tribolayers leading to a high COF value.     

Friction and wear of tribofilms formed by zinc dialkyl dithiophosphate antiwear additive in low viscosity engine oils

Friction and wear characteristics of low viscosity SAE 5W-20 engine oils containing different amounts of phosphorus were studied using two different test devices. One was a laboratory high frequency reciprocating rig (HFRR) testing new and used oils at low and elevated temperatures. A direct acting mechanical bucket (DAMB) sliding valvetrain bench test apparatus was used to measure the friction and wear performances of fresh engine oils containing 0, 0.05 and 0.1 wt% phosphorus for a cam lobe rubbing against a tappet insert. The tester was coupled with a radioactive tracer machine (RTM). The results show that in the region of low phosphorus concentration, friction is inversely correlated to temperature. The friction coefficient slightly drops with increasing temperature and increases with increased phosphorus concentration at elevated temperatures. Significant wear is produced at phosphorus concentrations lower than 0.02 wt% at most temperatures. Friction and wear are reduced with the addition of supplemental antiwear additives. MoDTC reduces wear more effectively than ZnDTC in the presence of ZDDP.     

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.     

Sliding wear characteristics of plasma-sprayed Al2O3 and Cr2O3 coatings against copper alloy under severe conditions

Al₂O₃ and Cr₂O₃ coatings were deposited by atmospheric plasma spraying and their tribological properties dry sliding against copper alloy were evaluated using a block-on-ring configuration at room temperature. It was found that the wear resistance of Al₂O₃ coating was superior to that of the Cr₂O₃ coating under the conditions used in the present study. This mainly attributed to its better thermal conductivity of Al₂O₃ coating, which was considered to effectively facilitate the dissipation of tribological heat and alleviate the reduction of hardness due to the accumulated tribological heat. As for the Al₂O₃ coating, the wear mechanism was plastic deformation along with some micro-abrasion and fatigue-induced brittle fracture, while the failure of Cr₂O₃ coating was predominantly the crack propagation-induced detachment of transferred films and splats spallation.     

A study of the friction and wear performance of MoSx thin films produced by ion beam enhanced deposition and magnetron sputtering

MoS_x thin films were deposited by ion beam enhanced deposition (IBED) and magnetron sputtering (MS) onto the surface of IBEN Si₃N₄ and TiN thin films. The friction and wear performances of thin films and 52100 steel were compared using an SRV model reciprocating testing machine. The results showed that all MoS_x films exhibit good tribological behavior. The MS MoS_x thin film has better wear resistance and the IBED MoS_x film has a longer wear life. The wear resistance of IBED Si₃N₄ and TiN thin film plus MoS_x film is 3–4 times and 8–20 times that of single IBED Si₃N₄ and TiN thin films and 52100 steel respectively. The analyses indicate that the difference in friction and wear performance between the two kinds of MoS_x thin film is determined by the x value of MoS_x, its microstructure and the atom mixing effect at the interface.     

Friction and wear behaviour of Thordon XL and LgSn80 in sliding against plasma-sprayed Cr2O3 coatings

This paper studies the friction and wear behaviour of two important bearing materials, Thordon XL and LgSn80, in dry and lubricated sliding vs. plasma-sprayed Cr₂O₃ coatings. As a reference, AISI 1043 steel is also studied under the same conditions. SEM, EDS and surface topography were employed to study the wear mechanisms. The results indicate that the Thordon XL/Cr₂O₃ coating pair gives the lowest dry friction coefficient (0.16) under a normal load of 45.3 N (pressure 0.453 MPa) at a velocity of 1 m/s. The dry friction coefficient of Thordon XL/Cr₂O₃ coating increases to 0.38 under a normal load of 88.5 N (pressure 0.885 MPa). The dry friction coefficients of the LgSn80/Cr₂O₃ coating are in the range of 0.31–0.46. Secondly, both dry wear rate under low normal load (45.3 N) and lubricated wear rate under a load of 680 N for Thordon XL are lower than those of LgSn80 in sliding against plasma-sprayed Cr₂O₃ coatings at a speed of 1 m/s. However, under a normal load of 88.5 N the dry wear rate of Thordon XL is much higher than that of LgSn80. Thirdly, a high viscosity lubricant (SAE 140) leads to lower wear for Thordon XL and LgSn80 than a low viscosity lubricant (SAE 30). Finally, the dominating wear mechanism for Thordon XL is shear fracture when against the plasma-sprayed Cr₂O₃ ceramic coating. For LgSn80 against plasma-sprayed Cr₂O₃ ceramic coating, abrasive wear is the governing failure mechanism.     

Sliding wear behaviour of Ca α-sialon ceramics at 600 °C in air

As an extension of a previous investigation on the wear behaviour of Ca α-sialon ceramics of differing microstructures at room temperature, wear testing was conducted at 600 °C in air to explore the effects of microstructure, contact pressure and sliding speed on the wear behaviour. Under all loading conditions from 1 MPa to 1 GPa, a constant high friction coefficient was observed and a severe wear process was dominant, in which the sliding contact induced cracks were observed in different microstructures. Wear particles were generated along the wear track, but no tribofilm was detected. Increasing the sliding speed from 10 to 23 cm/s was found to significantly increase wear rate. However, variations in microstructure had little impact. That is, large elongated-grained α-sialon exhibited only a slightly lower wear rate than fine equiaxed-grained α-sialon.