Friction and wear behavior of flame-sprayed PEEK coatings

Polyetheretherketone (PEEK) becomes of great interest to applications as bearing and slider materials. In this paper, PEEK coatings with three kinds of crystallinities were deposited using the flame spray process. Employing a uniform design experiment, the friction and wear behavior of the three PEEK coatings were systematically investigated under dry sliding conditions against a 100C6 counterbody on a ball-on-disc arrangement for several loads and sliding velocities. For the three coatings, the friction coefficient significantly followed the normal distribution. The average friction coefficients appeared to decrease while increasing the sliding velocity, but were insensitive to the applied load in the range of investigation. Among the three coatings, the higher the crystallinity of the coating, the lower its average friction coefficient was. The wear rate of the coating with the lowest crystallinity decreased with an increase in the load and a decrease in the sliding velocity. The wear rate of the coating with the intermediate crystallinity decreased with an increase in the load, but increased with an increase in the sliding velocity at lower loads, and then decreased with an increase in the velocity at higher loads. The wear rate of the coating with the highest crystallinity decreased with the increase of both the load and the sliding velocity. The wear mechanisms of the different coatings are explained in terms of plastic deformation, plogh marks and fatigue tearing.     

Friction of lubricated soft surface layers

Elastohydrodynamic lubrication of solids covered by a thin layer of material of low elastic modulus is studied theoretically and experimentally. The pressure and film thickness lie between the limits set by rigid and bulk elastic solids. The sliding friction shows a marked difference from that of rigid solids.     

Friction in starved hydrodynamically lubricated line contacts

The hydrodynamic friction coefficient, which determines the friction coefficient at high Sommerfeld numbers (viscosity⁎velocity/load) has a unique solution for the fully flooded case. As such the friction coefficient can be predicted as a function of the above number. As shown by Cameron (1976) [1] the friction coefficient in the fully flooded regime increases as the square root of the Sommerfeld number. For very low Sommerfeld numbers, the asperity interaction causes the friction coefficient to increase when entering the mixed lubrication regime.Even though a unique (and low) friction coefficient exists in the fully flooded regime, the situation is more complex in the starved regime. First of all the friction coefficient is higher in the starved regime, and secondly the coefficient depends on the degree of starvation.This paper analyses the load carrying capacity, the Poiseuille flow based friction and the Couette flow based friction, as a function of the degree of starvation. It is shown that the Poiseuille friction force diminishes fastest with starvation, followed by the load carrying capacity, and finally the Couette term diminishes slowest of all three terms.As a consequence the friction coefficient for sliding starved conditions is dominated by the Couette term.The current paper analyses the friction coefficient evolution as a function of starvation for line contact conditions, using analytical and numerical tools. Finally, curve fitted equations are given for the friction as a function of the starvation level.     

Friction and wear behavior of electroless Ni-based CNT composite coatings

Ni-based carbon nanotube (CNT) composite coatings with different volume fraction (from 5 to 12 vol.%) of CNTs were deposited on medium carbon steel substrates by electroless plating. The friction and wear behavior of the composite coatings were investigated using a pin-on-disk wear tester under unlubricated condition. Friction and wear tests were conducted at a sliding speed of 0.0623 m s⁻¹ and at an applied load of 20 N. The experimental results indicated that the friction coefficient of the composite coatings decreased with increasing the volume fraction of CNTs due to self-lubrication and unique topological structure of CNTs. Within the range of volume fraction of CNTs from 0 to 11.2%, the wear rate of the composite coatings showed a steadily decreasing trend with increasing volume fraction of CNTs. Because of the conglomeration of CNTs in the matrix, however, the wear rate of the composite coatings increased with further increasing the volume fraction of CNTs.     

Friction torque in grease lubricated thrust ball bearings

Thrust ball bearings lubricated with several different greases were tested on a modified Four-Ball Machine, where the Four-Ball arrangement was replaced by a bearing assembly. The friction torque and operating temperatures in a thrust ball bearing were measured during the tests. At the end of each test a grease sample was analyzed through ferrographic techniques in order to quantify and evaluate bearing wear.A rolling bearing friction torque model was used and the coefficient of friction in full film lubrication was determined for each grease, depending on the operating conditions.The experimental results obtained showed that grease formulation had a very significant influence on friction torque and operating temperature. The friction torque depends on the viscosity of the grease base oil, on its nature (mineral, ester, PAO, etc.), on the coefficient of friction in full film conditions, but also on the interaction between grease thickener and base oil, which affected contact replenishment and contact starvation, and thus influenced the friction torque.     

Tribological behaviour of polymeric coatings lubricated with salt water

In many applications, such as marine industries, pumping of underground water, and in some chemical industries, sliding elements are lubricated with the process fluid (e.g. salt water). Therefore, the effect of salt water on the tribological behaviour of metallic friction couples has been a subject of study. In the present work, the influence of salt water solution (3 wt. % sodium chloride) on the tribological behaviour of some polymeric coatings is investigated, using a pin-on-disc testing machine, at different sliding speeds and normal loads. The coating consists of a base-matrix of high-density polyethylene (HDPE) and polyamide (PA6) with different concentrations of copper- and tin-powder fillers. The effect of the formation of the oxide films, sodium hydroxide microlayers (NaOH) and the polymeric transformed films on the friction mechanism, is discussed. The results show that squeeze of the lubricant takes place in the presence of the NaCl-solution; and, despite the relatively low viscosity of the lubricant, hydrodynamic pressure affected the friction mechanism at the sliding speed of 0.50 m/s, resulting in a significant reduction in the coefficient of friction. The value of the coefficient of friction seems also to be affected by the generation of sodium hydroxide, as well as the oxide films and material transfer layers. At a low speed (0.20 m/s), the PA6-based composites with copper-filler showed an optimal wear resistance at normal loads up to 15N.     

Friction and wear studies of an internally lubricated polyetherimide composite

Polyetherimide (PEI) is one of the latest generic high-performance engineering thermoplastics. PEI (developed by General Electric (USA) under the trade name ULTEM) is an amber and amorphous polymer with a heat distortion temperature between those of polyarylate resin and thermally stable crystalline polymers such as polyether-ether ketone (PEEK) and polyamideimide (PAI). It has excellent thermal, mechanical and electrical properties along with easy processability. In the work reported here, a wear-resistant formulated composite supplied by GEC (ULTEM 4001) was selected for tribological investigations on a pin on disc machine under unlubricated conditions, against mild steel. Analysis of the composite revealed that this grade contained PTFE (13–15%), which is the most promising polymeric lubricant. A very low and stable frictional coefficient was observed against moderately finished surfaces. However, its specific wear resistance (⋍10⁻¹⁴ m³/Nm) was comparatively lower than that of fibre-reinforced thermoplastics. The wear mechanism was found to be significantly dominated by the presence of PTFE. The friction coefficient was in the range of 0.2 and reduced to a still lower value (0.1) as the apparent contactpressure increased. Scanning electron microscopy was used to investigate the underlying wear mechanism. Film transfer of PTFE was observed to be the principal factor responsible for reduced friction.     

Diesel‐lubricated ta‐C coatings

Samples coated with tetrahedral amorphous carbon were investigated in two different oscillating test setups regarding their friction and wear properties. As lubricants petrodiesel, rapeseed methyl ester biodiesel and a mixture thereof were used. Compared with a petrodiesel‐lubricated uncoated system, friction was reduced for more than 50% when a tetrahedral amorphous carbon coating was applied and biodiesel added to the lubricant. Impurities of monoglycerides are assumed to be the cause for the friction‐modifying effect. Copyright © 2013 John Wiley & Sons, Ltd.     

Microstructural aspects of lubricated mild wear with zinc dialkyldithiophosphate

The microstructure of wear particles formed under boundary lubrication with a zinc dialkyldithiophosphate (ZDDP) is investigated in the analytical scanning transmission electron microscope. This is made possible by the generation of wear particles in foil form. Physical and chemical data are reported and correlated with surface analysis data obtained on reaction films. The measurements show that the thin foil particles consist mainly of an amorphous material containing iron, phosphorus, sulphur, zinc and oxygen (carbon was not detected). These particles are the result of a reaction of the ZDDP with metals under friction.     

Influence of viscoelastic coatings on the contact of lubricated bodies

The contact of elastic bodies with viscoelastic coatings is considered, in the presence of lubrication. The one-dimensional Kelvin model is employed for the viscoelastic coatings. The pressure distribution in the lubricant layer and the layer thickness are investigated on transition from low to large loads.     

Friction characteristics of a ferritic-austenitic stainless steel during lubricated reciprocating motion

This report describes friction measurements of stainless steel against stainless steel during lubricated, small-amplitude reciprocating motion. The experimental investigation was divided into two parts. First, four different lubricants were evaluated using a response surface design, during which the average contact pressure and the sliding velocity were varied. Secondly, a 2⁴ factorial design with three replicate runs was performed. Here, the coefficient of friction in the initial stage and the duration of that stage were studied. The independent variables were the average contact pressure, sliding velocity, surface roughness and type of lubricant. In the early state (stage I), the value of the frictional force is controlled by plowing of the surfaces by asperities. In many lubricated contacts, this is the practically useful stage. The experimental results from the response surface design show that the duration of stage I depends on the type of lubricant. Adhesive wear can take place before 100 cycles. The factorial design indicates that the coefficient of friction in the initial stage is affected by the type of lubricant, surface roughness and the simultaneous change of the surface roughness and type of lubricant. The duration of the initial stage is affected by a change in the surface roughness, average contact pressure and a simultaneous change in average contact pressure and surface roughness. A two-parameter Weibull analysis was performed on the data from the factorial design. For the tests where lubricant no. 3 was used, a mixed distribution was indicated for the duration of stage I. This mixed distribution indicates that a weakest-link process as well as a healing process were involved.     

Friction of brushlike self-assembled monomolecular coatings

Friction modes of self-assembled monomolecular coatings (SAM) based on dodecyclophosphoric acid ester and its derivatives on a silicon substrate are investigated. The antifriction properties of the films are shown to be due to their ability to decrease the interfacial energy of friction contact. Models are suggested for friction force computation for single and multiple molecularly smooth contacts. They are based on consideration of the thermodynamic cycle of formation and break-off of the friction junction. On the example of composite coatings having different surface energy, a possible means of regulation of the tribological parameters of the silicon-SAM-silicon system is discussed.     

Friction models for sliding dry, boundary and mixed lubricated contacts

Friction, lubrication, and wear have a strong influence on the performance and behavior of mechanical systems. This paper deals with different friction models for sliding contacts running under different conditions. The models presented are suited to different situations, depending on the type of contact, running conditions, and the behavior of interest. The models will be discussed from simulation and tribological points of view. The different types of friction models considered are:

• friction models for transient sliding under dry, boundary and mixed lubrication conditions,

• friction models for micro-displacements of engineering surfaces subjected to transient sliding,

• friction models often used in the simulation and control of technical systems,

• combined friction models that represent physical behaviors fairly well but are also suitable for use in simulating systems,

• friction models that take into account the stochastic nature of interacting surface asperities.

Friction torque of thrust ball bearings lubricated with wind turbine gear oils

Planetary gearboxes used in wind turbines very often have premature bearing and gear failures, some of them related to the lubricants used. Five fully formulated wind turbine gear oils with the same viscosity grade and different formulations were selected and their physical characterization was performed. The lubricant tribological behaviour in a thrust ball bearing was analyzed. A modified Four-Ball Machine was used to assemble the bearings. They were submitted to an axial load and the tests were performed at velocities ranging between 150 and 1500 rpm. Experimental results for the operating temperatures and for the internal friction torque are presented.     

Friction of ultrathin Si,F-containing coatings

The friction of an ultrathin coating (~5 nm) formed as a result of the modifier segregation towards the surface of modified epoxy polymer in the form of multiple short chains of Si, F modifier has been investigated in a range of loads from 0.1000 to 0.0005 N. During the investigation of the reverse friction of ceramic (Al₂O₃) ball on the coating, the sharp decrease in coefficient of friction at a load of less than 0.002 N has been shown, which is probably caused by the change in the mechanism of friction during the transition of friction to the zone of a self-lubricating coating. Under these conditions, the difference between the static coefficient of friction (stop effect) and dynamic coefficient of friction increases.     

Friction and wear in silicon nitride-steel and cemented carbide-steel pairs in lubricated sliding

Friction and wear tests between a stationary block and a rotating ring under high contact pressure of about 200 MPa were carried out at room temperature under lubrication with a light mineral oil at a sliding distance of 500 m. The block was silicon nitride and cemented carbide, and the ring was bearing steel. The effect of phosphorus and sulphur contained in the mineral oil on the friction, the roughness of the worn surface and the wear of the steel ring is discussed in relation to both pairs. Sulphur was effective in reducing the coefficient of friction of the cemented carbide block-steel ring pair, while phosphorus was successful in decreasing the wear of the steel ring paired with the silicon nitride block. The surface analysis of the steel ring using X-ray photoelectron spectroscopy (XPS) shows that the peak intensities of sulphur or phosphorus beneath the surface depend upon the material of the counterpart, silicon nitride or cemented carbide blocks.     

Tribological behavior of reciprocating friction drive system under lubricated contact

The dynamic friction and wear behaviors are investigated in reciprocating friction drive system using a 0.45% carbon steel pair. The effects of various operating parameters on the traction force, stick and slip time, and friction modes are examined under the lubricated contacts. Moreover, the critical operating conditions in classifying three friction modes are also established. Results show that the fluid friction induced by the shearing of lubricant dominates the variation of traction force and produces the positive slope γ at the first period of slip in the traction force–relative sliding velocity curve. The γ value decreases at higher driver speed during stick-slip motion due to the thicker fluid film and shear thinning effect. The γ value increases due to the asperity interactions as the friction region is transferred from stick-slip to sticking with normal load from 196 to 980 N. Furthermore, it is also found that the static friction force is independent of stick time for the tangential loading rate ranged from 1.12 to 16.8 s⁻¹. The transition region produces the severest wear under the different driver speeds, but the wear is insensitive to the friction regions and the severe wear only occurs at higher normal load due to the action of Hertzian contact.     

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.