Ultra-low Wear Rates Research Articles

A novel method for isolation and recovery of ceramic nanoparticles and metal wear debris from serum lubricants at ultra-low wear rates

Ceramics have been used to deliver significant improvements in the wear properties of orthopaedic bearing materials, which has made it challenging to isolate wear debris from simulator lubricants. Ceramics such as silicon nitride, as well as ceramic-like surface coatings on metal substrates have been explored as potential alternatives to conventional implant materials. Current isolation methods were designed for isolating conventional metal, UHMWPE and ceramic wear debris. In this paper, we describe a methodology for isolation and recovery of ceramic or ceramic-like coating particles and metal wear particles from serum lubricants under ultra-low and low wear performance. Enzymatic digestion was used to digest the serum proteins and sodium polytungstate was used as a novel density gradient medium to isolate particles from proteins and other contaminants by ultracentrifugation. This method demonstrated over 80% recovery of particles and did not alter the size or morphology of ceramic and metal particles during the isolation process. Statement of SignificanceImprovements in resistance to wear and mechanical damage of the articulating surfaces have a large influence on longevity and reliability of joint replacement devices. Modern ceramics have demonstrated ultra-low wear rates for hard-on-hard total hip replacements. Generation of very low concentrations of wear debris in simulator lubricants has made it challenging to isolate the particles for characterisation and further analysis. We have introduced a novel method to isolate ceramic and metal particles from serum-based lubricants using enzymatic digestion and novel sodium polytungstate gradients. This is the first study to demonstrate the recovery of ceramic and metal particles from serum lubricants at lowest detectable in vitro wear rates reported in literature.

Tribological Performance of Polyamide-Imide Seal Ring Under Seawater Lubrication

High-performance engineering polymers are potential candidate materials for mechanical seals, especially in corrosive environments and environments where substantial shock and vibration loads are present. This study addresses the friction and wear performances of polyamide-imide (PAI) sliding against tungsten carbide (WC) in seawater for seal applications. The experimental setup consists of two nominally flat rings rotating against each other to simulate the contact in mechanical seals. Three types of experiments were performed, i.e., measurement of friction and wear in a series of 8-h tests, measurement of the friction coefficient as a function of speed and load and measurement of PV (pressure times velocity) limit. Test results show that at lightly loaded conditions WC/PAI seal rings in seawater can reach hydrodynamic lubrication with ultra-low friction coefficient and wear rate.

The influence of the initial near-surface microstructure and imposed stress level on the running-in characteristics of lubricated steel contacts

The tribological behavior of polished and lapped 56NiMoCrV7 and Ck45 disks with lubrication was studied to link the impact of initial near-surface microstructure to the running-in behavior of the systems. The tests were performed using a pin-on-disk tribometer with radionuclide technique to resolve ultra-low wear rates. For the running-in different stressing regimes were applied. Whereas the lapped disks developed low friction and wear as response to a high-power running-in, the polished disks had to be stressed by a step-wise increase in load to achieve a similar result. In addition, the duration of some load steps and the sliding velocity had to be adjusted in order to obtain a proper running-in. With the help of focused ion beam and transmission electron microscopy the response of the microstructure to the stressing conditions was investigated. It turned out that the quality of the running-in crucially depends on a subtle equilibrium between material strengthening and softening. Strengthening by finishing and running-in was a prerequisite for the formation of the third body, whereas softening resulted in scuffing. When it was possible to charge the near-surface material in relation to its strengthening capabilities, low wear rates in the nanometer per hour regime were obtained.

“Ultralow” sliding wear polytetrafluoro ethylene nanocomposites with functionalized graphene

The dry friction and sliding wear behavior of sintered polytetrafluoro ethylene containing various amounts of functionalized graphene were studied in this work. Graphene was incorporated in 0, 0.25, 0.75, 1, 2 and 4 vol.%, respectively. Sliding wear tests were performed in ring(metal)-on-plate(polytetrafluoro ethylene) test rig under ambient temperature setting 1 m/s sliding speed and 1 MPa contact pressure. The dynamic coefficient of friction and specific wear rate (ws) data were determined. Very low coefficient of frictions (0.12–0.14) were measured for polytetrafluoro ethylene containing 2 or 4 vol.% graphene, which was attributed to the formation of a tribofilm on the countersurface. Specific wear rate went through a maximum (peaked at doubling that of the unmodified polytetrafluoro ethylene at about 0.75 vol.% graphene) as a function of graphene content. Ultralow wear rate data in the range of 10−6 mm3/(N.m) were measured for the polytetrafluoro ethylene nanocomposites with 2 and 4 vol.% graphene. This was reasoned by the formation of a robust tribofilm, the development of which was followed by scanning electron microscopy by inspecting the worn surface of polytetrafluoro ethylene nanocomposites and that of the steel ring of the ring(metal)-on-plate(polytetrafluoro ethylene) test rig. Fourier transform infrared spectroscopic results confirmed the formation of carboxyl groups in the tribofilm. They were supposed to react with the functional groups of graphene and to create complexes with the metal countersurface ensuring the tribofilm with high adhesion and cohesion strengths.

The running-in corridor of lubricated metal–metal contacts

In this paper the question is raised whether the coefficient of friction and the wear rate of a lubricated metal–metal system after passing the running-in can be deduced from the initial friction power density this tribological system was subjected to. This contribution defines a running-in corridor as specific energetic range in which the tribological system is able to develop ultra-low wear rates and small coefficients of friction. It will be shown that this corridor is associated with the formation of the third-body. The running-in corridor has a certain width which depends on external tribological stressing conditions, on materials, lubricants and mainly on the initial coefficient of friction. Using two different material pairings it will be demonstrated how tribological systems can be taught to find the route into the running-in corridor. Furthermore, levers of optimization employing friction-modifying additives or appropriate final machining routines will be discussed. The results of this contribution help to improve the understanding of ultra-low-wear systems. In addition comprehensive support for tribological optimization is given.

Transfer Film Tenacity: A Case Study Using Ultra-Low-Wear Alumina–PTFE

The transfer film, a protective barrier that forms when a solid lubricant slides against a hard and high-surface-energy counterface, plays an important role in friction and wear reduction. The transfer films of many solid lubricants are removed and replenished during sliding. However, one particularly low wear rate solid lubricant has been shown to produce persistent transfer films that thicken and homogenize over the course of a test. Based on a mass balance of the system, transfer film growth can only occur if its wear rate is less than that of the parent polymer. However, recent measurements of the wear rates of these transfer films show that they are likely orders of magnitude higher than those of the parent polymer. The goal of this paper is to elucidate the origins of this apparent contradiction. In this study, a high-density polyethylene (HDPE) probe was used in an effort to approximate the contact conditions under which the film originally formed. Transfer films formed during run-in of the parent solid lubricant were removed immediately by the HDPE pin (k ~ 100 mm3/Nm). However, transfer films formed after the solid lubricant had transitioned to ultra-low wear rates themselves exhibited ultra-low wear rates in the range from 10−8 to 10−10 mm3/Nm; to our knowledge, this is the first direct observation of ultra-low wear rate transfer films, a condition that was previously assumed necessary for ultra-low wear rates of the solid lubricant system. Follow-up measurements showed that the wear rate of the transfer film was extremely sensitive to the surface energy of the probe; the wear rate increased by orders of magnitude when the surface energy of the probe exceeded a critical value near ~35 mJ/mm2. These results provide fresh insights into the wear behaviors of transfer films and the processes governing ultra-low wear of solid lubricant materials.

Interrelated Effects of Temperature and Environment on Wear and Tribochemistry of an Ultralow Wear PTFE Composite

A particular alumina–PTFE nanocomposite has distinguished itself with unusually large wear reductions at trace filler loadings. Recent studies have shown that the formation of carboxylic acid end groups in humid environments is a critical part of the wear reduction mechanism. This finding has significant implications for the utility of the material for space and high temperature applications. In this paper, wear rate, morphology, composition, and chemistry of the wear surfaces were characterized as a function of environmental composition and surface temperature to better understand the environmental limitations of this solid lubricating system and the associated wear resistance mechanisms. The following results were found: (1) ultralow wear rates were retained with increasing interface temperature up to 100 °C, (2) all samples ran in to low wear regardless of the environmental conditions (dry conditions caused an eventual transition to high wear), (3) carboxylates were detected after sliding in dry enviro...

A near-wearless and extremely long lifetime amorphous carbon film under high vacuum.

Prolonging wear life of amorphous carbon films under vacuum was an enormous challenge. In this work, we firstly reported that amorphous carbon film as a lubricant layer containing hydrogen, oxygen, fluorine and silicon (a-C:H:O:F:Si) exhibited low friction (~0.1), ultra-low wear rate (9.0 × 10–13 mm3 N–1 mm–1) and ultra-long wear life (>2 × 106 cycles) under high vacuum. We systematically examined microstructure and composition of transfer film for understanding of the underlying frictional mechanism, which suggested that the extraordinarily excellent tribological properties were attributed to the thermodynamically and structurally stable FeF2 nanocrystallites corroborated using first-principles calculations, which were induced by the tribochemical reaction.

Influence of subsurface microstructure on the running-in of an AlSi alloy

The friction and wear behavior of a lubricated AlSi11Cu3 disk in contact with a 100Cr6 pin was studied by a radionuclide-assisted pin-on-disk tribometer. It is well known that shear forces change chemistry and microstructure of the near-surface material, thereby influencing friction and wear. To better understand the influences of the microstructure on the running-in behavior, disks with different silicon phase morphologies were tested under constant stressing conditions. Topography, chemistry and microstructure of pin and disk were characterized before and after tribological testing by white light interferometry, X-ray photoelectron spectroscopy, Auger electron spectroscopy and focused ion beam microscopy. Wear of pins was measured with a radionuclide technique to resolve ultra-low wear rates. To characterize near-surface deformations, Ga ion markers were implanted and Auger electron spectroscopy was applied to follow their shear-induced displacements. To monitor subsurface shear, the deformation of columnar markers was analyzed. The results were discussed using Godet׳s third body model. Furthermore, the results allowed to us extend the model to systems operated under ultra-low wear rates.

Ultra-High Tribological Performance of Magnetron Sputtered a-C:H Films in Sand-Dust Environment

The hydrogenated amorphous carbon (a-C:H) films were prepared on AISI 440C steel substrates using a RF magnetron sputtering graphite target in the CH4 and Ar mixture atmosphere. The friction and wear behavior of a-C:H films were comparatively investigated by pin-on-disc tester under dry sliding and simulated sand-dust wear conditions. In addition, the effects of applied load, amount of sand and sand particle sizes on the tribological performance of a-C:H films were systemically studied. Results show that a-C:H films exhibited ultra-high tribological performance with low friction coefficient and ultra-low wear rate under sand-dust environments. It is very interesting to observe that the friction coefficient of a-C:H film under sand-dust conditions was relatively lower when compared with dry sliding condition, and the wear rate under sand-dust conditions kept at the same order of magnitude (×10−19 m3/N m) with the increase of applied load and particle size as a comparison with the dry sliding condition. Based on the formation of “ridge” layer (composite transfer layer), a transfer layer-hardening composite model was established to explain the anti-wear mechanisms and friction-reducing capacity of a-C:H solid lubrication films under sand-dust conditions.

Message from the Scientific Organizers

This volume of Tribology Letters contains articles that were presented at the 428th Wilhelm and Else Heraeus Seminar ‘‘Physics of Tribology—Understanding Friction and Wear in technical systems’’. The seminar took place at the ‘‘Physikzentrum’’ in Bad Honnef, Germany from 23rd to 25th of March, 2009 with 60 participants from 14 countries. The purpose of the seminar was to present and discuss the physics of friction and wear in a multi-scale approach from the atomic level up to the size of an engine component. Within this topical setting, several contributions dealt with plastic deformations and third-body formation caused by sliding friction and by manufacturing (grinding). Insights into how nanocrystalline microstructures in bcc and fcc metals are generated as an effect of cyclic straining is steadily being gained. In addition, the importance of on-line wear measurement and ultra-low wear rates was discussed in several talks. Another important topic for macroscopic systems was the use of novel water based lubricants and of polyols. On the simulation side, several presentations dealt with elastic contact mechanics. A new theory now allows the exact calculation of the contact area even between millimeter-sized technical (e.g., aluminum–silicon alloys) surfaces. A novel reactive potential allows more realistic MD simulations of diamond-like carbon (DLC). On the atomic scale the experimental focus was on friction force microscopy. New experimental results showed a link between atomic friction measured on graphene and electron–phonon coupling for the first time. Another talk showed that the widely used Prandtl–Tomlinson model is over-simplifying the fast motion of the AFM tip apex. The poster session also allowed younger researchers to present their work. The poster prize was awarded to a contribution from Julich which dealt with experiments on rubber sealing and was an impressive example of intelligent research on a small budget. Overall the seminar showed how atomic-scale experiments and simulations are able to stimulate engineering tribology and vice versa. It therefore seems that future workshops of this kind may help to fertilize all disciplines of tribology. At this point we would again like to thank the Wilhelm and Else Heraeus foundation for funding, as well as their perfect organization of the seminar.

Imparting Ultra‐Low Friction and Wear Rate to Epoxy by the Incorporation of Microencapsulated Lubricant?

AbstractThe incorporation of microcapsules containing lubricant oil into epoxy composites leads to materials with ultra‐low friction and wear performance. During sliding wear tests, the capsules are damaged by the asperities of the counterface, releasing the oil to the contact area. The lubrication effect of the released oil and the entrapment of wear particles in the cavities left by the ruptured capsules leads to a significant reduction of both the frictional coefficient and the specific wear rate. The approach provides the merits of fluid lubrication but excludes the drawbacks of external lubrication. A minute amount of lubricant oil is sufficient to improve the tribological properties significantly.magnified image

A Novel Method for Quantitative Determination of Ultra-low Wear Rates of Materials, Part I: On Steels

A novel method for the quantitative determination of the wear of materials at very low wear rates is introduced here. The main concept involves implanting gold as a trace marker into the tested materials, and measuring the gold concentration and depth profile before and after the wear tests by Rutherford backscattering (RBS). The amount of gold loss is related to the material loss under the tested conditions. The background concentration of gold in AISI 1095 and 52100 steels has been determined by neutron activation analysis (NAA; ~3 and 33 wt. ppb, respectively). Several fluences (1 × 1015–1 × 1016 atoms/cm2) of gold were implanted into AISI 52100 steel samples in order to evaluate the changes in the mechanical and tribological properties induced by implantation. The new method shows the relative effectiveness for wear prevention of the different tested lubricants with and without antiwear additives. Even the thickness of the antiwear film and the amount of zinc (from zinc dialkyldithiophosphates used as antiwear additives) can be measured by this method.

A Novel Method for Quantitative Determination of Ultra-low Wear Rates of Materials, Part II: Effects of Surface Roughness and Roughness Orientation on Wear

A novel method of measurement of the very low wear-rates of materials in the ultra-mild wear regime, which involves the use of implanted gold as a marker, was used to understand the effects of surface roughness and roughness orientation on wear under reciprocating sliding conditions. AISI 1095 steel coupons with various Vickers hardness values and different surface roughness and roughness orientation relative to the sliding direction were tested under the same sliding conditions. It was found that parallel sliding causes more wear compared with transverse sliding for the harder samples (Vickers hardness (VH); 450 HV, 650 HV and 1000 HV). Furthermore, the average friction coefficient of parallel sliding is also higher than that of transverse sliding for these samples. Severe wear takes place when the samples are too soft (250 HV), resulting in the complete loss of implanted gold. Surface topographic images were taken before and after the wear tests. It was found that parallel sliding dramatically increases the surface roughness, while transverse sliding does not increase the surface roughness for harder samples (450 HV, 650 HV and 1000 HV). For the soft sample (250 HV), the surface roughness increases significantly under parallel or transverse sliding.

Microstructure of tribologically induced nanolayers produced at ultra-low wear rates

Wear in modern tribological systems is characterized by ultra-low rates in the order of a few nanometers per hour. Even under low wear conditions, morphology, crystalline arrangement, and chemical bonds of surface and subsurface are modified significantly and considerable plastic deformation occurs up to 1 μ m depth. We characterize these changes with the help of tribometer experiments employing the radionuclide technique and transmission electron microscopy (TEM). The tribological conditions lead to complex and strongly inhomogeneous layered structures. The top zone of the subsurface has a nano-crystalline structure with an average grain size of about 10 nm containing ferrite, austenite, and cementite phases. Below the subsurface material consists of a work-hardened, fine-grained structure. Our experiments lead us to conclude that the observed combination of a nano-crystalline top layer and a work-hardened subsurface favours high wear resistance and low friction.

The mechanical properties of freestanding near-frictionless carbon films relevant to MEMS

Amorphous, diamond-like carbon films with a mixture of sp2 and sp3 hybridizations have exhibited excellent material properties such as chemical stability, wear resistance and optical transparency resulting in their wide use as protective coatings in numerous applications. The hydrogenated forms of these films, a-C:H, and specifically the near-frictionless carbon (NFC) films developed at Argonne National Laboratory have exhibited the lowest ever recorded friction coefficient, 0.001, and ultra-low wear rates of 10−11−10−10 mm3 N−1 m−1, even under dry sliding conditions and at very high contact pressures (Robertson 2002 Diamond-like amorphous carbon Mater. Sci. Eng. R 37 129). Application of these films to sliding or rotating microelectromechanical systems (MEMS) would open up an entirely new class of commercialized MEMS devices. With this in mind, this paper reports on thin-film mechanical property measurements of the NFC films relevant to MEMS. The membrane deflection experiment was employed to subject microfabricated freestanding films to pure tension and measure mechanical properties such as Young's modulus, residual stress and fracture strength. Young's modulus was consistently measured at 35.13 ± 2.29 GPa. The fracture strength varied from 0.12 GPa to 0.90 GPa and the residual stress state was compressive and ranged from 79 MPa to 310 MPa. Width and thickness effects of the membranes were also observed in this work, where fracture strength increased with decreasing membrane width and thickness. Weibull analysis of the fracture strength is also presented in the paper.

A low friction and ultra low wear rate PEEK/PTFE composite

This paper presents a PEEK filled PTFE composite that exhibits low friction and ultra-low wear. It is hypothesized that a synergistic effect shuts down the dominant wear mechanism of each constituent. The friction coefficient and wear rate of this composite material on lapped stainless steel were evaluated for samples with PEEK wt.% of 0, 5, 10, 20, 30, 40, 50, 70 and 100 using a linear reciprocating tribometer. Tests were performed in filtered, standard laboratory conditions with a nominal contact pressure of 6.35 MPa, a speed of 50 mm/s and total sliding distances ranging from 0.5 km for the unfilled PTFE to 140 km for a 20 wt.% PEEK filled sample. The friction coefficients, averaged over an entire test, ranged from μ ¯ = 0.111 for a 50 wt.% composite to μ ¯ = 0.363 for unfilled PEEK. Wear rates ranged from K = 2.3 × 10 −9 mm 3/(Nm) for a 20 wt.% PEEK sample to K = 6 × 10 −4 mm 3/(Nm) for unfilled PTFE. Scanning electron microscopy revealed a unique interfacial connection between the PTFE and PEEK that is likely responsible for the ultra low wear rates observed in these experiments.

Characterization of wear debris of systems operated under low wear-rate conditions

Wear particles found in ultra-low wear rate experiments were analyzed in order to understand the wear mechanism. Using a sophisticated particle separation method, the debris were separated from the oil and then analyzed using transmission electron microscopy accompanied by electron diffraction for chemical characterization. The majority of the wear particles have a size of about 250 nm, only very few are larger than 1 μ m. The particles appear mostly amorphous and contain chemical elements of both interacting solids as well as the oil. In addition, almost all particles have a thickness of 30 nm or less. The proposed particle generation mechanism is a squeezing process of flowed material under pressure.

Mechanical properties of tribologically modified nanolayers

Wear in modern mechanical systems is characterised by ultra-low wear rates in the range of a few nanometres per hour. Both the surfaces of the materials involved and their volumes undergo modification as regards morphology, crystalline arrangement, and chemical composition. The depth of tribological interactions for given conditions is in the order of a micrometre or less. Synchrotron-based X-ray diffraction and nano-indentation were used to probe these thin modified zones. The specimens were taken from tribometric experiments employing the radionuclide technique as the ultimate tool to measure ultra-low wear rates. To obtain different degrees of tribological interaction various loads and sliding velocities were applied. The specimens were classified according to the total power dissipated during the tribometric experiment.

Structure and mechanical properties of tribologically induced nanolayers

Ultra low wear rates are common for lubricated state-of-the-art mechanical devices used in machines and cars. The main paths of energy dissipation are heat and wear generation as well as a significant change of near-surface material within the range of a few hundred nanometers. Due to mechanical intermixing at the asperity level the involved materials change with respect to chemical composition, morphology and mechanical properties. By means of focused ion beam analysis the morphology of the near-surface material of tribologically stressed and unstressed samples was investigated. In addition, nanoindentation was applied to characterize the mechanical properties. In the presented study mechanical intermixing leads to nanocrystalline material which is softer than bulk material. Depending on the level of tribological stressing, the modified material exhibits smaller energy dissipation, thus lower friction and wear.