Ultra-low Wear Research Articles

The Running-in of Lubricated Metal-Metal Contacts—A Review on Ultra-Low Wear Systems

The running-in of lubricated metal–metal contacts leading to ultra-low wear is inseparably connected with the formation of the third body and vice versa. Adequate tribological stressing provides the system with a power density that leads to complex changes of topography, near-surface morphology and chemical composition. During the running-in these changes proceed until the system shows small friction and ultra-low wear rates and performs stable with low sensitivity to external perturbations. By means of high-resolution wear measurement as well as physical and chemical analysis the capability of a tribological system to develop the third body can be determined. Moreover, the running-in can be controlled by sample finishing, oil additivation and the sequence of initial stressing steps. This contribution summarizes 20 years of own research on ultra-low wear systems and its applications.

Tribological Behavior of PTFE Composites Filled with PEEK and Nano-Al2O3

ABSTRACT In this study, the tribological properties of polytetrafluoroethylene (PTFE) composites filled with polyetheretherketone (PEEK) and nano-Al2O3 particles were studied using a block-on-ring wear tester. The tribological performance of the composites was affected by the experimental parameters (sliding speed, normal load, and environmental temperature) and the composites achieved a high-speed sliding friction state. The results showed that the PEEK and nano-Al2O3 particles significantly improved the wear resistance of the PTFE composites. In addition, the nano-Al2O3 particles increased the hardness of the composites and enhanced the mechanical properties to enable applications in a wider range of industrial fields. The effects of the sliding speed and normal load on the tribological properties were more significant than that of the environmental temperature. In addition, the entire wear process was divided into three stages (the initial wear stage, severe wear transition stage, and ultralow stable wear stage), according to the evolution of the tribological characteristics (wear rate, morphology of the worn surface and transfer film, and wear debris morphology).

Hydrogen ion induced ultralow wear of PEEK under extreme load

As a high-performance engineering polymer, poly(ether ether ketone) (PEEK) is a perfect candidate material for applications under extreme working conditions. However, its high wear rate greatly shortens its service life. In this study, ultralow friction and wear between PEEK and silicon nitride (Si3N4) under extreme-load conditions (with a mean contact pressure above 100 MPa) are found in acid lubricating solutions. Both friction and wear decrease sharply with decreasing pH. At pH = 1, the friction coefficient decreases by an order of magnitude and the wear rate of the PEEK decreases by two orders of magnitude compared to the results with water lubrication. These reductions in friction and wear occur for different speed, load, and surface roughness conditions. The underlying mechanism can be attributed to the formation of hydrogen-ion-induced electrical double layers on the surfaces of PEEK and Si3N4. The combined effect of the resulting repulsive force, electro-viscosity, and low shear strength of the water layer dramatically reduces both friction and wear.

Composite Material Based on Polytetrafluoroethylene and Al–Cu–Fe Quasi-Crystal Filler with Ultralow Wear: Morphology, Tribological, and Mechanical Properties

Samples of composites with polytetrafluoroethylene as the matrix and a powder of 0, 1, 2, 4, 8, 16, and 32 vol % Al–Cu–Fe quasi-crystal as the filler are prepared. Electron microscopy studies of the sample structure are carried out, the influence of the filler on the degree of crystallinity and the melting and destruction temperatures of the samples is investigated; mechanical tensile tests and tribological tests are performed. The composite samples with filler contents of 4, 8, 16, and 32 vol % show ultralow wear with the coefficient K < 5 × 10–7 mm3/N m. The highest wear resistance exceeding that of unfilled polytetrafluoroethylene by 2200–3100 times is recorded in composites with 16 vol % filler. An increase in the wear resistance is associated with formation on the friction surface of a thin crust containing quasi-crystal particles 0.2–0.3 μm in size, revealed by scanning electron microscopy in combination with energy dispersive analysis.

The Competing Effects of Counterface Peaks and Valleys on the Wear and Transfer of Ultra-Low Wear Alumina–PTFE

The wear rates of tribological polymers are strongly influenced by counterface roughness through competing mechanisms such as polymer abrasion by counterface asperities and mechanical engagement of debris into the features of the rough surface. Previous studies have shown that an ultra-low wear (k ~ 10−7 mm3/Nm) alumina–PTFE solid lubricant loses wear resistance and the ability to form stable transfer films when the counterface roughness exceeds a critical magnitude or aligns with the sliding direction. In this paper, we aimed to test the independent effects of counterface peaks and valleys on polymer wear and transfer using this well-studied alumina–PTFE system. Wear tests were performed against 304 stainless steel counterfaces of systematically varied peak height and valley depth. Interrupted microscopy measurements were used to record the details of debris engagement, migration, aggregation, and removal. Preferential removal of the tallest peaks on the counterface helped stabilize the transfer films and dramatically reduced the transient wear volume of the polymer composite even on very rough surfaces. The results illustrate the independent and competing effects of counterface peaks, plateaus, and valleys on the wear and transfer of this ultra-low wear polymer composite. While increased peak height promoted primary material removal from the polymer composite and inhibited the formation of the transfer film, intersecting valleys and smooth plateaus helped nucleate and stabilize transfer films. On rough surfaces with tall peaks, the composite eventually achieved ultra-low wear by gradually removing the tallest asperities to achieve a more favorable topography for transfer film formation.

Phase‐Transited Lysozyme as a Universal Route to Bioactive Hydroxyapatite Crystalline Film

AbstractA key factor for successful design of bioactive complex, organic–inorganic hybrid biomaterials is the facilitation and control of adhesion at interfaces, as many current synthetic biomaterials are inert, lacking interfacial bioactivity. In this regard, the development of a simple, unified way to biofunctionalize diverse organic and inorganic materials toward biomineralization remains a critical challenge. In this report, a universal biomimetic mineralization route that can be applied to virtually any type and morphology of scaffold materials is provided to induce nucleation and growth of hydroxyapatite (HAp) crystals based on phase‐transited lysozyme (PTL) coating. Surface‐anchored abundant functional groups in the PTL enrich the interface with strongly bonded calcium ions, facilitating the formation of HAp crystals in simulated body fluid with the morphology and alignment being similar to that observed in natural HAp in mineralized tissues. By the adhesion of amyloid contained in the PTL, such protein assembly could readily integrate HAp on ceramics, metals, semiconductors, and synthetic polymers irrespective of their size and morphology, with robust bonding stability and corresponding ultralow wear extent under normal bone pressure. This strategy successfully improves the in vivo osteoconductivity of Ti‐based implant, underpinning the expectation for such biomaterial in future biointerface and tissue engineering.

Moisture dependent wear mechanisms of gallium nitride

Abstract Ultralow wear nature of gallium nitride (GaN) has been revealed recently. The wear rate for GaN has a significant dependence on humidity, ranging from 9 × 10−9 mm3/Nm to 9.5 × 10−7 mm3/Nm; the mechanisms responsible for this variation in wear remain unclear. Here, we performed reciprocal sliding test on GaN under different environments and characterized the chemical compositions of corresponding worn surface by energy dispersive X-ray spectroscopy (EDS). We show that the surface chemistry of GaN responded differently to various testing environments; this gave rise to the wear rate of GaN spanning over orders of magnitude. Additionally, the EDS conducted on the countersample (alumina probe) clearly evidenced a material transfer from GaN to the ruby countersample. Atomic force microscopy (AFM) and secondary electron microscopy (SEM) inside the wear scar indicated a grooving abrasion wear mechanism of GaN tested under humid environment and adhesive wear mechanism for dry nitrogen testing environment; this is confirmed by SEM/EDS of the ruby probes. In addition, transmission electron microscopy (TEM) was employed to image the defects formed underneath the worn surface, due to the tribological sliding, and the results suggested a possible evolution of wear debris formation.

Effect of carbon concentration and argon flow rate on the microstructure and triboperformance of magnetron sputtered WS2/a-C coatings

WS2/a-C coatings with various carbon contents (0–65at.%) were deposited on silicon wafers by magnetron co-sputtering under different Ar flow rates. Increasing the argon flow rate increases the chemical stoichiometric S/W ratio, but the coating gradually becomes porous and columnar-like. The S/W stoichiometry is less influenced by the variation in carbon concentration. TEM and XRD confirm well crystallized basal (002) planes in the sputter deposited pure WS2 or low-carbon WS2/a-C coatings. The hardness of the composite coatings increases with increasing carbon content or decreasing Ar flow rate. The coating at 40at.% C exhibits the highest hardness (10.6GPa). Tribotests show that, together with a ultralow wear rate of 10−7mm3m−1N−1, the coefficient of friction can be as low as 0.02 in dry air (5% RH) and around 0.15 in moisture (55% RH) and remains stable within a sliding distance of 1000m. The wear resistance is strongly affected by the carbon concentration, deposition pressure and the testing atmospheres.

Functional Multi-Nanolayer Coatings of Amorphous Carbon/Tungsten Carbide with Exceptional Mechanical Durability and Corrosion Resistance

A novel functional multilayer coating with periodically stacked nanolayers of amorphous carbon (a:C)/tungsten carbide (WC) and an adhesion layer of chromium (Cr) was deposited on 304 stainless steel using a dual magnetron sputtering technique. Through process optimization, highly densified coatings with high elasticity and shear modulus, excellent wear resistance, and minimal susceptibility to corrosive and caustic media could be acquired. The structural and mechanical properties of the optimized coatings were studied in detail using a variety of analytical techniques. Furthermore, finite element method simulations indicated that the stress generated due to contact against a steel ball was distributed well within the coating, which allowed the stresses to be lower than the yield threshold of the coating. Thus, an ultralow wear rate of ∼10-12mm3/N mm could be achieved in dry sliding conditions under relatively high Hertzian contact pressures of ∼0.4-0.9 GPa. The amorphous and pinhole-free structure of the individual layers, sufficient number of pairs, and the relatively dense stacked layers resulted in significant polarization resistance (Z″ = 5.5 × 106 Ω cm2) and increased the corrosion resistance of the coating by 10-fold compared to that of recently reported corrosion-resistant coatings.

Ultralow Friction and Wear of Polymer Composites under Extreme Unlubricated Sliding Conditions

Dependence of the tribological behaviors of polyimide‐ and polyetheretherketone‐based composites on pv (pressure × speed) factors is investigated in air ambience. It is demonstrated that the hybrid composites filled with nanosilica/carbon fibers/graphite exhibit ultralow friction and wear under extreme conditions. In particular, the friction coefficients of the hybrid nanocomposites at 40 MPa m s−1 are in the range of 0.03–0.04, which are even lower than those obtained with poly alpha olefin lubrication. Moreover, the friction coefficients are lower than those of carbon fibers reinforced polymer composites ever reported in literatures. In order to reveal the underlying mechanisms of ultralow friction and wear, tribochemistry and tribofilms' nanostructures are comprehensively analyzed. It is identified that chelation of polymeric molecular radicals with steel counterface occurs enhancing tribofilm's bonding strength. Striking orientation of the molecules of remnant polymer in tribofilm is indicative that the film exhibits an easy‐to‐shear characteristic under extreme pv conditions. Nanosilica released onto sliding interface and iron oxide particles abraded from the counterface are thereafter tribosintered into a compact layer, which accounts for the high load‐carrying capability of the tribofilm.

Nanocrystalline Graphite Formed at Fullerene‐Like Carbon Film Frictional Interface

High performance solid lubricant materials with long wear life remain as one of the challenges in space technology. It is of great significance to lower the solid film's friction and wear and to prolong its service life in vacuum. Hydrogenated fullerene‐like carbon film produced by dc‐pulsed power source behaves ultralong wear life (≫1.8 × 105 cycles) and ultralow wear rate (2.2 × 10−8 mm3 Nm−1) as well as low friction (≈0.14) in high vacuum (2.3 × 10−4 Pa). To understand its long wear life reason, the interfacial structural evolution is investigated after steady state friction is achieved. By observing the interfacial nanostructure evolution, it is found that the friction and wear properties are related to the friction‐induced phase transformation from fullerene like structure to nanocrystalline graphite and the deposition of tribofilm on Al2O3 counterface. And this process is analyzed by X‐ray photoelectron spectroscopy, micro‐Raman spectra and is observed through transmission electron microscopy. The results enrich hydrogenated fullerene‐like carbon film friction mechanisms and expand its potential application in space.

Interpretation of friction and wear in DLC film: role of surface chemistry and test environment

In spite of the large amount of tribological work carried out to explain the friction and wear mechanism in diamond-like carbon (DLC) films, some of the core issues relating to the evolution of reactive species across sliding interfaces and their role on the friction and wear mechanism remain unclear. The phase composition, film density and hydrogen content present in a DLC film can be tailored by substrate biasing during film deposition to achieve a nearly vanishing friction coefficient. Furthermore, nitrogen doping in DLC films significantly improves wear resistance, and sliding occurs in a nearly wearless regime. Undoped and nitrogen-doped DLC films exhibit a nearly frictionless value with ultra-low wear behavior when tests are performed in argon, nitrogen and methane atmospheres. The antifriction and antiwear properties of the DLC films were improved with the reduction of adsorbed oxygen impurities on the film surface. This behavior was understood by correlating the oxygen impurities present at the surface/subsurface region of the DLC film while using x-ray photoelectron spectroscopy and depth-resolved Auger electron spectroscopy.

Ultralow wear of gallium nitride

Here, we reveal a remarkable (and surprising) physical property of GaN: it is extremely wear resistant. In fact, we measured the wear rate of GaN is approaching wear rates reported for diamond. Not only does GaN have an ultralow wear rate but also there are quite a few experimental factors that control the magnitude of its wear rate, further contributing to the rich and complex physics of wear of GaN. Here, we discovered several primary controlling factors that will affect the wear rate of III-Nitride materials: crystallographic orientation, sliding environment, and coating composition (GaN, InN and InGaN). Sliding in the ⟨12¯10⟩ is significantly lower wear than ⟨11¯00⟩. Wear increases by 2 orders of magnitude with increasing humidity (from ∼0% to 50% RH). III-Nitride coatings are promising as multifunctional material systems for device design and sliding wear applications.

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.

Ultralow wear Perfluoroalkoxy (PFA) and alumina composites

Fluoropolymers have unique mechanical, chemical, and tribological properties (low friction coefficients) but their use as solid lubricants is inhibited by high wear rates (1–5×10−4mm3/Nm). The addition of certain types of α-alumina has been shown to reduce the wear rate of PTFE by over three orders of magnitude, but due to its extremely high molecular weight PTFE cannot be screw injection molded. However, PFA, a perfluorinated copolymer of tetrafluoroethylene (TFE) and a perfluorinated alkylvinyl ether (PAVE), can be. Teflon® PFA 340 samples with various weight fractions of α-alumina (0%, 5%, 7.5%, 10%) were injection molded, and samples from each mold were wear tested against stainless steel (P=6.3MPa, v=50.8mm/s). Experiments showed that the friction behavior of the PFA 340-α alumina composite was very close to that of both unfilled PFA 340 and PTFE-α alumina composites. The wear rate of unfilled PFA 340 was 1.4×10−4mm3/Nm, and dropped to 4.0×10−8mm3/Nm for the PFA-α alumina composites. Just as in the case of PTFE-α alumina composites, these PFA composites generated brown-colored tribofilms on both the polymer and metal surfaces, which were indicative of tribochemical changes. ATR-IR and FTIR spectra of each surface showed evidence for the generation of perfluorinated carboxylate salts and waters of hydration. This spectral similarity between PTFE and PFA 340 samples shows that the same tribological mechanism found in PTFE-α alumina composites is responsible for ultralow wear in PFA-α alumina composites as well.

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.

Influence of subsurface plastic deformation on the running-in behavior of a hypoeutectic AlSi alloy

The present publication elaborates on the influence of the initial microstructure preset by machining on the friction and wear behavior of a hypoeutectic AlSi9Cu3 alloy. During running-in the subsurface microstructure of tribological contacts is subjected to shear followed by plastic deformation. Aluminum disks were tested with a steel pin on a pin-on-disk tribometer with focus on running-in and sensitivity phenomena. The systems were operated in the ultra-low wear regime. Wear was measured continuously using Radionuclide Technique. The microstructure of the samples was characterized by nanoindentation, x-ray photoelectron spectroscopy and focused ion beam microscopy. The visualization of subsurface shear by markers in the disks showed a significant correlation of subsurface deformation and wear. With the experiments the relationship between machining and tribological behavior was analyzed and ways to optimize tribological systems were shown.

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.