Tribological characteristics of greases with and without metallo-organic friction-modifiers

ABSTRACT

An important focus of grease development is to minimize friction and wear while improving load bearing capacity. ASTM D2266 test method is commonly used to evaluate performance of grease at 75°C, 40?kg and 1200?rpm for 1 hour. However, actual applications may require bearings to be subjected to cyclic loading and variable frequency conditions wherein rotations per minute (rpm), load and duration of test are variables. Five different blends of greases were formulated using ZDDP (3?wt.%), PTFE (2?wt.%), MoDTC (2?wt.%), combination of ZDDP/PTFE in a weight ratio of 3:2 and a combination of ZDDP/PTFE/MoDTC in 3:2:2 weight ratios. They were tested under ASTM D2266 test method as well as under cyclic loading and variable frequency conditions where loads, frequency and duration of the tests were treated as variables. It was found that the combination of ZDDP/PTFE/MoDTC results in significant improvement in the wear and friction under cyclic loading as well as ASTM D2266 test conditions. It was also demonstrated that MoDTC accelerated the tribochemical degradation of ZDDP that resulted in the formation of a protective tribofilm layer on the interacting surfaces. The analysis of the tribofilm formed indicated that when MoDTC was used together with ZDDP and PTFE, a combination of MoS₂, phosphates and sulfates of Zn and Fe are formed whereas when only ZDDP and PTFE was used the tribofilms were largely composed of phosphates and sulfates of Zn and Fe.     

Environmental dependence of ultra-low wear behavior of polytetrafluoroethylene (PTFE) and alumina composites suggests tribochemical mechanisms

Composites of polytetrafluoroethylene (PTFE) and alpha phase alumina produce wear rates that can be nearly five orders of magnitude less than the wear rates of virgin PTFE. The mechanism for this reduction in wear cannot be explained solely by mechanical effects. The influences of oxygen and humidity on the tribological performance of both unfilled PTFE and PTFE/alumina composites were studied. The wear rate of PTFE/alumina composites is dependent on the humidity of the environment; this dependence suggests a tribochemical mechanism is responsible for the ultra-low wear behavior of these PTFE/alumina composites.     

Effect of Activated Carbon and Various Other Nanoparticle Fillers on PTFE Wear

It has long been known that a breadth of materials in microscale filler form reduce wear of polytetrafluoroethylene nominally by a couple orders of magnitude through prevention of large plate-like debris delamination. Though hypothesized that this wear reduction mechanism should halt as particle size is reduced to the nanoscale, alumina fillers not only maintained their wear reducing capability at the microscale but improved when employed as nanoparticles. In a survey of other nanofiller materials it is found that improved performance at the nanoscale is special not only to this alumina but also to its alpha phase, as most other materials and phases of alumina at best maintained microcomposite levels of wear resistance, more often losing some and in several cases fully returning to prohibitively high wear rates of unfilled polytetrafluoroethylene (PTFE). However, activated carbon emerged as exceptional, providing levels of wear resistance as a nanofiller well beyond that of typical microcomposites and even alpha-alumina itself. Against polished steel countersurfaces this activated carbon nanofiller began showing sizeable reductions in PTFE wear at contents as low as 0.18%, attaining with 0.8% content reduced wear rates ～3 * 10^?7 mm³/Nm comparable to alpha-alumina nanofiller, further decreasing with increased content to ～10^?8 mm³/Nm levels at 20% filling. It is believed that exceptional filler materials such as alpha-alumina and activated carbon possess an additional wear reduction mechanism complementing that operating at the microscale, one involving their specific surface chemistry that triggers fibrillation deformation processes in neighboring PTFE and becomes increasingly active at reduced filler particle size where surface area and affected interfacial polymer are augmented.     

TEM-EELS study of low-friction superlattice TiAlN/VN coating: the wear mechanisms

A 20–50 nm thick tribofilm was generated on the worn surface of a multilayer coating TiAlN/VN after dry sliding test against an alumina counterpart. The tribofilm was characterized by applying analytical transmission electron microscopy techniques with emphasis on detailed electron energy loss spectrometry and energy loss near edge structure analysis. Pronounced oxygen in the tribofilm indicated a predominant tribo-oxidation wear. Structural changes in the inner-shell ionization edges of N, Ti and V suggested decomposition of nitride fragments.     

ZDDP and MoDTC interactions and their effect on tribological performance – tribofilm characteristics and its evolution

In this work, the interactions between two key additives in current lubricants (ZDDP and MoDTC) and the effect on tribofilm formation and tribofilm evolution under boundary lubrication are studied. The chemical and tribological characteristics of the tribofilms are probed using measurement of friction, wear and film characteristics. Tribofilms have been examined by energy dispersive X-ray analysis (EDX) and X-ray photoelectron spectroscopy (XPS). In order to investigate the morphology of the reaction films formed, atomic force microscopy (AFM) was used. In this work, for the first time, a link between a proposed MoDTC breakdown mechanism and MoDTC tribofilm characteristics, measured on experimentally derived tribofilms, is made.     

The influence of polymer composition on the wear of the metal surface during fretting of steel on polymer

The effects of the nature of the polymer on the amount of metal wear during fretting of steel on polymers in laboratory air have been studied under a range of loads (130–330 g), amplitudes (3–10 μm) and frequencies (30–60 Hz).A number of polymers can cause damage to the metal, which takes the form of adhesive transfer of α-Fe₂O₃ particles to the polymer surface. The amount of metal wear depends on the polymer counterface and, under a given set of experimental conditions, increases in the order polytetrafluoroethylene (PTFE) and polyethylene, polyvinylidene fluoride (PVDF), polychlorotrifluoroethylene (PCTFE), polysulphone, Polyvinylchloride (PVC), polymethylmethacrylate (PMMA), polycarbonate, nylon 66. No metal wear occurs when the counterface is PTFE and only occurs with polyethylene when the amplitude is greater than 7 μm. These differences are explained in terms of the adhesive properties of the polymers, as determined by their surface energetics. Wear of the polymer during fretting takes the form of fibre formation for polysulphone, PVC, polycarbonate, nylon 66 and, to a small extent, polyethylene, while it takes the form of a transfer of a polymer film to the metal for PTFE and PVDF. No polymer wear occurs for PCTFE or PMMA.     

蒸馏水润滑下Si_3N_4-白口铸铁摩擦面上表面膜形成过程的观察

为了观察蒸馏水润滑下的Ｓｉ３Ｎ４—白口铸铁摩擦面上表面膜的形成过程,在环—块磨损试验机上进行了不同磨程的磨损试验,通过扫描电镜（ＳＥＭ）对不同磨程的铸铁磨面进行了观察,对表面膜的形成机理进行了初步探讨。结果表明,当Ｓｉ３Ｎ４与白口铸铁配副摩擦时,由于铸铁中碳化物的剥落而形成剥落坑,Ｓｉ３Ｎ４磨屑嵌入剥落坑并氧化和水解,其反应产物富集于剥落坑中,脱水聚合后形成硅胶,从而在磨面形成了含硅胶的表面膜。表面膜的形成保护了陶瓷和铸铁磨面,使其变得很光滑,从而使摩擦系数降至０．０２,并使陶瓷和铸铁的磨损几乎接近于零。     

The Role of Microstructure in Ultralow Wear Fluoropolymer Composites

Past studies have shown that the inclusion of fillers in a polytetrafluoroethylene (PTFE) matrix can improve wear resistance by nearly four orders of magnitude. These discoveries have prompted several tribological experiments over the past decade that have highlighted the importance of particle size, tribofilm formation, filler percentage, and environment. To evaluate the effect that microstructure plays on a composite’s tribological performance, PTFE-filled polyamide-imide (PAI) composites were made and tested. To investigate the role of microstructure on the tribological performance of fluoropolymer composites, 12 composite formulations of PTFE and PAI over a range of 0 to 100 vol% PAI were tested. PTFE–PAI composite samples were slid against a stainless steel countersample using a linear reciprocating tribometer under a nominal 6.35?MPa contact pressure at 50.8?mm/s sliding speed. Of the samples tested, the 25 vol% PAI showed a remarkable mean steady-state wear rate of k?=?3?×?10^?9 mm³/Nm over an extreme distance of 360?km. A serial imaging investigation revealed that a mechanical interlocking of the two polymers occurred during the sintering process, which possibly contributed to the ultralow wear rates observed in this polymer–polymer composite.     

Tribological Behavior of TiAlN,AlTiN, and AlCrN Coatings at Boundary Lubricating Condition

In this study, ~?3.5 µm thick multilayer titanium alumina nitride (TiAlN), alumina titanium nitride (AlTiN), and alumina chromium nitride (AlCrN) coatings were deposited on the H13 steel surface by cathodic arc physical vapor deposition (CAPVD) method. The tribological performance of the coatings was evaluated by a tribometer at boundary lubrication condition. Then, coating surfaces were observed by optical microscope, optical profilometer, and atomic force microscope to evaluate the morphological changes, wear volumes, and tribofilm thickness. Also, scanning electron microscopy (energy dispersive X-ray) and X-ray photoelectron spectrometry analyses were applied to coating surfaces for the tribochemical evolution of the tribofilm. Results showed that AlCrN coating performed the best tribological behavior at boundary lubricated condition, when compared to TiAlN and AlTiN coatings and it can be used as a wear resistant cam tappet coating in internal combustion engines.     

A test method to investigate the tribological behaviour of friction materials under ultrasonic fretting conditions

To investigate and understand the tribological behaviour of high-frequency tribosystems such as ultrasonic motors, a specific test method is necessary. This work reports on the construction of a test machine to evaluate the friction and wear behaviour of friction materials under ultrasonic fretting conditions, as well as giving some representative experimental results. Hard/soft (steel/polymer) and hard/hard (steel/alumina, alumina/alumina) couples were studied with respect to their application as contact materials in ultrasonic motors. Investigation of friction behaviour at high frequencies showed that friction-induced vibrations lead to friction forces of much lower magnitude than predicted by quasistationary friction coefficients obtained for sliding friction. The wear behaviour is characterised by abrasive, adhesive, fatigue and oxidative mechanisms, depending on the mating materials. For polymeric friction materials, the influence of fibre reinforcement and the incorporation of PTFE as a solid lubricant were evaluated. The presence of PTFE resulted in a strong improvement of both friction and wear behaviour.     

Effect of Blowing Air and Floating Sand-dust Particles on the Friction and Wear Behavior of PTFE, UHMWPE and PI

The friction and wear behaviors of polytetrafluoroethylene (PTFE), ultra-high molecular weight polyethylene (UHMWPE), and polyimide (PI) have been comparatively evaluated under dry sliding, blowing air, and simulated sand-dust conditions. The tribological tests were conducted on an improved block-on-ring test rig equipped with an attachment for simulating the sand-dust environment. The reason for the difference in the tribological behavior of these polymers under the three test conditions was also comparatively discussed, based on scanning electron microscopic examination of the worn polymer specimens and counterfaces. Under blowing air conditions, the decrease of the contact temperature produced by blowing air led to the increase in the shearing strength of the sliding surface when compared with dry sliding conditions and hence to cause an increase in the friction coefficient and a remarkable decrease in the wear rate of PTFE and UHMWPE. On the contrary, blowing air produced a decrease in the friction coefficient of PI because of the formation of transfer film on the counterfaces, and an increase in the wear rate, because the blowing air considerably promoted the transfer of PI onto the counterfaces when compared with dry sliding conditions. Both PTFE and UHMWPE registered the lowest wear rate under sand-dust conditions, owing to the tribolayer formation on the worn surfaces, while PI exhibited the highest wear rate because no tribolayer was formed during the abrasive wear process.     

An electron microscopy study of worn ceramic surfaces

An electron microscopy investigation of worn surfaces of monolithic alumina, whisker-reinforced alumina, monolithic silicon carbide and silicon-silicon carbide has been performed. The emphasis has been to explore the mechanisms of wear debris generation and tribofilm formation and their influence on the tribological behaviour in closed contacts of the flat-on-flat category.     

Numerical Study of the Effect of Tribofilm Kinetics and Its Hardness on the Roughness Evolution of the Substrate in Boundary Lubrication Regime

Abstract

A tribochemical modeling framework that considers the growth of a tribofilm on the contacting surfaces has been used in this work. The model couples a fast contact mechanics model with the thermodynamics of interfaces and captures the growth of the tribofilm on the asperities. The model was shown to be able to capture the dynamics of a tribosystem and the evolution of surface topography. The model considers the effect of plastic deformation and wear in modifying the surface geometries. In a recent work by the authors (Ghanbarzadeh et al., Wear, 362–363, 2016), the same numerical model was validated against experiments in a micropitting rig (MPR) and the wear, topography, and tribofilm thickness results were compared. In this work, validation of the model is presented and the effect of tribofilm kinetics and its hardness have been numerically studied to assess the evolution of surface roughness in a rolling sliding contact. Results suggest that the kinetics of the tribofilm growth significantly influence the roughness evolution with higher kinetics resulting in a rougher interface. Similarly, the tribofilm hardness affects the roughness evolution and is more influential in the later stages of roughness evolution.     

Effect of Particle Size on the Wear Resistance of Alumina-Filled PTFE Micro- and Nanocomposites

It was long supposed that the ability of hard particle fillers to reduce the wear rate of unfilled PTFE (typically ～ 10^{? 3} mm ³ /Nm) by an order of magnitude or more was limited to fillers of microscale or greater, as nano-fillers would likely be encapsulated within the large microscale PTFE wear debris rather than disrupting the wear mechanism. Recent studies have demonstrated that nano-fillers can be more effective than microscale fillers in reducing wear rate while maintaining a low coefficient of friction. This study attempts to further elucidate the mechanisms leading to improved wear resistance via a thorough study of the effects of particle size. When filled to a 5% mass fraction, 40- and 80-nm alumina particles reduced the PTFE wear rate to a ～ 10^?7 mm ³ /Nm level, two orders of magnitude better than the ～ 10^?5 mm ³ /Nm level with alumina micro-fillers at sizes ranging from 0.5 to 20 μm. Composites with alumina filler in the form of nanoparticles were less abrasive to the mating steel (stainless 304) countersurfaces than those with microparticles, despite the filler being of the same material. In PTFE containing a mixture of both nano- and micro-fillers, the higher wear rate microcomposite behavior predominated, likely the result of the continued presence of micro-fillers and their abrasion of the countersurface as well as any overlying beneficial transfer films. Despite demonstrating such a large effect on the wear rate, the variation of alumina filler size did not demonstrate any significant effect on the friction coefficient, with values for all composites tested additionally falling near the μ = 0.18 measured for unfilled PTFE at this study's 0.01 m/s sliding speed.     

Pre-Polishing the Metal Counterface of Metal–UHMWPE Wear Pair with Filler-Filled UHMWPE Composites to Generate Counterface Changes for an Effective Reduction in Pure UHMWPE Wear

Ultra-high molecular weight polyethylene (UHMWPE) is well known for high-wear-resistance applications. Its long-chained easy sliding molecules and semi-crystalline structures enable the polymer’s great wear resistance. UHMWPE composites made for higher wear resistance study have been analyzed in this paper. Pure UHMWPE, 1 wt% CNT UHMWPE, 1 wt% PEEK UHMWPE, 1 wt% alumina (nano)–UHMWPE composites were made to be tested against metal disk on pin-on-disk tribometer. The metal disk surface conditions were found to have significant influence on the UHMWPE–polymer wear than the composite itself. This result indicates a simple and industrial applicable method that involves transfer film on the counterface to reduce polymer wear for metal–polymer wear pair applications.     

In Vacuo Tribological Behavior of Polytetrafluoroethylene (PTFE) and Alumina Nanocomposites: The Importance of Water for Ultralow Wear

Polytetrafluoroethylene (PTFE) is widely regarded as an excellent candidate for solid lubrication in vacuum. However, it is often precluded from many practical applications due to its intrinsically high wear rate. Over the past decade, it has been discovered that small loading fractions of alumina nanofillers can increase the wear resistance of PTFE by three to four orders of magnitude. This dramatic increase in wear resistance has in turn prompted numerous tribological studies to examine the robustness of this performance. In this study, the wear and friction behavior of unfilled PTFE and PTFE and alumina nanocomposites were evaluated under a broad range of vacuum environments from 760 to 4 × 10^?6 Torr. The nanocomposites of PTFE/alumina showed a dramatic increase in wear of over two orders of magnitude at the highest vacuum conditions. There appears to be an optimal vacuum environment around 1–10 Torr, in which these samples achieved the lowest wear rates of approximately 2.5 × 10^?7 mm³/(Nm).     

Tribological Performance of EP Lubricants with Phosphorus-Based Additives

Lubricants containing additives that protect mechanical components against extreme pressure by reducing friction and wear are known as extreme pressure (EP) lubricants. In the current study, phosphorus-based EP lubricants with different additives (amine phosphate and phosphate ester) were tested in a steel ball-on-disc assembly under different EP conditions. The phosphate ester–steel interaction resulted in significantly higher wear and marginally lower friction than the amine phosphate–steel interaction. The tribological performance (especially wear) depended on the contact conditions. The tribofilm that formed on the steel surface with both EP lubricants consisted of organic compounds, oxides, and phosphates. The greater formation of the wear-resistant iron phosphate for the amine phosphate–steel interaction resulted in lower wear. The friction and wear performance for both EP lubricants depended upon surface roughness parameters along with the compounds that formed in the tribofilm.     

Tribochemical interactions between Zndtp,Modtc and calcium borate

Tribochemical interactions between antiwear zinc dithiophosphate (Zndtp), friction modifier molybdenum dithiocarbamate (Modtc) and overbased detergent calcium borate (OCB) lubricant additives have been investigated. Friction tests were performed in mild wear conditions under boundary lubrication, in order to enhance tribochemical surface effects. The nature of tribofilms formed was studied by coupling high‐resolution TEM on wear fragments and inside‐wear‐scar, micro‐spot XPS in the same location of the wear track (so‐called dual analysis). The performance of the Modtc/Zndtp mixture is mainly due to the generation of MoS₂ single sheets and the digestion of MoO₃ in the zinc polyphosphate glass formed. The final result of the tribochemical reaction is a two‐phase tribofilm composed of (i) non‐oriented MoS₂ sheets (friction modifier) embedded in a carbon‐rich phase and (ii) a mixed Zn/Mo polyphosphate glass (antiwear). The Modtc/OCB mixture has a similar antiwear mechanism except that the oxide is not completely eliminated, due to the softer action of borate anion compared with phosphate one. Compared to the data obtained with binary combinations (Modtc/Zndtp, Modtc/OCB and Zndtp/OCB), we show here that the ternary system Modtc/Zndtp/OCB provides both a low wear rate and an ultralow friction value, while adding detergent and anti‐corrosive properties to the formulation. Our analytical data indicate that the synergistic effect can be attributed to an outstanding nanostructure of the tribofilm formed. It is composed of a single‐phase material containing perfectly oriented MoS₂ single sheets embedded in a calcium and zinc borophosphate glass. The ternary system produces a smart material in the interface, because both functions (antiwear and friction reduction) are correlated. Compared to phosphate alone, the mechanism by which MoS₂ sheets have been oriented in the borophosphate could be related to aligned molecules of the glassy polymer in the direction of sliding. This revised version was published online in July 2006 with corrections to the Cover Date.     

Transfer of semicrystalline polymers sliding against a smooth steel surface

The interrelationships between transfer and wear in polymers were studied using a pin-disk-type wear testing apparatus. The wear rates of polymers except polytetrafluoroethylene (PTFE) were high for up to about the first 100 revolutions of the disk and decreased gradually until the steady low wear rates which generally occurred after about 2000 revolutions. However, PTFE exhibited an almost constant high wear rate throughout the wear process. The thickness of transferred polymer increased rapidly with increasing number of revolutions in the initial wear stage but after about several hundred revolutions remained constant. A coherent transfer film was formed in most parts of the friction track after about 100 revolutions. It was found that polymer wear could occur in polymers sliding on a transferred polymer layer. All polymers except PTFE exhibited smaller wear rates when sliding on the transferred layer. The load dependence of the thickness was very small compared with that of the wear rate. PTFE produced a very dense and coherent transferred layer compared with that of other polymers. However, there was no clear relationship between friction and the thickness of the transferred polymer layer.     

ZDDP and MoDTC interactions in boundary lubrication—The effect of temperature and ZDDP/MoDTC ratio

Tribofilms formed under boundary lubrication from ZDDP and MoDTC additives alone or in different ratios in the lubricant have been studied. The tribological performance is linked to the tribofilm properties and consequently to the lubricating conditions. Tribofilms are formed using a reciprocating pin-on-plate tribometer. Surface sensitive analytical techniques, such as energy dispersive X-ray analysis (EDX) and X-ray photoelectron spectroscopy (XPS) have been used for tribofilm characterisation. The XPS peaks have been deconvoluted to characterise the species formed in the wear scar. The formation of species with different tribological properties, due to the decomposition of ZDDP and MoDTC molecules as a result of testing temperature, is shown. Surface analyses have shown that MoDTC decomposes, even in low-lubricant bulk temperature tests (30 °C), forming the same species as in high-lubricant bulk temperature tests (100 and 150 °C) but the tribofilms give different tribological performance. The effectiveness in friction reduction is shown to depend on the ratio between what are defined as high- and low-friction species in the tribofilm.