Metal Counterface Research Articles

Fluorinated graphite reinforced polytetrafluoroethylene/poly(butylene terephthalate) composites as friction materials for deep‐sea applications

AbstractThe application of polymer composites is one of the most effective methods for preventing lubrication failure in deep‐sea engineering. In this study, fluorinated graphite (FGr) microsheets were incorporated into polytetrafluoroethylene (PTFE) microparticle‐filled poly(butylene terephthalate) (PBT) composites. The tribological performance in the simulated deep‐sea environment at elevated seawater pressure equivalent to 3000 m ocean depth was investigated. The results showed that FGr enhances the flexural and compressive strengths, thermal stability, and seawater resistance of the PTFE/PBT composite. The wear rate was decreased to 96% at the critical FGr content of 4% by improving the transfer onto the metallic counterfaces, preventing direct contact and shear between friction couples. Moreover, the seawater pressure impedes composite transfer, leading to a 29% increase in the wear rate.

Graphene oxide-encapsulated baghdadite nanocomposite improved physical, mechanical, and biological properties of a vancomycin-loaded PMMA bone cement

Polymethyl methacrylate (PMMA) bone cement is commonly used in orthopedic surgeries to fill the bone defects or fix the prostheses. These cements are usually containing amounts of a nonbioactive radiopacifying agent such as barium sulfate and zirconium dioxide, which does not have a good interface compatibility with PMMA, and the clumps formed from these materials can scratch metal counterfaces. In this work, graphene oxide encapsulated baghdadite (GOBgh) nanoparticles were applied as radiopacifying and bioactive agent in a PMMA bone cement containing 2 wt.% of vancomycin (VAN). The addition of 20 wt.% of GOBgh (GOBgh20) nanoparticles to PMMA powder caused a 33.6% increase in compressive strength and a 70.9% increase in elastic modulus compared to the Simplex® P bone cement, and also enhanced the setting properties, radiopacity, antibacterial activity, and the apatite formation in simulated body fluid. In vitro cell assessments confirmed the increase in adhesion and proliferation of MG-63 cells as well as the osteogenic differentiation of human adipose-derived mesenchymal stem cells on the surface of PMMA-GOBgh20 cement. The chorioallantoic membrane assay revealed the excellent angiogenesis activity of nanocomposite cement samples. In vivo experiments on a rat model also demonstrated the mineralization and bone integration of PMMA-GOBgh20 cement within four weeks. Based on the promising results obtained, PMMA-GOBgh20 bone cement is suggested as an optimal sample for use in orthopedic surgeries.

On the anisotropy in the dry-sliding behaviour of a self-lubricating PTFE composite and the effects from surface texturing

ABSTRACT In self-lubricating sliding contact facilitated with polytetrafluoroethylene (PTFE) composites, the anisotropy in tribological performance was often attributed to the oriented structures of the polymer. However, in this research, it is found that the non-homogeneity introduced by the hard fibres or stiffeners may be the main contributor to the anisotropic tribological performance of the polymeric composites. The interaction between the non-uniformly arranged hard fibres/stiffeners and the metallic counterface was found to be the leading phenomenon of this tribological behaviour. Moreover, the effects from a surface texturing scheme was found capable of establishing intervention to this interaction and reducing the anisotropy. The mechanism of the texturing effects was found related with back-transfer of PTFE and trapping of wear debris.

The relations between wear behavior and basic material properties of graphene‐based materials reinforced ultrahigh molecular weight polyethylene

AbstractThis article aims to investigate the influence of reinforcing graphene oxide (GO) and graphite flakes (GF) fillers into ultrahigh molecular weight polyethylene (UHMWPE) for orthopedic application. These fillers were expected to physically bond to UHMWPE, thus can enhance the subsurface strength, improving the wear behavior of the composites. UHMWPE/GO and UHMWPE/GF composites were prepared at 0.1 and 1.0 wt% by melt‐blending, followed by a compression molding technique. A multidirectional pin‐on‐disc wear test was performed to simulate the kinematic of hip application. Whilst getting exposed in the artificial in‐vivo lubricant bath (30 v/v% diluted bovine serum). Following this, the wear mechanism fostered by each filler (GO and GF) was determined by wear features obtained from the optical microscope and scanning electron microscope (SEM). The crystallinity degree and crystal defect were assessed using x‐ray diffraction (XRD). The mechanical properties of fabricated composites were evaluated by using a universal testing machine and Vickers microhardness. We found that UHMWPE/GO has the lowest specific wear rate due to the improved subsurface strength, as the reduction of a weak adhesive point was observed on the worn surface. Meanwhile, higher GF content (1 wt%) in UHMWPE displayed a lower specific wear rate than neat UHMWPE after completing the 10 km sliding distance attributed to the filler resurfaced, responsible for providing a strong resistance of the shear stress applied upon sliding with the metal counterface. Interestingly, the hardness and tensile strength for both UHMWPE/GO and UHMWPE/GF increased, although the crystallinity percentage was declining compared to neat UHMWPE.

Effect of humidity and counterface material on the friction and wear of carbon fiber reinforced PTFE composites

The tribological performance of PTFE composites is affected by both the composition of the counterface material and the sliding environment. However, no comprehensive investigation has been conducted on the combined effect of the humidity and counterface material on PTFE composites. This study investigates the tribological behavior of carbon fiber reinforced PTFE composites sliding in a dry nitrogen environment at different humidity levels. Experiments were conducted in an enclosed tri-pin-on-disc tribometer against two different metallic counterface materials. Tests in laboratory air were used for comparison. Results indicate that the tribological performance of carbon fiber reinforced PTFE composites are sensitive to environmental changes. The impact of humidity on both the coefficient of friction and specific wear rate was up to about 40%.

Interfacial interaction of PTFE sub-micron particles in oil with steel surfaces as excellent extreme-pressure additive

This work investigates on the plausible interfacial mechanisms of PTFE (submicron particle) SMPs with the metal counterface during the growth of PTFE tribo-film. A series of submicron-oils was prepared to examine the effect on oil performance with variation in concentration of SMPs (0–9 wt%) in a Group III base-oil. Commercially available submicron suspension containing an adequate amount of dispersant for suspending the PTFE particles (avg size 230 nm) was used in various doses. In order to roughly estimate the shelf life of the formulated oils; UV–Vis spectroscopy technique was employed on the diluted oil samples since original samples were too thick. Dynamic light scattering (DLS) studies were carried in order to estimate the changes in the particle size with storage time. Nano-oils were characterized for their physical properties viz. (density, viscosity and viscosity index) and tribo- performance. Results revealed that 6 wt% of PTFE SMPs proved to be an optimum concentration for the highest EP performance with 392% enhancement compared to the base oil, which is not reported in the literature. To understand the mechanisms behind this performance, worn surfaces of pre-weld balls were inspected by FESEM (Field emission scanning electron microscope), 3D profilometry, goniometry and XPS (X-ray photo-electron spectroscopy), which revealed exact reasons for the varied performance of nano-oils as a function of increasing concentration of SMPs. XPS spectroscopy on worn surfaces revealed the formation of chemically modified tribo-film over the asperities. Furthermore, the thickness of tribo-film formed was also evaluated. Since EP trends were so promising, preliminary oils were characterized for AW and AF performance also. AW and AF performance got enhanced by 30% and 40% respectively.

The transfer behavior of poly(methylsilsesquioxane) microspheres reinforced poly(butylene terephthalate) composites in the deep sea

The role of deep sea environment in polymer transfer was investigated by studying poly(methylsilsesquioxane) (PMSQ) microspheres reinforced poly(butylene terephthalate) (PBT) composites. The results show that besides reinforcement in mechanical strengths, the incorporation of PMSQ enhances PBT transfer due to its unique chemical structure, appropriate size and soft characteristic. The transfer process of composites starts with PMSQ particle transfer, forming the initial transfer layer and growing to optical visible transfer film. The seawater pressure prolongs the transfer process and decreases adhesion of transfer film by preferentially saturating hydroxyl groups at PMSQ surface and freshly created dangling bonds at metallic counterface.

Experimental study on triblogical behaviors of PA6 and PTFE polymers in the case of rolling friction

Rolling/sliding contacts, such as rolling-element bearings or gears, are a keystone of many machines, which could not properly operate in absence of these components. Machine elements of this kind are typically made of certain variants of steel, whereby hardening processes are carried out in order to increase lifetime and reduce wear. Furthermore, proper lubrication of these tribo-systems is inevitable in order to guarantee a safe operation throughout the component’s lifetime. In recent years high performance polymers, such as Polyether ether ketone (PEEK) or polyamid 46 (PA46), have become an option for application in rolling/sliding contacts. Due to the low density and effective manufacturing processes, such as injection moulding, polymers enable lightweight design solutions under relatively low cost. A comparative investigation based on tribological properties including the friction and wear behaviour of two polymers (Polyamide 6, Polytetrafluoroethylene) was researched in this study. Where samples were prepared by machining for precast cylinders of these materials, and experiments were carried out using a device designed for this purpose, with two parameters (rolling speeds and loads) in select. The morphology of the polymeric transfer film which plays a lubricating role in the dry operation conditions was observed for both materials. The obtained results showed that the friction feature of PTFE was better than that of PA 6 which released high sounds during testing duo to adhesion bonds with metallic counterface. The main wear mechanism of polyamide was micro mechanical machining and deformation, while was adhesive wear for PTFE, and the wear rate of PTFE was very high compared to PA 6 which suffered from delamination wear in some cases

Formation of Tribofilm in the Friction of Fluorinated Diamond-Like Carbon (FDLC) Film against Ti6Al4V in Bovine Serum Albumin (BSA) Solution

A route to reducing the wear of the metal counterpart in the friction of meatal against diamond-like carbon (DLC) is to form a lubricating tribofilm on the metal counterface. However, in liquid lubricating conditions, the formation of tribofilm can be influenced by both the lubricating medium and the counterpart material. Here we report the effect of lubricating biomolecule and doping fluorine element on the formation of tribofilm in fluorinated DLC (FDLC)-Ti6Al4V friction system. A group of ball-on-disc frictional experiments with different sliding speeds and normal loads were performed in phosphate buffer solution (PBS) and bovine serum albumin (BSA) solution. The results showed the formation of tribofilm was inhibited by the absorption of biomolecules on the frictional surface, thus improving the friction coefficient and wear of Ti6Al4V counterpart. Doping fluorine into DLC film also can restrain the formation of tribofilm on Ti6Al4V counterface. As a result, tribofilm is difficult to form when Ti6Al4V counterface slides against FDLC in BSA solution. Fluorinated DLC film should be considered carefully for the anti-wear use in body fluid containing biomolecules because it might cause severe wear of the counterpart material.

Competitive Binding of Bilirubin and Fatty Acid on Serum Albumin Affects Wear of UHMWPE.

Total Joint Replacement (TJR) devices undergo standardized wear testing in mechanical simulators while submerged in a proteinaceous testing solution to mimic the environmental conditions of artificial joints in the human body. Typically, bovine calf serum is used to provide the required protein content. However, due to lot-to-lot variability, an undesirable variance in testing outcome is observed. Based on an earlier finding that yellowish-orange serum color saturation is associated with wear rate, we examined potential sources of this variability, by running a comparative wear test with bilirubin; hemin; and a fatty acid, oleic acid, in the lubricant. All these compounds readily bind to albumin, the most abundant protein in bovine serum. Ultrahigh molecular weight polyethylene (UHMWPE) pins were articulated against CoCrMo discs in a pin-on-disc tribometer, and the UHMWPE wear rates were compared between lubricants. We found that the addition of bilirubin increased wear by 121%, while hemin had a much weaker, insignificant effect. When added at the same molar ratio as bilirubin, the fatty acid tended to reduce wear. Additionally, there was a significant interaction with respect to bilirubin and hemin in that UHMWPE wear rate decreased with increasing fatty acid concentration. We believe the conformational change in albumin by binding bilirubin makes it more likely to form molecular bridges between UHMWPE and the metal counterface, thus increasing adhesive wear. However, fatty acids compete for binding sites on albumin, and can prevent this conformational change. Hence, the protein is stabilized, and the chance for albumin to form bridges is lowered. Ultimately, UHMWPE wear rate is driven by the competitive binding of bilirubin and fatty acid to albumin.

Interfacial Gradient and Its Role in Ultralow Wear Sliding

A particular nanosized alumina (α-Al2O3) filler reduces the wear rate of polytetrafluoroethylene (PTFE) by nearly 4 orders of magnitude in ambient environments through the formation of stable interfacial tribofilms. One key to the unusual success of this system is the tribochemical degradation of the polymer and subsequent formation of carboxylate salts, which directly bond the PTFE to both the alumina nanofiller and the metallic counterface. However, previous studies have shown that the exceptional wear resistance of these cross-linked, stable, and well-anchored surfaces vanishes when slid against surfaces of slightly higher surface energy. In this paper, we elucidate the effects of these interfacial gradients within the native ultralow wear composite-on-transfer film system using interrupted wear tests and intermittent surface analysis. As anticipated, the transition from high wear to ultralow wear was accompanied by small adherent debris, tribochemical formation of carboxylates, increased surface energy, and increased adhesion. Interestingly, we observed significant differences on either side of the interface during low wear sliding; compared to the running films on the composite surface, the transfer films on the counterface exhibited consistently greater tribochemical degradation, surface energy, and adhesion to a model alumina probe. This interfacial gradient, we propose, is a necessary feature of the ultralow wear system and functions by setting the direction and driving force for transfer wear. In this case, the interfacial gradient stabilizes the transfer film and minimizes the driving force for running film wear.

Dry sliding tribological properties of PI/UHMWPE blends for high speed application

PI/UHMWPE blends are going to be used as the bush material of sliding bearings under high speed dry condition in one project. Prior to the fabrication of these bearings, the tribological properties PI/UHMWPE blends with various component ratio under the specific service condition was investigated by using a pin-on-disk tribometer. The results revealed that the compression modulus of PI/UHMWPE blends increases with the increase in PI content. Furthermore, the coefficient of friction and wear rate decrease firstly and then increase slightly with the increase in PI content, both of which reach the minimum value when PI content is 70%. The analyses of transfer membrane indicate that both of UHMWPE and PI can transfer to the metal counterface, while the increase of PI content is more conducive to the increase of transfer membrane.

Self-competing and Coupled Effect of Laser-Engraved Counterface Groove Depth and Density on Wear of Alumina PTFE

Polymeric solid lubricants rely on the deposition of a debris-formed transfer film on the usually metallic counterface to reduce wear and friction. Debris retention at the sliding surface is determined by the interactions between debris and roughness asperities which are complex functions of debris size and roughness profile. Recent works found pre-existing, sandpaper-lapped counterface roughness grooves perpendicular to the sliding direction could significantly improve debris retention and reduce the wear rate of an alumina PTFE solid lubricant by 70%. In this paper, we aimed to test the independent effects of roughness groove depth and density on debris retention and wear performance of a well-studied alumina PTFE solid lubricant using laser-textured counterfaces with independently varied groove depth and interval. All grooves in this study were engraved perpendicular to the sliding direction. The results suggested both groove depth and interval have self-competing effect on wear due to the in situ grounding and roughness alignment of the counterface topography during sliding and may compete or cooperate with each other in determining solid lubricant wear. A new surface directionality parameter was defined to quantify the in situ counterface roughness alignment which increased proportionally with increased composite wear volume at low-wear transition. A conceptual framework was proposed to illustrate the relations between counterface texture, transient wear volume, surface directionality and counterface abrasion.

The Effect of PTFE Powder Adhesive Activation on the Wear Resistance of Block PTFE

The effect of adhesive activation of polytetrafluoroethylene (PTFE) powder by sodium ammonia solution on the structure and tribological behavior of sintered polymer is studied. The friction coefficient of the adhesion-activated PTFE (PTFE-A) increases, while the rate of wear decreases compared to the original PTFE by about 50-fold on friction against 40X13 stainless steel and by up to 800-fold on friction against 40X chromium steel. The difference is attributed to the dependence of the rate of wear on activation of the tribochemical mechanism, which is responsible for chemisorption of the transfer film on the metal counterface and increases its life, wear resistance and, eventually, the wear resistance of PTFE-A. When the friction conditions preclude its activation (friction against 40X13 stainless steel), the modest improvement in wear resistance is achieved through increased intermolecular interaction in PTFE-A. On friction against corrosion-active 40X steel, an iron oxide layer facilitates chemisorption of the transfer film providing for additional reduction in the wear rate by more than an order of magnitude. Functionalization of PTFE-A also inhibits the dependence of the wear rate on the relative humidity of the surrounding atmosphere observed during friction of some PTFE-based composites, which may be useful for practical applications.

Measuring Evolution of Transfer Film–Substrate Interface Using Low Wear Alumina PTFE

Polymeric solid lubricants lay down their own wear debris onto hard metallic counterfaces to form a protective transfer film which reduces friction and wear effectively without lubrication. Adhesive shear strength at the hidden interface between the film and substrate determines the film persistence and correlates with system wear qualitatively. Previous studies showed that an ultralow wear (k ~ 10− 7 mm3/Nm) alumina-PTFE solid lubricant forms an extremely adherent and complete transfer film, and strong chemical bonds between wear debris and counterface perpetuate the film–substrate adhesion very early in the sliding. In this paper, we aimed to test the permanence of such adhesion by removing pre-developed transfer films using sliding rubber contact and measuring the topographical evolution of the interface throughout the course of a standard wear test using the well-studied alumina-PTFE system. The results unexpectedly showed continuous wear of the counterface across the wear track, and counterface wear rate decreased proportionally from 3 × 10− 7 to 3 × 10− 8 mm3/Nm with increased film area fraction and sliding distance. A proposed rule-of-mixtures wear model coincided closely with the experimental results and strongly suggested a coupled mechanism of adhesive and fatigue wear of the counterface. The upper limit of the interfacial counterface fatigue wear rate was predicted to be 3 × 10− 8 mm3/Nm.

Influence of polyphenyl ester and nanosized copper filler on the tribological properties of carbon fibre–reinforced ultra-high-molecular-weight polyethylene composites

Ultra-high-molecular-weight polyethylene (UHMWPE) reinforced with carbon fibre (CF) and filled with polyphenyl ester (POB) and nanosized copper (Cu) fillers was prepared by compression moulding. The tribological behaviours and the synergism of the incorporation of fibre and particulates were studied. The proportions of the reinforcement material ranged from 5 wt% to 25 wt%, the filler material of POB varied from 5 wt% to 25 wt% and the nanosized filler was from 4 wt% to 12 wt%. In the sample with CF only, the lowest wear rate was observed for the UHMWPE + 15% CF composite. The particulate filler further reduced the composite wear rate, and the lowest wear rate was found for the hybrid with CF, POB and nanosize Cu particles, that is, for the UHMWPE + 15% CF + 15% POB + 12% Cu composite. The particulate filler was added, and the coefficient of friction slightly increased. The transfer film formed on the metal counterface was studied using optical microscopy, and the topography of the transfer film was investigated using atomic force microscopy. Results showed that the transfer films were thin, compact and uniform on the metal counterfaces of the UHMWPE + 15% CF + 15% POB + 12% Cu composite. Worn surface morphologies of composites were studied using scanning electron microscopy. Results showed that the worn surface of the UHMWPE + 15% CF + 15% POB + 12% Cu composite was smoother and had better wear resistance than that of other composites.

Ways of Improving the Tribological Characteristics of Composite Polymeric Materials and Lubricants in Friction Units

The regularities of the formation of a friction transfer film (FTF) of polymeric composite materials on a metallic counterface are found. It is shown that in the friction process, the lubricants based on biodegradable base oils are able to form antifriction films, which are related to surface absorption structures.

The Dependence of Aftereffects and Contact of Rough Surfaces Composite – Metal from Orientation Factors of Dispersed Phase of Polymer Matrix

Results of researches of influence orientation effects on the friction properties of carbon-plastics and wear mechanism of unidirectional carbon-plastics have been brought in a work. The optimal structure of carbon-plastics and fiber orientation layers in it relative to the surface friction and sliding direction have been provided. The influence of schemes reinforcing by fibrous carbon materials of polymer composites and schemes contacting of samples composites with the surface of metal counterface on the intensity of wear of friction pair and on the changing of microroughness parameters of metal surface have been researched. It has been shown that the topography of metal counterface surface changes in friction carbon plastics with oriented fibers, but the allocation of heights and curvatures of microroughness peaks of friction surface far removed from the Gaussian allocation and close to Rayleigh allocation.

Influence of contact pressure, cross-shear and counterface material on the wear of PEEK and CFR-PEEK for orthopaedic applications

Total joint replacement is a successful surgical intervention for the treatment of the degeneration of many joints, particularly the hip and knee. As the demand for joint replacement grows, and the life expectancy of the population increases, the performance requirements of these implants also changes. New materials, to improve longevity and enhance performance have been explored including PEEK and CFR-PEEK.This study investigated whether CFR-PEEK and PEEK were appropriate materials for total joint replacement by examining wear performance in simple configuration studies articulating against cobalt chrome under a range of cross-shear and contact pressure conditions. Simple geometry pin on plate studies were conducted for one million cycles for each test condition, with the contact pressure and cross-shear conditions representing a range in which the material may need to operate in-vivo.The wear factor for PEEK was significantly higher than CFR-PEEK and conventional polyethylene under all test conditions. Both PEEK and CFR-PEEK wear were influenced by contact pressure, with the highest wear factors for both materials measured at the highest pressure conditions. PEEK appeared to have a cross-shear dependent wear response, but this was not observed for the CFR-PEEK material.This study has further characterised the wear performance of two materials that are gaining interest for total joint replacement. The wear performance of the PEEK material showed poorer wear performance compared to polyethylene when articulating with a metal counterface, but the performance of the CFR-PEEK material suggested it may provide a suitable alternative to polyethylene in some applications. The wear performance of CFR-PEEK was poorer than polyethylene when it was used as the plate, when there was translation of the contact zone over the surface of the CFR-PEEK plate. This has implications for applications in low conforming contacts, such as lower conformity knee replacement.

A Review of Transfer Films and Their Role in Ultra-Low-Wear Sliding of Polymers

In dry sliding conditions, polytetrafluoroethylene (PTFE) composites can form thin, uniform, and protective transfer films on hard, metallic counterfaces that may play a significant role in friction and wear control. Qualitative characterizations of transfer film morphology, composition, and adhesion to the counterface suggest they are all good predictors of friction and, particularly, wear performance. However, a lack of quantitative transfer film characterization methods and uncertainty regarding specific mechanisms of friction and wear control make definitive conclusions about causal relationships between transfer film and tribological properties difficult. This paper reviews the state of the art in the solid lubricant transfer film literature and highlights recent advances in quantitative characterization thereof.