Polymer Wear Research Articles

高分子材料の摩擦・摩耗面の分析と解析

Friction and wear properties of polymeric materials are reviewed with special reference to morphologies analyzed by transmission electron microscopy (TEM) of carbon replica films obtained from the worn surfaces. Tribological properties of polyethylene as functions of molecular weight and lubricants are related to mechanism for the formation of fine wear powders as revealed by TEM, including mechanism of self-lubrication of ultrahigh molecular weight polyethylene in water. Tribologies of isotactic polypropylene, polyacetal-based polymer alloy and other polymers are also reported based on the carbon replica TEM method. It is concluded that TEM analysis using carbon replica film obtained from the worn polymer surfaces is a powerful method for clarifying polymer wear mechanisms.

Scratching of polymers—Modeling abrasive wear

This paper aims to model abrasive wear for polymers using single-track and intersecting scratching techniques. Scratch and pin-on-disc wear tests were conducted on five different commercial polymers. Wear debris generated by multiple-pass single-track and intersecting scratching tests were compared and correlated with the specific wear rates of the same polymers in a pin-on-disk test using ground steel surface ( R a = 1.34 μm) as the counterface. It is the purpose of this paper to establish a correlation between scratching phenomenon and abrasive wear of polymers. Confirming previous studies, for polymers, the traditionally computed scratch hardness does not follow any trend with the abrasive wear. However, if the amount of wear debris in scratching is quantitatively measured and plotted against specific wear rate in pin-on-disc test, a reasonable linear relation is obtained.

Sliding wear performance of polyamide 6–clay nanocomposites in water

Polyamide 6 (PA6) clay nanocomposites, a new class of specialty polymer, shows improved tribological properties under dry sliding conditions. In food and chemical industry, the polymeric materials are widely used in the machine elements due its excellent chemical inertness. Water, being a widely used solvent, tends to have contact with the machine elements and can either act as a lubricant or as a contaminant. Many factors such as the interaction of polymer with water, surface wettability, etc., play a major role on the friction and wear of polymers in aqueous conditions. This work reports the sliding wear performance of PA6 clay nanocomposites in water. Addition of clay affects the crystallinity of the nanocomposites, which in turn affects the plasticization. Plasticization of the surface by water causes increase in wear and decreases the coefficient of friction.

Sliding wear, viscoelasticity, and brittleness of polymers

We have connected viscoelastic recovery (healing) in sliding wear to free volume in polymers by using pressure-volume-temperature (P-V-T) results and the Hartmann equation of state. A linear relationship was found for all polymers studied with a wide variety of chemical structures, except for polystyrene (PS). Examination of the effect of the indenter force level applied in sliding wear on the healing shows that recovery is practically independent of that level. Strain hardening in sliding wear was observed for all materials except PS, the exception attributed to brittleness. Therefore, we have formulated a quantitative definition of brittleness in terms of elongation at break and storage modulus. Further, we provide a formula relating the brittleness to sliding wear recovery; the formula is obeyed with high accuracy by all materials including PS. High recovery values correspond to low brittleness, and vice versa. Our definition of brittleness can be used as a design criterion for choosing polymers for specific applications.

Aging Performance of a Compliant Layer Bearing Acetabular Prosthesis in an Ovine Hip Arthroplasty Model

The wear performance of a polyurethane compliant layer (CL) material formed into an acetabular component and implanted into a sheep model of cemented total hip arthroplasty was assessed at 6, 12, 24, and 48 months. Four (11%) of 36 acetabular components debonded from the cement and one component was slightly loose at the cement-bone interface. There was no macroscopic evidence of fracture, wear, or deformation of the CL material on the articular surface of the acetabular components. Small numbers of polymeric wear particles was found in the hip synovial tissues of 10 sheep, most commonly in the early time groups, and were likely associated with initial wear of surface asperities. The wear performance of the CL was unchanged during a 48-month implantation period.

Biological Impact of Polyacetal Particles on Loosening of Isoelastic Stems

The aim of our study was to identify biological factors responsible for premature loosening of polyacetal hip stems. The results of histological analyses of the tissue around 11 total hip prostheses with loosened polyacetal femoral stems were compared to those obtained in a group of 11 total hip prostheses with loosened metal (CoCr) femoral components. A higher number of polymer wear particles surrounded by giant cells, more bone chips, and a more extensive necrosis were found around loosened polyacetal stems. Histomorphological characteristics of polyacetal wear particles containing BaSO(4) granules in the tissue around loosened polyacetal stems were described. Radiological evaluation of the wear of polyethylene cups suggested that elastic modulus of the stem had no influence on the wear of polyethylene cups. This study indicates that polyacetal wear particles have a great biological potential accelerating the process of loosening.

Application of a stereometric structural analysis for an evaluation of surface and for a forecast of the wear of endoprothesis acetabular cups

To improve the resistance to wear and permanent deformation of polyethylene operating in a polymer/metal friction couple, initial plastic deformation of the polyethylene and its electron irradiation was applied. This contributed to a change of the polymer structure, visible already while machining when the sample surfaces were being prepared for a tribological test. The study shows that the interactions that shape the structure of polyethylene, at the same time cause adequate changes to the stereometric structure of its surface. The parameters of surface microgeometry characterize the future tribological behavior of polyethylene during its operation in a friction couple. It has been shown that an analysis of stereometric parameters may constitute the first projection of polymer wear resistance.

In vivo wear of polyethylene glenoid components in total shoulder arthroplasty

AbstractLoosening of the glenoid component after a total shoulder arthroplasty, leading to pain and the need for a revision operation, accounts for one third of all shoulder replacement complications. It has been shown that glenoid loosening could be a result of osteolysis of periprosthetic bone induced by polymeric wear debris. The aim of this study was to determine the amount and pattern of in vivo wear of polyethylene glenoid components by examining a set of so-called retrievals. Six glenoid components were retrieved and examined (5 all-polyethylene and 1 metal-backed). The average time of functioning of these components in human bodies was 10 years with the shortest time of 7 and longest 14 years. Four components were scanned using a laser scanner. The results of measurements have been used to generate geometrical models of the retrievals using CAD software. These models have been subsequently used to determine the maximum depth and volumetric wear rate for each component. All six retrievals were examined using scanning electron microscopy (SEM). The maximum volumetric wear rate and wear depth for the analyzed retrievals was 30 mm3/year and 0.2 mm/year, respectively. From SEM analysis of the retrievals it was found that at macroscopic level the worn surfaces consist of (a) polished and (b) scratched regions. Intensive wear areas and fine multidirectional scratches were dominant in all retrievals. Random scratches were also observed, most probably due to third body wear (cement particles). In two implants pitting areas and surface micro-cracks have been observed, which most likely result from subsurface fatigue.

The Influence of Process and Material Variables on the Surface Properties of Polystyrene Matrix Reinforced with Silica Sand

The surface properties of polymer matrix composite materials reinforced with Jordanian silica sand and cement subjected to abrasive wear has been studied in this investigation. Process variables, such as velocity, load, and time and material variables, such as volume fraction of the reinforcement and particle size are studied using weight loss measurements. Wear mechanisms responsible for the observed variation in wear rate are also studied using a scanning electron microscope. It is concluded that the introduction of SiO2 hard particles significantly increases the abrasive wear resistance of polystyrene. The higher each of the process variables (load, velocity, and time), the higher the dimensionless wear rate, and the difference between the highest and the lowest wear rates decreases as the volume fraction content and the particle size increase. The higher each of the material variables (reinforcement content and particle size), the lesser the wear rate for fixed process conditions. It is also found that abrasive wear of SiO2-reinforced polymers occurs due to three wear mechanisms: microploughing, microcutting, and microcracking.

Tribology of polymers: Adhesion, friction, wear, and mass-transfer

Tribological behavior of polymers is reviewed since the mid-20 th century to the present day. Surface energy of different coatings is determined with new contact adhesion meter. Adhesion and deformation components of friction are discussed. It is shown how load, sliding velocity, and temperature affect friction. Different modes of wear of polymers and friction transfer are considered.

Wear Mechanisms of Untreated and Gamma Irradiated Ultra-High Molecular Weight Polyethylene for Total Joint Replacements

Modification of the surface microstructure of ultra-high molecular weight polyethylene (UHMWPE) is essential for improving the wear resistance of orthopedic implants. A common approach is to cross-link the polymer by gamma irradiation. The objective of this study was to examine the tribological behaviors of untreated and gamma irradiated UHMWPE under physiologically relevant contact conditions. Emphasis was placed on the identification of the dominant wear mechanisms in the early stage of polymer wear. The irradiation dose exhibited a strong effect on the tribological properties of UHMWPE sliding against Co–Cr alloy in a bath of bovine serum. Transmission electron microscopy (TEM) and environmental scanning electron microscopy (ESEM) were used to examine the microstructure and morphology of the worn surfaces. Regularly spaced folds with average spacing depending on the irradiation dose (i.e., cross-link density) formed on the wear tracks. Surface folding was related to plastic flow and the degree of mobility of the crystalline lamellae. SEM and TEM results elucidated the roles of the cross-link density and crystalline lamellae in the wear process. Based on the experimental evidence, a deformation model was obtained that provides explanation for the dependence of surface folding on the cross-link density and lamellae reorientation during sliding.

Large-scale tests on friction and wear of engineering polymers for material selection in highly loaded sliding systems

Tribological properties of polymer materials are traditionally tested on small-scale test rigs because of cost-efficiency and flexibility. For highly loaded applications (e.g., sea lock doors, retaining walls, positioning systems) however, large-scale tests should be performed for estimating the frictional behaviour and wear lifetime of different engineering polymers more accurately under nearly working conditions. In present paper, two filled polymer grades covering a broad range of mechanical properties polyethylene terephtalate (PET)/teflon (PTFE) and ultra high molecular weight polyethylene (UHMWPE)/carbon are slid against high alloy steel, stainless steel and epoxy resin, in order to make a selection for its use as bearing material under dry and lubricated conditions. A general thermal model is evaluated for present test rig. The coefficients of friction on large-scale tests are lower than those obtained from small-scale testing. Polymer wear rates are calculated from weight loss and compared to dimensional measurements (thickness reduction) including elastic deformation, thermal expansion and creep. Microscopic observation of the worn surfaces shows different wear mechanisms of PET/PTFE after sliding against stainless steel, developing a transfer layer onto the polymer surface, and after sliding against high alloy steel, causing polymer transfer onto the steel surface. The better adherence of a polymer transfer film onto a steel counterface is explained by higher attractive forces resulting from its high surface energy, while there is little adherence on stainless steel counterfaces in accordance with its lower surface energy and lower friction. Wear of PET/PTFE after sliding against stainless steel is partly caused by the incorporation of steel particles. After sliding of UHMWPE/carbon, no wear debris was observed as its higher toughness allows for tearing of the surface without particle detachment. The high surface toughness and abscence of a transfer film results however in higher friction.

Determination of Friction and Wear of Engineering Polymers by Means of Large-Scale Specimen Testing

Although tribological tests on polymers are traditionally performed on small-scale pin-on-disc or bloc-on-ring configurations, present sliding tests under high load provide more accurate de-sign data. For wear tests on large samples edge effects, stress concentrations and the moveability of wear debris into the contact zone are simulated close to practice. It is illustrated that friction is gene-rally lower compared to small-scale tests, while overload and deformation occur more frequently.

A note on the effect of temperature on the friction and wear behaviour of carbon-fibre-reinforced Polyetheretherketone (PEEK-CF30) at dry sliding

In this study, an evaluation was made of the composite PEEK-CF30 tribological (friction and wear) behaviour at selected temperatures in a range of 60–180°C and at pv = 0.5, 1.5 and 3.0 MPa m/s. The friction and wear tests were performed on a pin-on-disc machine. As testing temperature increased from below to above the glass transition temperature (Tg) of the polymer matrix, wear and friction transitions occurred. An increase in testing temperature resulted in higher specific wear rates and, in general, in lower friction coefficient.

Influence of yield strength and toughness on friction and wear of polycarbonate

Polycarbonate was chosen as a model material to study the influence of yield strength and toughness on the tribological behaviour of the polymer. Yield stress and toughness of polycarbonate were varied by controlled physical ageing or by mechanical or thermal rejuvenation. The yield stress ranged from about 40 MPa for mechanically rejuvenated to 70 MPa for annealed polycarbonate. The area under the stress-strain curve is considered to be a measure for the toughness. The strain-to-break decreases with physical ageing, which results in a decrease of the toughness of the polymer. It was shown that yield strength and toughness did not affect friction and abrasive wear of polycarbonate under several different test conditions. These findings are compared with the usual views in literature upon friction and wear of polymers. There are strong indications that the polymer network of polycarbonate plays an important role in the wear resistance. However, no wear improvement was found with an increase in molecular weight in the range of polycarbonates studied.

An easy technique to digest and isolate UHMWPE wear particles from a hip joint simulator.

The size and morphology of polyethylene wear particles isolated from in vitro tests were analysed in this study. There are some major controversies emerging in the literature that will only be resolved by careful particles analysis. Since it is difficult to identify the factors that affect size and morphology of the wear particles, the goal of this study was to compare four different polymer wear debris isolation techniques (base method and acid treatment) under identical conditions in a hip joint simulator to obtain polyethylene wear particles. We investigated achieving particulate isolation by using a different approach to the one reported in the literature that involved an easier and streamline method of particle debris isolation. The new method, compared to the previous one, used a strong base, normal centrifugation, and filtration to digest the serum constituents thus isolating the polyethylene particle debris from lubricant. In all four methods the isolation involved some or all of the following steps: density gradient separation, centrifugation or ultracentrifugation, and washing. However, a requirement for all these techniques was that the lubricant had to be devoid of organic compounds in order to effectively image and count the particle debris. The results from these studies clearly show that this new method of particle isolation is easier and more streamlined than the three methods analysed and reported in literature because it did not involve ultracentrifugation and is quicker.

Artificial neural network predictions on erosive wear of polymers

In the present paper, an artificial neural network (ANN) approach was applied to the erosive wear data of three polymers, i.e. polyethylene (PE), polyurethane (PUR), and an epoxy modified by hygrothermally decomposed polyurethane (EP-PUR). Three independent datasets of erosive wear measurements and characteristic properties of these polymers were used to train and test the neural networks. For the first two material examples, the impact angle of solid particle erosion and some characteristic properties were selected as ANN input variables. Whereas the third one, material compositions, i.e. epoxy and HD-PUR weight contents, were also involved as additional ANN input variables. In all these cases, the output parameter was the erosive wear rate. Acceptable ANN predictive qualities were reached, demonstrating that ca. 35–80% of the randomly selected test dataset had a coefficient of determination B≥0.9 for these three cases, respectively. Ranking of the importance of characteristic properties to erosive wear rate could offer some information about which property has a stronger relationship to wear in each polymer case. Even though the ANN approach is only a phenomenological method, a well-trained ANN is believed to be also of help for a mechanistic understanding of the problem considered.

Modeling of Fatigue Crack Propagation During Sliding Wear of Polymers

The propagation of a crack initiating at the surface was analyzed to simulate the fatigue wear behavior of glassy polymer materials. A crack in a material half plane was assumed to propagate along a predefined path as a result of contact loading by a cylinder sliding on the polymer surface. The crack path consisted of a vertical straight-line segment and a declined straight line originating at a branch point on the vertical crack segment. The stress intensity factors KI and KII along the crack path were computed by using finite element methods, and their values utilized in the Paris law to determine crack propagation rates. Because this process simulates surface pitting, component fatigue life is assumed to be proportional to the time needed for the propagating declined crack to intersect a neighboring vertical crack, a condition known to lead to pitting. This fatigue life is estimated by integrating the Paris law. Numerical results show that the branch point where the declined crack path originates can effectively hinder crack propagation, and that the rate limiting step in fatigue is crack propagation along a small segment of the declined crack near the branch point. Some important factors that affect the reliability of numerically predicted fatigue life cycles are discussed. Experimental crack propagation paths and lifetimes are shown.

Modelling the three-body abrasive wear of UHMWPE particle reinforced composites

In this paper, some of the complexities in developing predictive models for polymer composite materials are discussed with particular reference to models based on surface deformation energy. Although there are models available for abrasive wear of metals, polymers and composite materials, models are suitable for a particular wear situation. Most of the cases, the wear models are based on traditional test methods, such as two-body abrasive wear test or tests in a controlled environment. Moreover models for polymer composite materials are very limited. This is due to the fact that the vast range of polymers and polymer composite materials present different wear behaviour in similar wear situations. This makes the modelling wear of polymer composite materials complicated. This paper presents and discusses recent test results on ultra high molecular weight polyethylene (UHMWPE) particle reinforced composites with varying particle concentration from a unique test rig that exposes the wear surface to sliding bulk solids (granular materials). The wear phenomena of resin and UHMWPE particle reinforced resin surfaces abraded by bulk solids are discussed with respect to the interaction between the particles and the wear surfaces. The effects of elastic and plastic deformation energy of the surfaces on the abrasive wear resistance are also discussed in the context of an appropriate wear model for these surfaces and wear events. Finally a model is proposed to predict the wear of these polymer composites based on the deformation energy of the surfaces. Good qualitative agreement was obtained between the model and the experimental wear rates.

Nano-rheological properties of polymeric third bodies generated within fretting contacts

The accumulation and compaction of polymer wear particles into coherent third body compacts has been investigated in a contact between steel and poly(methylmethacrylate) under small amplitude cyclic micro-motions, i.e. fretting. Under such contact conditions, the preferential accumulation of the PMMA wear particles at the centre of the contact was found to result in the formation of a distinct third body agglomerate. Hardness and modulus measurements carried out at the nanoscale revealed that the polymer wear particles within this third body formation were progressively compacted to form an agglomerate, for which the mechanical properties were ultimately close to those of the initial PMMA substrate. The efficiency of the third body compaction process was interpreted by considering the evolving load-carrying capacity of the contact, which was characterised by progressive redistribution of the contact pressure within the third body layer. In addition, an assessment of the dissipation of frictional energy within the worn contact area demonstrated that particle compaction was enhanced by the highly energy-intensive nature of the fretting process.