Softening and melting mechanisms of polyamides interfering with sliding stability under adhesive conditions

The thermal stability of polymers is a main issue when used as friction elements under dry sliding. Cast polyamide grades processed with either natrium or magnesium catalysors are slid on a small-scale and a large-scale test configuration to reveal the effect of softening or degradation on the sliding stability and to investigate possibilities for extrapolation of friction and wear rates between both testing scales. The combination of softening and afterwards transition into the glassy state is detrimental for the sliding stability of natrium catalysed polyamides, characterised by heavy noise during sliding. A transfer film formed under continuous softening also provides high friction. Melting during initial sliding is necessary for stabilisation in both friction and wear, and eventual softening of a molten film near the end of the test then not deteriorates the sliding stability. Softening of magnesium catalysed polyamides is favourable for the formation of a coherent transfer film resulting in more stable sliding than natrium catalysed polyamides. The differences in softening mechanisms of both polyamide grades is correlated to structural changes investigated by thermal analysis and Raman spectroscopy: the γ crystalline structure prevails in magnesium catalysed samples and the α crystalline structure is predominant in natrium catalysed samples. For internal oil lubricated polyamides, a time dependent degradation of the polyamide bulk deteriorates the supply of internal oil lubricant to the sliding interface, resulting in high friction and wear under overload conditions. As the degradation mechanisms during sliding are strongly correlated to the test set-up, extrapolation is only possible for friction in a limited application range, while wear rates cannot be extrapolated.     

Tribological behavior of pure and graphite‐filled polyimides under atmospheric conditions

As the use of common engineering plastics in tribological systems is limited to low sliding velocities and low loads because of creep and insufficient temperature resistance, there is increasing interest in application of high‐performance polymers such as polyimides, characterized by their ability to maintain favorable mechanical properties up to their melting point. However, for practical design, tribotesting remains necessary for determination of the material's performance under a given contact situation. In this article, two commercially available polyimides are tested at relatively high sliding velocities and contact pressures under atmospheric conditions of temperature and humidity. A consistent overview of tendencies in friction and wear for pure polyimides as a function of applied normal loads and sliding velocities is given. Addition of 15% by weight graphite powder as internal solid lubricant strongly influences friction and wear. Its behavior is compared with pure polyimide grades and differences are discussed in relation with experimental measured bulk‐temperatures. A linear temperature law is derived as a function of pv‐levels and a steady‐state condition is found at different temperature levels, in accordance with thermal conductivity of the polymer bulks. In case of graphite additives, a steady state in temperature coincides with the regime condition of wear rate.     

Fracture Assessment of Carbon Fibre/Epoxy Reinforcing Rings through a Combination of Full-Scale Testing, Small-Scale Testing and Stress Modeling

Carbon fibre/epoxy rings are used as radial reinforcement for polymer bearing elements with nominal diameter 250 mm functioning under 150 MPa. Full-scale static and dynamic testing revealed no catastrophic failure for loading to 400 MPa, although there was circumferential splitting of carbon fibres at the machined top edge causing counterface wear under sliding. A combined numerical–experimental analysis was applied for design improvement with a representative small-scale qualification test on the real ring geometry, inducing additional stress concentrations compared to ASTM standards. Full-scale modelling revealed high radial–axial shear stresses (33 MPa) in non-hydrostatically loaded zones, while it increased towards 104 MPa under hydrostatic load conditions. The former is the most critical and should be simulated either on a small-scale unidirectional compression test or on a representative short beam shear test, respectively, measuring the radial–axial or radial–tangential shear strength. A relation between both small-scale states of stress was experimentally and numerically studied, experiencing that the composite ring has lower radial–tangential shear stress compared to radial–axial shear stress as a different hydrostatic stress state is observed in the bulk of the composite ring. As a compressive test is however more difficult to perform than a short-beam-shear test, a representative design criterion for shear fracture is determined from failure at 27 kN normal load in a short-beam-shear test. Finally, fracture is avoided by optimising the cross-sectional geometry of the composite reinforcing ring and close control of the processing parameters.     

Friction and Thermal Effects of Engineering Plastics Sliding Against Steel and DLN-Coated Counterfaces

Polymers are increasingly used in tribological applications, because of their self-lubricating ability, corrosion resistance and chemical compatibility. However, their performance depends strongly on the parameters of the total tribological system. Not only polymer characteristics, but also counterface properties become important because of their influence on friction and wear, on surface energy and on the thermal conductivity of the total system. Applying a Diamond-Like Nanocomposite (DLN) coating on a steel counterface can improve the tribological behaviour of the sliding couple under certain conditions. In the case of metal sliding against DLN, the high hardness and the wear resistance of the coating is advantageous for better tribological properties. However, for polymers sliding against DLN, the lower thermal conductivity of the DLN coating compared with a steel mating surface dominates friction and wear. In case of polyamides this results in worse tribological performance in contact with the DLN coating, because of polymer melting. In the case of more rigid polymers, such as, e.g., POM-H and PETP, lower coefficients of friction lead to lower frictional heat generation. In these cases, the thermal characteristics of the counterface are less important and the lower surface energy of the DLN coating is favourable for decreased adhesion between the polymer and the coating and consequently better tribological properties.     

Should a bicuspid aortic valve be replaced in the presence of subvalvar or supravalvar aortic stenosis?

BACKGROUND: A bicuspid aortic valve is commonly associated with other levels of left ventricular outflow tract obstruction. Providing the bicuspid aortic valve is competent and nonobstructive, repair of subvalvar or supravalvar stenosis usually focuses on the obstructive lesions, leaving the valve in situ. The aim of this report was to examine the impact of a bicuspid aortic valve on the risk of reoperation for patients undergoing operation for subvalvar or supravalvar aortic stenosis. METHODS: Since 1976, 47 patients with supravalvar or subvalvar aortic stenosis have undergone repair. The median follow-up is 5.1 years (range, 2 months to 20.1 years). Sixteen patients (34%) had a bicuspid aortic valve that was competent and nonobstructive, and 31 (66%) had a tricuspid aortic valve. RESULTS: Reoperation was required in 9 patients (56%) with a bicuspid aortic valve, in each involving aortic valve replacement with an autograft (3), homograft (2), or prosthesis (4). Six patients (19%) with a tricuspid aortic valve required reoperation, yet only 1 required aortic valve replacement. The freedom from valve replacement was 43% (70% confidence interval, 31% to 55%) in the bicuspid aortic valve group versus 100% (70% confidence interval, 94% to 99.5%) in the tricuspid group at 5 years (p = 0.0001). The freedom from any reoperation at 5 years was 43% (70% confidence interval, 31% to 55%) in patients with a bicuspid aortic valve versus 86% (70% confidence interval, 80% to 93%) in the tricuspid group (p = 0.02). CONCLUSIONS: The data suggest that patients with subvalvar or supravalvar aortic stenosis and a bicuspid valve may be better palliated with a more definitive operation such as the Ross or Ross-Konno procedure.     

Friction of polyoxymethylene homopolymer in highly loaded applications extrapolated from small-scale testing

Most studies on tribology of polymer materials are traditionally performed on small-scale test specimens. However, to obtain data relevant for practical design of polymer parts in highly loaded bearings or sliding systems one must simulate real working conditions as close as possible on laboratory scale. In present work, a large-scale test rig has been used for determinating friction of a commercial polyoxymethylene homopolymer (POM-H). Test samples with contact area 22500 mm² are submitted to a reciprocating motion with stroke 230 mm under different contact pressures from 8 to 150 MPa and sliding velocity of 5 mm/s. Test results are compared to those obtained on a traditional cylinder-on-plate configuration and reveal lower friction on large-scale tests. However, general laws predicting the coefficient of friction as a function of normal loads cannot be used for extrapolation. Bulk and flash temperatures are calculated to explain transitions in friction mechanisms on both testing scales. Local surface temperatures are higher under large-scale sliding and allow for surface melting, which is not observed on small-scale tests. Although, sliding conditions implying an identical flash temperature induce polymer transfer only on large-scale tests, while no transfer occurs on small-scale tests. Even an artificial increase in small-scale bulk temperatures allowing for polymer transfer, is not able to provide low friction as observed on large-scale. An appropriate combination of load and sliding velocity is used for linear extrapolation of friction values, indicating the additional importance of bulk temperatures. When the polymer softening point is exceeded, creep at the polymer surface contributes to low friction.     

Friction, wear and material transfer of sintered polyimides sliding against various steel and diamond-like carbon coated surfaces

Polyimide cylinders are slid under 50 N normal load and 0.3 m/s sliding velocity against carbon steel (R_a=0.2 and 0.05 μm), high-alloy steel (R_a=0.05 μm), diamond-like carbon (DLC, R_a=0.05 μm) and diamond-like nanocomposite (DLN, R_a=0.05 μm). Only for a limited range of test parameters, the friction of polyimide/DLN is lower than for polyimide/steel, while polyimide shows higher wear rates after sliding against DLN compared to steel counterfaces. The DLN coating shows slight wear scratches, although less severe than on DLC-coatings that are worn through thermal degradation. Therefore, also friction against DLC-coatings is high and unstable. Calculated bulk temperatures for steel and DLN under mild sliding conditions remain below the polyimide transition temperature of 180 °C so that other surface characteristics explain low friction on DLN counterfaces, as surface energy, structural compatibility and transfer behaviour. Friction is initially determined through adhesion and it is demonstrated that higher surface energy provides higher friction. After certain sliding time, different polyimide transfer on each counterface governs the tribological performance. Polyimide and amorphous DLC structures are characterised by C–C bonds, showing high structural compatibility and easy adherence of wear debris on the coating. However, it consists of plate-like transfer particles that act as abrasives and deteriorate the polyimide wear resistance. In sliding experiments with high-alloy steel, wear debris is washed out of the contact zone without formation of a transfer film. Transfer consists of island-like particles for smooth carbon steel and it forms a more homogeneous transfer film on rough carbon steel. The latter thick and protective film is favourable for low wear rates; however, it causes higher friction than smooth counterfaces.     

Review: nanoparticles and nanostructured materials in papermaking

The introduction of nanoparticles (NPs) and nanostructured materials (NSMs) in papermaking originally emerged from the perspective of improving processing operations and reducing material consumption. However, a very broad range of nanomaterials (NMs) can be incorporated into the paper structure and allows creating paper products with novel properties. This review is of interdisciplinary nature, addressing the emerging area of nanotechnology in papermaking focusing on resources, chemical synthesis and processing, colloidal properties, and deposition methods. An overview of different NMs used in papermaking together with their intrinsic properties and a link to possible applications is presented from a chemical point of view. After a brief introduction on NMs classification and papermaking, their role as additives or pigments in the paper structure is described. The different compositions and morphologies of NMs and NSMs are included, based on wood components, inorganic, organic, carbon-based, and composite NPs. In a first approach, nanopaper substrates are made from fibrillary NPs, including cellulose-based or carbon-based NMs. In a second approach, the NPs can be added to a regular wood pulp as nanofillers or used in coating compositions as nanopigments. The most important processing steps for NMs in papermaking are illustrated including the internal filling of fiber lumen, LbL deposition or fiber wall modification, with important advances in the field on the in situ deposition of NPs on the paper fibers. Usually, the manufacture of products with advanced functionality is associated with complex processes and hazardous materials. A key to success is in understanding how the NMs, cellulose matrix, functional additives, and processes all interact to provide the intended paper functionality while reducing materials waste and keeping the processes simple and energy efficient.     

Tribological properties of PTFE‐filled thermoplastic polyimide at high load,velocity, and temperature

The effect of 20 wt% polytetrafluoroethylene (PTFE) fillers on the friction and wear properties of thermoplastic polyimides (TP) are investigated, under dry sliding in line contact against steel under 50 to 200 N, 0.3 to 1.2 m/s, and 60 to 260°C. Besides the lubricating mechanisms of PTFE based on mechanical shear, the thermal and tribophysical interactions in the sliding interface are considered in this research by using thermoanalytical measurements, Raman spectroscopy, and calculating the maximum polymer sliding temperature T*. The effect of hydrolysis of the TP bulk material, causing high friction at 100 to 140°C, is covered by PTFE. A transition at pv‐values 2.2 MPa m/s (T* = 120°C) is due to thermally controlled sliding of PTFE, while a transition at pv‐values 3.2 MPa m/s (T* = 180°C) remains controlled by degradation of the TP bulk material into monomer fractions. The reduced coefficient of friction in the presence of PTFE leads to smaller degradation and orientation of the molecular back‐bone and side‐chains within the TP structure. The formation of a homogeneously mixed transfer film is only observed at 180 to 260°C. The PTFE forms a fibrillar structure during wear at high sliding velocities, while they wear as separate particles at high normal loads. POLYM. COMPOS., 2009. © 2009 Society of Plastics Engineers     

Wear behavior of carbon fiber‐reinforced poly(phenylene sulfide)

On a pin‐on‐disc test rig, online measurements of the wear and friction of steel sliding against carbon fiber‐reinforced polyphenylene sulfide were done. Instead of the standard set‐up, a rotating composite disc and steel pin are used. The frictional behavior of this material pair results in a friction coefficient of 0.33, while a carbide film is formed in the wear track. This results in the lowering of the frictional behavior. The wear rate is rather low, but when the wear track is covered with a carbide film, suddenly the wear rate raises. This is not due to the wear of the composite material but only as a result of the start of terrible wear of the steel counter face. Moreover, the wear of the pin is strongly related to the wear track. The pin is flattened at the sides of the formed wear track, but in contact with the carbide film there is less wear, resulting in a pin with two flat sides, making contact with the original surface, and a rig in the middle of the pin following the roundness of the wear track. The frictional behavior is strongly dependent on the weft‐warp direction. POLYM. COMPOS., 27:92–98, 2006. © 2005 Society of Plastics Engineers