Tribological Materials Research Articles

윤활영역에서 멀티크기 Laser Surface Texturing 효과

Laser Surface Texturing(LST) is a surface engineering process used to improve tribological characteristicsof materials by creating patterned microstructures on the mechanical contact surface. In LST technology, a pulsatedlaser beam is used to create arranged dimples on a surface by a material ablation process, which can improve such asload capacity, wear resistances, lubrication lifetime, and reduce friction coefficients. In the present study, the effect ofmulti-scale LST on lubricant regime was investigated. A pulsed Nd:YAG laser was applied on the bearing steel(AISI52100) to create arranged dimples. To optimize the surface texturing effect on friction, multi-scale texture dimples withsome specific formula arrays were fabricated by combining circles, ellipses and the laser ablation process. The tri-bological testing of multi-scale textured surface was performed by a flat-on-flat unidirectional tribometer under lubri-cation and the results compared with that of the non-textured surface. Through an increase in sliding speed, thebeneficial effect of multi-scale LST performance was achieved. The multi-scale textured surface had lower frictioncoefficient performances than the non-textured surface due to the hydrodynamic lubrication effect.Keywords: laser surface texture (레이저 표면 텍스처), lubrication regime (윤활영역), friction (마찰), surfacemodification (표면개질)

Tribology and Micromechanics of Chromium Nitride Based Multilayer Coatings on Soft and Hard Substrates

The tribological protection of carbon fiber reinforced epoxy composites (CFC) is essential for broadening their use from structural to functional applications, e.g., to linear bearings in mechanical engineering. However, their wear resistance in sliding and rolling contacts is low. This work focusses on the possibility of improving their tribological properties by the application of thin hard multi-layered coatings. Chromium nitride (CrN) single layer and chromium-CrN multilayer coatings of ~4 µm thickness, partly finished with a 1 µm diamond-like carbon (DLC) top layer, were deposited by magnetron sputtering at low temperatures on soft CFC and for comparison of the mechanical behavior on comparatively hard austenitic steel substrates. Structural investigations showed especially that the multilayer coatings possess a very fine grained, columnar microstructure and a very low density of intercolumnar micro-cracks, while the single layer coatings possess a coarse structure. The indentation testing and the analysis of the deformed and fractured cross-sections revealed a tougher behavior with improved plastic deformability of the multilayers in comparison to CrN single layers. However, in wear testing only coatings with DLC top layers significantly improved the tribological material properties of CFC. This is due to the reduced shear forces in sliding on low-friction DLC coatings on the soft epoxy-based CFC, decreasing the total dynamic stresses during sliding under high loads.

Modeling and Contact Analysis of Composite (FRP) Material Lamination on Cylindrical Roller Bearing

Fiber reinforced polymer (FRP) composites are an important class of tribological materials. They possess unique self -lubrication capabilities and low noise which make them suitable for applications like seals, bearings, gears and artificial prosthetic joints. The FRP composite bearings are ideal for high load low speed applications or where normal lubrication is difficult or costly. The friction and wear behavior of FRP composites varies with fiber orientation and sliding direction. For the purpose of fully utilizing the beneficial contact characteristics of FRP composite, it is necessary to obtain an in-depth knowledge of their contact behavior. In this work, the compliance behavior of FRP composite bearings is studied. The frictional sliding contact between a FRP composite and a rigid parabolic cylinder is analyzed. The influence of sliding direction, fiber and matrix material combinations, volume fraction of the fiber, frictional coefficient and fiber ply orientation on the contact pressure distribution and the contact area for unidirectional FRP composite bearings are evaluated. A finite element model is developed using ANSYS12.0 and the results obtained from the analysis are compared with the analytical results. The influence of sliding direction on the contact pressure distribution for cross FRP composite bearings is studied and compared with unidirectional FRP composite bearings.

Real time acoustic emission methodology in effective tribology testing

The last two decades saw an explosive growth of research activities in non–destructive testing and quality control of a broad variety of tribological materials and units. The substantial amount of data has been accumulated to date, proving the efficiency of the modern acoustic emission (AE) technique for tribological examinations of various materials and coatings. In the present work, we review the available knowledge on this topic and align it with our original investigations, bridging standard tribological practice and modern AE technologies aimed at enhancing reliability and effectiveness of testing. Special attention is paid to quantification of the correlation between the AE features and friction induced damage.

自動車エンジン用トライボロジー材料技術の動向

自動車エンジン用トライボロジー材料技術の動向

Novel W-based metallic glass with high hardness and wear resistance

An attempt has been made to develop a new metallic glass (MG) that combines high hardness with wear resistance. Refractory metallic films of W33Ni32B35 (at.%) have been deposited on stainless steel and Si substrates by dc magnetron sputtering. The alloy films are glassy, have a high crystallization temperature of 873 °C and rank among the very hard metallic materials (∼24 GPa). Importantly, this MG also shows excellent wear resistance, approaching that of standard tribological materials like TiN and hence it represents one of the most wear-resistant known metallic materials. Based on its unique combination of high strength and low elastic modulus, other potential applications are also discussed.

Study on the Tribological Behavior of the Foamed Copper Filled with Epoxy Matrix

In order to improve the thermal performance and wear resistance of the polymer, foamed copper filled by the curable epoxy-matrix composite are developed as tribological materials. Graphite and multi-walled carbon nanotubes (MWCNTs) were incorporated the contents as friction additives. The tribological properties of the foamed-copper reinforced composites were investigated on an UMT-2 friction and wear tester. An electric field was imposed between the specimen and disc to monitor the formation of transfer film by means of contact resistance. It is found that the foamed-copper reinforced composites possess better wear resistance than homologous polymers, and the smaller is the aperture, the better the wear resistance. While the friction coefficients increase for the local direct contact between the copper and the steel disc. The copper skeletons contribute to the timely transfer of friction heat and the load-sharing. The foamed-copper unit was modeled from the foaming mechanism of the polyurethane foam. Two parameters of the surface area density of foamed copper and the area ratio of copper at the friction interface were quantized based on two kinds of typical contact models. The modeling analysis can lead to a better understand the influence of metallic skeletons on the wear performance of composite.

An analytical model to predict interstitial lubrication of cartilage in migrating contact areas

For nearly a century, articular cartilage has been known for its exceptional tribological properties. For nearly as long, there have been research efforts to elucidate the responsible mechanisms for application toward biomimetic bearing applications. It is now widely accepted that interstitial fluid pressurization is the primary mechanism responsible for the unusual lubrication and load bearing properties of cartilage. Although the biomechanics community has developed elegant mathematical theories describing the coupling of solid and fluid (biphasic) mechanics and its role in interstitial lubrication, quantitative gaps in our understanding of cartilage tribology have inhibited our ability to predict how tribological conditions and material properties impact tissue function. This paper presents an analytical model of the interstitial lubrication of biphasic materials under migrating contact conditions. Although finite element and other numerical models of cartilage mechanics exist, they typically neglect the important role of the collagen network and are limited to a specific set of input conditions, which limits general applicability. The simplified approach taken in this work aims to capture the broader underlying physics as a starting point for further model development. In agreement with existing literature, the model indicates that a large Peclet number, Pe, is necessary for effective interstitial lubrication. It also predicts that the tensile modulus must be large relative to the compressive modulus. This explains why hydrogels and other biphasic materials do not provide significant interstitial pressure under high Pe conditions. The model quantitatively agrees with in-situ measurements of interstitial load support and the results have interesting implications for tissue engineering and osteoarthritis problems. This paper suggests that a low tensile modulus (from chondromalacia or local collagen rupture after impact, for example) may disrupt interstitial pressurization, increase shear stresses, and activate a condition of progressive surface damage as a potential precursor of osteoarthritis.

Effect of Annealing Temperature on Tribological Properties and Material Transfer Phenomena of CrN and CrAlN Coatings

This study evaluates the effects of annealing temperature and of the oxides produced during annealing processes on the tribological properties and material transfer behavior between the PVD CrN and CrAlN coatings and various counterface materials, i.e., ceramic alumina, steel, and aluminum. CrAlN coating has better thermal stability than CrN coating in terms of hardness degradation and oxidation resistance. When sliding against ceramic Al2O3 counterface, both CrN and CrAlN coatings present excellent wear resistance, even after annealing at 800 °C. The Cr-O compounds on the coating surface could serve as a lubricious layer and decrease the coefficient of friction of annealed coatings. When sliding against steel balls, severe material transfer and adhesive wear occurred on the CrN and CrAlN coatings annealed at 500 and 700 °C. However, for the CrAlN coating annealed at 800 °C, much less material sticking and only small amount of adhesive wear occurred, which is possibly due to the formation of a continuous Al-O layer on the coating outer layer. The sliding tests against aluminum balls indicate that both coatings are not suitable as the tool coatings for dry machining of aluminum alloys.

TRIBOLOGY OF BIO-INSPIRED NANOWRINKLED FILMS ON ULTRASOFT SUBSTRATES

Biomimetic design of new materials uses nature as antetype, learning from billions of years of evolution. This work emphasizes the mechanical and tribological properties of skin, combining both hardness and wear resistance of its surface (the stratum corneum) with high elasticity of the bulk (epidermis, dermis, hypodermis). The key for combination of such opposite properties is wrinkling, being consequence of intrinsic stresses in the bulk (soft tissue): Tribological contact to counterparts below the stress threshold for tissue trauma occurs on the thick hard stratum corneum layer pads, while tensile loads smooth out wrinkles in between these pads. Similar mechanism offers high tribological resistance to hard films on soft, flexible polymers, which is shown for diamond-like carbon (DLC) and titanium nitride thin films on ultrasoft polyurethane and harder polycarbonate substrates. The choice of these two compared substrate materials will show that ultra-soft substrate materials are decisive for the distinct tribological material. Hierarchical wrinkled structures of films on these substrates are due to high intrinsic compressive stress, which evolves during high energetic film growth. Incremental relaxation of these stresses occurs by compound deformation of film and elastic substrate surface, appearing in hierarchical nano-wrinkles. Nano-wrinkled topographies enable high elastic deformability of thin hard films, while overstressing results in zigzag film fracture along larger hierarchical wrinkle structures. Tribologically, these fracture mechanisms are highly important for ploughing and sliding of sharp and flat counterparts on hard-coated ultra-soft substrates like polyurethane. Concentration of polyurethane deformation under the applied normal loads occurs below these zigzag cracks. Unloading closes these cracks again. Even cyclic testing do not lead to film delamination and retain low friction behavior, if the adhesion to the substrate is high and the initial friction coefficient of the film against the sliding counterpart low, e.g. found for DLC.

Failure mode in sliding wear of PEEK based composites

Polymers and polymer composites are commonly used as solid lubricants. Polyether–ether–ketone (PEEK) is an engineering thermoplastic with a very good combination of thermal and mechanical properties. Although PEEK presents relatively high friction coefficients during unlubricated sliding, the wear rates are remarkably low. Despite this potential as a good tribological material, the high performance abilities of PEEK have not been entirely realized in practice because PEEK-ferrous metal contacts may fail by scuffing and/or two-three body abrasion for certain tribosystems. The traditional design approach to reinforce PEEK has been to use fibres for strengthening and filler particles for lubrication. In this work three materials were analysed: PEEK, carbon-fibre (CF) reinforced PEEK and PTFE+Graphite+CF filled PEEK. A series of different tribo-tests was carried out: (i) Reciprocating linear scuffing test with 7N incremental loading every 10min up to 910N; (ii) constant load (10N, 5h duration), reciprocating sliding tests assessing friction coefficient and wear rates of both specimen and counter body; (iii) free ball micro-abrasion tests using soft, fine abrasive particles. The wear tracks were analysed using SEM and light interferometry in order to assess the morphology of the wear tracks during scuffing, abrasion and sliding tests. The aim was to identify whether the failure mode in the sliding wear of these PEEK based materials is dominated by scuffing or by abrasive mechanisms. Unfilled PEEK exhibited very low scuffing resistance and a large wear rate during reciprocating sliding wear and abrasion, presenting a clear tendency to fail by abrasion mechanisms. CF-reinforced PEEK also presented very low scuffing resistance but higher sliding and micro abrasive wear resistance. It seemed that the presence of CF enhanced protection against abrasion by minimizing plastic deformation, although flaky debris probably originated from subsurface cracking were observed. The addition of PTFE and Graphite to the CF-reinforced PEEK produced a sharp decrease in friction coefficient, high scuffing and abrasion resistance, with an almost non-measurable wear rate in the reciprocating sliding test. The behaviour observed for PTFE+Graphite+CF filled PEEK suggested the transfer of a protective tribo-layer from the composite to the counter-body and vice-versa.

Tribo-pair dependence of friction and wear moisture sensitivity for a-C:Si:Al carbon-based coating

Providing a low, predictable and stable friction as well as low wear for mechanical components across a wide range from dry to wet atmosphere is of great importance in many engineering applications, the urgent challenge is to control tribological moisture sensitivity of their protective coatings. In this paper, the a-C:Si:Al carbon-based coating was fabricated successfully using multifunctional magnetron sputtering system. The microstructure and mechanical properties of as-fabricated coating were investigated; especially the friction and wear of coating sliding against Si3N4, SiO2, GCr15, brass and aluminum counterparts were evaluated under different relative humidity atmospheres. Results showed that the as-fabricated a-C:Si:Al carbon-based coating presented typical amorphous microstructure and good combined mechanical properties. Under low relative humidity conditions, the graphitized carbon tribo-layers on the contact surfaces were responsible for low friction; under high relative humidity conditions, the low shear strength colloidal silica on the contact surfaces induced by tribo-chemical reaction were mainly responsible for low friction. In particular, the lowest tribological moisture sensitivity can be achieved by the a-C:Si:Al/SiO2 pair, which was mainly attributed to the formation of tribo-layers on the contact surfaces induced by the graphitization and tribo-chemical reaction under different relative humidity conditions. These indicate that the a-C:Si:Al carbon-based coating might be a good candidate as low tribological moisture sensitivity material in engineering applications.

Wear of ceramic interfaces: Multiscale statistical simulation

When developing new tribological materials, such as high performance ceramics, the accurate estimation of their expected wear rate is a challenging goal due to the complexity of wear mechanisms and surface morphology and ultimately the multiscale nature of wear. The present work proposes a combination of modelling techniques, addressing these challenges. A dual scale model is formulated, with single-asperity interactions in the microscale and the interaction between stochastic surface morphologies in the macroscale. The microscale model is based on numerical simulations or alternatively to analytical models. The macroscale model, presented in this work, transforms surface topography into a multivariate distribution of asperity parameters and maps the microscale model's response; then it simulates the wear process via a Monte Carlo simulation, by querying the map and integrating over the interface. Challenges such as variable selection, mapping and profile evolution are addressed. The dry sliding interfaces of silicon nitride, tungsten carbide and zirconia, exhibiting abrasive wear mechanisms, are simulated and compared to a macrogeometric wear model based on Archard's law and experimental results.

Effect of reinforcement on wear debris of carbon nanofiber/high density polyethylene composites: Morphological study and quantitative analysis

Polymeric nanocomposites are promising tribological materials. However, the wear debris generated on the sliding surface of composite materials is highly affected by the nanofillers used (types, surfaces, etc.). In this study, based on the high density polyethylene (HDPE) composites with various carbon nanofibers (CNFs) prepared, the effects of CNF concentration and surface modification on morphology and size distribution of wear debris were examined. The individual wear particle morphology was observed with a scanning electron microscope (SEM). The particle size distributions of the wear debris were statistically analyzed based on the optical microscope imaging for all the debris collected after 24h of wear testing. In addition, the influence of various CNFs (untreated and organosilane treated) on debris components was analyzed by both differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) measurements. The results revealed that there were less mechanical damage and a lower proportion of large particles in the wear debris for the nanocomposites with heavily organosilane coated CNFs.

Vibration and noise in brake systems of mobile vehicles. Part 3: Forced low-frequency vibration (A Review)

The paper presents a review of studies of forced low-frequency vibration and noise in brakes. A phenomenological classification of brake noise is given. Ideas on the mechanisms of the origination of low-frequency vibration and the corresponding methods of study are considered. Reduction of the low-frequency vibroactivity of brakes is analyzed from the viewpoint of design factors and the development of tribological materials.

Friction and Wear of Pyrophyllite on the Atomic Scale

Pyrophyllite, Al2Si4O10(OH)2, a layered aluminosilicate material, has several properties which suggest that it might exhibit desirable frictional properties for solid-state lubrication. These include a lamellar structure and a low-energy basal plane. Here, we utilize atomic force microscopy to investigate the tribological performance of mineralogical pyrophyllite samples and compare the results to those of other minerals and solid lubricants. The results indicate that pyrophyllite exhibits low friction forces and shear stresses, on the same order of other commonly used solid lubricant materials, and a high resistance to deformation within a single layer. These fundamental findings highlight the potential use of pyrophyllite as a possible tribological material and alternative solid lubricant.

Wear and Life Characteristics of Microwave-Sintered Copper-Graphite Composite

Copper-graphite composite is an important tribological material used in electrical sliding contact applications like electrical brushes in motors and generators. The electrical sliding contact experiences multiple stresses such as mechanical pressure and temperature. Traditional life tests under normal operating condition would be a time-consuming process due to the longer expected life of the composite. Accelerated wear testing was carried out to evaluate the life characteristics of the composite. This work focuses on evaluation of tribological performance of microwave-sintered copper-graphite composite using accelerated wear testing methodology using high temperature pin-on-disc tribometer. Microstructural studies of worn out surfaces were carried out using SEM with EDAX. Reliability and analysis on life characteristics were performed on the time-to-failure data using temperature-nonthermal-accelerated life-stress model. The obtained times-to-failure data from the accelerated wear testing was extrapolated to normal usage condition. Temperature and pressure are significantly affecting the wear performance. Self-lubricating action of graphite and improvement in wear resistance is helpful in extending the life of copper graphite composite. The life of the composite obtained through testing at mean and 99% reliability are 18,725 and 16,950 h, respectively.

The Discussion about Thermal Stress Analysis of Spherical Plain Bearings

Spherical plain bearing was applied widely in industry. In the paper, with the finite element code Abaqus, a sequentially coupled, 3D, and thermal-mechanical finite element analysis model of the bearing system was built up based on the theories of heat transfer, tribology and composite material mechanics. The paper focused on effects of the mean sliding velocity and the load. In the end, the paper mentioned a simulation method of computing wear.

Friction of composites based on titanium carbide produced by the process combustion method

The method of SHS force compaction is used to produce composites that include TiC + 50 vol % inert. Copper, iron, nichrome, and steel G13 served as the inert. It is noted that the lattice parameter, the TiC grain size, and the mass ratio Ti/C in TiC grains are practically independent of the inert composition. It is shown that some of the composites possess high mechanical properties. The friction of the composites that contain Ni-Cr is characterized by a friction coefficient of up to 0.5 and a high wear owing to their strong adhesion to the counterbody. The materials whose compositions are TiC + (Cu, Fe) and TiC + (Cu, G13) are effective under a pressure of about 100 MPa and can serve as a basis for developing tribological materials.

Sliding bearings used as seismic dampers with composite self-lubricating materials

The scientific basis for the design of self-lubricating self-aligning sliding bearings with reciprocal motion is presented for the first time. The bearings are used as seismic dampers for bridges, industrial and civil buildings, and oil and gas offshore structures; they are superior to known bearings in carrying and damping capacities. An original experimental-calculation method for determining the life of a frictional pendulum sliding bearing used as a seismic damper is elaborated. It implies the use of the dependences for the wear rate presented in generalized variables. A method for estimating the friction coefficient of frictional pendulum bearings is also developed. The outlook for domestic development of an antifriction self-lubricating coating with preset tribological characteristics is analyzed and the present state of the art is discussed. The studies are based on known developments in the field of tribological materials with account for modern advances in nanotribology. The most promising heat-resistant polymers and fillers, as well as combined mixed antifriction fabrics on their basis, are considered.