Search

Tribological properties of tread rubber is a key problem for the safety and durability of large aircraft tires. So, new molecular models of carbon nanotube (CNT) reinforced vulcanized natural rubber (VNR) composites have been developed to study the enhanced tribological properties and reveal the reinforced mechanism. Firstly, the dynamic process of the CNT agglomeration is discussed from the perspectives of fractional free volume (FFV) and binding energy. Then, a combined explanation of mechanical and interfacial properties is given to reveal the CNT-reinforced mechanism of the coefficient of friction (COF). Results indicate that the bulk, shear and Young’s modulus increase with the increasement of CNT, which are increasement of 19.13%, 21.11% and 26.89% in 15 wt.% CNT/VNR composite compared to VNR, the predicted results are consistent with the existing experimental conclusions, which can be used to reveal the CNT-reinforced mechanism of the rubber materials at atomic scale. It can also guide the design of rubber material prescription for aircraft tire. The molecular dynamics study provides a theoretical basis for the design and preparation of high wear resistance of tread rubber materials.

Up-to-date predictive rubber friction models require viscoelastic modulus information, thus, the accurate representation of storage and loss modulus components is fundamental. This study presents two separate empirical formulations for the complex moduli of viscoelastic materials such as rubber. The majority of complex modulus models found in the literature are based on tabulated dynamic testing data. A wide range of experimentally obtained rubber moduli are used in this study, such as SBR (styrene-butadiene rubber), reinforced SBR with filler particles and typical passenger car tyre rubber. The proposed formulations offer significantly faster computation times compared to tabulated/interpolated data and an accurate reconstruction of the viscoelastic frequency response. They also link the model coefficients with critical sections of the data, such as the gradient of the slope in the storage modulus, or the peak values in loss tangent and loss modulus. One of the models is based on piecewise polynomial fitting and offers versatility by increasing the number of polynomial functions used to achieve better fitting, but with additional pre-processing time. The other model uses a pair of logistic-bell functions and provides a robust fitting capability and the fastest identification, as it requires a reduced number of parameters. Both models offer good correlations with measured data, and their computational efficiency was demonstrated via implementation in Persson’s friction model.

The understanding of friction on soft sliding biological surfaces at the nanoscale is poorly understood as hard interfaces are frequently used as model systems. Herein, we studied the influence of elastic modulus on the frictional properties of model surfaces at the nanoscale for the first time. We prepared model silicone-based elastomer surfaces with tuneable modulus ranging from hundreds of kPa to a few MPa, similar to those found in real biological surfaces, and employed atomic force microscopy to characterize their modulus, adhesion, and surface morphology. Consequently, we used friction force microscopy to investigate nanoscale friction in hard-soft and soft-soft contacts using spherical colloidal probes covered by adsorbed protein films. Unprecedented results from this study reveal that modulus of a surface can have a significant impact on the frictional properties of protein-coated surfaces with higher deformability leading to lower contact pressure and, consequently, decreased friction. These important results pave the way forward for designing new functional surfaces for serving as models of appropriate deformability to replicate the mechanical properties of the biological structures and processes for accurate friction measurements at nanoscale.

A multifunctional beta TiNb surface, featuring wear-resistant and antibacterial properties, was successfully created by means of open-air fibre laser nitriding. Beta TiNb alloy was selected in this study as it has low Young's modulus, is highly biocompatible, and thus can be a promising prosthetic joint material. It is, however, necessary to overcome intrinsically weak mechanical properties and poor wear resistance of beta TiNb in order to cover the range of applications to load-bearing and/or shearing parts. To this end, open-air laser nitriding technique was employed. A control of single processing parameter, namely duty cycle (between 5% and 100%), led to substantially different structural and functional properties of the processed beta TiNb surfaces as analyzed by an array of analytical tools. The TiNb samples nitrided at the DC condition of 60% showed a most enhanced performance in terms of improving surface hardness, anti-friction, anti-wear and anti-bacterial properties in comparison with other conditions. These findings are expected to be highly important and useful when TiNb alloys are considered as materials for hip/knee articular joint implants.

With the development of 3D printing technology, there is a need to produce printable materials with improved properties, e.g., sliding properties. In this paper, the authors present the possibilities of producing composites based on biodegradable PLA with the addition of graphite. The team created composites with the following graphite weight contents: 1%, 2.5%, 5%, 7.5%, and 10%. Neat material was also subjected to testing. Tribological, mechanical, and chemical properties of the mentioned materials were examined. Measurements were also made after keeping the samples in ageing and climatic ovens. Furthermore, SEM observations of samples before and after friction tests were carried out. It was demonstrated that increasing graphite content caused a significant decrease in wear (PLA + 10% graphite had a wear rate three times lower than for a neat material). The addition of graphite did not adversely affect most of the other properties, but it ought to be noted that mechanical properties changed significantly. After conditioning in a climatic oven PLA + 10% graphite has (in comparison with neat material) 11% lower fracture stress, 47% lower impact strength, and 21% higher Young&rsquo, s modulus. It can be certainly stated that the addition of graphite to PLA is a step towards obtaining a material that is low-cost and suitable for printing sliding spare parts.

Diamond-like carbon (DLC) films were deposited on nitrile butadiene rubber (NBR) by the DC magnetron sputtering under different bias voltages. Raman spectra revealed that the variation of bias voltage could tune the carbon bond structure in DLC films. Both the hardness and Young's modulus increased with the increasing bias voltage. Tribological tests revealed that the DLC-coated NBR prepared at the bias voltage of −200 V exhibited low wear rate due to its high hardness. The sealing property was studied by evaluating the leakage rate of volatile liquid in a simple apparatus. All DLC films resulted in less leakage rate as compared to the raw rubber under large stress. The lowest leakage rate occurred in the DLC-coated NBR prepared with a bias voltage of −200 V, which was associated with the theoretical calculations (Persson's theory). It was attributed to the synergetic effects of the variations of the Young's modulus and root-mean-square (Rms) roughness. The low Young's modulus and Rms, controlled by regulating bias voltage, could enhance actual contact area and reduce the leakage rate.

In the study we experimentally examine the influence of elastic properties and surface morphology on the inter-particle friction of natural soil grains. The experiments are conducted with a custom-built micromechanical apparatus and the database is enhanced by testing engineered-reference grains. Naturally-occurring geological materials are characterized by a wide spectrum of mechanical properties (e.g., Young’s modulus) and surface morphology (e.g., roughness), whereas engineered grains have much more consistent characteristics. Comparing to engineered materials, geological materials are found to display more pronounced initial plastic behavior during compression. Under the low normal load range applied in the study, between 1 and 5 N, we found that the frictional force is linearly correlated with the applied normal load, but we acknowledge that the data are found more scattered for natural soil grains, especially for rough and weathered materials which have inconsistent characteristics. The inter-particle coefficient of friction is found to be inversely correlated with the Young’s modulus and the surface roughness. These findings are important in geophysical and petroleum engineering contents, since a number of applications, such as landslides and granular flows, hydraulic fracturing using proppants, and weathering process of cliffs, among others, can be simulated using discrete numerical methods. These methods employ contact mechanics properties at the grain scale and the inter-particle friction is one of these critical components. It is stressed in our study that friction is well correlated with the elastic and morphological characteristics of the grains.

International audience; The aim of this study conducted on carbon tows and single fibres is to highlight some friction behaviours to help better understand the friction mechanisms that occur during the manufacture of carbon composites. These mechanisms are responsible for damage that reduces the specifications and lifetime of mechanical parts. An experiment has been developed in order to rub together two carbon tows, or two single carbon fibres (with a diameter down to 5 mm), at an angle of 90°. The influences of friction velocity, normal load, and type of carbon fibre have been studied. For both tows and fibres the friction follows the Coulomb’s law because there is no influence of the velocity and the normal load in the tested range. The rearrangement of fibres within the tow has been shown to be fundamental. For the single fibre, the role of the Young’s modulus and the sizing treatment is important.

Cartilage repairing grafts have been widely studied, and osteochondral replacement hydrogels have proven to be an excellent method in research and clinical fields. However, it has been difficult to simultaneously solve three main issues in osteochondral replacement preparation: surface lubrication, overall mechanical support and good simulations of cell regeneration. A novel integrated bilayered hydrogel osteochondral replacement was constructed by blending polyvinyl alcohol (PVA) and β-tricalcium phosphate (β-TCP) in this study. Separated nano-ball milling with ultrasound dispersion prepared β-TCP demonstrated suitable properties of tiny particle size, high purity and ideal distribution, improving the mechanical properties of the novel integrated hydrogel, and providing a cartilage-like lubrication effect and high biocompatibility, including cytocompatibility and osteogenesis. The reinforcement of β-TCP and integrated molding technology enabled the hydrogel to demonstrate excellent component compatibility and good bonding between the two layers, which promoted the strengthening of the compression modulus and tensile modulus up to three times by mechanical testing. The surface lubrication properties of the novel osteochondral hydrogel were similar to the natural cartilage by friction coefficient characterization. The two layers of the novel integrated graft provided a considerable bio-function by co-culturing with chondrocytes and synovium mesenchymal stem cells: chondrocytes promoted adherence achieved by the upper density layer and better osteogenesis performance of the porous lower layer. The design of the bilayered β-TCP/PVA osteochondral hydrogel is promising for use in articular cartilage repair.

The conventional development of a coating system for cutting tools includes a variety of test series with elaborate experimental parameter studies. In particular, experimental investigations of the cutting behavior cause a significant consumption of cost, time and resources. In order to adapt the coating properties to the specific requirements of the cutting process, it is desirable to reduce the experimental effort of coating development by simulation of the machining process. Therefore, the main factors of the thermo-mechanical coating load in machining AISI 4140 were identified by means of 2D FEM chip formation simulations. In order to provide the required thermal and mechanical coating properties for the simulations, CrAlN-based coatings were deposited onto cutting inserts and extensively characterized. Within the simulations, the coating properties were varied between the physical and technological boundaries of CrAlN-based coatings. It was shown that the Young's modulus, the coating thickness and the friction coefficient significantly influence the thermomechanical load and the stress distribution within the coating. Finally, the cutting performance of the coated inserts was experimentally investigated and compared with the results of the simulations. Here, it was shown that delamination of the coating is particularly influenced by coating thickness. © 2017 The Authors.

Hydroxyapatite (HA) coatings were deposited onto Ti6Al4V substrates via atmospheric plasma spraying under systematically varying spray parameters, leading to different in-flight particle velocities. Morphology, composition, and tribological properties of the coatings were then studied. The coatings deposited at higher in-flight particle velocities exhibited smoother surface topography, better inter-particle bonding and higher Young's modulus and hardness. Ball-on-disc tribological results showed that the friction and wear of the HA coatings significantly decreased with increased in-flight particle velocity under both dry and wet (Hanks' solution) conditions. All HA coatings exhibited lower friction and wear during the wet sliding due to the lubricating effect of the solution.

Phys. Rev. E ISI Document Delivery No.: AE1BR Times Cited: 0 Cited Reference Count: 33 Capron, M. Tordjeman, Ph. Charru, F. Badel, E. Cochard, H. Amer physical soc College pk; The xylem vessels of trees constitute a model natural microfluidic system. In this work, we have studied the mechanism of air flow in the Populus xylem. The vessel microstructure was characterized by optical microscopy, transmission electronic microscopy (TEM), and atomic force microscopy (AFM) at different length scales. The xylem vessels have length approximate to 15 cm and diameter approximate to 20 mu m. Flow from one vessel to the next occurs through similar to 10(2) pits, which are grouped together at the ends of the vessels. The pits contain a thin, porous pit membrane with a thickness of 310 nm. We have measured the Young's moduli of the vessel wall and of the pits (both water-saturated and after drying) by specific nanoindentation and nanoflexion experiments with AFM. We found that both the dried and water-saturated pit membranes have Young's modulus around 0.4 MPa, in agreement with values obtained by micromolding of pits deformed by an applied pressure difference. Air injection experiments reveal that air flows through the xylem vessels when the differential pressure across a sample is larger than a critical value Delta P-c = 1.8 MPa. In order to model the air flow rate for Delta P >= Delta P-c, we assumed the pit membrane to be a porous medium that is strained by the applied pressure difference. Water menisci in the pit pores play the role of capillary valves, which open at Delta P = Delta P-c. From the point of view of the plant physiology, this work presents a basic understanding of the physics of bordered pits.

International audience; Carbon fiber reinforced polymers (CFRP) are increasingly employed within the aerospace industry, particularly within the aircraft sector. However, machining of fiber reinforced composites can be quite complex, first due to the inherent heterogeneity resulting from the reinforcements/matrix assembly and second due to the presence of high modulus/high strength fibers. Therefore, a lot of Finite Element models have been developed in order to understand their material removal mechanisms. Among the scientific issues faced by these works, the identification of friction coefficients between CFRP and cutting tool materials remains a strategic field of research. This paper aims at characterizing the friction properties between composite and cutting tool materials. More precisely, the paper focuses on the context of a randomly structured CFRP, called HEXTOOL (TM), machined with a carbide tool under dry conditions. The specific tribological conditions during machining of such heterogeneous materials are discussed in the paper, especially the configuration of the tribosystem ('opened tribosystem'). The great lack of friction coefficient is mainly due to the absence of relevant tribometers simulating the tribological conditions occurring in cutting. This paper presents the development of a new tribometer designed to simulate conditions corresponding to machining of randomly structured CFRP materials. It provides quantitative values of friction coefficient and heat partition coefficient depending on sliding velocities. This work has revealed that friction coefficients are very low in dry regime compared to those obtained in metal cutting. Moreover, experimental results confirm that friction coefficient decreases from 0.25 to 0.1 when sliding velocity increases. Finally this works establishes that a TiN layer deposited on carbide tools is not able to modify friction properties.

We calculate the friction force acting on a hard cylinder or spherical ball rolling on a flat surface of a viscoelastic solid. The rolling friction coefficient depends non-linearly on the normal load and the rolling velocity. For a cylinder rolling on a viscoelastic solid characterized by a single relaxation time Hunter has obtained an exact result for the rolling friction, and our result is in very good agreement with his result for this limiting case. The theoretical results are also in good agreement with experiments of Greenwood and Tabor. We suggest that measurements of rolling friction over a wide range of rolling velocities and temperatures may constitute an useful way to determine the viscoelastic modulus of rubber-like materials., 7 pages, 6 figures

Depth sensing indentation is used to study the effect of grain refinement to submicro and nan ograins on the mechanical properties (hardness, plasticity, Young’s modulus) of armco iron subjected to severe plastic deformation by attrition in argon. In contrast to fcc metals, where the hardness increases and the plastic ity decreases as the grain size decreases to 20 nm, the hardness of bcc iron decreases from 5.8 to 3.7 GPa and plasticity δA increases from 0.82 to 0.87 as the grain size decreases from 50 to 20 nm

Use of thermoplastic composite material for load bearing components is increasing due to economical processing of complicated shapes in large quantities. Addition of fibre improves the strength and modulus of composites. Although the tribo-behaviour of thermoplastic composites were investigated, the friction and wear mechanisms are not yet fully understood. Friction and wear behaviour of injection unfilled Nylon 66, glass fibre reinforced Nylon 66 and carbon fibre reinforced Nylon 66 is investigated under dry sliding conditions. Tests were conducted at different normal loads and sliding velocities at room temperature. Coefficient of friction, wear loss and heat generation during the wear tests were quantified. Presence of fibre affects coefficient of friction and wear resistance of Nylon 66 matrix composites. The formation and stability of the transfer films affects the wear resistance. The rise in temperature during sliding was also calculated and also measured. The contact temperature rise is influenced by the composition which in turn influences the fibre adhesion and thereby the wear resistance. Glass fibre reinforced Nylon exhibited the lowest wear rate among the materials investigated. Both adhesive and abrasive wear mechanisms were observed in polymer matrix composites. ? Springer 2005.