Experimental study of contact angle and surface energy of a single aramid fibre and its relation on frictional behaviour of fibre-fibre contact

Surface energy has a significant impact on the adhesion and friction behaviour of fibres in composite materials. Finding the surface energy of a single fibre, on the other hand, maybe challenging. In this work, the surface energy of single aramid fibre is calculated using dynamic contact angle measurements. The contact angles of three different types of Twaron® aramid fibres with varying treated materials and thicknesses are tested in a series of test liquids. As a result of the changing surface characteristics of the fibre, the surface energy of treated fibre is found to be approximately 37% greater than that of untreated fibre. The thickness of the fibre also affects its surface energy because thicker fibres have a larger contact area, which increases surface energy in the region of contact. At specific applied normal load conditions, it can be found that the coefficient of friction increased as the total surface energy in the area of the contact interface increased.</p

Existence of a Tribo-Modified Surface Layer on SBR Elastomers: Balance Between Formation and Wear of the Modified Layer

In most of the tribological contacts, the composition and tribological properties of the original interface will change during use. The tribo-films, with modified properties compared to the bulk, are dynamic structures that play a significant role in friction. The existence of a tribo-modified surface layer and its importance on the overall friction of elastomers has been shown both theoretically and experimentally before. The characteristics of the modified surface layer deserve specific attention since the tribological properties of elastomers in contact with a rough counter-surface are determined by these modified surfaces together with the properties of bulk of the material. Both the formation of the modified layer and the break down (wear) of it are of importance in determining the existence and thickness of the tribo-modified layer. In this study, the importance of the wear is emphasized by comparing two styrene butadiene rubber-based elastomers in contact with a granite sphere. A current status of perception of the removal and the stability of the modified surface layers on rubbers is introduced as well as experimental work related to this matter and discussion within literature. Pin-on-disk friction tests are performed on two SBR-based samples in contact with a granite sphere under controlled environmental conditions to form the modified surface layer. Although the hysteresis part of the friction force which has a minor contribution in the overall friction is not markedly different, the total measured friction coefficient differs significantly. Mechanical changes both inside and outside the wear track are determined by atomic force microscope nano-indentations at different timescales to examine the modified surface layer on the test samples. The specific wear rates of the two tribo-systems are compared, and the existence of the modified surface layer, the different measured friction coefficient and the running-in distances toward steady-state friction are explained considering different wear rates. A conceptual model is presented, correlating the energy input into the tribo-system and the existence of a modified surface layer

Numerical Analysis of a Journal Bearing with Chemical Roughness

In this paper, performances of an oil-lubricated journal bearing are investigated with special attention paid to slip occurrence. The chemical roughness at which slip occurs is introduced. The slip length model which is a theoretical scheme for taking the slip occurrence into hydrodynamic lubrication theory is applied to the analyses by a finite volume treatment of the Reynolds equation that deals with the geometry of a journal bearing. The calculated results are compared with other literature under certain eccentric state, and verified in terms of Sommerfeld number and the friction force. The study shows that the presence of judicious chemical roughness in deterministic way can result in a significant improvement in decreasing power loss of the bearing performance

On an Elastoplastic Sliding Model for a Coated Single Asperity

In this study, a sliding friction model for coated single asperity contacts is proposed. A displacement-driven layered contact algorithm is firstly introduced and verified by the finite element method. Then, this algorithm is applied to simulate the contact between two semispherical asperities. The full sliding contact process is discretized into a series of transient steps, and each of these steps are calculated by the displacement-driven contact algorithm. The effects of the interference depth and the properties of, respectively, the tribofilm (thickness, elastic modulus, and yield strength) and the nanocrystalline layer on the sliding coefficient of friction are investigated. The results suggest that when surface adhesion and asperity damage are ignored, the plastic deformation of the tribofilm is the main source of the sliding friction. Greater interference depth, tribofilm with greater thickness, higher elastic modulus or lower yield strength, and the presence of a nanocrystalline layer will lead to a higher coefficient of friction in single asperity sliding

A multi-technique characterization of the tribofilm formed by a fully formulated CVT fluid

In this work, the morphology, thickness, chemical composition and mechanical properties of the tribofilm formed by a fully formulated CVT fluid are investigated by multiple techniques and linked to the frictional and wear characteristics of a pin-on-disc tribosystem. It is found that the tribosystem shows higher friction and wear when tested at 150 °C than at 80 °C. The main reason is that although the morphology and thickness are similar, the tribofilms formed at different temperatures have different chemical compositions and mechanical properties. The tribofilm formed at 150 °C is Fe richer and has greater hardness and shear strength, which leads to a higher tribochemical wear rate and a greater interfacial shear force

Misalignment-Induced Micro-Elastohydrodynamic Lubrication in Rotary Lip Seals

In literature the lubrication of rotary lip seals is explained by hydrodynamic action on a microscopic scale. This theory assumes perfect concentricity between the seal and the shaft which in reality seldomly occurs. Focusing on the stern tube seals application, an analysis is performed on the phenomena distorting the axisymmetric operation of rotary lip seals. Radial and angular shaft misalignments together with pressure and temperature gradients have been modelled. The model predictions are validated using a dedicated setup. Additionally, applying the soft-EHL film thickness expressions at the asperity level, an equivalent film thickness along the circumferential direction is estimated. The Reynolds PDE is solved to predict the misalignment-induced hydrodynamic pressure build-up. The film thickness variation derived and accompanying non-uniform contact pressure distribution was shown to be sufficient for hydrodynamic action and, depending on the minimum film thickness, the hydrodynamic pressure build-up can exceed the static contact pressure. Additionally, significant differences were observed between the radial and angular misalignment configurations

Rheological and wetting properties of Environmentally Acceptable Lubricants (EALs) for application in stern tube seals

The use of Environmentally Acceptable Lubricants (EALs) for stern tube lubrication is increasing. Although the machine components of a sailing vessel are designed to operate together with mineral oil-based lubricants, these are being replaced by the less environmentally harmful EALs. Little is known about the rheological performance of EALs in particular at the high shear rates that occur in stern tube seals. In this study, the viscosity and wetting properties of a set of different EALs is analysed and compared to traditional mineral oil-based lubricants using a set of experimental techniques. Some of the EALs present Newtonian behavior whereas other show shear thinning. No significant difference in surface tension was observed between the different lubricants

Low Friction in CuO-Doped Yttria-Stabilized Tetragonal Zirconia Ceramics: A Complementary Macro- and Nanotribology Study

The tribological behavior of CuO-doped yttria-stabilized tetragonal zirconia (3Y-TZP) ceramics in the absence of additional lubricants was characterized by macroscale pin-on-disk measurements and nanoscale atomic force microscopy (AFM) for a broad range of velocities. The previously observed low shear strength interfacial layers generated in pin-on-disk tracks by Al2O3 ball counter surfaces on CuO-doped 3Y-TZP, as well as virgin surfaces, were probed quantitatively by AFM with Si3N4 tips as the counter surface. The observed trends in nanoscale coefficient of friction determined by AFM were found to be in agreement with data acquired using a pin-on-disk tribometer. The combined data support the notion that a layer of surface contaminations is removed during the initial sliding, and wear of high asperities occurs. Subsequently, an interfacial layer with low shear strength is generated during sliding. While these results do not provide an exhaustive explanation for the process of layer formation, they represent the first report of bridged nano- and macrotribological analysis of a compositionally heterogeneous low-friction, low-wear ceramic material and further confirm some of the key assumptions for the deterministic model reported previously by Pasaribu and Schipper