Can increase or decrease friction. No effect. Piercing and penetration of the soft surface by the asperities of the hard surface. Increasing in friction. None of these. Greater than coefficient of sliding friction. Lesser than coefficient of sliding friction. Equal to sliding friction. May be greater or smaller compared to sliding friction. Micro-slip effect within the contact area. Elastic hysteresis of the contacting materials. All of the above.
Both I and II. Both I and III. There would be less hysteresis loss. Rolling friction would be lower. More steering controllability. Increase the operation speed. Decrease the operation speed. Perhaps there is an error? It is always useful to make progress from these apparently striking results. Introduction In fracture mechanics, it is well known that mixed mode enhances the toughness observed in pure mode I, due to dissipative effects.
However, the Menga, Carbone and Dini MCD, paper finds this increase of toughness surface energy, in particular , due to a new effect, purely from energy balance at the interface, without the need to assume any irreversible process dissipation.
Fracture mechanics concepts were firstly applied by Johnson, Kendall and Roberts JKR-theory, to adhesion between elastic bodies, are applicable to contact even in the presence of friction, as mixed-mode fracture mechanics problem, as done first by Savkoor and Briggs who also conducted experiments between glass and rubber. They found an effective surface energy which is reduced due to tangential force.
Experiments clearly evidenced a reduction of the contact area when tangential load was applied, but less than expected from the prediction. Experimental findings also found development of Schallamach wave which tend to permit slip without affecting the contact. More recent experiments continue to confirm contact area reduction at both macroscopic and even smaller scales Sahli et al.
But the contact area still decreases with tangential load. MCD obtain, instead, from a thermodynamic treatment, an effective surface energy given by their eq. However, this result appears paradoxical on various grounds.
Where does this energy come from? Under large compressive normal forces, the contact area would be large, which implies an additional energy in any contact which would violate Hertz theory. Yet, Hertz theory has been largely validated in many machines even in sliding contacts, without the need to consider this possibly unbounded increase of the effective energy; why the present experiments do not show this area enhancement?
Conclusion The tribology community should gain some insigth if this paradoxical result is better understood. Author M. DMMM dept. V Orabona, 4, Bari. Fracture mechanics simple calculations to explain small reduction of the real contact area under shear.
Facta universitatis, series: mechanical engineering, 16 1 , Johnson, K. A , — Menga, N. Do uniform tangential interfacial stresses enhance adhesion?. Journal of the Mechanics and Physics of Solids, , Papangelo, A. Shear-induced contact area anisotropy explained by a fracture mechanics model.
Physical Review E, 99 5 , On mixed-mode fracture mechanics models for contact area reduction under shear load in soft materials. Sahli, R. Shear-induced anisotropy in rough elastomer contact.
Physical Review Letters, 21 , Savkoor, A. Pallares, G. Al , Guibert, M. Proc R Soc A — MIT researchers developed a frictional interface at the atomic level. The blue corrugated surface […]. Graphene has unique properties and is being extensively used in various applications. It got a deserved attention in the field of tribology as well and was reported to lead to the states of superlubricity see […]. In technical terminology, a slurry is a mixture of solid particles in a liquid that can be readily pumped.
The following article describes the use of tribology in understanding slurry-related wear prevalent in drilling operations. Hello, the topic addressed by Ciavarella is very important and interesting but still mysterious to me. Here are some comments:.
This was for a rough rubber surface in contact with a smooth glass plate, but the rubber asperities can be approximately considered as small spherical or elliptic bumps. Applying the theory of Ciavarella et al [3] a similar theory was already presented and used in Ref. I believe this is due to mechanical non-linearity and could be studied using FEM.
This problem can probably only be studied using accurate numerical methods e. Even PDMS, which is usually considered as a nearly perfect elastic material, exhibit viscoelastic properties which increases the adhesion[5].
However, in the study in [2] viscoelasticity appears not to be important for the contact area; the contact area is perfectly constant at the JKR-value up to rather high sliding speeds corresponding to high frictional shear stress , which could only occur if the influence of viscoelasticity on the contact mechanics would be perfectly compensated by some other mechanism, which appears unlikely to me. Krick, J.
Vail, B. Though, while continuous sliding with real contact area is notably increases with increase in friction. Further, the applied load is considered as major input parameter in the ceramic material. If the applied load is increased, the brittle contact may establish, and this will increase the coefficient of friction as much as high around 0. Specifically, the brittle contact can be occurred while the tangential stresses owing to higher friction due to the occurrence of critical microcracks on the surfaces.
This type of microcracks can be seen on the ceramics surfaces normally and producing the defects such as porosity, flaws and inclusions. The cracks can be initiated from the development of asperity because of continuous applied load during the tribological study.
Coefficient of friction and applied load for alumina sliding on the alumina. The polymer like polytetrafluoroethilene PTFE is produces very low friction around below 0. Thus, this material behaves as solid lubricant while sliding with counterpart [ 13 ]. Generally, most of the polymer material friction coefficient ranges from 0. In the case of the work of adhesion in polymer was lesser than in ceramics and metals.
However, their stiffness and hardness of the material is lesser, and these two effects are nearly proportional. These experiments were conducted on flat to flat surface contacts with lower sliding speed of 0. So that the thermal effects on the polymers can be avoided [ 14 ]. It can be observed that the coefficient of friction was increased with the work of adhesion. In this kind of working condition, the adhesion was considered as a most important factor in friction determination.
In the case of point or line interactions, the produced deformations may be higher and therefore the effect of viscoelastic can play a major role.
Work of adhesion and friction coefficient for some polymers sliding against PA 6. The clean surfaces without any rust is freely adsorb traces of other substances from the atmosphere. In addition, the newly manufactured surfaces normally produce the lesser wear and coefficient of friction while compared with clean surfaces.
However, there may be chances of external material on the interface of bulk material which can increase the coefficient of friction during continuous sliding process. Hence, the lubricants can be applied to reduce the wear rate and coefficient of friction [ 8 ]. The term called lubrication can be applied to two various conditions: namely solid lubrication and fluid film lubrication liquid or gaseous.
In any kind of material, the solid lubricant such as solid film and powder was used to protect the sliding surface from the unexpected damages during the sliding process and reduce the wear rate and coefficient of friction. The solid lubricants were used in sliding applications.
For example, the bearing was operated with low speeds and higher loads, and the hydrodynamically lubricated bearings demanding the start and stop processes.
The solid lubricant holds the higher variety of material which can produce the lower wear rate and coefficient of friction [ 15 ]. In addition to that the hard materials also were used as lubricant to reduce the friction and wear in extreme working atmosphere. A thick film of lubrication was maintained in the region between two solid surfaces with no relative motion or lesser motion through an external pumping agency is called as hydrostatic lubrication [ 16 ].
This curve has a lowest with providing the recommendation that higher than one lubrication system is presented.
In some cases, the lubrication regimes can be recognized through lubricant film parameter [ 17 ]. Coefficient of friction and lubricant film parameter as a function of Stribeck curve showing various lubrication regimes observed in fluid lubrication without an external pumping agency. In hydrostatic bearings, the supporting load on the thicker film provided from an external source, a pump, which can induce the fluid pressure toward the film. Based on this reason, those kinds of bearings are called externally pressurized.
Generally, the hydrostatic bearings are considered for usage in compressible and incompressible fluids. Subsequently, the hydrostatic bearings are no need of any relative motion on the surface of bearings to create the load supporting pressures as it essential in the hydrostatic bearings [ 18 ].
Further, this type of hydrostatic bearings is used in application with no relative motion or lesser motion between the sliding surfaces. Besides, the hydrostatic bearings offer great stiffness though, this type of lubrication needed the high-pressure equipment and pumps for the cleaning of fluid, which is occupying more space with higher cost.
The hydrodynamic lubrication is called as thick film or fluid film lubrication. The convergent type of bearings starts to move in the direction of longitudinal from the initial position, a less thickness of layer is pulled due to viscous entrainment.
This hydrodynamic lubrication mechanism is necessary for the effective working of the hydrodynamic journal and the thrust bearings are extensively used in the modern manufacturing industry [ 19 , 20 ]. Also, the friction coefficient of the hydrodynamic contacts is as lesser as 0.
Sometimes the frictional force can be increased slightly while increase in sliding speed due to the viscous drag. Generally, the physical interaction can be occurred while starting and ending up with lesser sliding speeds.
The behavior of the interaction is directed through the lubricant bulk properties i. Besides, the corrosive wear can be occurred on the bearing surfaces due to the presence of lubrication. The adhesive wear also was possible while initial and ending up the processes. The corrosive wear can be reduced through lubricant precipitation and formation of film on the bearing surface. In this situation, the friction coefficient can be increased as high as 0.
This situation may also arise in a starved contact. When the solid surfaces are nearby that surface contact between the multimolecular or monomolecular films of gases or liquids and the dense solid asperities may rule the contact.
Further, with the absence of gases and boundary lubricants without oxide films , the produced frictional forces may be higher than one. In case with failure in boundary lubrication, causes corrosive and adhesive wear. Generally, the boundary lubricants can easily form the sheared film on the bearing surfaces.
Therefore, this formed shear film can be reduced the corrosive and adhesive wear. The major physical properties of the films are hardness, melting point and shear strength. The other properties are cohesion, tenacity or adhesion and formation rates.
Also, the viscosity of the lubricant can show a minor impact on the wear and friction behavior [ 22 , 23 ]. The transition between the boundary lubrication and the hydrodynamic regimes is a gray zone known as mixed lubrication. In this regime, the both mechanisms such as boundary lubrication and hydrodynamic lubrication may be in operational condition. There might be possible for the higher solid contacts, however, a minimum portion of the bearing surface leftover through partial hydrodynamic film.
The hard solid interaction between the new metal surfaces can cause to a wear debris formation, adhesion of particle with counterpart, metal transfer from bulk to counterpart and eventual seizure.
But, in the case of liquid condition, the chemically formed films protect the surfaces from adhesion during the sliding experiment. This mixed regime is called as thin film lubrication, partial fluid and quasi-hydrodynamic lubrication [ 21 ]. Generally, the term wear is defined as material removal or surface damage on the one or two surfaces while rolling, sliding or impact motion relative to one another.
Particularly, the wear happens through surface interactions at asperities. Consequently, the properties of the material may be changed at least or interface region. But, there is a possibility for less or no material losses. Then, the displaced material can be removed from the interacting surfaces and may cause the material transfer to the counterpart surface or may break as small wear debris.
When material transfer from bulk to counterpart, the net mass or volume loss of the interacting surface is zero while the bulk material surface is worn. The wear loss leads the real material loss, and this may occur sometimes independently. Generally, the wear is a system output and it is not a material property. In addition to that the working atmosphere affect the interface wear. In some cases, mistakenly assumed that the higher frictional force displays the increase in wear rate. For example, the polymers and solid lubricant interfaces showed with higher wear and lesser friction, whereas ceramic material showed the lower wear but moderate frictional force.
In all the dynamic machine components such as cams, bearings and seals, the wear is almost undesirable one. Those components or machines need to be replaced after a small damage or material loss or if the surface showed with higher roughness. If the system is well defined in tribology, the material removal will be very slow and at the same time it must be continuous with steady process. The wear occurs chemically or mechanically means and is normally induced through frictional heat.
Mainly, the wear contains six principal quite distinct phenomena that have only one thing in common; the removal of material from the rubbing interface [ 24 , 25 ].
The wear can be classified as follows: 1 abrasive; 2 adhesive; 3 fatigue; 4 impact by erosion; 5 corrosive; 6 electrical-arc-induced wear. The other commonly raised wear is fretting corrosion and fretting.
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