Tribology - Science topic

Influence coefficients are used in numerical simulations to determine how much influence a given node or point in the domain has on other nodes or points. These coefficients are often calculated based on the nodes' relative positions and the system's physical laws, such as the heat equation or Navier-Stokes equations for fluid dynamics.

As you've noticed, one common issue with influence coefficients is handling the boundaries or corners of the domain. In many cases, the influence coefficients near the boundaries will differ from those in the interior because the boundary conditions affect the system's behaviour.

One way to handle this is by using different formulas or methods to calculate the influence coefficients near the boundaries and interior. For example, you might calculate the first row of the influence coefficient matrix using x1 and y1 for the corners, and then use a different method for the interior points.

Another potential issue is that some numerical methods assume periodic boundary conditions, meaning that the system wraps around from one domain edge to the other. As you've described, this can result in 'continued influence' on the other edge of the domain. If your system doesn't have periodic boundary conditions, you might need to use a different method that properly handles the actual boundary conditions of your system.

It's hard to give more detailed advice without knowing the specifics of your system and the method you're using. However, I hope this gives you a starting point for understanding and resolving your issues.

Hi guys,

Can anyone help me to understand, how shall i simulate the isotropic confining pressure in PFC3D using the wall servo command for triaxial testing in soil?

@ U. Ashok kumar

Dear sir,

I have sent an email on your respective email. But it is undelivered.

Kindly submit your abstract and title before 20thApril at amitmahajan291@gmail.com.

Thanks

The influence of the carbon content on the tribology of its alloys with iron is very strong. The carbon content changes the hardness of steels, creating both very hard carbide inclusions and very soft graphite inclusions in cast irons and steels, and all this is reflected in both the coefficient of friction and wear resistance. The influence of the carbon content is also manifested in the ability of steel to harden, which also has a dramatic effect on tribology.

No, the pin of the indenter was not broken.

A single indenter was used.

What would be the answer if hard particles were embedded in the matrix?

It depends on which kind of tribological test you did (pin on disk, ball on ring, ...), depends on the shape of the pin/ball after the test (if damaged or not) and it depends on the substrate-materials you used for your experiments. In all cases you can use the classical technique by calculating the tribo area by optical microscope, or using the Optical 3D Profilometer, or using the mecanichal Profilometer, etc ...

I suggest you to do not wash you sample after testing even if you use lubricant in order to no destroy the thin layer of tribofilm formed during the test.

@mohamadou Diew Thank yo so much for the explanations. A manual calculation by adding all set points of mm3 and dividing by total distance covered.

I have 2 load 10 and 40 newton. Same sliding speed for alls amples. Should i quoute specific wear rate or just wear rate

Hello Mr. Mohammed Hussein j. H. Al'Atia

I would want to thank you again for your thorough response to my query.

Per your suggestion, we have performed an FT-IR analysis on generated wear debris for both Mg alloy and its composite with B4C particles. The detected tribochemical compounds were MgCO₃, Mg(OH)₂, and MgO. As anticipated, the CO₃^2- bands in MgCO₃ increased. Now, I would appreciate it if you could clarify why increasing CO₃^2- bands in composites might lessen their oxidative wear probability.

Best regards

Well thank you, but that paper doesn't answer my question, as the authors just put moderate bulk stress ("We restrict attention to cases in which the direction of slip is the same at either end of the contact") and calculate from there. And for now, I am only interested in the Cattaneo loading, not the cyclic case.

My question is, what happens, if the contact switches from non-moderate to moderate bulk stress? Consider Q = 0. Then, the "moderate bulk stress condition" is that there is actually no bulk stress; you can look it up in Barber's book, but it is logical, as any bulk stress will lead to opposite slip directions at the two contact ends.

Now, does that change anything about the contact solution if Q gets large enough during the Cattaneo loading for the bulk stress to be actually moderate?

I guess, the point is, that slip will always just propagate inside from the contact edges, unless loading is reversed, even if the bulk stress is not moderate, and so, no point will ever switch from slip to stick (although at one edge the slip direction is continously reversed), but I am not able to prove that rigorously.

The significance of the film thickness less than 0 in this case is that it indicates that the contact is operating in the mixed EHL regime.

In the reference, the authors are discussing the solution for the lubricant load carrying capacity in an EHL contact simulation. The negative film thickness is occurring as the fluid film pressures alone are not sufficient to support the contact loads (in the preceding paragraph, the authors are discussing mixed EHL, with a load carrying capacity provided by the fluid film and direct asperity interactions).

If the fluid film thickness calculation at a node is less than 0, the authors suggest that the lubricant load carry capacity is neglected and the film thickness is set to 0, but the elastic deformation of the solid is retained for the given node. The elastic deformation of the solid at the node gives the solid-to-solid load carrying capacity to balance the loads applied to the contact in addition to the fluid film pressures at other nodes.

Work from Patir and Cheng and others has shown that the lubricant film load carrying capacity can be significant in the mixed regime of lubrication, but here they seem to be neglected for simplification.

Your question spans quite a big field, but I will try to answer succinctly.

There is no simple formula relationship between the coefficient of friction and the lubrication regime. You could look at the Stribeck curve and see the trend for coefficient of friction as the contact moves through the regimes, but this is more indicative than conclusive. To calculate it, you would probably need numerical simulations of the lubricant and surface load carrying capacities.

One approach would be to evaluate the ratio of the film thickness relative to the composite surface roughness. As a rough guide, where this ratio is large the regime is probably hydrodynamic, between 3 and 1.5 is probably mixed and less than 1.5 is probably boundary.

Please also bear in mind that that regime of lubrication will vary in the contact, as the surfaces are non-conformal.

The coefficient of friction that you measure can be dependent on a lot of factors. The lubricant viscosity, surface roughness and material hardness are probably the main factors, but the importance of these will change depending on what regime of lubrication you are in. Any additive within the lubricant, temperature, pressure, directionality of the surface roughness and slide-to-roll ratio may also be important factors.

There is a nice paper here that might help; https://www.researchgate.net/publication/335573001_Modelling_Transitions_in_Regimes_of_Lubrication_for_Rough_Surface_Contact

your body is not always ground, it could have a charge. As our body stores charge like a capacitor, there should be always a device that gives a tapping movement. it should not touch with any other materials or a material that is near to zero in triboelectric series like wood. That is the only solution to separate your signal from each other( if it is an independent tapping system without the involvement of the body). As our body skin has highly negative in triboelectric series. in the link, I have sent you our independent characterization device that has the ability to characterize samples without the involvement of the body.

Idir Abdelhek

Thank you, sir. But I don't understand your language.

Hello Rafael,

Even I am working on the same problem, please let me know if you have found the solution. It would be a great help.

you may find these cite valuable:

Yes, it is possible. It depends against which material the friction occures. For instence, PI against SS creats a new chemical bond, etc.

A pretty large topic... hopefully will not became a troll.

Concerning current researches on the topic, more concerned with mechanical engineering, I will nevertheless make a bit of an advertising (sorry): you can have a look on the topics of the World Tribology Conference series (the next one, number 7, is planned in France) see https://www.wtc-2022.org and previous issues of this series (2017 in China, 2013 in Italy...)

Dear Muhammad Chhattal, the following RG thread may brings some help. My Regards

Great question Andreas Almqvist! One which I've debated a lot!

The complementarity problem is an excellent and effective way of solving the problem of hydrodynamic cavitation. Have you (or someone else) applied this method to highly-loaded contacts (GPa pressures) such a line and point contact geometries out of interest? Great to see answers already from authors adapting the approach for their specific problems here too.

I see the expected pressure magnitude being one of the main drivers behind the choice of model for this phenomena. I have reviewed (a little) the modelling approaches to hydrodynamic cavitation with the late, great Duncan Dowson as part of this publication: . In this article we developed a 'moving mesh' type of approach for modelling the onset of cavitation but did not consider film reformation. This method can be applied to low pressure (MPa) and high pressure (GPa) problems without any difference in the implementation.

For lower pressure environments (MPa) the Elrod-Adams algorithm is still widely used as the baseline method for simulating cavitation including papers published by my group this year: . There are several publications optimising and adapting this method with varying degrees of success in the literature, too many to list here.

More recently this method has been combined with the complementarity condition to formulate the problem as described by Andreas and others. This also links to recent improvements in the solution approach by Woloszynski and the use of the Fischer-Burmeister equation to formulate the complementarity problem (see very recent work by Ron van Ostayen at TU Delft).

The complementarity method is a more stable than Elrod-Adams approach numerically because of the lack of switching functions and more robust mathematical formulation, but this does require a LCP solver which may be prohibitive to some. This is likely why Elrod-Adams remains widely used given it can be used with a standard PDE solver and setup readily with commercial codes, albeit with switching functions included making the problem numerically stiff.

There is also the issue of the change of variable approach from pressure to a derived field used explicitly for the cavitation problem, which then conflates the solution to the lubrication flow in terms of one variable. Therefore several authors use a pressure-only type approach such as that which I explored with Andreas here: . This article uses the Söderfjäll model to create a blending function reducing the density and viscosity of the lubricant rapidly to almost zero in the presence of negative pressure. This simulates the effect of cavitation solely on the lubricant properties and produces the required pressure distribution efficiently with rupture and reformation included. A similar but different approach was used by Gao et al in this paper: . The approach is very useful for where there are many regions of cavitation occurring within the domain.

Note that all the applications discussed so far are conventionally developed for low pressure (MPa) environments, and that in the classic high pressure (GPa) contact problems cavitation is managed with less rigour of the pressure profile shape/value. This is because of the large separation in magnitude between the max pressure and the vapour pressure in such contacts, with the requirement only being to resolve the pressure and pressure gradient to zero at the location of the onset of cavitation (JFO conditions).

For GPa problems the JFO conditions are conventionally achieved either with the use of a free boundary problem (see the classic multilevel solutions of Venner) or with the penalty approach of Wu (see more recent solutions by Habchi). Neither of these approaches fully resolve the problem of cavitation and this has led to several two-phase type models being developed which require very fine grids to resolve the flow. Multi-phase CFD simulations have also been developed recently by Imperial College London to investigate cavitation in much more depth for highly-loaded contacts and resolving all the flow (see Hartinger et al. and Hajishafiee et al.). These two- or multi- phase models also allow variance in the film thickness direction and are therefore not necessarily based on the Reynolds equation, making them more complex descriptions of the flow that are harder to solve numerically.

So the question comes down to whether resolving the full pressure shape/value is of importance to the outcome required, which itself is connected to the magnitude of the expected pressure. There is also the question of what type of solver you have readily available, whether a change of variable doesn't conflate the problem formulation, and if you're interested in very fine detail about the flow features.

Personally I would be in favour of the ALE or complementarity approaches for any problem/geometry but this certainly needs more development, hence my question at the start. I also think a review paper on the topic is well overdue!

dear Ziyauddin Seikh

what are the drawbacks of the addition of waste eggshells with (MMCs) composite?

It is possible to tell the advantages and disadvantages of Al-EggShell only after knowing the properties of the waste Egg Shell.

However the Egg Shell consists of "calcium carbonate (CaCO3)" having Density 2.71g/cc, and Melting point 1,300 °C.

Thus it can be used with aluminum (bcoz its density is 2.65g/cc). But dont know whether the bonding between matrix and reinforcement take place or not.

Hello Sumit Bhowmick

Re the explanation of , see the following reference.

[1] Jim Woodhouse; Chapter 9 - Vibrations: tuning forks and train wheels, squealing brakes and violins; in Christine Bondi (editor); Penguin Books; 1991; pp. 202-222.

Regards,

Tom Cuff

This link may be useful

Dear Mohamed iben houria,

The penetration of particles into the surface depends on the type of penetrating particles - electrons or a mechanical element. Penetration also depends on the type of the investigated base (material), its condition (polished, rough, hardened, annealed, platinum deformed, casting, the way of fastening, etc.), which parameters have a strong influence. In addition, the environment has an impact (environment, or vacuum with controlled gas). Of course, the limits of penetration also depend on the scanning mode (frequency, retention time, power, etc.).

thank you so much

Dear Muhammad Chhattal,

Tribological properties depend on the hardness, roughness, types of layers and the difference between them. In addition, this method is often influenced by the test method. It is known that with increasing hardness, wear resistance increases at the expense of reducing impact resistance. Friction is highly dependent on roughness. At high roughness and high hardness, brittle fractures are observed. At low roughness and high hardness the appearance of cracked distributed subforms of scales and chipping is established.

Dear Alexandra,

the paper "Antifriction and Mechanical Properties of the Thermoplastic Matrix

of Polyetheretherketone-Based Composites" may be useful for you.

Abhishek Parida as suggested in my previous reply, you can find some equations as a starting point in the paper:

"Physical analysis of the state- and rate-dependent friction law. II. Dynamic friction" T. Baumberger and P. Berthoud, 1999

Dear Achraf Tchanderli Braham

You have to explain some more about your questions.

For example, you said your current generator has the frequency of 33 kHz and the power of 300 watts. So the question is if such a generator is acceptable for your study? If so, it is ok. If not, you will need to a generator with suitable power/amplitude/frequency. For heavy applications, your will need more power.

You would need a higher level power if you are studying those tribological applications wherein the applied forces or frictions are considerable. Lower ultrasonic energy is usually damped/disappeared in heavy applications.

Let me know should you have still question.

Massoud

Germa'n,

I am not particularly familiar with the samples you are working with as I specialize in biological materials. However, a quick search online may provide you with some guidelines on how to resolve the peak positions for your measurements and make tentative assignment.

Since you explained that you rubbed a steel ball against a Bi2S3 coatings, I think it might be prudent looking up Raman peak tables for

1) steel (assuming - Chrome alloy steel balls, these comprise about 90% of all balls manufactured. This material is a high carbon (1.00%), chrome (1.36%) alloy steel) and

2) Bismuth sulfide

Additional, I would research a good internal standard or compounds that I would run to try to confirm peaks.I have uploaded a couple of reads you may find helpful. I suspect you are picking up on the chrome from the steel alloy and/or perhaps the D- G- bands formed from the sulfide-carbon bond ( )

Best of luck!

Dear Atef S. Hamada , may be electroplating would work for your composite, and if not you could paint it with some conductive paint to favour the electroplating of Ni. Alternatively you could try plasma sputtering or thermal evaporation. Probably the most cost effective coating method would be electroplating, but it would depends on piece sizes, volume and throughput required.

Hope it helps.

COMSOL can solve tribological problems. Some tutorials are already available.

Dear sir

You can try RMexpert and Mawell. Maxwell is very much suitable for time varying electromagnetic simulations, However RMexoert will gives you a rough idea

As the cell voltage and currents are low, you can high end DC programmable electronic loads for measurements. It will measure mV and mA with very small loads

ASTM C1624 test standard is the most used standard for this. You can read the article below, this may help you and clear your doubts

Stephen T. Gonczy Nicholas RandallFirst published: 05 October 2005 https://doi.org/10.1111/j.1744-7402.2005.02043.x

Oscar O'Dwyer Lancaster-Jones the friction need not always be with the lubricating surfaces right. It will be between the two bodies in sliding as in pin on disc case or ball on disc. In that case it may be wear asperities may get introduced between two surfaces as an abrasive this may also induce the higher friction right?

What do you think about it?

Thanks for this nice question Kashif Usman

Theoretically you can not calculate these two parameters just using UCS. But, usually it is not necessary to consider the confining pressure effect of concrete. So you can fix the internal friction angle and calculate the cohesion accroding to MC failure criterion. Friction angle of concrete does not change a lot, in the range of 20°~30°.

Thanks Soumitra Paul sir for your extensive in depth answer. I will surely go through the articles.

Many variables can affect the mechanism and kinetics of wear of sliding surfaces such as: normal load, sliding speed, surface geometry, relative surface movement, surface roughness, type of material, hardness of material, vibration, third body particles and their characteristics, temperature, lubrication (chemical interaction between AW/EP additives and surfaces and formation of a structural layer, etc.).

Your requirement is assessing the wear rate of sintered alumina. There would be an application in mind which is prompting you to carry out the tribometry. Try to choose the counter-body (ball/pin), keeping the application in mind. Try to find out the counter-body in the actual application against which the sintered alumina component is interacting.

There are two way that you can consider performing experiments.

1. The test were performed based on the standard ASTM D 4172. In the standard it clearly states that the load variations should be +/- 0.2 kg that means +/-1.92 N variation to consider test to be admissible and results to be reliable. Hence, the variation in 17 n is definitely a significant variation. This is condition is considering the variation in load during the testing.

2. If you are planning to design your experiments such that variation in the load is considered one of the parameters then you have freedom to do so.

Hence, if your question is based on 1 or 2, you have the answer for that.

Hope this helps

Thank you

I take your terms ‘tribolayer’ and ‘glaze layer’ to refer respectively to diffusely and mirror-like reflecting surfaces. In effect the surface layer of the sample being tested is being smoothed at the micro level by the rub test and is becoming more mirror-like. In consequence light reflectance is more diffuse before and more directionally oriented after rubbing. In practice the surface reflectance is typically a proportionate combination of both modes. The light grey shiny wear-track after rubbing thus reflects more light directionally, and may appear slightly lighter and the rougher un-rubbed surface may appear slightly darker due to light scattering.

If I capture what you are after correctly, then.

If you are struggling to use Archard Wear Law for your parameters, you can look into other predictive tools like a DOE to aid you. However, you need data on the parameters you are evaluating to create predictions. This is not any better than Archard Wear Law, just a different way.

There are three Laws of Wear; wear increases with sliding distance, wear increases with normal load, and wear decreases as the hardness of the sliding surface increases. These build Archard Wear Law. You can use these individually for predictions, but may not yield much value, unless you build your bespoke equation for your case. Archard Wear Law can be very useful if you have data on the wear coefficient.

Hope this helps.

Oscar

Lithium is a soft metal, with a Young's modulus in the range of 5 - 8 GPa. Yield strength is about 0.6 - 0.8 MPa. Elongation is about 50%. Therefore it is plastically deformed at very low stress levels. When exposed to air it will immediately oxidize to Li₂O which is a brittle ceramic. Li will oxidize even at very low oxygen partial pressures (~10^-40 bar). Therefore, if lithium is used as a solid state lubricant to reduce the friction between two mating surfaces, the lithium needs to be coated with an impervious layer of lubricant that completely prevents oxidation of lithium. Formation of Li₂O will cause scoring of the surface.

I think abrasive wear (both two and three body) and sliding wear must be looked at differently. I am not sure that Hemanth and Sergei are talking about the same tribosystem.

The plausible charge separation mechanism in convective clouds, leading to lightning, was suggested by Baker et al. (1987) (Q. J. R. Meteorol. Soc. 113, 1987, pp 1193–1215). The essence of the process is the interaction of particles with different growth rates and different electrical charges. The succinct summary is presented by Saunders in the paper “Charge Separation Mechanisms in Clouds”

>>Begin quote:

“A thunderstorm charging mechanism based on vapour deposition rate, first proposed by Baker et al. (1987), has been successful in helping to account for differences between the results from various laboratory studies. The concept follows on from the result described earlier that, during collisions leading to the removal of some surface mass from the larger particle, fast growing ice surfaces charge positively and conversely, sublimating surfaces charge negatively. Baker et al., suggested that an additional variable comes into play when two ice surfaces having different vapour diffusional growth rates come into brief contact, namely the surface state of the smaller particle in the collision process”.

End quote<<

(A copy of the paper is attached)

In summary, the traditional concepts of charge separation in triboelectric effect, observed in experiments with the Van de Graaff generator, do not have direct application in clouds due to thermodynamic and diffusion processes.

Additional reference:

Lightning: Physics and Effects (Vladimir Rakov and Martin Uman, Cambridge University Press, 2003).

In precision glass molding PtIr coating is used with a Cr adhesion layer, which works upto 600 °C. You can search something in that area.

In the general (macroscopic) understanding of friction outlined in the previous answers it is important to distinguish between sticking (the surfaces in contact are not in relative tangential motion to each other, for example in ideal rolling of a wheel or a tyre) and sliding (the surfaces are in relative tangential motion).

Under sticking conditions the friction force is a reaction force resisting motion, that means, the value of the force is exactly the one required to inhibit relative tangential motion. The (static) coefficient of friction \mu_s gives the maximum reaction force, \mu_s*N, before the surfaces start sliding.

So, a higher coefficient will allow you to apply a larger tangential force; in the tyre example you wish to have a higher coefficient of friction to get a higher torque "on the street".

Under sliding conditions the friction force opposing the relative motion is fixed at \mu_d*N (the dynamic coefficient of friction \mu_d often has a slightly smaller value than the static one). In sliding the friction force is dissipating mechanical energy to heat and wear, which in many applications will be undesired. That is why sliding surfaces are often lubricated to reduce the coefficient of friction.

PS: a COF close to zero is referred to as superlubricity, not superfluidity. Infact, the linguistic "similarity" to quantum effects like superfluidity and superconductivity is quite misleading.

If I understand your question and the term "soft EHL" (soft solid materials?) correctly, there will be some differences.

In traditional EHL, solids are made from hard materials. The contact surface is therefore relatively low, the contact pressure is high (1 GPa or more) and the film thickness is very thin (100-200 nm).

The situation is significantly different in soft EHL: large-size contacts, medium pressure, thicker film thickness.

This will necessarily have consequences on how to solve the problem numerically even if the fundamental equations are the same or almost the same. Reasons among others: the effect of pressure on lubricant viscosity, the extension of the deformed solids zone will be very different, which will very likely require a re-examination of the adimensionalisation and numerical resolution strategies,

Any wear process is very complex and depends on many factors, including material properties, ambient conditions, regime of lubrication and so on. So, wear rate reflects rather properties of the whole tribological system than properties of any single material. The only case of positive correlation between strength of material and wear factor has been stated for intensive abrasive wear (see Khrushchev's works ). In general, there is no relation between compressive strength and wear rate.

Hi Diogo,

It backs to how you use E* and how the contacting curvatures are defined. You need both equivalent radius (Re) and Equivalent stiffness (E*) to calculate the contact pressure.

however, as Johnson has mentioned in "Contact Mechanics" or :

an equivalent radius Re defined by

Re = (R'R")^0.5 = 1/2 (AB)^-0.5

If the two bodies are solids of revolution, then

R', = R ; = R 1 and R; = R i = R 2 , whereupon A = B =1/2 ( l / R 1 + 1/R 2).

Thus contours of constant separation between the surfaces before loading

are circles centred at 0. After loading, it is evident from the circular symmetry

that the contact area will also be circular, Two cylindrical bodies of radii R 1

and R 2 in contact with their axes parallel to the y-axis have R ; = R 1 , R1'' ; = inf,

R2 = R 2 , R 2'' = inf, so that A =1/2 ( l / R 1 + 1/ R 2), B = 0. Contours of

constant separation are straight lines parallel to the y-axis and, when loaded,

the surfaces will make contact over a narrow strip parallel to they-axis. A special case arises when two equal cylinders both of radius

R are in contact with their axes perpendicular. Here R 1 = R , R ; = inf, R ; = R ,

R2'' = inf) a = n/2, from which A = B = 1/2R. In this case the contours of constant

separation are circles and identical to those due to a sphere of the same radius

R in contact with a plane surface (R2´ = R2´´ = inf).

My email id is

In the tribology for each there is a called a critical load. Wich the load Will be without effect on the friction coefficient. And for each wear test there a steady state in the evolution of friction coefficient.

Your order is in the lubricant centers of interest to you

If the applied load and the sliding distance are the same for all tribo-tests, I think it is better that you analyze directly the wear volume and not the wear rate.

Oil film thickness in the lubrication

Surface roughness

Tribofilm formed on the worn surfaces

Morphology of the worn surfaces (wear mechanisms)

Hello Murali. You could coat with a diamond carbon PECVD (Plasma Enhanced Chemichal Vapor Deposition) coating or another of your choice. You would have to leave exposed only the region that will receive the coating.

You need to consider residual stresses of the coating as well.

It depends what kind of application process are you targeting...but in most of the cases it's an only way to go. I would do that ;)

I think this link can help you,

https://www.researchgate.net/post/How_might_I_calibrate_the_lateral_force_of_AFM_cantilevers

Dear Abdenour Guerraoui

Please find rules and regulations of your university, how it is possible to have co-supervisor from outside country. if yes, then you may find well suitable persons

From my point of view the implants that has poor wear resistance are the hip and knee implant replacement.

Claude Le Gressus

There are two distinct mechanisms of sliding friction. One is related to solids with pronounced surface asperity when only largest asperities are in a physical contact between each other. Then friction losses are caused predominately by plastic and elastic deformations as well as wear of these asperities. Another mechanism of friction is governed predominately by surface energy and chemical bonding between solids in the contact wherein friction losses are caused by creation of new surfaces in the back of a sliding solid body with heat release in removing the surfaces at the front of it as well as in propagation of self-healing shear cracks or voids aka Schallamach waves along the interface between the solids. Occurrence of such voids we observed in extrusion of clay paste through a transparent tubular die, see [ Kulikov OL, Hornung K. (2002) J. Non-Newtonian Fluid Mech., 107:133–144 ]. We demonstrated in our experiments that flow instability known as sharkskin as well as stick-slip occurrences are suppressed and molten polyethylene slips along walls of a tubular die if the die's inside is coated with silicon rubber. Remarkably, that even a very thin elastic coating can be used to ignite slip, see [ Kulikov O, Hornung K. (2004) J. of Non-Newt. Fluid Mechanics.V.124 (1-3), pp.103-114 ]. Moreover, an onset of slip and magnitude of friction losses depend on elasticity of the coating, see [ Kulikov O, et al. (2007) Rheologica Acta, V. 46(5) p.741 ]. That is, with more elastic material of the coating the slip onset happens at lower shear stress with lesser friction losses. These findings were developed later on into advanced lubricants for engines and machines that almost eliminate wear and friction losses, see my project and references in it. What is bad is that this engine oil extends longevity of engines and saves fuel up to 30% - nobody needs it.

Among three kinds of polymeric materials; thermoplastics, thermosets, and elastomers, only thermoplastics are the weldable polymers. It is due to their ability to be reshaped after heating below their degradation temperature.

Several researchers such as Bilici and Yukler are depending tensile strength especially lab shear.

Bilici MK, Yukler AI (2012) Effects of welding parameters on friction stir spot welding of high density polyethylene sheets. Mater Des 33:545–550

I hope this help you.

Regards

We need 4 elements: 1. body, 2. counterbody, 3. interfacial medium, and 4. environment medium. For example, humidity affects significantly the Friction behavior of ceramics and polymers. Regards.

Rather than load what is more relevant is contact pressure. The contact area may increase due to wear of the ball and this as a consequence decreases the the contact pressure.

Alternatively it might be that both loads and contact pressure are increasing but wear dimities due to adhesive wear as already mentioned... basically if the contact pressure is high enough, the wear particles are "welded" back to the disc or ball rather than being removed from the contact area.

I think abrasion will cause the friction to increase as a function of the road. Thus there will be frictional component that is perpendicular to the tire thread and this will help in the peeling off of large particular that will be highly abrasive as the speed increases.

Yes. However, the concentration of asphaltenes varies considerably from light oil to heavy and extra-large heavy oil such as bitumen.

Hello! It could happen because in the friction pair with increasing of sliding speed the friction coefficient, as rule, decreases and therefore there is possible the decreasing of volume wear too, due to the direct-proportional dependance of friction coefficient and wear.

Again the biomechanics differ from young to old age. Older people suffer from high friction and we should adjust the flexibility

Two points. First, While the modules is a function of atomic properties such as electronic bonding, the hardness is a function of film properties such as thickness and grain structure, e.g., dislocation mobility. Secondly, when you report hardness and modulus values, be sure to quote the appropriate precision, based on the statistics of the measurement, and don’t just quote the output from the machine. Your table shows the values to one, two, three, and four significant figures. I doubt the precision is three significant figures, let alone four. Also, the accuracy of the values, which is another issue altogether, depends on the models used to compute hardness and modulus.

Diiodomethane is typically used to determine the surface energy of materials and coatings. It is used because it is a non-polar liquid - for the calculation of surface energy (and its polar and dispersive components) you need at least two liquids, a polar one (typically water) and a non-polar one (typically diiodomethane). See e.g.: https://www.kruss-scientific.com/fileadmin/user_upl