Try to determine the coefficient of friction as a function of the relative film thickness λ. probably will change the relationship. Show the paper: Hamrock B.J., Dowson, D.: Ball Bearing Lubrication. John Wiley & Sons. New York, Chichester, Brisbane, Toronto, Singapore: 1981.

It may happen that vegetable oil degrades very fast during the tribology experiment.

Hi, you need to provide more details of the experimental setup. Perhaps, if the velocities are low and if vegetable oil is of lower viscosity than the mineral oil, you may fall into mixed lubrication regime and the corresponding log-log plot is no longer linear.

Shubrajit, as mentioned more information is needed to properly discuss this. For instance the experimental setup with contact pressure, properties of the contacting pair etc. 

I interpret what you are saying in the way that the mineral oil showed a decrease in friction with speed/film thickness etc, and with the vegetable oil you instead had an increase in friction?

We have seen similar results in experiments with ester and mineral oil based lubricants in hydraulic motors: The apparent friction coefficient (based upon mechanical efficiency measurements) initially increased with Stribeck number for the ester before it plateaued and then decreased. (blue and green symbols in figure) The mineral oil showed normal Stribeck behavior with no initial increase in friction coefficient.(black blue red) The tribological surfaces in the orbital motor undergo sliding and rolling contact.

Do you measure the lubricant viscosity during the trial? It may change during the trial.

It depends on your test conditions. For example, if you speed range used can provide lubrication regimes from boundary to full film HL, you can get a standard Stribeck curve, other wise, only part of it is obtainied. Even lubrication regimes from boundary to full film HL are included in your test, some factors can revise your stribeck surve,  which should be carefully identified. 

It depends on the amount of fatty acids in your vegetable oil.  Some vegetable oil has high amount of fatty acids and glycerides and they function as friction modifiers.  It is not a fair comparison with mineral oil if you friction modifiers in one and none in the other.  

The experiment has been carried out with a vegetable oil of 239cSt at speeds 500,800 and 1100rpm with loads of 140N,150N and 160N. I am getting an increase in COF with the increase in load. The oil was supplied dropwise to attain boundary lubrication regime. While with the same experimental setup with mineral oil of 460cSt the COF trend is decreasing. Kindly shar your views. the tests were carried according to ASTM G 99.

You might check this work:

The effect of clearance upon friction of large diameter hip resurfacing prostheses using blood, clotted blood and bovine serum as lubricants

Chapter · January 2010

DOI: 10.1007/978-3-642-03889-1_112

In book: World Congress on Medical Physics and Biomedical Engineering, September 7 - 12, 2009, Munich, Germany, pp.418-420

OR

The Effect of cup deflection on friction in metal-on-metal bearings

Article in The Journal of Bone and Joint Surgery 90-B:552 · June 2008

Impact Factor: 4.31 ·

Saeed Afshinjavid

You are absolutely right! The matter is that the curve of Stribeck is that other as attempt to connect two different processes and two different ideas of friction in one theory. The adhesion of the greased surfaces arises in the field of depression or in the field of a vacuum which always arises in contact at friction. Hydrodynamic friction when in an oil film pressure increases - it are other theory and other area of contact. I recommend to you to examine with compromise Adhesion-Hydro-Dynamic (AHD) model of friction and wear

What does “opposite trend” mean? You mean friction increases with velocity at low range relative velocity?

Perhaps if you increase the load and decrease the speed you will have a normal Strribeck curve.

Stribeck curve measurements more relate to mineral oils and such anti-wear additives like ZDDP assuming that roughness of the surface do not change in time. Meanwhile in conditions of polishing wear and with friction modifiers that may be present in vegetable oils the friction coefficient is changing (dropping) in time.

Олег! Наличие в масле модификаторов трения может сузить интервал чисел Герси, в котором имеет место заметное повышение коэффициента трения. Попробуйте ввести в чистое минеральное масло олеиновую кислоту (где-то порядка 1%) и сравните полученную кривую Штрибека с кривой для того же масла, но без присадки ПАВ. Естественно, для масла с присадкой переход к граничной смазке будет наблюдаться при меньших значениях числа Герси, чем для чистого минерального масла, и увеличение коэффициента трения будет меньше, но всё-таки будет. Закон природы - при граничной смазке трение практически всегда выше, чем при гидродинамической или ЭГД-смазке.

Все таки я не о столько о модификаторах трения, сколько о нано-полировке поверхностей в процессе трения. С некоторыми добавками можно добиться выравнивания неровностей до нано-масштаба в процессе трения без задиров и холодной сварки поверхностей металлов. А уж тогда на гладких поверхностях модификаторы трения помогут снизить коэффициент трения практически до ноля, как это происходит у нас в суставах. Поэтому кривая Штрибека теряет смысл - коэффициент трения в граничных условиях меняется со временем вслед за выравниванием поверхностей. А вот для минеральных масел с традиционными добавками типо ZDDP никакого выравнивания поверхности не наблюдается по определению. Скорее происходит коррозионный износ поверхности и появление пористой неровной пленки фосфатов. Которая удерживает минеральное масло на металлической поверхности, но и которая не позволяет снижать коэффициент трения до ноля или около того. я пытался это донести до народа, но практически всем пох. Как ни странно.

https://www.youtube.com/watch?v=0VuDtGHSt9k

https://www.youtube.com/watch?v=4B5LCLGvQFU&t=827s

снижение коэффициента трения при нанополировке как первой стадии притирания деталей дает увеличение эффективности двигателя до 30% и соответственно экономию топлива. не говоря о том, что скорость износа снижается в 5-50 раз по сравнению с ЛУЧШИМИ сортами синтетического моторного масла. И максимальная нагрузка до задиров куда выше, чем для традиционных минеральных и синтетических масел.

Привет из Таиланда. Тут можно в качестве основы моторного масла использовать пальмовое масло, в котором много лучше растворяются добавки и стоимость моторного масла тогда ниже 1,5 доллара США.

Так что пора бы традиционный подход с кривыми Штрибека сдавать в утиль и переходить на натуральные способы снижения трения, как это в Природе делается легко, дешево и просто. Но кому это надо?

Plus the above mentioned posts, it also depends on the nature and type of the mating surfaces for example in case polymeric materials or viscoelastic materials.....

Диаграмму Штрибека никогда не сдадут в утиль. Она позволяет разграничить зоны граничной, смешанной и гидродинамической смазки. Увеличьте нагрузку, снизьте скорость - и вы перейдёте в режим граничной смазки. Естественно, выглаживая поверхности трения, вы расширяете зоны жидкостной и смешанной смазки. Это как раз подтверждает ваши соображения. Но не требуйте от диаграммы Штрибека, чтобы она дала вам то, для чего она предназначена.

"In a friction test under lubricated condition, do the results always follow the Stribeck curve?" - the answer is NO. As an example - lubrication in joints of mammals' bones. I have demonstrated in my old publications that flow with slip along an elastic smooth surface can happen. And in my recent research an engine oil was proposed that allows 1) to produce nano-polishing of adjacent surfaces in conditions of boundary friction by special additives and by diluting the oil with acetone. As acetone evaporates the engine oil viscosity recovers but due to presence of special additives slip of the lubricant may happen along the smooth surfaces. That is flow of the lubricant along the surfaces goes virtually without friction losses similar to what we may observe in the bones' joints. So the Stribeck curve is changing in time and friction losses virtually disappear. The Stribeck curve is for a "stone age" of the industrial revolution. Time is coming when people would learn from the Mother Nature on how to eliminate friction losses and wear in engines and machines. Yet only a few of scientists care to bring it to wider use, alas.

@Shubrajit Bhaumik

@Tekkouk Nazih

@Oleg Kulikov

@Ahmed Morsy

An important question concerns the physico-chemical processes explaining the Stribeck curve which represents the variation of the coefficient of friction as a function of the Hersey number. At low speed and in the absence of wear, it is experimentally verified that the coefficient of friction varies as the inverse of the applied pressure. It was then shown that the coefficient of friction varies with the density of point defects and with the annihilation of dislocations in the friction interface. The next step was to link the stick slip phenomenon to energy processes in dielectrics and interfaces. These works are cited in the publication

Electron-trapping and energy localization in insulating materials. Technological impact of space charge electron-beam characterization

AIP Advances 8, 095228 (2018); https://doi.org/10.1063/1.5047673

Regards

Claude Le Gressus

Claude Le Gressus The stick-slip phenomenon is related to crack propagation and friction mechanism is related to propagation of cracks too according to Gerde E & Marder M (2001) Friction and fracture. Nature 413, 285-288. DOI: 10.1038/35095018

Thank you for the reference you send me and for your comment.

I prefer to say that the phenomena of (1) friction, (2) stick slip and (3) fracture or wear, appear in this order and are due to different processes. They are usually modeled on the basis of the Griffith fissure hypothesis, but this does not require that stick-slip and fracture be related to the propagation of fractures. Moreover, it is surprising that the simulations are still based on equations unrelated to the microscopic aspects of adhesion and lubrication.

Sincerely

Le Gressus

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.

Oleg Kulikov

We have addressed the issues of adhesion, stick slip and wear without using the Griffith crack model which cannot explain observations made at high speed of deformation. In a fundamental research step we investigated on model materials (single crystals) 1- defects capable of localizing an energy comparable to that of defects "imagined" by Griffith, 2- the nature of the transformation of this energy into surface energy in the zone of decohesion, 3- the mechanisms of the instabilities of the surface energy at the origin of stick-slip and wear, 4- a relation between the maximum internal energy accumulated in the material and the wear. These studies were carried out on insulating materials because on the one hand, all the materials exposed to the air or in contact with lubricants become insulating and on the other hand, the study of the lubrication of engines shows that these dielectric layers have a vital role on wear. This work involved the development of experimental techniques to analyze processes, from energy localization to instabilities. These analyzes highlight the effect of the entropy of the system on the instabilities of surface energy and their consequences on the damage and in particular the thermal and fracto-emissive processes. I am preparing a project in which this logic will be explained and illustrated.

We can think that for ecological needs the interest will come back for the energy saving or the increase of longevity of the engines. Otherwise, the innovations that will emerge from the progress made, remain to come.

Claude Le Gressus

Thanks for your opinion that interest in energy savings will come. Well. Nature is using it for millions (billions?) years. Actually it is not a big issue to follow Nature. Bad luck for humankind was and is that mineral oils replaced bio-lubricants because they used to be cheap but we continue to utilize them in times when bio-oils are cheaper or compatible in cost with mineral oils.