I guess the misunderstanding that the COF should increase with surface roughness stems from the misleading colloquial usage of the term "rough". In general the COF doesn't correlate too much (at least not consistently) with surface roughness. However, if you have a very smooth surface, you might have adhesive components to the normal force in contact, which will result in an increase of the macroscopic apparent COF.

Note that one parameter is not sufficient to characterize roughness. And "zero" is just "smaller than measurement accuracy" :)

Best regards

The increase of COF may be due to the increase of contact area after surface finishing.

Of course increasing surface roughness lead to increase the Coefficient of friction and also the value of surface roughness depended on field of applications for surface of part .

Dear Piyapong Buahom

Follow are the test conditions and procedures before and after surface finishing

1) Ball on disk test rig

2) Steel ball dia 3 mm

3) wear track dia 3 mm

4) No of laps - 1000

5) frequency - 5 Hz

6) Speed - 5cm/s

Number of repetitions - 3 times

Average the value and calculated std.dev.

I want to know if a fluid is flowing on top of the finished surface, how will be the resistance to the flow. As my COF increases as surface roughness reduces, how do i explain that fluid flow will experience less resistance?

Dear Emanuel Willert and Daniel Bulgadaryan

Thanks for the reply. After reading few articles, I understand that COF may increase with decrease in surface roughness due to increase in contact area. Below is my application. I am unsure on how to explain it.

Q) I want to know if a fluid is flowing on top of the finished surface, how will be the resistance to the fluid flow. As my COF increases with decrease in surface roughness, how do i explain that fluid flow will experience less resistance after surface finishing? or how will the finished surface benefit the fluid flow properties using my COF results?

If there are any reference, pls suggest or provide your own suggestions too. Thanks.

The COF is not really useful to predict the friction between your material and a liquid, as the liquid-solid tribological interaction is governed by fluido-dynamic equations and is quite different from a solid-solid interaction. If your scope is measuring or estimating the resistance of a solid in a fluid, i would change test type. Many factors enter in the fluido-dynamic equation among others the viscosity of the fluid and relative speed and also the roughness of your specimen...

Surface finishing does not automatically mean lower friction. At first you will increase the contact area and you will have increased friction at the onset. Afterward the lubricant will take over and you will have lesser friction. You should study all these factors such as: lubricant, temperature, roughness, EHL or boundary lubrication and measure your responses with respect to Ra before trying to get a conclusion. One factor does not tell you anything about friction. You should optimize the interactions of several stated factors above to be able to draw a conclusion. You should use DOE to build a matrix then optimize your process.

This result seems to be logical: in the case of a dry contact between metallic surfaces, the friction mechanism is plasticity-based. In this case, high roughness leads to smaller real contact area and high local pressures at asperity contacts, which reduces the ability of these micro-contacts to carry additional tangential load (before the onset of sliding), which finally yields lower friction coefficient.

This result remains true in boundary lubrication, mostly because of reduced real contact area with higher roughness. In mixed lubrication, however, coarser roughness normally results in higher friction due to a higher fraction of solid-to-solid micro-contacts within the nominal contact area. Finally, in EHL the friction is mostly defined by the physical properties of the lubricant while roughness plays minor role.

How long does it stay like that in boundary lubrication. At the onset I agree with you, but once we have long term durability testing temperature will take over and dictate the friction mechanism and not the small aperities alone. Factors such as temp, asperity, and lubricant additives all interact and understanding this mechanism in boundary regime is important to understand the overall frictional issues, especially when you are using bearing steel materials

Kinetics of the COF should be taken into accout, as in the course of the wear process COF is widely chainging (mainly because of the running-in ). Mostly, despite differences in surface roughness at the beginning of tribological tests, after some friction distance we get so called balanced roughness characteristic for used materials.

Nagalingam Arun Prasanth sir

Regarding your 1st question rough and hard surface responsible for abrasive wear mechanism and formation of wear debris also effect the friction value.

In general we cannot comment about COF without knowing wear debris morphology and wear mechanism etc.

2nd Question trapping of debris and heat decide final COF apart from aforementioned points.

Formation of wear debris will affect the friction value if you have no tribofilm formations. But my experience tells me that certain additives will react with the surface and depending on the temperature that you can optimize you will have certain tribofilm that will lessen the frictional events. Please review my publications in wear and tribology transactions.

Gabi Nehme sir are you talking about EP additives.

Extreme pressure additives are the most important specially at high reaction temp

Welcome to tribology!

Enrico Corniani Thank you for the suggestion. After doing a series of tests i understood that my COF becomes constant at the end of the test. Surface finishing does not play a major role in altering the COF. However, the contact angle of a liquid droplet would be a much better estimate to find the relation between liquid contact with a surface before and after surface finishing. Do have any suggestions on the type of test that can be conducted to determine this?

@Eugeniusz Sajewicz : You are correct. After surface finishing COF change is noticed only for the first few minutes. COF reaches a constant at the end of the test for all surface roughness conditions.

Some acadmit people would suggest to use the entire fractal power spectrum of roughness. I warn you that this will not lead you anywhere except publish papers ;)

Coefficient of friction does not only depend on surface roughness but on several variables depending on your test.

If the mentioned intitial COF is a run-in COF(and if there is no lubrication), the decrease of initial roughness height increases adhesive wear and adhesive component of friction force. However to “feel” difference in intitial COF the roughness average height should vary in wide range. At sufficient loads and smooth surface of the sample alumimium can transfer on counterbody surface non uniformely. I have carried out some experiments with aluminium alloy - steel pair at dry friction. The effect of ititial roughness was small and only at high loads. The formation of balanced (equilibrium) roughness at the run-in is a complex and unsolved problem now, depending on the wear mechanism.

The run in process is not very effective in measuring coefficient of friction specially in the boundary regime. there are several variables that could intervene to lower or increase such coefficient. you should look at the film thickness through time and optimize your conditions accordingly.

As I know the surface roughness increase with increasing the coefficient of friction during mechanical machining .

Sometime surface roughness will be benificial to form a solid tribofilm that will happen at certain temperature depending on your lubricant and additives which will contribute to lessening friction and wear later or for long term duration tests. All depend on how you optimize your variables and conditions

In general under unlubricated conditions, lower Ra means that your real area of contact is higher (compared to higher Ra), under this condition higher adhesion forces are expected, due to the fact that you need to shear contact spots of higher area; therefore you will experience an increase in the friction force and proportionately on the friction coefficient

Adhesion and interactions with additives can be beneficial at high temperature since you will have tribofilm reaction with surface and this can enhance antiwear resistance