i think it depends on the hardness ratio of the counter body and the tested materials, but in many cases the friction coefficient and the wear rate decrease with the increase of the sliding velocity. That is because of the increase of the strain hardening of the surface withe the increase of the velocity which lead to smoother surfaces and high strength surface layer. So, the wear mechanism tends to be fatigue mechanism at high speeds.

Dear Ibrahim thank you very much for the answer.

Higher sliding speed usually means higher surface temperature in dry sliding so that the subsurface layer may totally peel off or flow like a fluid. In this case wear rate will be high enough if measured by the size change. So both friction and wear will depend on the properties of friction-induced transformations and chemical changes. For instance, carbon steel will intensely oxidize at high siding speed and friction  will be reduced. In the case of stainless steel there will be strong adhesion between the samples and very high friction.

Dear Dr. Tarasov, thank you very much for the answer.

For many material connections wear rate and coefficient of friction show some inversion in relation to sliding speed, i.e. for low speed we observe high friction and wear, then increasing speed leads to deacreasing of tribological characteristics (some optimal state of tribological system), and finally rapid increase tribological charakteristics are seen when the speed icreases.

In general,changing of wear rate and coefficient friction  are complicated due to tribological processes. That mean that, increase of velocity can increase coefficient friction  due to heat generated at  conterface which, in turn increase the contact true area based formula µ=(F=A.t/W). (A=contact true area)

On the other hand, oxides existence (iron oxide) at the conterfcae can act as a self lubricant resulting in a decrease of µ.

Dear Zafar,

                     you should consider that the friction coefficient or wear resistance are not intrinsic properties of the investigated material but the result of chemical, physical and mechanical interactions of a system that includes two or more bodies.

So as mentioned by Ahmed it is important to consider also the counter part material.

However as pointed out before in general the friction coefficient, in general, decreases with increasing sliding speed. This is due to many factors including tribochemical reactions.

The wear rate, is a bit more complicated to describe. lt can decreases or increases depending on may factors including experimental conditions and microstructural features of the two sliding bodies .

Dr. Sajewicz, Dr. Portu and Dr. Rajabi thank you for your valuable answers.  

The answer is not simple.  Many of our students have done PhD's answering this question for any given pairing of material, stress/geometry, roughness, and lubricant.   

I am impressed by the answers given to your question. Friction and wear are too complicated to be characterized by a single parameter, like velocity. They are not materials properties; they can only be understood in terms of external conditions (pressure, temperature, sliding speed, atmosphere, . . .) as explained by H. Czicos in his 1978 book 'Tribology: a system approach.'

Wear behavior vs. sliding speed is best summarized by 2D (sometimes 3D) wear maps, developed by S.C. Lim and M.F. Ashby (Cambridge University) in the mid-1980s. They are also know as wear-mechanism maps or wear-mode maps or wear-transition maps; they give wear rates and wear modes in a plot of normalized pressure vs sliding speed.

Friction coefficients, equally complicated, are rarely correlated with wear. Don't be fooled by tables in engineering handbooks that list friction coefficients for various counterface materials. Many of my metallurgy colleagues asked me to measure the friction coefficient of this-versus-that material. I pointed to a chart that said:

COST OF A FRICTION COEFFICIENT:

$30 per hour for 0.3 to 0.5

$60 per hour for 0.2 or 0.6 to 0.8

$200 per hour for 0.1 or > 1

Dear Dr. Stewart and Dr. Singer thank you very much sharing the valuable knowledge.

Dr. Ibrahim please send the link of your article.

Dr. Ibrahim thanks alot for the paper.

Thank you all, however still I have another problem with friction coefficient as related to wear rate. If we think logically, considering the same counter disc, as the sample hardness increases, the friction coeff. is expected to increase since higher friction force will be generated and the wear rate is believed to decrease. However, this is not always the case. I am currently working on high temperature wear of magnesium and aluminum alloys and I got mad from the results. The relationship between friction coefficient and wear rate varies significantly with test temperature. So, if any one have good experience with friction coeff. at high temperature, please help.

https://iopscience.iop.org/article/10.1088/1757-899X/1059/1/012020/meta

I agree with Dr Irwin Lee Singer, and am also negatively impressed by the answers provided to such a difficult question. For instance, saying things like: "in general friction coefficient decreases with increasing sliding speed", is so inaccurate and wrong. This is a very complex question and the answer changes depending on the tribological conditions (tribopairs, temperature, atmosphere, normal load, lubrication regime, etc).

The decrease in friction coefficient with increasing sliding speed can be associated with the rise in the interface contact temperature leading to softening, fragmentation, and reorientation of the self-lubricating layer.

https://iopscience.iop.org/article/10.1088/1757-899X/1059/1/012020/meta

Effects of the Velocity Sequences on the Friction and Wear Performance of PEEK-Based Materials | SpringerLink