Friction coefficient and pressure

[Luiscb]

Hello, I am student of Master degree about geotecnic here in Chile. So I am traing to solve the next problem.

What happened with static coefficient of friction when the normal pressure increase from zero to very high values.

For any material, Is it the same value of the coeffient of friction always or decrease before kinematic coeffient stars.

Could someone help me and send me papers or publication about that.

thank a lot

 

[Cyrus]

The coefficient of friction is a constant value.

The friction force is proportional to the pressure. Increase the pressure, and the friction force increases. But the coefficient of friction does not.

$$F_{f}= \mu N$$

N is the normal force (Pressure x area) at the area of contact.

The value of $$\mu$$ will change when relative motion starts between the two surfaces. It is always smaller than the coefficient of static friction.

 

[rcgldr]

In the case of tires, the coefficient of friction decreases with an increase of normal force. The friction increases with normal force, but not linearly.

There are a few solid materials that have almost the same static and kinetic coefficient of friction which would be useful for controlling moition (virtually no jerk), but it's easier to accomplish this by forcing fluids upwards onto a track where an object can move. The fluid resists horizontal motion, so the effect is the equivalent of having higher kinetic friction than static friction, but it's really the drag of the fluid. Again, this is good for precise controlled movements.

 

[Cyrus]

Interesting post Jeff.

 

[Q_Goest]

In the case of tires, the coefficient of friction decreases with an increase of normal force. The friction increases with normal force, but not linearly.

To be more specific, I assume you mean the coefficient of friction decreases with an increase of contact stress.

I know this is also true of Teflon and any Teflon compound such as Teflon reinforced with glass or carbon fiber, bronze, etc...

I'd assume the same holds true of any plastic, and perhaps even most materials. I've never researched the reason for this before, but I've always assumed it's because of wear (ie: the surface layer of the plastic sheds some minute amount). Note that the same relationship holds true for wear, the higher the contact stress the higher the rate of wear. I wonder if that's correct or if there's another explanation.

 

[rcgldr]

To be more specific, I assume you mean the coefficient of friction decreases with an increase of contact stress.

This is probably more accurate.

In the case of a car, the tendency of one end of the car to slide before the other can be controlled with anti-roll bars. By making one end of the car stiffer, more downforce is applied to the outer, and stiffer end tire, with less downforce on the outer, looser end tire, (and less downforce on the inner tires) and the non-linearity of friction versus downforce results in the stiffer end having less conering grip than the looser end when at the limits. This is commonly used for setting up a race car.

 

[Luiscb]

I need datas and examples, not just words. please help me

 

[arildno]

Well, here's a link for you:
http://www.roymech.co.uk/Useful_Tables/Tribology/co_of_frict.htm
It should be noted, however, that any such tables are fraught with inaccuracies since any impurities in the substances may have a significant effect on the actual friction coefficient.

 

[Luiscb]

thanks for the link, but I saw that before.

 

[Luiscb]

i can see that from macro scale to nano scales the friction coefficiente is totally diferent, the amonton's law it can't use it at nano scale, because the plastic deformations.

 

[Luiscb]

http://www.oetg.at/website/wtc2001cd/html/index-topic-paper.htm

i found this and they have a lot of papers
voula

 

[Q_Goest]

Hi Luiscb, That's a nice link! Thanks for the info.

Did you find anything about how coefficient of friction varies depending on contact stress in here?

 

[PerennialII]

Luiscb, one closed form way to study the impact of contact stresses to coefficient of friction would be to apply continuum models derived to account for effects arising from finite deformations, mismatching material properties near the region of contact (like in coatings), shape and geometry of the contact and inelastic deformation. These are typically studies the likes of "an indenter on a half-space" and thereon. I can hook you up with some latest papers if you're interested. If start going multiscale all the way to nanotribology then it'll get interesting :!) (but finding concrete answers a degree more difficult).

 

[Luiscb]

Luiscb, one closed form way to study the impact of contact stresses to coefficient of friction would be to apply continuum models derived to account for effects arising from finite deformations, mismatching material properties near the region of contact (like in coatings), shape and geometry of the contact and inelastic deformation. These are typically studies the likes of "an indenter on a half-space" and thereon. I can hook you up with some latest papers if you're interested. If start going multiscale all the way to nanotribology then it'll get interesting :!) (but finding concrete answers a degree more difficult).

yes please send me that papers
thank

 

[PerennialII]

Sure thing, I'll get back to you later this week when get back from the current "roadtrip". I'll PM you links to papers will upload.