# Friction coefficient

1. Jul 12, 2006

### Luiscb

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

What happend 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

2. Jul 12, 2006

### 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.

3. Jul 13, 2006

### 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.

Last edited: Jul 13, 2006
4. Jul 13, 2006

### Cyrus

Interesting post Jeff.

5. Jul 13, 2006

### Q_Goest

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.

6. Jul 14, 2006

### rcgldr

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.

7. Jul 14, 2006

### Luiscb

8. Jul 14, 2006

### arildno

9. Jul 27, 2006

### Luiscb

thanks for the link, but I saw that before.

10. Jul 31, 2006

### Luiscb

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

11. Aug 1, 2006

### Luiscb

12. Aug 1, 2006

### 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?

13. Aug 2, 2006

### 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).

14. Aug 10, 2006

### Luiscb

yes please send me that papers
thank

15. Aug 10, 2006

### 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.