How Does Surface Roughness Influence Cold Welding and Friction?

[Zorodius]

If I'm understanding it correctly, my book says that frictional forces are the result of cold welds - that when two surfaces are in contact with one another, the highest points on the surfaces on an atomic scale weld together to form a single object, and these welds are what resists movement when some object tries to slide over some other object.

This seems intuitive, but it raises a question in my mind: Usually, I would think of a rougher surface as one with greater friction, and a smoother surface as one that is easier to slide across. By my book's explanation, a smoother surface has the opportunity for more atom-to-atom contact, and therefore more points of cold-welding, leading to greater friction.

Why the discrepancy? Is it because I'm thinking of "rough" and "smooth" on entirely the wrong scale, or is it something else?

And, on a mildly related note, does this mean that any time friction opposes the movement of two surfaces across one another, some atoms of each surface are ripped off and stuck to the other surface?

Thanks for taking the time to answer my neophyte questions.

 

[arildno]

To the first (I'm on thin ice here..):
I would think that a macroscopically "rough" surfaces is microscopically characterized by lots of "protuberances" (spikes)
These protuberances can more easily slide into the other material, and hence come in close contact with more molecules (more difficult to dislodge).

Secondly, I would think that occasionally, such spikes will break off..

 

[Integral]

I work in a wafer fab clean room, a few weeks ago 2 200mm raw wafers were mis handled by our tool and ended up in full surface contact, these raw Silicone faces which are extremely flat and highly polished were impossible to separate. There was no glue other then a bit of moisture, no applied pressure other then atmospheric. Yet is was as if the wafers were glued together. Any time the unprocessed face of Si wafers come in contact it is very difficult or impossible to separate them, this is an example of high surface contact friction. You simply do not encounter this phenomena in the everyday world. Most surfaces are neither smooth nor flat enough for this type of bonding to occur.

 

[Zorodius]

You simply do not encounter this phenomena in the everyday world. Most surfaces are neither smooth nor flat enough for this type of bonding to occur.

So is it correct or incorrect to say that all friction is the result of cold welding, but that there are usually several orders of magnitude less cold welding occurring between regular surfaces than occurred with your silicone wafers?

 

[KingNothing]

You know, this really angers me. My physics teacher said that frcition had nothign to do with how much of the surface is in contact...I intuitively argued, it woulda been nice to have this stuff.

 

[Zorodius]

You know, this really angers me. My physics teacher said that frcition had nothign to do with how much of the surface is in contact...I intuitively argued, it woulda been nice to have this stuff.

I think he was pointing out that by reducing the area of contact between two surfaces, you increase the pressure across that area by the same amount, so the net change is nothing. Therefore, friction between two surfaces does not depend on the surface area, only the magnitude of the normal force and coefficient of friction.

 

[Zorodius]

So is it correct or incorrect to say that all friction is the result of cold welding, but that there are usually several orders of magnitude less cold welding occurring between regular surfaces than occurred with your silicone wafers?

I'm still curious about the answer to the above question.

 

[Integral]

So is it correct or incorrect to say that all friction is the result of cold welding, but that there are usually several orders of magnitude less cold welding occurring between regular surfaces than occurred with your silicone wafers?

The word "all" bothers me I am reluctant to draw that conclusion. I am not sure I understand the last statement, nor am I willing draw an conclusion with such an ill defined statement. Please clarify.

 

[Zorodius]

The word "all" bothers me I am reluctant to draw that conclusion. I am not sure I understand the last statement, nor am I willing draw an conclusion with such an ill defined statement. Please clarify.

I'm trying to understand why, when two surfaces attempt to slide across one another, a force resists that movement. It seems intuitive that the nature of friction can be best understood by considering the interaction between the two surfaces at a microscopic level. My physics textbook seems to say that frictional forces result from the difficulty of breaking the bonds between two surfaces that form as a result of cold welding. I realize that, if correct, this cold welding would normally be on a much lesser scale than that which occurred between your silicone wafers.

What I am looking for is a confirmation or denial that, when two mostly flat surfaces attempt to slide across one another, friction results from the difficulty of breaking the cold welds that form between the atoms of one surface and the atoms of the other.

 

[Gokul43201]

Friction is not a result of cold welding alone. Friction between rough surfaces is dominated by another effect. I don't know if this has a name but it can be "roughly" described by an analogy. Consider a road with irregular undulations on it, each of which has a radius of curvature of magnitude a foot. Clearly, it will be harder to drive your car over this road (not just from a passenger comfort point of view) than over a relatively smooth one. And sometimes, if the undulation (or pit) exactly matches the size of one of your wheels (what ? a car with different sized wheels ?!), you might find it easier to get past that pit if you leave that wheel behind. This is the "breaking off" effect that Arildno described. For most normally rough surfaces, I believe the above effects dominate the cold welding action.

 

[z_ahutd]

Hi guys. As a mechanical engineering final year student, I was assigned to design and build a cold welding prototype to weld just two sheets of metal together by lap joint. The thing is I have no idea how to design this since I haven't had any experience in building these kind of things. Anyone did similar kind of project? Can you show me examples or samples? Thanks in advance!