Sliding Friction between two Plates as a function of surface roughess

[MrDomino]

For our Manufacturing class, one of our take-home quizzes is to find out why, as surface roughness continues to decrease (think of very finely polished materials) the static friction between the plates as you try and slide them will increase. Basically, really rough surfaces will be hard to slide and really fine surfaces will also be hard to slide.

We can use any sources and the explanation must be in-depth (driven down to pretty much the molecular level).

As of now, we're having difficulty finding information and I figured some of you guys here might know where to look (we've been searching Google Scholar, Web of Science, etc.).

 

[Savant13]

The intermolecular forces are stronger if there is more area over which to act.

Let us consider a rough surface with grooves, the surface of which looks like this in cross-section:

_----_----_----_----_----_

The contact surface area between too such surface would be decreased, but the the grooves are not wide enough for the hills in between to go in them and catch.

Another possible surface would be like this:

__-__-__-__-__-__-__

When two of these surfaces are in contact, the hills of one enter the valleys of another. This results in a resistance to lateral movement. It is friction, but a different kind.

 

[astrokreng]

I would approach this question by first examining the concept of sliding friction and how it relates to surface roughness. Sliding friction is the resistance between two surfaces in contact that are moving relative to each other. It is caused by the microscopic irregularities on the surface of the materials, known as asperities, which come into contact and resist movement. The rougher the surfaces, the more asperities there are, resulting in a higher resistance to sliding.

To understand why decreasing surface roughness leads to an increase in static friction, we need to look at the molecular level. When two surfaces are in contact, there are intermolecular forces at play. These forces, such as Van der Waals forces, are responsible for holding the two surfaces together. As surface roughness decreases, the number of contact points between the two surfaces decreases as well. This means that there are fewer intermolecular forces acting between the surfaces, resulting in a decrease in the overall adhesive force holding the surfaces together.

Moreover, as the surfaces become smoother, the contact area between them decreases. This means that the same amount of force is now acting on a smaller area, resulting in an increase in pressure. This increase in pressure leads to an increase in the deformation of the asperities, making them more likely to come into contact with each other and resist movement.

Furthermore, as surface roughness decreases, the asperities become more closely packed together. This leads to an increase in the number of points of contact between the two surfaces, resulting in an increase in the overall frictional force.

In addition to the molecular level, we must also consider the effect of lubrication on sliding friction. As surface roughness decreases, the lubricant is less able to fill in the gaps between the asperities, resulting in a decrease in its effectiveness. This means that the surfaces are now in closer contact, resulting in an increase in the frictional force.

In summary, as surface roughness decreases, there are fewer intermolecular forces and a decrease in the contact area between surfaces. This leads to an increase in pressure and an increase in the number of points of contact, resulting in an increase in sliding friction. Additionally, the decrease in lubricant effectiveness also contributes to this increase in friction. This phenomenon is important to consider in manufacturing processes, as it can affect the efficiency and effectiveness of sliding mechanisms. Further research in this area could potentially lead to the development of new materials or lubricants