When two surfaces rub against each other, do they lose atoms?

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Main Question or Discussion Point

When two surfaces of any kind rub against each other which happens everywhere every moment, do they lose atoms/particles from the surface theoretically ? Is it what we understand as "wear"?

However, looks like in our daily life, something doesn't seem to wear over time (even after many years) if we keep them in good condition, for example, we clean it, we wipe it, but for both "cleaning" and "wiping" we are practically rubbing against the surfaces, in which it should "wear" faster.

How about scratch? What is the difference from the microscopic point of view between wear and scratch? Are they the same but scratch could be understood as a greater extent of wear?? Or Aret they different??

I once studied Chemistry but have left school for many years.
 

Answers and Replies

  • #2
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do they lose atoms/particles from the surface
They can.

something doesn't seem to wear over time (even after many years)
What about tires?
 
  • #3
256bits
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How about scratch
You must have heard of the Mohs scale for hardness.
It lists some materials in the order such that a mineral with a higher Mohs number will scratch a material with a lessor Mohs number.
https://en.wikipedia.org/wiki/Mohs_scale_of_mineral_hardness
Other scales are also mentioned.

If you want to consider the wear of a material - a softer material will "wear" out faster than a harder material.

Wear is not always a bad thing. Sandpaper a piece of wood - the wood wears out faster than the sandpaper, and one goes from coarser grit to finer, obtaining a desired surface finish. An abrasive procedure.

Same for polishing some objects. One removes some material from the object ( or transports some material to a different location ) to obliterate imperfections.
https://en.wikipedia.org/wiki/Polishing
I am not sure of this sentence, as there is no citation.
Polishing with very fine abrasive differs physically from coarser abrasion, in that material is removed on a molecular level, so that the rate is correlated to the boiling point rather than to the melting point of the material being polished.
The term " very fine abrasive" would have to be key.

The Wiki's are rather sparse on the subject, but I am sure one can find more in depth discussion.
 
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Kenny, since you've studied chemistry, I think that if you review ionic bonding, metallic bonding, covalent bonding, and the different types of intermolecular bonding you might have a better microscopic picture for why materials behave differently. Make sure to take note of the strengths of the bonds and which materials deform and which 'break apart'.
 
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You must have heard of the Mohs scale for hardness.
It lists some materials in the order such that a mineral with a higher Mohs number will scratch a material with a lessor Mohs number.
https://en.wikipedia.org/wiki/Mohs_scale_of_mineral_hardness
Other scales are also mentioned.

If you want to consider the wear of a material - a softer material will "wear" out faster than a harder material.

Wear is not always a bad thing. Sandpaper a piece of wood - the wood wears out faster than the sandpaper, and one goes from coarser grit to finer, obtaining a desired surface finish. An abrasive procedure.

Same for polishing some objects. One removes some material from the object ( or transports some material to a different location ) to obliterate imperfections.
https://en.wikipedia.org/wiki/Polishing
I am not sure of this sentence, as there is no citation.
The term " very fine abrasive" would have to be key.

The Wiki's are rather sparse on the subject, but I am sure one can find more in depth discussion.
Am I right that the material with higher Mohs number could also get scratched by the lower Mohs number but the extent is less?? Or they just...couldn't get any scratched by the lower number material at all??

btw, I have just googled about Mohs hardness, but most sites give only a list of hardness of minerals or jewellery ,is there any list of more common materials that we can find easily surrounding us??
 
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256bits
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Am I right that the material with higher Mohs number could also get scratched by the lower Mohs number but the extent is less?? Or they just...couldn't get any scratched by the lower number material at all??

btw, I have just googled about Mohs hardness, but most sites give only a list of hardness of minerals or jewellery ,is there any list of more common materials that we can find easily surrounding us??
I would think that since the level of hardness between each item in the Mohs list is quite widespread - its not an absolute scale , just a qualitative scale, and readily available for geologists in the field - the scratching is one sided, the harder scratches the softer and not the other way around, otherwise it would be not very useful for the field engineer - at least that is the assumption.
Yet, ( did you read the wiki )
Frequently, materials that are lower on the Mohs scale can create microscopic, non-elastic dislocations on materials that have a higher Mohs number. While these microscopic dislocations are permanent and sometimes detrimental to the harder material's structural integrity, they are not considered "scratches" for the determination of a Mohs scale number.[8]
Some of the other scales where the hardness from one material to the next is absolute, each can produce a scratch on the other, if the materials are close together in hardness.

For a list of materials see the wiki again
 

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