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About materials wear

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shvcko99:
Hi all. I am new to this forum but I've finally got here where this is a sub-forum with a title the most appropriate to my question.

I'd like to have some general and conceptual idea about wear of materials.

Theoretically, is it true that when two objects of any kind collide, rub, slide or a just soft touch/contact, there should be some wear on the microscopic level (even if it's a very small extent that we won't notice) ?

When an object gets wear for its first time, will it still keep wearing (either quickly or slowly) in the air even without other factors that keep it wear?
i.e. two identical objects put under the same environmental conditions but one of them got collided or a hammering, which may lead to some wear. After that, both of them are kept stationary without any collision, would the one once get wear keep wearing in the air at a little bit higher rate than the one never got wear?

Borek:
Unless there was some special coating that got destroyed - not.

Most materials that are stable in the air are stable because they either don't react with air oxygen, or they use the oxygen to rebuild the protective layer. So, once they get hit and the oxide layer gets destroyed it gets rebuild rather quickly and things don't change any further.

shvcko99:

--- Quote from: Borek on March 13, 2020, 04:03:51 PM ---Unless there was some special coating that got destroyed - not.

Most materials that are stable in the air are stable because they either don't react with air oxygen, or they use the oxygen to rebuild the protective layer. So, once they get hit and the oxide layer gets destroyed it gets rebuild rather quickly and things don't change any further.

--- End quote ---

Hello. Do most materials form a layer with oxygen at the surface? How about if it doesn't react with oxygen? For example, how about a plastic surface, I don't think it will react with oxygen will it?

Borek:

--- Quote from: shvcko99 on March 14, 2020, 12:40:13 AM ---Hello. Do most materials form a layer with oxygen at the surface? How about if it doesn't react with oxygen? For example, how about a plastic surface, I don't think it will react with oxygen will it?
--- End quote ---

I already addressed that:


--- Quote ---are stable because they either don't react with air oxygen, or
--- End quote ---

Think gold or stone (or concrete). Minerals do erode, but (apart from those taking part in karst creation) in most cases it is mostly a mechanical erosion, not a chemical one, and not related to oxidation (you can think of minerals as of a mixture of oxides, they are already oxidized and thermodynamically stable).


--- Quote ---how about a plastic surface
--- End quote ---

All plastics do react with oxygen and they slowly oxidize/decompose. Those stable do it slowly, but they do. In general mechanical wear doesn't change the decomposition by much unless it introduces cracks which increase the surface on which the oxidation can take place.

shvcko99:
Could oxygen form a protective oxide layer with plastic rather than attacking it?

Isn't plastic generally inert that it shouldn't react with oxygen? I only know that UV / light could attack plastic but will oxygen do so as well?

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