[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]

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.

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]

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?

I already addressed that:

are stable because they either don't react with air oxygen, or

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).

how about a plastic surface

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?

[Corribus]

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

No because polymers are permeable to oxygen. Compare to metals, in which oxygen diffusion is very low. Therefore reaction between oxygen and a metal is limited to the surface. Oxygen generally can't permeate far through the oxide layer so the oxide layer protects the interior from further oxidation unless it flakes (i.e., iron/rust) or is mechanically removed (e.g., by polishing, like with silver)

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?

Light catalyzes reaction with oxygen. Some polymers are more reactive than others. Polypropylene is far more prone to oxidation than polyethylene, for example. Polymers with benzene rings tend to be UV-sensitive because benzene rings absorb UV irradiation more efficiently.

[shvcko99]

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

No because polymers are permeable to oxygen. Compare to metals, in which oxygen diffusion is very low. Therefore reaction between oxygen and a metal is limited to the surface. Oxygen generally can't permeate far through the oxide layer so the oxide layer protects the interior from further oxidation unless it flakes (i.e., iron/rust) or is mechanically removed (e.g., by polishing, like with silver)

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?

Light catalyzes reaction with oxygen. Some polymers are more reactive than others. Polypropylene is far more prone to oxidation than polyethylene, for example. Polymers with benzene rings tend to be UV-sensitive because benzene rings absorb UV irradiation more efficiently.

Hello.

Is it correct that - Oxygen can keep penetrating plastic / polymers to react with it so that it's difficult to form a protective layer on the surface, is it correct?

Another question is - How about stainless steel, could it form any oxide layer?

A further question: Could plastic itself slowly lose its atoms to the air by itself??

Thanks for answers.

[Borek]

Is it correct that - Oxygen can keep penetrating plastic / polymers to react with it so that it's difficult to form a protective layer on the surface, is it correct?

Isn't it just what Corribus wrote?

Another question is - How about stainless steel, could it form any oxide layer?

That's exactly how it works.

A further question: Could plastic itself slowly lose its atoms to the air by itself??

Define what you mean by "lose its atoms to the air by itself". And please remember there are many types of plastics and they have quite different properties.

[Enthalpy]

when two objects just touch, there should be some wear ?

This question is still debated in mechanical engineering where unsound loads on ball bearings serve to measure a life expectancy which is then extrapolated to normal load. Books give formulas for that, but SKF, the biggest manufacturer, tells "no wear at all below some threshold load".

I tend to believe SKF's "zero wear" because they experimented more than books and standards authors did, and because nuclear engineering can detect nearly single atoms ripped from the parts and carried by the lubricant.

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

Most polymers consist of carbon, hydrogen and little more like nitrogen, chlorine or fluorine. None of these atoms makes a solid oxide. Very unpromising start.

Isn't plastic generally inert that it shouldn't react with oxygen?

Many polymers react with oxygen, even without light. Air contains also more reactive species like ozone and nitrogen oxides that harm polymers quickly. I already found plastic bags in a dark attic that were destroyed after 10 years.

[Corribus]

Many polymers react with oxygen, even without light. Air contains also more reactive species like ozone and nitrogen oxides that harm polymers quickly. I already found plastic bags in a dark attic that were destroyed after 10 years.

Case in point: our ICP-OES just sprang a catastrophic leak on the interior, water everywhere. We opened it up and found the plastic tubing, 10 yrs old, cracked into pieces when you flexed it. Completely deteriorated. No light exposure.

[shvcko99]

when two objects just touch, there should be some wear ?

This question is still debated in mechanical engineering where unsound loads on ball bearings serve to measure a life expectancy which is then extrapolated to normal load. Books give formulas for that, but SKF, the biggest manufacturer, tells "no wear at all below some threshold load".

I tend to believe SKF's "zero wear" because they experimented more than books and standards authors did, and because nuclear engineering can detect nearly single atoms ripped from the parts and carried by the lubricant.

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

Most polymers consist of carbon, hydrogen and little more like nitrogen, chlorine or fluorine. None of these atoms makes a solid oxide. Very unpromising start.

Isn't plastic generally inert that it shouldn't react with oxygen?

Many polymers react with oxygen, even without light. Air contains also more reactive species like ozone and nitrogen oxides that harm polymers quickly. I already found plastic bags in a dark attic that were destroyed after 10 years.

Hello

You said there are many reactive species in the normal air that could react with plastic, but it looks like if I have a plastic box or product it won't go bad after many many many years. Is it because the reaction is extremely slow and the concentration of the reactants in the air is also very low?

[Corribus]

The concentration of "reactants" in the air - mostly oxygen and I guess ozone unless you'rein a really industrial environment - is generally fairly constant. The variable is the plastic. Depending on (1) what kind of plastic it is, (2) how it was made, and (3) how it was stored, a plastic material can have a highly variable lifespan. Polyethylene with UV stabilizers can last virtually forever without exhibiting signs of deterioration if it's stored at reasonable temperature away from direct sunlight. On the other hand if you store a polypropylene or nylon container on your deck subjected to extremes of sun, moisture and temperature, it may not last a month. All depends.

[pnacze199204]

If it's true that when two objects of any kind collide, then what is the point of making a sphere like that in The Avogadro Project if you could not even touch it because you brush the atoms off? I once asked about diamond and got a very intersting answer from Enthalpy about cosmic rays that can rip off a carbon atom from the crystal. I'd like to ask if the same would happen with silicon? Does that mean that at any moment the world's roundest object can lose its its honorary title of the most roundest thing? 

[pnacze199204]

In one of the Internet forums I read: "Every contact results in surface changes to both materials". Is it really so? Does that mean that if I touch a diamond with my hand, the surface of it changes? Or the change is so small that it would take a lot of time to notice it? I'm confused...