COF change when adding water between metals

[Phil121324]

Hello, I have posted another question regarding valves and high torques on the valve section of the forum (

https://www.eng-tips.com/viewthread.cfm?qid=492603)

but im thinking the issue is not a valve issue and more of a material issue / COF change..... anyways, hear me out, I'm mechanical....

Im trying to get an explanation of why the COF between two stellite surfaces might change due to a layer of water between them. Iv looked into tribology and found that in some cases the introduction of water can have an adverse effect on the COF i.e frictional resistance can go up. This is despite the theory that adding water will lubricate. The COF might change due to strong capillary force creating a strong adhesion force......is this correct or im I talking BS. If it is a potential phenomenon can someone please explain the basics....

In the valve issue, I have the sliding speed between materials is slow, say 0.5inch per second, but the contact force is approx. 50,000N. Maybe the tests in tribology iv looked at are not relevant.....

Is there any study, research papers etc that can assist with my question or perhaps even explain what I think might be going on.

Keen to know any theories on this,

thanks, Phil

 

COF ?

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[Phil121324]

Coefficient of friction (COF) is a dimensionless number that is defined as the ratio between friction force and normal force

 

Oh.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[MintJulep]

Well, it's vastly complicated. Far more complicated than can be described in a single dimensionless number.

Material matters - down to the atomic level of electrons and protons. Both substrates and film.

Surface finish matters.

Film properties matter.

Film thickness matters.

Relative velocities and normal forces matter.

Go to your kitchen and cut out two pieces of flat plastic from something in your recycle bin.

Wet them with water and rub them together. See how things change as the water film changes.

 

[IRstuff]

Those pesky plastic bags in the vegetable section of the supermarket are extremely difficult to open with dry fingers. A teensy amount of moisture makes short work of that, there's just a right amount of moisture to make your fingers tacky, but over-wetting your fingers makes them too slippery

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Those pesky plastic bags in the vegetable section of the supermarket are extremely difficult to open with dry fingers legislate out of existence.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[IRstuff]

Regardless, some sort of bag needs to be present; putting all your tomatoes into a shopping cart that might have been used move pig carcasses to the meat department is hardly sanitary.

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[EdStainless]

IR, are you trusting the cleanliness of any fruit or veg?
I have watched them pick straight into the shipping boxes, which might be better than washing with contaminated water.

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[IRstuff]

No, I don't but I'd rather not add more Salmonella, E. Coli, etc., than I absolutely have to. Shopping carts might be free of COVID, but they're otherwise nasty places

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Could also grow your own tomatoes.
Looking at all the factors it's a win-win-win-win.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[IRstuff]

Kind of; we've done that twice and both time we wound up with nothing until they ripened and then we got sick and tired of eating tomatoes for a solid week, and then nothing again. But, we're not exclusively tomato eaters, so we'd need a full blown farm and hothouse to get both the variety and year-round availability

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We will all need to become small farmers again.
You heard it here first.

p.s., a little more friction (higher COF) would have prevented us sliding so far off topic.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[3DDave]

IRstuff - that's why you instead get sick of canning them, but have them at decent intervals the rest of the year.

 

[EdStainless]

OK, I am sorry, back to the topic.
At high loads I can't see water having any impact.
Co alloys are weird, they have great wear and erosion resistance, but they are not slippery.
We had disc valves that were Sellite faced.
They were used in high pressure ultra-pure water.
We had to cycle them hundreds of times before they were put into service.
As the faces wore against each other the surfaces burnished and work hardened they would become smoother and slick enough to survive at pressure.

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[mrfailure]

Adding to MintJulip:

Weight/Load matters
Movement directions matters
Shape matters (e.g. piston in cylinder versus flat plates against each other

I was enlightened a number of years ago when I had to look at wear of a "self lubricating" alloy against austenitic stainless steel. We engaged a wear lab who performed a number of tests to study wear. COF was surprisingly high given the self lubricating nature of the alloy.