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The lower viscosity of water reduces friction. Besides, the water can transfer heat way better than oils. So why not use water as a lubricant in wind or hydro turbines, especially on equipment which is operating in the ocean ...

It seems to be a perfect material. It's the most abundant liquid on Earth, and it causes no ecological damage.

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    $\begingroup$ Ocean water is salty increasing corrosion issues unless you purify it... $\endgroup$ – Solar Mike Nov 17 '19 at 20:34
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    $\begingroup$ I think my question on physics.SE might be of interest for you: physics.stackexchange.com/questions/259501/… $\endgroup$ – ahemmetter Nov 18 '19 at 7:22
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    $\begingroup$ Why do you think a lower viscosity would be a good thing in a bearing lubricant? :) $\endgroup$ – Luaan Nov 18 '19 at 8:54
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    $\begingroup$ @Luaan Smaller friction! $\endgroup$ – Sam B Nov 18 '19 at 19:50
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    $\begingroup$ The chemical properties of water being a nearly universal solvent that can also easily become an acid or oxidizer as well as vaporize or freezer under most working temperatures make it less than an ideal lubricant in most long duration applications. $\endgroup$ – Matthew Whited Nov 19 '19 at 10:36
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The whole point of lubrication is to keep the two surfaces almost apart.

Water has low viscosity in comparison to oil-based lubricants, which means that it forms a thinner film (electrohydrodynamic film thickness) between the two surfaces; the thinner the film, the greater the chance of direct physical contact between the parts.

So, the higher viscosity actually can be useful here. Another thing about water is that its viscosity doesn’t increase by pressure, unlike oil-based lubricants, as usually, the two surfaces are operating under pressure.

Oils are better vibration dampers than water.

As @SolarMike mentioned in the comments, water is extremely corrosive and can also dissolve many destructive elements.

Thanks to the capillary effect, water molecules can penetrate into micro-cracks on/beneath the surface of parts. Once there, due to extreme pressure, those molecules can decompose into hydrogen and oxygen, and build up pressure in the cracks, which leads to crack propagation and failure. This phenomenon is called Hydrogen embrittlement.

Water also provides a very good environment for fungi and other living microorganisms, which decay and changes the PH of the water, usually a shift toward an acidic environment, which in turn also corrodes the steel.

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    $\begingroup$ @SamFarjamirad do you have a source for H2O decomposition under high pressure? I never heard of such an effect, and I find it very surprising that such an effect exists, but then if it does, I'd like an authoritative confirmation. (In general I'd worry more about the oxygen radical than the hydrogen embrittlement in that situation, but then again I do not have enough background about physical and chemical dynamics under high pressure to be certain.) $\endgroup$ – toolforger Nov 18 '19 at 12:14
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    $\begingroup$ @SamFarjamirad hydrogen embrittlement is a well-known effect. It's H2O decomposition that I am questioning; if you do not have a reference, you should probably remove it from your answer. $\endgroup$ – toolforger Nov 18 '19 at 12:48
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    $\begingroup$ @SamFarjamirad The reason to remove it would be because it is providing incorrect information, when the information isn't even required. Some quick searching seems to suggest that high pressure actually prevents the decomposition; while heat and low pressure are actually more likely to cause it. $\endgroup$ – JMac Nov 18 '19 at 14:35
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    $\begingroup$ @SamFarjamirad Wow... aggressive response much? Your answer doesn't mention electrocatalysts. I don't see why you're attacking me for calling out things lacking in your answer. I'm specifically questioning the claim that increased pressure leads to decomposition; because from what I understand, it is the opposite. I don't see why you would go on the offensive instead of just supporting the information you've provided. $\endgroup$ – JMac Nov 18 '19 at 14:49
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    $\begingroup$ I don't think that's an accurate characterization of hydrogen embrittlement, either. You described a phenomenon with several stages, none of which is hydrogen absorption or brittleness (a type of weakening) being caused. $\endgroup$ – piojo Nov 19 '19 at 2:26
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Sea water is about the worst thing in the world to put in a metallic mechanical device. As noted , corrosion would be horrendous ; dissimilar metals, stray currents , etc. Wind turbine gearing is an exceptionally demanding application ; EP ( extreme pressure ) lubes are needed with graphite and/or moly sulfide, etc. I am sure a major problem addressed by wind machine engineers for off shore applications , is sealing the lube systems to keep salt and moisture out. Not the least of problems is that water has little/no lubricity qualities like film strength.

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    $\begingroup$ Aqua regia would be worse ... $\endgroup$ – Martin Bonner supports Monica Nov 18 '19 at 9:00
  • $\begingroup$ @MartinBonnersupportsMonica My intuition immediately found hydrochloric acid. Sure, aqua regia destroys the surface fast. But hydrochloric acid makes any surfaces that it's vapors reach corrode, if they can. Slow, but not much better. If the surfaces are complex, much worse. $\endgroup$ – Volker Siegel Nov 18 '19 at 10:22
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Great question! Water absolutely is used as a lubricant in some power generating systems. For example many Francis style turbines use their feed water as the primary lubricant and coolant of the bottom main bearing. (In the old Loeffel turbines, that bearing was typically made of lignum vitae wood, and could last up to 100 years in near constant use). Good engineering takes advantage of the cheap availability of water whenever applicable.

But water freezes and expands, blowing apart your engines, so it's not suitable for every application. This is enough of an everyday issue that automobiles are designed with "freeze plugs" and block heater ports and typically use toxic and environmentally unfriendly antifreeze chemicals in their liquid coolant systems. If your generator fails in cold weather, you don't want it to freeze solid before you can manage to repair it.

(A freeze plug, also called a Welch plug, is a core hole plug purposely made weak enough that it will fail before the block cracks due to frozen coolant. Core holes are left by the block casting process.)

And although absolutely pure water is an electrical insulator in the laboratory, in real life it's not, because even the slightest amount of contamination makes water an excellent electrical conductor. Oils, on the other hand, typically have consistently low conductivity until they are so loaded with metal contaminants that they have lost lubricity. If your generator has an oil leak, the oil won't provide a conductive path to repair technicians and other system parts.

And finally, as others have already noted, water is very corrosive, especially seawater. Since rust expands to five times the size of the steel it came from, you can't use any of the cheap iron alloys in water, which drives up the cost of materials pretty quickly.

Petroleum oils are a superb lubricant, able to be purified and recycled many times. At least as early as 1915 intelligent people were writing that burning fuels like petroleum is a criminal waste of a fantastically valuable and limited resource; it's robbing future generations in order to turn our shared treasure into atmospheric poisons. It's nice to see people like yourself thinking about these things!

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    $\begingroup$ Great point about insulation, I forgot it in my post. @SamB $\endgroup$ – Sam Farjamirad Nov 19 '19 at 16:23
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    $\begingroup$ Might be good to mention the low boil point for water as well. $\endgroup$ – axsvl77 Nov 20 '19 at 18:01
  • $\begingroup$ @axsvl77, yes, very true. Not a problem in high flow rate turbines but certainly a big issue in closed loop systems. $\endgroup$ – Medievalist Nov 22 '19 at 4:35
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Lubricity is a material quality

Look into the difficulties they have had removing sulfur from diesel fuel. Sulfur was added (or not removed) to increase the lubricity of the diesel fuel, to improve freedom of motion in injection pumps. That being missing is a hardship for older diesel engines whose pumps depend on it. The fuel is basically the same in other respects, but for this lubricity factor. This is easily restored with as little as 2% biodiesel.

So here you have these two liquids, mineral diesel and biodiesel - that are both seemingly the same. But one is "slick" and the other not. That property is lubricity, and that is the very thing you need in a lubricating oil.

Water has none.

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