Solid lubricants for engineering ceramics!

I believe graphite is a good lubricant to well above 1000C, provided you do not have a strongly oxidizing environment.

Even if environment is oxidizing graphite doesn't burn very readily or quickly - things like iron oxidize more rapidly.

Good luck

Thank you for your input. I've looked into graphite, but in this application it differently would oxidise [degrade] and therefore is not suitable. Luckily I have found a department in the university in China which studies solid lubrication, so fingers crossed that they can give me the latest [if it's not too commercially sensitive].

Many thanks... JD

graphite is good lubricant for above 1000C? can we use any kind/grade of graphite for reducing friction and wear inside a hole?

I'm not an expert in this field, but I believe fine graphite can be used virtually anywhere in a reducing or neutral atmosphere.

As it is worked, graphite breaks up and tends to be adsorbed into irregularities of the surface and stick to the surface, so it gets better with use, at least for a while.

Did you consider MoS₂ ?

I have just read up on molybdenum disulphide. It says that it only works up to 600°F which is only 300°C unless it is in an oxygen free atmosphere, then it will work up to 1300°F which is 700° C.

Boron will work up to 600°C [1113°F] without an oxygen free environment; so it looks like boron is the benchmark to be improved on. Thx JD

did you consider Cu powder, or metal oxides, like ZnO, TiO, etc?

I can't see why you consider Cu powder. The melting point of Cu is close to the working temperature, and the particles would coalesce.

Having said that however, the question is why do you need lubricant at all? Ceramics do not fuse in friction. If at all, you would need a liquid lubricant to remove solid particles from created in the initial rubbing and the only possible liquid that comes to mind for the temperature is gallium.

Try using Cerium oxide!

Spencer.

My learned friend tells me that, lubrication would not be needed if no microscopic particles of ceramic were present between the two moving surfaces. Unfortunately this is not possible in the real world.

I am checking out gallium and some of the other suggestions [but not copper]. Just to make it clear, there are two ways to solid lubricate, [1] to use standard engineering ceramics and place some solid lubrication between the two surfaces, or [2] to integrate one or more solid lubricant(s) with the ceramic material. My intuition is telling me that, solid lubricants of different types in the two different surfaces, would work best. JD

Since the need you are attempting to address is for a dry lubrication under rather extreme conditions the prospective solution I propose is also rather extreme.

This link will give you a background on the general idea.

http://en.wikipedia.org/wiki/Detonation_nanodiamond

This link is to a specific vendor that may have a superior product in this field. Although comparative testing is really the only method of evaluation with any meaning in this sort of performance range.

http://www.diamondlube.com/Testing.php

In this product the nano-diamond is in an oil carrier, but the lubrication is not dependent upon the oil.

Here is their additional contact information.

Midwest Office
NanoLube, Inc.
9 N. Main St
Lombard, IL 60148

630-706-1250

Mr. Gee

'Extreme' is not in my vocabulary, 'appropriate' is Thank you for the information, I have e-mailed diamondlube to find out its operating temperature limitations to see if they have used it with engineering ceramics before. I'll keep you posted when they get back to me. JD

The problem you are confronted with is not trivial so that it has to be analysed with great care.

1- What is ther reason to use a lubricant? In general a lubricant is used to separate two similar or different materials at their contact-sliding surface and avoid a bond under pressure and higher temperatures on those surfaces. A 2nd reason is to reduce the generated heat by reduction of friction. The 3rd reason is to eliminate from the said surfaces the heat generated during sliding. A 4th reason is to eliminate wear particles.

2- In a high temperature application most of materials loose their cohesion so that pressures must be reduced in order to avoid particles separation from the contact surfaces. Experiment show that if shear stresses are small enough wear is very limited, it can be even spoken about "zero wear". But if pressures are very low heat generation will also decrease and the need for a lubricant is also reduced if the 2 materials in contact do not have the tendency to "weld".

3- If a lubricant is used and it fulfils its goal as mentioned above there is no need for a second since the 2 surfaces are already separated by the first.

I do not expect to change your mind that the use of 2 lubricants will be THE solution since such a priori opinions cannot be changed, I only wanted to make clear how the need for a lubricant is determined.

So if

- pressures are under a limit (experimental)

- if sliding speed is low

- materials do not have the tendency to build up micro-welds

- local temperature is not over a limit (experimental)

- friction is not so high that even at low speed and low pressure the heat generations and the shear stresses are too high

then there is no need for a lubricant!

Up to you to decide.

This is interesting.. there is a project to make a ceramic Wankel engine, and they seem very sure that they will need some kind of [solid] lubrication. Obviously in a Wankel engine there is no change of direction of the rotating 'piston' [rotor], so it should be less in need of lubricating than a reciprocating engine where the moving parts actually stop and change direction.

By the way, they may be experts on materials, but they don't seem to know much about engine design. A naturally aspirated Wankel engine running at well over 600° C. will have a greatly reduced intake of air due to the internal heat affecting induction. The [cool] air would need to be blown/pumped in.

However, I am an inventor-design and was inspired by the efficiency of running an engine at what ever temperature it rises to with no cooling. The reasons internal combustion engines are so inefficient is because they are made out of inappropriate materials and limited by the maximum operating temperature of engine oil lubrication.

My design is for a very compact and lightweight reciprocating two-stroke diesel engine [temperatures are too high for petrol/gasoline etc] which would use ceramic cylinders and pistons. Obviously I can't go into more detail other than to say it uses a combination of old and new technologies and is nothing like any engine currently available.

So…

Pressures and temperatures would be extremely high. Assume 1000° centigrade and a 22:1 maximum compression ratio. I do have an idea for cooling the cylinder and pistons if for example there was a fantastic lubricant which worked up to say 800° C. but obviously 'wasting heat' is 'wasting energy'

Do two engineering ceramic surfaces need separating at ambient to very high temperatures?

Generation of heat is not an issue.

Reduction of friction is desirable.

The elimination/removal of wear particles is necessary.

My IC engine could be in start-up conditions of -20°C and can run at 1000°C.

The piston could move back and forth up to 80 times a second [5000 rpm].

Scotch yoke design means no side trust but the piston will rest longer at BDC and TDC.

I am more than willing to have my intuition or theories proved wrong so that they are more likely to be right next time.

I look forward to your comments. JD

After you explained your goal it seems to me that only the experimental path can validate a solution since you will be confronted with high sliding speeds. I think that what should be done is to find a design able to eliminate wear particles without the need of a lubricant or to find a way to modify surfaces so that particles will not separate. The temperature interval is very large and I doubt that an only material could fulfil the function all over it. This is the reason to adapt the design to the lack of a lubricant.

There will be initial 'shaving' of surface asperities once you start the operation, like in the 'run in' of new engines in the old days. This will cause wear debris particles that can be quite aggressive.

This initial debris will have to be removed before you can go without lubrication. You also have to make sure that there is always a gap between the two surfaces (i.e. they are not touching).

What kind of clearance gap do you think is necessary between a ceramic piston and ceramic cylinder wall? I believe that both parts should expand almost uniformly as the temperature gets very hot, but having said that, expansion should be minimal compared to metal engines.

The only other question I have is, can friction be significantly reduced by use of a lubricant? Reduced friction means reduced temperature and more efficiency. Nano-diamonds are also very interesting. http://www.nabond.com/Nanodiamond.html

It looks like 'dirt collectors' is the way to go! JD

Debris result only if surfaces do touch and shear stresses appear.

No touch no debris so that...

In an engine if a connecting rod is used then a side load is imposed and a contact between surfaces is present since the force loop goes through the contact zone piston-cylinder.

It is also possible and I consider that this should be investigated by the inventor to separate the side load transmission from the sliding sealing property. This is what was done in steam engines and the idea was not bad at all. Such a design will permit a contact free pair piston-cylinder with a limited gap and with a nil risk of debris generation.

It is not impossible that minute debris come due to thermal stresses. The design of the two surfaces should be so that those will be recovered and taken away from the gap.

Side thrust is not an issue, as I have previously stated, my engine will have a Scotch yoke design not a con rod type crank. 'Debris removal' is defiantly on the design palette. The information I am lacking right now is, the clearance gap between the piston and the cylinder. Got any idea what the margins should be? [Use, 350cc Bore 75.00 mm stroke 79.10 mm, as a guide]. What about some kind of piston rings? Is the clearance you are thinking of so small that rings are not worth bothering with? Thx JD

The gap cannot be estimated but computed. Depends on many factors it is not a simple exercise. The rings will slide under load since the pressure will press the ring on the cylinder.

As I said it is not a trivial problem! There are ways to reduce leaks but also quite complex as computation; unfortunately it is not enough to have qualitative ideas some times quantifying is a must.

During the 80's, the Japanese experimented with an uncooled ceramic engine. I believe they used partially stabilized zirconia rings and I think walls and piston were alumina. I'm hazy on the use of alumina. I was working on something else using alumina at the time so I may have confused the two.

re my earlier post (#17) I've found an article "Ceramic engines are hot" (Laurel M. Sheppard; Mechanical engineering October 1984. The subtitle reads "The ceramic engine may be available commercially before 1990".

Nissan tested a ceramic gas turbine, Isuzu a diesel with zirconia based components, GM's Opel division a 2.3L diesel with ceramic liner and valve heads (ceramic not specified); Carborundum in joint venture with Volkswagen had completed a 13,000 mile road test with diesel which seems to have been an opposed piston 2 stroke with SiC liner and pistons. Others were testing ceramic components such as tappets etc.

Obviously I had some of my previous info in #17 a little scrambled.

Hope this helps

Hi Jon D

you may try " Glass " as the lubricant ( micro glass beads ). Please find more info.about Rheologically controlled glass lubricant for hot metal working in

US Patent 524506. Pl. refer www.freepatentsonline.com/ 5242506.html

This is OK for HOT EXTRUSION processes.I am not sure about ceramics.

Regards. P.Rangasamy