Boron Nitride as a lubricant

[vile_fly]

Sounds like 100% sintered hBN is the way to go for piston rings if it has the same strength. This should solve many problems, indeed.

A CAD program I like that is free is from www.emachineshop.com. I use it regularly to draft .STL files for my 3d printer. It also allows you to estimate costs of different materials for various designs that you want machined for you. Basically, you design it, check your estimate, send it in, confirm the order of a number of units to be produced, pay, and they ship them to you. Their prices can be beat, though. I like to use it to export .stl files for my other programs.

[Aviator168]

Sounds like 100% sintered hBN is the way to go for piston rings if it has the same strength. This should solve many problems, indeed.

I am not sure. You do know how piston rings work. The high pressure gas in the cylinder gets into the ring groove and expends the ring against the cylinder wall to create a perfect seal. I am not sure if 100% sintered hBN rings have the same elasticity as the cast iron piston rings.

[vile_fly]

certified master mechanic, 25yrs. But I don't know avionic technology and methods. There may be a great number of things I have never seen, and Ian could probably shoot me down any day of the week if I am not careful.
However, automotive a/c compressors use ceramic piston seals vs aluminum cylinders with no end gap. The dykes ring design with drilled gas ports in the piston crown at this point is optional. What they did do was bond the ceramic seals in place on the pistons and turn the outside in a lathe to expose the ceramic .002" above the aluminum piston. The only real purpose I have in mind for the BN seal is to take the heat off of the teflon "x" seal behind it. So, we are at the "learn by doing stage" at this point. We simply won't know till we try. I might be wrong, I might be right, we both might be wrong, heh. such is life.
What would be also interesting is output shaft seals for the crankshaft made of hBN coated vee packing seals like this:

[Ian S C]

All I can say is that the aluminium cylinders are probably hard anodised, or maybe as in some IC engines chromed.
All this is called R&D, industry spends billions on it. My R&D budget goes up to many dollars, has been know to go up to $NZ 10.
Ian S C

[Aviator168]

In general cylinder liners are made of cast iron.

vile_fly, interesting ring configuration. Compressor seals do seal better Just made sure you have a way to remove the heat from the hBN ring and that is big challenge. The alpha stirlings I seen all have the teflons seal bellow the hot end displacer and they are usually in the area of the cold exchanger (I call those fake alpha). My thought was to do a way with the teflon seal all together. Even IC engine ring seals have small amount of blow by.

[vile_fly]

The IC engine doesn't have gapless top rings because they would not survive. Many racers have tried it, but even Total Seal stopped offering a gapless top ring and went with a gapless 2nd ring. The aluminum compressor piston bores ARE anodized (overhauled plenty of them) and so are the pistons in case you wanted to know for sure.
I really think using automotive engine technology on a stirling engine is not the way to go. Apples and oranges. A compressed fluid system is the best tech to rob from, hydraulic or gas. The only thing automotive can offer is variations on piston design or crankshaft design. Oh, and cylinder liners....I tend to collect them for future projects.
As to the heat removal, a good heat dam built into the piston should work. Some designs even use heat reflectors (sheet metal) on the underside of the piston to keep the infrared heat at the top of the piston.
The vee-seals can be stacked until the target rating is achieved, and are made of teflon. The pressure ratings for these teflone shaft seals are up to 3000psi. here is a better picture.

[Aviator168]

If your alpha engine is one of those where the heater, regenerator, and cooler on one cylinder, heat is not a problem for teflon seals.

Their seals can go up to 600F
http://www.coorstek.com/materials/plast ... _seals.php

[vile_fly]

V-twin configuration, 80mm bore, 30mm stroke. 600F isn't enough to satisfy me, 450F over a long time period will eventually decompose the teflon to make it fail. I need to go 2x over the design limit for longevity's sake. Totally impervious would be nice, but this is a search for the best material, and this is today's zenith.....until someone finds something better. I think ceramics are the key. I will not debate that the design is less than optimal, but most of the parts are mass produced already.

[Aviator168]

I hope you guys find this research paper useful.

Unidirectional carbon fiber-reinforced hBN composites were fabricated by a filament winding technique, followed by hot-pressing at 1900, 30MPa for 1h in N2 atmosphere. An hBN powder containing 8 wt% oxygen and a high-purity hBN powder admixed with 30 wt% Al2O3 were used as starting powders. The bending strength of hBN composites and monolithic hBN was evaluated by three-point bending test. The strength of hBN composites was twenty-five times and six times larger than that of monolithic hBN (B2O3) and hBN (Al2O3), respectively. The hBN composites exhibited typical fracture behavior of fiber-reinforced composites.

https://www.jstage.jst.go.jp/article/jc ... 81_58/_pdf

[vile_fly]

Nice research paper, I just wish it wasn't mostly in Japanese, but it still contains useful info for sure.
I found one on ceramic-hBN composites. Unfortunately, it is on google books, and I can't post the conclusion word-for-word. Scroll down to the bottom to see it.

https://books.google.com/books?id=tzFIA ... es&f=false

The conclusion is that ceramic composites work great against other ceramics, but perform poorly against steel. They say the testing is not thorough enough, as they were testing the feasibility of the composite instead of testing against steels and alloys. I think it was .6 coefficient of friction vs steel. They mentioned that porosity is the enemy, and causes more friction if present.

Looks like we both are doing some digging. This is both fun and productive research, thanks.

[Aviator168]

I think it was .6 coefficient of friction vs steel

That's very bad; like grinding rocks against each other.:=) Teflon vs steel could be as low as 0.05. I want to know how they come up with the conclusion that porosity is the problem since they haven't produce a ceramic that isn't porous.

BTW. Do you know the friction coefficient of ceramic vs ceramic?


Edit: I wouldn't worry about too much about the high friction coefficient. As long as the side load is low enough, it wouldn't matter.

[vile_fly]

Ceramic vs ceramic friction depends greatly on the ceramic, of course. ~.27 is what the paper found. rough estimate.
About the same as nylon vs nylon friction. I guess hBN is not all the great in ceramic mixes, but helps in small quantities, like 5%.

[Aviator168]

There is information on this page the contains friction coefficient of some materials. Look at graphite. Twice the coefficient of teflon, not bad at all.
http://www.engineeringtoolbox.com/frict ... d_778.html

[vile_fly]

Have 80mm graphite slugs ready to go already thought of that. Have enough to shut down a small reactor. I find glassblowing suppliers to have plenty of graphite for sale. Graphite + hot air = CO2. CO2 is very bad for a working fluid. The design target is for air as a working fluid, all other considerations secondary. I can switch to more efficient fluids later on at any time. Graphite wears too quickly for my taste.

[Aviator168]

Trying to burn nuclear grade graphite with a 1500c fire.
https://www.youtube.com/watch?v=PqmuVaHpw-Y