Wednesday, June 25, 2008

"Anodised" aluminium, alumina over aluminum pistons

Friction. Friction is bad for engine performance. Aluminium piston in cast iron liner is a relatively long lasting and low friction combination.Aluminium puston in straight aluminium cylinder is a bad idea. Dies fast.

What is usually done is coating, like iron coated alu piston in alu cylinder (Porsche), or better solution, nikasil, kanigen or similar coating in the cylinder and alu piston.

These solutions are good, but technologically complex and not easy to implement for a hobbyist racer.

Mechanical watches of the past used texts like "17 jewels", what that meant there were 17 bearings consisting of ruby or sapphire. And indeed, steel over ruby or sapphire forms a very good bearing. Sapphire (and ruby) are minerals which re Al2O3, aluminium oxide. Guess what! Pistons are mostly from aluminium!

What can be done is that you can make the sapphire-like coating over most aluminium pistons! Standar method is anodic process where the aluminium part is submerded into sulphuric acid (H2SO4) or oxalic acid (in my opinion far better for this purpose!!) and you turn on the voltage source for several hours. The result is a aluminium oxide coating that is somewhat porous (and this may help us) and should copy original alu surface. The problem is mostly in thickness achieved: it is small, micrometers, maybe up to 10, 20. Next problems is dimension change: the aluminium oxide CAN be cissolved in the acid too, so sometimes after the maximum thickness is achieved, new aluminium oxide is created only at the expense of dissolving some of the top layer.

In any case, the piston surface must be very smoothly polished to be of benefit to us. Sapphire is hard substance and any loose aluminium oxide would only serve as an abrasive. While still on the aluminium piston, it can be a very helpful ally, and while OFF the surface a very nasty enemy. So the mechanical surface preparation is of the most importance to us. Also, manu aluminium alloys can not be anodised! The last point is that the Mercedes pistons use steel inserts which don't like oxidising in acid at all, the steel surface would need to be well protected from the bath and fumes!

INVENTION: Caustic "anodising" aluminium oxidation process

I did that some years ago. Basic compound in the electrolyte is water and caustic soda. Yes, the very same caustic soda that dissolves aluminium and attacks aluminium oxide on its surface. However we can stop that process by applying electric current. I folgot the current density needed, but it was very high, optimal (for quality) temperature was 91°c-93°C, the bath also contained some dissolved aluminium in the process, clean fresh bath was a lower uuality producer. The other electrode was stainless steel. Lots of bubbling, bath temperature is kept by the current alone. One more point: I forgot which electrode was "+" and which "-" pole, it may not even be an anodising process then!

Disadvantages: The part diameter will be less than it was at the start of the process. Some aluminium alloys will totally spoil the bath.

Advantages: Many, it needs some observation, but it is very quick. The resultant coating is transparent, shiny with milky-white look. Very slick surface after the process depending on bath state can have high environmental resistance (TESTED! some of them were thrown out on ground, in contact with soil, rain and all the dirt you can get). After applying oil, the surface is oleophilic (attracts oil, yup, we want that!) and the friction factor on the surface drops a lot too. The material used were forged aluminium spoons and the surface of the later bath specimens had milky pearl-like look and after oiling kept incredibly slick surface due to the oil filled the pores.

INVENTION: Use elecroless ""anodising"" process

And now for something I do in the recent two days. I made a path and submerged a slightly worn aluminium piston. I cleaned and degreased it with acetone. Surface was polished with 6-micron SiC powder which also removed contaminants and aluminium oxide. After dipping the piston in the bath (some hours), a coating of grayish metallic color became evident in all exposed aluminium parts. The coating is hard and slick when tested with fingernail, not up to the slickness of the electrolytic coating above, but the thickness is also very thin. (it gets better as the coating gets thicker)

Maing ingredients besides water are two neutral organic compounds and a minority of some salts (weakly caustic). So far I could explain it only as that the compunds serve as an oxygen transfer agent, as I used tap water, and the sodium ions attack the aluminium to form hydroxide which gets mostly converted into aluminium oxide. (Aluminium hydroxide would cause the gray coloring for example). The two organic compounds would serve as an oxygen transfer helper, the aluminium itself acts as a catalyst for the reaction and the organic compounds propably help during the oxide formation to promote the formation of nanopores trough which the reaction is able to continue further down into the metal. To prove this hypothesis a presuurised container would have to be used and filled with air, say ordinary compressed at 10 bar (~11x the pressure we breathe) would be a good start. So far, I was adjusting the bath and the coating is thicker, slicker and darker. As a side note: with one liter of air you can turn 0.16g af aluminium into oxide (quick math, take care).

P.S. I am already improving the process, the result so far looks like a very slick colorless transparent sapphire!


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