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Aluminum sticks to ruby spheres like it's an aluminum magnet

I work in an industry that uses synthetic sapphire (ruby) spheres to measure metal components made of aluminum. Ruby is very hard and aluminum is soft. When the two rub together during the measuring process aluminum is deposited onto the ruby surface. The aluminum deposit rubs off with scotch-brite, however the deposit changes the ball size and spoils the measured result.

What makes the aluminum stick to the ruby? There must be a scientific explanation.

Jim Yegge
Applications Engineer - San Jose, California, USA
April 9, 2010

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The ruby surface is nowhere as smooth as you think that it is, so it is acting like a very very fine sandpaper and filling the pores with aluminum. Other aluminum will smear onto this.
Don't sand the spheres. Dissolve the aluminum out with a solution of one teaspoon of lye per gal of water. Use the solution warm. Save the solution and add another teaspoon of lye as required. Lye is tough to get nowadays. You can use easy off oven cleaner [on eBay or Amazon] by spraying the balls and then soaking the sprayed balls in a minimum amount of water. You need protective equipment!

James Watts
- Navarre, Florida
April 13, 2010

Silicon Nitride is also used as an alternate material for the measuring spheres. It's hardness is relatively the same, the surface finish is the same, but it does not "scrape" the aluminum off. I'm sure that scraping is the right answer.

Jim Yegge
- San Jose, California, USA
April 15, 2010