Coated Tune cassette

Some months ago, we got a full titanium Ti-6Al-4V Tune cassette. We wanted it to last as long as possible to take the best out of its properties, in term of material and weight.  Thus, we sent it to Surface Solutions Incorporated, for a special very hard coating called Alpha. This coating is about sending metal vapors containing ionized particles to react with the material and increasing its hardness.  It is realized at high temperature (800°F) and the thickness of the coating is about 100 to 200 microinches.

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Originally, the titanium teeth hardness is around 320HV (Hardness Vickers, a hardness unit). The Alpha coating increases the surface hardness up to 4400/4600HV, which is close to half the diamond hardness, the hardest material of the earth. As information, stainless steel is close to 200HV.

Theorically, such a coating makes cogs that last eternally. No wear, even after a few dozen thousands kilometers… However after a couple of thousands kilometers, the coating seems to have disappeared where the chain pins enters in contact. So is this a simple color change or a did the hard coating run away? We continue the long term test for more precisions.

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What’s the hardness of a material?
The material hardness represents its ability to resist to permanent and plastic deformations. There are several standardized tests to determine it. The Vickers hardness test uses a diamond square based pyramid that enters the material via a load. Once the load is took off, the pyramid goes back because of the material elasticity. However it doesn’t go back to the original position. The difference in depth between the original and the final position ( h on the drawing) represents the hardness.

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