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Old 12-27-2009, 08:52 PM   #1
ocean
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Question Titanium Brake Shim Coloration

I've been running the StopTech Ti brake shims in my STI Brembos for a few months, and recently pulled the pad / backing plate assembly out of the caliper to take a look at it. The caliper piston side of the shim looked like it was brand new, but the pad side of the shim was mostly straw / yellow with some streaks of light to vivid blue through it.

My question is this: is there any kind of titanium coloration to max temperature scale? I've done a fair amount of research and everything I can find discusses welding, which so far as I can gather has more to do with weld purity and oxidization, or anodizing, which only gives colors in terms of the electrode voltage. I ask because I'm curious what temperatures the shims have seen so I have an idea of how hot the overall system is getting.

I know I haven't boiled the brake fluid so I know the shims are insulating well, but I'd like to know just how hot the pads and rotors are getting under load when I can't go at them with an IR pyrometer and figured this would be a good place to start. I do know the rotors have almost certainly seen temps upwards of 1200 - 1300 deg F based on ~950 deg F pyrometer measurements once the car is stopped, if that's of any help. It's probably worth mentioning I'm developing a ducting system to get these temps way down - will post a thread on that when I have more progress.

If any of you can bust out the mat sci skills I'd really appreciate it
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Old 12-28-2009, 01:33 AM   #2
williaty
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No, there's not a direct correlation. There's many, many factors involved in determining how thick the oxide layer is. However, the color scale for head-induced oxidization is the same as for electrically-induced oxidization. So yellow is "not that hot". Well, depends some on which yellow/gold/straw color.

Lemme tell you, reliably oxidizing titanium to the same color is a PITA.
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Old 12-28-2009, 06:52 AM   #3
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Titanium has a lower heat handling capability than stainless. Depending on the alloy, you may be operating in a range that is very near the limit of whatever those shims are made of.

Jay
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Old 12-28-2009, 12:47 PM   #4
williaty
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No, if he's only getting to yellow, he has A LOT of thermal headroom.

The important point is that Ti has a lower thermal conductivity. You're not going to melt either the Ti or the SS in a car. The Ti is just going to keep your fluid cooler.
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Old 12-28-2009, 12:48 PM   #5
ocean
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Quote:
Originally Posted by williaty View Post
No, there's not a direct correlation. There's many, many factors involved in determining how thick the oxide layer is. However, the color scale for head-induced oxidization is the same as for electrically-induced oxidization. So yellow is "not that hot". Well, depends some on which yellow/gold/straw color.

Lemme tell you, reliably oxidizing titanium to the same color is a PITA.
Yeah, I've heard much the same about achieving specific colors intentionally. I guess what I'm asking is, if we consider only temperature as a factor, is it possible to correlate the existing anodizing voltage tables to an actual thermal measurement? This is what I've been unable to find so far. And perhaps it's because no such thing exists, although it seems a general scale should be possible to construct. I may have to buy another shim set and go at it with some brazing torches . Just seems like someone else would have done this and published the results before, even if they're subject to a lot of fluctuations.

Quote:
Originally Posted by Storm View Post
Titanium has a lower heat handling capability than stainless. Depending on the alloy, you may be operating in a range that is very near the limit of whatever those shims are made of.

Jay
Can you elaborate on what you mean by heat handling ability? From what I understand, in this application the thermal conductivity is the really important aspect, but your point is well taken and is also a concern with respect to this application.

I would assume the shims are 6AL-4V, which I've read is generally used up to 750 F. Question is, what happens if it's exposed to twice that temperature? What little I've read seems to suggest that most hot working is done around 1,200 F and that almost half the yield strength is lost by 800 F. I'm not sure this is all that crucial however, but I also don't know what kind of pressure the shims are exposed to from the pistons. Given that they could likely see 1,400+ F, this may be a bit of a worry.

It's also possible they are a higher temperature alloy - I would hope StopTech used something much more thermally resistant given the extreme temperatures where their shims will be used. I will try to get in contact with them and find out the specific alloy they use.
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