Ceramic coating for Titanium - High Temp Stability 1

[explorervs265r]

I'm looking for a ceramic coating to be used on Titanium brake rotors. The coating needs to be stable at high temps (high being relative, I don't have an exact temperature unfortunately). The coating is over brake rotors so the coefficient of friction can not change adversly at high temperature. I've been told W-Cu, W-Carbide, TiAlN and TiN were possibilities but all had downsides. Do any of you know of a coating that could work?

 

[JimMetalsCeramics]

How thick a coating do you need? If you can live with a few microns, I suggest that you have the part plasma nitrided or nitrided (or carburized) in a furnace at any standard heat-treating house.

 

[kenvlach]

A significant thickness is needed for a wearing part such as a brake rotor. For chemical and thermal compatibility, use a Ti-based coating, incorporate either TiB₂ or TiC for hardness/wear resistance, and apply via a reactive plasma deposition process for a fine-grained, uniform structure.

Have you considered making the entire rotor as a Ti matrix composite (with either TiB₂ or TiC particles)? TiC_particulate-reinforced Ti-6Al-4V composite prepared by powder metallurgy technique had superior strength and creep resistance compared to the base alloy, although fracture toughness was lower. – Composite Materials, Vol. 1, M. M. Schwartz, p. 181 (1997).

 

[explorervs265r]

Thanks for the TiB2 idea. I did a little research and it looks like that may work well. I do have another question now though. How do I figure out what compounds have a high coefficient of friction with TiB2 (i.e. what material of break pads do I use with it)? I don't want to use incompatible pads and end up with a rotor that took so much effort to make and barely works. Thanks again for the good advice so far.

 

[JimMetalsCeramics]

A company like VaporKote (

http://www.vaporkote.com)

can boronize the surface of a titanium part, converting it to TiB2.

Coefficient of friction is primarily a function of surface roughness and texture. For a brake pad you want good fracture toughness and wear resistance. You also want materials that mate without one phase severely wearing the other. If you can afford it, C-C composite is a common, high performance brake pad material. It is great for flat, sliding wear.

If you roughen or texture the TiB2 you should probably have is rub against another piece of TiB2. If you lap it, you might still try surface treating the mating surface so that it will not preferentially wear so much faster than the TiB2 that it wears out quickly.

 

[kenvlach]

I meant that the coating (or maybe the entire rotor) be made of a Ti metal matrix composite, reinforced with either TiB₂ or TiC particulate. The coating should be thick enough to contribute to the overall rotor strength, with the substrate thickness reduced accordingly.

An ultrahard ceramic thin film would be unsuitable for long-term use –cannot do any finish machining or ‘trueing,’ there is a CTE mismatch, and eventually some scratch or gouge or wear would result in spalling and unpredictable behavior, i.e., a rotor which cannot be turned so must be scrapped. As brakemaker stated in thread342-60704, MMC’s are a proven material for friction and brake applications. With a metal matix composite rotor or rotor coating, you will have predictable ‘pedal feel.’

To produce a TiB₂-reinforced Ti matrix composite, use a plasma deposition process with Ti (or Ti alloy such as Ti-6-4) and B powder feeds of desired ratio to produce ultrafine TiB₂ in situ within the Ti matrix. If producing the entire rotor of Ti metal matrix composite, a P/M method can be used; I don’t know whether other methods are commercially feasible.

As JimMetalsCeramics stated, C-C composite is a common, high performance brake material [common for F1 racers and aerospace and perhaps the French TGV]. Rather expensive, though.

 

[JimMetalsCeramics]

A carburing, nitriding or boronizing process would produce a coating on Ti which has similar expansion and good adhesion (low spalling). TiC, TiN and TiB2 are very abrasion-resistant. A metal-matrix composite has the plus that you can disperse hard phases into it. If you want to conversion coat the MMC, that might be tricky, however. Sputtered TiN is so common and cheap, that it's worth a try.

 

[kenvlach]

JimMetalsCeramics,
The surface hardening processes would work better than coatings, but the hardened layer is still quite thin. I am unaware of any brake rotors which rely upon such thin coatings or surface hardening; all are solid material (except for coatings on bicycle rims). Have they been tested for brake applications? Long-term wear? Friction when wet as well as dry? What is the maximum sputtered TiN thickness before running into spalling problems? Please give citations or links.

‘Conversion coat’ indicates reacting something with a substrate to form a thin surface coating, e.g., chromating, phosphating, black oxide, nitride formation, etc. The application method for metal matrix composite coatings to which I was referring is known as ‘reactive plasma spray.’ Several examples, from Proceedings of the 1993 Thermal Spray Conference:
pp. 439-444: Ti-TiC and Ti-TiN formation using Ti powder fed into a plasma gun, along with hydrocarbon or nitrogen reactive gases, respectively.
pp. 429-432: Formation of Cu-TiB₂ (wear resistant electrical conductor) by feeding a prepared Cu-10%Ti+B powder into the plasma spray. The TiB₂ crystallites formed were less than 1 micron in size.
The principle involving the direct formation of TiB₂ within a ferrous matrix via the plasma spraying of reactive materials had been previously demonstrated.

explorervs265r,
The Canadian government is funding research on lightweight MMC’s for heavy duty brake applications. Initial results indicate that low-cost aluminum matrix composites (squeeze castings) are suitable for drum brakes but not disks, due to the higher operating temperature of the latter. Other MMC materials [unspecified; one might presume Ti] are being evaluated for disk brake applications.

http://www.tc.gc.ca/tdc/projects/road/b/5294.htm

There are some newer, high tech approaches than carbon-carbon:

Carbon-Ceramic brake discs using a carbon fiber reinforced ceramic silicon carbide composite:

http://www.sglcarbon.com/sgl_t/brakedisc/index.html

Porsche is currently using this type of brake on the 911. The new ceramic composite brake disc is perforated and internally ventilated, like the conventional metal brake disc, but weighs 50 % less.

http://www.autointell.com/news-2000-2/September-05-00-p5.htm

The US Air Force is developing a new generation of aircraft brakes with better characteristics (C-C is low friction when damp or wet) and lower cost than carbon-carbon. The ceramic matrix composites (CMCs) being optimized for these brake applications are carbon fiber reinforced silicon carbide (C/SiC) and carbon fiber reinforced boron carbide (C/B4C).

http://www.ml.afrl.af.mil/publications/pdf/Ceramic_Brake_Discs_TechFS.pdf

Improvements to carbon-carbon composite brake disks by incorporating SiC nanocrystals reduces the ‘slippery when wet’ and oxidation at high temperature (500^oC) problems.

http://www.siu.edu/~perspect/01_sp/carbon.html

P.S. There are many ceramic composite and semi-metallic brake pads available. Just do a search.

 

[JimMetalsCeramics]

If you need a thick coating try a plasma deposited TiB2. Same expansion coefficient as titanium.

 

[explorervs265r]

My oringal idea was to make a ceramic composite brake, similar to Porsche's Carbon Fiber in a SiC matrix. However, I'm on a team building a formula car in college and we are limited by a budget or what we have the ability to fabricate. I still have to look into whether or not a CMC is a possibility for us. I was told by a team leader that Ti rotors are our most feasible option, only they want a coating to provide a harder breaking surface, increase coefficient of friction and prevent galling (coating is not uncommon for Ti and Al rotors).

A lot of the options that have been mentioned on this thread are either expensive or won't work for our situation. For example, C-C brakes don't have very good low temperature performance. Our races aren't long so we'd need very thin rotors to get them to heat up quickly. If Ti is the option we most likely will be going with, I want to do whatever I can to make the rotors perform in the best way possible.

So, if Ti rotors are what we have to work with, would you recommend surface hardening the Ti to TiB2? I don't think the surface layer needs to realy add any strength to the rotor...

 

[JimMetalsCeramics]

I'm not aware of C-C having a low temperature problem. You should check with BF Goodrich in Santa Fe Springs, CA. They make C-C brakes for 747s.

 

[explorervs265r]

Carbon rotors like those used in F1 cars have a higher operating temperature than normal brakes and need to be kept at higher temperature in order to be effective.

 

[JimMetalsCeramics]

I do believe, however, that C-C on 747s is used ver effectively, even when there are icy conditions. Another option might be SiC-SiC.

 

[kenvlach]

Suggest reading 'Engineering Wear-Resistant Surfaces
in Automotive Aluminum,' pp. 32-34 in JOM Feb. 2003.
It is about metal matrix composite rotors, where the outer layer is more wear-resistant. Effects of CTE and elastic modulus mismatch are discussed, with the solution sometimes involving the use of an interlayer. Three manufacturing methods were used. The 3rd involved a TiB₂ preform, the outer layer was infiltrated with liquid gray cast iron, then the core was infiltrated by molten aluminum alloy.
Page one can be viewed at

http://doc.tms.org/ezMerchant/prodt...OM-0302-32/$FILE/JOM-0302-32F.pdf?OpenElement

 

[Titanium]

A very interesting thread, folks. A couple clarifications. The SGL brakes have not been working very well on the Porsche. Fires in the wheel wells, etc. C-C and C-SiC rotors do require higher temperatures to achieve effective stopping mu's. The "morning sickness" is presently put up with because on aircaft such as a B-747 it doesn't last very long during the landing process.

The world's expertise in both MMC and C-SiC rotors, and compatible pads, resides with GOM Technologies. Call them at 302 861 0120.