Brake Shoes

[Big_Boy_4005]

Over on the Clasic Toy Trains forum, we were having a discussion about ballast, and what material to use on a model railroad. One suggestion was to use discarded metal shavings found at an automotive brake shop. Suddenly the conversation turned to ASBESTOS. As it turns out some automobiles still use pads that contain it, even though it was supposedly outlawed back in the 1980's.

That got me to wondering. What is that 2" high friction comopsition made of anyway???? Does it contain asbestos?

[Big_Boy_4005]

Thanks Mark. If my memory serves, that date actually coincides fairly well with all of the law suits brought by people who had worked in industries that used asbestos.

Do you know what that stuff is? I see old shoes discarded along the right of way fairly often.

[Overmod]

Mark, I'm almost 100% certain that what you're seeing is actually intended as "non-metallic, ceramic" -- meaning essentially 'not cast iron any more'.

There is a definition of 'non-metallic ceramic' in some areas of engineering, referring to refractory materials made up of non-metallic elements (e.g., silicon or boron carbide). This is not, I think, the way the term is being applied to brake practice. It's possible that a 'metallic ceramic' is a material like potassium titanide (which is indicated as being a key ingredient in some high-performance automobile brakes) and that for some reason -- perhaps extreme spot heating in railroad service -- such materials are not favored.

Materials like 'cermets' are blends of metallic and ceramic materials... but I doubt these are favored materials for "cost-effective" brakeshoes. I have seen some studies that discuss a sintered bronze matrix with infilling of graphite and ceramic material; this does seem better suited than 'organic' materials for heavy brake service... but again I would question the cost-effectiveness.

Brake pad materials, as one might expect, can be something of a 'trade secret'. High-performance brakes, for example, can use carbon fiber (or carbon/carbon composites) and aramid fiber (Kevlar) to give support to an actual friction material which has the appropriate characteristics for the speed and load anticipated. Braking demand for high-speed passenger work will almost certainly be very different from what's best for freight; intermodal-train composition material may be quite different from what would be best, say, for coal trains. I will not bore you any further with details.

"Semi-metallic" brakes use metal fibers in some sort of tailored binder or matrix. If carefully designed, with the 'right' metal composition, these might provide just the right amount of tread 'scrub' when used on clasp brakeshoes.

The backing of the brakeshoes or pads, whether for disc or tread brakes, will probably continue to be a metal casting, of course.

I look forward with avidity to a discussion of the specific materials used, especially those for high-speed train braking...

[Anonymous]

The unabridged Random House Dictonary defines ceramic thusly:

ceramic adj.-1. of or pertaining to products made from clay and simlar materials, as pottery, brick, etc., or to their manufacture 2. ceramic material

Now a few disclaimers: I am by no means an expert, and this edition of the dictionary is copywrited 1973.

You will notice that the definition includes brick as a ceramic material. Presuming they mean ordinary brick, dried mud is a ceramic material. Mud is essentially wet dirt, and dirt (or soil if you prefer) contains at least trace elements of metals. So I don't think that ceramic means 100% metal-free.

The solid nonmetals which are safe to use (I think) are: Boron, Carbon, Silicon, Phosphorus, Sulfur, Germanium, Selenium, Antimony, and Tellurium. For those of you who are chemically inclined (I by no means am), most of these can form ionic compounds with metals (probably transition metals in my layman's opinion), and some would probably form compounds such as carbonates.

I hope I have not confused you any more than I have confused myself, and I'm probably wrong about this anyway.

See you around the forums,
Daniel

[Overmod]

Daniel -- a primary item of importance for ceramics is that they are FIRED until vitreous. Mud doesn't qualify. "Brick" is not a ceramic, but some FIREbrick is.

We shouldn't forget that asbestos is at least ceramic-like (one brake manufacturer claims to use "basalt fibers" as a constituent of brake linings, and this material presumably has similar characteristics to chrysotile or amphibole asbestos.)

The issue with 'metal freedom' is somewhat peripheral; this isn't a situation where trace concentrations of metallic ions cause problems and have to be chelated. Many ceramics DO contain "metals", of course (as do glasses), but they're strongly bound as oxides, carbides, etc. -- not characterized by metallic bonding, typical alloy grain structure, etc.

Keep in mind that some of the materials you mention are usually termed 'metalloids' rather than 'nonmetals' -- boron, silicon, and germanium being the 'classic' ones, and Linus Pauling notes (referring to selenium and tellurium) that "the increase in metallic character with increase in atomic number is striking".

Having said that, I would note that many of the 'nonmetals' you mention are anything BUT "safe" in the application we're discussing! One very important characteristic of *railroad* brakes is that the composition be ablative, because preserving a rather accurate wheel-tread contour is very important for effective railroad operation. If asbestos dust is viewed as a critical health hazard, what should we think about dust containing selenium, antimony, or tellurium? Not to mention the relative expense of such materials, even before we stop to ask what purpose they could contribute to effective brake performance...

Ionic bonding is almost certainly not going to give you the necessary bond strength for the desired combination of good strength and high temperature stability -- heck, part of the problem is that even metallic bonding isn't good enough to prevent undesirable alloying to the chilled iron in the wheel tread at high temperatures, so a stable refractory material (this btw being a much better working definition of what 'ceramic' means in an engineering context) is preferable.

I have been assured that the 'ceramic' compositions for brake material contain a general average of something like 19-20 distinct materials. Some of these will be binders, some of them friction enhancers or moderators, some of them material that moves heat away from the friction face, BUT some of them materials that insulate the pad or shoe backing from friction-heat softening (as asbestos did)... etc.

An interesting consideration is that the composition requirements for disc brakes can be very different from those involved for tread brakes. In particular, it's possible to use some of the modified carbons, such as graphite, in ways that either form carbides or embed carbon in the brake rotor surface. Thereafter, there is a working equilibrium between two carbon-bearing surfaces rather than a 'pad' working on a 'metal' surface... and, for one thing, this can result in longer effective rotor surface life under heavy braking requirements. I have not so far found any indication of whether or not this effect is desirable in wheel-tread surfaces, particularly insofar as subsequent transfer to the (very thin and by definition highly-stressed) martensitic layer in the contact areas of work-hardened high-strength alloy rail may contribute either way to observed cracking and stress problems there...

I noticed with some interest, a few days back, that experiments are going on with short-duration railhead lubricants that appear to reduce running resistance by substantial amounts 'without interfering with braking'. Presumably these don't interfere with railhead metallurgy either... but are their researchers looking for that?

More yada yada yada... hey, it's the middle of the night and you're SUPPOSED to be sleeping, so don't complain if I give you MEGO syndrome with these posts... ;-}