What Are Brake Pads Made Of?
While the exact composition of any given brake pad is a closely guarded secret, the information below is generally applicable to brake pads produced within the last decade.
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TABLE OF CONTENTS
- List of Common Pad Materials
- Organic vs. Semi-Organic vs. Metallic vs. Ceramic Brake Pads
- Which Brake Pad Material(s) Are Best?
- Asbestos Warning
- Brake Pad Performance Requirements
- What Are Transfer Film Brake Pads?
- Useful Links
First, The Best Brake Pad Advice You'll Ever Get
There is one very important point that every brake pad buyer should commit to memory:The best brake pads you can get are the pads that are designed for your system.
Whether you're driving a 1977 AMC Pacer or a brand new race car, brake pads, calipers, and drums/rotors are a balanced system. Changing one component in the system - even if that change is an upgrade - can sometimes have the opposite effect. Likewise, saving a few dollars with some cheap replacement parts can downgrade the performance of the entire system.
Now, onto the specifics.
What Materials Are Used To Make Brake Pads
A rough break-down of brake pad composition by material type.
Brake pads are a mix of five types of materials:
- Binding materials (binders)
- Abrasive materials
- Performance materials that are included in precise amounts to enhance certain braking characteristics, including temperature specific lubricants
- Filler materials
- Structural materials, which help the pad maintain proper shape during use
These five types of materials encompass more than 2,000 substances, and only each brake pad manufacturer knows a specific pad's exact composition.
For our purposes, we can narrow the list of 2,000 down to the following compounds and elements and cover most of the bases.
A List of Common Brake Pad Materials
|Material Name||Main Function||Other Functions||Percentage Found In Pads||Notes|
|Fiberglass||Binder||Works both as a binder and as a structural material||5-25%|
|Phenolic Resin, which may or may not be derived from Cashew Nut Shells||Binder||Works a binder but also as performance additive||10-20%|
|Cashew Resin which may or may not be used in "friction dust"||Performance||A special type of phenolic resin that improves brake performance, resists fade, and can help make brakes operate more quietly||0-20%||Yes, this really is derived from cashew nut shells.|
|Mineral wool fibers||Structure||These fibers typically reinforce the structure of the pad, but they can also help manage temperatures and reduce fade. Fibers can be spun from silica, alumina, calcia, and magnesia. Vermiculite is also a mineral fiber.||10-20%||Mineral fibers are unlikely to be found in a metallic pad.|
|Mineral fillers, typically quartz or synthetic silicates||Abrasive||Unlike mineral fibers, fillers are used as an abrasive to boost friction.||5-35%||Mineral fillers are unlikely to be found in metallic pads.|
|Metal Oxides, typically aluminum oxide and iron oxide||Abrasive||An abrasive that boosts friction, but also a filler/binder in a metallic or semi-metallic pad.||0-70%||It's likely that all brake pads have a small percentage of metal oxides, even "organic" pads.|
|Carbon (graphite)||Performance||Carbon comes in many forms. It may be used to increase friction, but could also be used as a lubricant.||0-35%||Very common because it's very cheap.|
|Brass filings or chips||Abrasive||A cheap abrasive that boosts pad performance in wet weather.||0-5%|
|Friction Dust or Friction Powder||Performance||A proprietary mix of compounds that have a wide variety of uses - everything from reducing noise to acting as a flame retardant.||0-25%||Check out the link to Cardolite's friction particle page to see just how mysterious the descriptions can be.|
|Metal sulfides, including copper sulfide, antimony sulfide, and lead sulfide||Performance||Work to stabilize friction coefficients at various temperatures||0-5%|
|Petroleum coke||Performance||Stabilizes and modifies friction performance - very cheap material.||??%|
|Asbestos||Structure||Filler, structural reinforcement, and heat resistance||0-35%||Believe it or not, it's perfectly legal for brake pads to contain asbestos. See our notes about asbestos below.|
|Barium Sulfate, a.k.a. Barytes||Filler||Very common filler material.||0-35%|
|Lime (Calcium Hydroxide)||Performance||Inhibits corrosion in metallic pads||0-5%|
|Potassium Titanate||Filler||Common filler material||0-30%|
|Steel Wool||Filler||Another common filler material||0-30%|
|Rubber or Rubber Scrap from recycled tires||Filler||Filler material can increase wear resistance.||0-10%||Mostly used because it's cheap.|
|Ceramic Microspheres||Performance||Works as a filler, friction modifier, and brake dust inhibitor||0-20%|
|Finely ground bituminous coal, known as Sea Coal||Filler||Cheap filler material||??%|
|Kevlar„¢ and Kevlar„¢ Pulp||Performance||Friction enhancer - see Dupont website for more info.||0-3%|
|Copper||Performance||Copper provides heat resistance, improves cold weather performance, and acts as a lubricant to prevent squeaking. Frequently found in ceramic brake pads. May be replaced with hexagonal boron nitride (pending legislation).||0-25%||Legislation passed in Washington State and California will effectively outlaw copper in brake pads in the next few years.|
|Ceramics||Structure||Ceramic compounds come in an incredible number of varieties and are capable of performing all functions.||0-100%||The main benefit of ceramic pads is reduced noise, reduced weight, and solid performance. Over the last 10 years, ceramic materials have become increasingly common in brake pads. HOWEVER, "ceramics" are very loosely defined. Some pads are marketed as ceramic when they are in fact more like a semi-metallic pad or even an organic pad. Some brake pad manufacturers think adding clay to an organic pad automatically makes them ceramic...|
|Carbon Fiber||Binder||Carbon fiber is an up-and-coming pad material that acts as a binder and abrasive. However, it's very expensive and relatively exotic.||0-100%||PURE carbon fiber brake pads are only used in racing, but some after-market performance pads boast that they contain carbon fiber. This is likely only a very small amount of carbon fiber, as the material is very expensive.|
As you've undoubtedly noticed, the percentages don't add up to 100%. This is because the exact percentage of each material in any given brake pad is a trade secret - no brake pad manufacturers will disclose exactly how much of each material they're using.
As a matter of fact, the exact mix of materials is determined as much by design as it is by trial and error. Creating the best brake pad mix is still very much an art form.
Organic vs Semi-Organic vs Metallic vs Ceramic Brake Pads
First, let's dismiss the notion that a typical brake pad is completely organic, completely metallic, or completely ceramic - there are too many materials available, and mixing and matching organics with metallics (or vice versa) is common. In fact, brake pads should be called "mostly organic," "mostly metallic," or "mostly ceramic."
Next, here are the key features of organic, semi-metallic, and metallic pads.
- Increased wear resistance (this is their biggest benefit)
- Better performance at high temperatures
- Very likely to make noise
- Wear more quickly, especially in a racing application
- Quiet operation (this is their biggest benefit)
- Better performance at lower temperatures
- Depending on their composition, organic pads can "outgas" at high temperatures (aka decompose due to high heat - it smells really bad) and essentially lose their braking ability
- Decent compromise between wear resistance, noise, and performance
- Somewhat expensive compared to metallic or organic pads
- Semi-metallic pads are the 2nd most common type of pad found on new vehicles
- Most quality after-market brake pads are semi-metallic
- While semi-metallic pads don't shine in any one particular area, their well-rounded attributes make them a great option
- Excellent performance characteristics in all areas - they're quiet, they last a long time, and they're very effective at most temperatures
- Ceramic semi-metallic pads are the new industry standard - more than half of all new passenger vehicles come equipped with these pads (as of 2012)
- Unfortunately, ceramic pads are often the most expensive option available
Which Brake Pad Material(s) Are Best?
Generally speaking, the very best brake pad you can buy in terms of normal driving performance is an OEM brake pad. The OEM pad compound was designed and tested specifically for your vehicle. While OEM pads are not ideal for racing situations, they perform well when they're cold, they don't wear too quickly, and they're tuned for your vehicle so they're quiet.
If you don't want to go with an OEM pad, the next best option would be to invest in a premium after-market brake pad - preferably a semi-metallic or ceramic after-market pad. Also, keep in mind that a brake pad designed for racing or high performance isn't going to perform that well on your daily driver until the pads are warm.
High-performance brake pads designed for racing may sound like a good idea, but unless you get them up to operating temperature (like the red-hot system above), they're not going to stop your vehicle any better than OEM brake pads. Image & copy; Paul Crumlish.
Some Notes About High Performance After-Market Brake Pads:
1) A true racing brake pad is designed for sustained operation at higher temperatures. As you can see in the image above, brake rotors and pads can get red hot during a race. Unless the rotors and pads on your car are getting just as hot (and staying that hot) for extended periods, racing brake pads probably aren't for you.
2) Some brake pad materials - such as metallic pads with a high percentage of sintered metal - can damage factory spec brake rotors.
3) Cheap after-market pads should not be used if you value performance, durability, and/or quiet operation...not to mention minimizing your exposure to asbestos (see below).
After-Market Brake Pads May Contain Asbestos!
While it's rumored that one European auto manufacturer may still use asbestos in their brake pads, it's a safe bet that any OEM brake pads you buy (including genuine Toyota brake pads) do not contain asbestos. However, some after-market brake pad manufacturers still use asbestos as a brake pad filler material because:
- Asbestos is a pretty decent pad material, at least if you ignore the glaring health risks
- Asbestos is really inexpensive
This information may run contrary to what you've heard about brake pads. Many people believe that asbestos was banned as a brake pad material back in the 1980's. While it is true that the Environmental Protection Agency (EPA) proposed a ban on asbestos in 1986, the ban never took effect. In 1991, the Fifth Circuit Court of Appeals in New Orleans ruled that asbestos could not be banned from use in brake pads, and to this day (February 2014) it is perfectly legal to use asbestos in brake pads and clutches.
The EPA *tried* to ban asbestos brake pads, but they still exist. However, no Toyota OEM pads contain asbestos.
What's more, there are no regulations requiring that brake pad manufacturers disclose the presence of asbestos. This is more than a bit disconcerting, as inhaling asbestos brake dust can have serious health repercussions Therefore, it's a good idea to buy pads that are labeled "NAO", which stands for "non asbestos organic." Again, all Toyota OEM brake pads are asbestos free. The risk of asbestos exposure is limited to after-market pads (at least as far as Toyota is concerned).
Finally, it's a very good idea to wear a mask and/or use a wet brake cleaner of some kind whenever you change your brake pads. It's likely that all brake dust is bad for you, even if it doesn't contain asbestos.
» Now that we've covered the basics, we'll dive into some more detailed info and answer some common questions.
Brake Pad Performance Requirements
If you think about the engineering requirements of a modern brake pad, it's a small wonder that brake pads work as well as they do. Consider that a brake pad must perform all of the following tasks:
- Have a consistent amount of "bite" at a wide range of temperatures
- Keep the rotor/drum clean of debris, but not so clean that the rotor/drum becomes polished
- Resist fade as temperatures rise
- Be minimally impacted by water, both in terms of maintaining structure but also in terms of performance, because brake pads get wet
- Minimize the amount of dust/particulate matter they generate so the vehicle wheels don't always look dirty
- Operate as quietly as possible
- Maintain structural integrity despite being used to stop a fast rotating drum or rotor...they can't fracture or break apart at any point
- Be as non-toxic as possible - some chemicals, such as lead oxide, asbestos, and antimony trisulfide - have positive attributes that have to be balanced against health risks
- Last as long as possible
This is no small list of requirements. Think about this the next time you buy a set of brake pads.
What Are Transfer Film Brake Pads?
"Transfer film" brake pads function by leaving a semi-permanent residue on the brake rotor surface once they reach operating temperature. This residue - or film - then works in concert with the pad material to provide solid and consistent resistance. The disadvantages of transfer film pads are:
- They require the vehicle owner to follow a very precise "bedding" procedure after the pads are installed, otherwise they won't work correctly.
- They require the vehicle owner to regularly get the pads nice and hot...something that's difficult to do unless you're racing or towing on a daily basis.
As a result, we don't recommend after-market transfer film brake pads for most vehicle owners. If you're a normal driver, OEM brake pads are best.
This article leans very heavily on a great report written by Peter Blau at Oak Ridge National Labs back in August of 2001. If you have more interest in this topic, you owe it to yourself to read the report cover to cover (you can find it here).
Other useful and interesting links
- Asbestos Brake Dust is Still a Hazard - http://www.aa1car.com/library/trtu796.htm
- EPA Advice on Dealing with Brake Pads That May Contain Asbestos - http://www.epa.gov/asbestos/pubs/brakesbrochure.html#Why
- Brakepad Issues - Leading Edge Motorsports University - http://www.lemd.com/motorsport/previous.cfm?id=25
- Cardolite Uses - Auto Industry - http://www.cardolite.com/?q=automotive
- Brake NVH Technology - Akebono brake systems - http://www.akebonobrakes.com/company/media_center/white_papers/akebono_NVH.pdf