Are carbon ceramic brake discs better than iron?04.19.2021
by Jeff Ritter
One of the most common debates we see raging on high-end sportscar forums today is carbon ceramic vs. iron brake discs for road and track cars. This debate has been playing itself out over many months and years, with proponents on both sides of the argument fiercely defending their viewpoint. Since carbon ceramic discs are typically an $8,000+ option, the financial stakes are high, thus dictating that the emotions attached to the iron vs. carbon ceramic decision run hot as well.To put it simply, nobody wants to be the guy who made a five-figure mistake!
The purpose of this article is to first explain the differences between the three main types of brake discs on the market today. After examining the technical differences and limitations of each disc type, we will then consider what you can do to ensure that you have the brake system on your road and track car that best matches your intended usage.
Three types of automotive disc, not two!
While the complete materials science and design parameters behind each disc type is far outside the scope of this article, we will first take a brief look at what exactly is being compared before getting too deep into the merits of choosing any single disc material.
A brake disc is a fairly simple implement in the grand scheme of automotive components. Its job is to transform the kinetic energy of the car’s linear motion into heat when clamped by the brake caliper and pads. The material used for a disc’s construction must have the capacity to absorb and dissipate heat, while also having enough mechanical strength to handle the clamping force of the caliper and the brake torque. It must also transmit an appropriate and controllable amount of friction as it interfaces with the brake pads. That is clearly a big ask, which is why we have such a limited range of materials that are up to the task. For automotive applications, there are three primary types of brake discs:
- Gray (ductile) Iron
- Carbon-Carbon (C/C)
- Carbon Ceramic Matrix (CCM) / Ceramic Matrix Composite (CMC) aka (CCM, CMC, PCCB, etc.)
Gray iron brake discs, which earn their name from their graphite content, gained widespread automotive use in the 1960s.While you more frequently see the term ‘steel’ used to describe the gray iron brake discs most of us have on our cars, that is not the proper word to describe them. They have remained popular for so long because we have yet to find a material that offers a superior blend of performance, durability, and cost. Iron discs have excellent thermal conductivity, mechanical strength, wear resistance, and can be manipulated to meet specific needs via alloying and easily forming into different shapes. Small amounts of other elements such as carbon, molybdenum, and silicon can be added to the mix to tailor the performance to meet specific needs, making iron discs especially versatile. The processing costs of casting and machining iron brake discs is relatively low vs. other disc materials, making them cost-effective for broad use on a wide range of applications.
Carbon-carbon (C/C) brake discs are primarily used in aircraft brakes, with extremely limited use in certain automotive racing series such as Formula 1 and IndyCar. The key reasons for their usage in these applications are low weight, thermal shock resistance, low thermal expansion, ability to withstand very high temperatures, and a high coefficient of friction at elevated temperatures. To form a carbon-carbon disc, fiber fabric is first laid-up in the general shape of a disc (aka a preform). The fibers can either be chopped, or they can be woven into layers (sometimes referred to as ‘continuous strand’). The disc preforms then go through a series of processes such as heating in an inert gas, chemical vapor deposition (CVD), and/or liquid phenolic impregnation (LPI), with the goal of creating a pure carbon structure. Uniquely, the pads that mate to carbon-carbon discs are made from the identical material. These processes are complex, time-consuming, and extremely energy-intensive, taking weeks to process at extremely high temperatures and requiring very expensive equipment. Logically, they result in an incredibly expensive final product. While they work well when a massive jumbo jet slams on the brakes and instantaneously generates tremendous heat, Carbon-carbon discs are not suitable for road cars because they do not generate adequate friction at low temperatures seen during daily driving, and they also tend to have a high wear rate.If you think that you have a brake dust problem on your wheels now, multiply that by 20 with Carbon-Carbon!
Carbon Ceramic Matrix (CCM) or Ceramic Matrix Composite (CMC) brake discs are a derivative of carbon-carbon discs that are becoming more and more popular on road-going sportscars today. Carbon Ceramic discs are different from carbon-carbon discs because they are manufactured by melting silicon powder and drawing it into the pores of a carbon fiber disc mold at extremely high temperatures. This process of Liquid Silicon Infiltration creates a ceramic matrix, known as silicon carbide (C/SiC). The discs are then coated with a thin layer of material to protect them from oxygen, because oxygen turns solid carbon into carbon dioxide gas at high temperatures. Carbon ceramic discs are superior to carbon-carbon discs for road cars because the ceramic matrix allows them to generate friction at daily driving temperatures. They are also more abrasion resistant to brake pads, and they are less expensive to produce vs. carbon-carbon because of the reduced processing time. That said, the process of creating a carbon ceramic matrix disc is still much more labor-intensive and time-consuming vs. iron discs, resulting in considerably higher costs.
A key point to understand is that Carbon-Carbon and Carbon Ceramic Matrix are very different materials with very different performance characteristics, intended for very different applications.Carbon-Carbon discs used on F1 cars are worlds apart from the black discs that come from the factory on a Porsche 911.Yet, enthusiasts often generically refer to all non-iron composite discs as “ceramics” or “carbons”, just like they mistakenly refer to iron discs as “steelies” or “steels”.
What is the source of this confusion and misunderstanding? Vehicle manufacturers. They perpetually leverage the racing pedigree of carbon-carbon discs to sell expensive optional carbon ceramic brake packages. Their websites, literature, and salespeople use marketing phrases such as, “proven in motorsport”, “maximum durability”, and “racing-inspired” to describe their composite disc brake systems. Unsuspecting enthusiasts assume that if the carbon-carbon discs they saw in a Formula 1 race can handle that type of abuse, then carbon ceramic discs are certainly the ideal choice for track abuse on their ZR1 or 911 Turbo. Unfortunately, they are comparing two completely different product types! While Carbon-Carbon brakes have long been recognized as the ultimate racing brake solution since Gordon Murray first applied them to a Brabham F1 car in 1976, carbon ceramic brakes have not been developed with that intention. They were instead designed with the objective of being the ultimate solution in road brakes.
What are the positives of carbon ceramic brakes?
Carbon Ceramic discs are a great option for street driving because they have the following characteristics:
- Large unsprung and rotational weight reduction- A CCM disc assembly typically weighs approximately 60% of its iron counterpart. A reduction in unsprung and rotational mass of that magnitude has a positive impact on all aspects of vehicle dynamics including acceleration, braking, and handling. Fuel economy, agility, shock absorber response, and vehicle comfort can also all be improved by reducing the unsprung weight.However, the true weight reduction is usually much smaller than advertised.Yes, the disc is much lighter.But, the heavier caliper and pads often needed to work with the larger required friction area (more on that later) are usually left out of the comparison.
- High abrasion resistance- The ceramic matrix is hard, has low porosity, and is very resistant to wear from brake pads rubbing against it. If the surface coating is not burned off at the extreme temperatures experienced on a racetrack, and the discs are not damaged by some other means, carbon ceramic discs can potentially last the vehicle’s lifetime with normal street driving. Some manufacturers forecast as many as 200,000 road miles on a single set of discs.
- Low brake dust- With an iron brake system, most of the accumulated brake dust on the wheels is the iron disc material that was scraped off the disc surface by the pads, not the pad material itself. Carbon ceramic pad and disc pairs typically have low dust output since the hardness of the CCM disc surface prevents it from being abraded. That means you will not be washing or wiping down your wheels as often.
- Low NVH- While we do hear some owner complaints about groaning and squealing, carbon ceramic brake packages tend to have lower associated NVH (noise, vibration, and harshness) issues than their iron disc counterparts.
- Corrosion resistance- Carbon ceramic discs won’t rust or corrode due to road salt and water.
- Resistance to warpage and deformation- Carbon ceramic discs don’t grow and deform at high temperatures like those experienced on a racetrack. The size of the discs remains stable, which makes them less likely to cone, warp, or create uneven pad wear.
If the above benefits are important to you and you can afford them, carbon ceramic composite discs should certainly be a consideration when purchasing your next vehicle. While some enthusiasts cannot live with the wooden feel of a carbon ceramic brake system, they can offer superior road manners and performance.
What are the downsides to carbon ceramic discs?
While carbon ceramic discs do excel in street usage, their performance on the racetrack is an altogether different story. On the track, repetitive stops from high speeds generate massively higher disc temperatures vs. what could ever be legally or sanely achieved on the street. Everything below applies to not only OEM carbon ceramic discs, but to current aftermarket offerings as well.
- Oxidation at track temperatures- While they may be less resistant to warping or deformation at repeated elevated temperatures, the single biggest problem with carbon ceramic discs is that they oxidize at track temperatures. If you refer to the section above on how carbon ceramic discs are manufactured, you’ll remember that the final step is to paint a coating on the discs that protects the carbon fiber strands from burning up and turning into carbon dioxide gas at high temperatures. Unfortunately, the current technology embedded in that coating is not sufficient to protect carbon ceramic discs on today’s crop of heavy, powerful sportscars under severe track conditions. The surface coating erodes when the discs are repeatedly heated to track temperatures, and rough eruptions appear on the disc face. Those voids on the disc face indicate that your carbon ceramic discs are literally turning into gaseous form via oxidation. In some cases, the oxidation is terminal (chopped fiber discs), and the discs must be scrapped once it occurs. In other cases (continuous fiber discs), the discs can be resurfaced, but only a limited number of times and at a high cost. Carbon ceramic discs are therefore measured for wear in terms of minimum mass, rather than the traditional minimum thickness used to measure iron discs. Once the minimum mass is reached, the carbon ceramic disc is trash. While the technology continues to evolve and improve, we still regularly see carbon ceramic discs begin to severely oxidize in as few as a dozen track sessions (which can sometimes be had in a single day at the track)!
- High disc surface temperatures- The temperatures on a carbon ceramic disc face can run a couple hundred degrees C higher than a similarly sized iron disc under comparable track conditions. The result is more heat pouring into your pads, caliper pistons, piston seals, and brake fluid, which necessitates more frequent caliper rebuilds, and a higher likelihood of boiling the brake fluid.
- Low thermal conductivity- Heat does not flow through carbon as readily as it does through iron, which causes numerous issues. First, carbon ceramic discs rely on radiation from a large surface area to cool. Whereas an iron disc can leverage intricately shaped internal vanes to introduce cooling air and carry away heat, the heat is not as evenly dispersed throughout a carbon disc. Carbon ceramic discs therefore are not very effective at leveraging brake ducts. Instead, carbon ceramic discs have a very wide friction face, or swept area, to radiate as much heat as possible. Another downside to the larger swept area is that the pads required to mate to the discs are very large and expensive, as pad prices are typically proportional to size.
- Expensive and limited range of compatible brake pads- In addition to being very large and expensive, there are not many brake pad compounds that are compatible with carbon ceramic discs. The pad compound must be compatible with the specific disc material being used and can destroy the discs in a hurry if it is not. Since brake pads are a very personal choice for most track junkies, carbon ceramic discs do not provide many options for the driver to pursue a desired feel.
- Poor feel- Experienced drivers will tell you that cast iron discs provide superior pedal feel. Some drivers find that carbon ceramic discs feel abrasive at lower temperatures, and like stone with little modulation once they reach track temps. Brake pedal feel and the resulting confidence is rather important when hurtling towards a guardrail at 150mph!
- High replacement disc cost- Carbon ceramic replacement discs can be hideously expensive. If you do wear out or damage a disc, it can cost thousands of dollars to replace each one, rather than hundreds of dollars for a comparable iron disc. When running carbon ceramic discs hard on a racetrack, the odds of having to replace one or more of them increases exponentially vs. if you only drive your car on the street.
- Less weight reduction than claimed- Yes, the disc assembly is lighter.However, the wide swept area requires larger and heavier pads, as well as larger and heavier calipers.Add up the weights of the total systems and they are many times not that far apart.
- Damage-prone- Many manufactures suggest covering or padding their carbon ceramic discs when handling them, so they are not chipped or fractured. We have seen many instances in which a rock or other piece of track debris lodges itself between the caliper and disc, destroying the CCM disc. One knock when changing a wheel can ruin a disc. Additionally, some chemical wheel cleaners or abrasives used in car detailing can damage carbon ceramic discs.
- Splinters- Carbon ceramic discs should not be handled with bare hands, as they can leave carbon splinters in the skin.
- Greater sensitivity to burnishing/bedding-in- Most manufacturers have an explicit, and sometimes intricate, set of instructions for bedding-in their carbon ceramic discs. I remember reading page after page of comments from owners who were frustrated with the bedding procedure recommended for the CCM discs on their C6 ZR1. Conversely, a set of iron discs and pads can typically be bedded in as little as ten minutes on an empty stretch of road, or in minutes on a brake dyno.
- Poor wheel fitment- CCM discs generally need to be at least 25 mm larger in diameter to have comparable cooling capacity to an iron disc, and even then cooling can suffer.Huge discs limit wheel and tire selection, and force the owner to use larger, heavier wheels and tires, counteracting the weight savings offered by the discs.
Despite the promise of superior racetrack performance by vehicle manufacturers, the real-world results tell a very different tale. A quick search of Rennlist, McLaren Life, Audizine, or Corvetteforum will reveal a lengthy list of frustrated owners who have been literally and figuratively burned by carbon ceramic discs on the racetrack in one form or another. You’ll find comments about oxidation/burning up, rock chips, wheel interference, difficulty bedding, lack of pad choice, etc. We’ve seen this play out over and over again for the past twenty years since Porsche first brought PCCB to the market on the Carrera GT and 996 GT2. Some internal Porsche personnel have even finally admitted that PCCB may not be the best choice if you track your car!
What are my options?So, you ordered your latest track car with the carbon ceramic disc option, and now you’re feeling a bit despondent about that decision.Fear not, you have options! In Part 2 of this article, we will examine how you can avoid the destruction of your very expensive carbon ceramic brake system on the racetrack.
A carbon ceramic disc that was either dropped or hit by a rock/track debris. It is now a very expensive paper weight.