Using Ceramics, Brakes Are Light but Cost Is Heavy

[Yaz]

I thought I'd post this article on ceramic brakes in the Tech Section. A little interesting to read.

By KEVIN CAMERON
Published: June 18, 2006



CHECKING boxes on an option list can quickly run up the price of a new car, but few add-ons are in the league of the ceramic brakes on a Ferrari F430: at $16,808, they cost about the same amount as a nicely equipped Honda Civic.

The high-tech brakes, an option on the $172,505 F430 but standard on some other Ferraris, are an upgrade for iron discs that already deliver impressive stopping power. Only those drivers who spend weekends on the racetrack are likely to notice a big difference in performance.

Car-savvy pedestrians may take note of the owner's selection, though. Instead of a metallic gleam visible through the spokes of the alloy wheels, they will see discs that look like patio stones.

Porsche was the first automaker to use ceramic brakes on a production car; in 2001, it offered discs made of a novel ceramic composite material to reduce the weight of a special sport model. Several Porsches now offer them as an option; on the Cayman sports car they cost $8,150.

Brake discs, also called rotors, of similar ceramic material are optional on the Audi S8 (although not yet in the United States). The $210,000 Bentley Continental GT Diamond Series will also be equipped with ceramic discs.

Why replace metal discs, which have served well for many years and are easy to make? Using a ceramic composite takes advantage of a material with outstanding hardness (and potentially long life) and an ability to retain its strength and shape at temperatures that would melt conventional iron brake material into a glowing puddle.

Simple single-ingredient ceramics tend to be brittle like dinnerware, though some types work well in turbochargers or as bearings for jet engines. To make ceramics that are tough enough for a brake disc, the material is manufactured as a composite: strands of carbon fiber, which are highly resistant to stretching, are embedded in the material, using a process developed by the Mitsubishi Chemical Company.

Production begins with a disc-shaped "preform" of carbon fibers, essentially a bundle of woven cloth in the approximate shape of the finished disc. The preform is saturated with a liquid polymer containing carbon and silicon. It is then heated to convert the polymer into silicon carbide, an extremely hard ceramic. The finished surface looks like stone.

Today, ceramic brakes are of interest for their performance advantage — maintaining their stopping power even when extremely hot. But because ceramic discs will last four times as long as iron ones, according to automakers, their use could increase.

More important, ceramic discs weigh about half as much as iron discs — a valuable benefit for handling and acceleration.

There are other good reasons to seek lighter, more durable brake disc materials. Since the introduction of CAFE — the federally mandated corporate average fuel economy standards that an automaker's fleet of models must collectively meet— there has been strong pressure to reduce the weight of automobiles.

As a vehicle accelerates, its rotating parts require more energy to accelerate than nonrotating parts like seats or engine blocks. This is because they gain energy from both their accelerating forward motion and from their increasingly rapid rotation. This gives brake discs a special importance in fuel economy.

Because so much driving is stop-and-go, and because it takes more fuel to accelerate a heavy car than a light one, reducing weight can help automakers meet the CAFE standards. Heavy iron brake discs are a favorite target of weight-conscious auto engineers.

When a car is braked, friction between the disc and the pads that grip it converts the kinetic energy of forward motion to heat. The heat is absorbed mainly by the discs, and eventually dissipated to the surrounding air. The higher the speed, the more kinetic energy there is, and the hotter the brakes become.

As discs have been made lighter, their average operating temperature has risen, leading to more rapid pad and disc wear. Braking force increases with disc diameter, so any attempt to remove weight by reducing disc diameter also lengthens stopping distances. Carmakers sometimes compensate by installing higher-friction pads — which in turn may wear more rapidly.

Discs from the pre-CAFE era included extra material that allowed worn discs to be machined one or more times and re-used. Today's lighter discs have little extra; often, they must be replaced when worn.

Brake pads designed for use with ceramic composite discs may contain ceramic powder along with metal in the form of wire or particles. The ceramic provides the hardness to resist wear while the metal forms a so-called "transfer coating" on both the pad and the disc surfaces during the break-in period. Much of the friction generated between the pad and the disc occurs between and within these metallic films.

To permit safe brake operation at very high disc and pad temperatures, the hydraulic pistons, brake fluid and seals in the brake caliper must be insulated from the heat. This can be accomplished by installing heat shields, assuring good circulation of cooling air over the parts and blocking the path that the heat would travel.

Aircraft and Formula One racecars have the luxury of a more expensive solution: the carbon-carbon discs seen in dramatic racing photos, glowing red or even bright orange under hard braking. Discs and pads made of this material are able to operate routinely at temperatures that would melt most metals.

The carbon-carbon name is engineer-speak for a material consisting of two forms of carbon; crystalline carbon fibers of immense strength, reinforcing a structure of amorphous carbon (like the solid black carbon found in the brushes that carry electrical current in a motor or generator).

The carbon-carbon manufacturing process is enormously expensive — carbon-carbon discs cost thousands of dollars, but in racing their benefits are worth the price. Not only does braking improve, but the low weight of the disc mass lets the racecar accelerate slightly more quickly.

Such light discs can absorb large amounts of energy because their temperature can safely rise much higher than that of iron discs. Unfortunately, wear is fairly rapid and brake torque is limited at low disc temperature. Still, for aircraft, a pound of weight saved is a pound of payload (and revenue) gained, on every flight.

Ceramic composite disc materials are a big step in the right direction, costing only about one-fourth as much as carbon-carbon.

That may persuade more high-end automakers to offer ceramic brakes, which could help to reduce costs and make them available on more cars.

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And here's a list of best and worst stopping distance from 60 MPH:

Stopping Short

By SHAUN BAILEY
Published: June 18, 2006

In its tests of new cars, Road & Track magazine routinely measures stopping distances from 60 miles an hour. The following lists include the best and worst results reported within the last four years in the magazine's Road Test Summary. All of the cars had antilock braking systems.

Modern passenger cars have such powerful brake systems that in a panic stop, when the driver slams on the brake pedal, there are two possible outcomes:

In a car without antilock brakes, the wheels will lock up. From a cruising speed of 60 m.p.h., a car with locked brakes will slide nearly 200 feet. An aware and skilled driver can avoid lock-up and modulate brake pressure to achieve distances closer to 150 feet.

In a car with antilock brakes, the distance can usually be shortened to 130 feet or less. When the antilock system is activated, a modern car's stopping distance has more to do with the tires than with the size of the brake rotors. Sedans with all-season tires do not stop as well as sports cars with high-performance tires that provide more grip.

When it was tested 21 years ago, a Honda Civic without antilock brakes stopped in 166 feet from 60 m.p.h. And while a more recent Civic EX (with an antilock system) had one of the worst performances in Road & Track tests over the last two years, it still managed, at 133 feet, an impressive 20 percent improvement over the old Honda.

It is also noteworthy that a Mazda RX-8 stops nearly as well as some superexotic sports cars. The Mazda's braking benefits from sticky tires, antilock technology and a lightweight chassis. SHAUN BAILEY

BEST
Aston Martin V-8 Vantage 107 ft.
Ferrari F430 Spider 107 ft.
Lotus Exige 107 ft.
Mercedes-Benz SLR McLaren 107 ft.
Porsche Boxster S 107 ft.
Porsche 911 Carrera S 108 ft.
Ferrari Enzo 109 ft.
Chevrolet Corvette Z06 109 ft.
Maserati Quattroporte 109 ft.
Mazda RX-8 110 ft.

WORST
Ford Escape XLT 145 ft.
Volkswagon Passat 3.6 138 ft.
Volvo S60 T5 138 ft.
Cadillac CTS 138 ft.
Audi A3 2.0 T. 138 ft.
Mitsuibishi Eclipse GT 137 ft.
Toyota Camry Hybrid 136 ft.
Subaru Legacy GT Limited 135 ft.
Scion xB 134 ft.
Honda Civic EX 133 ft.