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Tech Talk > Ceramic Matrix Composite (CMC) Brake Rotors


Material Overview and Manufacturing Processes
Manufactured by Starfire Systems Inc. under exclusive contract
for BrakeTech USA, Inc.

Starblade™ is a highly advanced composition brake rotor outer blade manufactured by Starfire Systems Inc. from Ceramic Matrix Composite material (CMC). Like many other composites it is reinforced with graphite fibers giving high strength and stiffness, similar to metallic materials, but at a much lower weight and capable of higher temperatures. What differentiates CMC from typical composites is the matrix, the material that holds the fibers together. The matrix in the most typical composites, those used in sports equipment and low temperature aerospace components, is epoxy. Essentially it is the same resin used in epoxy glues. The resulting matrix from curing the epoxy resin and the composite that results, is called graphite reinforced epoxy or just graphite/epoxy (or sometimes carbon/epoxy).

The Starblade™ matrix is formed by high temperature heat-treating of a pre-ceramic polymer patented by Starfire Systems. When the cured resin is heat treated it converts to a hard ceramic. The ceramic it converts to is Silicon Carbide aka: SiC. The composite that results is called graphite or carbon reinforced silicon carbide or C/SiC for short. The material is also known as ceramic matrix composite, because SiC is a ceramic material. It is similar to those used in some abrasives like grinding wheels and sandpaper and next in hardness only to diamond. It is also used in high temperature furnace hardware and defensive tactical armor.

A ceramic matrix composite is tough when compared to what we typically think of as ceramics, like china or decorative ceramics. The fibers provide that toughness by holding the matrix together, like rebar in concrete. Toughness is actually a technical term and measurable as the amount of energy required to brake a material. Typical fine ceramics have a toughness of 1-2ksi(in)1/2. The CMC material used in Starblade™ has a toughness of 18-20ksi(in)1/2 , nearly 10 times tougher.

Ceramic Matrix Composites have been in development for approximately 25 years. Most of the development has been in the Aerospace Industry for hot components like turbine engine components and spacecraft reentry surfaces and are in limited use in aircraft. The engine exhaust washed aft deck of the B-2 Stealth Bomber is manufactured from a CMC as well as the turbine engine exhaust parts for the F/A-18E,F Super Hornet Navy Fighter. Much of the development has been supported and performed by NASA for use in reentry and the Air Force especially for use in turbine engines. Starfire’s material has been approved for use as an in-space repair method for the Shuttles Leading Edge Material as part of the Return to Space Program, and is being developed for use in the next generation space planes and defense systems.

The BrakeTech Ceramic Matrix Composite material is manufactured in 6 basic steps:

Pre-pregg: The resin, sometimes including ceramic fillers, is coated on to woven graphite fiber cloth. This is similar to the process used for making graphite/epoxy pre-pregg except that it doesn’t need a solvent to wet the fabric.

Lay-up: The cloth is cut into plies in shapes necessary to fabricate the component and then laid-up or stacked to build thickness. The lay-up for the 5mm Starblade consists of 10 plies of pre-pregged fabric [is depicted]. The plies are rotated to get an even distribution of fibers in each direction. The plies are pre-cut to near-net shape to reduce the amount of machining required later.

Press Cure: The plies are pressed in a warm press which heats the resin under a slight pressure to help it to flow and bond the plies together and thermally cures or sets the resin to a plastic consistency. The result is a near-net shape rotor. The matrix after cure is essentially hydrogen rich non-crystalline, SiC.

Pyrolysis: The rotor is heat treated at high temperatures converting the hydrogen rich plastic SiC to pure ceramic SiC. The matrix shrinks, increasing in density, creating porosity in the part, but the near-net shape is maintained. The SiC is either very fine crystals or large crystals depending on pyrolysis temperature. Hardness can be controlled by pyrolysis temperature as well.

Densification: The porous rotor is re-impregnated with polymer and pyrolized again several times until there is approximately 5-8% porosity left in the part.

WaterJet Cutting and Machining: Details like the “inverted-fingers” in mating up to the BrakeTech AXIS carrier design are first waterjet cut then precision CNC machined using diamond cutters. The final step in the process, the rotor is then diamond ground for flatness and a smooth surface finish.

This whole process encompasses several weeks to complete versus several months for the best of Carbon/carbon assemblies

Comparison to Carbon/carbon

The material is similar to Carbon/carbon used in Moto GP and Formula 1 racing in that it is graphite fiber reinforced, giving the composite material toughness, the matrix is stable at elevated temperatures. But that is where the similarities end. The Carbon matrix in C/c is either formed by pyrolysis of phenolic resin or by depositing Carbon by a process known as chemical vapor deposition. In either case the material is soft compared to a ceramic matrix. As a result, only soft C/c pads can be used against Carbon rotors, relying on a soft carbon wear debris layer to form the friction film. C/c wears much faster than conventional materials as well as CMC. They are also very susceptible to “morning sickness”: the C/c will actually adsorb moisture that weakens the bond to the Carbon surface giving relatively low friction until the brake heats up enough to liberate water from the surface. This and other related factors make C/c not suitable for street use. The Starblade™ CMC is not hygroscopic and only slightly denser; see Material Density Comparison Chart.

Another area of significant difference is the very destructive effects caused by oxidation, Carbon/carbon rotors suffer from this phenomenon at an alarming rate. This is particularly acute as temperatures approach 350 degrees (C). The advent of elevated thermal load increases wear dramatically, which structurally weakens the assembly. This becomes a focal point in early retirement. To improve reliability and lessen these rapidly debilitating effects, C/c rotors are typically made thicker (than either conventional rotors or the new generation CMC composites) in effort to increase their working lifespan.

Lastly, late developments in compatible friction material technology combined with the ability to operate at a much lower thermal threshold make the new generation CMC rotors not only remarkably more user friendly for street operation than C/c, but at a significantly higher level of performance as well.



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