34 00... Assessing the appearance of carbon ceramic brake discs

Structure of a carbon ceramic brake disc:

1. Ceramic friction layer
2. Carbon base
3. Cooling duct

To ensure that the ceramic brake disc has a long service life, the base is provided with a separate friction layer. The friction layer is bonded by means of a special joining paste. The siliconization results in a bonded connection.

The base and the friction layer are made from different mixtures/recipes. For this reason, the two components differ in appearance.

Fig 1: Identifying Ceramic Friction Layer, Carbon Base And Cooling Duct

Courtesy of BMW OF NORTH AMERICA, INC.

Tension relief structure: 

Due to the different material compositions of the friction layer and base, the friction surfaces (even when new) are characterized by a harmless tension relief structure. Some relief cracks pass along the perforation bore holes.

The tension relief structure arises during the manufacturing process, starting from the brake disc surface, and is not a safety risk. There is also no danger that the friction layers may split off the base in large pieces since because the friction layers are virtually tension free due to the tension relief structure.

The tension relief structure is partially visible on new components and may vary considerably between the two brake disc chamber sides.

Thus, a brake disc with this feature is OK.

Fig 2: Identifying Cracks In The Brake Disc Friction Surfaces

Courtesy of BMW OF NORTH AMERICA, INC.

Rough surfaces in cooling duct: 

The surfaces (1) in the cooling duct have a different structure. The cooling ducts are not finished and therefore have a rough structure.

Thus, a brake disc with this feature is OK.

Fig 3: Identifying Surfaces In Cooling Duct

Courtesy of BMW OF NORTH AMERICA, INC.

Processing traces: 

The surface of the ceramic brake disc is characteristic of the type of grinding applied. The cross and spiral pattern (see arrows) created by the type of grinding process has no influence on the performance of the component.

Thus, a brake disc with this feature is OK.

Fig 4: Identifying Cross And Spiral Pattern

Courtesy of BMW OF NORTH AMERICA, INC.

Pores in base: 

Due to the carbon fibres used, with a defined length of 10 mm without a short fibre portion, visible pores arise in the base during the manufacturing process. These pores have no negative influence on the function of the ceramic brake disc.

e.g.:

1. Pores on brake disc chamber screw connection
2. Pores on the brake disc face
   Fig 5: Identifying Pores

   Courtesy of BMW OF NORTH AMERICA, INC.

Possible surface flaws on the brake disc: 

Flaws on perforation bore holes 

1. Brake disc OK
2. Brake disc not OK -> nest-shaped pittings
   Fig 6: Identifying Flaws On Perforation Bore Holes

   Courtesy of BMW OF NORTH AMERICA, INC.

Brake disc not OK

-> Nest-shaped pittings (3 or more holes on each side)

Fig 7: Identifying Nest-Shaped Pittings On Brake Disc

Courtesy of BMW OF NORTH AMERICA, INC.

Flaws in friction layer: 

Pittings in friction layer: 

Fig 8 Picture A: Brake disc OK

Fig 8 Picture B: Brake disc not OK

Fig 8: Identifying Pittings In Friction Layer

Courtesy of BMW OF NORTH AMERICA, INC.

Partial spalling (1) of the friction layer has no influence on the function or safety of the brake system (no comfort disadvantages or uneven braking force distribution).

The separation is a purely visual defect, but may cause customer disappointment and uncertainty.

If the pitting/spalling (1) has reached 2x2 mm, these ceramic brake discs can be replaced. They must be returned through the special defective parts channel, as that is the only way that a refund (applicable only to the warranty period) can be made.

Return address:

BMW AG

Sonderschadteil Gewahrleistungsteile

GWTZ

Heidemannstr. 164

Werk 01.01/Gebaude 23

DE 80935 Munich

Fig 9: Identifying Partial Spalling Of Friction Layer

Courtesy of BMW OF NORTH AMERICA, INC.

Friction layer detachment: 

Brake disc with friction layer detachment

(1) -> brake disc not OK.

Fig 10: Identifying Friction Layer Detachment

Courtesy of BMW OF NORTH AMERICA, INC.

Fiction layer not fully finished: 

Brake disc not OK (see Fig 11)

Fig 11: Identifying Unfinished Fiction Layer

Courtesy of BMW OF NORTH AMERICA, INC.

Gap between friction layer and base 

Brake disc not OK (see Fig 12)

Fig 12: Identifying Gap Between Friction Layer And Base

Courtesy of BMW OF NORTH AMERICA, INC.

Splitting off on outer edges of friction ring: 

Brake disc OK (see Fig 13)

Fig 13: Identifying Splitting Off On Outer Edges Of Friction Ring (1 Of 2)

Courtesy of BMW OF NORTH AMERICA, INC.

Brake disc not OK (see Fig 14)

Fig 14: Identifying Splitting Off On Outer Edges Of Friction Ring (2 Of 2)

Courtesy of BMW OF NORTH AMERICA, INC.

Edge pittings in rib area: 

Brake disc not OK (see Fig 15)

Fig 15: Identifying Edge Pittings In Rib Area

Courtesy of BMW OF NORTH AMERICA, INC.

Radial cracks on inside circumference of brake disc ring 

Brake disc OK (see Fig 16)

Fig 16: Radial Cracks On Inside Circumference Of Brake Disc Ring

Courtesy of BMW OF NORTH AMERICA, INC.

Cracks on rib transition: 

Cracks (1) in rib area of brake disc

Brake disc OK (see Fig 17)

Fig 17: Identifying Small Cracks In Rib Area

Courtesy of BMW OF NORTH AMERICA, INC.

Brake disc:

Cracks (dimension A) in rib area >20 mm deep -> brake disc not OK (see Fig 18).

Fig 18: Identifying Cracks In Rib Area >20mm Deep

Courtesy of BMW OF NORTH AMERICA, INC.

Phosphate residue in cooling duct and perforation bore holes/dirt contamination on ASSY 

Fig 19: -> brake disc not OK.

Fig 19: Identifying Contamination

Courtesy of BMW OF NORTH AMERICA, INC.

Brake discs not OK (see Fig 20)

1. Perforation bore hole
2. Contamination of ASSY

Fig 20: Identifying Contamination

Courtesy of BMW OF NORTH AMERICA, INC.