A Study of Carbon-Carbon Composites for use in Airplane

Disc Brakes

Greg Oberson

Advisors: Dr. Bowman and Dr. Trice

How a disc brake works

Desired properties for an airplane brake

• High thermal conductivity

• Consistent coefficient of friction

• High strength at high temperatures

• Oxidation and wear resistance

Project objectives

• To characterize the microstructure of the composites and relate it to oxidation behavior and mechanical properties

• To develop a framework for further testing of the composites

Two common microstructures

1. Laminated carbon fiber matte

2. Chopped carbon fibers in a graphitic matrix

Honeywell Carbenix 2000 Series

Honeywell Carbenix 4000 and 4100 Series

Fabricated via CVD

Fabricated via impregnation in thermosetting resin

Brake surface

Laminated Matte Chopped Fiber

Cross section

Laminated Matte Chopped Fiber

How are the microstructures similar?• Density (1.7 g/cm3) and porosity (10%)• Thermal conductivity (70 W/m/K)• Heat capacity (1.5 J/g/K)

• Oxidation and wear resistance• Strength and stiffness

How are the microstructures different?

TGA comparison

80

85

90

95

100

105

0 200 400 600 800 1000

Laminated matteChopped fiber

% of Original Mass

Temp (C)

Graphite crystal structure

Edges are susceptible to oxidation

Basal planes are resistant to oxidation

Hexagonal unit cell

(100) is perpendicular to basal edges and will be detected when the edges are exposed to the surface of the material.

XRD comparison

Planes perpendicular to basal planes are detected

Planes perpendicular to basal planes are not detected

Mechanical properties of carbon-carbon composites…

• Are largely controlled by the properties, volume fraction, and geometry of the fibers.

• Are affected by interactions that occur during processing.

Four-point bend testing (ASTM standard C1161-94)

• Imposes tensile and compressive loading simultaneously

• Measures the relative structural soundness of the test material

Comparison of flexure strength versus microstructure and fiber orientation

0

50

100

150

200

250

300

Flexure Strength (MPa)

LongitudinalTransverse

2400 4000 4100

Four point bending comparison

0

20

40

60

80

100

120

0 0.2 0.4 0.6 0.8 1 1.2

Laminated Matte

Stress (MPa)

Crosshead (mm)

Maximum stress = 76.7 MPa

0

50

100

150

200

250

300

0 0.5 1 1.5 2

Chopped Fiber

Stress (MPa)

Crosshead (mm)

Maximum stress = 290.2 MPa

Fibers are randomly aligned

Fibers are parallel to tensile axis

Conclusions

• The chopped fiber microstructure shows better oxidation resistance and flexure strength than the laminated matte microstructure.

• The fiber orientation largely controls the thermal and mechanical properties of the composite.