a study of carbon-carbon composites for use in airplane disc brakes greg oberson advisors: dr....
TRANSCRIPT
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.