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 Laminated carbon fiber matte Chopped carbon fibers in a graphitic matrix Honeywell Carbenix 2000 Series Fabricated via CVD Honeywell Carbenix 4000 and 4100 Series Fabricated via impregnation in thermosetting resin

Brake surface Laminated Matte Chopped Fiber

Cross section Chopped Fiber Laminated Matte

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

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

Four point bending comparison 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.