Abstract of Recent Work: Petrology of Automobile Brakes

Background: Because coal, coke, graphite, char, and carbon fibers are used in a wide variety of modern brakes for automobiles, trucks, and aircraft, there are applications for petrographic analysis in the manufacture an use of these materials. Brakes for automotive and light aircraft use generally consist of a mixture of particles of metal (iron, steel, copper, brass), crystalline compounds (antimony trisulfide, potassium titanite, magnesium oxide, elemental sulfur), non- crystalline compounds ( glass and plastic), fibers (aramid polymers, cellulose, carbon) and carbonaceous materials (metallurgical coke, petroleum coke, graphite, coal, and char) set in a phenolic resin. The composition of these brakes has been characterized by a petrographic point- count analysis in both white light and blue light. The blue light analysis is both necessary and possible because many of the plastics and fiber components which are difficult to observe in white light fluoresce distinctively when excited with blue light.

Objectives: The objective of this project is to determine both the type and amount of Carbon based components present in automobile brakes.

Procedures: A set of seventy-five automobile brake samples were sectioned and mounted in one inch diameter epoxy pellets and polished using standard petrographic methods. The polished pellets were analyzed with a Leitz Orthoplan optical microscope set up for both white light and fluorescence viewing. For white light viewing a polarizer, analyzer, and interference plate were used to enhance the anisotropy of the samples. For fluorescence viewing a mercury arc lamp with a BG-23 red suppression filter, a BG-12 blue excitation filter, and a K-530 barrier filter were used. In this fluorescence mode no polarizer, analyzer, or interference plate were used, and the light observed was only that light that was excited from the sample. The samples were analyzed by standard point-count methods where 500 points were identified by two operators with a mean difference of two percent or less. The identity and composition of many of these components were confirmed by scanning electron microscopy (Hitachi 25 SEM) and EDX(Voyager system).

Results: lThe petrographic examination of over seventy-five samples of automobile brakes reveal the presence of a variety of carbons including, coal, metallurgical coke (see Figure 1), petroleum coke (see Figure 2), graphite (see Figure 3), and chars in varying proportions. Other components identified optically were particles of metal (copper, iron, steel, brass), a number of glassy phases, plastic, sulfur, antimony trisulfate, potassium titanite, magnesium oxide, aramid polymer fibers, and phenolic resin. In petrographic terms the brakes are a porphyritic aphanite with phenocrysts (particles) of the above mentioned materials in a phenolic resin groundmass. Because the carbon and other organic components are not amenable to SEM or x-ray analysis they are sometimes difficult to identify.

Fig. 1

Fig. 2

Fig. 3

The carbonaceous components were also separated from the brake matrix with density gradient centrifugation (DGC) so that they are available for gas chromatography and/or gas chromatography mass spectrography analysis. The results of this work show that the various brake components can be quantitatively identified using standard petrographic techniques and that the carbon phases can be separated from the brake matrix with proper grinding and density gradient centrifugation.

Conclusions:

• The preliminary results of this investigation show that petrographic analysis with a polarizing microscope set up with fluorescence capability supported with a scanning electron microscope allows the successful qualitative and quantitative compositional analysis of automobile brake materials.
• The DGC results show that the carbon and other organic components found in automobile brakes can be successfully separated and made available for further characterization.

References:

Crelling, John C., Filip, Peter, and Wright, M. A., 1997, Characterization of carbon and other components in automobile brakes with density gradient centrifugation: Extended abstracts and program,-23rd Biennial Conference on Carbon- American Carbon Society, v. II, p. 226-227224-225.

Crelling, John C., Filip, Peter, and Wright, M. A., 1997, Petrographic characterization of carbon and other components in automobile brakes: Extended abstracts and program,- 23rd Biennial Conference on Carbon- American Carbon Society, v. II, p. 224-225.