Some research highlights of CAFS research associates are:

(1) Designed prototypes that characterize both bulk and surface properties at various scales (macro, micro, and nanometric).

We possess full capabilities to characterize thermal, mechanical, and chemical properties of bulk materials at various temperatures.

We have designed and constructed specialized equipment. One technique allows measurement of surface thermal diffusivity and another apparatus examines the shear of particulates at temperatures up to 600^o C.

For instance, we have measured the thermal properties of bulk materials and calculated the amount of heat transmitted per unit time through carbon-carbon materials. We have found that the thermal diffusivity distribution on the surface depends on the degree of homogeneity of the bulk material. The variation of thermal conduction with positron alters the temperature profile through bulk material during a sliding process. In this regard, thermal diffusivity mapping of friction materials before and after a friction process is important. We have observed that friction surfaces become very wavy when carbon-carbon is subjected to the braking process, especially after rapid deceleration. These grooves alter the contact characteristics at the sliding interface and may reduce the friction performance. We have shown that the formation of macro-grooves on the friction surface is a function of thermal and/or mechanical property variations, crystallinity distribution, and crystallite orientation with respect to the rubbing direction. However, if the distribution of crystallites is non-uniform, only micro-grooves are observed.

(2) Performed in situ quantitative wear measurements on the subscale aircraft dynamometer.

Employing a laboratory grade portable profilometer attached to a custom three axis positioner, CAFS researchers measure areas on the rotor and stator before and after a stop or a number of stops. The thickness of the brake ring at each point in a topographical area is accurately determined and provides a means of determining the evolution and wear rate at each location.

(3) Developed an understanding of friction and wear changes.

In the friction of carbon materials, it has been found that the coefficient of friction is dependent on the total energy absorbed by a given mass and the rate at which the energy is dissipated. Usually, low energy absorption and low rate of energy dissipation produces relatively high coefficients of friction. Conversely, high energy absorption and high rate of energy dissipation leads to lower coefficients of friction. In addition, testing at low energy input in a moist environment produces a low coefficient of friction, which may increase significantly during the course of the test. Similar friction transitions occur during high energy tests. However, these do not appear related to moisture effects. In some higher velocity tests, very high local temperatures are detectable even though the bulk temperature rise is relatively low.
Examination of the friction surfaces of carbon-carbon display two distinct surface characteristics: shiny and dull bands. Structural changes of friction film have been observed indicating changes in carbon from the graphitic allotropic form to an amorphous type structure. These changes in crystal type lead to corresponding changes in coefficient of friction. It seems that the inter-laminar shearing and disruption of the film might cause the friction variation.
Both friction transitions and hot spots are produced because of mechanical or thermal disturbances. Our published studies show that any type of failure (mechanical or thermal fatigue) of the friction film or bulk material was sufficient to generate either frictional transition or thermal instabilities.

(4) Developed other hybrid materials, such as ceramic-based carbon materials.

C-C composites possess excellent mechanical properties at high temperatures, a low thermal expansion coefficient, and a high thermal conductivity. These properties make them attractive materials for applications such as airframe structural materials, aerospace engines, and brakes. However, it is well-known that carbon is oxidized in air at temperatures as low as 500 †C, and extensive research has been carried out to improve the oxidation resistance of carbon-carbon composites. CAFS researchers have extensively studied the oxidation of carbon-carbon materials. There are two major ways to protect these materials against oxidation. The first method makes use of oxidation-resistant coatings, such as SiC. The major problem with this method is the fact that coatings usually induce stresses and often lead to crack formation. The other major method of protecting C-C composites is by using matrix inhibitors, such as boron or boron carbide. They reduce the carbon oxidation by spreading a sealant borate glass within the composite. However, due to their low melting point, such inhibitors introduce temperature limitations for composite applications and are effective only after an appreciable fraction of carbon has been gasified. This is unacceptable in certain cases, since as little as a few percent of weight loss may drastically reduce the mechanical properties of the composite.
In the braking process and at high humidity, a carbon composite loses much of its friction property. Instead, it becomes greasy--more like a lubricant. Making carbon composite brakes last longer and perform better is a reasonable goal. We have developed a nano-composite material which uses ceramic particles to protect carbon from high heat in an oxidizing environment. Adding ceramic prevents this effect. In fact, dynamometer testing shows that the ceramic-enhanced carbon composite has about a 20-fold higher coefficient of friction than a standard carbon composite. For certain friction applications, ceramic doped carbon materials exhibit more braking capability.

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