Patent Application
20070235126
The present invention will be more fully understood by reference to the detailed description given hereinbelow and to the accompanying drawing. The drawing is not to scale, and is provided by way of illustration only. It does not in any way limit the present invention.
Application Docket No. H0010773-1160, filed concurrently herewith, entitled ‘APPARATUS AND METHOD FOR BONDING CARBON-CARBON COMPOSITES THROUGH A REACTANT LAYER’ describes an apparatus and methods for bonding carbon-carbon composites through a reactant layer. The apparatus and methods described in H0010773-1160 may be employed to practice the present invention. Accordingly, the entire disclosure of H0010773-1160 is expressly incorporated by reference into this application.
The present invention employs liquid titanium carbide as a hot “glue”. The melting point of titanium carbide is significantly higher than the operating temperatures of aircraft brakes. Also, the heat capacity of titanium carbide on a volume basis is comparable to that of carbon-carbon composites, so the bonding material does not degrade the thermal properties of the brakes.
In accordance with this invention, carbon-carbon composite pieces to be joined together are selected and machined to have a good fit. That is to say, a large surface area relative to the overall size of the pieces should be present complementarily on both pieces. The simplest example of complementary surfaces is two surfaces that are flat. However, irregularly shaped pieces, such as pieces having corresponding convexities and concavities, can be joined together by the method of the present invention. Two complementary surfaces are considered to be matched up when they are arranged such that there is a minimum amount of space between them.
A mixture of titanium powder and carbon powder is placed on one of the complementary mating surfaces. The particle size of the titanium and carbon powders should be small, in order to create a large powder surface area that will support a rapid combustion reaction. Generally, the average diameter of the titanium powder particles will range from 25 to 250 microns, and the carbon powder will powdered graphite in which the average diameter of the graphite particles will range from 1 to 10 microns. Typical specific examples would be titanium powder particles having an average diameter of about 45 microns, and graphite particles having an average diameter of about 2 microns. Such powders can be prepared as needed, but they are commercially available from a variety of sources, including Alfa Aesar (a Johnson Matthey Company) of Ward Hill, Mass. The titanium (Ti) and carbon (C) powders will generally be mixed in molar ratio of titanium powder to powdered graphite ranging from 3:1 to 1:1. We have found that a molar ratio of Ti to C of about 2:1 works well.
To effect bonding, the other complementary mating surface is placed onto the complementary mating surface to which the powder mixture has been applied, and the two pieces joined at their complementary surfaces with a layer of titanium and carbon powder between them are pressed tightly. A typical pressure would be on the order of 7400 pounds per square inch, applied for 10 seconds. While the construct is under pressure, an electrical current is applied to the construct in order to begin a combustion reaction. A typical electric current would be 600 Amps applied for 5 seconds. The oxidation-reduction reaction, once started, is highly exothermic, and has an adiabatic product temperature of over 3000° C. The product of the reaction is titanium carbide at a temperature so high that the TiC is in liquid form.
The powder mixture may be placed in the joint area by any convenient method. For instance, when an aircraft brake disc is being refurbished, the bottom piece will lie in a horizontal plane and the mixed powder can be shaken out onto the flat surface from a “salt shaker” type dispenser. Pressure on the carbon-carbon composite pieces forces small amounts of the liquified TiC into the pores of the composite pieces. The operator will normally distribute the powder mixture as evenly as possible. However, a precisely uniform layer of powder is not essential, because during the bonding procedure, the liquefied powder is distributed evenly across the joint plane and partially forced into pores of the composite materials being bonded together. The heat of the reaction is rapidly absorbed by the carbon-carbon composite, lowering the temperature of the reaction product. As the liquid reaction product material cools below 2500° C., the liquid “freezes”, bonding the carbon-carbon composite pieces together.
Employing an apparatus as described in copending application H0010773-1160, a controller is programmed with an initial loading force of 500 lbs/in2, a maximum loading force of 7400 lbs/in2, a delay time of maximum force application of 1 second, a time of maximum force application of 10 seconds, a maximum current of 600 Amps, a time of current of 5 seconds, and an initial temperature of 30° C. A worn brake disc friction surface portion is removed from a carbon-carbon composite aircraft brake disc, leaving a first flat disc-shaped piece approximately half the thickness of the original brake disc. Next, the area from which the worn portion was removed is covered with a thin layer that is a mixture of titanium carbide particles and carbon powder. Subsequently, a corresponding second flat disc-shaped piece of carbon-carbon composite material is placed on top of the layer of TiC/graphite mixture on the first disc-shaped piece, to form a “sandwich” construct having two “slices” of carbon-carbon composite material with a “filling” of titanium/graphite powder. The resulting construct is placed in the apparatus and the TiC/graphite mixture is ignited, bonding the new friction surface in place and providing a refurbished full size brake disc.
While the present invention has been described with respect to a detailed example of its implementation, the invention is capable of numerous modifications, rearrangements, and alterations, and such are intended to be within the spirit and scope of the disclosure and claims.
