Patent Application
20080041674
The present invention is directed to a removable insert for a disc brake made from a higher strength carbon-carbon composite. The carbon-carbon composite insert is treated with antioxidant solution. Preferably, the carbon-carbon composite is vacuum pressure impregnated with antioxidant solution. With this approach the lug strength and the oxidation resistance can be simultaneously improved. The removable insert also allows for replacement if excess wear occurs.
In one embodiment, CARBENIX® 4000 carbon-carbon composite is treated with P-13K, P-39 or other high performance phosphoric acid based anti-oxidant solutions that are known in the art. The materials are then cured by a standard process. The inserts can then be tack bonded into a mating surface in the stator. Preferably the carbon-carbon composite insert is vacuum pressure impregnated with antioxidant solution.
Oxidation treatments. Methods of treating carbon-carbon composites with an antioxidant are known in the art. One such method is to impregnate the carbon-carbon composite with an aqueous solution comprising standard antioxidants. The antioxidant solution may be administered to the carbon-carbon composite by, for example, painting the solution onto the surface of the carbon-carbon composite. The carbon-carbon composites may also be sprayed or soaked with antioxidant solution. The carbon-carbon composite is then heated to high temperature under nitrogen. The antioxidant solution can also be heated before being applied to the carbon-carbon composite.
Any antioxidants that can be used to inhibit oxidation in carbon-carbon composites, that are generally known to those of skill in the art can be used with this invention. Antioxidants that can be used with this invention include phosphate coatings of Al, Zn, or Mn, that is brushed on the edges of the brake discs and then charred. Another class of anti-oxidant treatments for carbon and graphite materials, including carbon-carbon composites, is based on using compounds that form a stable complex with active sites on the carbon composite in order to prevent oxidation.
Halogen and organohalogen compounds have been used as oxidation inhibitors at temperatures of up to 900° C. A concentrated aqueous solution of H3PO4 ZnO, Al(OH)3, CuSO4 and Cu(NO3)2 can also be used to inhibit oxidation at high temperatures. Similar antioxidants are described in U.S. Pat. No. 4,837,073 to McAllister et al., which is incorporated herein by reference in its entirety.
A further class of antioxidants that are especially preferred include phosphoric acid penetrants, which are coated on the carbon/carbon material. These antioxidants significantly improve the oxidative resistance of the carbon-carbon composite at the high end of the typical operating temperature range. These antioxidants are also effective in the presence of high concentrations of known oxidation catalysts, such as potassium acetate, a common constituent in aircraft runway deicers.
Such a phosphoric acid-based penetrant salt solution may contain the ions formed from one or more of the following: 10-80 wt % H2O, 20-70 wt % H3PO4, 0-25 wt % MnHPO4.1.6H2O, 0-30 wt % Al(H2PO4)3, 0-2 wt % B2O3, 0-10 wt % Zn3 (PO4)2 and 0.1-25 wt % alkali metal mono-, di-, or tri-basic phosphate. Antioxidants suitable for use with this invention are described in U.S. Pat. No. 6,455,159 to Walker et al., which is incorporated herein by reference in its entirety.
U.S. Pat. No. 5,759,622 and U.S. Pat. No. 6,551,709 both to Stover discuss methods and materials for treating carbon-carbon composites with antioxidant. U.S. Pat. No. 2,685,539 to Woodburn et al.; U.S. Pat. No. 4,439,491 to Wilson et al.; and U.S. Pat. No. 4,837,073 to McAllister et al. also discuss methods and materials for treating carbon materials with antioxidant solutions. Each of U.S. Pat. No. 6,551,709 to Stover; U.S. Pat. No. 5,759,622 to Stover; U.S. Pat. No. 2,685,539 to Woodburn et al.; U.S. Pat. No. 4,439,491 Wilson et al.; and U.S. Pat. No. 4,837,073 to McAllister et al. are each herein incorporated by reference in their entireties.
Vacuum impregnation. The articles may be impregnated with antioxidant using any vacuum impregnation technique that is known in the art. Generally, vacuum impregnation involves first placing the article under a vacuum, then applying antioxidant to the surface of the article. Pressure is then re-applied, which forces the antioxidant into the pores of the article. Excess antioxidant is then wiped from the surface of the article.
Vacuum impregnation results in the pores of the article becoming filled with antioxidant, but does not necessarily result in a change in the size or shape of the article. Other methods of applying antioxidant to an article generally affect the surface of the article, and adversely affect the friction properties.
In one embodiment of the present invention, the carbon-carbon composites are placed into a vacuum chamber. The air is evacuated, opening fine paths within the carbon-carbon composite, which makes the pores receptive to filling with antioxidant. The antioxidant is then introduced onto the surface of the carbon-carbon composite. The chamber is then pressurized, which forces the antioxidant into the pores of the carbon-carbon composite. This is followed by rinsing to clear excess antioxidant from the external surface of the carbon-carbon composite, and removal from the chamber. The process can be done relatively quickly. Typical impregnation times are about 20-25 minutes.
The impregnation process does not alter the external surface, so carbon-carbon brake pads require no additional tooling or shaping after the process. Vacuum impregnation is effective in filling surface pores of the carbon-carbon composite as well as cracks and holes that penetrate the part.
Carbon-carbon composites. Any carbon carbon-carbon composites that are well known in the art can be used with this invention. Carbon-carbon composites are generally made of fibers, and carbonaceous polymers and/or pyrocarbon as the matrix. Carbon-carbon composites and methods of their manufacture are well known to those in the art. Carbon-carbon composites are described in Carbon-Carbon Materials and Composites, John D. Buckley and Dan D. Edie, Noyes Publications, 1993, which is incorporated herein by reference. The carbon-carbon composites of the present invention can be made with thermosetting resins as matrix precursors. These materials generally possess low densities 1.55-1.75 g/cm3 and have well-distributed microporosity. Composites made with resins as the matrix generally exhibit high flexural strength, low toughness, and low thermal conductivity.
The carbon-carbon composites of the present invention can also be made with pitch as the matrix precursor. These materials, after densification, can exhibit densities in the range of 1.8-2.0 g/cm3 with some mesopores. The carbon-carbon composites of the present invention can also be made by chemical vapor deposition (CVD). This technique uses hydrocarbon gases, and the carbon-carbon composites that are produced possess intermediate densities, and have matrices with close porosities. Composites with pitch as the precursor, and the CVD-based composites, can be made with very high thermal conductivity (400-700 W/MK) in the fiber direction.
In one preferred embodiment, the carbon-carbon composites of the present invention are prepared from carbon preforms. Carbon preforms are made of carbon fibers, which can be formed from pre-oxidized polyacrylonitrile (PAN) fibers. The carbon fibers can be layered together to form a shape, such as a friction brake pad. The shape is heated and infiltrated with methane, or another pyrolyzable carbon source, to form the carbon-carbon composite. A carbon-carbon composite prepared in this manner is preferred, and will have a density in the range of about 1.6 g/cm3 to about 1.9 g/cm3. More highly preferred is a carbon-carbon composite with a density of approximately 1.75 g/cm3.
One highly preferred carbon-carbon composite is CARBENIX™ 4000. This carbon/carbon composite material is manufactured by Honeywell International, Inc. as an aircraft brake carbon/carbon composite friction material. CARBENIX™ 4000 is made of non-woven polyacrylonitrile precursor carbon fibers, densified with carbon utilizing chemical vapor deposition.
Another highly preferred carbon-carbon composite is CARBENIX™ 2400, also manufactured by Honeywell International, Inc. CARBENIX™ 2400 is an aircraft brake carbon/carbon composite friction material, consisting of pitch precursor carbon fibers, densified with carbonized phenolic resin and with carbon from chemical vapor deposition.
Preferred embodiments.
The Recesses 16 of disc 10 are generally annular shaped slots extending radially outwardly from opening 14 of disc 10. Annular drive inserts 20 couple a torque tube 6 with disc 10. The torque tube is coupled with the axle of an aircraft (not shown).
The preferred embodiments given above and shown in the Figures are examples of the invention, and are not intended to define the full scope of the invention. One skilled in the art would recognize many variations that would also be encompassed by the claims.
