METHOD OF PROVIDING IMPROVED STATIC FRICTION FOR CARBON BRAKE MATERIAL

Abstract: A carbon/carbon composite friction material having high static friction characteristics is provided for an aircraft brake. The improved static friction brake material is produced by adding a hard-phase addi­ tive, generally a ceramic, to the carbon/carbon composite material during processing.

Description: The invention relates generally to a carbon composite material having improved static friction and for use in aircraft brakes. BACKGROUND OF THE INVENTION Carbon aircraft brakes generally comprise carbon/carbon composite discs which rub against one another and generate frictional heat, such as those dis­ closed in U. S. Patent No. 3,639,l97 incorporated by reference herein. Under normal braking conditions, this carbon composite braking system works effectively and yields a dynamic and static coefficient of friction of approximately 0.2 to 0.5. However, in some static brake applications, the coefficient of friction can be as low as 0.l0. For some aircraft, it is desirable to provide a carbon composite material having an improved static fric­ tion. This increases the braking control that the pilot is able to effect, and is particularly useful in elimi­ nating the slight movement or "creep" of the aircraft which may occur while the brakes are applied. Therefore, it is desirable to provide a carbon composite brake material having improved static friction in order to increase the braking capability of the aircraft operator. It is desirable that the static coefficient of friction be increased by 20-l00% for the carbon composite brake material. BRIEF STATEMENT OF THE INVENTION The present invention comprises a process which produces a carbon brake component having carbon material with improved static-friction, and comprises the steps of molding a mixture of carbon fiber, thermosetting resin, and hard-phase additive into the shape of the carbon brake component, heating the carbon material in order to convert the thermosetting resin to glassy carbon, densi­ fying the carbon material in order to minimize porosity by depositing additional carbon therein, heating the carbon material to a temperature in the range of l600-2800° C, and machining the carbon material to final dimensions of the carbon brake component. If carbon fabric or felt is used, the hard-phase additive may be incorporated during the resin impregna­ tion step, and the resulting prepreg molded and processed as above. A non-resinous carbon composite brake material can be made by evenly distributing the hard-phase addi­ tive over each layer of fabric or felt reinforcement prior to CVD processing. DESCRIPTION Static friction in carbon composite friction materials is influenced by the surface condition of the carbon material and by the presence of adsorbed gaseous impurities. The wear life development of a carbon fric­ tion surface occurs in three stages, each stage being characterized by the surface condition of the respective stage. In the first stage, the surface is new and rough and it is charcterized by the presence of surface asperities. Friction at this stage is caused by inter­ ference with sliding motion by, and shearing of, the asperities on opposing surfaces. This friction mechanism is called "asperity interaction" and the coefficient of friction in this condition is high. In the second stage, after successive braking engagements, the asperities are worn off and leave a smoother surface and some wear debris. This surface condition is characterized by the presence of wear debris. The static friction in this stage is lower than the first stage and is caused by fracture and deformation of, and plowing by, the wear debris. In the final stage of surface friction develop­ ment, the wear debris is firmly compacted on the friction surface to provide a smooth surface. This smooth surface condition is characterized by the presence of a smooth friction film. The friction mechanism in this case is adhesion. However, the adhesive forces are very low in carbon due to adsorbed impurities. A typical aircraft brake disc surface is in this condition and the static friction is low. In other conditions, the static friction of a brake disc is high when either the friction film has been disturbed to generate some loose wear debris or the sur­ face has been desorbed of impurities. These conditions are generally temporary. A few static braking engage­ ments would smear the debris back into the surface and the value of static friction would drop to the original level. Also, the carbon disc surface would adsorb oxygen and water vapor from the atmosphere, and thereby lower static friction. The present invention comprises a method that increases the static friction of the carbon brake disc by keeping the friction surface in the disturbed state, or by delaying the formation of compacted friction film, or by increasing asperity contact. This is accomplished by adding a hard-phase material, generally a ceramic, to the carbon composite material. Addition of this hard-phase material increases the asperity interaction in the ini­ tial stages. In the intermediate stages, it delays the formation of smooth compacted friction film. In the final stage, the debris from the hard-phase material maintains the friction surface in a disturbed state. A key element of the present invention is that the hard-­ phase additive is of such a nature that it is well anchored in the matrix and will not be dislodged from the matrix during braking. Prematurely dislodged particles will cause excessive wear and this is not desired. The present invention provides hard-phase asperities on the one hand, and fine wear debris on the other. EXAMPLES There are several methods for making carbon composite materials for use as a carbon friction com­ ponent in aircraft brakes. The compositions of such components are comprised typically of carbon fiber (chopped, felt, or fabric) 30-70% by volume, chemical vapor deposited carbon 10-70% by volume, and glassy car­ bon derived from thermosetting resins 0-60% by volume of the entire composition. The resin may be any resin which yields a carbonaceous residue upon pyrolysis. The vari­ ous heating cycle times are dependent upon the material of the article, size of the article, and load of the furnace, as is well known in the art. A method of making a carbon composite brake material for aircraft brakes comprises molding a mixture of (l) chopped carbon fiber, (2) a thermosetting resin of high char-yield type, and (3) a hard-phase additive in the amount of l-l0% by volume, into the shape of a carbon brake material. The molded carbon brake material comprising the above three constituents is thermally treated in a carbonization cycle in order to convert the resin to glassy carbon. The carbonization cycle is typically at a temperature in the range of approximately 500-ll00° C. Because of the porosity of the carbon brake material, the brake material is densified by either a chemical vapor deposition pro­ cess (also called a chemical vapor infiltration process) or by a liquid resin/pitch impregnation, or by both methods. If the liquid impregnation method is used for densification of the carbon brake material, then the brake material must undergo another carbonization cycle at the above temperature range. The densified carbon brake material is then thermally treated for graphitiza­ tion and stress relief, at a temperature in the range of approximately l600-2800° C, or to the maximum temperature permitted by the thermal stability of the hard-phase additive. Then the carbon brake material is machined to provide a carbon brake friction component having the required dimensions. Additionally, an oxidation protec­ tion may be applied to the carbon friction component if so desired. Typical oxidation protection substances comprise chemical penetrants or coating such as those disclosed in U. S. Patent No. 4,465,777. In the above process, the hard-phase additive is added in the amount of l-l0% by volume of the mixture during the initial step of processing. This is achieved by mixing it mechanically with the fibers in the resin so that the molding compound obtained has three separate phases -- fibers, hard-phase additive, and the thermo­ setting resin. The mixture is then molded into the car­ bon brake material and further processed as described above. The resulting carbon friction component has a composition typically as follows: Carbon Fiber: 25-70% by weight CVD Carbon: 0-50% by weight Glassy Carbon: l0-60% by weight Hard-Phase Additive: l-25% by weight The composition of the carbon composite brake material (exclusive of the hard-phase additive) may de­ pend on a particular application desired by the user, and can vary according to the desired properties for that particular application. If the carbon composite brake material is to be based on fabric or felt, then the hard-phase additive can be incorporated in the resin-impregnation step. Initi­ ally, the fabric or felt material is placed within a resin vat having therein the hard-phase additive. The following staging step serves to heat the impregnated fabric or felt at a low temperature in order to remove volatiles. Then the resulting prepreg is cut to shape and heated under pressure in a molding step, followed by a carbonization cycle during which the molded carbon brake material is heated to a temperature in the range of approximately 500-ll00° C. This is followed by a densi­ fication step (typically CVD carbon deposition) in order to minimize porosity and deposit further carbon thereon. Then the carbon brake material is heat treated for graphitization and stress relief at a temperature in the range of approximately l600-2800° C, and followed by machining of the carbon brake material into the final dimensions of a carbon friction component. Additionally, the oxidation protection may be applied to the carbon friction component, as described above. In order to provide a non-resin based carbon composite brake material, the hard-phase additive can be incorporated by even distribution on each layer of carbon fiber reinforcement prior to CVD processing. The rein­ forcement fixture, which may comprise a fabric material, and the hard-phase additive evenly distributed thereon, are placed within the chemical vapor deposition appara­ tus, and the deposition process completed. This is followed by the heat treating for graphitization and stress relief, and then machining to the final dimensions and with an optional application of oxidation protection if so desired. The resulting non-resin based carbon brake material has a composition comprising: Carbon Fiber: 25-70% by weight CVD Carbon: 25-70% by weight Hard-Phase Additive: l-25% by weight Table A below provides an example of hard-phase additives utilized in the chopped carbon fiber process to provide a carbon composite brake material with improved static friction. As can be seen in the Table, the materials with round particle additives exhibit a higher wear rate than those materials containing irregular par­ ticles or fibers. However, in all cases the static fric­ tion is improved to varying degrees. Composition No. l comprises a typical carbon composite brake material not having any hard-phase addi­ tive included therein, and provides a baseline figure for comparison with the carbon composite materials having a hard-phase additive added thereto. Table A illustrates that the dynamic friction of the carbon composite brake material is also affected, but remains in the usable range for aircraft brakes. Also, the TiB₂ additive has been found to improve the oxida­ tion resistance of the carbon/carbon composite brake component. Although the present invention has been illus­ trated and described in connection with the examples, it will be understood that this is illustrative of the invention, and is by no means restrictive, thereof. It is reasonably to be expected that those skilled in the art can make numerous revisions and additions to the invention and it is intended that such revisions and additions will be included in the scope of the following claims as equivalents of the invention.

Claims: 1. A process for providing a carbon brake compon­ ent having carbon material with improved static friction, comprising the steps of molding a mixture of chopped carbon fiber, thermosetting resin, and hard-phase additive into the shape of the carbon brake component, heating the carbon material in order to convert the thermosetting resin to glassy carbon, densifying the carbon material in order to reduce porosity thereof by depositing additional carbon therein, heating the carbon material to a temperature in the range of l600-2800° C, and machining the carbon ma­ terial to final dimensions of the carbon brake component. 2. The method in accordance with claim l, wherein the hard-phased additive comprises a ceramic material. 3. The method in accordance with claim 2, wherein the additive is one of SiC and TiB₂. 4. The method in accordance with claim l, wherein the densification of the carbon material further comprises a liquid resin/pitch impregnation followed by heating to a temperature in the range of 500-ll00° C. 5. A method for providing a carbon brake com­ ponent having carbon material with improved static fric­ tion, comprising the steps of impregnating a carbon fabric material in a resin bath having therein a hard-phase addi­ tive, heating the fabric material to a low temperature in order to remove volatiles therefrom and provide a prepreg, forming the prepreg to a desired shape, molding the prepreg under pressure and heat to form a carbon fabric brake ma­ terial, heating the carbon fabric brake material to a tem­ perature in the range of 800-ll00° C to produce a porous substrate with a glassy carbon matrix, densifying the porous substrate by a chemical vapor process in order to reduce porosity and add additional carbon thereto, heat-treating the resulting composite to a temperature in the range of l600-2800° C, and machining the composite to dimension to provide the carbon brake component. 6. The process in accordance with claim 5, where­ in the carbon fabric material comprises 30-70% by volume, carbon provided by densification comprises l0-70% by vo­ ume, glassy carbon comprises 0-60% by volume, and the hard-­ phase additive comprises l-l0% by volume of the entire composition. 7. The method in accordance with claim 6, wherein the additive is one of SiC and TiB₂. 8. A process for providing a carbon friction component having improved static friction, comprising the steps of distributing a hard-phase additive on a carbon fabric material and placing the carbon fabric material within a fixture for chemical vapor processing, densify­ ing the carbon fabric material by adding carbon thereto in order to reduce the porosity thereof and provide a friction material, heat-treating the densified carbon friction ma­ terial to a temperature in the range of l600-2800° C, and machining the carbon friction material in order to provide the carbon friction component. 9. A carbon/carbon composite article with im­ proved static friction comprising carbon fiber in the amount of 25-70% by weight of the article, CVD carbon in the amount of 0-50% by weight of the article, glassy car­ bon in the amount of l0-60% weight of the article, and hard-phase additive in the amount of l-25% by weight of the article. l0. A carbon/carbon composite article with im­ proved static friction comprising carbon fiber in the amount of 25-70% by weight of the article, CVD carbon in the amount of 25-70% by weight of the article, and hard-­ phase additive in the amount of l-25% by weight of the article.

Inventors: AWASTI, SHRIKANT
Applicants: ALLIED CORPORATION
Application year: 1987
Main classification: F16D69
IPC codes: C04B35, F16D69