Patent Application
20080090064
The present invention will become more fully understood from the detailed description given herein and from the accompanying drawings. The drawings are not to scale, and are given by way of illustration only. Accordingly, the drawings should not be construed as limiting of the present invention.
The present invention is directed to an improved carbon-carbon composite material. The present invention is also directed to an improved method for manufacturing a carbon-carbon composite material wherein at least one of the densification steps with carbon involves rapid infiltration with a liquid resin/pitch carbon followed by infiltration with a ceramic additive which results in lower wear rates with good friction properties.
A composite is a material created from a fiber (or reinforcement) and an appropriate matrix material in order to maximize specific performance properties. The individual constituents do not dissolve or merge completely but retain their identities as they act in concert. In composites, the matrix material acts to hold the fibers together and protect them, and to transfer the load to the fibers in the fabricated composite part. The reinforcing fiber imparts strength and other required properties to the composite. Among composites, carbon-carbon composites are used in a variety of applications and consist of a composite of carbon fiber in a carbon matrix.
Carbon fibers are reinforcing fibers known for their lightweight, high strength, and high stiffness properties. They are generally produced by pyrolysis of an organic precursor fiber in an inert atmosphere at high temperatures. These fibers may be produced from several different types of precursor fibers, such as polyacrylonitrile (PAN), rayon, and petroleum pitch. The carbon fibers are produced by the controlled burning off of the oxygen, nitrogen, and other non-carbon parts of the precursor fiber, leaving only carbon in the fiber. During the burning off (carbonization/graphitization) steps, the fibers are run through a furnace to produce carbon or graphite fibers. The carbon or graphite fibers are produced at furnace temperatures of 1000-3000° C.
A carbon preform is a fibrous reinforcement pre-shaped to the approximate contour and thickness desired in the finished part. Processing carbon-carbon composite preforms consists of building up of the carbon matrix around the carbon fibers. There are two common ways to create the matrix: through chemical vapor deposition/infiltration and through impregnation with a resin/pitch. Chemical vapor deposition/infiltration begins with a preform in the desired shape of the part, usually formed from multiple layers of woven carbon fabric. The preform is heated in a furnace pressurized with an organic gas, such as methane, acetylene, or benzene. Under high heat and relatively low pressure, the gas decomposes and deposits a layer of carbon onto the carbon fibers. The gas must diffuse through the entire preform to make a uniform matrix, so the process is very slow, often requiring several weeks and several processing cycles to make a single part.
In the second method, a liquid resin such as phenolic or pitch is infiltrated into the preform to fill the open porosity, The preform is then pyrolized at high temperature to rigidize the composite and remove volatiles. Alternatively, a preform can be built up from woven or nonwoven resin-impregnated carbon textiles or yarns, then cured and pyrolized.
In both methods of resin infiltrated preforms the resin shrinks during subsequent carbonization which results in a network of fine cracks and pores in the matrix carbon and a reduction in density. The part must then be re-infiltrated with resin or CVD to fill in the small cracks and to achieve the desired density.
The carbon-carbon composite material of the present invention comprises a nonwoven preform that is carbonized and subjected to a combination of CVD and liquid resin infiltration prior to the addition of a ceramic additive.
Previous efforts to improve the wear rates of carbon composites with the introduction of additives have utilized short chopped fibers and resin matrices. The problem with these materials is that they have lower strength compared with carbon-carbon composites made from nonwoven preforms that contain continuous fibers. In addition, the z-oriented fibers generated during needling in the nonwoven preforms provide good interlaminar strength properties as well as good axial thermal conductivity properties that are required in friction applications where carbon-carbon composites are used.
Previous efforts to produce a low wearing carbon-carbon composites using nonwoven preforms have utilized melt infiltration of silicon (Si). The problem with these materials is that they generate large particles of ceramic (SiC) that lead to high wear rates due to the abrasive nature of the large SiC particles.
In the present invention the use of a nonwoven preform is combined with the infiltration of silica (SiO2) particles to generate a low wearing material. In the present invention the use of liquid infiltration of resin/pitch is used to provide a fine network or pores into which the fine particle of silica are infiltrated. Because of the small size of the porosity the size of the final SiC particles that are generated during the Conversion Heat-Treatment process remain small, generally less than 5 microns. It is the fine size of the SiC particles that leads to low wear rates of the carbon-carbon composites of the present invention.
The goal of the first two cycles of densification is to generate a network of fine cracks into which the colloidal silicon is introduced via liquid infiltration. The type of densification medium used in the first and second densification cycle and the use of an optional carbonization/heat treatment step depends on the original, pore-size distribution of the preform as well as the pore size following the first cycle densification. If CVD is used first, then the second cycle has to be a liquid densification process with pitch or resin. The liquid infiltration fills the large pores. The existence of large pores is undesirable since they allow high levels of particles to agglomerate during subsequent heat treat conversion to form large particles of SiC (>10 microns) which cause high wear rates due to their abrasive nature. For low wear rates the ceramic particles (SiC) have to be smaller than 5 microns and preferably 1 microns or less.
Following the liquid infiltration of resin or pitch the materials are carbonized or heat treated to form the network of cracks prior to infiltration with Ludox (colloidal silica). Following the infiltration of the colloidal silica the materials are dried to remove the liquid medium used to infiltrate the colloidal silica into the composite. The composite is then densified with either CVD or resin/pitch to help bind the silica particles into the carbon prior to a heat-treatment conversion step where the silica particles are converted to fine particles of silicon carbide.
In one embodiment of the present invention, a carbonized nonwoven material is subjected to a first chemical vapor deposition procedure, followed by infiltration of pitch and then the addition of a ceramic additive by infiltration of a colloidal suspension containing silica. After the nonwoven material is dried and heat-treated, it is subjected to a second cycle of chemical vapor deposition to bind the SiO2/SiC into the composite material.
In another embodiment of the present invention, the nonwoven material is a nonwoven preform which is densified by subjecting it to rapid impregnation by means of a resin transfer molding (RTM) process, preferably utilizing a high viscosity pitch resin.
In a preferred embodiment of the present invention, the step of densifying the nonwoven material is followed by a step of stabilizing the pitch and a further step of carbonizing the pitch, and the step of adding a ceramic additive comprises infiltrating the nonwoven material with a ceramic additive solution, preferably one comprising silica.
In another preferred embodiment of the present invention, the nonwoven material is infiltrated with a ceramic additive solution comprising a solution of Silica suspended in water.
In a most preferred embodiment of the present invention, a carbonized nonwoven preform is subjected to rapid densification using RTM with a high viscosity pitch resin and is subsequently infiltrated with a solution of Silica suspended in water. The nonwoven preform is preferably subjected to two separate chemical vapor deposition procedures. The first chemical vapor deposition procedure takes place prior to the step of RTM densification with a pitch, while the second CVD step occurs subsequent to the step of infiltration with a silica solution.
In one embodiment of a present invention, the preform is formed by needling nonwoven materials. After the nonwoven preforms are created, they are carbonized and subjected to a first chemical vapor deposition process followed by a step of densification.
In one embodiment of the present invention, the densification step comprises subjecting the nonwoven material to a rapid transfer molding (RTM) process utilizing pitch. In a preferred embodiment of the present invention, the RTM process utilizes a high viscosity pitch resin.
In a preferred embodiment of the present invention, the densification step further comprises a step of stabilizing the pitch and a step of carbonizing or charring the pitch.
In a most preferred embodiment of the present invention, a ceramic additive is added to the material in order to enhance performance characteristics such as wear, and friction stability. The ceramic additive may comprise silica or a solution of silica suspended in water and may be infiltrated into the nonwoven material. After addition of the ceramic additive, the nonwoven material is preferably dried in a low temperature oven and heat treated in order to convert the silica to silicon carbide.
After heat-treatment, the nonwoven material is subjected to a second chemical vapor deposition procedure. After the second CVD step, the manufacturing process proceeds as is known in the art by, for example, machining the preform in order to meet application criteria. Normally, carbon-carbon composites produced in accordance with this invention will be coated with anti-oxidant before use.
The liquid resin/pitch impregnation process of the present invention fills the large pores in the nonwoven substrate. After stabilization and carbonization, approximately 15% of the pitch mass is lost, and the structure is opened up with fine connected porosity more similar to the random fiber materials. Infiltrating with a ceramic additive at this point provides a more uniform distribution of silica and prevents agglomeration. Heat treatment further shrinks the pitch matrix and opens up the porosity for improved densification during the final CVD step. Furthermore, final heat treatment converts the silica into fine silicon carbide particles. This, in turn, results in a very efficient final CVD cycle and leads to higher densities than all-CVD densified nonwoven materials.
This invention contemplates a specific order of processing (CVD and liquid densification) that results in the desired chemistry of the final C—C composite as well as the desired distribution and particle size of a ceramic filler that results in a low wearing C—C composite for friction applications. The general process may be described with reference to
Examples of the various embodiments of the low wear carbon-carbon composites provided by this invention are described below.
A nonwoven preform is made from carbon fiber using PAN precursors. The preform consists of continuous fibers arranged in the X and Y plane as well as short chopped fibers in the Z-direction. The nonwoven preform is then densified with carbon using CVD. The density of the composite is 1.1 g/cc. A second densification cycle is then applied with pitch using vacuum impregnation. The density of the composite is 1.6 g/cc. The preform is stabilized in air at 170° C. for 30 days. The composite is then carbonized at 600-1000° C. to remove the volatiles. The density of the composite is 1.5 g/cc). Following carbonization the composite is infiltrated with silica using vacuum infiltration of silica suspended in an aqueous slurry. Following the infiltration by the ceramic additive, the composite undergoes a third and final densification cycle of carbon with pitch by VPI. This is then followed by heat treatment to >1800° C. to convert the ceramic additive to SiC. The composite is then machined to the configuration of an aircraft brake disc. Finally, an oxidation protection system is applied to the composite brake disc.
A nonwoven preform is made from carbon fiber using pitch precursors. The preform consists of continuous fibers arranged in the X and Y plane as well as short chopped fibers in the Z-direction. The nonwoven preform is then densified with carbon using CVD. The density of the composite is 0.9 g/cc. A second densification cycle is then applied with a resin matrix carbon vacuum pressure impregnation. The preform is stabilized in air at 170° C. for 20 days. The composite is then carbonized at 1000° C. to remove the volatiles. The density of the composite is 1.3 g/cc). Following carbonization the composite is infiltrated with silica using vacuum infiltration of silica suspended in an aqueous slurry. Following the infiltration by the ceramic additive, the composite undergoes a third and final densification cycle of carbon with by CVD. This is then followed by heat treatment to >1800° C. to convert the ceramic additive to SiC. The composite is then machined to the configuration of an aircraft brake disc. Finally, an oxidation protection system is applied to the composite brake disc.
A nonwoven preform is made from carbon fiber using PAN precursors. The preform consists of continuous fibers arranged in the X and Y plane as well as short chopped fibers in the Z-direction. The nonwoven preform is then densified with pitch via vacuum pressure impregnation (VPI). The density of the composite is 1.0 g/cc. A second densification cycle is then applied with pitch-based carbon using vacuum pressure impregnation. The density of the composite is 1.6 g/cc. The preform is stabilized in air at 170° C. for 25 days. The composite is then carbonized at 600-1000° C. to remove the volatiles. The density of the composite is 1.4 g/cc). Following carbonization the composite is infiltrated with silica using vacuum infiltration of silica suspended in an aqueous slurry. Following the infiltration by the ceramic additive, the composite undergoes a third and final densification cycle of carbon with pitch by VPI. This is then followed by heat treatment to >1800° C. to convert the ceramic additive to SiC. The composite is then machined to the configuration of an aircraft brake disc. Finally, an oxidation protection system is applied to the composite brake disc.
A nonwoven preform is made from carbon fiber using pitch precursors. The preform consists of continuous fibers arranged in the X and Y plane as well as short chopped fibers in the Z-direction. The nonwoven preform is then densified with carbon using CVD. The density of the composite is 1.0 g/cc. A second densification cycle is then applied with the resin using VPI. The density of the composite is 1.6 g/cc. The preform is stabilized in air at 170° C. for 28 days. The composite is then carbonized at 600-1000° C. to remove the volatiles. The density of the composite is 1.6 g/cc). Following carbonization the composite is infiltrated with silica using vacuum infiltration of silica suspended in an aqueous slurry. Following the infiltration by the ceramic additive, the composite undergoes a third and final densification cycle of carbon with pitch by VPI. This is then followed by heat treatment to >1800° C. to convert the ceramic additive to SiC. The composite is then machined to the configuration of an aircraft brake disc. Finally, an oxidation protection system is applied to the composite brake disc.
A nonwoven preform is made from carbon fiber using PAN precursors. The preform consists of continuous fibers arranged in the X and Y plane as well as short chopped fibers in the Z-direction. The nonwoven preform is then densified with carbon using CVD. The density of the composite is 1.4 g/cc. A second densification cycle is then applied with pitch using VPI. The density of the composite is 1.6 g/cc. The preform is stabilized in air at 170° C. for 22 days. The composite is then carbonized at 600-1000° C. to remove the volatiles. The density of the composite is 1.6 g/cc). Following carbonization the composite is infiltrated with silica using vacuum infiltration of silica suspended in an aqueous slurry. Following the infiltration by the ceramic additive, the composite undergoes a third and final densification cycle of carbon with pitch by VPI. This is then followed by heat treatment to >1800° C. to convert the ceramic additive to SiC. The composite is then machined to the configuration of an aircraft brake disc. Finally, an oxidation protection system is applied to the composite brake disc.
A nonwoven preform is made from carbon fiber using pitch precursors. The preform consists of continuous fibers arranged in the X and Y plane as well as short chopped fibers in the Z-direction. The nonwoven preform is then densified with carbon using CVD. The density of the composite is 0.8 g/cc. A second densification cycle is then applied using an RTM process. The density of the composite is 1.6 g/cc. The preform is stabilized in air at 170° C. for 20 days. The composite is then carbonized at 600-1000° C. to remove the volatiles. The density of the composite is 1.5 g/cc). Following carbonization the composite is infiltrated with silica using vacuum infiltration of silica suspended in an aqueous slurry. Following the infiltration by the ceramic additive, the composite undergoes a third and final densification cycle of carbon with pitch by VPI. This is then followed by heat treatment to >1800° C. to convert the ceramic additive to SiC. The composite is then machined to the configuration of an aircraft brake disc. Finally, an oxidation protection system is applied to the composite brake disc.
A nonwoven preform is made from carbon fiber using PAN precursors. The preform consists of continuous fibers arranged in the X and Y plane as well as short chopped fibers in the Z-direction. The nonwoven preform is then densified with carbon using CVD. The density of the composite is 1.3 g/cc. A second densification cycle is then applied with pitch using an RTM process. The density of the composite is 1.4 g/cc. The preform is stabilized in air at 170° C. for 21 days. The composite is then carbonized at 750° C. to remove the volatiles. The density of the composite is 1.5 g/cc. Following carbonization the composite is infiltrated with silica using vacuum infiltration of silica suspended in an aqueous slurry. Following the infiltration by the ceramic additive, the composite undergoes a third and final densification cycle by vacuum impregnation of pitch-based carbon matrix material. This is then followed by heat treatment to >1800° C. to convert the ceramic additive to SiC. The composite is then machined to the configuration of an aircraft brake disc. Finally, an oxidation protection system is applied to the composite brake disc.
The present invention has been described herein in terms of preferred embodiments. However, obvious modifications and additions to the invention will become apparent to those skilled in the relevant arts upon a reading and understanding of the foregoing description. It is intended that all such modifications and additions form a part of the present invention to the extent that they fall within the scope of the several claims appended hereto.
