High performance carbon fiber

A system is disclosed comprising a carbon/carbon brake disk comprising a carbon fiber having a crystal orientation (CO) between 80% and 100% and a coefficient of friction of more than 0.183 in response to a rejected takeoff condition.

BACKGROUND

A rejected takeoff (RTO) condition may refer to a condition wherein an aircraft applies braking during takeoff in response to a pilot aborting takeoff. A large amount of torque is applied at the aircraft brakes during an RTO condition, causing a large amount of heat to be produced and, in turn, causing an increase in temperature. It is important that this increase in temperature experienced during an RTO condition does not create an undesirably large drop in the coefficient of friction of carbon/carbon (C/C) brake disks in the aircraft braking system.

SUMMARY

According to various embodiments, a system is disclosed comprising a carbon/carbon brake disk comprising a carbon fiber having a crystal orientation (CO) between 80% and 100% and a coefficient of friction of more than 0.183 in response to a rejected takeoff condition.

According to various embodiments, a method is disclosed comprising a method manufacturing of a carbon/carbon brake disk comprising forming a fibrous network from an oxidized carbon fiber precursor, carbonizing the oxidized carbon fiber precursor by heating the oxidized carbon fiber precursor to form a carbon fiber, wherein the carbon fiber has a crystal orientation (CO) between 80% and 100% and a coefficient of friction of more than 0.183 in response to a rejected takeoff condition.

According to various embodiments, a method of improving rejected takeoff (RTO) performance comprising manufacturing a first densified preform using a first manufacturing process, the first manufacturing process being free of a crystal orientation (CO) improvement process, manufacturing a second densified preform using a second manufacturing process, the second manufacturing process being the same as the first manufacturing process except the second manufacturing process includes the crystal orientation (CO) improvement process, measuring a first coefficient of friction in an RTO condition of the first densified preform, and measuring a second coefficient of friction in the RTO condition of the second densified preform, wherein the second coefficient of friction is greater than the first coefficient of friction.

DETAILED DESCRIPTION

All ranges and ratio limits disclosed herein may be combined. It is to be understood that unless specifically stated otherwise, references to “a,” “an,” and/or “the” may include one or more than one and that reference to an item in the singular may also include the item in the plural.

Carbon/carbon parts (“C/C”) in the form of friction disks (also referred to as a carbon/carbon brake disks) are commonly used for aircraft brake disks, race car brakes, and clutch disks. Carbon/carbon brake disks are especially useful in these applications because of the superior high temperature characteristics of C/C material. In particular, the carbon/carbon material used in C/C parts is a good conductor of heat and is able to dissipate heat generated during braking away from the braking surfaces. Carbon/carbon material is also highly resistant to heat damage, and thus, is capable of sustaining friction between brake surfaces during severe braking without a significant reduction in the friction coefficient or mechanical failure.

One method of manufacturing C/C materials involves fabrication of an oxidized polyacrylonitrile (PAN) or carbon fiber preform, followed by carbonization and chemical vapor infiltration (CVI) densification. The CVI cycles are continued, in conjunction with machining the preform between infiltration cycles if desired, until the desired part density is achieved.

With reference toFIG. 3, carbon fiber production lifecycle300is shown. Acrylonitrile302may be treated with addition polymerization303to produce PAN304. PAN304may be oxidized via oxidation stabilization at between 200° C. and 400° C. (392° F. and 752° F.) to produce oxidized PAN fiber (OPF)306. PAN304and OPF306may be referred to as a carbon fiber precursor. OPF306may undergo carbonization/graphitization308at 1,000° C. (1,832° F.) or more in an inert environment to form carbon fiber310. Carbon fiber310may be structured as sheets of graphene.

In general, C/C parts produced using the OPF, carbonization, and CVI densification method are made in three successive manufacturing steps. First, a fibrous preform is made utilizing a variety of textile manufacturing techniques. Typically, the fibrous preform is made from OPF or carbon fiber. Although numerous techniques are known in the art for making fibrous preforms from OPF, a common technique involves stacking layers of OPF to superimpose the layers. The added layers may then be needled perpendicularly to the layers with barbed textile needles. The needing process generates a series of z-fibers through the fibrous preform that extend perpendicularly to the fibrous layers. The z-fibers are generated through the action of the needles pushing fibers from within the layer (x-y or in-plane) and reorienting them in the z-direction (through-thickness). Needling of the fibrous preform may be done as one or more layers are added to the stack or may be done after the entire stack is formed. The needles may also penetrate through only a portion of the preform or may penetrate through the entire preform. In addition, resins are sometimes added to the fibrous preform by either injecting the resin into the preform following construction or coating the fibers or layers prior to forming the fibrous preform. Fibrous preforms may also be made from pitch based carbon fiber tows and/or from lignin carbon fiber tows.

After the fibrous preform is made, it is carbonized to convert the OPF into carbon fibers, such as in carbonization/graphitization308. Typically, fibrous preforms are carbonized by placing the preforms in a furnace with an inert atmosphere. As is well-understood by those in the art, the heat of the furnace causes a chemical conversion which drives off the non-carbon chemicals from the preform. Carbonization/graphitization308may be conducted in a vacuum or partial vacuum (e.g., at pressures of 1-10 millitorr) or in an inert atmosphere at a temperature in the range from about 1,400° C. to about 2,800° C. (2,552° F. to about 5,072° F.), and in various embodiments in the range from about 1,600° C. to about 2,200° C. (2,912° F. to about 3,992° F.), and in various embodiments in the range from about 1,600° C. to about 2,500° C. (2,912° F. to about 4,532° F.)(wherein the term about in this context only means+/−100° C.) for a period of time in the range of up to about 60 hours, and in various embodiments, in the range up to about 10 hours (wherein the term about in this context only means+/−2 hours). The resulting preform generally has the same fibrous structure as the fibrous preform before carbonizing. However, the OPF have been converted to 100% carbon or very near 100%, for example from 95% carbon to 99.9% carbon. The resulting preform may be referred to as having a fibrous network.

After the preform has been carbonized, the preform is densified. In general, densification involves filling the voids, or pores, of the fibrous preform with additional carbon material. This may be done using the same furnace used for carbonization or a different furnace. Typically, chemical vapor infiltration and deposition (“CVI/CVD”) techniques are used to densify the porous fibrous preform with a carbon matrix. This commonly involves heating the furnace and the carbonized preforms, and flowing hydrocarbon gases (e.g., at least one of methane, ethane, propane, butane, and/or the like, as described herein) into the furnace and around and through the fibrous preforms. The hydrocarbons may comprise alkanes, for example, straight chain, branched chain and/or cyclic alkanes, having from 1 to about 8 carbon atoms, and in various embodiments from 1 to about 6 carbon atoms, and in various embodiments from 1 to about 3 carbon atoms. Methane, ethane, propane, cyclopentane, or mixtures of two or more thereof may be used. The gas may comprise one or more alkanes of 2 to about 8 carbon atoms, and in various embodiments from 2 to about 6 carbon atoms. Mixtures of one or more alkanes of 1 to about 8 carbon atoms with one or more alkenes of 2 to about 8 carbon atoms may be used. In various embodiments, fibrous preform may undergo an infiltration with a resin or pitch material to form a matrix therein.

As a result, carbon from the hydrocarbon gases separates from the gases and is deposited on and within the fibrous preforms. When the densification step is completed, the resulting C/C part has a carbon fiber structure with a carbon matrix infiltrating the fiber structure, thereby deriving the name “carbon/carbon.”

The resulting C/C part may thus be referred to as a C/C brake disk. A C/C brake disk may be used in an aircraft braking system. With reference toFIG. 5, an aircraft braking system, an in particular, multi-disk brake system20is illustrated. In various embodiments, multi-disk brake system20may be used to reduce the speed of a wheel of an aircraft. An A-R-C axis has been included in the drawings to illustrate the axial (A), radial (R) and circumferential (C) directions. The system may include a wheel10supported for rotation around axle12by bearings14. Wheel10includes rims16for supporting a tire, and a series of axially extending rotor splines18(one shown). Rotation of wheel10is modulated by disk brake system20. Disk brake system20includes torque flange22, torque tube24, a plurality of pistons26(one shown), pressure plate30, and end plate32. Torque tube24is an elongated annular structure that includes a reaction plate34and a series of axially extending stator splines36(one shown). Reaction plate34and stator splines36may be integral with torque tube24as shown inFIG. 5, or attached as separate components.

Disk brake system20also includes a plurality of friction disks38, which may comprise C/C brake disks. The plurality of friction disks38includes at least one non-rotatable friction disk40, also known as a stator, and at least one rotatable friction disk42, also known as a rotor. Each friction disk38includes an attachment structure. As shown inFIG. 5, each of four non-rotatable friction disks40include a plurality of stator lugs44at circumferentially spaced positions around non-rotatable friction disk40as an attachment structure. Similarly, each of five rotatable friction disks42include a plurality of rotor lugs46at circumferentially spaced positions around rotatable friction disk42as an attachment structure. As shown inFIG. 5, pressure plate30, end plate32, and friction disks38are all annular structures made at least partially from a carbon composite material (i.e., a C/C material, which may be referred to as a C/C brake disk).

Torque flange22is mounted to axle12. Torque tube24is bolted to torque flange22such that reaction plate34is near an axial center of wheel10. End plate32is connected to a surface of reaction plate34facing axially away from the axial center of wheel10. Thus, end plate32is non-rotatable by virtue of its connection to torque tube24. Stator splines36support pressure plate30so that pressure plate30is also non-rotatable. Stator splines36also support non-rotatable friction disks40. Non-rotatable friction disks40engage stator splines36with gaps formed between stator lugs44. Similarly, rotatable friction disks42engage rotor splines18with gaps formed between rotor lugs46. Thus, rotatable friction disks42are rotatable by virtue of their engagement with rotor splines18of wheel10.

As shown inFIG. 5, rotatable friction disks42are arranged with end plate32on one end, pressure plate30on the other end, and non-rotatable friction disks40interleaved so that rotatable friction disks42are adjacent to non-rotatable friction components. Pistons26are connected to torque flange22at circumferentially spaced positions around torque flange22. Pistons26face axially toward wheel10and include pucks52that contact a side of pressure plate30opposite rotatable friction disks42. Pistons26may be powered electrically, hydraulically or pneumatically.

An aircraft braking system, such as multi-disk brake system20, may be used to slow an aircraft while the aircraft is on the ground, for example during landing. Typically, by regulation, aircraft braking systems are designed to withstand an RTO condition. An RTO condition may refer to a condition wherein an aircraft applies brakes during takeoff in response to a pilot aborting takeoff. An RTO condition may involve a fully laden aircraft (i.e., an aircraft carrying its maximum rated weight) without benefit of the use of thrust reversers. In that regard, an RTO condition may involve the application of a large amount of torque on the aircraft braking system. For example, during an RTO condition, multi-disk brake system20may exert over 1,000 lbs ft of torque and, in various embodiments, over 20,000 lbs ft of torque.

An RTO condition may be simulated using a dynamometer. An aircraft braking system, such as multi-disk brake system20, may be mechanically coupled to a dynamometer. The dynamometer may simulate the speed of an aircraft wheel prior to an RTO under a load equivalent or nearly equivalent to an aircraft. The aircraft braking system may then be activated to exert a braking torque. During simulation, a coefficient of friction for a C/C brake disk (typically referred to as the Greek letter mu (μ)), may be determined. As may be understood, as load and heat increases, the coefficient of friction tends to decrease in conventional systems.

The ability to maintain an acceptable coefficient of friction during an RTO condition may be a measure of RTO performance and it is important to aircraft braking system development. Thus, C/C brake disks with acceptable coefficients of friction under RTO conditions are desirable.

As discussed above, carbon fiber may be comprised of graphene sheets with varying levels of alignment about a given plane. With reference toFIG. 1, a carbon fiber100is shown in accordance with various embodiments with varying levels of alignment about the xy plane. Stated another way, carbon fiber100exhibits varying levels of alignment about the xy plane along the z direction. In a carbon fiber, the x direction may be the length of the carbon fiber. For example, if a carbon fiber is cylindrical in geometry, the x direction may be thought of as the axial direction. As an example of alignment, at point102, the graphene sheets of carbon fiber100are aligned along with the xy plane and stacked in the z direction. However, at point104, the graphene sheets tend to be disposed at a first angle relative to the xy plane along the z direction. At point106, the graphene sheets tend to be disposed at a second angle relative to the xy plane along the z direction, the first angle being less than the second angle. At point108, the graphene sheets tend to be disposed at a third angle relative to the xy plane along the z direction, the second angle being less than the third angle.

The degree of alignment with regard to a plane in a carbon fiber may be referred to as crystal orientation (CO). CO may be expressed as a percentage of graphene sheets in a given fiber that are aligned with the xy plane. At point102, for example, CO is 100% in that all graphene sheets are aligned with the xy plane. At point104, however CO may be only 90%, as a portion of the graphene sheets are aligned at the first angle with the xy plane. At point106, however CO may be 70% and at point108, CO may be 40%.

With reference toFIG. 2, exemplary CO illustration200is shown for two samples of carbon fiber. Sample202illustrates graphene sheets206. Sample202has a CO of 80%. Sample204illustrates graphene sheets208. Sample204has a CO of 55%. In that regard, graphene sheets206of sample202have a higher degree of alignment with respect to a plane (here, the horizontal plane) than graphene sheets208of sample204.

The present inventors have discovered that carbon fibers having a high level of CO exhibit enhanced RTO performance. Stated another way, carbon fibers having a high level CO, once densified into a C/C brake disk, exhibit larger coefficients of friction under an RTO condition than carbon fibers having a lower level CO. As used herein, a high level of CO may be from 80% to 100%, 82% to 100%, from 87% to 100%, from 87% to 98%, from 87% to 95%, from 88% to 95%, and from 90% to 99%. Moreover, the present inventors have discovered that performing one or more CO improvement processes (as described herein) during or after the manufacture of PAN fibers or OPF fibers may improve the level of CO in the resultant carbon fibers, which would exhibit enhanced RTO performance relative to the carbon fibers produced without the one or more CO improvement processes. Stated another way, a conventional PAN fiber or OPF fiber manufacturing process may be improved by increasing the level of CO in the resultant carbon fibers. For example, a conventional PAN fiber or OPF fiber manufacturing process may lead to carbon fibers having a CO of 58%. The PAN fiber or OPF fiber manufacturing process may be improved by adding one or more CO improvement processes. The resulting carbon fibers may have a CO of 65% or higher.

CO may be measured using wide angle X ray diffraction (wide angle XRD or WAXD). Wide angle XRD may be used to examine the crystalline structure of polymers. Generally, wide angle XRD may comprise projecting X rays at a target and observing the diffraction patterns that occur as a result. For example, d spacing may be discerned using wide angle XRD. D spacing may be defined as the spacing between planes of a crystalline structure. For example, with momentary reference toFIG. 1, d spacing may be thought of as the spacing in the z direction of the graphene sheets.

For example, with reference toFIG. 4, rig400in accordance with various embodiments is shown for use with wide angle XRD. Support card402is provided as a support for carbon fiber404. Support card402may comprise any suitable stiff structure that is compatible with wide angle XRD. For example, support card402may comprise heavy duty paper, cardboard, construction paper, or other stiff paper product. Support card402may be take any suitable geometry, for example, support card402may be circular, square, rectangular, or the like. Support card402as shown inFIG. 4is rectangular.

Carbon fiber404may be secured to support card402. As shown, carbon fiber404is disposed across the length (L) of support card402. In various embodiments, L is between 10 mm and 40 mm. Carbon fiber404is anchored, for example, at point406and is pulled taught so that carbon fiber404is at right angle408to support card402. Carbon fiber404may be attached to support card402in any suitable manner. For example, in various embodiments, carbon fiber404may be glued, stapled, tied, taped, or otherwise fastened to support card402.

Support card402may be placed proximate an X ray source. X rays may be projected at support card402from the X ray source. A sensor, film, or other recording media may capture the X ray diffraction patterns for analysis. The CO of carbon fiber404may be determined from the X ray diffraction patterns. For example, the X ray diffraction patterns obtained during testing of carbon fiber404may be compared to other known reference values. For example, the WAXD patterns generated by graphite may be used to compare to the X ray diffraction patterns obtained during testing of carbon fiber404. In that regard, the differences between the reference graphite and carbon fiber404may be used to calculate CO.

For example, two-dimensional wide angle X-ray diffraction (2D WAXD) experiments were conducted on a RIGAKUA 18 kW rotating anode X-ray generator using Cu Ka radiation in the wavelength of 0.1542 nm. The X-ray beam shoot is perpendicular to fiber length direction. A R-AXIS-IV image plate system was used as a detector. The exposure time to obtain suitable patterns was 20 minutes to 40 minutes for each test. The peak positions were calibrated using silicon crystals. A two dimensional pattern was integrated into a one dimensional powder pattern. The angle scan range is about 2 degrees to 60 degrees.

To determine CO, the Hermans uniaxial orientation function may be used. The Hermans uniaxial orientation function specifies an average orientation of the vector rjwith respect to a reference axis z in terms of the angle ϕjz. The function may be represented as:
f=(3<cos2ϕjz>−1)/2

In that regard, f=1 corresponds to perfect orientation in the ϕ=0 direction and f=0 corresponds to random orientation. If the orientation function is calculated for an orthogonal axis, fa+fb+fc=0.

For a given crystalline plane (hkl), the evaluation of <cos2φhkl,z> with respect to the reference axis z can be carried out by the following equations:

Conventional analysis may yield high testing variations. However, by using the process described herein, the variations across samples decreases. Thus, one may now more reliably detect fiber differences.

Samples 1, 2, 3 and 4 in TABLE 1 represent four different C/C brake disks. Each sample had its CO assessed via wide angle XRD as described herein. Each sample was then formed into a preform and densified via CVI, as described herein. The CVI technique performed on each sample was identical or nearly identical. Each sample was then subjected to an RTO condition simulation on a dynamometer. Each sample was subjected to the same RTO condition test in that testing parameters such as applied torque load, etc., were identical and/or substantial identical. TABLE 1 contains the CO value and coefficient of friction for each sample, normalized to account for textile differences. As shown, the coefficient of friction under an RTO condition is shown to be related to CO value.

As described herein, increasing CO of a carbon fiber is beneficial to a C/C brake disk. In that regard, several approaches may be used in carbon fiber production to improve CO. In various embodiments, a CO improvement process may comprise any process performed on a carbon fiber and/or carbon fiber precursor prior to carbonization that tends to improve CO. For example, a CO improvement process may comprise methods to increase polymer isotacticity content, methods of adjusting fiber stretching during spinning, fiber washing, drying, oxidation and/or carbonization and reducing polymer molecular weight distribution. In various embodiments, a CO improvement process comprising at least one of melt spinning, air gap spinning, or wet jet spinning may be used in carbon fiber production. In melt spinning, a feed material may be processed through a barrel heater and fed to a gear pump. The gear pump may supply a die that extrudes a carbon fiber precursor fiber. In air gap spinning, a tow of carbon fiber precursor fiber may be spun in a solvent bath. In wet jet spinning, a tow of carbon fiber precursor fiber may be spun in a solvent bath, fed through a spinneret and jet stretched. A carbon fiber precursor fiber may also be formed by catalyst supported vapor growth. Catalyst supported vapor growth involves catalyst-mediated carbon fiber precursor fiber growth. In various embodiments, a CO improvement process may comprise performing carbonization (such as carbonization/graphitization308, discussed above) at 1,600° C. to about 2,500° C. (2,912° F. to about 4,532° F.) (wherein the term about in this context only means+/−100° C.).

In various embodiments, a first CO improvement process and a second CO improvement process may be performed on a carbon fiber precursor. In that regard, one or more CO improvement processes may be used during manufacture of a carbon fiber precursor. In this manner, a carbon fiber precursor may experience an improved CO. By performing first CO improvement process and a second CO improvement process, the CO of a carbon fiber produced from the carbon fiber precursor may improve. In that regard, a carbon fiber precursor may be made in a given manufacturing process (e.g., a manufacturing process that is free of a CO improvement process) to have, for example, a 55% CO after carbonization. However, one or more CO improvement processes may be added to the given manufacturing process which may improve CO by 5% or more.

Benefits, other advantages, and solutions to problems have been described herein with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any elements that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as critical, required, or essential features or elements of the invention. The scope of the invention is accordingly to be limited by nothing other than the appended claims, in which reference to an element in the singular is not intended to mean “one and only one” unless explicitly so stated, but rather “one or more.” Moreover, where a phrase similar to “at least one of A, B, or C” is used in the claims, it is intended that the phrase be interpreted to mean that A alone may be present in an embodiment, B alone may be present in an embodiment, C alone may be present in an embodiment, or that any combination of the elements A, B and C may be present in a single embodiment; for example, A and B, A and C, B and C, or A and B and C. Furthermore, no element, component, or method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the claims. No claim element herein is to be construed under the provisions of 35 U.S.C. 112(f) unless the element is expressly recited using the phrase “means for.” As used herein, the terms “comprises”, “comprising”, or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus.