High density boron nitride matrix composites
A method of manufacturing a composite material comprises forming a mixture comprising a plurality of fibers and a borazine oligomer; subjecting the mixture to a first heating, for 12 hours to 56 hours; and subjecting the mixture to a second heating. The temperature of the first heating is 60° C. to 80° C., and the pressure during the first heating is at least 0.5 MPa, the temperature of the second heating is at most 400° C., and the greatest pressure of the second heating is at least 15 MPa.
This invention was made with Government support under Contract Numbers DMR-9523735 and DMI-0127834 awarded by the National Science Foundation (NSF). The government has certain rights in the invention.
BACKGROUNDMaterials which do not deteriorate mechanically and continue to function in extreme conditions of temperature, friction and wear, are desirable and necessary in a wide range of modern applications. The materials of aircraft brakes experience some of the harshest service conditions seen in any application. The material experiences high friction and temperatures while the aircraft is taxiing and landing. In addition, all aircraft brake materials are required to withstand the extreme conditions of rejected take-off (RTO) trials.
One class of materials used for aircraft brakes is composites of carbon fibers reinforcing a carbon matrix (C/C composites). These materials are described in J. Economy, H. Jung and T. Gogeva, Carbon, Vol. 30, No. 1, pp 81-85 (1991).
C/C composites are considerably stronger and lighter than steel. Such materials increase in strength with increasing heat treatment and resist thermal shock caused by rapid temperature changes. However, they suffer from a number of drawbacks including poor oxidation resistance, highly variable wear rate and coefficient of friction, and costly manufacturing. The lack of high temperature stability requires an expensive supplementary processing step to coat the non-frictional surfaces with an oxidation barrier. In addition, fabrication is very time consuming (1-2 months) due to the long periods required for chemical vapor infiltration of the carbon matrix. Thus, the fabrication is a slow, expensive process. The carbon matrix is usually introduced among the carbon fibers by liquid impregnation and charring of organic materials. In some applications, chemical vapor deposition is used as a final step in processing. The steps in the processes are repetitive and can take months to complete. These materials are also only moderately oxidation resistant, thus requiring the addition of an expensive oxidation barrier coating to the non-frictional surfaces. Furthermore, they have a highly variable coefficient of friction, especially in the presence of water, which causes variable brake feel to pilots (nicknamed “morning sickness”).
Carbon fibers reinforcing a boron nitride matrix (C/BN composites) have the potential to overcome some of the shortcomings of C/C composites. These materials are prepared by first polymerizing borazine (B3N3H6), to yield an oligomer of appropriate viscosity that is used as a boron nitride precursor for impregnating the carbon fibers. The impregnated fibers are then heated under pressure, yielding a solid boron nitride matrix reinforced by the carbon fibers. The preparation time for such composites requires days, as opposed to the months needed for C/C composites, with concomitant cost savings (U.S. Pat. No. 5,399,377). However, due to low density, the resulting C/BN composites do not have acceptable heat capacity and thermal conductivity values to substitute for C/C composites in aircraft brakes. The preparation typically yields a composite with a density in the range of 1.38 g/cc to 1.43 g/cc, and with multiple impregnations (up to six) the density increases up to 1.61 g/cc. Additional impregnations do not appear to effectively increase the density of the composites.
Consequently, C/BN composites have higher oxidation resistance than C/C composites, but do not have the desired density of about 1.8 g/cc which is expected to be necessary for the materials to have the heat capacity and thermal conductivity needed for good braking.
The performance of C/C composites may also be improved by the addition of a boron nitride matrix, yielding carbon fiber composite materials with a boron nitride layer, or C/C/BN composites. These materials are prepared by immersing lower density C/C composites in borazine oligomers and subjecting the system to the same procedure used for C/BN composites (U.S. Pat. No. 5,399,377). The boron nitride coating renders the product more oxidation resistant than C/C composites. Nevertheless, the density of C/C/BN composites is still below the level required for aircraft brakes applications, and their wear rate is too high once the thin BN coating is worn away (Brian Fabio, M. S. Thesis, The University of Illinois at Urbana-Champaign).
SUMMARYIn a first aspect, the present invention is a method of manufacturing a composite material comprising forming a mixture comprising a plurality of fibers and a borazine oligomer; subjecting the mixture to a first heating, for 12 hours to 56 hours; and subjecting the mixture to a second heating. The temperature of the first heating is 60° C. to 80° C., and the pressure during the first heating is at least 0.5 MPa, the temperature of the second heating is at most 400° C., and the greatest pressure of the second heating is at least 15 MPa.
In a second aspect, the present invention is a composite material comprising carbon fibers in a boron nitride matrix. The composite material has a density of at least 1.62 g/cc.
In a third aspect, the present invention is a composite material comprising carbon fibers in a boron nitride matrix. The composite material has a wear rate of at most 0.4 mg/m at an energy level of 100 kJ/kg to 1100 kJ/kg, and a coefficient of friction of at least 0.22 at an energy level of 100 kJ/kg to 1200 kJ/kg.
In a fourth aspect, the present invention is a method of manufacturing a composite material comprising boron nitride, comprising forming a mixture comprising a preform and a borazine oligomer; subjecting the mixture to a first heating, for 12 hours to 56 hours; and subjecting the mixture to a second heating. The temperature of the first heating is 60° C. to 80° C., and the pressure of the first heating is at least 0.5 MPa, and the temperature of the second heating is at most 400° C., and the greatest pressure of the second heating is at least 15 MPa.
In a fifth aspect, the present invention is a composite material comprising a 3D needled carbon fiber preform impregnated with boron nitride having a density of at least 1.63 g/cc.
In a sixth aspect, the present invention is a composite material, comprising CVI-infiltrated carbon fiber preform impregnated with boron nitride having a density of at least 1.62 g/ cc.
In a seventh aspect, the present invention is a composite material comprising a 3D needled carbon fiber preform impregnated with boron nitride having a wear rate of at most 0.05 mg/m at an energy level of 100 kJ/kg to 1000 kJ/kg, and a coefficient of friction of at least 0.12 at an energy level of 100 kJ/kg to 900 kJ/kg.
BRIEF DESCRIPTION OF FIGURES
The present invention provides a new type of C/BN composites with densities averaging 1.75 g/cc, and methods for their fabrication. These C/BN composites were found to outperform C/C composites in testing, and their density renders them a viable substitute for C/C composites in applications of friction and wear. The invention is based on the discovery that higher density C/BN composites can be formed when carbon fibers impregnated with borazine oligomers are heated under a pressure exceeding 0.5 MPa, and the pressure is applied before the temperature reaches 400° C.
These high density C/BN composites are prepared by first heating the borazine monomer in a pressure vessel, with occasional venting, at a temperature of 60° C. to 80° C., more preferably 65° C. to 75° C., and, most preferably, 68° C. to 72° C. The borazine reacts with itself to yield oligomers while releasing hydrogen as a primary by-product, and the heating is applied until a viscosity of 500 cP to 2500 cP is attained, usually within 24 to 48 hours. The oligomers are used to impregnate carbon fibers, and the resulting system is heated to 60° C. to 80° C., more preferably 65° C. to 75° C., and most preferably, 68° C. to 72° C., under a pressure ranging from 0.5 to 8 MPa, more preferably 1 to 5 MPa, and most preferably 2.2 to 4.4 MPa, for preferably 12 to 48, hours to induce further oligomerization of the borazine without formation of voids.
The composite is then subject to increasing temperature and pressure under an inert atmosphere. The heating promotes further borazine polymerization while the pressure is controlled to achieve a high density product. The temperature is increased to 300° C. to 400° C. at a rate of 0.25° C./min to 3° C./min, more preferably to 325° C. to 375° C. at a rate of 0.75° C./min to 1.25° C./min, yet more preferably to between 340° C. to 360° C. at a rate of 0.9° C./min to 1.1° C./min. The pressure is ramped to a final pressure of 14 MPa to 30 MPa, more preferably from 18 MPa to 26 MPa, and most preferably, 21 MPa to 23 MPa. The composite is then preferably held at the final temperature and pressure for an additional 10 to 30 hours, more preferably 16 to 24 hours.
The resulting composite is preferably heated at 5° C./min to 15° C./min to a final pyrolysis temperature of 1100° C. to 1500° C., for 1 to 3 hours, and then cooled, more preferably heated at 10° C./min to a final pyrolysis temperature of 1150° C. to 1250° C., for about 2 hours. This process is optionally carried out two or three times to maximize the density of the product.
The process described above was also applied to 3D needled carbon fiber preforms. The hydrogen and other by-products appear to easily escape from the preform, yielding a new type of high density 3D C/BN material with as little as a single impregnation. In addition, it was found that optimal wear properties were obtained when the 3D carbon preform was first subjected to carbon CVI to a density of ˜1.3 g/cc and then impregnated with borazine oligomer and subjected to the process above, yielding a new higher density 3D C/C/BN material.
Objects of various shapes may be fabricated by using the materials of the invention in resin transfer molding processes (RTM). In RTM, a fiber preform, with the shape of the desired object, is loaded into a mold without the borazine. Alternatively, the preform may be bent into the desired shape by the walls of the mold. The mold is closed, and borazine or the borazine oligomer as described above is injected or transferred into the mold and impregnates the preform. The resulting system is then subjected to the same temperature and pressure treatment as described above, yielding objects with increased densities and complex shapes in faster processing times.
EXAMPLES1) Thermogravimetric Analysis of the C/BN Composite Production Process
One possible explanation for the higher density of the materials of the invention may be that the pressure prevents the hydrogen evolving from the transformation of the borazine into boron nitride from forming bubbles that would lead to a porous, lower density structure. To investigate this possibility, a series of thermogravimetric analysis (TGA) experiments were done using a 1:1 mixture of carbon fibers and borazine.
2) Preparation of High Density C/BN Composites
Borazine monomer was oligomerized in an inert atmosphere at 70° C. for 36 hours, until an oligomer of viscosity of 500 to 2500 cP was attained. In a dry box, chopped carbon fibers were placed into a two inch diameter mold and a measured amount of the borazine oligomer was added. Graphite foil was then placed on top of the sample (as a releasing agent) followed by a tightly fitting disk of TEFLON® (DuPont, Wilmington, Del.) to prevent potential leakage of borazine. To provide extra mechanical support to the TEFLON® a precisely machined brass disk was placed on top of the TEFLON®. A steel plunger was then placed on top of the brass and the entire assembly transferred into a controlled atmosphere hot press. The mold and the sample were then heated to 70° C. and held isothermally for an additional 48 hours under pressure. Pressure was applied while the temperature was increased to 350° C. at a rate of 1° C./min under dry nitrogen. The composite was then held at 350° C. for 20 hours under constant pressure. Following this final oligomerization, the composite was removed from the mold and separated from the graphite foil. The composite was then placed in a mullite tube furnace which was backfilled with dry nitrogen. The furnace was ramped at 10° C./min to a final pyrolisis temperature of 1200° C., held for two hours, and then cooled. The processing schedule, shown in Table 1, describes the temperatures and the pressures applied. From a single impregnation bulk densities in the range of 1.3 to 1.55 g/cc were achieved using 40% Vf (fiber volume fraction) chopped pitch based carbon fibers. This process was carried out three times to maximize the density to approximately 1.75 g/cc.
During the reimpregnations a similar temperature-pressure process was used with one additional step prior to hot pressing. This involved placing the composite in a steel pressure vessel and heating under vacuum for 30 minutes, backfilling with nitrogen and then introducing enough borazine oligomer to submerge the composite. The system was then held at 70° C. for 12 hours, with occasional venting to reduce the partial pressure of hydrogen. This allowed for better permeation of the composite and caused the borazine oligomer molecular weight to increase while filling in the porous structure. As an example, one composite exhibited an initial density of 1.46 g/cc and one, two and three impregnations yielded increased densities of 1.62, 1.66 and 1.75 g/cc, respectively.
3) X-ray Diffraction Data of C/BN Composites
The X-ray diffraction data depicted in
4) Wear Rate Testing of High Density C/BN Composites
Once the C/BN composites reached a density of approximately 1.75 g/cc, friction and wear testing was performed on an inertial brake dynamometer. This is the most important parameter since it determines the lifetime and safety of the brake material. The resulting wear rate plotted versus energy level for previously tested C/C, C/C/BN, and the new C/BN composites of the invention appear in
5) Comparison with Traditional C/BN Materials.
The wear rate of the C/BN material of the invention is also markedly better than that of a traditional, lower density C/BN material produced according to the process of U.S. Pat. No. 5,399,377. As illustrated in
6) Friction Testing of High Density C/BN Composites
The average coefficient of friction (COF) is plotted versus energy levels in
7) Preparation of High Density 3D C/BN Composites.
The procedure previously described for the C/BN composite of Example 1 was applied on a 3D needled preform instead of the chopped carbon fibers. The preform had a 28% fiber volume and a bulk density of about 0.45 g/cc. The fiber of the preform was polyacrylonitrile-derived, with an average fiber diameter of 9 microns and a density of about 1.78 g/cc. Samples of the preform were provided by the Goodrich Corporation (Brecksville, Ohio) or Albany International Techniweave (Rochester, N.H.). The process was repeated up to three times, yielding final 3D C/BN composites with a density of 1.63 g/cc to 1.72 g/cc.
8) Preparation of High Density 3D C/C/BN Composites.
A sample of 3D needled preform such as that used in Example 7 was carbon vapor infiltrated to increase density to ˜1.3 g/cc, then placed in a steel pressure vessel and heated under roughing vacuum for 12 hours. The pressure vessel was backfilled with dry nitrogen and borazine oligomer was added. The preform and borazine were reacted for an additional 12 hours in the steel pressure vessel. In a dry box, the borazine soaked sample was placed in a mold and an excess of borazine oligomer was added. The rest of the procedure then followed the same steps as previously described for the C/BN composite of Example 1. This process was typically carried out four times, yielding a product with a density of 1.62 g/cc to 1.80 g/cc.
9) X-ray Diffraction Data of High Density 3D C/C/BN Composites.
As seen in
10) Wear Rate Testing of High Density 3D C/C/BN Composites.
As seen in
11) Friction Testing of High Density 3D C/C/BN Composites.
At an interfacial pressure of 0.25 MPa, the 3D C/C/BN displays a COF of approximately 0.2 (
Claims
1. A method of manufacturing a composite material comprising:
- forming a mixture comprising a plurality of fibers and a borazine oligomer;
- subjecting the mixture to a first heating, for 12 hours to 56 hours; and
- subjecting the mixture to a second heating;
- wherein the temperature of the first heating is 60° C. to 80° C., and the pressure during the first heating is at least 0.5 MPa,
- the temperature of the second heating is at most 400° C., and the greatest pressure of the second heating is at least 15 MPa.
2. The method of claim 1, further comprising subjecting the mixture to a third heating, wherein the temperature of the third heating is at least 1200° C.
3. The method of claim 1, wherein the borazine oligomer is obtained by heating borazine for 24 to 48 hours, at a temperature of 60° C. to 80° C.
4. The method of claim 1, wherein the fibers are carbon fibers.
5. The method of claim 1, wherein the pressure during the first heating is 1 MPa to 6 MPa.
6. The method of claim 1, wherein the temperature of the first heating is 65° C. to 75° C., and the pressure during the first heating is 1.5 MPa to 5 MPa.
7. The method of claim 1, wherein the temperature of the first heating is 68° C. to 72° C., and the pressure during the first heating is 2.0 MPa to 4.6 MPa.
8. The method of claim 1, wherein the temperature of the second heating is increased at a rate of 0.25° C./min to 3° C./min.
9. The method of claim 1, wherein the temperature of the second heating is increased at a rate of 0.75° C./min to 1.25° C./min.
10. The method of claim 1, wherein the temperature of the second heating is increased at a rate of 0.9° C./min to 1.1° C./min.
11. The method of claim 1, wherein the greatest temperature reached during the second heating is 130° C. to 170° C., and the greatest pressure is 12 MPa to 32 MPa.
12. The method of claim 1, wherein the greatest temperature reached during the second heating is 140° C. to 160° C., and the greatest pressure is 16 MPa to 26 MPa.
13. The method of claim 1, wherein the greatest temperature reached during the second heating is 148° C. to 152° C., and the greatest pressure is 21 MPa to 23 MPa.
14. The composite material made according to the method of claim 1.
15. The composite material made according to the method of claim 2.
16. The composite material made according to the method of claim 3.
17. A composite material comprising carbon fibers in a boron nitride matrix, wherein the composite material has a density of at least 1.62 g/cc.
18. The composite material of claim 17, wherein the composite material has a density of 1.62 to 1.75 g/cc.
19. A composite material comprising carbon fibers in a boron nitride matrix, wherein the composite material has a wear rate of at most 0.4 mg/m at an energy level of 100 kJ/kg to 1100 kJ/kg, and a coefficient of friction of at least 0.22 at an energy level of 100 kJ/kg to 1200 kJ/kg.
20. A method of manufacturing a composite material comprising boron nitride, comprising:
- forming a mixture comprising a preform and a borazine oligomer;
- subjecting the mixture to a first heating, for 12 hours to 56 hours; and
- subjecting the mixture to a second heating;
- wherein the temperature of the first heating is 60° C. to 80° C., and the pressure of the first heating is at least 0.5 MPa, and
- the temperature of the second heating is at most 400° C., and the greatest pressure of the second heating is at least 15 MPa.
21. The method of claim 20, further comprising subjecting the mixture to a third heating, wherein the temperature of the third heating is at least 1200° C.
22. The method of claim 20, wherein the borazine oligomer is obtained by heating borazine for 24 to 48 hours, at a temperature of 60° C. to 80° C.
23. The method of claim 20, wherein the preform is a 3D needled carbon fiber preform.
24. The method of claim 20, wherein the preform is a CVI-infiltrated 3D needled carbon fiber preform.
25. The composite material made according to the method of claim 20.
26. The composite material made according to the method of claim 21.
27. A composite material comprising a 3D needled carbon fiber preform impregnated with boron nitride having a density of at least 1.63 g/cc.
28. The composite material of claim 27, having a density of 1.63 g/cc to 1.72 g/cc.
29. A composite material, comprising CVI-infiltrated carbon fiber preform impregnated with boron nitride having a density of at least 1.62 g/ cc.
30. The composite material of claim 29, having a density of 1.62 to 1.80 g/cc.
31. A composite material comprising a 3D needled carbon fiber preform impregnated with boron nitride having a wear rate of at most 0.05 mg/m at an energy level of 100 kJ/kg to 1000 kJ/kg, and a coefficient of friction of at least 0.12 at an energy level of 100 kJ/kg to 900 kJ/kg.
32. A brake for aircraft comprising the composite material of claim 17.
33. A brake for aircraft comprising the composite material of claim 27.
34. An aircraft comprising the brake of claim 32.
35. An aircraft comprising the brake of claim 33.
36. A method for decelerating an aircraft comprising braking the aircraft with the brake of claim 32.
37. A method for decelerating an aircraft comprising braking the aircraft with the brake of claim 33.
Type: Application
Filed: Nov 19, 2003
Publication Date: May 19, 2005
Inventors: James Economy (Urbana, IL), Christian Mangun (Urbana, IL), Steven Seghi (Edgewater, MD)
Application Number: 10/717,303