FIELD OF THE INVENTION

[0001] This invention relates to the carbonization of pitch-infiltrated fibrous preforms. Such pitch infiltrated fibrous preforms are manufactured into carbon-carbon composites having utility in such demanding applications as aircraft brake discs.

BACKGROUND OF THE INVENTION

[0002] Carbonization of pitch-infiltrated fibrous preforms during the manufacture of carbon-carbon composites poses many difficulties. Among the difficulties are the phenomena that when the fibrous pitch- infiltrated preform is held in a constraint fixture during a carbonization process, (1 ) the pitch may bond the preform to the fixture, (2) unequal coefficients of thermal expansion between the preform and an adjacent wall of the constraint fixture may break or stress the preform, and (3) the pitch may foam during carbonization, causing backpressure from the constraint fixture which may break or stress the preform. US

2005/00931 88 A1 discloses a method for making a carbon-carbon composite preform using e.g. a steel constraint fixture. The method disclosed therein is operative, but in some cases foaming of pitch causes untidiness in the carbonization furnace. Also, it is sometimes difficult to remove the preform from the constraint fixture due to unwanted bonding.

SUMMARY OF THE INVENTION

[0003] The present invention provides embodiments that overcome problems such as those discussed above. In one aspect of the present invention, a friable inert material such as activated carbon is placed between the preform and the constraint fixture. The activated carbon or other inert friable material adsorbs the pitch molecules that escape the

preform during carbonization, which reduces problems with foaming. The layer of activated carbon or other friable material between the fibrous preform and the constraint fixture also provides an easily crushable region that facilitates removal of the preform from the constraint fixture after carbonization.

[0004] In one embodiment, this invention provides a method of carbonizing a pitch-infiltrated fibrous annular preform by: infiltrating the fibrous annular preform with pitch; placing the pitch-infiltrated annular fibrous preform in a constraint fixture having an ejector base plate, an inner wall, an outer wall, and a top press plate, the relative sizes of the preform and the constraint fixture being selected so that a layer of inert friable material may be situated between the preform and walls of the constraint fixture; placing inert friable material (e.g., activated carbon, typically having a bulk density of from 0.09 g/cc to 0.25 g/cc) between the preform and the top, bottom, and walls of the constraint fixture; and subjecting the pitch-infiltrated fibrous preform to carbonization in the constraint fixture. In the practice of this embodiment of the invention, the relative sizes of the preform and the constraint fixture may be selected to leave a gap of any convenient size (e.g., 1 /8 inch to 1 inch) between the preform and the walls of the constraint fixture.

[0005] To practice this method of the invention, the step of placing the inert friable material between the preform the top, bottom, and walls of the constraint fixture may be accomplished, for instance, by dusting a layer of the material up to %inch thick onto the ejector base plate, placing the annular preform on top of the layer of material on the base plate, placing the inner wall inside of the annular preform, placing the outer wall around the outside of the preform, pouring the inert friable material into the gaps between the inner and outer walls of the constraint fixture and the preform, dusting a layer of the material up to %inch thick onto the top of the preform, and placing the top press plate on top of the layer of material on top of the preform.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] The present invention will be more fully understood by reference to the detailed description given hereinbelow and to 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 the scope of the present invention. [0007] Figure 1 is an exploded perspective view of a mold incorporating a constraint fixture in accordance with the present invention.

[0008] Figure 2 is a cut-away perspective view of a mold bottom and constraint fixture.

[0009] Figure 3A is a sectional side view of a preform in a constraint fixture surrounded by friable material in accordance with this invention. [0010] Figure 3B is a sectional top view of a preform in a constraint fixture surrounded by friable material in accordance with this invention. [0011] Figure 4 is a perspective view of a mold bottom and constraint fixture used in conventional processing. [0012] Figure 5 is a perspective view of a mold bottom and constraint fixture used in processing in accordance with the present invention.

DETAILED DESCRIPTION QF THE INVENTION [0013] One type of constraint fixture which may be used in the practice of this invention is illustrated in Figure 1 . In the practice of the present invention, the inner surface of wall (1 2) will be e.g. from 0.1 25 to 1 inch in diameter larger than the outside diameter of an annular pitch- infiltrated preform to be carbonized in the fixture, and the outer surface of wall (10) will be e.g. from 0.1 25 inch to 1 inch in diameter smaller than the inside diameter of an annular pitch-infiltrated preform to be carbonized in the fixture. Activated carbon or another suitable inert friable material

would be loaded into the gaps between walls (10), (12), and the inside and outside diameters, respectively, of the annular preform situated in the constraint fixture.

[0014] However, this invention is not limited to this type of constraint fixture. Any constraint fixture having appropriate gaps between the walls of the constraint fixture and the edges of the preform may be used. For instance, another type of constraint fixture that can be used to practice the present invention is a flexible constraint fixture made up of a lower plate, a' top plate, a ring for the outside diameter of an annular preform being treated within the constraint fixture, and a ring for the inside diameter of the annular preform. In this flexible constraint fixture, the outside diameter and inside diameter rings are made of thin flexible sheet metal strips or thin flexible carbon-carbon composite strip material. The thin flexible strips are joined together by deformable joints, so that the flexible constraint system retains pitch within the fibrous matrix of the preform. The outside diameter strips are joined together, e.g., by expandable joints and the inside diameter strips are joined together, e.g., by collapsible joints. [0015] Referring to the drawings, Figure 1 shows an exploded perspective view of a mold constraint fixture that can be used in the practice of this invention. The constraint fixture as depicted in Figure 1 comprises a bottom plate 2, an ejector base plate 3, a mold outer ring 4, a mold inner ring 10, and a top press plate 1 1 . Figure 1 also identifies locking cams 5, cam retainer plates 6, retainer bolts 7, and cam locking bolts 9. The manner in which two cam retainer bolts 7 and one cam locking bolt 9 connect each cam retainer plate 6 to a corresponding locking cam 5 can be seen from their relative positions in Figure 1 . In this example, the locking cams are turned clockwise to lock the constraint fixture during use, and counterclockwise to unlock. After a preform being manufactured has been subjected to the pressing step, the locking cams 5 are swung down on the top of press plate 1 1 and are locked in position

by locking bolts (9). Alternatively, a ratcheting device (not shown) can be used to automatically lock the press plate into position once the plate is pressed down to achieve the desired compaction or pressure. Finally, Figure 1 shows outer wall 12 and inner wall 13. [0016] Figure 2 is a perspective view of a mold constraint fixture, identifying outer ring 4, outer wall 12, and top press plate 1 1 . Also visible in Figure 2 is a preform 15 inside the mold. [0017] US 2005/0093188 A1 , entitled BINDERLESS PREFORM

MANUFACTURE, provides more details concerning the configuration and use of such mold constraint fixtures.

[0018] Figure 3A is a sectional side view of a preform in a constraint fixture surrounded by friable material in accordance with this invention. In Figure 3A, ejector base plate 3 is covered by a layer of friable material 21 , preform 15 is located onto of layer 21 , and another layer of friable material 24 is located on top of preform 15. Top press plate 1 1 is situated on top of friable material layer 24.

[0019] Figure 3B is a sectional top view of a preform in a constraint fixture surrounded by friable material in accordance with this invention. In Figure 3B, layers of friable material 22 and 23 are located respectively between preform 15 and inner wall 13 and outer wall 12.

[0020] Figures 4 and 5 may be compared to illustrate some of the benefits of the present invention. Figure 4 is a perspective view of a mold bottom and constraint fixture used in conventional processing. In Figure 4, carbonized pitch can be seen on the inside diameter of the constraint fixture. Patches (layers) of carbonized pitch can be seen on top of the constraint fixture. Overflow carbonized pitch can also be seen on the outside of the constraint fixture. This carbonized pitch may be as much as % inch thick. Figure 5 is a perspective view of a mold bottom and constraint fixture used in processing in accordance with the present invention. Figure 5 illustrates that significantly fewer and smaller patches of pitch will be formed inside of, on top of, and outside of the constraint

fixture when pitch-infiltrated fibrous annular preforms are carbonized in accordance with the present invention.

[0021] For the sake of comparison, comparable pitch-infiltrated fibrous annular preforms may be carbonized by similar processes, with one iteration employing activated carbon in accordance with the present invention and another iteration not employing activated carbon between the preform and the top, bottom, and walls of the constraint fixture. Significantly more (e.g., approximately 50% more) residual carbonized pitch will be found on the outside of the constraint fixture that does not employ activated carbon (illustrated in Figure 4) than will be found on the outside of the constraint fixture which uses activated carbon in accordance with the present invention (illustrated in Figure 5). A preform carbonized by the process iteration using activated carbon releases easily from the mold. It does not fall out loosely, but the fixture comes free of the preform with little effort. Less carbonized pitch has to be removed from the constraint fixture in which activated carbon is used before the fixture is re-used. Moreover, removal of carbonized pitch from that constraint fixture is easier than removal of carbonized pitch from the constraint fixture which did not employ activated carbon. The activated carbon reduces the ability of the pitch to bond to the fixture.

[0022] This invention has been described hereinabove in terms of preferred or typical embodiments. However, modifications and additions to this invention will be readily apparent to those skilled in the relevant arts upon 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.