Disclosed are methods for preparing a carbon/carbon composite having protective layers suitable for precluding oxidative weight loss and self-healing carbon/carbon composites prepared according to the disclosed methods.

BACKGROUND

Carbon/Carbon (C/C) composites are lightweight, highly thermal conductive, with low coefficient of thermal expansion materials that find use in niche applications, such as rocket nozzles, heat shield of space vehicles, and disk brakes of airplanes. Unfortunately, when operating at temperatures greater than 500° C. and under oxidizing conditions, such as in air, C/C composites experience significant mass loss due to oxidation. Ceramic coatings may be used to inhibit oxidation; however, such coatings must be selected for their ability to adhere to the C/C composite and must have a coefficient of thermal expansion similar to the C/C composite. To overcome the problems of currently available methods and the products produced by the same, the present disclosure provides novel methods for applying a protective coating to C/C composites and C/C composites with novel, self-healing, protective coatings.

SUMMARY OF THE INVENTION

In one embodiment, the methods disclosed herein provide a C/C composite with a self-healing surface having at least three combined or intermingled layers. These three layers include:

In another embodiment, the methods disclosed herein provide a C/C composite with a self-healing surface having at least three combined or intermingled layers. These four layers include:

In one embodiment the current disclosure provides a method for providing an improved C/C composite. The method may be summarized as follows:

In another embodiment the current disclosure provides a method for providing an improved C/C composite. The method may be summarized as follows:

DETAILED DESCRIPTION

The drawings included with this application illustrate certain aspects of the embodiments described herein. However, the drawings should not be viewed as exclusive embodiments. The subject matter disclosed is capable of considerable modifications, alterations, combinations, and equivalents in form and function, as will occur to those skilled in the art with the benefit of this disclosure.

The present disclosure may be understood more readily by reference to these detailed descriptions. For simplicity and clarity of illustration, where appropriate, reference numerals may be repeated among the different figures to indicate corresponding or analogous elements. The following description is not to be considered as limiting the scope of the embodiments described herein. The drawings are not necessarily to scale and the proportions of certain parts may have been exaggerated to better illustrate details and features of the present disclosure. Also, the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting except where indicated as such.

Throughout this disclosure, the terms “about”, “approximate”, and variations thereof, are used to indicate that a value includes the inherent variation or error for the device, system, or measuring method being employed as recognized by those skilled in the art.

As used herein the term carbon/carbon composite (C/C composite) refers to a composite material prepared from materials such as polyacrylonitrile (PAN) carbon fibers, pitch-based carbon fibers or carbonized fibers, combined with a carbon matrix. The matrix material may be any conventional material commonly used in C/C composites. Matrices commonly include materials such as but not limited to: pitches, polymeric resin, chemical vapor infiltration of the composite material by a hydrocarbon gas, coke, sintered carbon, graphite and other high molecular carbon materials suitable for binding the other carbon materials. The following disclosure provides a method of applying a multilayer coating to C/C composites. As such, the following disclosure does not relate to the formation of the initial C/C composites; rather, the preparation of such materials is well known to those skilled in the art. The resulting multilayer coating protects the C/C (Carbon/Carbon) composite from mass loss during operation in air at temperatures in excess of 500° C.

The disclosed multilayer application method provides a C/C composite carrying multiple layers of silicon carbide (SiC). As depicted by the process flow diagram of FIG. 1, the present method utilizes a six-step method which includes separate chemical vapor deposition (CVD) steps. Typically, the method utilizes three separate CVD steps.

FIGS. 2 and 5 provide representations of the final C/C composite with the resulting protective layers. As reflected in the FIGS., the first CVD-SiC layer is intermingled with the pack cementation layer of SiC. The second CVD-SiC layer is intermingled with the first ZrB2—SiC layer and the third CVD-SiC layer is intermingled with the second ZrB2—SiC layer. The resulting carbon/carbon composite has self-healing properties as the layers of ZrB2—SiC will form oxides when exposed to high temperatures under oxidizing conditions. The resulting oxides, primarily in the form of ZrO2 and SiO2 will expand and fill any surface cracks in the carbon/carbon composite resulting from heating stresses.

The initial Step involves the pack cementation (PC) of the C/C composite in a closed container containing a mixture of powders. The powders used are graphite, silicon, silicon carbide and aluminum oxide. The percent by weight for each component is:

A SiC layer is deposited by heating the C/C composite and powders within the closed container to a target temperature between about 1600° C. and about 2000° C. at a rate of about 5° C./minute to about 15° C./minute, under an inert atmosphere. Suitable inert atmospheres include argon and other gases which are non-reactive with the components within the closed container. During this Step, the components in the reactor are held at the target temperature for a period of about 60 minutes to about 180 minutes. Typically, during initial heating, the components are first allowed to dwell at around 1450° C. for a hold time of about 30 minutes. The desired hold time should be sufficient to permit substantially complete melting of the silicon powders. During this Step, SiO(g) evolves from the powders and reacts with the surface carbon of the C/C composite forming a SiC buffer layer on the surface of the C/C composite. The SiC buffer layer on the surface of the C/C composite is also referred to as a pack cementation SiC layer or PC-SiC layer.

In Step 2, a CVD process is used to fill any pores and cracks in the initial pack cementation SiC buffer layer with SiC. The CVD process utilizes a SiC precursor such as liquid HMDS (hexamethyldisilane). The layers of SiC resulting from CVD are referred to as CVD-SiC (chemical vapor deposition-silicon carbide layer). Additional SiC precursors include HMDS, methyltrichlorosilane (MTS) and silane gas which may be deposited at the appropriate deposition temperatures for formation of SiC as known to those skilled in the art.

To initiate the CVD process of Step 2, the C/C composite with the PC layer of SiC is placed in a reactor suitable for carrying out the CVD Steps. Prior to initiating the CVD process, the reactor must be pre-heated to a target temperature of about 750° C. to about 850° C. Typically, the pre-heat step will occur at a rate of about 5° C./min to about 25° C./min with the passage of a first carrier gas through the reactor.

The typical first carrier gas for this process is hydrogen or a mixture of Ar/H2. The preferred hydrogen will have a purity of about 99.9%. The hydrogen or hydrogen portion of the carrier gas aids in the formation of SiC. Generally, the reactor is pre-heated to about 800° C. at a rate of 20° C./minute. Upon reaching the target temperature, a second carrier gas along with the HMDS precursor is injected or passed into the reactor. For example, the injection may be achieved using a bubbler operating at pressure between about 30 kPa and about 80 kPa with a second hydrogen carrier gas flowing through the bubbler at a rate between about 5 sccm and about 30 sccm. Typically, the HMDS addition with the second carrier gas takes place at a bubbler pressure of about 60 kPa and a hydrogen flow rate through the bubbler of about 20 sccm. In most operations, the first carrier gas flows to the reactor at a rate between about 50 sccm and about 150 sccm. More typically, the flow rate of the first carrier gas will be about 100 sccm. During the CVD process, the pressure within the reactor will typically be between 10 Pa and about 2000 Pa. Typically, the pressure within the reactor will be less than 1000 Pa.

Following initiation of HMDS addition, the reactor is heated to the desired deposition temperature of about 900° C. to about 1000° C. by increasing the temperature of the reactor at a rate between about 0.5° C. and 2° C. per minute. Typically, the target temperature is about 950° C. and the rate of increase in temperature occurs at a rate of about 1° C./minute to provide for deposition of CVD-SiC in any pores and cracks in the initial PC-SiC buffer layer. Thus, the flow of the SiC precursor continues until the resulting CVD-SiC provides a layer which overlays the initial PC-SiC layer and intersects or intermingles with the initial PC-SiC layer. The total thickness of the combined PC-SiC layer and the CVD-SiC layer may range from about 20 microns to about 25 microns.

Following generation of the pack cementation SiC buffer layer and sealing of the PC layer with CVD-SiC on the C/C composite, Step 3 of FIG. 1 provides for the deposition of an initial layer of ZrB2—SiC over the prior layers on the C/C composite. The deposition of the ZrB2—SiC layer begins with the preparation of a slurry of ZrB2 powders with a SiC precursor. One suitable precursor is a polycarbosilane (such as SMP-10), which forms SiC upon pyrolysis. One source of polycarbosilane is SMP-10 from Starfire Systems, Inc. of Glenville, NY. SMP-10 is a liquid at room temperature which produces a silicon to carbon atomic ratio of 1:1. Another source is poly(dimethylsilane) from Gelest, Morrisville, PA as well as derivatives of polycarbosilane (PCS) such as liquid polycarbosilane (LPCS) or mixtures containing polycarbosilane with xylene or divinyl benzene. According to Starfire Systems, Inc., SMP-10 converts to SiC under pyrolysis with a ceramic yield of 72 to 78 percent by weight. To complete the slurry, tetrahydrofuran (THF) is added to the mixture of SMP-10 and ZrB2 as a solvent. The weight percentage of each component in the resulting slurry is:

In Step 3, the resulting slurry is applied by any convenient method to the C/C composite already coated with the PC SiC layer and the CVD-SiC layer provided by Steps 1 and 2. For example, the slurry may be applied by spin coating, brushing, controlled dip coating, or even directly dipping of the C/C composite into the slurry. The C/C composite with the applied slurry is allowed to “rest” at room temperature for a period of time sufficient enough to allow the THF solvent to dry or evaporate. Typically, this time period is about 15 minutes to about 60 minutes.

Following drying, the C/C composite with the slurry coating is placed into a suitable reactor, e.g. a quartz tube and heated. Heating in an inert atmosphere, such as an argon (Ar) atmosphere, takes the C/C composite with the slurry coating from room temperature to about 600° at a rate of about 1° C./min. During this heating step, the C/C composite is held at each of the following temperatures for about 1 hour each: about 100° C., about 300° C., and about 600° C. Following these heating steps, the C/C composite is then heated to about 1000° C. at a rate of about 2° C./min, with a hold time at the final temperature of about 2 hours. The resulting layer (Step 3 in FIG. 1) of ZrB2—SiC provides self-healing ability to the coating through the formation of oxides (mainly ZrO2 and SiO2) when exposed to air at high temperatures. Cracks formed at these high temperatures will be self-sealed as these oxides will expand and fill the resulting empty spaces formed by surface cracking.

Following the application of the ZrB2—SiC layer, Step 4 provides for the addition of another CVD-SiC layer to the C/C composite. Step 4 repeats the same method as those described above in Step 2. The resulting layer of CVD-SiC fills any pores or cracks present in the as deposited slurry layer of ZrB2—SiC. The total thickness of the combined first ZrB2—SiC layer and second CVD-SiC layer may range between about 20 microns to about 25 microns.

Step 5 provides a second layer of ZrB2—SiC on the C/C composite. Step 5 repeats the same steps as described above in Step 3. Finally, Step 6 provides an external or top layer of CVD-SiC. The method for applying the top layer of CVD-SiC corresponds to the process described in Step 2 except with regard to the target temperature. In Step 6, the typical target temperature is about 1000° C. The hold time at the target temperature should be sufficient to provide deposition of the desired external layer of CVD-SiC. Typically, the hold time will be between about 30 minutes to about 120 minutes. More typically, the hold time at the target temperature is about 60 minutes. The resulting combined layer of ZrB2—SiC layer and CVD-SiC will have a thickness between about 25 microns and about 30 microns with an additional overlay of CVD-SiC having a thickness of about 15 microns to about 20 microns.

Sample C/C composites were prepared according to the foregoing methods. In Step 1, the pack cementation utilized a mixture of powders containing graphite (C, 10 wt %), silicon (Si, 55 wt %), silicon carbide (SiC, 30 wt %), and aluminum oxide (Al2O3, 5 wt %) prepared by grinding for 2 hours using ball milling. The C/C samples to be coated were embedded in the pack powders using a graphite crucible closed with a graphite lid. The crucible assembly was then heat treated under atmospheric argon by first heating at 10° C./min to 1450° C., with a dwell time of 30 minutes to allow the silicon to melt, followed by a ramp to 1900° C. at 10° C./min with a hold time of 2 hours to form the PC-SiC layer. During this process, SiO(g) evolved from the powders and reacted with the carbon from the surface of the C/C composite, forming the SiC buffer layer.

Steps 2 and 4 were carried out by preparing a slurry by mixing ZrB2 powders with SMP-10 (a SiC polymeric precursor) acting as a binder, and THE as a solvent. Upon pyrolysis (under atmospheric argon), SMP-10 converts into SiC with a ceramic yield of 72-78 wt % according to the manufacturer. The proportions for each component of the slurry were optimized to obtain the best coating integrity. During experimentation, observation of the samples indicated that an excessive amount of polycarbosilane, the pre-ceramic precursor (SMP-10), can cause mud-cracking due to the shrinkage of the SiC precursor upon pyrolysis. On the other hand, not enough SMP-10 causes the coating to be powder-like due to the lack of binder. The amount of THF solvent is also important, where too much solvent causes the slurry to be unstable and the powder decants, while not enough solvent will produce a slurry with clumps of ZrB2 powders. The best proportion of ZrB2:SMP-10:THF was found to be about 71:16:13 wt %. This ratio will be adjusted in response to additives to the slurry such as but not limited to other phenolic resins, graphite powders and other oxidation resistant ceramics like Al2O3, ZrO2, and Y2O3. The C/C composite specimens are then dipped in the slurry, and placed in a quartz crucible, and heated in a quartz tube furnace using the program described in Table 1.

Rate
Dwell

Steps 2 and 4, CVD steps, were carried out using liquid HMDS as the CVD precursor. Samples were heated in a quartz crucible, placed inside a quartz tube furnace. High purity hydrogen was used as the carrier gas at 100 sccm under an absolute pressure of <10 mbar. The system was first heated to 800° C. at 20° C./min with flow of carrier gas only (no precursor). After reaching 800° C., deposition was initiated by opening the precursor line. HMDS liquid precursor was injected using a bubbler kept at 600 mbar, and through which H2 was flowed at a rate of 20 sccm. The system was then heated to the final deposition temperature (either 950 or 1000° C., depending on the coating step) at a rate of 1° C./min. The slow heating to the final deposition temperature allows SiC to be first deposited in the pores and cracks of the previous PC-SiC (step 2 in FIG. 1) and/or slurry ZrB2—SiC (steps 4 and 6 in FIG. 1) layers, increasing the overall coating density. The CVD temperature in Steps 2 and 4 was 950° C. with no hold time. In Step 6, the CVD temperature was increased to 1000° C. with a hold time of 1 hour. The longer hold time used in the last CVD step allows the growth of an external CVD-SiC layer. FIGS. 4 and 5 provide images of the C/C composite produced by the foregoing example. FIG. 5 shows the cross-section of the final C/C composite taken by slicing the C/C composite at line 5-5 of FIG. 4.

FIG. 6 compares the weight loss of the coated sample prepared in the above example to samples prepared by methods lacking the key steps of the present invention. In FIG. 6, Sample I reflects testing on a material prepared with a pack cementation step using SiC followed by slurry coating with ZrB2—SiC but lacking the CVD steps. Sample II reflects testing on a material prepared with a pack cementation step using SiC followed by CVD, a single slurry coating of ZrB2—SiC and a final CVD step. Sample III reflects material prepared according to the process described above, i.e. the product of the inventive method. As reflected in FIG. 6, C/C composites prepared according to the disclosed method experience minimal weight loss even after 150 hours of exposure to air at 850° C. In this test, Sample III experienced no more than 1% weight loss due to oxidation. In contrast, Sample I experience greater than 25% weight loss due to oxidation in about 38 hours and Sample II experienced a weight loss of less than about 15% in about 65 hours.

The ability of ZrB2 to form oxides (mainly ZrO2 and SiO2) improves the oxidative protection of the coating at temperatures below 800° C. To achieve the self-healing nature of the final C/C composites with the described layers, samples were first heat treated in air at 850° C. for 10 hours to “condition” the coating, which allows oxides (mainly ZrO2 and SiO2) to form and fill any eventual cracks that were formed. FIG. 7 demonstrates oxidation, at different temperatures, of C/C composite samples with the novel multilayer ZrB2—SiC coating applied as disclosed above. The multilayer ZrB2—SiC coating developed in this work also proved to be effective in protecting unconditioned C/C composites from oxidation not only at 850° C. (line B) but also at a lower temperature of 800° C. (line A) in air, as shown in FIG. 7, with no significant weight change after 150 hours. However, as reflected by the graph, when oxidizing unconditioned samples at lower temperatures, cracks in the coating allows air to react with the substrate underneath, and thus a weight loss is observed (line C for sample 750° C. unconditioned in FIG. 7). In contrast, FIG. 7, line D demonstrates the benefits provided by conditioning the sample. For line D, a conditioned sample was placed in a muffle furnace at 750° C. for oxidation testing. The conditioned sample was able to withstand exposure to air at 750° C. with no weight loss even after 150 hours.

FIG. 8 provides the X-ray diffraction (XRD) results for a C/C composite prepared according to the disclosed method, i.e. the samples carry multiple layers of ZrB2—SiC. In FIG. 8, the lower portion of the graph, identified as A, reflects the XRD of the C/C composite prior to oxidation at 850° C. for 150 hours. Before oxidation, only the β-SiC (from CVD) and ZrB2 phases are present. Since XRD analysis occurs only at the surface of the coating, the α-SiC phase from the first layer (PC-SiC) does not appear. The upper portion of the graph (identified as B) reflect the XRD of the C/C composite after oxidation at 850° C. for 150 hours. Following the oxidation step, the oxide phases for the self-healing layers of ZrB2 appear as ZrO2 and SiO2. The resulting oxidation of these layers prohibits loss of carbon from the C/C composite thereby preserving the integrity of the composite. Thus, the resulting composites are considered to be self-healing. This self-healing characteristic provides a composite with better structural integrity under operational conditions in an oxidizing environment.

For comparison purposes, a sample was prepared using only the PC and slurry steps. The resulting sample carrying an oxidized coating was examined using XRD analysis. As depicted in FIG. 10 the resulting formation of ZrO2 is clear with only traces of ZrB2 detected. Thus, as expected, most of the coating at the surface was oxidized. The results depicted in FIG. 10 further demonstrate that the multilayer ZrB2—SiC described above prevents excessive formation of ZrO2 as shown in FIG. 8. Additionally, the results of FIG. 10 explain the failure of samples coated only with PC and slurry coating as reported in FIG. 6, line I as the developed cracks are not able to self-heal fast enough to prevent oxidation.

While the foregoing method was describe as having the six steps described above, modification of this method will also produce improved C/C composites. The six steps described in detail above may be summarized as follows:

While the above steps will provide an exceptional C/C composite having a self-healing surface structure and improved structural integrity, Steps 4 and 5 can be omitted and still provide a C/C composite having a self-healing surface structure. Thus, in one embodiment, the foregoing method provides a C/C composite with a self-healing surface having at least three combined or intermingled layers. These three layers include:

Other embodiments of the present invention will be apparent to one skilled in the art. As such, the foregoing description merely enables and describes the general uses and methods of the present invention. Accordingly, the following claims define the true scope of the present invention.