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Rapid Chemical Vapor Infiltration of Silicon Carbide Minicomposites at Atmospheric Pressure Kenneth Petroski, Shannon Poges, Chris Monteleone, Joseph Grady, Ram Bhatt, and Steven L. Suib ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17098 • Publication Date (Web): 18 Jan 2018 Downloaded from http://pubs.acs.org on January 19, 2018

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ACS Applied Materials & Interfaces

Rapid Chemical Vapor Infiltration of Silicon Carbide Minicomposites at Atmospheric Pressure Kenneth Petroski,† Shannon Poges,† Chris Monteleone,‡ Joseph Grady,§* Ram Bhatt,§ Steven L. Suib†‡┴* †Dept. of Chemistry, University of Connecticut, Storrs, CT 06269, USA. ‡Dept. of Materials Science and Engineering, University of Connecticut, Storrs, CT 06269, USA. §NASA Glenn Research Center, Cleveland, OH 44135, USA. ┴Institute of Materials Science, University of Connecticut, Storrs, CT 06269, USA. *To whom correspondence should be addressed.

Keywords: Silicon carbide, chemical vapor deposition, chemical vapor infiltration, atmospheric pressure, minicomposite

ABSTRACT: The chemical vapor infiltration technique is one of the most popular for the fabrication of the matrix portion of a ceramic matrix composite. This work focuses on tailoring an atmospheric pressure deposition of silicon carbide onto carbon fiber tows using the methyltrichlorosilane (CH3SiCl3) and H2 deposition system at atmospheric pressure to create minicomposites faster than low pressure systems. Adjusting the flow rate of H2 bubbled through

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CH3SiCl3 will improve the uniformity of the deposition as well as infiltrate the substrate more completely as the flow rate is decreased. Low pressure depositions conducted at 50 torr deposit SiC at a rate of approximately 200 nm*h-1 while the atmospheric pressure system presented has a deposition rate ranging from 750 nm*h-1 to 3.88 µm*h-1. The minicomposites fabricated in this study had approximate total porosities of 3% and 6% for 10 sccm and 25 sccm infiltrations, respectively. INTRODUCTION Refractory ceramic materials have been a promising candidate for replacing superalloys in a number of extreme environments where high temperatures, mechanical stresses, and chemical attack are of concern.1-5 Silicon carbide is a prime candidate for use in these systems due to a high melting point, chemical inertness, and mechanical strength.6 The main drawback is that silicon carbide and other ceramics lack the toughness of alloys. This is due to the strong network of covalent bonds which is responsible for the refractory properties of these materials. To overcome this, the materials are incorporated into a system called a ceramic matrix composite (CMC). The CMC is a three component system consisting of a matrix, interphase, and fiber. The matrix is the continuous phase in the composite, and is the ceramic material which needs to be toughened. The fibers are the reinforcing material; these are usually continuous fibers that run the length of the composite. If the matrix bonds too strongly to the fiber, the system will still exhibit brittle behavior.7-8 The interphase acts to de-bond the fiber from the matrix which allows for various toughening mechanisms to take place, such as crack deflection and fiber pullout.9 The interphase also stops high temperature sintering of the fiber and matrix, which would lead to the composite exhibiting monolithic behavior.10


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Chemical vapor infiltration (CVI) has many distinct advantages over other matrix forming techniques. CVI is a non-line-of-sight coating process which can infiltrate fiber preforms in complex shapes. The relatively lower processing temperatures used by this technique minimize damage to the fibers.11-12 There are several drawbacks to CVI: long processing times and specialized equipment make this a costly process. The formation of closed porosity is also unavoidable with this technique, and the machining of parts to re-expose the pores for further infiltration adds more time and may damage the components.13 The CVI process is composed of a main flow of reactant gasses, substrate, and boundary layer. The boundary layer between the substrate and reactive gasses is composed of laminar flowing gasses. At atmospheric pressure this boundary layer is thick enough to be the rate limiting factor for the deposition. The process begins with a reactive gas molecule diffusing through the boundary layer, adsorbing to the substrate, and then reacting to form the products and byproducts. The reaction finishes with the byproducts desorbing from the substrate and rediffusing through the boundary layer. As the byproducts are diffusing away from the substrate, they contribute to the thickness of the boundary layer. This increases the amount of time necessary for reactant gasses to diffuse through the layer and is the source of the poor step coverage in atmospheric depositions.14 Atmospheric depositions also have very poor infiltration ability due to the extremely rapid deposition rate: the coating forms quickly on the surface and rapidly seals all of the pores, preventing further infiltration of the substrate.15 Atmospheric pressure CVI is capable of depositing a matrix at least an order of magnitude faster than low pressure methods; these types of reactors are also much simpler to maintain and operate than the low pressure systems.14


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The deposition of silicon carbide is usually carried out using the well-studied system of CH3SiCl3 in a hydrogen atmosphere, shown in scheme 1.16-21

Scheme 1. Thermal decomposition of methyltrichlorosilane into silicon carbide and hydrogen chloride in the presence of hydrogen gas. CH3SiCl3 is a single source precursor that contains a stoichiometric amount of silicon and carbon and thermally decomposes into silicon carbide. H2 is usually used as a process gas to facilitate the formation of SiC, increasing the deposition rate of the reaction.13, 22 Typically, CVI is carried out at low pressure (~50 Torr) to alter the deposition kinetics into a surface reaction limited process instead of a diffusion limited process which occurs at higher pressures (~760 Torr). Lower pressures also give better step coverage, or coating uniformity, throughout the furnace.14 Several experimental23-28 and computational29-35 studies have been conducted in an effort to find the optimal conditions to minimize the porosity and maximize the infiltration rate, but the isothermal isobaric technique is still the most commercially viable.36 Despite low pressure infiltrations having lower deposition rates and requiring more expensive equipment, to this point atmospheric depositions have not had the infiltration ability or the step coverage to produce minicomposites acceptable for the modelling of full CMCs. This has limited the production of minicomposites strictly to low pressure techniques. The goal of our current work is to prove atmospheric pressure CVI can produce a carbon fiber SiC matrix minicomposite with porosity and step coverage comparable to low pressure systems much faster and without the expensive equipment. This is to be accomplished by steadily altering the flow of reactant molecules into the furnace to produce a uniform boundary layer throughout the reactor. This uniform layer will provide even step coverage throughout the deposition zone.


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The uniform layer will also steadily become thicker as the flow decreases, which will decrease the deposition rate and promote enhanced infiltration into the substrate. The substrate will be a tow of carbon fiber which will form a minicomposite after the matrix has been deposited.15 Experimental Section Materials. Methyltrichlorosilane (CH3SiCl3, 98%) was purchased from Gelest, Inc. Ultra high purity nitrogen and ultra high purity hydrogen were purchased from Airgas, Inc. All chemicals were used without further purification. The carbon fiber used in this study was THORNEL T300 purchased from Amoco (CYTEC). The fiber tow has a filament count of 3000, the average filament diameter is 7 µm, and is supplied with 1% UC.309 epoxy-compatable sizing. The fibers were not modified before being placed in the reactor.

Figure 1. (A) Schematic diagram of chemical

vapor infiltration apparatus. (B) SiC coated graphite boat.

Furnace Profiling Reactions. Equipment for the CVI of silicon carbide onto the carbon fiber tows is shown schematically in Figure 1A. A 47 mm inner diameter, 50 mm outer diameter, 53.3 cm long fused silica tube was lined with a graphite foil liner. A single tow of carbon fiber was then run the length of the fused silica tube. The ends of the tube were sealed using Cajon compression fittings. The tube was placed so that 5.7 cm of the tube protruded from the front of


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the furnace. The reactor was then purged with flowing N2 gas for 20 minutes to remove any ambient moisture and O2. The furnace was heated under a N2 atmosphere at a rate of 20°C·min-1 until reaching 1050°C. The flow of N2 was then stopped and a metered flow of hydrogen gas was then passed through a bubbler filled with CH3SiCl3. The hydrogen was allowed to flow through the bubbler for 1 h, and then stopped. Afterwards, the N2 flow was restored and the furnace was allowed to cool to room temperature. Matrix Infiltration Studies. After the deposition zone in the reactor was established, the carbon fibers were cut into 25.4 cm sections, weighed, and centered on a graphite boat coated in silicon carbide. A schematic of the boat is included in Figure 1B. The boat and fibers were then placed 15.2 cm from the end of the fused silica tube. The length of reaction time was varied for each of the different flow rates to examine how extending the infiltration time would alter the final infiltrated fiber tow throughout the deposition zone. Characterizations. Powder X-ray diffraction (PXRD) patterns were collected using a Rigaku Ultima IV diffractometer (Cu Kα radiation, λ= 1.5406 Å) with an operating voltage of 40 kV and a current of 44 mA. Scanning electron microscopy was conducted using an FEI Teneo LVSEM with an accelerating voltage of 5 kV. The atomic composition of the coating was determined with an RBD Instruments PHI 660 scanning Auger electron spectroscopy (AES) system. The Xray computed tomography (XCT) experiments were conducted with a ZEISS Xradia 510 Versa 3D X-ray microscope. Results


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Figure 2: Deposition zone when decomposing CH3SiCl3 in H2 at 1050°C, 25 SCCM H2, 4 hours, on graphite foil. Furnace profiling – deposition zone. The graphite foil is grey before the coating process. After the silicon carbide deposition, different sections of the reactor left behind a distinctly colored coating (Figure 2). The exact depositions left behind varied with the different flow rates of H2 through the CH3SiCl3 that were used. At a flow rate of 200 standard cubic centimeters per minute (sccm) the first 8.9 cm of the graphite foil had no deposit. The next 1.3 cm had a very lustrous silver deposit. The following 10.2 cm were composed of a striped birefringent coating consisting of striped blues, yellows, greens and reds. After the birefringent section was a 12.7 cm length of a solid grey coating. This was followed by another 7.6 cm birefringent coating. The remainder of the tube had no evidence of a coating outside of the last 7.6 cm of the fused silica tube, which was a tan powder. The 150 sccm depositions had no coating for the first 7.6 cm. The next 5.1 cm was a silver deposit, followed by a 25.4 cm solid grey deposit. Next was a 2.5 cm birefringent


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Figure 3. SEM micrographs of carbon fiber tow infiltrated with SiC at flow rates of (A-C) 200 sccm, (D-F) 150 sccm, (G-I) 100 sccm, (J-L) 50 sccm. The images were taken from different distances from the front of the tube (A, D, G, J) 10th cm, (B, H, K) 20th cm, (E) 15th cm, (C, F, I, L) 30th cm. coating. The remainder of the tube had little evidence of a coating except for the last 3.8 cm which had a white powder. Decreasing the flow to 100 sccm produced a small amount of white


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powder in the first 7.6 cm of the reactor. This was followed by 2.5 cm of a silver coating and then 2.5 cm of a birefringent deposit. The next 5.4 cm was a birefringent coating that steadily transitioned into a solid grey deposition which was 15.2 cm long. A 7.6 cm birefringent coating was directly after the grey deposit. The remainder of the tube had a small amount of white powder spread throughout. The 50 sccm deposition had a slightly more prevalent amount of white powder in the first 7.6 cm of the tube. The next 5.4 cm was a lustrous silver deposit followed by a 5.4 cm birefringent deposit. About 12.7 cm of a grey deposit came next, which then had a 7.6 cm birefringent coating directly after that coating. The remainder of the tube showed no evidence of a coating except for the last 3.8 cm which had white powder similar to the front of the tube. Decreasing the flow to 25 sccm lengthened the deposition of white powder in the beginning of the reactor to 8.9 cm. An 8.9 cm birefringent deposit came next, which was then followed by a 7.6 cm grey deposit. Another 8.6 cm birefringent coating followed this grey deposit. The rest of the tube showed no evidence of a coating besides the last 5.4 cm, which was a tan powder. A flow rate of 10 sccm continued to produce an 8.6 cm deposit of white powder in the front of the reactor. The next 5.4 cm was a birefringent coating which led into a 20.3 cm grey deposit. The end of this grey deposit had a dark black band which was 2.5 cm long. The remainder of the tube had a small amount of white powder dispersed throughout. Furnace profiling – Infiltration. The most significant differences between the flow rates were observed in the SEM analysis of the fiber tows. The infiltration at 200 sccm can be seen in Figures 3A, 3B and 3C, which approximately correlate to the 10th, 20th, and 30th cm from the front of the fused silica tube, respectively. Figure 3A exhibits a thick outer coating of roughly 40 µm. The center of the fiber tow is poorly infiltrated, and is porous in the center. Figures 3B and 3C have significantly less coating present on the outermost portion of fibers in the tow, 4.45 µm


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and 2.92 µm, respectively. The 150 sccm deposition can be seen in Figures 3D, 3E, and 3F, which correspond with the 10th, 15th, and 30th cm from the front of the tube, respectively.

Figure 4. SEM micrographs from the (A-G) 10th cm, (h) 20th, (I) 30th, from the front of the tube at different flow rates (A-C) 50 sccm, (D-F) 25 sccm, (G-I) 10 sccm, and different infiltration times (A) 1 h, (B) 2 h, (C) 4 h, (D) 1 h, (E) 4 h, (F) 8 h, (G-I) 60 h.

Figure 3D shows that the lower flow rate has improved the infiltration as the voids are much less prevalent as compared to Figure 3A. Figure 3E and 3F displayed poorer infiltration than Figure 3B and 3C, with only sparse groups of fibers grouping together due to the formation of the


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matrix and having a coating of 3 µm on the outside of the fiber tow. The effects of decreasing the flow rate further to 100 sccm are displayed in Figures 3G, 3H, and 3I which correspond to the 10th, 20th, and 30th cm from the front of the tube, respectively. A significant difference is present in the 10th cm zone. Instead of a well infiltrated fiber tow that forms into a single continuous piece, only groups of fibers were connected with the SiC matrix. The poor infiltration further in the reactor continued with the 20th and 30th centimeter samples, where the thin coating of 3 µm continued to be present for both sections. Further decreasing the flow to 50 sccm again altered how well the 10th cm of the reactor was infiltrated (Figure 3J). Instead of producing a matrix that bundled groups of fibers together, there are spheres that have been deposited on the fibers. Proceeding to the 20th cm (Figure 3K) the thickness of the coating has increased from 3 µm to 4.5 µm and larger portions of fibers have been brought together. The 30th cm zone in the reactor (Figure 3I) has a 2 µm thick outer coating, slightly thinner than previous infiltrations. Matrix Synthesis. The deposition zone at 50 sccm has a much more even coating thickness throughout the deposition zone than at higher flow rates, and infiltrations for longer periods of time started at this flow rate. Figures 4A, 4B, and 4C show the 10th cm of the furnace after infiltration times of 1 h, 2 h, and 4 h, respectively. Increasing the deposition time to 2 h infiltrated the fiber tow much more completely and produced a matrix that encompassed the entire fiber tow. Extending the infiltration to 4 h produced a fiber tow with little porosity throughout, although the center of the tow has one large void. The infiltration was not as successful through the rest of the composite despite the 10th cm being so well infiltrated. Even after 4 h the 20th and 30th cm zones had bundles of fibers with large voids present. Decreasing the flow to 25 sccm required that the following infiltrations be performed for twice the length of


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time compared to the 50 sccm infiltrations for comparison. Figure 4D shows that although a matrix is present, the spheres are still present on the fibers. Increasing the infiltration time to 4 h (Figure 4E) produced a matrix that spread through the entire fiber tow but a large void is present in the center of the tow. Infiltrating for 8 h (Figure 4F) produced a matrix that spread throughout the tow with relatively low porosity present. The difference between the 50 sccm and 25 sccm infiltration is that the quality of the infiltration was much more uniform throughout the length of the deposition zone, and the 20th and 30th cm zones had a porosity similar to that found at the 10th cm. The success of the 25 sccm infiltration prompted a 10 sccm experiment. Figures 4G, 4H, and 4I represent the 10th, 20th, and 30th cm from the front of the tube in a 60 h infiltration of carbon fiber. Not only has the porosity decreased compared to experiments with higher flow rates, but the step coverage throughout the reactor is much more uniform, with each zone appearing to have roughly the same porosity. The deposition rates and porosity measurements for the 25 SCCM and 10 SCCM infiltrations are listed and compared to low pressure infiltration in Table 1. Table 1. Deposition Rate and Porosity Data for Different Flow Rates Compared to Low Pressure Infiltration PRESSURE 760 TORR

50 TORR

H2 FLOW RATE (SCCM)

DEPOSITION RATE

POROSITY

25

3.88 µm*h-1

6%

10

750 nm* h-1

3%

*

200 nm* h-1

**

* H2 is not flowed through bubbler for low pressure, CH3SiCl3 is pulled into furnace by vacuum at 80 SCCM. ** Porosity data not available.


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Characterization. Powder X-ray diffraction was performed on the coatings to examine what phase of material was being produced (Figure 5). The reflections of the PXRD experiment as shown in Figure 4 show several representative diffraction peaks that are located at 35.6°, 41.3°, 60.1°, 71.9°, and 75.5° can be readily indexed, respectively, to the (111), (200), (220), (311), and (222) planes of cubic SiC, while the other broad diffraction peak at 25.7° (002) is matched with the peak seen from carbon fiber. Three dimensional reconstruction of a SiC infiltrated carbon fiber tow was nondestructively examined using the XCT technique (Figure 6). The sample fiber tow

Figure 5. PXRD pattern for SiC deposited onto carbon fiber.


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Figure 6. XCT 3D reconstruction of carbon fiber tow infiltrated with SiC. (A) Vertical view (B)

Example of a single XCT cross section (C)Horizontal View.

Table 2. AES determination of atomic percent at different parts of the composite POSITION IN FURNACE

FRONT

MIDDLE

REAR

CARBON ATOMIC PERCENT

57.4

50.0

47.9

SILICON ATOMIC PERCENT

42.6

50.0

52.1

Front: 5 cm from front end of composite Middle: 12 cm from front end of composite Rear: 23 cm from front end of composite. Front of composite is defined as end closest to gas inlet of furnace.


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Figure

7.

AES

line

scan

of

a

C/SiC

minicomposite cross section beginning on the

SiC matrix, scanning over a carbon fiber, then returning to the matrix.

was infiltrated for a total of 12 h at a flow rate of 25 sccm of H2. The compiled image shows that there is not a significant change in the overall porosity of the deposition in the local area. The XCT had the resolution to distinguish fibers, matrix, and voids. The chemical composition of the coating was examined using AES. Three separate scans were conducted on the front, middle, and rear of a minicomposite, with the atomic percentages listed in Table 2. Line scans were conducted on a cross-section of carbon fiber infiltrated for 18 hours with SiC (Figure 7). The spectrum shows that the composition of the coating is relatively consistent throughout the coating. The atomic percentages change dramatically when the scan passes over the carbon fiber, where the atomic percent of carbon goes to 100% and the atomic percent of silicon goes to 0%. DISCUSSION In this work, the effect of altering the flow rate of hydrogen gas through CH3SiCl3 for the deposition of SiC onto carbon fibers was studied. The initial profiling experiments produced different deposition zones that were evident on the graphite foil liner. The birefringent deposits


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leading up to the uniform grey zone in the center are from differences in coating thickness, with the coating steadily becoming thicker towards the center of the furnace. The length of grey deposition increases as the deposition time is increased, which can be attributed to the coating becoming too thick to exhibit birefringence. The 200 sccm profile exhibited typical atmospheric pressure deposition characteristics. The pores on the outermost portion of the tow were quickly sealed off and the deposition continued to form a thick coating on the outside of the tow. The poor step coverage throughout the rest of the deposition zone is also typically observed in atmospheric pressure reactions. The rapid deposition in the front of the reactor quickly produces a much thicker boundary layer that reactive gasses must diffuse through. Decreasing the flow rate of H2 carrier gas through CH3SiCl3 steadily produced a more uniform deposition zone, and also allowed the fibrous substrate to be more well infiltrated, which is evidenced by the decreased porosity present. Both factors changing with the flow rate can be explained by the boundary layer becoming more uniform throughout the deposition zone. At high pressures, the kinetics of deposition are controlled by the diffusion of reactive gasses through a boundary layer formed of laminar flowing gasses around the substrate. A uniform deposition is evidence of reactive gasses diffusing through a uniform boundary layer. The poor infiltration of atmospheric depositions is due to the very high concentration of gasses quickly depositing and sealing off pores on the surface and preventing further infiltration of the substrate. As the flow rate decreases the boundary layer is becoming thicker throughout the furnace. The thicker layer allows more time for reactive molecules to diffuse through the porous substrate before depositing, which accounts for the improved infiltration at lower flow rates. The 25 SCCM and 10 SCCM infiltrations both produced porosities lower than 10%, which is the target for minicomposites.10


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PXRD analysis of coated carbon fiber revealed that the cubic phase of silicon carbide is present for all samples produced using these deposition parameters. The SEM cross sections used to analyze the depositions were validated through the use of the XCT technique, which takes a number of cross sectional images (Figure 6B) which are similar to the SEM micrographs. The 3D compiled images show that the infiltration is smooth within the fiber tow, and the infiltration changes gradually throughout the reactor. Auger spectroscopy revealed that there is a stoichiometric gradient of SiC being produced. The gradient of carbon to silicon rich in the reactor is a product of a changing CH3SiCl3 to H2 ratio in the reactor.37 The front of the reactor is carbon rich due to the higher concentration of CH3SiCl3. As the CH3SiCl3 is consumed through the furnace, the ratio changes and the product steadily transitions into a silicon rich SiC by the end of the composite, with stoichiometric SiC lying between these ends. The line scan confirms that the stoichiometry of the coating is consistent throughout the coating. CONCLUSIONS The effect of infiltrating tows of carbon fiber at atmospheric pressure using different flow rates of H2 bubbled through CH3SiCl3 was examined. The deposition of these reactions can be examined by the coloration of the graphite foil lining in the reactor. Decreasing the flow rate both increased how well infiltrated the fiber tows were, and also lengthened the uniformity of the deposition zone. β-SiC was confirmed to be produced through PXRD analysis of coatings. XCT was used to validate the SEM cross sections, and show that the coatings are smooth and change gradually throughout the reactor. The method presented in this work is capable of infiltrating a fiber tow with SiC matrix much faster than low pressure infiltrations,38-39 but AES confirmed that there was a stoichiometric gradient in the furnace. Deposition of SiC using the CH3SiCl3 and H2 reaction at low pressure (50 torr) has a deposition rate of approximately 200 nm*h-1, while


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atmospheric deposition at 10 sccm and 25 sccm has deposition rates of 750 nm*h-1 and 3.88 µm*h-1, respectively. The minicomposites had porosities of 3% for 10 sccm and 6% for the 25 sccm infiltration.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] ORCID Steve Suib: 0000-0003-3073-311X

ACKNOWLEDGMENTS We thank NASA for support of this research (NASA contract 4200600107). The SEM studies were performed using the facilities in the UCONN/Thermo Fisher Scientific Center for Advanced Microscopy and Materials Analysis (CAMMA). We thank Peter Kerns for performing the Auger spectroscopy. REFERENCES (1) Huda, Z.; Edi, P. Materials selection in design of structures and engines of supersonic aircrafts: a review. Materials & Design 2013, 46, 552-560. (2) Naslain, R. Design, preparation and properties of non-oxide CMCs for application in engines and nuclear reactors: an overview. Composites Science and Technology 2004, 64 (2), 155-170. (3) Ohnabe, H.; Masaki, S.; Onozuka, M.; Miyahara, K.; Sasa, T. Potential application of ceramic matrix composites to aero-engine components. Composites Part A: Applied Science and Manufacturing 1999, 30 (4), 489-496. (4) Roth, R.; Clark, J. P.; Field, F. R. The potential for CMCs to replace superalloys in engine exhaust ducts. JOM Journal of the Minerals, Metals and Materials Society 1994, 46 (1), 32-35.


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Rapid Chemical Vapor Infiltration of Silicon ... - ACS Publications

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