Advances in manufacture of ceramic matrix composites by infiltration techniques

Advances in manufacture of ceramic matrix composites by infiltration techniques

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Dmitri Kopeliovich SubsTech (Substances & Technologies), Meitar, Israel

Abstract Ceramic matrix composites reinforced with long fibers are commonly fabricated by infiltration methods, in which the ceramic matrix is formed from a fluid infiltrated into the fiber structure. The infiltration techniques differ from each other in the types of the fluids and the processes of conversion of the fluid into a ceramic: polymer infiltration and pyrolysis, chemical vapor infiltration, reactive melt infiltration, slurry infiltration, and solegel infiltration. Formation of the ceramic matrix microstructure, properties of the interface, and benefits and drawbacks of the composites prepared by different techniques are discussed in this chapter. Fabrication routes including the stages of preform preparation, interphase deposition, preceramic fluid infiltration, and thermal processing are described.

Keywords: Ceramic composites; Fiber reinforcement; Infiltration; Interphase; Preceramic polymers.

5.1

Introduction

The most serious drawback of monolithic ceramics is brittleness (i.e., low toughness). The ceramic material toughness may be significantly increased by incorporating a reinforcing phase (particularly continuous fiber structure) into the ceramic matrix. The resulting ceramic matrix composite (CMC) materials have a unique combination of properties: • • • • • • • •

Hardness; Strength; Toughness; Low density; High modulus of elasticity; Creep resistance; Oxidation resistance; Thermal shock resistance.

Due to their exceptional properties ceramic composites are used in the applications where reliable operation under extreme conditions is required: aerospace jet Advances in Ceramic Matrix Composites. https://doi.org/10.1016/B978-0-08-102166-8.00005-0 Copyright © 2018 Elsevier Ltd. All rights reserved.

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engines, gas turbine engines, wear resistant parts (e.g., braking systems), heat exchangers, and furnaces. Ceramic composites may be produced by traditional ceramic fabrication methods including mixing the powdered matrix material with the reinforcing phase followed by processing at elevated temperature: hot pressing, sintering. Such fabrication routes are successfully employed for preparing composites reinforced with a discontinuous phase (particulate or short fibers). However, the composites reinforced with continuous or long fibers are rarely fabricated by conventional sintering methods due to mechanical damage of the fibers and their degradation caused by chemical reactions between the fiber and matrix materials at high sintering temperature. Additionally, sintering techniques result in high porosity of the fiber-reinforced composites. CMCs reinforced with long fibers are commonly fabricated by infiltration methods. In this group of fabrication techniques the ceramic matrix is formed from a fluid (gaseous or liquid) infiltrated into the fiber structure (either woven or nonwoven). Prior to the infiltration with a ceramic-derived fluid, the reinforcing fibers surface is coated with a debonding interphase providing weak bonding at the interface between the fiber and matrix materials. Weak bonding allows the fiber to slide in the matrix and prevents brittle fracture.

5.2

Classification of infiltration techniques

Generally any infiltration technique employs the following fabrication stages: • • • •

Preform preparation. At this stage the fiber reinforcing phase is laid up and molded forming a preform of the required shape. Interphase deposition. The interphase may be deposited over the fiber surface during either the filament production or after the preform fabrication. Infiltration. The reinforcing fiber preform is infiltrated with a preceramic fluid. The fluid contains either ceramic matrix particles (slurry) or a substance, which may be converted into a ceramic as a result of chemical reaction. Thermal processing. The preceramic fluid incorporated into the fiber reinforcing structure converts into ceramic, filling the space between the fibers.

Ceramic composites may be fabricated by a number of infiltration techniques differing from each other in the types of the fluids and the processes of conversion of the fluid into a ceramic: • • •

Polymer infiltration and pyrolysis (PIP): infiltration with a low viscosity preceramic organometallic polymer followed by its pyrolysis when the polymer converts into a ceramic. Chemical vapor infiltration (CVI): infiltration with a preceramic gaseous precursor (vapor), which produces ceramic as a result of chemical decomposition. Reactive melt infiltration (RMI): infiltration with a liquid metal, which converts into a ceramic when reacting with a surrounding substance. • Liquid silicon infiltration (LSI): a type of RMI utilizing molten silicon reacting in the preform with the porous carbon and forming the matrix of silicon carbide (SiC).

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• Direct melt oxidation (DIMOX): a type of RMI, in which a molten metal (commonly aluminum) forms an oxide ceramic matrix as a result of the reaction with the surrounding air. Slurry infiltration: infiltration with slurry containing fine ceramic particles, which forms a ceramic matrix after drying and hot pressing. Solegel infiltration: infiltration of the preform with a sol preceramic precursor, which undergoes polymerization (gelation) and is then converted into a ceramic at an elevated temperature. Combined infiltration methods. • Slurry infiltration þ PIP: infiltration with a preceramic polymer blended with fine ceramic particles (slurry) followed by pyrolysis. • Slurry infiltration þ LSI: partial filling of the preform with SiC particles by infiltration of SiC slurry followed by infiltration of molten silicon, which reacts with the surrounding carbon. • CVI þ LSI: preparation of porous carbon preform by CVI method followed by its infiltration with molten silicon reacting with the surrounding carbon and forming SiC matrix. • CVI þ PIP: partial fabrication of SiC matrix by CVI followed by infiltration of preceramic polymer and pyrolysis.

5.3 5.3.1

Reinforcing fibers Fiber architecture

Reinforcing phases of CMCs may be in different forms: particles, platelets, whiskers, chopped fibers, and continuous fibers. The ceramic composites fabricated by infiltration methods are commonly reinforced with continuous fibers, which may be arranged in different structures (unidirectional, 2D, 3D): • • • • •

Tows (fiber bundles). Tows are denoted by the number of filaments (ends) in the bundle: 1K, 3K, 6K, 12K. Tows are used for reinforcing in one direction. Unidirectional fabrics (tapes). If the number of warp yarns (yarns running in the roll direction) is much greater than the number of picks (fill yarns running crosswise to the roll direction), the fabric is called unidirectional. Bidirectional fabrics (2D architecture) have the woven structure with similar numbers of warp yarns and picks. 3D fabrics are woven of yarns running in three different directions. 3D bulk structures are produced from the fabric. Braided architecture is used for the fabrication of 3D net shape composites.

5.3.2

Fiber materials

The reinforcing fibers are manufactured from different ceramic materials providing a required combination of properties: strength, modulus of elasticity, flexibility, creep resistance, chemical stability, and oxidation resistance. The most popular fibers materials are SiC and carbon; however, oxide fibers (alumina, silica) are also used. SiC

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fibers are manufactured from organosilicon precursor filaments, which are cured (cross-linked) and then heated in a nitrogen atmosphere at a temperature of about 1200 C (w2200 F). The properties of SiC fibers (Hi-Nicalon, Nicalon S, Sylramic, Tyranno) (MIL-HDBK-17-5, 2002) include the following: • • • •

Filament diameter: 10e15 mm; Modulus of elasticity (stiffness): up to 420 GPa (61 Msi); Tensile strength: 3.4 GPa (493 ksi); Thermal stability: up to 1450 C (2642 F).

Carbon fibers are manufactured from precursor filaments, which are carbonized in a nitrogen atmosphere at a temperature of about 1200 C (w2200 F) and then graphitized at about 2500 C (w4500 F). The properties of carbon fibers (BP Amoco, Conoco, Grafil, Granoc, Toho, Toray, Zoltek) (MIL-HDBK-17-5, 2002) include the following: • • • • •

Filament diameter: 5e15 mm; Modulus of elasticity (stiffness): up to 920 GPa (133 Msi); Tensile strength: up to 6.9 GPa (1000 ksi); Density: 1800 kg/m3 (114 lb/ft3); High chemical inertness.

Despite excellent oxidation resistance in inert environments, carbon fibers have low oxidation resistance in air. This is their main disadvantage. Carbon oxidizes in air at temperatures exceeding 500 C (932 F). The most popular oxide fibers are made of alumina (aluminum oxide). Oxide fibers are manufactured from an organic alumina precursor. The precursor fibers after spinning are heated to 800 C (1472 F). At this temperature the organic precursor transforms to alumina. If silica is added into the precursor, the mullite structure forms. The properties of alumina fibers (Nextel 610, Nextel 720) (Wilson, 2006) include the following: • • • •

Filament diameter: 5e15 mm; Modulus of elasticity (stiffness): up to 373 GPa (54 Msi); Tensile strength: 3.3 GPa (479 ksi); Density: up to 4100 kg/m3 (256 lb/ft3).

Brittleness, insufficient creep resistance due to grain growth at high temperatures and relatively high density are the main drawbacks of alumina fibers.

5.4

Interphases

A CMC is commonly stronger and stiffer than the matrix material in the monolithic state that is a result of reinforcing action of the strong and stiff fibers. Additionally, the composite is tougher than the matrix material. Higher toughness of the fiberreinforced ceramic composites is provided by the mechanism of crack deflection at

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Figure 5.1 Pullout of fibers in a fracture of a SiC/SiC composite fabricated by chemical vapor infiltration method. Courtesy Dr. Karin Handrick, MT Aerospace AG, Augsburg, Germany.

the matrixefiber interface: a crack advancing through the matrix material reaches a fiber; the weak matrixefiber interfacial bonding enables sliding (pullout) of the fiber in the matrix preventing the fiber fracture and bridging the cracked material (Fig. 5.1). The fracture behavior of ceramic composites is described by R-curve: the crack propagation resistance curve. The strength of the interfacial bonding plays the key role in the crack deflection mechanism. If the bonding is strong, the composite material fractures similar to brittle (low toughness) monolithic ceramics. The fibers are not able to slide in the matrix and the advancing crack passes through, breaking the fibers. A photograph of a brittle fracture of a fiber-reinforced ceramic composite is presented in Fig. 5.2. Most ceramic composites form strong bonds between the matrix and the fibers during the infiltration processes. The bond is formed as a result of chemical interaction of the materials or their diffusion into each other. To provide debonding (weak bonding) capability the fibers are coated with a layer of interphase separating the matrix and fiber surfaces and preventing their interaction. The functions of the interphases include the following: • • •

Debonding; Protection of the fibers from oxidation and other environmental attacks (Si’an Chen et al., 2013); Protection of the fibers from the aggressive action of the matrix material at elevated temperatures and during its infiltration.

Good debonding ability is achieved if the interphase material has low shear strength. Such material allows easy slippage between the neighboring sublayers. The most suitable materials having the microstructures with easy cleavage planes (similar to graphite) are pyrolytic carbon (C) and hexagonal boron nitride (BN). Pyrolytic carbon is composed of graphene planes weakly bonded to each other. Hexagonal

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4/14/2006 WD Mag HV Det Spot Pressure 1:41:25 p.m. 10.4 mm 5000x 20.0 kV ETD 3.0 ---

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Figure 5.2 Brittle fracture of a C/SiC composite fabricated by the method of liquid silicon infiltration. Courtesy Prof. Victor Kulik, Baltic State Technical University “Voenmeh,” Russia.

BN also has a layered structure, in which the atoms of boron and nitrogen within a layer are strongly bonded to each other. However, the neighboring layers are bonded to each other by weak van der Waals forces. The interphase film is deposited by the method of CVI prior to the infiltration of the matrix. The interphase layer thickness may vary in the range 0.1e1 mm. Thicker interphase film results in weaker matrixe fiber bonding. Carbon interphase works excellent in nonoxidizing environments, but in air its operation temperature is limited: not higher than 500 C (932 F). Interphases from high purity hexagonal BN may survive in a dry oxidizing atmosphere up to 1200 C (2192 F) (MIL-HDBK-17-5, 2002). To protect both the interphase film and underlying fiber from the interaction with the matrix material during its infiltration, the interphase layer may be overcoated with 0.5e5 mm of SiC using the CVI technique (DiCarlo and Bansal, 1998). The overcoating protects the interphase from the aggressive environmental action during the subsequent use as well.

5.5 5.5.1

Polymer infiltration and pyrolysis Introduction

PIP is the method of fabrication of CMCs comprising an infiltration of a low viscosity polymer into the reinforcing ceramic structure (e.g., fabric) followed by heating them

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in the absence of oxygen when the polymer decomposes and converts into a ceramic. The ceramic materials obtained from the polymers as a result of their pyrolysis are called polymer-derived ceramics. The polymers, which can be converted into ceramics, are called preceramic polymers or polymer precursors.

5.5.2

Preceramic polymers

Preceramic polymers are referred to as organometallic or organoelement compounds. Their molecules commonly contain carbon (C) and/or silicon (Si); however, there are also polymers containing nitrogen (N), oxygen (O), boron (B), aluminum (Al), and titanium (Ti). Carbon matrices of carbonecarbon (C/C) composites are fabricated by pyrolytic conversion of precursors made of either carbon thermosetting resins (Savage, 1993): phenolic resin, furan resin, oxidized polystyrene, polyvinyl alcohol, or thermoplastic resins such as pitches and coal tar. The carbon yield of these resins is 50%e60%. However, PIP method is used mainly for fabrication composites with SiC and other silicon-based matrices (SiCN, SiBCN, Si3N4) (Li et al., 2013). The silicon-derived polymer precursor available on the market are as follows: polysilazane (Seraset VL-20, Kion Corp.), polycarbosilanes: polymethylsilane, and allhydridopolycarbosilane (Starfire) (Yoon et al., 2010). Polycarbosilanes may be converted into SiC ceramic (ceramic yield 65%) and polysilazane into SiCN or Si3N4 (ceramic yield up to 90%).

5.5.3

Polymer infiltration and pyrolysis process description

The principal scheme of a PIP process of fabrication of a continuous fiber-reinforced ceramic composite is presented in Fig. 5.3. •

• •

Prepreg fabrication. Preimpregnated (prepreg) fiber reinforcing material (tow, tape, weave) combines the fibers with a resin. After the resin impregnation, prepregs may be either dried or partially cured (B-stage). The viscosity of the resin binder is increased after the B-stage curing. In such plastic condition the prepregs can be laid up to shape the desired architecture. Lay-up. The prepreg is laid up on a tooling (mold) or wrapped around it. Molding. The laid-up prepreg is molded by one of the molding techniques: vacuum bag molding, gas pressure bag molding, or compression molding. A bag mold consists of a rigid lower mold and a flexible upper mold. The flexible mold (bag) is pressed against the prepreg by Reinforcing fibers

Prepreg fabrication

Resin

Lay-up Molding Preceramic polymer

Polymer infiltration Pyrolysis

4–10 times

Figure 5.3 Fabrication of ceramic matrix composites by polymer infiltration and pyrolysis.

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either the atmospheric pressure in the vacuum bag mold or increased air pressure in the gas pressure bag mold. The prepreg preform pressurized in the bag mold is cured in an autoclave. Compression molding combines pressure with high temperature producing cured preform. Polymer infiltration. The preform is immersed into a low viscosity solution of a preceramic polymer, which infiltrates the porous reinforcement structure filling the spaces between the fibers. The infiltration process is driven by the capillary forces at normal pressure but it may also be vacuum assisted or pressure assisted. Pyrolysis. Pyrolysis is the chemical decomposition of polymers by heat in the absence of oxygen. Most preceramic polymers may be converted into ceramics at a temperature in the range 800e1300 C (1472e2372 F). Argon atmosphere is commonly used in the pyrolysis; however, silicon nitride (Si3N4) ceramic matrices are obtained in the atmosphere of nitrogen (N2) or ammonia (NH3). The chemical decomposition of polymer precursors results in a release of volatile products such as CO, H2, CO2, CH4, and H2O. The ceramic material obtained in the pyrolysis of a polymer has a porous structure due to the shrinkage caused by the release of the volatiles. The weight loss determines the value of the ceramic yield. Multiple reimpregnation and pyrolysis. To densify the composite matrix the impregnatione pyrolysis cycle is repeated 4e10 times.

Pyrolysis in nitrogen/ammonia (NH3) atmosphere is used for fabrication of nitride matrices. Qi et al. (2005, 2006) have fabricated a three-dimensional silica fiberreinforced silicon nitride matrix composite by the infiltration and pyrolysis of perhydropolysilazane in anhydrous ammonia at 800 C (1472 F). The PIP method is also used for fabrication of particulate-reinforced composites. Lee (2009) used SiC particles (0.51 mm mean particle size) compacted into cylindrical pellets. The pellets were infiltrated with a Si-C-N based polymer precursor and heated for pyrolysis to 1350 C (2462 F) in the atmosphere of high purity argon. The resulted composite showed excellent creep resistance at 1400 C (2552 F). Fabrication of nuclear fuel elements from mixed metal carbides of uranium, niobium, and zirconium using PIP technique was reported by Singh (2008). Preparation of C/C-SiC composite by PIP process was reported by Swaminathan et al. (2010). Porous C/C composite structure was prepared by pyrolysis of a carbon fiber fabric impregnated with phenolic resin. At the next stage the material was infiltrated with silicon-derived precursor polymers and pyrolized. It was shown that the maximum flexural strength was reached after 8 infiltrationepyrolysis cycles. A method of fabrication of novel SiBNC/SiC composites for high-temperature applications (e.g., combustion liners of gas turbines) by PIP was proposed by Klatt et al. (2010). Rapid CVI process was used for coating relatively new amorphous SiBNC fibers with a pyrocarbon interphase.

5.5.4

Advantages and disadvantages of polymer infiltration and pyrolysis

PIP is the main method of fabrication of ceramic composites with SiC matrices. PIP technique has the following advantages: • •

The ceramic matrices are formed at relatively low temperature that prevents fiber damage; Good control of the ceramic matrix microstructure and composition;

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Net-shape fabrication of composite parts; Different types of reinforcement may be used (particulate, short fibers, continuous fibers); Wide range of matrices may be fabricated (in contrast to melt infiltration (MI) method); No free silicon in the matrices (in contrast to MI method).

The drawbacks of the PIP method include the following: • • •

Long fabrication time as a result of multiple infiltrationepyrolysis cycle; The production cost is higher than in MI method; The residual porosity present in the matrix microstructure affects the mechanical properties of the composite.

5.6

Chemical vapor infiltration

5.6.1

Introduction

CVI is the method of fabrication of CMCs based on the conversion of a gaseous precursor (vapor) into a ceramic at an elevated temperature. The precursor is infiltrating into the reinforcing ceramic continuous fiber structure (preform), driven by either diffusion process or an imposed external pressure. The gaseous precursor dissociates on the fiber surface forming a ceramic layer. Commonly the vapor reagent is supplied to the preform in a stream of a carrier gas (H2, Ar, He). The most popular preceramic gaseous precursor used for fabrication composites with SiC matrix is methyltrichlorosilane, which is decomposed according to the reaction: CH3 Cl3 Si/SiC þ 3HCl The gaseous hydrogen chloride (HCl) is removed from the preform by the diffusion or forced out by the carrier stream. Carbon matrix is formed from a methane precursor (CH4). The ceramic deposition is continuously growing as long as the diffusing vapor is reaching the reaction surface. The porosity of the material is decreasing being filled with the formed solid ceramic. In the course of the CVI process the accessibility of the inner spaces of the preform is getting more difficult due to filling the vapor paths with the forming ceramic matrix. The precursor transportation is slowing down. The growing solid phase separates spaces in the material from the percolating network of the vapor precursor. Such inaccessible pores do not decrease any longer forming the residual porosity of the composite. The matrix densification stops when the preform surface pores are closed. The final residual porosity of the ceramic composites fabricated by CVI method may reach 10%e15% (Singh and Levine, 1995; Katoha et al., 2004). The structure constituents of a composite fabricated by CVI are shown in Fig. 5.4; it consists of fibers, matrix, and pores (black spots). To decrease the porosity of SiCf/SiC composites and increase the infiltration rate, the whisker growing assisted CVI (WACVI) process was developed (Park et al., 2010a).

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Figure 5.4 Microstructure of SiC/SiC composite fabricated by chemical vapor infiltration method. Courtesy Dr. Karin Handrick, MT Aerospace AG, Augsburg, Germany.

Higher density and shorter infiltration time were achieved by growing SiC whiskers into a SiC fiber preform before matrix CVI filling. According to Naslain et al. (2010) two phenomena play a key role in the CVI process: heat/mass transport in fiber preform and chemical reaction kinetics. Numerical simulation of CVI process including optimization of the parameters of the resulting microstructure (e.g., porosity, density) was performed by Jin and Wang (2003) and Besmann et al. (1996). The presented mathematical models take into account the kinetic parameters of the vapor decomposition reaction, the process of diffusion and forced convection of the gaseous precursor through a porous preform, the rate of matrix growth, and the topology of the pore network. However, Naslain et al. (2010) believe that CVI process can be modeled but a large number of data are required. Composites with SiC and carbon (C) matrices are commercially fabricated by CVI.

5.6.2

Types of chemical vapor infiltration processes

CVI process has different versions, which may be classified into five types (Mazdiyasni, 1990): •

•

Isothermal/isobaric (I-CVI) is the most commonly studied and used version of CVI process (Naslain, 2001). The preform infiltrated in I-CVI process is kept at a uniform temperature (no temperature gradient). The gas surrounding the preform has a uniform pressure (no pressure gradient). Temperature gradient (TG-CVI). In this process the vapor precursor diffuses through the preform from the cooler surface to the hotter inside regions. The temperature gradient increases the diffusion speed. Since the rate of the chemical reaction is greater at higher temperatures, the precursor decomposes mostly in the inner regions. TG-CVI method prevents early closing of the surface pores and allows better densification of the ceramic matrix. The method allows fabrication of complex and integrally shaped in situ stiffened fiber-reinforced ceramic composites (Handrick et al., 2010).

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Preform fabrication Interphase deposition

Gaseous precursor

Deposition of ceramic matrix by chemical vapor infiltration

Repeated cycles

Surface machining Surface coating

Figure 5.5 Fabrication of ceramic matrix composites by chemical vapor infiltration. • • •

Isothermal-forced flow (IF-CVI) utilizes forced flow of the gas precursor through the uniformly heated preform. The forced flow results in enhancing the rate of infiltration and the matrix deposition. Thermal gradienteforced flow (F-CVI) combines the advantages of the temperature gradient CVI with the forced flow CVI. The infiltration rate of the vapor precursor is enhanced by both the gradient of temperature and the gradient of pressure. Pulsed flow (P-CVI). In P-CVI process the cycle of rapid changes of the surrounding precursor gas pressure is repeated many times. A cycle of the pressure change consists of the evacuation of the reactor vessel followed by its filling with the reactant gas.

5.6.3

Chemical vapor infiltration process description

The principal scheme of a CVI process of fabrication of a continuous fiber-reinforced ceramic composite is presented in Fig. 5.5. • • •

• • •

Fabrication of fibrous preform. Deposition of a debonding interphase. A thin (commonly 0.1e1 mm) layer of pyrolytic carbon (C) or hexagonal BN is applied on the fiber surface by CVI method. Deposition of the ceramic matrix by CVI. The preform is heated and placed into a reactor with a gaseous precursor (Kopeliovich, 2010). The preform is infiltrated with the gas, which decomposes and forms a ceramic deposit on the fiber surface. The process continues until the open porosity on the preform surface is closed. Machining the preform surface to open the paths of the percolating network, which allow further densification of the matrix. Repeated reinfiltrationemachining cycles until maximum densification is achieved. Surface coating. The open porosity is sealed to prevent a penetration of the environmental gases into the composite during the service. Additional layer protecting the composite surface from the oxidation may be applied over the seal coat. The coatings are deposited by CVI.

5.6.4

Advantages and disadvantages of chemical vapor infiltration

CVI method of fabrication of CMCs has the following advantages: • •

High purity matrices; Relatively low temperatures of the matrix formation that prevents fiber damage;

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Low residual mechanical stresses; Excellent mechanical properties (strength, elongation, toughness); Good thermal shock resistance; Good creep and oxidation resistance at temperatures of 1400 C/2642 F (Kiser et al., 2010); Interphases may be deposited in situ; Wide range of matrices may be fabricated (SiC, C, Si3N4, BN, B4C, ZrC, etc.).

The disadvantages of CVI are as follows: • • •

Very slow process (may take several weeks); High residual porosity (10%e15%); High capital and production costs.

5.7

Reactive melt infiltration

5.7.1

Introduction

In RMI technique the ceramic matrix forms as a result of chemical interaction between the liquid metal infiltrated into a porous reinforcing preform and the substance (either solid or gaseous) surrounding the melt. Commonly the liquid metal is infiltrated at a normal pressure or in vacuum. The melt penetrates into the porous structure due to the capillary force. RMI method of fabrication of CMCs is similar to MI technique of fabrication of metal matrix composites, in which the infiltrated metal solidifies and forms metallic matrix. In RMI the liquid metal converts into a ceramic compound: carbide, oxide, or nitride of the metal. Two versions of RMI method are commercially used: LSI and DIMOX.

5.7.2

Liquid silicon infiltration

LSI is used for fabrication of SiC matrix composites (Patel et al., 2012). The process involves infiltration of carbon (C) microporous preform with molten silicon (Si) at a temperature exceeding its melting point 1414 C (2577 F). The liquid silicon wets the surface of the carbon preform. The melt soaks into the porous structure driven by the capillary forces. The melt reacts with carbon forming SiC according to the reaction: SiðliquidÞ þ CðsolidÞ/SiCðsolidÞ A mechanism of growth of the SiC phase was proposed by Schulte-Fischedick et al. (2002): silicon diffuses through the already formed SiC and then reacts with carbon forming numerous nucleation sites, which result in fine-grained structure of SiC. SiC produced in the reaction fills the preform pores and forms the ceramic matrix. Since the molar volume of SiC is less than the sum of the molar volumes of silicon and carbon by 23%, the soaking of liquid silicon continues in course of the formation of

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SiC. According to Singh and Behrendt (1992) the initial pore volume fraction providing complete conversion of carbon into SiC is 0.562. If the initial pore volume fraction is lower than 0.562, the infiltration results in entrapping residual free silicon. Commonly at least 5% of residual free silicon is left in SiC matrix. The pores’ dimensions are important for complete infiltration. If the pores are too small, the infiltration passages are choked off resulting in prematurely stop of the infiltration (Sangsuwan et al., 1999). Too large pores help complete infiltration but may result in noncomplete chemical interaction and formation of a structure with high residual free silicon and unreacted carbon. Excellent control of the pores size and the pores’ volume fraction in the microporous preform may be achieved by the pyrolysis of a polymerized resin (Singh and Levine, 1995). The porous preform may also be prepared by CVI from a gaseous precursor (Zhuan et al., 2010). In contrast to the composites fabricated by PIP and CVI, ceramic matrices formed by LSI are fully dense (have zero or low residual porosity). The infiltrated at high temperature molten silicon is chemically active and may not only react with the carbon porous preform but also attack the reinforcing phase (SiC or C fibers, whiskers, or particles). A protective barrier coating (interphase) of SiC, C, or Si3N4 prevents the damage of the fibers by the melt. The barrier coatings are applied over debonding coatings (C, BN). The interphases may be deposited by CVI. The protective barrier from pyrolytic carbon is formed by PIP. Kulik et al. (2006) proved that a protective barrier of pyrolytic carbon deposited over the bundles of carbon fibers prevented an infiltration of liquid silicon into the interfiber space, which resulted in a formation of the matrix containing significant amount of unreacted pyrolytic carbon surrounding the carbon fibers (Fig. 5.6). Such structure provided good debonding ability of the C/SiC composite. LSI is commonly used for fabrication of ceramic composites reinforced with continuous fibers (SiC or C); however, particulate-reinforced composites may also be produced by LSI method. Fabrication of B4C-SiC composites by infiltration of a sintered B4C preform with liquid silicon is described by Hayun et al. (2008). The resulting composite structure is presented in Fig. 5.7. The final phases are B4C (dark gray), SiC (light), and residual silicon (gray).

5.7.2.1

Liquid silicon infiltration process description

The principal scheme of LSI process of fabrication of a continuous fiber-reinforced ceramic composite is presented in Fig. 5.8. •

• •

Deposition of interphases. To provide debonding coating of pyrolytic carbon (C) or hexagonal BN is deposited over the fiber surface. Additionally, the fibers used for fabrication ceramic composites by LSI should be protected from the highly reactive liquid silicon by a barrier coating (commonly SiC). The interphases are deposited by CVI. Prepreg fabrication. The fiber reinforcing material (tow, tape, weave) is impregnated with a resin. The resin contains carbon, which further will react with molten silicon. Lay-up. The prepreg is laid up on mold. The resin application may be performed either before (Singh and Petko, 2003) or after lay-up operation (Singh et al., 1997).

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4/14/2006 WD Det Spot Pressure Mag HV 1:01:46 p.m. 9.5 mm 5000x 20.0 kV ETD 3.0 ---

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Figure 5.6 Nonbrittle fracture of a C/SiC composite fabricated by liquid silicon infiltration method. Courtesy Prof. Victor Kulik, Baltic State Technical University “Voenmeh,” Russia.

SiC

Si B4C

Figure 5.7 Microstructure of a particulate B4C/SiC composite. Courtesy Prof. N. Frage, Ben-Gurion University, Israel. • •

Molding. The laid-up prepreg is molded by one of the molding techniques and cured. Pyrolysis. During pyrolysis the resin decomposes by heat in the absence of oxygen (in an inert atmosphere). Pyrolysis is performed at a temperature 800e1200 C (1472e2192 F). Argon atmosphere is commonly used as an inert atmosphere. A porous carbon structure is formed as a result of the resin pyrolysis.

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Preform fabrication Interphase deposition Prepreg fabrication

Resin

Lay-up Molding Pyrolysis Liquid silicon

Silicon infiltration Machining

Figure 5.8 Fabrication of ceramic matrix composites by liquid silicon infiltration.

• • •

Primary machining. This operation may be performed after the steps of molding and/or pyrolysis. LSI. The prepreg is immersed into a furnace with molten silicon. The porous carbon structure is infiltrated with the melt driven by the capillary forces. Liquid silicon reacts with carbon forming in situ silicon carbide matrix. Final machining.

5.7.2.2

Advantages and disadvantages of liquid silicon infiltration

LSI method of fabrication of CMCs has the following advantages: • • • • • •

Low cost; Short production time; Very low residual porosity; High thermal conductivity: up to 40 W/mK (Heidenreich, 2005); High electrical conductivity; Complex and near-net shapes may be fabricated.

The disadvantages of LSI include the following: • • •

High temperature of the infiltration process, which may cause a damage of the fibers; Presence of residual free silicon in the carbide matrix; Lower mechanical properties: strength, modulus of elasticity (Heidenreich, 2005).

5.7.3

Direct melt oxidation

DIMOX of fabrication of a CMC involves a formation of the matrix in the reaction of a molten metal infiltrated into a porous reinforcing preform with an oxidizing gas (commonly air). Capillary effect forces the melt to wick into the porous reinforcing

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Oxygen

Barrier

Molten metal

Preform

Oxidation front

Finished composite

Figure 5.9 Direct metal oxidation.

preform toward the reaction front where the metal reacts with the gas forming the ceramic layer (Fig. 5.4). After an initial oxide layer has formed the liquid metal wicks through it to reach the reaction front. The melt advances to the reaction front continuously at a rate limited by the oxidation reaction. Some residual metal (about 5%e15% of the material volume) remains in the intergranular spaces of the ceramic matrix (Kopeliovich, 2011) (Fig. 5.9). The resulting materials have no pores and impurities, which are usually present in ceramics fabricated by sintering. Commonly DIMOX technique is used for fabrication of composites with the matrix from aluminum oxide (Al2O3). A reinforcing preform (SiC or Al2O3 in either particulate or fibrous form) is infiltrated with a molten aluminum alloy heated in a furnace to a temperature 900e1150 C (1652e2102 F) (Santhosh Kumar et al., 2012). The aluminum alloy is doped with additives improving the wettability of the reinforcing phase with the melt and enhancing the oxidation process. Addition of magnesium prevents passivation of aluminum melt. Silicon helps in dissolution of oxygen in the liquid aluminum (Karandikar et al., 2007). The typical rate of DIMOX process is 1e1.5 mm/h (0.04e0.0600 /h). In principle the direct oxidation process and the oxide growth may continue even after the reaction front has reached the outer surface of the preform. In this case the aluminum oxide will be deposited over the preform changing its dimensions. To prevent an advance of the reaction front beyond the preform surface, it is coated with a gas permeable barrier. The ceramic matrix growth stops when the reaction front reaches the barrier.

5.7.3.1

Direct melt oxidation process description

The principal scheme of DIMOX process of fabrication of a continuous fiberreinforced ceramic composite is presented in Fig. 5.10.

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Reinforcing fibers

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Preform fabrication Interphase deposition Application of gas permeable barrier

Molten aluminum

Direct metal oxidation

Oxygen

Removal of residual aluminum

Figure 5.10 Fabrication of ceramic matrix composites by direct melt oxidation.

• • • •

•

Lay-up. The preform made of the reinforcing fibrous phase is shaped at this stage. Deposition of interphases. A debonding coating of pyrolytic carbon (C) or hexagonal BN is deposited over the fiber surface by CVI. Application of a gas permeable barrier on the preform surface. The surface, through which the melt should wick into the preform, is not coated. Direct metal oxidation. The preform is put in contact with liquid aluminum alloy. The reinforcing structure is infiltrated with the melt through the noncoated surface. The oxidant (air) penetrates into the preform through the gas permeable barrier. Aluminum and oxygen meet at the reaction front and form the growing layer of the oxide matrix. The process terminates when the reaction front reaches the barrier coating. Removal of residual aluminum. The excessive aluminum is removed from the part surface.

DIMOX process of fabrication of particulate-reinforced ceramic composites is similar. The difference is in the preform shaping technique: sintering instead of layup. Of course debonding interphases are not deposited on the particulate-reinforcing phase.

5.7.3.2

Advantages and disadvantages of direct melt oxidation

Advantages of DIMOX process include the following: • • • • •

Near-net shape parts may be fabricated due to very low shrinkage; Low cost and simple equipment; Low cost raw materials; No impurities or sintering aids decreasing mechanical properties under elevated temperatures (e.g., creep resistance); Low residual porosity.

The disadvantages of DIMOX process include the following: • •

Slow process rate and relatively long fabrication time (2e3 days); Presence of residual free aluminum in the oxide matrix.

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5.8 5.8.1

Slurry Infiltration Introduction

In slurry infiltration method a slurry containing fine ceramic particles wicks into the porous reinforcing preform due to capillary forces. After drying and hot pressing the slurry particles form the ceramic matrix. Slurry infiltration is used for fabrication of fiber-reinforced glass and glasseceramic composites. The matrices, which may be fabricated by slurry infiltration include the following: alumina (Al2O3), silica (SiO2), glass, mullite (3Al2O3*2SiO2), yttrium alumina garnet (Wessel, 2004), SiC, and silicon nitride (Si3N4). Slurry consists of ceramic particles dispersed in a carrier (water, alcohol), which may contain an organic binder and wetting agents. Slurry infiltration technique is similar to solegel infiltration; however, due to low content of solids in sols large shrinkage forms during solegel processing. It was shown by Sim and Kerans (2008) that higher (than in solegel method) packing density of 3D woven reinforced composites may be achieved by slurry infiltration if the slurry is well dispersed and contains agglomerate-free submicron particles. The density of a slurry-infiltrated composite may be further increased using an infiltration technique with a pressure gradient and alternating tape insertion between fabrics (Yonathan et al., 2009). SiC fiber-reinforced SiC matrix (SiC/SiC) composites with 0.2 mm interphase of pyrolytic carbon were fabricated by this method. The matrices with small porosity, appropriate SiC grain sizes, acceptable amount of residual oxides, and relatively low level of fiber damage were obtained in the composites fabricated by nano-infiltration and transient eutectic-phase (NITE) processing route (Katoha et al., 2004; Kohyama et al., 2008). NITE technique incorporates infiltration of carbon-coated fiber preform with nano-phase SiC powder-based mixed slurry followed by a pressure sintering of the matrix at a temperature slightly above melting point of the transient eutectic phase. An interesting technique of using slurry infiltration for fabrication porous alumina ceramic structure is presented by Liu and Miao (2005). A structure built of expanded polystyrene beds was infiltrated with well-dispersed alumina slurry and then heated in air to 1550 C (2822 F). During the heat treatment the slurry particles formed the ceramic matrix when the polystyrene beds burnt out and formed the porous structure.

5.8.2

Slurry infiltration process description

The principal scheme of slurry infiltration process of fabrication of ceramic composites is presented in Fig. 5.11. •

Slurry impregnation (infiltration). The fiber reinforcing material (tow, tape, weave) passes through a slurry, which wicks into the porous structure of the reinforcing phase. The process is performed at a normal pressure but it may also be vacuum/pressure assisted (Yonathan et al., 2009).

Advances in manufacture of ceramic matrix composites by infiltration techniques

Reinforcing fibers

Prepreg fabrication by slurry infiltration

111

Ceramic slurry

Preform fabrication (lay-up) Densification by hot pressing

Figure 5.11 Fabrication of ceramic matrix composites by slurry infiltration.

• •

Lay-up. At this stage the preform is fabricated. The impregnated fibers (prepreg) are wound onto a mandrel. After drying they are cut and laid up. The prepreg in the form of a woven cloth is laid up on a tooling (mold) or wrapped around it. Hot pressing (densification and consolidation). Densification is conducted at high temperature and under high pressure. The ceramic particles incorporated into the reinforcing structure consolidate due to the diffusion of the ceramic material between the particles. During the diffusion process the pores taking place in the preform diminish or even close up resulting in densification of the part.

5.8.3

Advantages and disadvantages of slurry infiltration

Advantages of slurry infiltration include the following: • •

Relatively low porosity; Good mechanical properties.

Disadvantages of slurry infiltration include the following: • • • •

High pressure of the hot pressing operation may damage the fibers; The ceramic particles may damage the fibers; Expensive equipment is needed for hot pressing; Large and complex shapes are difficult to fabricate.

5.9 5.9.1

Solegel infiltration Introduction

Solegel method utilizes infiltration of a reinforcing structure with a sol preceramic precursor. Sol is a colloidal dispersion of fine solid particles in a liquid. Sols used in solegel processes contain organometallic compound (e.g., alkoxides), which under particular conditions (e.g., elevated temperature) undergo cross-linking (polymerization) by either polycondensation or hydrolysis mechanism. During polymerization sol transforms into gelda polymer structure containing liquid. One of the remarkable

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properties of gels is their ability to convert into ceramics at relatively low temperature, which reduces the probability of the fiber damage. Solegel systems contain low amount of ceramic, therefore they undergo significant shrinkage after drying. To increase the densification of the matrix, the infiltratione drying cycle is repeated several times. The volumetric yield of ceramic of a solegel may be increased by an addition of ceramic particles, which also reduce the number and severity of drying cracks (MIL-HDBK-17-5, 2002). Commonly solegel infiltration is used for fabrication of continuous reinforcement composites, but the method can also be used for composites with particulate and short fiber-reinforcing phase. Fabrication of a discontinuous mullite fiberreinforced composite with ZrO2-10 wt%Y2O3 matrix by a solegel vacuum infiltration technique was reported by Dey et al. (2002). Solegel method fabrication of CMC with oxide matrix and Nextel (610 and 720) oxide fibers was developed by Machry et al. (2010). A combination of the solegel technique with the freeze gelation process was used for production of the ceramic matrix of this new composite. The material’s microstructure is presented in Fig. 5.12. The resulting composite demonstrated excellent oxidation stability and high thermal shock resistance. A wood structure may serve as a reinforcing phase for fabrication of ceramic composites by solegel infiltration. Klingner et al. (2003) used the technique for fabrication of SiC biomorphic composite. A wood-derived carbon template was prepared by pyrolysis of wood. Then the carbon template was infiltrated with a silica gel (SiO2). The resulting composite was converted into SiC ceramic via carbothermal reduction.

1616–12 Probe: N610/SL15/m

0

(µm)

20

Figure 5.12 Microstructure of an oxide ceramic matrix composite fabricated by solegel method. Courtesy C. Wilhelmi, EADS, Innovation Works, Germany.

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5.9.2

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Solegel infiltration process description

The principal scheme of solegel infiltration process of fabrication of ceramic composites is presented in Fig. 5.13. • • • • •

Prepreg fabrication. The reinforcing material is immersed into the sol, which wicks into the porous fibrous structure. Vacuum/pressure may be applied to assist the infiltration process. Lay-up. The prepreg is cut and laid up on a tooling. Gelation and drying. The sol is converted into gel when heated to 150 C (302 F). The gel is then dried at a temperature up to 400 C (752 F). Water, alcohol, and organic volatile components are removed from the material. Multiple reinfiltration and gelation. The sol infiltrationegelation cycle is repeated several times until the desired densification is achieved. Firing. The ceramic matrix is consolidated (sintered) at the firing temperature.

5.9.3

Advantages and disadvantages of solegel infiltration

Advantages of solegel infiltration include the following: • • • • •

Low processing temperature, which results in less fiber damage; Controllable matrix composition; Near-net shape fabrication, which reduces the machining cost; Low cost of the equipment; Large and complex parts may be fabricated.

Disadvantages of solegel infiltration: • • • •

Large shrinkage causing matrix cracking; Low ceramic yield, which requires repeated infiltrationegelation cycle; Low mechanical properties; High cost of sols.

Reinforcing fibers

Prepreg fabrication by sol infiltration

Sol

Preform fabrication (lay-up) Gellation and drying Sol reinfiltration

repeated cycles

Consolidation by firing

Figure 5.13 Fabrication of ceramic matrix composites by solegel infiltration.

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5.10 5.10.1

Advances in Ceramic Matrix Composites

Combined infiltration methods Combination of slurry infiltration with polymer infiltration and pyrolysis

To minimize the formation of shrinkage cracks during pyrolysis process and reduce number of infiltrationepyrolysis cycles the precursor polymer may be blended with particulate fillers (Naslain, 2001). Fabrication of SiC/SiC composite utilizing infiltration of allylhydriopolycarbosilane combined with 25 vol% of SiC powder (<1.8 mm size) is described by Hurwitz (1998). It was shown that pressure application increases the infiltration rate. Another approach was presented by Rak et al. (2000, 2001) and Rak (2004). Prior to the infiltration with a polymer the continuous carbon fiber matrix was infiltrated with a preceramic slurry containing fine SiC particles (average size of 0.5 mm). After the slurry infiltration the green body was dried and then calcinated at 400 C (752 F). A porous framework obtained after the calcination was then infiltrated with a liquid preceramic polymer and pyrolyzed in argon atmosphere. Low porosity (about 7%) and good tribological properties of the composite were obtained.

5.10.2

Combination of slurry infiltration with liquid silicon infiltration

In LSI method the SiC matrix forms in the chemical reaction of the silicon melt with a porous carbon structure obtained as a result of pyrolysis of a polymer precursor. The amount of the precursor and liquid silicon required for the matrix fabrication may be considerably reduced if the precursor contains particles of SiC. The SiC particles introduced into the free space between the reinforcing fibers also modify the matrix microstructure and shorten the processing time. It was shown by DiCarlo and Bansal (1998) that the composites prepared by this technique were characterized by high density and closed porosity. The SiC powder may be either blended with the precursor resin or infiltrated into the fiber preform in form of slurry prior to the resin impregnation. A fast method of fabrication of short carbon fiber-reinforced composite with SiC matrix obtained by microwave pyrolysis of resol resin blended with SiC powder followed by siliconizing (liquid silicone infiltration) is reported by Park et al. (2010b).

5.10.3

Combination of chemical vapor infiltration with liquid silicon infiltration

The method of CVI is commonly used for deposition of debonding and protective coatings (interphases) on the reinforcing fiber surface. Reinforcing fibers with CVI fabricated interphases are utilized in different matrix infiltration techniques: LSI, PIP, CVI, DIMOX, slurry, and solegel. However, CVI may also be used for preparation of porous carbon preform, which is then infiltrated with liquid silicon forming the composite matrix.

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Fabrication of carbon fiber-reinforced carbonesilicon carbide dual matrix composites by a combination of CVI with LSI was described by Zhuan et al. (2010). C3H6 gaseous precursor was used for obtaining porous pyrocarbon structure in the carbon fiber preform. The porous material was then infiltrated with silicon melt, which reacted with the pyrocarbon.

5.10.4 Combination of chemical vapor infiltration with polymer infiltration and pyrolysis The matrices produced by CVI method have the best mechanical and thermal properties; however, slow infiltration rate and high production cost limit the wide application of the method. In addition to this, the CVI-fabricated matrices contain 10%e15% of residual porosity. Bhatt and DiCarlo (2004) have achieved a significant improvement of thermomechanical properties of SiC matrix composites by a combination of CVI with other infiltration methods: PIP and LSI. In one of the methods the SiC matrix was partially fabricated by CVI and then the porous matrix structure was filled by repeated PIP.

5.11

Future trends in fabrication of ceramic matrix composites by infiltration methods

Long fiber-reinforced CMCs have been used since 1970s in many applications where a combination of good mechanical properties, low density, and thermal stability is demanded. New grades of reinforcing fibers, interphase compositions, liquid and gaseous precursors, and infiltration techniques for ceramic matrix formation have been developed and investigated for this period. Today the Research and Development works in this field are focused on further improvement of the ceramic composite properties via elaboration of the fabrication processes and creating composite materials with new compositions and microstructures of the matrix and the reinforcing phase. One of the most ambitious directions is a development of ultrahigh-temperature CMCs for the work temperatures above 1650 C (3002 F) and up to 3000 C (5432 F). Potential candidates for the matrices of such composites are MC or MB2 (M]Zr, Hf, Nb, or Ta) fabricated by the CVI (Naslain et al., 2010). SiC/SiC and C/SiC composites are prone to degradation in the combustion environment containing moisture (Wang et al., 2010). Environmental barrier coatings (EBC) are designed to inhibit the corrosive effect of the combustion gases. Normally silica is used for EBC. However, it volatilizes at increased temperatures (Lin, 2010). EBC with better environmental and thermal stability are required for composite parts operating in gas turbine engines (nozzle vanes, blades, combustor liners). Better durability in oxidizing atmosphere and low cost may be achieved by using oxideeoxide fiberreinforced ceramic composites (Janssen et al., 2010), which may replace in a future SiC composites in some applications.

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A ceramic matrix composition and the microstructure are determined by the preceramic precursors (polymer or gaseous). Commercial development of a broad range of high ceramic yield combined with low cost will enable new applications in the electronic, automotive, and aircraft industries (Beurich, 2010). In parallel to the development of new ceramic composites with enhanced mechanical and thermal properties the cost-effectiveness of the materials fabrication is being continuously improved. For example, shortening the CVI time (lower production cost) and reducing the residual matrix porosity may be achieved by incorporating SiC nanowires and whiskers in the reinforcing preform (Park et al., 2010a). Another example of lowering production cost is the fabrication of ceramic brake pads. Optimization of infiltration routes and use of short fiber reinforcement will significantly increase the friction applications of ceramic composites in the automotive industry.

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Singh, R.P., 2008. Novel Processing of Unique Ceramic-Based Nuclear Materials and Fuels. Report of School of Mechanical and Aerospace Engineering. Oklahoma State University. Singh, M., Behrendt, D.R., 1992. Studies on the Reactive Melt Infiltration of Silicon and SiliconMolybdenum Alloys in Porous Carbon. NASA, Glenn Research Center, Cleveland, OH, USA. Document ID: 19930003210. Singh, M., Levine, S.R., 1995. Low Cost Fabrication of Silicon Carbide Based Ceramics and Fiber Reinforced Composites. NASA, Glenn Research Center, Cleveland, OH, USA. Document ID: 19950025787. Singh, M., Petko, J.F., 2003. Prepreg and Melt Infiltration Technology Developed for Affordable, Robust Manufacturing of Ceramic Matrix Composites. NASA, Glenn Research Center, Cleveland, OH, USA, pp. 25e26. Report: Research & Technology. Singh, M., Dickerson, R.M., Olmstead Forrest, A., Eldridge, J.I., 1997. SiC (SCS-6) Fiber Reinforced-Reaction Formed SiC Matrix Composites: Microstructure and Interfacial Properties. NASA, Glenn Research Center, Cleveland, OH, USA. Document ID: 19970021683. Si’an Chen, Zhang, Y., Zhang, C., Zhao, D., Hu, H., Zhang, Z., 2013. Effects of SiC interphase by chemical vapor deposition on the properties of C/ZrC composite prepared via precursor infiltration and pyrolysis route. Mater. Des. 46, 497e502. Swaminathan, B., Painuly, A., Manwatkar, S.K., Packirisamy, S., 2010. Preceramic polymer derived C/C-SiC composites for high temperature applications. In: 7th International Conference on High Temperature Ceramic Matrix Composites (HT-CMC 7) Bayreuth, Germany, September 20e22. Wang, Y., Cheng, L., Zhang, L., 2010. Fabrication of carbon fiber reinforced ultrahigh temperature ceramic composites. In: 7th International Conference on High Temperature Ceramic Matrix Composites (HT-CMC 7) Bayreuth, Germany, September 20e22. Wessel, J.K., 2004. Continuous fiber ceramic composites. In: Handbook of Advanced Materials: Enabling New Designs. John Wiley & Sons, Inc., pp. 89e128 Wilson, D.M., 2006. New high temperature oxide fibers. In: Krenkel, W., Naslain, R., Schneider, H. (Eds.), High Temperature Ceramic Matrix Composites. Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, FRG. https://doi.org/10.1002/3527605622.ch1. Yonathan, P., Lee, J.-H., Yoon, D.-H., Kim, W.-J., Park, Ji-Y., November 2009. Improvement of SiCf/SiC density by slurry infiltration and tape stacking. Mater. Res. Bull. 44 (11), 2116e2122. Yoon, T.-H., Hong, L.-Y., Kim, D.-P., 2010. Fabrication of SiC-based ceramic microstructures from preceramic polymers with sacrificial templates and softlithography techniques. In: Wang, M. (Ed.), Lithography. InTech, Croatia, pp. 427e446. Zhuan, L., Peng, X., Xiong, X., Huang, Bo-Y., June 2010. Manufacture and properties of carbon fibre-reinforced C/SiC dual matrix composites. China N. Carbon Mater. 25 (3), 225e231.

Further reading Mucha, H., Langhof, N., Krenkel, W., 2010. Ceramic friction pads e manufacturing and performance. In: 7th International Conference on High Temperature Ceramic Matrix Composites (HT-CMC 7) Bayreuth, Germany, September 20e22.