Showing papers on "Ceramic matrix composite published in 2001"

High performance oxide fibers for metal and ceramic composites

Abstract: A family of oxide fibers, Nextel™ 610 Ceramic Oxide Fiber, Nextel™ 720 Ceramic Oxide Fiber and a new fiber, Nextel™ 650 Ceramic Oxide Fiber, has been developed specifically for the reinforcement of metal and ceramic matrix composites. This paper summarizes room and high temperature properties for these fibers. The strength of both single filaments and multi-filament rovings of Nextel 610, 650 and 720 fibers was determined between 25 and 250 mm gauge length. Weibull analysis was used to compare the statistical fracture distribution and gauge length dependence of strength. Fiber fracture statistics were in accord with Weibull theory; the effect of diameter variability on the statistical analysis was found to be small. Fractographic analysis on Nextel 610 fiber was used to identify primary fracture-causing defects; defect size was correlated with Griffith fracture predictions. High temperature single filament strength measurements were performed on Nextel 610, 650 and 720 fibers between 800 and 1400°C. High temperature strength varied inversely with strain rate. In combination with tensile creep tests at 1100 and 1200°C, these were used to compare the elevated temperature capability of each fiber and determine maximum use temperatures. The development of crystalline yttrium aluminum garnet fibers that demonstrate further improvements in creep performance relative to Nextel 720 fibers is also discussed.

Semiconductor processing equipment having improved particle performance

Abstract: A ceramic part having a surface exposed to the interior space, the surface having been shaped and plasma conditioned to reduce particles thereon by contacting the shaped surface with a high intensity plasma. The ceramic part can be made by sintering or machining a chemically deposited material. During processing of semiconductor substrates, particle contamination can be minimized by the ceramic part as a result of the plasma conditioning treatment. The ceramic part can be made of various materials such as alumina, silicon dioxide, quartz, carbon, silicon, silicon carbide, silicon nitride, boron nitride, boron carbide, aluminum nitride or titanium carbide. The ceramic part can be various parts of a vacuum processing chamber such as a liner within a sidewall of the processing chamber, a gas distribution plate supplying the process gas to the processing chamber, a baffle plate of a showerhead assembly, a wafer passage insert, a focus ring surrounding the substrate, an edge ring surrounding an electrode, a plasma screen and/or a window.

Comparative study of high temperature composites

Abstract: Two classes of composite made using either ceramic matrix with high temperature fibers or carbon/carbon have been used for various applications that require high temperature resistance, over three decades. However, their use has been limited to special applications because of the high costs associated with fabrication. Typically the composites are cured at more than 1000°C, and in most instances the heating has also to be carried out in controlled environments. In addition, because of the high processing temperature, only certain type of expensive fibers can be used with the ceramic matrices. A recently developed inorganic matrix, called polysialate can be cured at temperatures less than 150°C, making it possible to use carbon and glass fibers. Composites made using carbon, glass and combinations of carbon and glass fibers have been tested in bending and tension. This paper presents the comparison of processing requirements and mechanical properties of carbon/carbon composites, ceramic matrix composites made with silicon carbide, silicon nitride and alumina fibers and carbon/polysialate composites. The results indicate that carbon/polysialate composite has mechanical properties comparable to both carbon/carbon and ceramic matrix composites at room and high temperatures. Since the polysialate composites are much less expensive, the authors believe that it has excellent potential for more applications in aerospace, automobile and naval structures.

Ceramic matrix nanocomposites containing carbon nanotubes for enhanced mechanical behavior

Abstract: A ceramic matrix nanocomposite having enhanced mechanical behavior is made up of a nanotube filler composed of at least one nanotube material, and a ceramic matrix composed of a nanocrystalline ceramic oxide. A method for producing ceramic articles having improved fracture toughness includes combining of a nanotube filler made up of a nanotube material and a ceramic matrix made up of a nanocrystalline ceramic oxide, forming an article therefrom, and sintering the article under elevated pressure at elevated temperature.

Combustor having a ceramic matrix composite liner

Abstract: A combustor having liners made from ceramic matrix composite materials (CMC's) that are capable of withstanding higher temperatures than metallic liners. The ceramic matrix composite liners are used in conjunction with mating components that are manufactured from superalloy materials. To permit the use of a combustor having liners made from CMC materials in conjunction with metallic materials used for the mating forward cowls, and aft seals with attached seal retainer over the broad range of temperatures of a combustor, the combustor is designed to allow for the differential thermal expansion of the differing materials at their interfaces in a manner that does not introduce stresses into the liner as a result of thermal expansion and also balances the flow of cooling air as a result of the thermal expansion.

Use of electrophoretic deposition in the processing of fibre reinforced ceramic and glass matrix composites: a review

Abstract: Electrophoretic deposition (EPD) is a simple and cost-effective method for fabricating high-quality ‘green’ composite bodies which, after a suitable high-temperature treatment, can be densified to a composite with improved properties. In this contribution, we describe the use of EPD technique in the fabrication of fibre reinforced composites, with an emphasis on composites with glass and ceramic matrices containing metallic or ceramic fibre fabric reinforcement. EPD has been used to infiltrate preforms with tight fibre weave architectures using different nanosized ceramic particles, including silica and boehmite sols, as well as dual-component sols of mullite composition. The principles of the EPD technique are briefly explained and the different factors affecting the EPD behaviour of ceramic sols and their optimisation to obtain high infiltration of the fibre preforms are considered. In particular, the EPD fabrication of a model alumina matrix composite reinforced by Ni-coated carbon fibres is presented. The pH of the solution and the applied voltage and deposition time are shown to have a strong influence on the quality of the infiltration. Good particle packing and a high solids-loading were achieved in most cases, producing a firm ceramic deposit which adhered to the fibres. Overall, the analysis of the published data and our own results demonstrate that EPD, being simple and inexpensive, provides an attractive alternative for ceramic infiltration and coating of fibre fabrics, even if they exhibit tight fibre weave architectures. The high-quality infiltrated fibre mats are suitable prepregs for the fabrication of advanced glass and ceramic matrix composites for use in heat-resistant, structural components.

High Thermal Conductivity Silicon Nitride Ceramic

Abstract: Since the confirmation that nonmetallic single crystals with a diamond-like structure, such as SiC, BP, and AlN, have high intrinsic thermal conductivities of over 300 W m−1 K−1,1,2 a great deal of effort has been focused on the development of nonoxide polycrystalline ceramics with high thermal conductivity.

Mechanical properties and tribology of Si3N4–TiB2 ceramic composites produced by hot pressing and hot isostatic pressing

Abstract: Ceramic matrix composites containing TiB2 as a particulate phase have been produced by hot pressing and hot isostatic pressing. The problems of the reactivity of TiB2 with the Si3N4 matrix and with the sintering environment have been successfully addressed. A novel dual atmosphere sintering profile combined with low temperature hot pressing has been used to successfully produce fully dense materials. The effect of TiB2 addition on mechanical properties has been investigated and the tribological behaviour of optimised compositions has been examined using a ball-on-disc apparatus. Wear coefficients and friction coefficients for Si3N4–TiB2 composites have been measured and compared to monolithic Si3N4 materials. Composites containing TiB2 show significant improvements in hardness, fracture toughness and wear whilst also exhibiting electrical conductivity.

Ceramic Matrix Composites: Matrices and Processing

Abstract: Ceramic matrix composites (CMCs) generally consist of ceramic fibers or whiskers in a ceramic matrix. CMCs are designed to overcome the main drawback of monolithic ceramics, namely their brittleness. They are referred to as inverse composites, which is to say that the failure strain of the matrix is lower than the failure strain of the fibers, whereas it is the reverse in most polymer or metal matrix composites. Hence, under load it is the matrix which fails first. In order to prevent an early failure of the brittle fibers when the matrix starts to microcrack, the fiber/matrix (FM) bonding should be controlled during processing. CMCs are tough materials and display a high failure stress when the FM bonding is not too strong or too weak, which is usually achieved through the use of a fiber coating referred to as the interphase. The fabrication of CMCs requires specific processing techniques. Gas-or liquid-phase routes (or a combination of both), in which the interphase and the matrix are formed around the fibers from gaseous or liquid precursors, are usually preferred. CMCs are used as thermostructural materials under severe service conditions, for example, high temperatures under load and in corrosive atmospheres, such as combustion gases. The most commonly used CMCs are nonoxide CMCs, namely carbon/carbon (C/C), carbon/silicon carbide (C/SiC), and silicon carbide/silicon carbide (SiC/SiC), the fibers being specified first.

Mechanical Properties of Two Plain‐Woven Chemical Vapor Infiltrated Silicon Carbide‐Matrix Composites

Abstract: The elastic and inelastic properties of a chemical vapor infiltrated (CVI) SiC matrix reinforced with either plain-woven carbon fibers (C/SiC) or SiC fibers (SiC/SiC) have been investigated. It has been investigated whether the mechanics of a plain weave can be described using the theory of a cross-ply laminate, because it enables a simple mechanics approach to the nonlinear mechanical behavior. The influences of interphase, fiber anisotropy, and porosity are included. The approach results in a reduction of the composite system to a fiber/matrix system with an interface. The tensile behavior is described by five damage stages. C/SiC can be modeled using one damage stage and a constant damage parameter. The tensile behavior of SiC/SiC undergoes four damage stages. Stiffness reduction due to transverse cracks in the transverse bundles is very different from cross-ply behavior. Compressive failure is initiated by interlaminar cracks between the fiber bundles. The crack path is dictated by the bundle waviness. For SiC/SiC, the compressive behavior is mostly linear to failure. C/SiC exhibits initial nonlinear behavior because of residual crack openings. Above the point where the cracks close, the compressive behavior is linear. Global compressive failure is characterized by a major crack oriented at a certain angle to the axial loading. In shear, the matrix cracks orientate in the principal tensile stress direction (i.e., 45° to the fiber direction) with very high crack densities before failure, but only SiC/SiC shows significant degradation in shear modulus. Hysteresis is observed during unloading/reloading sequences and increasing permanent strain.

Ceramic matrix composite having improved interlaminar strength

Abstract: A ceramic matrix composite material (10) having a plurality of interlaminar stitches (16) as shown in figure. The stitches are formed by directing laser energy into the material to melt and recast zones of the material in a direction transverse to the layers of reinforcing fibers (12). The stitches not only improve the interlaminar strength of the material, but they also increase the through-thickness thermal conductivity of the material, thereby reducing thermal-induced stresses. The zones of recast material (18) may define holes (20) extending at least partially through the thickness of the material. The holes may be filled with a filler material (24), thereby mitigating any adverse loss-of-area effect created by the holes.

Flexural properties of a hybrid polymer matrix composite containing carbon and silicon carbide fibres

Abstract: The flexural properties of hybrid unidirectional fibre reinforced polymer (FRP) composites containing a mixture of carbon (C) and silicon carbide (SiC) fibres were evaluated at span-to-depth (S/d) ratios of 16, 32, and 64. The flexural strength generally increased with increasing S/d ratio with a maximum value of 2316 MPa being achieved for the specimen with nominally equal volume fractions of C and SiC fibre. However, even replacing 12.5 vol% of the C fibres by SiC fibres increased the flexural strength by 22%. The mechanical property most strongly influenced by the incorporation of SiC fibres was the work of fracture with a maximum value of 206.5 kJ m-2 (compared to 78.8 kJ m-2 for the specimen containing only C fibres). First estimate values for the SiC fibre compressive strength, elastic modulus, and strain to failure were 3.46 GPa, 157 GPa, and 0.018, respectively.

Analytical modelling of 3-3 piezoelectric composites

Abstract: A piezoelectric composite of the ‘3-3’ type consists of interpenetrating active piezoceramic and passive polymer phases. Applications for these materials include low frequency hydrophones, due to the improved properties and figures of merit under hydrostatic conditions. Previous research on analytical modelling 3-3 materials has considered polymer infiltrated equiaxed open pores in a piezoelectric ceramic and has assumed complete stress transfer of an applied load into the stiffer ceramic matrix. This research extends the analytical modelling of 3-3 piezocomposites to include the load experienced by the polymer phase. The model is a useful aid to fabricate and optimise piezocomposite 3-3 structures for specific applications.

Structure and high-temperature stability of compositionally graded CVD mullite coatings

Abstract: Dense, uniform and crack-free mullite (3Al 2 O 3 ·2SiO 2 ) coatings were deposited on SiC by chemical vapor deposition. The coatings were compositionally graded, with the Al/Si ratio increasing towards the outer surface of the coatings for improved corrosion resistance. The coatings were found to start out as a nanocrystalline layer, which is an intimate mixture of γ-Al 2 O 3 nanocrystallites imbedded in a vitreous silica-rich matrix at the substrate/coating interface. Mullite grains nucleated when the surface composition of the growing coating was in a narrow range close to that of stoichiometric mullite. The phase transformations occurring in these coatings during high-temperature anneals in the range 1100–1400 °C were studied. These phase transformations, which include a tetragonal-to-orthorhombic transformation, mullitization and devitrification of silica in the nanocrystalline layer, and α-alumina precipitation and twinning of the alumina-rich mullite, are discussed in light of the adhesion and corrosion resistance of the coatings.

Microstructural finite-element modelling of a ceramic matrix composite to predict experimental measurements of its macro thermal properties

Abstract: A new experimental test rig has been used to measure the thermal diffusivity of a fibre-reinforced ceramic matrix composite material (C/C-SiC) using laser pulse heating under conditions of both one-dimensional and three-dimensional heat transfer. To predict these measurements, a microstructural geometric unit cell model of the material has been developed from optical micrographs and which incorporates fibre tows, matrix and interface materials. With the selection of a suitable finite-element mesh to model each material phase and the prescription of appropriate boundary conditions, this model has then been analysed for conditions of steady-state and transient heat conduction. In this way macro-thermal properties have been calculated from the micro-thermal properties of each phase of the composite. The micro-thermal properties deduced from the analyses for these thermal conditions are in close mutual agreement, and within the bounds of numerical error. These results are presented in this paper alongside the experimental measurements. It is concluded that with careful geometric modelling and sensible property data selection, the micro-thermal properties obtained from the unit cell model can accurately predict the experimentally measured macro-thermal properties.

Method of forming cooling holes in ceramic matrix composite turbine components

Abstract: A method for producing apertures in hot section components of gas turbine engines made from ceramic matrix composites that have at least one oxidizable component. The method involves forming the apertures using a laser beam controlled by parameters that ablate the ceramic matrix composite in the path of the beam, while simultaneously heating the matrix material, SiC or SiN, to a sufficient temperature to oxidize it to form a silica. Sufficient heat is supplied by the beam to melt the silica to cause it to flow. The melted silica is quickly solidified as recast silica along the walls of the newly created aperture before it has an opportunity to flow and form undesirable geometries. The wall of the aperture is formed of recast silica that is a smooth surface and that forms an oxidation barrier to inhibit any further oxidation of the underlying composite as it is exposed to the high temperatures and oxidative, corrosive atmosphere of an operating gas turbine.

Environmental Barrier Coatings for Silicon‐Based Ceramics

Abstract: Silicon-based ceramics, such as SiC fiber-reinforced SiC (SiC/SiC ceramic matrix composites (CMC) and monolithic silicon nitride (Si3N4), are prime candidates for hot section structural components of next generation gas turbine engines. Silicon-based ceramics, however, suffer from rapid surface recession in combustion environments due to volatilization of the silica scale via reaction with water vapor, a major product of combustion. Therefore, application of silicon-based ceramic components in the hot section of advanced gas turbine engines requires development of a reliable method to protect the ceramic from environmental attack. An external environmental barrier coating (EBC) is considered a logical approach to achieve protection and CP long-term stability. The first generation EBC consisted of two layers, mullite (3Al2O3-2SiO2) bond coat and yttria-stabilized zirconia (YSZ, ZrO2-8 Wt.% Y2O3) top coat. Second generation EBCs, with substantially improved performance compared with the first generation EBC, were developed in the NASA High Speed Research-Enabling Propulsion Materials (HSR-EPM) Program. The first generation EBC consisted of two layers, mullite (3Al2O3-2SiO2) bond coat and yttria-stabilized zirconia (YSZ, ZrO2-8 wt.% Y2O3) top coat. Second generation EBCs, with substantially improved performance compared with the first generation EBC, were developed in the NASA High Speed Research-Enabling Propulsion Materials (HSR-EPM) Program (5). They consist of three layers, a silicon first bond coat, a mullite or a mullite + BSAS (BaO(1-x)-SrO(x)-Al2O3-2SiO2) second bond coat, and a BSAS top coat. The EPM EBCs were applied on SiC/SiC CMC combustor liners in three Solar Turbines (San Diego, CA) Centaur 50s gas turbine engines. The combined operation of the three engines has accumulated over 24,000 hours without failure (approximately 1,250 C maximum combustor liner temperature), with the engine in Texaco, Bakersfield, CA, accumulating about 14,000 hours. As the commercialization of Si-based ceramic components in gas turbines is on the horizon, a major emphasis is placed on EBCs for two reasons. First, they are absolute necessity for the protection of Si-based ceramics from water vapor. Second, they can enable a major enhancement in the performance of gas turbines by creating temperature gradients with the incorporation of a low thermal conductivity layer. Thorough understanding of current state-of-the-art EBCs will provide the foundation upon which development of future EBCs will be based. Phase stability and thermal conductivity of EPM EBCs are published elsewhere. This paper will discuss the chemical/environmental durability and silica volatility of EPM EBCs and their impact on the coating's upper temperature limit.

Fracture Behavior of SiC-Whisker-Reinforced Barium Aluminosilicate Glass-Ceramic Matrix Composites

Abstract: Barium aluminosilicate (BAS) glass-ceramic composites reinforced with various volume percents (0, 10, 20, 30, 40 vol%) of SiC whiskers were fabricated by hot pressing. The microstructure, the whisker/matrix interface structure, the phase constitution, and the mechanical properties of the composites were systematically studied by means of SEM, TEM, and XRD techniques as well as by indentation crack microfracture and single-edge-notched-beam bend testing. It was demonstrated that the incorporation of SiC whiskers could significantly increase the flexural strength and fracture toughness of BAS glass-ceramic matrices. The addition of active Al 2 O 3 to the BAS matrix reduced the amount of SiO 2 in the matrix, forming needlelike mullite, which further improved the mechanical properties.

Modelling of functionally graded materials by numerical homogenization

Abstract: In this contribution, the mechanical behaviour of different ZrO2/NiCr 80 20 compositions is analysed and compared with experimental findings. The microwave-sintered material is found to possess a slightly dominant ceramic matrix for intermediate volume fractions. Its thermal expansion coefficient deviates from the rule of mixture. The modulus and the stress strain behaviour can be simulated by a numerical homogenization procedure, and the influence of residual stresses is found to be negligible. A newly introduced parameter (matricity) describes the mutual circumvention of the phases and is found to strongly control the stress level of the composite, globally as well as locally. Finally, a graded component and a metal/ceramic bi-material are compared for thermal as well as mechanical loading.

Fracture Toughness and Reliability in High-Temperature Structural Ceramics and Composites: Prospects and Challenges for the 21st Century

Abstract: The importance of high fracture toughness and reliability in Si3N4, and SiC-based structural ceramics and ceramic matrix composites is reviewed. The potential of these ceramics and ceramic matrix composites for high temperature applications in defense and aerospace applications such as gas turbine engines, radomes, and other energy conversion hardware have been well recognized. Numerous investigations were pursued to improve fracture toughness and reliability by incorporating various reinforcements such as particulate-, whisker-, and continuous fiber into Si3N4 and SiC matrices. All toughening mechanisms, e.g. crack deflection, crack branching, crack bridging, etc., essentially redistribute stresses at the crack tip and increase the energy needed to propagate a crack through the composite material, thereby resulting in improved fracture toughness and reliability. Because of flaw insensitivity, continuous fiber reinforced ceramic composite (CFCC) was found to have the highest potential for higher operating temperature and longer service conditions. However, the ceramic fibers should display sufficient high temperature strength and creep resistance at service temperatures above 1000 'C. The greatest challenge to date is the development of high quality ceramic fibers with associate coatings able to maintain their high strength in oxidizing environment at high temperature. In the area of processing, critical issues are, preparation of optimum matrix precursors, precursor infiltration into fiber array, and matrix densification at a temperature, where grain crystallization and fiber degradation do not occur. A broad scope of effort is required for improved processing and properties with a better understanding of all candidate composite systems.

Production and EDM of Si3 N4–TiB2 ceramic composites

Abstract: Ceramic matrix composites containing TiB2 as a particulate phase have been produced by hot pressing. The problems of the reactivity of TiB2 with the Si3 N4 matrix and with the sintering environment have been successfully addressed. A novel dual atmosphere sintering profile combined with low temperature hot pressing has been used to successfully produce fully dense materials. The effect of TiB2 addition on mechanical properties has been investigated. Composites containing TiB2 show significant improvements in hardness and fracture toughness. The addition of TiB2 at a level of 40 vol.-% raises the conductivity of the composite to a level where electro-discharge machining (EDM) is possible. A comprehensive study of the application of EDM has been carried out and the optimum machining conditions have been identified. Under these conditions Si3 N4–TiB2 composites have been shaped with relative ease and proved to be ‘robust’ materials under EDM.