Showing papers on "Ceramic matrix composite published in 1996"

Oxidation protection for carbon fibre composites

Abstract: Carbon fibre-reinforced ceramic matrix composites are promising candidate materials for high-temperature structural applications such as gas turbine blades. In oxidizing environments at temperatures above 400°C, however, carbon fibres are rapidly oxidized. There is, therefore, a need to coat the composite in order to protect it against oxidation. This review identifies the requirements of an effective oxidation protection system for carbon fibre-reinforced ceramics and summarizes the work which has been carried out towards this goal over the last 50 years. The most promising coatings are those composed of several ceramic layers designed to protect against erosion, spallation and corrosion, in addition to possessing a self-healing capability by the formation of glassy phases on exposure to oxygen.

Models of High‐Temperature, Environmentally Assisted Embrittlement in Ceramic‐Matrix Composites

Abstract: The intermediate-temperature oxidation embrittlement, or pest, effect found in ceramic-matrix composites (CMCs) is shown to have features analogous to stress corrosion cracking. The behavior involves crack growth upon oxidation of the fibers or the fiber coatings to form an oxide that weakens the fibers. It has reaction- and diffusion-controlled regimes. The former occurs at low stresses. The latter occurs at higher stresses. It is controlled by oxygen ingress through the matrix cracks. There is also a crack growth threshold. Expressions for the crack velocity above the threshold are derived as well as the stress dependence of the rupture life.

Oxidation of BN‐Coated SiC Fibers in Ceramic Matrix Composites

Abstract: Thermodynamic calculations were performed to analyze the simultaneous oxidation of BN and SiC. The results show that, with limited amounts of oxygen present, the formation of SiO{sub 2} should occur prior to the formation of B{sub 2}O{sub 3}. This agrees with experimental observations of oxidation in glass-ceramic matrix composites with BN-coated SiC fibers, where a solid SiO{sub 2} reaction product containing little or no boron has been observed. The thermodynamic calculations suggest that this will occur when the amount of oxygen available is restricted. One possible explanation for this behavior is that SiO{sub 2} formation near the external surfaces of the composite closes off cracks or pores, such that vapor phase O{sub 2} diffusion into the composite occurs only for a limited time. This indicates that BN-coated SiC fibers will not always oxidize to form significant amounts of a low-melting, borosilicate glass.

Composite material protected against oxidation by a self-healing matrix, and a method of manufacturing it

Abstract: A composite material protected by oxidation at intermediate temperatures not exceeding 850°C comprises fiber reinforcement densified by a matrix which includes at least one self-healing phase including a glass-precursor component such as B4C or an Si-B-C system, together with excess free carbon (C) at a mass percentage lying in the range 10% to 35%. The, or each, self-healing phase can be interposed between two ceramic matrix phases, e.g. of SiC. While the material is exposed to an oxidizing medium, oxidation of the free carbon promotes oxidation of the precursor and transformation thereof into a glass capable of plugging the cracks in the matrix by self-healing.

Processing, structure, and properties of mullite fiber/mullite matrix composites

Abstract: Oxide fiber/oxide matrix composites form an important and very attractive subpart of ceramic matrix composites because of their inherent stability in oxidizing atmospheres at high temperature. In particular, mullite fiber/mullite matrix composites have the potential of high temperature usage in oxidizing atmospheres. The interface in mullite fiber/mullite matrix was engineered by using thick BN (1 μm) or BN/SiC double coating on mullite fibers, such that deformation mechanisms conducive to toughness enhancement could be brought to play. Significant improvements in the room temperature mechanical properties of these mullite fiber/mullite matrix composites could be achieved by incorporation of these interfacial coatings and by using a colloidal processing route to make dense mullite matrix.

Multilayer dielectric stack coated part for contact with combustion gases

Abstract: A metal or ceramic matrix composite part and corresponding method are provided exhibiting desired heat transfer characteristics The part has a metal or ceramic matrix composite substrate and a multilayer dielectric coating The coating has high reflectivity at wave lengths corresponding to radiation wavelengths of various combustion gases and has low reflectance at radiation wavelengths corresponding to the substrate The multilayer coating allows the heat generated external of the part at wavelengths corresponding to combustion gases to be reflected from the part while permitting radiation wavelengths associated with the substrate to pass through the coating The parts are useful for use in combustive gas atmospheres

Residual thermal stresses in ceramic composites. Part I: with ellipsoidal inclusions

Abstract: Residual thermal stresses in ceramic matrix composites containing ellipsoidal inclusions are analyzed using a modified Eshelby model. Closed-form analytical solutions are obtained; however, their formulations are formidable. When the inclusion is disc-shaped, spherical, or fiber-shaped, simple analytical solutions can be obtained using different models, and they are in excellent agreement with those obtained from the modified Eshelby model. The analytical solutions are compared with the experimental and finite element results. Also, effects of the aspect ratio and the volume fraction of inclusions on residual thermal stresses are examined.

Abrasive ceramic matrix turbine blade tip and method for forming

Abstract: An abrasive coating suitable for forming an abrasive blade tip of a gas turbine engine. The coating is characterized as being capable of abrading a ceramic shroud at elevated temperatures during the in-service operation of the engine, and being resistant to oxidation and hot corrosion within the engine environment. The abrasive coating includes an MCrAl alloy layer, a ceramic layer overlying the alloy layer so as to form an outer surface of the abrasive coating, and abrasive particles dispersed between the alloy layer and the ceramic layer so that at least some of the abrasive particles are partially embedded in the alloy layer and also partially embedded in the ceramic layer. In addition, at least some of the abrasive particles project above the outer surface of the abrasive coating formed by the ceramic layer.

Hysteresis loops and the inelastic deformation of 0/90 ceramic matrix composites

Abstract: Hysteresis measurements obtained on 0/90 SiC/CAS and SiC/SiC have been used to analyze the interface responses. Four parameters have been derived from these measurements. These relate to the compliance change caused by matrix cracking, the frictional resistance of the interface, the interface debond resistance, and the residual stress. These parameters have been used to predict the stress/strain curves. Preliminary estimates of stress partitioning between the plies have been used to estimate constituent properties, such as the friction stress.

Ablation resistant zirconium and hafnium ceramics

Abstract: High temperature ablation resistant ceramic composites have been made. These ceramics are composites of zirconium diboride and zirconium carbide with silicon carbide, hafnium diboride and hafnium carbide with silicon carbide and ceramic composites which contain mixed diborides and/or carbides of zirconium and hafnium, along with silicon carbide.

Advances in ceramic composites reinforced by continuous fibers

Abstract: Ceramic matrix composites reinforced with continuous fibers are on the verge of insertion into hot engineering structures. Yet current research is only beginning to attack some of the most critical problems. Key developments in the last 24 months include the formulation of constitutive laws for continuum mechanics analyses; the discovery of stable weak oxide—oxide interface systems; the analysis of how fiber creep limits life at high temperatures; confrontation of the problem of oxidation pesting at intermediate temperatures in SiC based systems; re-examination of the maxim that interfaces must be weak; and the advent of textile reinforcement as the solution to delamination problems.

Synthesis ofin situ TiB2/TiN ceramic matrix composites from dense BN-Ti and BN-Ti-Ni powder blends

Abstract: In the present research, near-net-shapein situ TiB2/TiN and TiB2/TiN/Ni composites were fabricated from cold-sintered BN/Ti and BN/Ti/Ni powder blends by pressureless displacement reaction synthesis or thermal explosion under pressure. In both approaches, the processing or preheating temperatures (≤1200 °C) were considerably lower than those typical of current methods used for the processing/consolidation of ceramic matrix composites. Microstructural characterization of the materials obtained was performed using X-ray diffraction, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Mechanical properties were evaluated by measuring microhardness, fracture toughness, and three-point bending strength. Application of a moderate external pressure (≤250 MPa) during self-propagating synthesis (SHS) synthesis was shown to be sufficient to ensure full density of the TiB2/TiN/Ni composite. The entire procedure of thermal explosion under pressure could be performed in open air without noticeable oxidation damage to the final product. The high fracture toughness of thein situ synthesized TiB2/TiN/Ni composite (20.5 MPa√m) indicated that the finely dispersed ductile Ni phase was effective in dissipating the energy of cracks propagating in the ceramic matrix.

Ceramic matrix composites

Abstract: SiC—SiC coposites exhibit a non-brittle behaviour when the fibre—matrix bonding is controlled by the use of an interphase with a layered structure or microstructure, such as pyrocarbon, hex-BN or (SiC—PyC)n multilayers. The best mechanical properties are achieved when there is a balance between the crack deflection and load transfer functions of the interphase. SiC—SiC composites are sensitive to oxidation but their stability in oxidizing atmospheres can be improved by tailoring their composition and microstructure.

Effect of Loading Rate on the Monotonic Tensile Behavior of a Continuous-Fiber-Reinforced Glass-Ceramic Matrix Composite

Abstract: The stress-strain behavior of a continuous-fiber-reinforced ceramic matrix composite has been measured over a wide range of loading rates (0.01 to 500 MPa/s). It was found that the loading rate has a strong effect on almost every feature of the stress-strain curve: the proportionality stress, the composite strength and failure strain increase with increasing loading rate. The microstructural damage varies also with the loading rate; with increasing loading rate, the average matrix crack spacing increases and the average fiber pullout length decreases. Using simple models, it is suggested that these phenomena are caused partly by time-dependent matrix cracking (due to stress corrosion) and partly by an increasing interfacial shear stress with loading rate.

Thermal Shock Behavior of Two-Dimensional Woven Fiber-Reinforced Ceramic Composites

Abstract: The thermal shock behavior of three types of two-dimensional woven, continuous fiber-reinforced (Nextel{trademark} 312 (3M Co., St. Paul, MN) or Nicalon{trademark} (Nippon Carbon, Tokyo, Japan)) ceramic matrix (silicon carbide matrix that had been processed by chemical vapor infiltration or polymer impregnation and pyrolysis) composites was studied using the water-quench technique. Thermal-shock-induced damage was characterized by a destructive technique of four-point flexure and a nondestructive technique of Young`s modulus measurement by the dynamic resonance method. Compared with monolithic ceramics, the continuous fiber-reinforced ceramic composites were capable of preventing catastrophic failure that was caused by thermal shock. Analysis of the results that were based on the stresses that were generated by thermal shock and the mismatch of thermal expansion between fibers and matrices suggested possible mechanisms of the thermal shock damage. Preliminary results showed evidence of matrix cracking and delamination because of the thermal shock damage. The feasibility of using the nondestructive technique to detect thermal shock damage also was demonstrated.

Ferromagnetic nanocomposite films of cobalt in a ceramic matrix formed by thermal decomposition of cobalt nitride, CoN, precursor

Abstract: Cobalt nitride films, CoN, in a pure form and also as a nanocomposite in boron nitride or silicon nitride were generated by reactive sputtering of cobalt metal, cobalt boride, or cobalt silicide as targets, respectively, in a nitrogen plasma. Cobalt nitride decomposes into the elements by heating under vacuum at 500 °C. The nanostructure of the composites was preserved in the heating treatment thus creating a fine dispersion (<10 nm) of cobalt particles, in a ceramic matrix. The magnetic properties of the nanocomposites were established. The precursor cobalt nitride is paramagnetic while the cobalt dispersions, having dimensions smaller than single magnetic domain, show characteristics typical of those systems such as superparamagnetism and, at temperatures lower than the blocking temperature, marked hysteresis. The coercive fields at 5 K for the BN and Si3N4 nanocomposites are 3250 and 850 Oe, respectively. These films are of interest as data recording media.

CVD SiC fiber-reinforced barium aluminosilicate glass—ceramic matrix composites

Abstract: Unidirectional CVD SiC (SCS-6) monofilament reinforced BaO Al 2 O 3 2SiO 2 (BAS) glass—ceramic matrix composites have been fabricated by a tape lay-up method followed by hot pressing. The glass matrix flows around fibers during hot pressing resulting in nearly fully dense (95–98%) composites. Strong and tough composites having first matrix cracking stress of 250–300 MPa and ultimate flexural strength as high as 900 MPa have been obtained. Composite fracture surfaces showed fiber pullout with no chemical reaction at the fiber/matrix interface. From fiber push out, the fiber/matrix interfacial debond strength and the sliding frictional stress were determined to be 5.9 ± 1.2 MPa and 4.8 ± 0.9 MPa, respectively. The fracture surface of an uncoated SiC (SCS-0)/BAS composite also showed fiber/matrix debonding, fiber pullout, and crack deflection around the fibers implying that the SiC fibers may need no surface coating for reinforcement of the BAS glass-ceramic. Applicability of micromechanical models in predicting the first matrix cracking stress and the ultimate strength of these composites has also been examined.

Fiber reinforced ceramic matrix composite and method of manufacturing the same

Abstract: A fibre reinforced ceramic matrix composite is disclosed which comprises a ceramic matrix and a fibre preform integrally assembled therein, the fibre preform comprising a braided fabric derivable from braiding a yarn which is composed of a plurality of fibre bundles formed of a plurality of monofilaments bundled together, the braided fabric being present in a volume fraction of fibre (Vf) of 10 to 40% by volume based on the total volume of the ceramic matrix composite. The yarns (fibre bundles) may preferably be disposed in oblique relation to each other. Initial matrix cracking strength, crack propagation resistance and fracture energy can be significantly increased. Fibre preforms even of relatively complicated shape can be easily obtained and used in forming the matrix composite.

Damage and fracture of a cross woven C/SiC composite subject to compression loading

Abstract: In order to understand the mechanical behaviours of ceramic matrix composites under various loading conditions, the compression stress-strain behaviour of a cross woven C/SiC ceramic matrix composite was characterized in terms of damage and fracture mechanisms, which were observed by scanning electron microscope (SEM) on a side-polished sample. The compression stress-strain curve was found to consist of two stages. The first stage, which extends to about 300 MPa, covers linear elastic deformation with a compression modulus of 140 GPa. The second stage starts with the occurrence of compression damage modes, which include ply delamination, bundle separation and transverse bundle cracking. Depending on the local structure of the sample, the second stage of the stress-strain curve can be either mostly linear or non-linear. The fracture of the composite under compression is by kinking, shearing and bending fracture of fibre bundles individually or in groups, forming a macroscopic shear band in the sample.

Tensile properties of a SiCf/SiC composite

Abstract: This work examines the mechanical behavior of a 2D woven, 0–90 SiC fiber-reinforced SiC matrix composite. Tensile experiments show that the short-term behavior is largely independent of test temperature below 1000 °C. Microscopic examination reveals that the extent of fiber pull-out and the integrity of the remaining material are also independent of temperature in this range. Conversely, at 1200 °C, the material retains much of its low-temperature stiffness and proportional limit, while the strength increases substantially. Micrographs of these specimens reveal little individual fiber pull-out and a higher density of matrix microcracks. Room-temperature tensile data show that the mechanical behavior is rate-dependent; higher strain rates lead to a lower Young's Modulus, higher proportional limit and higher ultimate strength. In-plane shear experiments demonstrate that the unreinforced matrix strength is approximately 10% of the composite tensile strength.

Porous ceramic and process thereof

Abstract: A process for assembling a porous ceramic coating to a substrate comprising: forming a ceramic matrix tape including a first ceramic powder having a first full density sintering temperature, a second ceramic powder having a second full density sintering temperature and a fugitive filler material; placing the ceramic matrix tape on the substrate; and heating the ceramic matrix tape and substrate to a sintering temperature above the first full density sintering temperature and below the second full density sintering temperature, wherein the fugitive filler material decomposes during said heating.

Mechanical behavior of several hybrid ceramic-matrix-composite laminates

Abstract: Several different hybrid laminated composites comprised of alternating layers of dense ceramic sheets (either SiC or Si3N4) and fiber-reinforced ceramic-matrix-composite (CMC) layers (Nicalon fibers with either glass or glass-ceramic matrices) have been fabricated and characterized. The effects of the reinforcement architecture (unidirectional vs cross-ply) and the relative volume fractions of the phases on the tensile and flexural properties have been examined. Comparisons have been made with the properties of the constituent layers. Rudimentary models have been developed to describe the onset of cracking and for the minimum volume fraction of CMC required to develop multiple cracks and thus obtain a high failure strain.

Aluminium nitride-molybdenum ceramic matrix composites : Characterization of ceramic-metal interface

Abstract: Pure aluminium nitride can be hot pressed with an addition of molybdenum powder. With this technique we obtain a ceramic matrix composite having a dispersed metallic phase. Composites produced in this manner present a homogeneous structure with very little open porosity. Electrical resistivity measurements done over a series of composites show a sharp decrease in electrical resistivity when the molybdenum volume fraction in the material increases from 0.2 to 0.22. This value corresponds to the percolation threshold of metallic phase in the ceramic matrix. The interface between molybdenum particles and aluminium nitride grains is established through a fine oxide or oxynitride layer present at the surface of AlN grains.

Effects of off-axis loading on the tensile behavior of a ceramic-matrix composite

Abstract: A 0°/90° ceramic-matrix composite (CMC) comprised of Nicalon fibers in magnesium aluminosilicate (MAS) has been loaded in tension in three orientations relative to the fiber direction: 0°, 30°, and 45°. The off-axis loaded samples exhibit inelastic deformation at appreciably lower stresses than samples loaded at 0°. Matrix cracking governs the inelastic strains in all orientations. But, important differences in the morphologies and sequencing of the cracks account for the differences in the stress levels. Off-axis failure also occurs at substantially lower stresses than on-axis failure. On-axis composite failure is governed by fiber fracture, but off-axis failure involves matrix-crack coalescence. To facilitate interpretation and modeling of these behaviors, the interface friction and debond stresses have been determined from hysteresis measurements.

Fibrous composites including monazites and xenotimes

Abstract: Thermodynamically stable ceramic composites are provided for use in high temperature reactive environments. A phosphate selected from monazites and xenotimes functions as a weak bond material in the composite. Monazite comprises a family of minerals having the form MPO4, where M is selected from the larger trivalent rare earth elements of the lanthanide series (La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, and Tb) and coupled substituted divalents and tetravalents such as Ca or Sr with Zr or Th. Xenotimes are phosphates similar to monazite where M is selected from Sc, Y, and the smaller trivalent rare earth elements of the lanthanide series (Dy, Ho, Er, Tm, Yb, and Lu). High temperature ceramic composites that include monazite or xenotime and exhibit damage tolerant behavior or non-catastrophic fracture may be fabricated in a variety of material systems and reinforcement morphologies, including multilayered laminar composites; fiber, whisker, and particulate reinforced composites; hybrid laminar composites; and fabric or fiber preform composites. In preferred embodiments, the ceramic matrix comprises a material similar to the reinforcement to improve compatibility of the composite materials. The weak bond material allows debonding and "frictional" sliding between the constituents of the composite and inhibits crack growth across the interface.

Ceramic matrix composite/organic matrix composite hybrid fire shield

Abstract: The present invention is a ceramic matrix composite/organic matrix composite hybrid fire shield and the methods and processes for fabricating such a hybrid fire shield. The hybrid fire shield is comprised of an organic matrix composite substrate co-bonded with a cured-ceramic matrix composite layer made of a plurality of cured-ceramic matrix composite plies. First, the surface of the organic matrix composite is appropriately prepared to prevent delamination and enable proper bonding between the surface and the ceramic matrix composite layer. Second, the ceramic matrix composite layer is applied to the surface of the organic matrix composite. Next, the organic matrix composite with the applied ceramic matrix composite layer is processed to produce a co-bonded hybrid structure. The ceramic matrix composite layer is intended to be in direct contact with heat and flames. During impingement of the flames and heat, the cured-CMC layer pyrolyzes and converts from a polymeric to a ceramic composite material.