Showing papers on "Ceramic matrix composite published in 1991"

Theory of mechanical-properties of ceramic-matrix composites

Abstract: A theory is presented to predict the pullout work and ultimate tensile strength of ceramic-matrix composite (CMC) materials tested under uniaxial tension as functions of the underlying material properties. By assuming that the fibers fracture independently and that global load redistribution occurs upon fiber fracture, the successive fragmentation of each fiber in the multifiber composite becomes identical to that of a single fiber embedded in a homogeneous large-failure-strain matrix, which has recently been solved exactly by the present author. From single-fiber fragmentation, the multifiber composite distribution of pullout lengths, work of pullout, and ultimate tensile strength are easily obtained. The trends in these composite properties as a function of the statistical fiber strength, the fiber radius and fill fraction, and the sliding resistance-tau-between the fibers and the matrix easily emerge from this approach. All these properties are proportional to a characteristic gauge length-delta-c and/or the associated characteristic stress-sigma-c, with proportionality constants depending only very weakly on the fiber Weibull modulus: the pullout lengths scale with delta-c, the work of pullout scales with sigma-c-delta-c, and the ultimate strength scales with sigma-c. The key length-delta-c is the generalization of the "critical length," defined by Kelly for single-strength fibers, to fibers with a statistical distribution of strengths. The theory also provides an interpretation of fracture-mirror measurements of pulled-out fiber strengths so that the in situ key strength sigma-c and Weibull modulus of the fibers can be determined directly. Comparisons of the theoretical predictions of the ultimate tensile strength to literature data on Nicalon/lithium aluminum silicate (LAS) composites generally show good agreement.

Boron Nitride Interphase in Ceramic‐Matrix Composites

Abstract: A BN interphase has been deposited, by isothermal/isobaric chemical vapor infiltration (ICVI) from BF3─NH3, within a preform made from ex-polycarbosilane (ex-PCS) fibers, at about 1000°C. In a second step, the BN-treated preform was densified with SiC deposited from CH3SiCl3─H2 at about the same temperature. From a thermodynamic standpoint, ex-PCS fibers could be regarded as unreactive vs the BF3─NH3 gas phase assuming they are coated with a thin layer of carbon or/and silica. The as-deposited interphase consists of turbostratic BN (N/B < 1) containing oxygen. The SiC infiltration acts as an annealing treatment: (i) the BN interphase becomes almost stoichiometric and free of oxygen; (ii) the fibers undergo a decomposition process yielding a SiO2/C layer at the BN/fiber interface. The weaker link in the interfacial sequence seems to be the BN/SiO2 interface. Deflection of microcracks arising from the failure of the matrix takes place at (or nearby) that particular interface.

Cyclic Fatigue of Ceramics: A Fracture Mechanics Approach to Subcritical Crack Growth and Life Prediction

Abstract: Cyclic fatigue, and specifically fatigue-crack propagation, in ceramic materials is reviewed both for monolithic and composites systems. In particular, stress/life (S/N) and crack-propagation data are presented for a range of ceramics, including zirconia, alumina, silicon nitride, SiC-whisker-reinforced alumina and a pyrolytic-carbon/graphite laminate. S/N data derived from unnotched specimens often indicate markedly lower lives under tension-compression compared to tension-tension loading; similar to metals, 108-cycle “fatigue limits” generally approach -50% of the tensile strength. Crack-growth results, based on studies on “long” (>3mm) cracks, show fatigue-crack propagation rates to be markedly power-law dependent on the applied stress-intensity range, ΔK, with a threshold, ΔKTH, of the order of -50% of Kc. Conversely, for “small” (<250μm) surface cracks, fatigue-crack growth is seen to occur at ΔK levels some 2 to 3 times smaller than ΔKTH, and to show a negative depen-dency on applied stress intensity. At ambient temperatures, lifetimes are shortened and crack-growth rates are significantly accelerated by cyclic, compared to quasi-static loading, although limited data suggest the reverse to be true at very high temperatures in the creep regime. Such results are discussed in terms of the primary crack-tip shielding (toughening) mechanisms and potential mechanisms of cyclic crack advance. Finally, implications are discussed of long and small crack cyclic fatigue data to life prediction and safetycritical design of ceramic components.

Composition-microstructure-property relationships in ceramic monofilaments resulting from the pyrolysis of a polycarbosilane precursor at 800 to 400 °C

Abstract: A 15 μm monofilament was extruded from a Yajima's type molten polycarbosilane, stabilized by addition of oxygen and heat-treated at 800 to 1400 °C under an argon atmosphere. Two important phenomena occur during pyrolysis. At 500 to 750 °C, an organic-inorganic state transition takes place with a first weight loss. It yields an amorphous material stable up to about 1100 °C. At this temperature, its composition is close to Si4C5O2. It can be described as a continuum of SiC4 and/or SiC4−xOx tetrahedral species (and possibly contains free carbon), with a homogeneity domain size less than 1 nm. The amorphous filament exhibits a high strength and semi-conducting properties. Above 1200 °C, a thermal decomposition of the amorphous material takes place with an evolution of gaseous species thought to be mainly SiO and CO, an important cross-section shrinkage and the formation of 7 to 20 nm SiC crystals which are surrounded with a poorly organized turbostratic carbon. The amorphous-crystalline state transition results in a drop in the tensile failure strength and an increase, by four orders of magnitude, in the electrical conductivity which becomes temperature independent. The former effect is due to the crystallization of the filament and the latter to a percolation phenomenon related to the intergranular carbon. The low stiffness is also due to the presence of carbon. It is anticipated that this transition is mainly related to the decomposition of the silicon oxycarbide species. Finally, a 40 to 50 nm layer of turbostratic carbon is formed at the filament surface at 1200 to 1400 °C whose origin remains uncertain. It is thought to be mainly responsible for the formation of the carbon interphase in the high-temperature processing of ceramic matrix composites.

Fractography of Glasses and Ceramics II

Abstract: Topics addressed include finite element stress analysis and crack path prediction of imploding CRT; fractography and fracture mechanics of combustion growth diamond thin films; the fracture behavior of machineable hydroxyapatite; a fractal approach to crack branching (bifurcation) in glass; the fracture of glass-ionomer cements; the effect of quartz particle size on the strength and toughness of whitewares; and a proposed standard practice for fractographic analysis of monolithic advanced ceramics. Also treated are thermal exposure effects on ceramic matrix composites, fractography applied to rock core analysis, fractography of flexurally fractured glass rods, the fractographic determination of K(IC) and effects of microstructural effects in ceramics.

Novel reinforced ceramics and metals: a review of Lanxide's composite technologies

Abstract: Lanxide Corporation is developing and commercializing novel technologies for ceramic matrix and metal matrix composites. The ceramic composite technology is based on the use of a unique directed-metal-oxidation process to grow ceramic matrices around pre-placed composite fillers or reinforcements. The metal matrix composites are made by a pressureless molten metal infiltration process involving excellent wetting of the reinforcement by the matrix alloy. This paper provides an overview of the processing methods, a description of some of the composites that have been made and their key properties, and an indication of some of the applications being addressed with these materials.

The mechanical properties of reinforced ceramic, metal and intermetallic matrix composites

Abstract: A perspective is presented concerning the mechanical properties of reinforced metal, intermetallic and ceramic matrix composites. The tensile properties in the longitudinal and transverse orientations are compared. Also, various modes of matrix cracking, subject to monotonic and cyclic loading, are described. Each behavior is related to such key variables as the interface properties, the matrix toughness and the thermal expansion misfit between the fiber and matrix. An attempt is made to provide a unified approach with emphasis on comparisons between different classes of composites.

Ceramic-Matrix Composites

Abstract: Introduction and basic principles, R.Warren and R.Lundberg fibres, A. Bunsell processing principles, G.C.Eckold principles of composite properties, R.Carlsson designing with ceramic composites, R.Warren particulate composites, V.Sarin and S.Buljan long-fibre reinforced ceramics, C.Philips CVI-composites, R.Naslain short-fibre and whisker reinforced ceramics, J.Homeny.

Experimental examination of the push-down technique for measuring the sliding resistance of silicon carbide fibers in a ceramic matrix

Abstract: The use of a push-down technique to measure the sliding resistance of NICALON fibers (SiC) in a lithium aluminosilicate (LAS) matrix has been examined experimentally. Tests have been conducted on more than 300 fibers in 2 different samples of the as-processed SiC/LAS III composite. Results show that the push-down measurements on an individual sample are reproducible, that the sliding resistance can vary significantly between samples, and that Poisson's expansion does not affect push-down measurements of fibers in this system.

Oxidation resistant ceramic matrix composites

Abstract: Ceramic matrix composites having a boron containing oxygen-scavenging sealant-forming region coextensive with the internal fibers and interface debonding layer and a ceramic matrix material provide superior resistance to oxidation.

The role of 90° domain wall displacements in forming physical properties of perovskite ferroelectric ceramics

Abstract: A proposed model which takes into account an elastic interaction between a polydomain grain and a surrounding ceramic matrix under weak external mechanical stresses has been used for evaluating a contribution from 90° domain wall displacements to piezoelectric and elastic constants of the perovskite ferroelectric ceramics. A difference between experimental and averaged constants of BaTiO3 ceramics can be explained by the estimated contribution.

Mode I Fracture Resistance of a Laminated Fiber-Reinforced Ceramic

Abstract: The mode I fracture resistance of a ceramic matrix composite has been measured. Simultaneous observations have revealed that the resistance is dominated by frictional dissipation upon the pullout of fibers that fracture in the crack wake off the crack plane. Numerical and analytical crack growth simulations have been compared with the experimental results. One important feature in this comparison concerns the occurrence of large-scale bridging. With these effects taken into account, the simulations and the experiments are found to be in good correspondence for acceptable magnitudes of the interface sliding stress.

Friction and Wear Characteristics of Silicon Nitride-Graphite and Alumina-Graphite Composites©

Abstract: To take advantage of the beneficial properties of advanced ceramics, it is necessary that their friction coefficient be reduced to an acceptable value. One method for achieving this goal is incorporation of a solid lubricant phase in the ceramic matrix. In this study, ceramic-matrix composites were fabricated by drilling a series of small holes in alumina and silicon nitride, which were subsequently filled with NiCl2 intercalated graphite under high pressure. It was found that the addition of graphite to silicon nitride considerably reduces the friction coefficient, but alumina-graphite composites exhibit only a marginal reduction in friction coefficient compared to alumina. The reduction in friction coefficient for silicon nitride-graphite composite can be explained by the formation of transfer films consisting of a mixture of materials from both contacting surfaces. However, for alumina-graphite composites the graphite regions were completely covered with steel wear particles, inhibiting the formation o...

Designing Interfaces in Inorganic Matrix Composites

Abstract: The key to controlling and predicting the properties of metal matrix composites lies in understanding and controlling the interface. When properly designed, the interface between reinforcing fibers and the matrix or protective coating can act as a mechanical fuse through a controlled delamination mechanism. Controlled delamination, in effect, results in the decoupling of fibers from early damage due to stress concentrations in the vicinity. The delamination event must precede the crack bridging and frictional pull-out mechanisms that have been so effectively demonstrated in ceramic matrix composites. The delamination event, therefore, is the necessary precondition, and so analysis of composite toughening must start with a definition of the conditions for interface debonding. This decoupling can be expressed in terms of cohesive strength of the interface, shear strength of the interface, and fiber fracture stress. In a related but alternative manner, debonding can be expressed in terms of the intrinsic work of fracture of the interface as compared to the transverse work of fracture of the fiber.

Effect of thermochemical treatments on the strength and microstructure of SiC fibres

Abstract: The room-temperature strength of commercially available polymer-derived SiC fibres degrades during the typical high-temperature thermal cycle used in ceramic matrix composite fabrication. Substantial improvements in retained room-temperature strength for two different commercially available fibres were observed after annealing in carbon powder at temperatures up to 1600 °C. Further improvements in strength were observed for both fibres when heat treated in CO atmospheres. X-ray diffraction, TEM, SEM, auger electron spectroscopy, and optical microscopy were used to characterize the microstructure and chemistry of these heattreated fibres in order to understand better the degradation mechanisms of the fibres as well as their improved strength retention.

An approach to the mechanical behaviour of SiC/SiC and C/SiC ceramic matrix composites

Abstract: The mechanical behaviour of two woven composites C/SiC and SiC/SiC was investigated at room temperature. The non-linear load-displacement curves and the damaging process were closely related to the specific structure of the composites, consisting of a network of impregnated bundles of fibres. The damage in the bundles proceeded by multiple cracking in the matrix before fibre failure, and dictated the response to the applied load. Other mechanisms, consisting mainly of distortions in bundles and their framework, induced a residual deformation and an energy dissipation. The behaviour was characterized according to the damaging process. Stress-electric strain curves revealed a mechanical response similar to those observed in unidirectional composites, although some effect of the specimen geometry on the curves was observed. Residual strains were similar in tensile and bending conditions. The work of fracture was consistently described by a volumetric rate of energy absorption, related to the applied strain, but the respective contributions of different damage mechanisms could not be determined.

Method for making ceramic matrix composites

Abstract: Fiber-reinforced ceramic matrix composite articles of complex shape are made with improved physical integrity and control over fiber alignment by a process wherein prepreg comprising fibers and powdered matrix is treated to shorten the average length of the fibers, then configured without binder removal into a preform of selected shape and fiber orientation, and finally consolidated with heat and pressure into a dense ceramic matrix composite article. Fiber breakage during the consolidation process is thereby avoided.

Fiber fragmentation in a single-filament composite

Abstract: The fiber fragment distribution obtained from tensile testing of a single‐filament composite contains vital information on the in situ strength of the fiber at short gauge lengths and on the shear stress across the fiber/matrix interface. Here, this fragmentation problem is mapped onto the problem of hard rods distributed randomly along a one‐dimensional line, but with a stress‐dependent rod length, and is then solved exactly. The solution utilizes the ‘‘car‐parking’’ problem of equal sized cars parked along a line and the theory predictions agree well with existing simulation results. The theory also applies to multiple cracking of brittle films on pliable substrates, film coatings on fibers, and matrix cracking in ceramic matrix composites, and now allows the in situ fiber or film statistical strength and interfacial shear strength to be derived from experimental fragment distributions.

Investigation of interfacial shear strength in SiC/Si3N4 composites

Abstract: A fiber push-out technique was used to determine fiber/matrix interfacial shear strength (ISS) for silicon carbide fiber reinforced reaction-bonded silicon nitride (SiC/RBSN) composites in the as-fabricated condition and after consolidation by hot isostatic pressing (HIPing). In situ video microscopy and acoustic emission detection greatly aided the interpretation of push-out load/displacement curves.

Polymeric routes to silicon carbide and silicon oxycarbide CMC

Abstract: An overview of two approaches to the formation of ceramic composite matrices from polymeric precursors is presented. Copolymerization of alkyl- and alkenylsilanes (RSiH3) represents a new precursor system for the production of Beta-SiC on pyrolysis, with copolymer composition controlling polymer structure, char yield, and ceramic stoichiometry and morphology. Polysilsesquioxanes which are synthesized readily and can be handled in air serve as precursors to Si-C-O ceramics. Copolymers of phenyl and methyl silsesquioxanes display rheological properties favorable for composite fabrication; these can be tailored by control of pH, water/methoxy ratio and copolymer composition. Composites obtained from these utilize a carbon coated, eight harness satin weave Nicalon cloth reinforcement. The material exhibits nonlinear stress-strain behavior in tension.

Synergism of Toughening Mechanisms in Whisker‐Reinforced Ceramic‐Matrix Composites

Abstract: The improved fracture resistance of whisker-reinforced ceramic-matrix composites involves more than one energy-absorbing mechanism. The possible mechanisms are reviewed and a micromechanical model evaluating the relative contributions to the overall toughness is presented. The mechanisms involve microcracking, load transfer, bridging, and crack deflection. The synergism of these mechanisms is examined using an energy release rate balance equation. The basic assumption of the proposed model is that the load transfer between the matrix and the whiskers is due to Coulomb friction. The model has been applied to an Al2O3/SiC whisker composite and shows reasonable agreement with reported experimental results. The role of the thermal residual stresses is also examined in light of the frictional load transfer assumption.

Protective material having a multilayer ceramic structure

Abstract: Sintered ceramic elements are associated to at least one composite material layer having a fibrous reinforcement arrangement and a ceramic matrix located behind the sintered ceramic elements.

Mechanical behaviour of a cross-weave ceramic matrix composite

Abstract: The deformation and fracture processes of a cross-weave carbon fibre/SiC composite prepared by a chemical vapour deposition process has been explored by interrupted-loading tests and SEM examination of cracking and fracture processes. The tensile stress-strain curves show non-linear behaviour associated with progressive matrix cracking and spalling, and the occasional fracture of a fibre. Re-loading curves and compressive stress-strain curves show linear behaviour. The fracture process does not involve cracking by a single dominant crack but occurs by the development of multiple damage sites operating around the transverse fractures of groups of four to eight fibres followed by longitudinal cracking at their fibre-matrix interfaces and temporary arrest of the cracks, until specimen failure occurs and there is massive fibre debonding and pull-out.

Method for making stiffened ceramic matrix composite panel

Abstract: An improved method for the fabrication of lightweight ceramic matrix composite panels, utilizing pressure forming with floating tooling to shape integral stiffeners such as raised integral surface channels on panel surfaces. The consolidation of green composite material against refractory floating tools by hot hydrostatic or isostatic (gas) pressing, followed by removal of the tooling, yields a strong, rigid, yet lightweight ceramic structure.

Thermal shock of fiber reinforced ceramic matrix composites

Abstract: Monolithic silicon carbide and silicon nitride and a Nicalon fiber reinforced silicon carbide composite were subjected to severe thermal shock conditions via impingement of a hydrogen-oxygen flame. Surface heating rates of 1000 C/sec to 2500 C/sec were generated. The performance of the monolithic reference materials are compared and contrasted to the significantly greater thermal shock resistance of the composite. Ultrasonic and radiographic NDE techniques were used to evaluate the integrity of the composite subsequent to thermal shock. Tensile tests were performed to determine the residual tensile strength and modulus. Physical property changes are discussed as a function of number and severity of thermal shock cycles.

Correlating the Mechanical Properties of a Continuous Fiber‐Reinforced Ceramic‐Matrix Composite to the Sliding Resistance of the Fibers

Abstract: The sliding resistance of NICALON fibers (SiC) in a lithium aluminosilicate (LAS III) glass-ceramic matrix has been correlated with the mechanical properties of the composite Push-down measurements of the sliding resistance for individual fibers show an order of magnitude increase when the ceramic composite is annealed in air for 4 h at 800°C Such increases are correlated with a loss of toughness, a reduction in the ultimate strength, and an increase in the matrix cracking stress of the composite, as measured using tension tests and bend tests of bulk samples

Mechanical identification of composites

Abstract: General Lectures.- Identification of the Rigidities of Composite Systems by Mixed Numerical/Experimental Methods.- Measurement of Complex Moduli of Composite Materials and Discussion of some Results.- Moire Interferometry for Composites.- A Damage Approach for Composite Structures: Theory and Identification.- Contributions.- Identification.- Identification of Temperature Dependence for Orthotropic Material Moduli.- Experimental Identification of Complex Stiffnesses of Composite Materials by Ultrasonic Wave Propagation.- A Hybrid Method to Determine Material Parameters of Composites.- Non Determined Tests as a Way to Identify Wood Elastic Parameters: the Finite Element Approach.- Measurement of Laminate bending Elastic Parameters from Non-Uniform Strain Fields.- Identification of the Shear Properties of the Lamina in the Framework of Plasticity.- Parametric Identification of Mechanical Structures: General Aspects of the Methods used at the LMA.- Static and Dynamic Characterization.- Material Parameters in Anisotropic Plates.- Evaluation of Global Composite Laminate Stiffnesses by Structural Wave Propagation Experiments.- Dynamic Measurements of Elastic Properties of Filament-Wound Cylindrical Shells.- Torsion Response Analysis of T300/914 and T800/914 Unidirectional Specimens.- A Computerized Test Setup for the Determination of the In-Plane and Out-of-Plane Shear Modulus in Orthotropic Specimens.- Some Experience from the Application of the Iosipescu Shear Test.- Constitutive Prediction for a Non-linear Orthotropic Media.- Creep and Relaxation of Coated Fabrics under Biaxial Loading.- Joints and Assemblies of Composite Material Structural Elements: Characterization of adhesives in bulk form and in composite structures.- Static and Dynamic Characterization of Composites using a Fourth Generation Programming Language on a Macintosh.- Impact Behaviour and Damage Characterization.- Use of the Shadow Moire Technique for the Investigation of the Impact Behaviour of Composite laminates.- Delamination Detection via Holographic Interferometry Techniques.- Dynamic Failure Processes in Fibre-Reinforced Composites.- Effect of Strain Rate on the Compressive Strength of Woven Glass-Reinforced/Epoxy Laminates.- Characterization of Highly Anisotropically Reinforced Solids in High Velocity Tension.- Micromechanics and Interfaces.- Determination of the Effective Mechanical Response of Polymer Matrix Composites via Microstructural Data.- A microstructural Method for Predicting the Damping of Lamina.- Theoretical and Experimental Study Technique of the Mechanical Properties of Fiber-Reinforced Laminates and of the Strain Features of Structural Elements.- Dynamic Properties of Layer Reinforced by Two Families of Fibres.- Micromechanical Modelling of Unidirectional Glassfiber Reinforced Polyester: Effect of Matrix Shrinkage.- Plastic Failure of Unidirectional Fibrous Composite Materials with Metal Matrix in Compression.- Analytical Model of Fibre Pull-Out Mechanism.- Nonlinear Viscoelastic Behavior of the Fibre-Matrix Interphase: Theory and Experiment.- Empirical Model Building and Surface Response Methodology Applied to Sclerometrical Investigation for Refractories Concrete.- Fracture and Fatigue Damage Mechanics.- Development of Fatigue Damage Mechanics for Application to the Design of Structural Composite Components.- Influence of Fibre Orientation on the Cracking and Fracture Energy in Brittle Matrix Composites.- Application of the Method of Caustics to Fibre-Reinforced Materials.- Modelling of Orthotropic Layered Structures Containing Cracks in the Interfaces.- A Criterion of Mixed Mode Delamination Propagation in Composite Material.- Fracture and Fatigue Damage in a CSM Composite.- Damage Modelling and Non-Destructive Testing.- Non linear Behaviour of Ceramic Matrix Composites.- Notched Strength and Damage Mechanism of Laminated Composites with Circular Holes.- Damage Modeling and Experimental Tools for 3D Composites.- Increasing Energy Absorption by 'Rope Effect' in Brittle Matrix Composites.- Specific Damping of a Carbon/Epoxy Laminate under Cyclic Loading.- The Application of automated Ultrasonic Inspection for Damage Detection and Evaluation in Composite Materials.- The Use of Vibrothermography for Monitoring Damage Evolution in Composite Materials.- Acoustic Emission Source Location in Anisotropic Plates.- List of Participants.- Index of Contributors.