Richard A. Lowden

Vapor-Phase Fabrication and Properties of Continuous-Filament Ceramic Composites

The continuous-filament ceramic composite is becoming recognized as necessary for new, high-temperature structural applications. Yet because of the susceptibility of the filaments to damage from traditional methods for the preparation of ceramics, vapor-phase infiltration has become the fabrication method of choice. The chemical vapor infiltration methods for producing these composites are now being studied in earnest, with the complexity of filament weaves and deposition chemistry being merged with standard heat and mass-transport relationships. Two of the most influential effects on the mechanical properties of these materials are the adhesion and frictional force between the fibers and the matrix, which can be controlled by a tailored interface coating. A variety of materials are available for producing these composites including carbide, nitride, boride, and oxide filaments and matrices. Silicon carbide-based materials are by far the most advanced and are already being used in aerospace applications.

Evaluation of neutron irradiated near-stoichiometric silicon carbide fiber composites

Composites have been fabricated by chemical vapor infiltration of silicon carbide (SiC) into SiC-based fiber preforms. Fibers were Ceramic Grade Nicalon TM , Hi-Nicalon TM and Hi-Nicalon TM Type-S. Results are presented for two parallel studies on the effects of neutron irradiation on these materials. In the first study, neutron irradiation induced changes in mechanical properties, as measured by bend testing, for Hi-Nicalon TM fiber materials of varied interphase structures is measured. Results indicate that both the Ceramic Grade Nicalon TM and Hi-Nicalon TM materials degrade substantially under irradiation, though the higher oxygen content Ceramic Grade fiber degrades more rapidly and more substantially. Of the three interfaces studied in the Hi-Nicalon TM system, the multilayer SiC is the most radiation resistant. At a dose of ∼1 dpa the mechanical property degradation of the Hi-Nicalon TM composite is consistent with a fiber densification-induced debonding. At a dose of 10 dpa the properties continue to degrade raising the question of degradation in the CVD SiC matrix as well. Low-dose results on the Hi-Nicalon TM Type-S fabricated material are encouraging, as they appear to not lose, and perhaps slightly increase, in ultimate bend strength. This result is consistent with the supposition that as the oxygen content in SiC-based fibers is reduced, the irradiation stability and hence composite performance under irradiation will improve.

Recent advances in forced-flow, thermal-gradient CVI for refractory composites

Chemical vapor infiltration (CVI) is simply chemical vapor deposition (CVD) on the internal surfaces of a porous preform and has been used to produce a variety of developmental and application materials. The greatest use of CVI is to infiltrate continuous-filament preforms taking advantage of the relatively low-stress CVD process. In CVI, reactants are introduced in the porous preform via either diffusion or forced convection and the CVD precursors deposit the appropriate phase(s). As infiltration proceeds, the deposit on the internal surfaces becomes thicker. Thus, after some length of time, the growing surfaces meet bonding the preform and fill much of the free volume with deposited matrix. The forced-flow/thermal-gradient technique (FCVI) developed at Oak Ridge National Laboratory overcomes the problems of slow diffusion and restricted permeability, and has demonstrated a capability to produce thick-walled, simple-shaped, SiC-matrix components in times of the order of hours. A model has been developed for the process that predicts flow, thermal and density profiles as a function of time. The results have been compared with an initial set of experiments and indicate qualitative agreement. It is expected that improved property relationships, such as permeability and thermal conductivity as a function of density, will allow the model to closely represent the FCVI process and be useful in fabrication and product optimization.

Moduli determination of continuous fiber ceramic composites (CFCCs)

Abstract NicalonTM/silicon carbide composites were fabricated by the Forced Chemical Vapor Infiltration (FCVI) method. Both through-thickness and in-plane (fiber fabric plane) moduli were determined using ultrasonic techniques. The through-thickness elastic constants (moduli) were found to be much less than the in-plane moduli. Increased porosity significantly decreased both in-plane and through-thickness moduli. A periodic model using a homogenization method was formulated to predict the effect of porosity on the moduli of woven fabric composites. The predicted moduli were found to be in reasonably good agreement with the experimental results.

The effect of fiber coatings on interfacial shear strength and the mechanical behavior of ceramic composites

Thin coatings (boron nitride and carbon) deposited on ceramic fibers prior to densification employing chemical vapor infiltration techniques have been used to limit fiber-matrix bonding. This has resulted in improvements in strength and toughness at room and elevated temperatures in Nicalon{reg sign} fiber-reinforced/SiC matrix composites. The properties of the fiber-matrix interface in fiber-reinforced ceramic composites have been examined utilizing an indentation method in which a standard microhardness indentor is used to push on fibers embedded in the ceramic matrix. Compositions and microstructures at the interface have been characterized employing analytical electron microscopy. Correlations between interfacial phenomena and observed mechanical behavior have been made. 16 refs., 5 figs., 1 tab.

Application of life cycle analysis: the case of green bullets

Life‐cycle analysis (LCA) provides a general framework for assessing and summarizing all of the information important to a decision. LCA has been used to analyze the desirability of replacing lead (Pb) with a composite of tungsten (W) and tin (Sn) in projectile slugs used in small arms ammunition at US Department of Energy (DOE) training facilities for security personnel. The analysis includes consideration of costs, performance, environmental and human health impacts, availability of raw materials, and stakeholder acceptance. Projectiles developed by researchers at Oak Ridge National Laboratory (ORNL) using a composite of tungsten and tin are shown to perform as well as, or better than, those fabricated using lead. A cost analysis shows that tungsten‐tin is less costly to use than lead, since, for the current number of rounds used annually, the higher tungsten‐tin purchase price is small compared with higher maintenance costs associated with lead. The tungsten‐tin composite presents a much smaller potential for adverse human health and environmental impacts than lead. Only a small fraction of the world’s tungsten production occurs in the USA, however, and market‐economy countries account for only around 15 per cent of world tungsten production. Concludes that stakeholders would prefer tungsten‐tin on the basis of total cost, performance, reduced environmental impact and lower human toxicity. However, lead is preferable on the basis of material availability. Life cycle analysis clearly shows that advantages outweigh disadvantages in replacing lead with tungsten‐tin in small‐caliber projectiles at DOE training facilities. Concerns about the availability of raw tungsten are mitigated by the ease of converting back to lead (if necessary) and the recyclability of tungsten‐tin rounds.

Interfacial Characterizations of Fiber-Reinforced Sic Composites Exhibiting Brittle and Toughened Fracture Behavior

A process has been developed for the fabrication of a ceramic composite of SiC fibers in a chemical vapor infiltrated (CVI) SiC matrix. Early specimens produced by this technique exhibited nonuniform fracture toughness behavior; regions of brittle fracture with no fiber pullout were interspersed with regions of good toughness where fiber pullout predominated. Microscopic and spectroscopic analyses of fiber surfaces revealed that this behavior may have been related to a thin, discontinuous layer of predominantly silica which, when present, prevented tight bonding of fibers and CVI matrix. Consequently, efforts to control interfacial bond strengths and enhance fracture toughness via fiber pretreatment with CVI overcoatings have had mixed success depending upon the overcoating specie. Chemical and microstructural characterizations by analytical electron microscopy of these composites are presented and correlated with composite mechanical property data.

Nondestructive characterization of woven fabric ceramic composites

Woven fabric ceramic composites fabricated by the chemical vapor infiltration method are susceptible to high void content and inhomogeneity. The condition of such materials may be characterized nondestructively with ultrasonic methods. In this work, longitudinal and shear waves were used in the quantitative determination of elastic constants of Nicalon{trademark}/SiC composites as a function of volume percent of porosity. Elastic stiffness constants were obtained for both the in-plane and out-of-plane directions with respect to fiber fabric. The effect of porosity on the modulus of woven fabric composites was also modeled and compared to the measured results. Scan images based on the amplitude and time-of-flight of radio frequency (RF) ultrasonic pulses were used for evaluating the material homogeneity for the purpose of optimizing the manufacturing process and for correlation with the mechanical testing results.

Fiber Coatings and the Fracture Behavior of a Continuous Fiber Ceramic Composite

Fiber coatings have been used to modify fiber-matrix interfacial forces, and thus control mechanical properties of continuous fiber ceramic composites. It has been shown that the properties and thickness of the interlayer influence composite properties such as matrix cracking and ultimate strength, toughness and interlaminar shear. The effects of fiber coating properties and thickness on fiber-reinforced SiC matrix composites fabricated employing CVI techniques have been examined. Correlations between interface condition, mechanical properties and failure mechanisms have been made.

Microstructural Characterization of Multiphase Coatings Produced By Chemical Vapor Deposition

Chemical vapor deposition has been utilized to produce ternary, multiphase coatings of various compositions of silicon carbide (SiC) with Ti, Cr, and Mo. Thermodynamic calculations have been performed for a variety of experimental conditions in each system. Scanning, transmission and analytical electron microscopy, and x-ray diffraction techniques have been used to characterize the microstructures and to determine compositions. 16 refs., 5 figs.