Microwave Assisted Chemical Vapor Infiltration
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MICROWAVE ASSISTED CHEMICAL VAPOR INFILTRATION D.J. Devlin, R.P. Currier, R.S. Barbero, B.F. Espinoza, and N. Elliott Materials Science and Technology Division Los Alamos National Laboratory Los Alamos, NM 87545
ABSTRACT A microwave assisted process for production of continuous fiber reinforced ceramic matrix composites is described. A simple apparatus combining a chemical vapor infiltration reactor with a conventional 700 W multimode oven is described. Microwave induced inverted thermal gradients are exploited with the ultimate goal of reducing processing times on complex shapes. Thermal gradients in stacks of SiC (Nicalon) cloths have been measured using optical thermometry. Initial results on the "inside out" deposition of SiC via decomposition of methyltrichlorosilane in hydrogen are presented. Several key processing issues are identified and discussed. INTRODUCTION Many materials used in high temperature service, including monolithic ceramics, tend to be brittle and susceptible to catastrophic failure through crack propagation. This has lead to concentrated interest in the fabrication of fiber reinforced ceramic matrix composites (CMCs). CMCs consist of a fibrous backbone, or substrate, whose void spaces are filled with a "matrix" material. The chemical composition of the matrix may or may not be the same as that of the fibers. While a random pile of individual fibers may be used as reinforcement, the tougher CMCs typically consist of continuous fiber bundles, or yam, woven together to form either two or three dimensional "textile" cloths. Processing techniques for CMCs differ primarily in the way matrix materials are infiltrated into the porous substrate. Infiltration may involve molten liquids, sol-gels, polymeric materials, powders, or vapors. Most of these techniques require final densification steps, e.g. sintering, involving matrix shrinkage. Large residual stresses can develop when a matrix shrinks around non-shrinking fibers. Chemical vapor infiltration (CVI) is an attractive alternative for matrix deposition since it avoids stressing the fibrous backbone during processing. Also, relatively low temperatures are used in CVI which limits adverse chemical attack on the fibers. Conventional CVI processes may be either isothermal or involve intentionally imposed thermal gradients. Deposition may be reaction or diffusion limited, or may rely on forced and/or pulsed reactant flows. The various configurations have recently been reviewed by Besmann et al [1]. To varying degrees, conventional CVI processes are typically subject to some or all of the following drawbacks: preferential deposition in the substrate's outer regions leading to pore blockage; long processing times with intermittent machining operations; non-uniform composite density; high residual porosity; and limitations on substrate geometry. However, despite inherent limitations commercial CVI operations are now in place. The idea of using electromagnetic radiation, in particular microwaves, to heat substrates during CVI has recently been explored
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