Improved uniformity of densification of ceramic composites through control of the initial preform porosity distribution
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Brian Heble Process Analysts, Inc., 3000 Young field Street, Lakewood, Colorado 80215
Herman C.T. Cheng Engineering Technology Laboratory, E. I. du Pont de Nemours and Company, Delaware 19898
Wilmington,
(Received 19 June 1989; accepted 26 March 1990)
Densification arising from chemical vapor deposition within a porous preform is modeled as a problem in diffusion and reaction. The effect of reaction rate on the nonuniformity of the final densified composite is explored. The possibility of achieving uniform densification by starting with an initially nonuniform porosity distribution is demonstrated with a simulation model. This model permits one to design a densification process from first principles using a simulation tool that yields results on a timescale far less than that of the processing laboratory.
I. INTRODUCTION
One method for production of ceramic coated and other composites begins with a "near shape" porous preform fabricated from woven carbon fibers. This textile-like body is then exposed to a reactive gas which promotes the deposition of a solid ceramic, metal, or metal oxide coating upon all surfaces of the preform. The purpose of the coating is to produce an object which can withstand a high temperature and/or harsh chemical environment while maintaining a high strength/low weight ratio. The deposition reaction can be stopped to yield a specified, desirable porosity of the final product, or the reaction can be continued until the product is (nearly) completely densified, i.e., having no (or very little) interior void space. This task is not easy to achieve economically. Typically, the deposition reaction proceeds from the exposed surface of the bulk preform into the interior, as a consequence of which the exposed surface porosity is closed off first, leaving an often undesired degree of porosity trapped within the bulk preform. In practice, the surface is "sanded" back to expose pores which provide access of the reactants to the interior, and the process begins a second cycle. Repetition of these cycles gives rise to long times to achieve the required degree of densification, with an attendant increase in manufacturing cost. This type of nonuniformity of densification is understood to arise from diffusion limitations to transport of reactants into the interior of the preform. A solution to this problem is to carry out densification at a slow rate so that chemical kinetics, and not diffusion, con1544
http://journals.cambridge.org
J. Mater. Res., Vol. 5, No. 7, Jul 1990
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trols the process. While this strategy leads to improved uniformity of densification, the long times required to achieve densification under these conditions again increase the manufacturing cost by decreasing the production rate of the finished, fully densified, preform. Since the technology exists to control the structure of the textile web from which a preform is constructed, it should be possible to begin with a preform which has a nonuniform porosity. The initial distribution of nonuniformity can b
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