Sol-gel processing of multi-directionally reinforced ceramic matrix composites.

摘要

Sol-gel processing is an attractive method for fabrication of multi-directionally reinforced ceramic matrix composites (CMCs). It offers versatility in terms of the matrix material, capability for net-shape manufacturing, and relatively low processing temperatures. The major drawback of the sol-gel route, however, is excessive shrinkage of the gel during the drying stage. In the case of a composite, the rigid fiber preform severely constrains the deformation of the gel, thus causing large drying stresses which often result in the cracking of the gel. One approach for the control of shrinkage and reduction of stresses is the addition of solid particles to the sol.; This dissertation investigates sol-gel processing of multi-directionally reinforced ceramic matrix composites, focusing on the drying behavior and drying stresses. A three-dimensionally (3-D) woven carbon fiber/silica composite was used as the model material system for the investigation. Composites were fabricated with and without solid particle additions to the sol and the drying rate, infiltration efficiency, and matrix cracking were characterized. The particle size was varied in order to study the role of particles addition in the processing. The preforms were infiltrated using liquid transfer molding and pressure infiltration techniques.; A pressure infiltration apparatus was designed to obtain data on kinetics of infiltration with the addition of solid particles to the sol. A parabolic rate kinetics model was developed and verified by experiments. The relationship among processing time, processing pressure, permeabilities of material system, and size of particles was investigated.; Models for the analysis of drying stresses, with and without solid particles addition in the sol, were developed. The contributions to the drying stresses were considered to be capillary pressure and constraint from the preform. Stresses were derived for two geometries of gels filling the pores within the preform: plate gels and cylindrical gels.; The addition of solid particles in the gel matrix influences its permeability and strain rate which both affect the drying stresses in a synergistic manner. The effects of the volume fraction of particles, viscosity ratio between particles and gel matrix, particle size, particles flocculation, and the change of pore size due to particles were investigated. The results indicate that the addition of solid particles to the sol during sol-gel processing of multi-directionally reinforced CMCs increases composite green density and reduces cracking caused by drying stresses.