Title

Synthesis and characterization of ceramic composites and coatings by sol-gel and chemical vapor infiltration processes

Date of Completion

January 1994

Keywords

Chemistry, Inorganic|Engineering, Chemical

Degree

Ph.D.

Abstract

This research is devoted to the development of a new method for the synthesis of nitrides by thermal nitridation of metal oxides or hydroxides in gaseous mixtures of ammonia and methane. Thermodynamic analysis was conducted for metal oxide-CH$\sb4$-NH$\sb3$ systems. Based on theoretical predictions, metal nitrides such as AlN, TiN, and their composites Al$\sb2$O$\sb3$ + TiN and AlN+ SiC were prepared experimentally. Crystalline AlN and TiN were obtained by this method. Temperature and time effects were studied. The lowest reaction temperatures for TiN and AlN were 750$\sp\circ$C and 950$\sp\circ$C, respectively. The particle sizes of the powders were studied using SEM and XRD. Properties such as electrical and oxidation resistance of the powder composites were investigated. Those properties vary with the compositions of the composites. The electrical resistances change from a good conducting material to a good insulator by increasing the Al$\sb2$O$\sb3$ or AlN content in their composites.^ Another major part of this research involves thin film coating on various substrates such as fibers, metal substrates, and silicon wafers. Alumina coatings on a silicon carbide monofilament, BP-SIGMA, a SiC coated boron monofilament, SICABO, and on SiC yarns Nicalon, and carbon Nicalon (C-Nicalon) were produced by a sol-gel process. The coated BP and SICABO fibers retained the strengths of the as-received fibers even after the coated fibers were heated to 1000$\sp\circ$C in air for 24 hours. The strengths of the as-received C-Nicalon fibers were also maintained after the coated fiber were heated to 1000$\sp\circ$C in air. However, the coated Nicalon fibers were totally degraded after the coated fibers were heated in air at 1000$\sp\circ$C. The degradation mechanism was investigated using Auger depth profiles. The carbon layer proved to be very important in preventing Al and O diffusion into the coated fibers during heat treatment. Evenness and thicknesses of the coating layers were studied by scanning electron microscopy (SEM) which showed that uniform and nonbridging coatings could be obtained if the coating, drying and heating processes were carefully controlled. ^