Spherical Rhabdophane Sols. II: Fiber Coating

A rhabdophane (LaPO4·nH2O) sol with fine spherical particles was used to coat Nextel™ 720 fiber tows continuously with monazite (LaPO4). The coatings are compared with those made previously from rod‐shaped particles. The coated fibers were heat‐treated at 1000°–1300°C for 1, 10, and 100 h. The effect of heat treatment temperature and time on coating microstructure was characterized by scanning electron microscopy and transmission electron microscopy, and the strengths of the coated fibers were measured after coating and heat treatment. Grain shapes and grain growth rates were measured. Coating thickness uniformity was quantified by a fit to a truncated extreme‐value distribution. Coating hermeticity was evaluated by analysis of grain growth rates. The spherical particles promote more rapid coating densification and local hermeticity, but introduce problems with sintering shrinkage cracking that are not present in coatings derived from rod‐shaped particles.