Precipitation coating of monazite on woven ceramic fibers: III - Coating without strength degradation using a phytic acid precursor

Multiple coatings of La-monazite (LaPO₄) were applied to Nextel^™ 610 and 720 oxide fibers using a heterogeneous nucleation and growth coating process with phytic acid solution precursors. The as-processed coatings were found to be largely amorphous. Coated fibers were heat treated at 1200°C for 1-2 h, whereupon the coatings crystallized to porous LaPO₄ with thin (50-100 nm) layers of AlPO₄ at the fiber-coating interface. Heat-treated fibers exhibited full strength retention, in marked contrast to fibers coated by precipitation using citric acid precursors as in a previous work. The microstructural evolution of the amorphous coatings was studied as a function of temperature. AlPO₄ forms at the fiber-coating interface between 700° and 800°C, before high-temperature outgassing of the coating as measured by TGA. NMR spectroscopy was used to study as-precipitated powders and as-processed coatings to gain insights into the structural nature of the materials. A possible explanation for the retention of fiber strength through the coating process is proposed and discussed in light of the experimental evidence.