Monazite Coatings on Fibers: II, Coating without Strength Degradation

Abstract

Washed and unwashed rhabdophane (LaPO4·xH2O) sols were used to apply monazite coatings to 3M Nextel 720 and 610 fibers. This precursor was designed to minimize stress corrosion from gaseous decomposition products at high temperature. The coatings were heat-treated in-line at 900°-1300°C, in air, using a continuous vertical coater with immiscible liquid displacement. Coatings were characterized by optical microscopy, scanning electron microscopy, and transmission electron microscopy. The sol was characterized with light-scattering and zeta-potential measurements. Precursor phase evolution was studied with differential thermal analysis/thermogravimetric analysis and X-ray diffractometry. The washed sol had a higher pH and lower weight loss than the unwashed sol. The as-coated fibers were tensile tested, along with coated fibers heat-treated in air at 1200°C for 100 h. The precursor was slightly phosphate-rich, and this excess phosphate reacted with alumina in the fiber to occasionally make very small (<10 nm) pockets of AIPO4 at the coating-fiber interface after 100 h at 1200°C. Both washed and unwashed sols made coated fibers with higher tensile strengths than those of coated fibers made from other precursors, and the washed sol may actually have slightly increased fiber strength when in-line heat treatments at <1200°C were used. A small amount of AIPO4 may also have helped seal preexisting flaws. Degradation mechanisms during fiber coating are discussed in this paper.