Monazite Coatings on Fibers: I, Effect of Temperature and Alumina Doping on Coated‐Fiber Tensile Strength

Abstract

Monazite was continuously coated onto Nextel 720 fibers, using an aqueous precursor and in‐line heat treatment at 900°–1300°C. Some experiments were repeated with alumina‐doped precursors. Coated fibers were heat‐treated for 100 h at 1200°C. Coatings were characterized by optical microscopy, scanning electron microscopy, and analytical transmission electron microscopy. Coated‐fiber tensile strengths were measured by single‐filament tensile tests. The precursors were characterized by X‐ray diffractometry, differential thermal analysis/thermogravimetric analysis, and mass spectrometry. Coated‐fiber tensile strength was lower for fibers coated at higher deposition temperatures. Heat treatment for 100 h at 1200°C decreased tensile strength further. The coatings were slightly phosphate‐rich and enhanced alumina grain growth at the fiber surface, but phosphorus was not detected along the alumina grain boundaries. Fibers with alumina‐doped coatings had higher tensile strengths than those with undoped coatings after heat treatment for 100 h at 1200°C. Alumina added as α‐alumina particles gave higher strengths than alumina added as colloidal boehmite. Alumina doping slowed monazite grain growth and formed rough fiber–coating interfaces after 100 h of heat treatment at 1200°C. Possible relationships among precursor characteristics, coating and fiber microstructure development, and strength‐degradation mechanisms are discussed in this paper.