Effect of coating deposition temperature on monazite coated fiber

Monazite (LaPO4) was continuously coated on 3M Nextel™ 720 fiber tows with an ethanolic precursor using hexadecane for immiscible liquid displacement. Coating deposition temperatures were varied from 900 to 1300°C. Fibers coated at 900°C were heat-treated up to 100 h at 1200°C. Coated fibers were characterized by analytical TEM, and tensile strengths were measured by single filament tensile tests. The monazite precursor was characterized by X-ray, DTA/TGA, and mass spectrometry. Microstructure evolution was complex and may have involved recrystallization of large defective grains into smaller grains and then subsequent growth of these grains, along with coarsening of porosity. After 100 h at 1200°C there was significant roughening of the coating–fiber interface, with facetting of alumina grains in the fiber and some lanthanum segregation to these facetted boundaries. Spheroidization of thin coatings was also observed. Tensile strength of coated fiber decreased with increasing deposition temperature and with time at temperature after deposition. Possible reasons for the strength decrease are discussed.