Continuous Sol-Gel Coating of Ceramic Multifilaments: Evaluation of Fiber Bridging by Three-Point Bending Test

Nextel™ 610 multifilaments were continuously coated with aqueous sols of yttria‐stabilized zirconia (8YSZ). For the transformation of the sol–gel precursor to a crystalline oxide coating detailed microstructural characterization is given. The main subject of the study was the bridging of fibers by interphase material, a general phenomenon of sol–gel coatings. A three‐point bending test is proposed as a semi‐quantitative method to determine the characteristic stiffening of bridged fiber bundles. This method is successfully used to evaluate the influence of sol concentration on fiber bridging and to prove the benefit of additional surfactants to aqueous sols. Differences in film formation on the individual fiber surfaces leading to homogeneous or inhomogeneous film thickness and variation in degree of fiber surface coverage are explained with a three‐fiber model. Based on model and experiment it is shown that even surface coverage and homogeneous film thickness can better be accomplished by multiple coatings from low concentrated sols than by single coatings from high concentrated sols. Finally, the three‐point bending test is used to qualitatively investigate the mechanical properties of the material bridges which can be correlated with results from structural investigations.