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

Abstract

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.

Influence of Precursor Densification on Strength Retention of Zirconia‐Coated Nextel™ 610 Fibers

“…Further synthesis details are given in the literature. [19][20][21] Coating sols with an oxide yield of 8 wt% respective to ZrO 2 or 8YSZ were prepared by dissolution of the amorphous precursor powder in water. After stirring over night they were filtered through a 0.45 mm membrane.…”

“…After stirring over night they were filtered through a 0.45 mm membrane. 20,22 The cationic surfactant hexadecyltrimethylammoniumbromide was added below the critical micelle concentration (0.1 wt% respect to water content) in order to improve the fiber wetting.…”

From Sol-Gel Processing to Bio-Inspired Materials Synthesis

“…Sol-gel coatings are generally possible, but the film quality will critically depend on the deposition of a uniform wet film (Table 1, 2.3). As shown in 2.2.2, solgel films can be prepared on fiber substrates, but during the coating of multifilaments often adverse material bridges are formed between adjacent fibers [62]. TiO 2 as accessible by LPD is not a favorable material for fiber integration into CMCs, but it has recently found much attention due to its photocatalytical activity.…”

“…In figure 6 SEM images of such a single NEXTEL™ 610 fiber are given. It must be noted, though, that the formation of ZrO 2 bridges between neighboring fibers is a technological problem and the removal of residual organics from the sol-gel coating may decrease the fiber strength [62].…”

“…SEM image of a NEXTEL 610 fiber sol-gel coated with ZrO2 (left), fiber surface at higher magnification[62].…”