The microstructures of the nanostructured YSZ coatings heat-treated at 1400 ◦C for 1, 5 and 20 h can be found in Fig. 3. It is possible to observe that at the magnification for taking the photos (500×), the coarse porosity of the coatings (i.e., the area of the black-colored regions) increased with increasing heat treatment time (Figs. 2 and 3).
Fig. 1. (a) Microstructure (cross-section) of the nanostructured YSZ coating made from a nanostructured feedstock. (b) Higher magnification of (a) darker-colored regions containing the previously semi-molten feedstock particles (scale bar: number represents combined distance across all tick divisions)
The microstructure of the as-sprayed nanostructured coating (Fig. 1) was described in Section as a bimodal one, formed by (i) a matrix of previously molten YSZ particles surrounding (ii) previously semi-molten nanostructured YSZ particles, i.e., the nanozones. It is expected that both the matrix and the nanozones will exhibit the traditional sintering effects, i.e., densification and shrinking, upon exposure to high temperatures. However, one may imagine that the densification rates of the matrix will be lower than those of the nanozones, because the matrix is basically formed by the previously molten and currently resolidified particles, i.e., its inner structure already exhibits high density after resolidification from the molten state. On the other hand, the porous nanozones, due to the presence of nanostructured particles and porosity (i.e., high surface area—Fig. 1b), will be under a higher driving force for sintering and densification. Therefore, the large nanozones tend to densify/shrink at faster rates than those of the matrix, thereby increasing the area (volume) of the coarse pores during heat treatment (Figs. 2 and 3).
Fig. 2. Variation of the porosity values from as-sprayed to heat-treated nanostructured and conventional YSZ coatings at 1400 ◦C for 1, 5 and 20
Fig. 3. Microstructures (cross-sections) of the nanostructured YSZ coating heattreated at 1400 ◦C for (a) 1 h, (b) 5 h and (c) 20 h (scale bar: number represents combined distance across all tick divisions).
Despite this sintering effect, regions that were identified as nanozones in the as-sprayed coating are observed at higher magnifications after 20 h of heat treatment. It is possible to distinguish their original microstructural characteristics or remains. Finely dispersed porosity zones are still present in the former nanozones after 20 h at 1400 ◦C. It is important to point out that not only the particle size, but also the density of the body to be sintered (i.e., the nanozones) play an important role during the densification process (among other factors). Therefore, very porous regions, despite exhibiting particles with nanostructural character (Fig. 1b), will not necessarily exhibit full densification upon sintering (Fig. 4). Fig. 4 also shows the effect of the higher sintering rates of the nanozones (compared to those of the matrix). It is possible to observe that the original nanozones shrank at higher rates upon sintering, thereby creating open voids within the matrix structure.
Fig. 4. Higher magnification view of the nanostructured YSZ coating heattreated at 1400 ◦C for 20 h (Fig. 3c) (scale bar: number represents combined distance across all tick divisions).
本文由桑尧热喷涂网收集整理。本站文章未经允许不得转载；如欲转载请注明出处，北京桑尧科技开发有限公司网址：http://www.sunspraying.com/
|
