Fig. 1a, b, and c show FESEM images of the surface of the AM5, AM10, and AM15 samples, respectively.
A uniform coating can be observed in all samples with similar morphological aspects, regardless of the proportion of 7YSZ added to the SiO2precursor.
Fig. 1. FESEM images of the surface of the samples (a) AM5, (b) AM10, and (c) AM15.
Apparently, all coatings are formed by fused particles, providing better adhesion between the overlapping particles. This effect is directly associated with the deposition system, which uses an axial injection of the precursor, ensuring a longer residence time of the precursor particles in interaction with plasma regions and, consequently, a better
heat transfer between plasma particles. The absence of semi-fused or unfused particles in the coating inhibits the embrittlement of the coating due to stressing points and does not compromise the protection against oxidation of the substrate, desired feature for passivation coating.
To confirm that the coatings are formed by fused particles, images were made with higher magnification (Fig. 2). Fig. 2 shows FESEM images from sample AM15. In Fig. 2a, it is observed that the material apparently was completely melted having a similar appearance to glassy materials such as SiO2. However, to verify that the material is completely melted an image with higher magnification was necessary (Fig. 2b). In Fig. 2b, the formation of nanospheres instead of splats was observed, this is characteristic of particles with low Weber number, small enough to not form a splat, suggesting that the particles are semimelted.
From this result, it is possible to affirm that it is a coating formed by semi-molten feedstock particles that are surrounded by fully molten particles that act as a binder, which maintaining the coating integrity. The dense structure (melted particles) and the fraction of semi-melted particles increase the toughness of the coatings. Besides that, a dense coating is desirable to allow the self-healing process, which is an intrinsic feature of silica, used to benefit the thermal protection system. This characteristic becomes active when the material is exposed to a high temperature (ablative) oxidizing environment, causing the silica to be melted and sealing the cracks/pores, inhibiting the degradation processes of the composite by oxidation.
Fig. 2. FESEM images from sample AM 15 highlighting (A) fused material and (B) nanostructures.
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/
|
