Fig.1 shows the microstructures of the Al2O3-doped t-YSZ coatings under the spraying parameter D. The SEM images reveal that the coatings contain dense regions, pores and some small cracks. On the one hand, some pores with complex shape originate from the loose raw powders that are unmolten and partly molten in the plasma flame. On the other hand, some pores are formed during the solidification process of the droplet. When a droplet impacts onto a solid surface,a thin air film is generally entrapped between the droplet and the substrate at the central portion which eventually evolves into a spherical air bubble for minimizing the surface energy. The internal gas in the spherical air bubble does not escape in time as a result of rapid solidification of the droplets under the high cooling rate provided by plasma spraying, which leads to the formation of some spherical pores in the thermal spray coatings. In addition, it is believed that the cracking occurs after solidification because the subsequent spray droplets stack, the splat thermally contracts, and the cracks are formed to relieve the high stresses during cooling. At the same time, subsequent high-velocity droplet impacts could also produce some cracks. The EBSD images indicate that the density regions are composed of equiaxed and columnar grains with random orientations. Furthermore, the porosity and grain boundary density of the coatings corresponding to Fig.2 were analyzed using the image method and EBSD, respectively.
Fig.1. SEM and EBSD orientation images of polished cross sections of Al2O3-doped t-YSZ coatings.(a,e)D5;(b,f) D10;(c,8)D15;(d,h)D25.
Fig.2.Coefficient of thermal expansion and thermal conductivity of different coatings.
The mean values of the coatings under different amounts of Al2O3 are shown in Fig.3. This indicates that the grain boundary density of the Al2O3-YSZ coatings increases gradually with the amount of Al2O3 addition. However, the change in porosity is irregular. An increase in the grain boundary density indicates a decrease in the grain size. The Al2O3 addition resulting in a decrease in the grain size of the YSZ coating can be summarized as follows. First and foremost,a previous study reported that AlzO3 addition caused the melting point of Al2O3-ZrO2 composites to decline steadily . This indicates that Al2O3 addition causes the composites to melt easily during spraying and recrystallize into fine grains because of the rapid solidification process. This phenomenon was proved based on Fig.4.
Fig.3.Mean value graph of grain boundary density and porosity of coatings under different addition amounts of Al2O3.
Fig.4.SEM and EDS images of polished cross section of D5 coating.
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