Fig.1 shows the SEM micrographs of sectioned surface of Nos.1,6,3 and 8 coatings. It is observed that the sectioned surface of Nos.3,6 and 8 coatings doped with rare earth is denser, more compact and less porous than that of No.I coating without rare earth. Figs.2 and 3 show the SEM micrographs of sectioned surfaces and corresponding MP results of Nos.1and 5 coatings, respectively, and Fig.4 shows the MP results of cobalt element distribution of Nos.7 and 2 coatings. It can be seen that cobalt, nickel and chromium elements distribution of No.I coating without RE are not uniform in the local region of sectioned surface, whereas the cobalt element distribution is uniform obviously. Furthermore, it can also be observed that cobalt and chromium elements appear alternatively with nickel element, indicating that only one of the three elements, such as cobalt, chromium and nickel, distribute uniformly, can the microstructure of the sectioned surface be drawn uniformly, thus the microstructure of coatings doped with RE is uniform credibly.
Fig.1. SEM micrographs of sectioned surface of (a) No.1,(b)No.6,(c)No.3 and No.8 coatings
Fig. 2. (a) SEM micrograph and MS results on the sectioned surfaces of (b) chromium element distribution, (c) cobalt element distribution, and (d) nickel element distribution of No. 1 coating.
Fig. 3. (a) SEM micrograph and MS result on the sectioned surface of (b) cobalt element distribution of No. 5.
Fig.4. MS results on the sectioned surface of cobalt element distribution of (a) No. 7 coating and (b) No. 2 coating.
So it is believed that rare earth(RE) can improve the microstructure of SPS coatings effectively, due to the characteristics of RE. For example, the RE is a surface active element with a rather large atomic radius(radii of cerium and lanthanum are 0.1824 and 0.1877 nm, respectively). As the RE reacts easily with other elements, stable compounds can be formed, which can increase the number of crystal nuclei and limit the growth of grains during crystallization of the molten cermets region, thus the microstructure of the coatings can be refined. On the other hand, as the atomic radius of RE is rather large, the existence of the RE within the solid solution would lead to great distortion of lattice, which would increase the energy of the system. In order to retain the lowest Helmholtz free energy, the RE distributes irregularly on the boundary, which would be required. When the grains grow, the RE atoms and compounds over the grain boundary would make a dragging effect on the movement of grain boundary, thus the growth of the grains will be suppressed. So the microstructure is further refined.
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