**5.1 Laser surface modification of gadolinium zirconate-based thermal barrier coatings**

Surface modification by using laser beam is a new and good technique for improving some of the properties of plasma-sprayed TBCs. According to the studies, laser remelting solidification process has provided a reduction in the surface roughness and specific surface area. Moreover, besides a columnar cross-sectional structure, controlled (varied depending on the laser parameters) segmented crack network that is perpendicular to the surface can be achieved with sealed porosities at the surface of the coatings. Therefore, hardness, thermal cycle lifetime, resistance to hot corrosion, CMAS, and erosion property of TBCs can be further improved by laser surface modification [1, 7, 11, 13, 18, 41–43]. After laser surface-modified process:


After laser surface modification, a TBC surface should:

1.Have a remelted layer having melting depth of 20–50 μm.

*State of the Art of Gadolinium Zirconate Based Thermal Barrier Coatings: Design, Processing… DOI: http://dx.doi.org/10.5772/intechopen.85451*

2.Be nonseparated from the as-sprayed layer.

3.Have a smooth surface (with surface roughness value (Ra) lower than 5 μm).

4.Have a distribution of the crack network that is equiaxed.

All of these properties can be obtained only at optimum laser surface modification parameters. A study of the laser surface modification was made to determine the optimum laser surface modification parameters of APSed GZ-based TBC [13]. The GZ-based TBCs were subjected to the laser surface modification process by using a continuous CO2 laser. Laser parameters were laser power density on the surface, scanning speed, and laser distance (distance to the surface from the laser torch). During studies, single laser beam tracks were generated on the specimens, and they were examined. As seen in **Figure 8a**, a very successful laser-glazed surface of GZ-based TBC was obtained having a denser microstructure than as-sprayed layer with an average thickness of molten layer ~37 μm, and a smooth surface (Ra: 2.9 μm) was accomplished at the optimum laser parameters (laser power density, 70 MW/m2 ; scanning speed, 150 mm/sn; laser distance, 14 mm). There was no separation from the as-sprayed layer. Furthermore, the network of cracks were very regular and excellent. These changes in the microstructure of the GZ-based TBC can be clearly noticed in **Figure 8a** when it is compared with **Figure 3a** belonging to the as-sprayed GZ-based TBC surface. **Figure 8b** shows high magnification microstructure of the fracture surface of GZ-based TBC. Individually, glazed layer and as-sprayed structure can be seen clearly. Characteristic cross-sectional microstructure of APSed TBC having splats, parallel cracks at splat boundaries, and pores transformed to dense and nonporous structure at the laser surface-modified layer. During the laser surface modification

#### **Figure 8.**

*(a) Surface and cross-sectional micrograph of the GZ-based TBC after laser surface modification process, (b) cross-sectional fracture surface micrograph of the GZ-based TBC after laser surface modification process, (c) grain size distribution of the laser surface-modified GZ-based TBC, (d) cross-sectional fracture surface micrograph of the laser surface-modified GZ-based TBC showing columnar grains.*

process, molten liquid GZ is replaced with microholes due to the surface tension force of liquid material. On the other hand, laser surface modification parameters affected the grain size and morphology of the GZ-based TBC due to directional solidification. At the optimum laser surface modification parameters (in terms of melting depth, cross-sectional damage, surface quality, and crack network regularity), distribution of the grain size was almost the same, and grains were equiaxed in the GZ-based TBC (see **Figure 8c**). Moreover, as seen in **Figure 8d**, these grains grew as perpendicular to the surface in the laser surface-modified layer of the GZ-based TBC. The structural change from the porous and lamellar plasma-sprayed structure to the columnar laser surface-modified structure could clearly be seen in **Figure 8d**. This structure was due to the directional solidification of laser-modified layer because the direction of heat flow was from the coating to the surface during solidification. This phenomenon was proved by phase analysis. XRD graphs showed that a preferred orientation on the direction took place after laser surface modification process of the GZ-based TBC. On the other hand, hardness value of the remelted regions was affected from the laser surface modification parameters because the grain size of the modified layer decreased as the parameters approach the optimum. The hardness value of laser-glazed GZ-based TBC increased from 10.66 to 12.73 GPa with decreasing grain size. The estimated cooling rate of the remelted layer of GZ-based TBC was about 103 –104 °C s<sup>−</sup><sup>1</sup> .
