**2.1 Production of gadolinium zirconate-based thermal barrier coatings**

The performance and many properties of the TBC mainly depend on the microstructure and hence the production techniques. In literature, gadolinium zirconatebased thermal barrier coatings have been produced by different techniques such as the air plasma spraying (APS) [8, 13, 14], suspension plasma spraying (SPS) [25, 30], and electron beam-physical vapor deposition (EB-PVD) [20, 33].

In a study, GZ-based TBC was produced as single layered on metallic bond coat using the APS process [13]. The surface and cross-sectional microstructure of plasma-sprayed GZ-based TBC were shown in **Figure 3**. The surface roughness (Ra) was 8.3 μm, and typical surface properties (such as unmelted particles and open porosities) of APSed TBCs could be seen in **Figure 3a**. The micrographs demonstrate typical characteristic microstructure defects of APSed thermal barrier coatings such as cracks, both parallel (at splats boundaries) and normal to the metal/ceramic interface, and porosities. The interface between ceramic top coat/ bond coats seems continuous and durable.

As seen in **Figure 4**, XRD patterns of GZ powder having an as-sprayed GZ-based coating proved that there was no decomposition after plasma spraying [8]. Both patterns had a cubic fluorite-type structure of GZ with the space group of Fm3m. Gd2Zr2O7 phase was maintained its stable position after APS process. Carpio et al. [23] showed that YSZ- and GZ-based TBCs had the same porosity value when

**Figure 3.**

*(a) Surface and (b) cross-sectional SEM images of atmospheric plasma-sprayed GZ coating.*

**Figure 4.** *The XRD patterns of powder and atmospheric plasma-sprayed GZ coating.*

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

the same APS parameters were used. These results indicate that there is no microstructural problem in producing GZ by APS.

In different studies [25, 30], GZ-based TBCs were successfully produced on YSZ by suspension plasma spraying (SPS) process. SPSed GZ-based TBCs had nonporous and crack-free interface between the different top coat layers. In addition, SPSed GZ-based TBCs had a columnar microstructure having orientation perpendicular to the top surface which is a characteristic of the process. On the other hand, it is possible to obtain denser coating morphology by increasing droplet size via changing SPS parameters. Bozbin et al. [33] produced TBCs having GZ-based ceramic top coat by EB-PVD technique. The morphology of TBCs was columnar. This morphology was a characteristic of the coating produced by EB-PVD process. They showed that morphology of the GZ-based TBCs could be controlled by deposition process and temperature. The results of these studies [8, 13, 14, 20, 25, 30, 33] proved that GZ is a suitable material to be coated with different TBC production processes (such as APS, SPS, and EB-PVD).

## **2.2 Design of gadolinium zirconate-based thermal barrier coatings**

Despite all the superior thermophysical properties of the single-layered GZ-based TBC, its thermal cycling lifetime is poor. Multilayered (as seen in **Figure 5a**–**c**) and functionally graded (as seen in **Figure 5d** and **e**) coating designs with a second TBC material have been used to overcome this problem [16]. In the multilayered (MLed) TBCs, there are two or more coating layers having different functions. On the other hand, a functionally graded (FGed) coating has gradual compositional variation in the layers. In these systems, a second material having higher coefficient of thermal expansion (CTE) balances the CTE of the system and improves TC performance of TBCs. The residual stress on the MLed and FGed coatings is lower than that of single-layered GZ [8, 14, 34]. The reaction between coating layer and TGO has been prevented owing to a second material adjacent to the top of the bond coat [8, 14].

**Figure 5.** *Schematic view of multilayered (a–c) and functionally graded (d, e) coating designs.*

In a previous study [8], MLed and FGed gadolinium zirconate-based TBCs were produced in 2, 4, 8, and 12 layered by APS process. In this study, CYSZ was used as the second material because its CTE (13 × 10<sup>−</sup><sup>6</sup> K<sup>−</sup><sup>1</sup> ) was better than that of GZ (10.4 × 10<sup>−</sup><sup>6</sup> K<sup>−</sup><sup>1</sup> ). While producing a MLed system, CYSZ or GZ was sprayed to each layer. On the other hand, GZ and CYSZ powders were mixed at different ratios in a turbula-type mixer to obtain different compositions, and these mixtures of powders were sprayed to each layer. The average thickness of the ceramic top coat was 350 μm. The interfaces between CYSZ and GZ layers were distinguishable, but there were no distinguishable interfaces between the different layers of FGed coatings. The following results were obtained from this study:


The increase in the porosity level was explained by discontinuous coating process of MLed and FGed designs (layer-by-layer spraying).

In a study, double-layered and functionally graded GZ/YSZ coatings having five layers were produced with the thickness of ~200 μm by APS process [23]. In the functionally graded GZ/YSZ TBC, ratio of the GZ to YSZ was different in each layer but bottom and top most layer were 100% YSZ and GZ, respectively. Researchers used two independent feeders to feed GZ and YSZ powders to the plasma flame, separately. Thus, there was no need to mix the powders before the APS process. Thanks to this system, different compositions could be obtained in each layer at the desired grade. It could be understood that it was possible to deposit the GZ and YSZ with functionally graded design by APS.

Mahade et al. [9, 29, 30] produced a double-layered GZ/YSZ and a triple-layered GZ dense/GZ/YSZ TBC by SPS process. The schematic view of their design was given in **Figure 6**. The morphology of the coatings was columnar, which was characteristic of the SPS, but dense in the topmost layer of the triple system. They aimed to increase the CMAS resistance of TBC thanks to a dense GZ layer on the topmost layer of triple-layered system. The density of coating layer was adjusted by changing the SPS parameters such as temperature and gas flow. The microstructure

#### **Figure 6.**

*Schematic view of GZ/YSZ (a) double-layered and (b) triple-layered systems [9, 29, 30].*

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

from the top surface of double-layered GZ/YSZ TBC was resembling a cauliflower, but top surface morphology of triple-layered TBC was denser due to dense GZ layer.

Mutilayered GZ/YSZ TBCs having ten layers were deposited by EB-PVD process [35]. In addition, nano-layered GZ/YSZ TBCs having ~200 nm layer thickness were produced by the same process. The production of multilayered TBCs was carried out by alternating layers of GZ and YSZ during deposition. As expected, the morphology of the coating was columnar. These results proved that GZ-based TBCs could be deposited with different techniques and designs to improve some of their properties by using a second coating material.
