**1. Introduction**

Temperature acting on hot section of gas turbine goes up to about 1300°C during engine operation. However, metallic materials cannot be used at these temperatures due to harsh effect of the high temperature. Therefore, a thermal barrier coating (TBC) system is needed to satisfy effective protection to the metallic components of the gas turbine engines [1–3]. Generally, a conventional TBC system consists of four layers having different functions, as seen in **Figure 1**.

The first part of a TBC system is metallic substrate. Ni-based superalloys (INCONEL) are the most widely used metallic materials in parts of the turbine blade and combustion chamber due to their resistance to high-temperature conditions [4]. Therefore, the substrate material of the TBC system is generally INCONEL alloys.

The bond coat layer is the second part of the TBC system. The MCrAlY (M = Ni, Co, or both of them alongside Fe) bond coating powders are coated with a thickness of 75–150 μm on INCONEL substrate in different techniques such as high-velocity oxy-fuel (HVOF), atmospheric plasma spray (APS), suspension plasma spray (SPS), low-pressure or vacuum plasma spray (LPPS or VPS), and electron beamphysical vapor deposition (EB-PVD). HVOF is the most suitable method to produce the MCrAlY bond coating layer on the substrate due to its cheapness and sufficient characteristic properties [5]. The basic properties expected from the bond coating layer are as follows:


The third part of the TBC system is a thermally grown oxide (TGO, predominately alpha-alumina) layer between the top coat and bond coat. A very thin TGO layer forms on the bond coat during coating process due to oxidation of the bond coat at the process temperatures, but it grows while the TBC system in gas turbine is operating at high temperatures. This layer has importance because failure of the TBCs mostly happens at the interface between TGO layer and ceramic top coat layer when the thickness of TGO layer reaches a critical value. At elevated temperatures

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

(>950°C), Al in the bond coat layer diffuses toward the bond coat layer/ceramic top coat layer interface. On the other hand, oxygen penetrates through the ceramic top coat layer and reacted with the Al. As a result, a TGO layer forms between the bond and the ceramic top coat [6–8].

The fourth and most important part of this system is ceramic top coat layer. Its principal role is to reduce the temperature of metallic substrate providing an insulation layer. Thus, efficiency of gas turbine engine is increased thanks to this thermal insulation layer, allowing higher turbine inlet temperatures (in the range of 100–300°C) and reduced cooling requirements. The thickness of the ceramic top coating layer ranges between 100 and 500 μm depending on the deposition method. Three different methods are used for the production of the ceramic top coat: (i) EB-PVD, (ii) APS, and (iii) SPS [1, 2, 7–14]. As seen in **Figure 2**, the main difference among these three methods is in the morphology of their microstructure.

APS coatings have a lamellar microstructure, while EB-PVD and SPS coatings have a columnar microstructure. Thus, ceramic top coating produced by APS technique has lower thermal conductivity due to microporosities between the lamellae. On the other hand, microporosities between the columns provide higher expansion tolerances in the ceramic top coating produced by EB-PVD and SPS techniques. The APS and SPS processes are more suitable for coating large parts and cheaper than EB-PVD process [10]. Static and stationary parts of the gas turbines are coated with a ceramic insulating material using APS process. A ceramic top coating layer suffers from the severe environmental effects of high temperature such as oxidation, hot

**Figure 2.** *Schematic general structures of TBC produced by (a) EB-PVD method, (b) APS method, and (c) SPS method.*

corrosion, wear, and flying ash damage. Therefore, a ceramic top coating material should have some important characteristic properties [15–17]. The basic properties expected from the ceramic top coating material can be listed as follows:


So far, no single coating material has been found which can fully meet all these properties listed above. However, it is believed that the best material that partially meets these properties is 6–8 wt% yttria-stabilized zirconia (YSZ) for more than 35 years now. Therefore, YSZ is widely and commercially used as a ceramic top coat for TBC in the gas turbine system at the present time. Although an alternative material has not yet been developed for YSZ, studies continue. These studies are important because YSZ has some undesirable properties, limiting working conditions of gas turbine listed below:


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

As a result of these undesirable properties:

1.The maximum use temperature use of YSZ is limited to below 1200°C.

2.YSZ is vulnerable to the hot corrosion and CMAS attack.

However, the TBC material of advanced next-generation powerful gas turbine engines must be able to operate without any damage both in harsh environments (under hot corrosion and CMAS attack) and at temperatures above 1200°C. The increase in efficiency in turbine engines is directly proportional to the increase in engine power and turbine inlet temperature. Therefore, an alternative ceramic top coat material having much better thermal properties than YSZ should be developed to be able to produce next-generation turbine engines. However, it is not easy to find a new material that will be an alternative to the YSZ [8, 13, 14, 18–21] because it requires a lot of experimental study, data, and evaluation.

Gadolinium zirconate (Gd2Zr2O7 or GZ) with pyrochlore or defect fluoritetype structure is a new and promising alternative ceramic top coating material to YSZ. The most important features that make it advantageous are given below:


Thanks to these advantageous features, GZ seems to be the most efficient alternative TBC material for advanced next-generation powerful gas turbine engines. However, besides all of these advantages, GZ has two poor properties affecting its thermal cycling (TC) behavior in a negative manner. One of them is its low coefficient of thermal expansion (CTE, 10.4 × 10<sup>−</sup><sup>6</sup> K<sup>−</sup><sup>1</sup> and 11 × 10<sup>−</sup><sup>6</sup> K<sup>−</sup><sup>1</sup> for GZ and YSZ, respectively), and another one is its high tendency to react with the TGO layer [8, 9, 13, 14, 19, 21–32]. These problems were solved by using multilayered (MLed) and functionally graded (FGed) designs. In these MLed and FGed systems, a second material balances the poor properties (i.e., CTE) of other materials and improves TC performance of TBCs. On the other hand, the reaction between coating layer and TGO which fails the TC performance of rare earth zirconate will be prevented owing to a third layer between GZ and TGO layers [8, 13, 14].

The purpose of this chapter is to summarize the properties of GZ-based thermal barrier coatings. Their production techniques, coating designs, thermal conductivities, thermal cycling behaviors, mechanical properties, hot corrosion and CMAS resistance, and laser surface modification process were compiled in the following sections to form a meaningful whole.
