TITLE:
A METHOD OF MAKING GADOLINIUM ZIRCONATE BASED TBC COATING USING SOLGEL BASED POWDER
FIELD OF THE INVENTION
This invention relates to a method of making gadolinium zirconate based TBC coating using solgel based powder.
BACKGROUND OF THE INVENTION
Hot gas path components of land based gas turbine like combustion liner and transition piece face the highest temperature during the combustion of fuel as high as 1400 ° C. This high temperature gives rise to oxidation to the base metal (superalloys) with time. In order to avoid the oxidation , the hot gas path components are protected by zirconia thermal barrier coatings (TBCs) which are insulating in nature. These coatings are used to extend the life of metal components by creating a temperature drop across the coating, permitting the underlying metal to operate at a reduced temperature.
Conventional TBCs are a two-layer system composed of a zirconia layer 300 µm thick, which faces the hot combustion gases, and a bottom layer of 100 µm bond coat (typically, NiCrAlY alloy). The bond coat provides strong coating adherence and enhances the oxidation resistance of the substrate metals. Ceramic coatings are brittle in nature. Due to brittleness, thicker coating easily cracks due to presence of micro cracks and porosity within the coating.

The coatings were applied by the plasma spray technique. In this process an electric arc ionizes an argon gas to form a plasma. Ceramic powders are injected into the plasma, heated to a "semiplastic" state, and accelerated toward the coating surface. When the particles impact the target, a very complex interlocking microstructure with roughened surface and semimelt powder. Pure zirconia is not used as top coat because pure zirconia in the room temperature has monoclinic structure which transform into tetragonal structure on heating. While cooling down, it comes back to monoclinic structure and with 3% volume expansion. Which finally give rise to cracking on the coating. In order to avoid this problem, the top coat is made of 8 wt% Yttria partially stabilized zirconia which does not change to monoclinic structure.. This material has much better thermal shock resistance than the fully stabilized zirconia and pure zirconia.
Gas turbine hot path components are made of cobalt/nickel based superalloy. These alloys can work at much higher temperature than the alloyed steel. These alloys are also high temperature oxidation resistant. Some of the higher rating of gas turbine operates near to melting point of superalloys or higher than melting point of superalloys. In those cases, it is very essential to coat the surface of the alloy with insulating material like alumina, zirconia, mullite, etc. Yttria stabilized zirconia (YSZ) has been very successful as thermal barrier coating (TBC) for GT components. YSZ contains 8-10% yttria which stabilize the structure of zirconia as cubic structure in room temperature. YSZ has low thermal coefficient compare to base metal. This result in thermal expansion mismatch during high temperature operation leads to cracking in the TBC coating. In order to reduce the mismatch of thermal expansion, an intermediate coat of MCrAlY(M = Ni/Co) is applied which is called bond coat.

In the conventional TBC coating Bond coat is applied to a thickness of 100 to 120 microns. Before bond coating, the substrate is prepared by grit blasting to remove the dirt and oxides present in the substrate as well as to have better adhesion of coating by mechanical anchoring. After grit blasting, bond coat is applied. Over the bond coat, top coat of YSZ is applied to a thickness of 200 to 250 microns. Both the coating is applied by plasma spray system like Metco 9 MB, Metco 4 MB system, etc. In these system argon gas is used as primary gas for generating the plasma. Hydrogen gas also added to increase the enthalpy of plasma. Powder can be fed internally as well as externally. Plasma produces high temperature which leads to overheating of the coating substrate. Overheating may lead to coating cracking as well as oxidation of coating substrate. To overcome such problem, compressed air is used to cool the coating substrate,
OBJECTS OF THE INVENTION
An object of the present invention is to propose a method of making gadolinium ZIRCONATE based thermal barrier coating for land based gas turbine components.
Another object of the present invention is to propose a gadolinium ZIRCONATE based thermal barrier coating to reduce oxidation.
Further object of the present invention is to propose a gadolinium ZIRCONATE based thermal barrier coating which can sustain high temperature as high as 1400oC.
Yet another object of the present invention is to propose a gadolinium ZIRCONATE based thermal barrier coating which does not change the phase or undergo degradation.

BRIEF DESCRIPTION OF THE INVENTION
According to this invention there is provided a method of making gadolinium ZIRCONATE
based thermal barrier coating for land based gas turbine components comprising:
preparing gadolinium zirconate powder by solgel route,
depositing the said gadolinium zirconate powder on gas turbine as a top coat by plasma spray
and
depositing Ni 22Cr 10A11.0Y alloy powder as a bond coat.
This invention also relates to power generation by land based gas turbine is one of the major sources in India. Advanced gas turbines operate at higher temperatures (>1400oC). Hot gas path components of these gas turbines get oxidized at this high temperature. In order to protect these components from high heat flux, gadolinium zirconate based thermal barrier coating is applied. Gadolinium zirconate powder is prepared by solgel route . This coating is made of two layers - one is a bond coat, which is made of CoCrAlY and the other is top coat, which is made of gadolinium zirconate. This coating is applied by plasma spray process, which is the only method by which such ceramic powder can be deposited. Plasma spray process produces lot of heat, which oxidize the coating substrate. In order to avoid such oxidation, sufficient forced air-cooling of the coating is undertaken. Coating is evaluated by X-ray diffraction (XRD) for phase integrity, by image analysing microscope, etc. as show in fig 3,

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Figure 1: shows the SEM micrograph of fine GDZ powder
Figure 2: shows the as-coated plasma sprayed samples of GdZ and bond coat on IN 718 substrate.
Figure 3: shows the XRD Pattern of Gadolinium zirconate powder after calcination at 1500°C. DETADLED DESCRIPTION OF THE INVENTION
In this study, gadolinium zirconate (Gd2Zr207) powder was prepared through solgel route by co precipitating Gadolinium nitrate (Gd(N03)36H20) and Zirconyl chloride octahydrate (ZrOCl28H20) in stoichiometric amounts. Citric acid and Ethylene glycol are added in the ratios of 1:1 to the mixture. This liquid mixture is heated on hot plate for 20 hours at 80°C till the gel is formed. This thick viscous gel is kept in oven at 100°C to remove moisture. Then gel is dried at 200°C to remove all the organic matter for 3 hours. This dried gel is taken in a Zirconia crucible and sintered at 1500°C to form Gadolinium zirconate Gd2Zr207
Plasma spray deposition was done by 80 KW plasma spray system with a Metco 7MC-II 9 MB gun. The powder injection was external from the nozzle using Argon as the carrier gas. The plasma was generated using the mixture of Argon and hydrogen gas. The powder feed rate is maintained constant at 6 rpm, using a volumetric powder feeder. Centrifugal pump (water cooling) is used for cooling the system, regulated at a temperature of 11-I4°C.

Argon is taken as the primary plasma gun gas and hydrogen as the secondary gas. The plasma operation is carried out robotically and the spraying angle is kept close to 90°. The properties of the coating products are dependent on the spray process parameters. The operating parameters during coating deposition are listed below
Plasma spraying was immediately carried out after cleaning to avoid oxidation. First the bond coat was deposited on the grit blasted samples. The plasma spray parameters for the bond coat and top coat are given in following tables
Bond Coat:
Ni-Cr-Al-Y alloy powder for plasma thermal spray application The particle size of the powder is -106 +53 microns. The composition of the powder is as follows: Ni 22Cr 10A11.0Y.

Total thickness of the bond coat was kept around 70-90 µm

Top Coat:
Gadolinium Zirconate (Gd2Zr207) plasma spray trials were carried out by varying the plasma current, hydrogen flow and stand-off distance are below. The particle size of the powder is 25 to 53 microns. Powder was spherodiesd by agglomeration using PVA binder followed by sieving operation.

Total thickness of coating 330-350 µm (25 passes)

WE CLAIM:
1. A method of making gadolinium based thermal barrier coating for land based gas turbine
components comprising:
preparing gadolinium zirconate powder by solgel route,
depositing the said gadolinium zirconate powder on gas turbine as a top coat by plasma spray
and
depositing Ni 22Cr 10A1 1 .OY alloy powder as a bond coat.
2. The method as claimed in claim 1, wherein the said solgel route comprising :
precipitating Gadolinium nitrate and zirconyl chloride octahydrate in stoichiometric amounts,
adding citric acid and ethylene glycol in a ratio of 1:1
heating the said liquid mixture on hot plate for 20hrs at 80 oC till gel is formed,
subjecting the said viscous gel to the step of removing mixture,
drying the gel at 200°C,
sintering the dried gel at 1500°C to form Gadolinium zirconate.
3. The method as claimed in claim 1, wherein the particle size of the Ni-Cr-Al-Y alloy powder
for bond coat is -106 + 53 microns.
4. The method as claimed in claim 1, wherein the particle size of the Gadolinium Zirconate
powder of top coat is 25 to 53 microns.

5. The method as claimed in claim 1, wherein the total thickness of bond coat is 70-90µm.
6, The method as claimed in claim 1, wherein the total thickness of top coat is 330-350µm.