The invention relates to a method to increase the thermal stress capability of a porous TBC and layer system.

Thermal Barrier Coatings (TBCs) are widely used on hot-gas- path metallic parts of Land-based Gas Turbines (LGTs) as well as of aero turbines in order to protect them from the exces sive temperature that commonly exceeds the melting point of the metallic alloys. The most resilient TBCs present a colum nar microstructure that allows the coating to expand rela tively freely accommodating any stress generated either from the Coefficient of Thermal Expansion (CTE) mismatch between the ceramic coating and the underlying metallic part or from the thermal gradient along the coating's thickness. The most well-known coatings deposited with this characteristic micro structure are the EB-PVD deposited partially stabilized YSZ, with an exceptional thermal strain capability and the Seg mented partially stabilized YSZ deposited with Air Plasma Spraying (APS) , with the former being a much more expensive option than the latter. Thus, the segmented coatings are typ ically used in the large land-based gas turbines and the EB- PVDs in the aero or aeroderivative turbines. However, the segmented, being a less costly option, they are still more expensive compared to the typical porous thermal sprayed ce ramic coatings. The reason is the more expensive torches re quired or the more frequent replace rate of hardware as well as the need for a tight temperature control that is required to generate the columnar microstructure during the spraying. The columnar microstructure presents itself as vertical cracks when looking at the coating' s cross section, which has given to this type of thermal spray coatings also the name Dense Vertical Cracked (DVCs) . Until now, on the parts sprayed for LGTs, the segmented coat ings have been deposited with the developed method associated with the higher cost of equipment and process.

It is therefore aim of the invention to overcome the problem mentioned above.

The problem is solved by a method according to claim 1 and a layer system according to claim 5.

In the dependent claims further advantages are listed which can be arbitrarily combined which each other to yield further advantages .

Figure 1, 3 shows a component and figure 2 how to produce cracks in the coating.

The figures and the description are only examples of the in vention .

It is suggested to generate vertical cracks after the deposi tion of the coating and that can be achieved by following up with a specialized heat treatment process. This method can be applied additionally on porous coatings, allowing in such a manner the combination of better thermal protection, as a result of the porous microstructure, with a columnar micro structure which can significantly increase the thermally re lated strain capacity of the coating, increasing thus its temperature of operation and its overall life expectancy.

The principle that the heat treatment is based on is rather simple. After its deposition the coating is typically charac terized by internal porosity. A heat source can be applied on the coating side bringing its surface temperature at over 1273K for long enough time in order to allow the ceramic coating to start sinter gradually from its surface towards the metal substrate. It is important that the heating rate is kept small in the order of less than 50K/s in order to pre- vent the formation of any horizontal cracks in the coating as a result of a much faster expanding ceramic coating compared to a still cooler metallic substrate. The residence time in the high temperature should be long enough to allow sintering to occur in the coating at a certain degree. That will mani fest itself with the disappearance of the finest pores and some of the fine splat boundaries close to the surface of the coating that is closer to the heat source. The residence time is related to the applied heat input, with higher heat input demanding less residence time to induce sintering.

The heat source can be the plasma burner itself which can be used post spraying, to scan the part to allow a relatively even temperature to be brought on the coating surface. Pre requisite is that the powder feed would be shut down, in order to avoid any additional coating deposition.

Alternatively, a laser beam can be used as the heat source. Following its exposure to high temperature, the coating should be cooled rapidly with the use of forced cooled air directed on the coating side. In this manner, the coating will shrink faster than the still hotter metal substrate, setting it under tensile stress. As the coating will have gone through sintering, which will have increased its Young' s modulus, inevitably it will reach rapidly its fracture point and vertical cracks will be generated alleviating in this manner the built-up stress. The depth or frequency of verti cal cracks can be adjusted by manipulating the heat rate, residence time and cooling rate.

Steps to create Vertical Cracks in a deposited porous ceramic coating :

1. Spray the component with the ceramic coating using a

plasma torch. The coating may or may not exhibit espe cially the typical porous microstructure.

2. The part can be cooled down by channeling forced cooling into its interior.

3. After it is cooled down, the forced cooling into its

interior can be maintained on. 4. Employ a thermal camera or any other device to monitor the temperature of the coating at the next steps.

5. With a laser or a plasma torch heat up the coating on the sprayed component with maintaining the temperature raise rate less than 50K/s, to minimize any horizontal cracks due to a fast-expanding hot coating on a much colder substrate. During the heat up the forced cooling in the components interior remains on.

6. After the coating on the component is brought overall in a temperature over 600 °C, the heat source can be focused on the section (s) of the components that the coating needs to present high thermal strain capacity, e.g. lead ing edge, suction side, pressure side etc.

7. The temperature over the section of interest should be brought ideally up to a temperature between 1000°C and 1500°C, especially 1300°C to 1500°C, in order to initiate the sintering of the ceramic coating. Again, the tempera ture is to be raised with a 50K/s max. rate.

8. After achieving the desired temperature, it should be

maintained for time between 5 min and 2 hours or 15 min to 2 hours in order to allow the sintering process to take place from outside (coating rim) towards the interi or of the part.

9. When the desired exposure time is achieved, remove the heating device and immediately initiate forced cooling from the coating side and kill the forced cooling from the interior. This will lead to faster shrinking of the coating compared to the substrate, which will build ten sile stress along its thickness and will create the ver tical cracks.

The depth and frequency of vertical cracks can be adjusted by manipulating heat rate, residence time and cooling rate. Advantages :

• The process can be brought in the thermal spray booth where the parts are sprayed. No need for additional capi tal investments.

• The process can be retrofitted to already sprayed coat ings .

• The process can be brought to follow the deposition of standard porous coatings, which are typically cheaper com pared to segmented coatings.

The porous coatings achieve better thermal protection com pared to the segmented, allowing thinner coatings and thus faster spraying on each part, which means reduced cost per part .

Figure 1 shows a component 1 with a substrate 4, which is es pecially metallic and very especially a nickel or cobalt based superalloy on which a bond coat 7, especially a metal lic bond coat 7, very especially a NiCoCrAlY bond coat, is applied on, which produces a TGO (Thermal Grown Oxid layer) at least during operation.

Yttrium (Y) , Tantalum (Ta) , Rhenium (Re), Iron (Fe) and/or Silicon (Si) , especially only Yttrium (Y) .

On top of this bond coat 7 or substrate 4 a ceramic coating 10 with a certain porosity is applied on.

The porosity of the ceramic coating 10 is preferably 8% to

22%.

Figure 2 shows a component 1 according to figure 1, wherein external heater 13, which can be a spray gun, a laser or any heating device, which heats up the surface 11 of the compo nent 1 which to produces the cracks as described above, be cause a sintered rim 12 is produced (Fig. 3) .