The present invention relates to coated turbo machine compo ^¬ nents and more particularly to a method of coating turbo machine components with bond coats.

Turbo machines like gas turbines or steam turbines are required to operate at high temperatures to achieve high efficiency. They operate at temperatures of the order of 900°C or more. However, as operating temperatures increase, certain components of the turbo machine face higher thermal loading and to overcome the harsh effects of high thermal loading the temperature durability of the turbo machine com ^¬ ponents must correspondingly increase. For example, the blades and the vanes of the turbo machines face more thermal loading as compared to other parts of the turbo machine.

Thermal barrier coatings are used to provide insulation for these turbo machine components that operate at very high tem ^¬ peratures .

Such thermal barrier coatings typically consist of four lay ^¬ ers: the metal substrate, metallic bond coat, thermally grown oxide, and ceramic topcoat. Turbine blades and vanes need the metallic coating as a bond coating or as an overlay coating. This metallic bond coat has a certain lifetime depending on its thickness and on the temperature level during the opera ^¬ tion time. However, bond coats of different qualities and with different properties are available. Turbine parts coated with a low quality bond coat, such as SC2231, must not be op ^¬ erated in high thermal loading conditions as the low quality bond coat cannot withstand such high thermal loads and would chip off or crack as a result of which the coat would not be able to provide the necessary protection to the parts. There ^¬ fore a high quality bond coat, such as SC2464, is used under such conditions. However, a high quality bond coat is more expensive than the low quality bond coat. So, when the high quality bond coat is applied on all parts of the turbo machine, the turbo machine parts become more expensive to manufacture. This increase in the manufacturing cost of the turbo machine parts leads to lower user satisfaction.

Therefore, there is a need to achieve a balance between per ^¬ formance and manufacturing cost for bond coats that can be applied to turbo machine components facing high thermal loads during operation of the turbo machine.

It is an object of the present invention to provide a solu ^¬ tion for coating turbo machine components which will give optimum performance and protect the turbo machine components adequately without increasing the manufacturing cost of the coated turbo machine components.

The object is achieved by providing a method for coating the turbo machine components with different bond coats based on the thermal load distribution on the turbo machine components and the thermal load bearing capacities of the bond coats.

The object is achieved by providing a method for coating a turbo machine component according to claim 1. The proposed solution overcomes the issue of chipping off, spallation or damage caused to bond coats at high thermal loaded operations of the turbo machine component, but at the same time the solution also limits the manufacturing cost of the turbo machine component to a reasonable amount by selec- tively applying high quality and more expensive bond coats to regions with higher thermal loads and low quality and less expensive bond coats to regions with lower thermal loads.

A turbo machine component and a method of coating a turbo machine component with bond coats is disclosed. The method comprises the steps of coating a first region of the turbo machine component with a first bond coat and coating a second region of the turbo machine component with a second bond coat, wherein during operation of the turbo machine component the first region has lower thermal loading than the second region and the first bond coat has a lower thermal load bear ^¬ ing capacity than the second bond coat.

The first bond coat can be SC 2231 and the second bond coat can be SC 2464.

The invention also discloses an interface zone on the turbo machine component where the first and the second regions in ^¬ terface. The interface zone is first coated with the first bond coat and thereafter coated with the second bond coat such that the two bond coats overlap and form a composite bond coat. The thickness of the composite bond coat in the interface zone matches a thickness of the first bond coat and the second bond coat on either sides of the interface zone.

The method for coating a turbo machine component comprises the step of coating a first region of the turbo machine com- ponent with a first bond coat and coating a second region of the turbo machine component with a second bond coat, wherein during operation of the turbo machine component the first region has lower thermal loading than the second region, and the first bond coat has a lower thermal load bearing capacity than the second bond coat.

The method has several advantageous characteristics. By coat ^¬ ing the turbo machine components according to its thermal loading, we can achieve the desired performance of the turbo machine components with superior and complete protection of the components from adverse effects of high temperatures and thermal loads and at the same time achieve cost optimization for manufacturing the components. Also by using this method of selectively applying high quality bond coat having higher thermal load bearing capacity to higher thermal loaded areas and low quality bond coat having lower thermal load bearing capacity to lower thermal loaded areas we can reduce the pow ^¬ der loss of the high quality more expensive bond coat as more powder loss takes place while applying bond coats to areas like platform region of a blade which has lower thermal load ^¬ ing . In one embodiment of the present invention, the first bond coat is SC 2231 and the second bond coat is SC 2464. These two bond coats together give the optimum protection to the turbo machine components. Moreover, these two bond coats overlap with each other to form a composite bond coat at an interface zone and provide protection to the components from thermal fatigue caused due to frequent rise and fall of tem ^¬ perature levels during operation of the turbo machine compo ^¬ nent. This prevents the bond coats from cracking due to ther ^¬ mal fatigue.

In one embodiment of the method, the first region and the second region overlap at an interface zone such that a part of the first region is at a first side of the interface zone and a part of the second region is at a second side of the interface zone, wherein the interface zone is first coated with the first bond coat and subsequently coated with the second bond coat to form a composite bond coat having a thickness. This provides a double layer protection for the interface zone which helps in adequately warding off the ad- verse effects of the hot gas and fluid on the interface zone during operation of the turbo machine.

In another embodiment of the method, the composite bond coat in the interface zone has a thickness which matches a thick- ness of the first bond coat at the first side of the inter ^¬ face zone and the second bond coat at the second side of the interface zone. The interface zone experiences more frequent temperature surges and drops and this leads to surface crack ^¬ ing of the bond coat due to thermal fatigue. The advantage of having a composite bond coat on the interface zone in com ^¬ parison to having a single layer bond coat is that the com ^¬ posite bond coat prevents surface cracking of the bond coat due to thermal fatigue as it provides a dual layer protection to the interface zone. The two bond coats together form a composite bond coat having a higher thermal load bearing capacity and a higher resistance to thermal fatigue. Thus an optimized material combination is achieved at the interface zone.

In another embodiment of the method, during the operation of the turbo machine component the first region is at a tempera ^¬ ture level less than 900°C and the second region is at a tem- perature level more than or equal to 900°C. This enables the turbo machine component to function in a long range of oper ^¬ ating temperatures and withstand the damaging effects of high temperature fluids or gases without getting damaged. In one embodiment of the method, the first and the second regions are coated with the first and the second bond coats respectively using a plasma spray process, wherein the first bond coat is provided via a first powder feed line and the second bond coat is provided via a second powder feed line. Plasma spray process will ensure that the bond coats are uni ^¬ formly spread over the turbo machine surface. The powder feed lines would ensure that there is a continuous supply of the bond coat material for spraying during the plasma spray pro ^¬ cess. Plasma spray process offers increased durability of the applied bond coats and provides a spallation resistant layer.

In one embodiment, the first and the second regions are coated with the first and the second bond coats respectively with the help of a first and a second spray gun, wherein the first powder feed line is connected to the first spray gun and the second powder feed line is connected to the second spray gun. The advantage of using spray guns is that the bond coat can be applied uniformly with precision. The bond coat material is ejected in a controlled flow from the spray guns and the coating process can be carried out smoothly with greater accuracy. Further in accordance with the instant disclosure, there is provided a turbo machine component which broadly comprises a surface having at least a first region and a second region, and a first bond coat on the first region and a second bond coat on the second region, wherein during operation of the turbo machine component the first region has lower thermal loading than the second region and the first bond coat has a lower thermal load bearing capacity than the second bond coat .

In one embodiment of the present invention, the first bond coat is SC 2231 and the second bond coat is SC 2464. These two bond coats together give the optimum protection to the turbo machine components and at the same time maintain a rea- sonable manufacturing cost of the turbo machine components.

In an embodiment of the turbo machine component the first region and the second region overlap at an interface zone such that a part of the first region is at a first side of the interface zone and a part of the second region is at a second side of the interface zone. The interface zone has the second bond coat coated over the first bond coat to form a composite bond coat having a thickness matching a thickness of the first bond coat on the first side of the interface zone and the second bond coat on the second side of the interface zone. As mentioned earlier, the interface zone ex ^¬ periences more frequent temperature variations and this leads to surface cracking of the bond coat due to thermal fatigue. The advantage of having a composite bond coat on the inter- face zone in comparison to having a single layer bond coat is that the composite bond coat prevents surface cracking of the bond coat due to thermal fatigue as it provides a dual layer protection to the interface zone. The two bond coats together form a composite bond coat having a higher thermal load bear- ing capacity and a higher resistance to thermal fatigue.

In another embodiment, the turbo machine component is a blade of a rotor of a turbo machine. The first and the second regions are located on an airfoil of the blade. The blades of a turbo machine rotor are impinged with very high temperature gases or fluids during operation of the turbo machine, there ^¬ fore the bond coating disclosed in accordance with this in- vention would provide the necessary protection to the blades during their operation.

In another embodiment, the rotor has an axis of rotation and the airfoil comprises a radial outer portion and a radial inner portion, wherein the radial direction is defined with reference to the axis of rotation of the rotor. The second region is located at least on the radial outer portion of the airfoil. Coating this second region with the high quality bond coat will protect the higher thermally loaded region, i.e. the radial outer portion of the airfoil, from the impact of high temperature gases and fluids thereby preventing spallation of the bond coat. The radial inner portion of the blade experiences lower thermal loading as compared to the radial outer portion of the blade. Hence, based on the ther- mal load distribution over the blade regions during operation of the rotor blades, a most effective way of coating the blade regions with the different types of bond coats can be achieved. This will ensure that the manufacturing cost of the blade is also minimized as lesser amount of the high thermal load bearing capacity bond coat, which is more expensive, will be used to coat only a selective region, i.e. the radial outer portion of the airfoil, instead of coating the entire airfoil . In another embodiment, the first region is located on the radial inner portion of the airfoil, or on a platform of the blade or on both. Both the radial inner section of an airfoil region as well as the platform region of the blade face lesser thermal loading and hence can be coated with a lower quality and less expensive bond coat having lower thermal load bearing capacity thereby reducing the manufacturing cost of the turbo machine components like airfoils and platforms of blades. In yet another embodiment, the turbo machine component is a vane of a stator of a turbo machine. The vanes of a stator also lie in the path of hot gases or fluids during operation of the turbo machine. The differential coating with two dif ^¬ ferent bond coats depending on the thermal loading of the regions on the vanes will ensure that the vanes are ade ^¬ quately protected and at the same time the manufacturing cost of the vanes are optimized.

In another embodiment, the vane comprises a surface having a length with a first end and a second end, wherein the first end is separated from the second end by the length of the vane. The vane is attached to the stator at the first end, and the first region is located on the surface at a portion adjacent to the first end and the second region is located on the surface at a portion adjacent to the second end. This helps in identifying and differentiating the regions of higher thermal loading from the regions of lower thermal loading on a vane so that based on the thermal load distribu ^¬ tion the different bond coats having different thermal load bearing capacities can be applied to the corresponding regions . In a further embodiment a turbo machine component coating is disclosed. The coating comprises a first bond coat to coat at least a first region of a turbo machine component and a second bond coat to coat at least a second region of the turbo machine component, wherein during operation of the turbo machine component the first region has lower thermal loading than the second region, and further the first bond coat has a lower thermal load bearing capacity than the second bond coat. In an embodiment of the coating, the turbo machine component coating is an airfoil coating. For example, the turbo machine component coating is an airfoil coating, comprising a first bond coat to coat at least a first region of the airfoil and a second bond coat to coat at least a second region of the airfoil, wherein when the airfoil is in operation, the first region has lower thermal loading than the second region, wherein the first bond coat has a lower thermal load bearing capacity than the second bond coat. Therein, the second region is located at least on a radial outer portion of the airfoil and the first region is located on the radial inner portion of the airfoil and/or on a platform of the blade. The above-mentioned and other features of the invention will now be addressed with reference to the accompanying drawings of the present invention. The illustrated embodiments are intended to illustrate, but not limit the invention. The drawings contain the following figures, in which like numbers refer to like parts, throughout the description and drawings.

FIG. 1 is a schematic diagram of an exemplary turbo machine component . FIG. 2 is a schematic diagram of a cross-section of the interface zone when seen from the side.

FIG. 3 is a schematic diagram of the interface zone when seen from the top.

FIG. 4 is a schematic diagram of a rotor of a turbo machine.

FIG. 5 is a schematic diagram of a vane of a turbo machine where the vane is attached to a stator at only one end of the vane .

FIG. 6 is a schematic diagram of a vane of a turbo machine where the vane is attached to a stator at both ends of the vane .

FIG. 7 is a schematic diagram depicting the powder feed lines and spray guns used during the plasma spray process of coat ^¬ ing the turbo machine components with the bond coats. FIG. 8 is a flowchart depicting the method for coating the turbo machine component. Embodiments of the present invention described below relate to a turbo machine component and more specifically to a blade component of a turbo machine. However, the details of the em ^¬ bodiments described in the following can be transferred to a vane component without modifications, that is the terms

"blade" or "vane" can be used in conjunction, since they both have the shape of an airfoil. The turbo machine may include a gas turbine, a steam turbine, a turbofan and the like.

Figure 1 is a schematic diagram of an exemplary turbo machine component 1. The turbo machine component 1 broadly comprises a surface 2 having at least a first region 3 and a second region 4. The turbo machine component 1 further comprises a first bond coat 5 on the first region 3 and a second bond coat 6 on the second region 4. During operation of the turbo machine component 1 the first region 3 has lower thermal loading than the second region 4 and the first bond coat 5 has a lower thermal load bearing capacity than the second bond coat 6. The first region 3 and the second region 4 interface at an interface zone 7. The first region 3 is at a first side 9 of the interface zone 7 and the second region 4 is at a second side 10 of the interface zone 7. The interface zone 7 is first coated with the first bond coat 5 and subse ^¬ quently coated with the second bond coat 6 to form a compos ^¬ ite bond coat 11.

In one embodiment of the invention, the turbo machine compo ^¬ nent 1 is a blade 12 of a rotor of the turbo machine. This will be explained in more detail with reference to FIG. 3. Thermal loading on a region here means the impact of gases or fluids at very high temperatures on that region. For turbo machines, the operating temperatures can be of the order of 900°C. When a region is subjected to such high temperatures during the operation of the turbo machine component, the region is said to have high thermal loading. Thermal load bearing capacity here means the ability to bear high thermal loads without breaking down or wearing out. In respect of bond coats, a high thermal load bearing capacity would mean that the bond coat is capable of withstanding high tempera ^¬ tures and thermal loads without leading to spallation, wear and tear or cracking of the bond coats due to the high tem ^¬ peratures during operation.

When the turbo machine is in operation, the first region 3 of the turbo machine component 1 reaches a temperature level of less than 900°C and the second region 4 of the turbo machine component 1 crosses or equals a temperature level of 900°C.

In accordance with the aspects of the present invention, the first bond coat 5 is a low quality bond coat. The second bond coat 6 is a high quality bond coat. The first bond coat has preferably a chemical composition of 29-31% Ni, 27-29% Cr, 7- 8% Al, 0.5-0.7% Y, 0.3-0.7% Si and rest is Co. The second bond coat has preferably a chemical composition of 24-26 Co, 16-18% Cr, 9.5-11% Al, 0.2-0.4% Y, 1.2-1.8% Re and rest is Ni .

Referring now to Figure 2, a schematic diagram of a cross- section of the interface zone 7 is disclosed when viewed from a side of the blade 12. In accordance with the aspects of the present invention, the first region 3 and the second region 4 interface at an interface zone 7 such that the first region 3 is at a first side 9 of the interface zone 7 and the second region 4 is at a second side 10 of the interface zone 7. The interface zone 7 is first coated with the first bond coat 5 and subsequently coated with the second bond coat 6 to form a composite bond coat 11. The composite bond coat 11 in the interface zone 7 has a thickness which matches a thickness of the first bond coat 5 at the first side 9 of the interface zone 7 and the second bond coat 6 at the second side 10 of the interface zone 7.

The bond coats 5, 6 merge over this interface zone 7 to coat the interface zone 7. Thus, the interface zone 7 is coated with dual layers of bond coats. The interface zone 7 experi ^¬ ences a wider range of operating temperatures as compared to the adjacent first 3 and the second 4 regions. For example, the first region 3 experiences operating temperature ranges below 900°C and the second region 4 experiences operating temperature ranges more than or equal to 900 °C. The interface region 7 invariably has to experience operating temperature ranges faced by both the first 3 and the second 4 regions as it lies adjacent to both the regions. The effect of coating the interface zone 7 with two bond coats 5, 6 is that the bond coats 5, 6 together form a composite bond coat 11 which is capable of protecting the interface zone 7 from thermal fatigue caused due to the wide range of operating tempera ^¬ tures. The individual properties of each layer of bond coats 5, 6 will also be retained. The thickness of the composite bond coat 11 matches the thickness of the bond coats at the adjacent first 3 and the second 4 regions so that there is no protrusion on the surface of the turbo machine component 1 at the interface region 7. Thus, the flow of hot fluid or gas during operation of the turbo machine is not disturbed.

Figure 3 shows the interface zone when seen from the top, i.e. in a direction perpendicular to the surface 2 of the turbo machine component 1 and the blade, respectively. The composite bond coat 11 covers the interface zone 7, whereas the first region 3 is covered by the first bond coat 5 and the second region 4 is covered by the second bond coat 6. The first 3 and the second regions 4 are located on respective first 9 and second sides 10 of the interface zone 7. Figure 4 shows a schematic diagram of a rotor 13 of a turbo machine. The rotor 13 has an axis of rotation 14 and an axial direction is defined with reference to the axis of rotation 14. According to FIG. 4, the turbo machine component 1 is a blade 12 of the rotor 13 of the turbo machine. The blade 12 comprises an airfoil 26, a platform 17, and a root portion (not shown) . The blades 12 of a turbo machine rotor 13 func ^¬ tion by rotating in the path way of hot fluids or gases. The blades 12 receive very high impact of hot fluids or gases when the hot fluids or gases impinge on the blades 12. There ^¬ fore the bond coatings 5, 6 disclosed in accordance with this invention and explained with reference to FIG. 1 and FIG. 3 would provide the necessary protection to the blades 12 dur ^¬ ing their operation.

In one embodiment, the first region 3 is coated with the first bond coat 5 and the second region 4 is coated with the second bond coat 6, wherein both the first as well as the second regions lie on the airfoil 26. In another embodiment the airfoil 26 is covered with the second bond coat 6 and the platform 17 is covered with the first bond coat 5.

The airfoil 26 comprises a radial outer portion 15 and a radial inner portion 16. The second region 4 with the second bond coat 6 is located at least on the radial outer portion

15 of the airfoil 26 and the first region 4 with the first bond coat 6 is located at least on the radial inner portion

16 of the airfoil 26. Hence, the second region 4 can be the radial outer portion 15 of the airfoil 26 and the first region 3 can be the radial inner portion 16 of the airfoil 26.

In another embodiment, the second region 4 can be a part of the radial outer portion 15 of the airfoil 26 and the first region 3 can extend from the radial inner portion 16 of the airfoil 26 to a part of the radial outer portion 15 of the airfoil 26 which is not a part of the second region 4. In yet another embodiment, the second region 4 can be both the radial outer portion 15 of the airfoil 26 and the radial inner portion 16 of the airfoil 26. In another embodiment, the first region 3 is located on the radial inner portion 16 of the airfoil 12 or on a platform 17 of the blade 12, especially on a radially outer surface of the platform 17. In another embodiment the first region 3 is located both on the radial inner portion 16 of the airfoil 12 and on the platform 17 of the blade 12.

Referring now to Figure 5, a schematic diagram of a vane 18 of a turbo machine is depicted. As can be seen from the dia ^¬ gram, only one end of the vane 18 is attached to a stator 21 of the turbo machine and the other end is free.

Figure 6 shows a schematic diagram of a vane 18 of a turbo machine where the vane 18 is attached to the stator 21 turbo machine at both ends of the vane 18.

The bond coat arrangement as disclosed in this invention can also be used for coating vanes of a turbo machine. The vanes also experience high thermal loading during the operation of the turbo machine, like the blades of the rotor. The vane 18 essentially comprises a surface 2 having two ends, a first end 19 and a second end 20 separated from each other by a length of the vane 18 in the radial direction. The vane 18 is attached to the stator 21 at the first end 19, and the first region 3 with the first bond coat 5 is located on the surface 2 at a portion adjacent to the first end 19 and the second region 4 with the second bond coat 6 is located on the sur ^¬ face 2 at a portion adjacent to the second end 19. Again, an interface region (not shown in FIG. 5 and FIG. 6) can be lo- cated between the first and second region. Referring now to Figure 7, the bond coats 5, 6 may be applied using any suitable techniques known in the art. In a pre ^¬ ferred embodiment, the bond coats are applied by a plasma spray process. In the plasma spray process the bond coats are supplied through powder feed lines to respective spray guns for spraying on desired regions. In accordance with the aspects of the invention, the first 3 and the second 4 regions are coated with the first 5 and the second 6 bond coats respectively using a plasma spray process, wherein the first bond coat 5 is provided via a first powder feed line 22 and the second bond coat 6 is provided via a second powder feed line 23.

The spray of bond coats is ejected from spray guns. In the disclosed invention, the first powder feed line 22 is con ^¬ nected to the first spray gun 24 and the second powder feed line 23 is connected to the second spray gun 25. The first 3 and the second 4 regions are coated with the first 5 and the second 6 bond coats respectively using the first 24 and the second 25 spray gun.

The second bond coat 6 has higher thermal load bearing capac ^¬ ity than the first bond coat 5 therefore the air plasma spray (APS) spallation time of the second bond coat 6 is longer than that of the first bond coat 5. Where the two bond coats 5, 6 overlap on the interface zone 7, the bond coat with the relatively longer APS spallation time, that is the second bond coat 6, is applied after the interface zone 7 is first coated with the first bond coat 5, which has a relatively shorter APS spallation time. This could be achieved by a pro ^¬ cess sequence of coating starting with the coat having lower APS spallation time, i.e. the first bond coat 5, followed by coating with the second bond coat 6. The surface 2 of the turbo machine component 1 which is facing the hot gas path or hot fluid path has the second bond coat 6 coated over the first bond coat 5 to provide a better protection to the interface zone 7 from the spallation effects of hot gas or fluid . Referring now to FIG. 8, a method 100 for coating a turbo machine component 1 is disclosed. The method 100 comprises a step 101 of coating a first region 3 of the turbo machine component 1 with a first bond coat 5 and a step 102 of coat ^¬ ing a second region 4 of the turbo machine component 1 with a second bond coat 6, wherein during operation of the turbo machine component 1 the first region 3 has lower thermal loading than the second region 4, and the first bond coat 5 has a lower thermal load bearing capacity than the second bond coat 6.

Although the invention has been described with reference to specific embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternate embodiments of the invention, will become apparent to persons skilled in the art upon reference to the description of the invention. It is therefore contemplated that such modifications can be made without departing from the embodiments of the present inven ^¬ tion as defined.