Method for manufacturing a gas-tubine blade

Abstract: The main body of an alloy for a gas-turbine blade has an outer surface which has concaved portions (10) except around through holes (4) allowing a cooling fluid to pass. The concave portions (10) holds thereon a heat-shielding coating which consists of an inner bonding layer and an outer ceramic layer.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention: This invention relates to a method for manufacturing a gas-turbine blade, particularly one having a heat-shielding coating layer formed on its surface. 2. Description of the Prior Art: The blades of a high temperature gas turbine are cooled to or below the temperature which the blade material can withstand. A cooling method, such as impingement or film cooling, is usually employed to cool the blades by utilizing a part of compressed air. The blade main body is made of an alloy and often has surfaces coated with a ceramic material, since the ceramic material is superior to the metallic material in heat resistance, though inferior in thermal shock resistance and mechanical strength. The ceramic material is used as a heat-shielding coating to lower the blade temperature. Figure 5 shows a gas-turbine blade of the known construction. The blade comprises a main body 1 made of an alloy and having a hollow interior 2 and a wall 3 having a plurality of through holes 4. Substantially the whole outer surface of the blade body 1, excluding the holes 4, is covered with a heat-shielding coating layer 5 formed from a ceramic material. Compressed air is blown into the hollow interior 2 and out through the holes 4 to cool the blade. The holes 4 are usually made by electric discharge machining, and have to be made before the coating layer 5 is formed, since the coating is a dielectric which does not permit electric discharge machining. The holes 4 have, therefore, to be masked when the coating layer 5 is formed. The removal of the masking material to open the holes 4 thereafter, however, results in an uneven blade surface which will cause an increased aerodynamic loss. A prior art gas-turbine blade described in US-A-5,030,060 has the ceramic heat-shielding coating formed in a one-layer structure directly on the blade surface. This prior art also discloses a method for manufacturing a gas-turbine blade in which, after forming of the main body with concave portions on the outer surface, a heat-shielding layer is deposited as one layer on the main body. Thereafter, the surface of the ceramic layer is polished until the projections surrounding the prepared holes are exposed. Another gas-turbine blade with a two-layer thermal barrier coating comprising a first MCrAlY type coating layer and a second ceramic oxide stabilized zirconia layer is disclosed in EP-A-0 253 754. During the manufacturing process a cleaned blade is spray-coated with the MCrAlY type coating layer, thereafter fugitive plugs are disposed in the cooling holes. After the deposition of the ceramic oxide layer the plugs are removed and the surface is polished. SUMMARY OF THE INVENTION It is the object of the present invention to provide a method for manufacturing a gas-turbine blade having an even surface not increasing aerodynamic loss and formed on a closely adhering heat-shielding coating layer which can be formed even before a plurality of holes are made in the blade wall by electric discharge machining. This object is attained by providing a method for manufacturing a gas-turbine blade as defined in claim 1 or 2. The blade manufactured according to this invention has an even or smooth outer surface not causing any undesirable aerodynamic loss, since its heat-shielding coating is so formed on the concave portions of its outer surface as not to protrude from the main body in which the through holes are made. A desired surface finish is easy to obtain if the entire surface of the blade, including its heat-shielding coating, is appropriately polished as required. The blade is, therefore, reliable in performance, and can be used to.make a gas turbine having an improved reliability in performance. The heat-shielding coating consists of a ceramic surface layer and an underlying bonding layer which adheres closely to the ceramic surface layer and the outer surface of the alloy main body of the blade to thereby ensure that the heat-shielding coating adheres closely to the blade wall. The coating is variable in thickness if the depth of the concavity on the outer surface of the blade main body is appropriately altered. The ceramic layer preferably has a thickness of 0.3 to 0.5 mm, since it is likely that a smaller thickness may result in a layer having a lower heat-shielding effect, while a larger thickness results in a lower thermal shock resistance. The bonding layer preferably has a thickness of 0.1 to 0.2 mm which is sufficient for its anchoring purposes, while a larger thickness calls for a concavity which may be too deep for the blade and results in reducing thickness of the blade. Other features and advantages of the invention will become apparent from the following description and the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a cross sectional view of a gas-turbine blade manufactured according to this invention; Figure 2 is an enlarged view of a part of the blade shown in Figure 1, showing its heat-shielding coating in detail; Figure 3 is a schematic perspective view of a hole formed in the wall of the blade shown in Figure 1, and a concave wall surface for holding its heat-shielding coating therein; Figure 4 is a schematic perspective view of a row of holes formed in the wall of the blade shown in Figure 1, and a concave wall surface for holding its heat-shielding coating therein; and Figure 5 is a cross sectional view of a known gas-turbine blade. DESCRIPTION OF THE PREFERRED EMBODIMENT A gas-turbine blade manufactured according to this invention is shown in Figures 1 to 4. Like numerals are used to denote like parts in Figures 1 to 4 and Figure 5, so that it may not be necessary to repeat the description of any of the features which have already been described with reference to Figure 5. The blade comprises a main body 1 formed of an alloy, such as a Ni-based or Co-based alloy, or an inter-metallic compound such as a Ti-Al alloy. The main body 1 has a wall 3 defining a hollow interior 2 and having a plurality of through holes 4. The main body 1 has concaved portions 10 on an outer surface except around the holes 4, and holds a heat-shielding coating 5 thereon. The heat-shielding coating 5 consists of two layers, i.e. an inner or bonding layer 11 formed on the outer surface of the main body 1 and an outer or ceramic layer 12 formed on the bonding layer 11, as shown in Figure 2. The bonding layer 11 is formed from a material as represented by the formula MCrAlY, where M stand for Ni or Co, or a combination thereof. This material undergoes diffusion with the alloy forming the main body 1 upon heat treatment and thereby enables the bonding layer 11 to adhere closely to the main body 1. The bonding layer 11 has a thickness of 0.1 to 0.2 mm. The bonding layer 11 has a surface which is sufficiently rough for anchoring the ceramic layer 12 thereon. The ceramic layer 12 is a heat-shielding layer formed from a ceramic material, such as alumina (Al 2 O 3 )or stabilized zir-conia (e.g. ZrO 2 •Y 2 O 3 , ZrO 2 •MgO or ZrO 2 •CO). It has a thickness of 0.3 to 0.5 mm and adheres closely to the bonding layer 11. The holes 4 may be formed separately from one another so that each hole 4 may be surrounded by the concave portion 10 of the blade wall 3, as shown in Figure 3, or in a row crossing to the direction of air flow as shown by arrows in Figure 4. Each hole 4, or each set of holes 4 forming a row are formed in a projection of the wall 3 of the blade. The holes 4 may be circular as shown, or may be of a different shape, such as square or oval. After the heat-shielding coating 5 has been formed, its outer surface is polished until each projection of the wall 3 surrounding a hole 4 is exposed, and an intended blade contour is obtained. The holes 4 can be made even after the heat-shielding coating 5 has been formed, since the alloy surfaces exposed by its polishing permit electric discharge machining. Thus, the blade can be manufactured by a process of this invention having a broader scope of variation.

Claims: A method for manufacturing a gas-turbine blade having a wall (3) formed with a plurality of through holes (4) allowing a cooling fluid to pass from the inside of its main body (1) defined by said wall to its outside, comprising the steps of: forming said main body (1) of an alloy so that its outer surface has concave portions (10) except around said holes (4); forming a bonding layer (11) from MCrAlY, where M stands for Ni and/or Co, on said concave portions (10); forming a ceramic layer (12) on said bonding layer (11); and polishing a surface of said ceramic layer (12) so that said main body is exposed around said holes (4) and so that said ceramic layer presents a desired blade surface contour. A method for manufacturing a gas-turbine blade comprising the steps of: forming a main body (1) of an alloy having concave portions (10) on its outer surface; forming a bonding layer (11) from MCrAlY, where M stands for Ni and/or Co, on said main body (1); forming a ceramic layer (12) on said bonding layer (11); polishing a surface of said ceramic layer (12) until surface wall portions except the concave portions (10) are exposed; and making a hole through each of said exposed wall portions. The method according to claim 1 or 2, wherein said bonding layer (11) is formed to have a thickness of 0.1 to 0.2 mm, and said ceramic layer (12) is formed to have a thickness of 0.3 to 0.5 mm.

Inventors: AOKI SUNAO, KIKKAWA SHOUICHI, TAKAHASHI KOUJI, AOKI, SUNAO, KIKKAWA, SHOUICHI, TAKAHASHI, KOUJI, Aoki, Sunao, c/o Takasago Res. &amp; Dev. Center of, Kikkawa, Shouichi, c/o Takasago Machinery Works of, Takahashi, Kouji, c/o Takasago Machinery Works of
Applicants: MITSUBISHI HEAVY IND LTD, MITSUBISHI JUKOGYO KABUSHIKI KAISHA
Application year: 1995
Main classification: C23C4
IPC codes: F01D5, C23C4, F01D9