Use of an oxide coating to enhance the resistance to oxidation and corrosion of a silicon nitride based gas turbine blade

Abstract: A gas turbine blade improved in terms of resistance to oxidation and corrosion is provided. A coat of an oxide excelling in high-temperature strength and heat resistance is formed onto the surface of a silicon nitride sintered part forming a gas turbine blade body. At least one oxide selected from the group consisting of alumina, mullite, zirconia, yttria and zircon is used as the oxide. The oxide coat is formed on the surface of the silicon nitride-based sintered part in the form of a thin film by plasma spray coating.

Description: The present invention relates to gas turbine blades. As well known in the art, gas turbine blades required to have high mechanical properties under high-temperature and severe conditions are formed of ceramic materials such as sintered silicon nitride and silicon carbide. However, even gas turbine blades built up of sintered silicon nitride or carbide materials of high heat resistance decrease in thickness as they are used, because they are exposed to a high-speed combustion gas flow which oxidises, corrodes or otherwise degrades them at high temperatures. A problem of particular significance associated with materials used for gas turbine blades exposed to the high-speed combustion gas flow is therefore to achieve good resistance to oxidation and hence good durability. The present inventors have been able to achieve good resistance to oxidation and corrosion by applying an oxide coat onto the surface of a nonoxide ceramic material having excellent high-temperature strength and heat resistance. In the prior art, JP-A-62/72582 (equivalent to EP-A-211579) discloses coating a zirconia layer onto a sintered silicon nitride member, eg. by plasma spraying or slurry coating, but only for the purpose of reducing the thermal conductivity of the member. FR-A-2268769 (equivalent to US-A-3953636) disclosed the use of a microcracked surface layer of polycrystalline ceramics which might be zirconia, to improve the impact resistance of an underlying polycrystalline ceramics which might be silicon nitride. The present invention provides the use of an oxide thin film, not thicker than 100µm and coated on the surface of a silicon nitride-based sintered part forming a blade body of a gas turbine blade, to enhance high-temperature resistance of the blade body to operational oxidation and corrosion. At least one oxide selected from alumina, mullite, zirconia, yttria and zircon may be used as said oxide. The thin film of oxide may be coated onto the surface of the silicon nitride-based sintered part by plasma spray coating. The method of production of the gas turbine blade preferably includes treating the sintered part's surface mechanically, thermally or chemically to a surface roughness of 1.5µm or more as expressed in terms of ten-point average roughness (Rz), or 0.2µm or more as expressed in terms of center-line average roughness (Ra), and then coating the surface of said part with at least one oxide selected from the group consisting of alumina, mullite, zirconia, yttria and zircon by plasma spray coating. A gas turbine blade embodying these concepts is thus obtainable by using, for the sintered part, silicon nitride or a silicon nitride composite material reinforced by dispersing, e.g., silicon carbide particles or whiskers throughout it and coating the surface of the part with at least one oxide selected from the group consisting of alumina, mullite, zirconia, yttria and zircon. The oxide coat has a thickness of 100µm or less. One reason for this is that because silicon nitride or a silicon carbide dispersion-reinforced silicon nitride composite material is different from the oxide layer in therms of the rate of thermal expansion, the oxide coat is liable to peel away or crack due to repeated heating and cooling, if it is too thick. Another reason is that, even at a thickness of the order of 100µm or less, the oxide coat is found to be effective to improve the resistance of a gas turbine blade to oxidation and corrosion. How to produce rotor blades and nozzle vanes for gas turbines will now be explained. A rotor blade or nozzle vane body for a gas turbine may be formed of either silicon nitride or a silicon carbide dispersion-reinforced silicon nitride composite material. In general, this body has an as-sintered or as-machined surface, which must be roughened or otherwise treated to increase its bond strength or its reactivity with respect to an oxide layer to be baked or spray-coated onto it. This may be by polishing with abrasive particles, oxidation by heating in the air and etching with hydrofluoric acid or other acids. Roughening or otherwise treating the part to increase its surface reactivity may be done with the best technique that is mainly determined by what material forms that part. According to our determination it is preferred that the surface roughness of the part be at least 1.5µm as expressed in terms of ten-point average roughness - Rz or at least 0.2µm as expressed in terms of center-line average roughness - Ra, both being measured according to JIS B 0601. This is because when the part has a ten-point average roughness Rz of at least 1.5µm or a center-line average roughness Ra of at least 0.2µm, as measured according to JIS B0601, it comes into intimate contact with the oxide. The oxide or oxides to be formed on the surface of the part may be selected from alumina, mullite, zirconia, yttria and zircon, and may be provided in the form of a single or multi-layer. In order to prevent the oxide coat from peeling away or cracking due to a thermal expansion difference between the oxide and the part, it is desirable to provide between them an intermediate layer having a rate of thermal expansion lying halfway between theirs. Preferably the oxide film is formed on the surface of the part by plasma spray coating, but may be achieved by coating an oxide slurry onto the part; dipping the part in the oxide; forming an oxide layer on the part by spraying, followed by firing and baking; coating a mixture of a certain metal element with its oxide onto the part or dipping the latter in the former, followed by lamination by spraying, reaction and sintering. In this way we find that it is possible to achieve excellent high temperature mechanical properties such as creep resistance, as well as good resistance to oxidation and corrosion at high temperature. We also find that it is possible to produce a gas turbine blade in which the oxide coat adheres well to the part and which has a high heat-resistance cycle property. The invention is now illustrated with reference to the following examples. Example 1 Provided were a sintered silicon nitride body containing Y 2 O 3 and Yb 2 O 3 as additives and a sintered composite part composed of a Y 2 O 3 and Yb 2 O 3 -containing silicon nitride material matrix and containing silicon carbide particles or whiskers as a reinforcing material. These parts were each cut to a 4-mm wide, 3-mm high and 40-mm long test piece, which was C-machined all over the surface and then oxidized at 1400°C for 100 hours in air to form an oxide layer on the surface, to which plasma spray coating was to be applied. Each of the spray coating materials referred to in Table 1 was applied to the entire surface of the test piece at varying thicknesses of 50-100µm by plasma spray coating. Oxidation and corrosion tests were done. The oxidation testing was carried out at 1400°C for 100 hours in the air according to JIS R 1609 - the oxidation resistance testing procedure for an nonoxide type of fine ceramics. The corrosion testing was done with a high-speed combustion gas burner; that is, the sample was exposed to the gas burner using light oil as fuel at at pressure of 1 ata, a gas flow rate of 200m/sec. and a temperature of 1400°C for 10 hours. The degree of corrosion was estimated by profiling the sample surface with a surface roughness meter before and after the exposure testing. The results of the oxidation and corrosion tests are shown in Tables 1 and 2. Substrate Spray Coating Material Weight Increase by Oxidation in mg/cm 2 Example 1 SN Alumina 0 Example 2 SN Mullite 0 Example 3 SN Zirconia 0 Example 4 SN Yttria 0 Example 5 SN Zircon 0 Example 6 C Alumina 0 Example 7 C Mullite 0 Example 8 C Zirconia 0 Example 9 C Yttria 0 Example 10 C Zircon 0 Comparative Example 1 SN - 0.5 Comparative Example 2 C - 0.3 Note that "SN" and "C" stand for silicon nitride and composite material, respectively, and that the "weight increase by oxidation" value is the average of three measurements. Substrate Spray Coating Material Thickness Decrease in µm Example 1 SN Alumina 0 Example 2 SN Mullite 0 Example 3 SN Zirconia 0 Example 4 SN Yttria 0 Example 5 SN Zircon 0 Example 6 C Alumina 0 Example 7 C Mullite 0 Example 8 C Zirconia 0 Example 9 C Yttria 0 Example 10 C Zircon 0 Comparative Example 1 SN - 5 Comparative Example 2 C - 6 Note that the "thickness decrease" is the average of three measurements. Example 2 A number of test pieces were formed of silicon nitride and investigated as to their surface roughness and their adhesion to oxide coats. The results are set out in Table 3. The test pieces of Examples 21-25 were heat-treated at 1400°C for 10 hours in the air after machining, and the test pieces of Examples 26-30 were sand-blasted with 50% by weight of #36 silicon carbide grit and 50% by weight of #36 alumina grit. In Comparative Examples 11-15 the test pieces were mechanically processed on their as-sintered surfaces with a #140 grinder, and in Comparative Examples 16-20 the test pieces were heat-treated at 1300°C for 1 hour in the air after processing. By plasma spray coating, these test pieces were coated with about 100-µm thick coats of oxides, namely alumina, mullite, zirconia, yttria and zircon, to obtain samples. The adhesion of the oxide coats to the surfaces of the test pieces and the heat-resistant cycle characteristics of the samples, when repeatedly subjected to a heating/cooling cycle at between room temperature and 1400°C, were measured for estimation. The results are shown in Table 3, as already mentioned. Surface Roughness in µm Spray Coating Material Rz Ra Example 21 1.7 0.2 Alumina Good Good Example 22 1.7 0.2 Mullite Good Good Example 23 1.7 0.2 Zirconia Good Good Example 24 1.7 0.2 Yttria Good Good Example 25 1.7 0.2 Zircon Good Good Example 26 12.0 2.2 Alumina Good Good Example 27 12.0 2.2 Mullite Good Good Example 28 12.0 2.2 Zirconia Good Good Example 29 12.0 2.2 Yttria Good Good Example 30 12.0 2.2 Zircon Good Good Comparative Example 11 0.7 0.1 Alumina Bad - Comparative Example 12 0.7 0.1 Mullite Bad - Comparative Example 13 0.7 0.1 Zirconia Bad - Comparative Example 14 0.7 0.1 Yttria Bad - Comparative Example 15 0.7 0.1 Zircon Bad - Comparative Example 16 1.1 0.1 Alumina Good Bad Comparative Example 17 1.1 0.1 Mullite Good Bad Comparative Example 18 1.1 0.1 Zirconia Good Bad Comparative Example 19 1.1 0.1 Yttria Good Bad Comparative Example 20 1.1 0.1 Zircon Good Bad The adhesion of the oxide coats was estimated by visually observing whether or not they were uniformly coated onto the surfaces of the test pieces. As can be understood from Table 3, the oxide coats adhered well to the test pieces in Examples 21-30 and Comparative Examples 16-20, but the other oxide coats adhered only locally to the test pieces with the rest peeling away. Estimating the adhesion of the oxide coats to the test pieces was done by subjecting the samples repeatedly to a heating/cooling cycle between room temperature and 1400°C and observing the test piece/oxide coat interfaces under an optical microscope after each cycle. In Examples 21-30 no peeling was observed even after the samples had been subjected to 200 heating/cooling cycles between room temperature and 1400°C, but in Comparative Examples 16-20 peeling occurred after one such a heating/cooling cycle. As can be understood from what has been described above, a gas turbine blade can be made which excels in surface resistance to oxidation and corrosion and so can be used without suffering any thickness decrease over extended periods of time, even under high-temperature and severe conditions. According to the methods described herein; it is possible to produce a gas turbine blade which can well stand up to use at high temperatures, and which is also good in terms of the adhesion of the oxide coat to the blade body and heat-resistance cycle characteristics.

Claims: The use of an oxide thin film, not thicker than 100µm and coated on the surface of a silicon nitride-based sintered part forming a blade body of a gas turbine blade, to enhance high-temperature resistance of the blade body to operational oxidation and corrosion. The use according to claim 1 in which the oxide material of the thin film is at least one selected from alumina, mullite, zirconia, yttria and zircon. The use according to claim 1 or claim 2 comprising coating the thin film onto the surface of the sintered part by plasma spray coating. The use according to claim 1 or claim 2 comprising coating the thin film onto the surface of the sintered part by coating an oxide slurry onto the sintered part, dipping the sintered part in the oxide, or spraying an oxide layer onto the sintered part and then firing and baking. The use according to claim 3 or claim 4 comprising treating the surface of the sintered part, before coating the thin film onto it, to give a surface roughness of at least 1.5µm as expressed in terms of ten-point average roughness (Rz), or a surface roughness of at least 0.2µm as expressed in terms of centre-line average roughness (Ra). The use according to claim 5 in which the surface is roughened by polishing with abrasive particles; oxidising by heating in air; or etching with acid. The use according to any one of the preceding claims in which the silicon nitride-based sintered part is of silicon nitride or of silicon nitride reinforced with dispersed silicon carbide.

Inventors: ENDO YASUYUKI, FURUSE YUTAKA, KOBAYASHI HIROMICHI, OHI HIROYUKI, SAKAI HIROAKI, ENDO, YASUYUKI,, FURUSE, YUTAKA,, KOBAYASHI, HIROMICHI, OHI, HIROYUKI, SAKAI, HIROAKI, Endo, Yasuyuki, c/o The Tokyo Elec. Power Co. Inc., FURUSE, YUTAKA, C/O THE TOKYO ELEC. POWER CO. INC.
Applicants: NGK INSULATORS LTD, TOKYO ELECTRIC POWER CO, NGK INSULATORS, LTD., THE TOKYO ELECTRIC POWER CO., INC.
Application year: 1992
Main classification: C04B41
IPC codes: C04B41, C23C4, F01D5