DURABLE THERMAL BARRIER COATING HAVING LOW THERMAL CONDUCTIVITY
This invention provides a thermal barrier ceramic coating for application to a metallic article, with the ceramic coating having a formula of RexZr1-xOy with Z dissolved in wherein Re is a rare earth element selected from the group consisting of Ce, Pr, Nd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb and Lu, where 0<x<0.5 and 1.75<y<2 and wherein Z is an oxide of a metal selected from the group consisting of Y, Mg, Ca, Hf and mixtures thereof. A preferred embodiment is wherein Re is Nd and Z is yttrium oxide.
The present invention relates generally to the field of thermal barrier coatings that are used in elevated temperature applications such as gas turbine engines. In particular, this invention relates to a thermal insulating ceramic coating, which has a low thermal conductivity as well as a long service life, and to the metallic articles such as turbine components, (e.g. blades and vanes) that the coatings are applied to prevent the components from overheating during high temperature operation.
BACKGROUND OF THE INVENTIONAdvanced gas turbine engines are continuously pursuing higher thrust and efficiency by the use of increased operating temperatures. However, the demand of increasing temperature is limited by the ability of most advanced nickel and cobalt based superalloy turbine blades and vanes to maintain their mechanical strength when exposed to the heat, oxidation, erosion and corrosion environment. Thus it is desirable to increase turbine engine operating temperatures, while decreasing the heat transfer to the metallic parts. One approach is to apply a thermal barrier coating onto the turbine blades and vanes to insulate the components from the high temperature operating environment. The ability of the thermal barrier coating to decrease the temperature to the metallic substrate depends upon the thermal conductivity of the thermal barrier coating. It is therefore desirable to develop thermal barrier coatings having low thermal conductivity to insulate effectively the thermal transfer to the components used in gas turbine engines, as well as providing a coated component having a long service life.
Efforts have been made to lower the thermal conductivity of thermal barrier coatings by modifying the chemistry and microstructure of current 7YSZ thermal barrier coating systems as disclosed in EP0816526A2, U.S. Pat. No. 6,071,628, U.S. Pat. No. 5,846,605, U.S. Pat. No. 6,183,884B1, U.S. Pat. No. 5,792,521, U.S. Pat. No. 5,687,679, W01/63008 and U.S. Pat. No. 6,284,323B1. These approaches decreased thermal conductivity of thermal barrier coatings to various extents with the minimum reported to be about half the thermal conductivity of typical 7 wt % yttria stabilized zirconia (7YSZ).
It is a main object of this invention to decrease thermal conductivity of thermal barrier coatings by providing a new ceramic material with low thermal conductivity. This invention also provides a method of applying such a thermal barrier coating system onto the metallic parts providing increased thermal insulation capability and prolonged durability.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 3(a) and 3(b) shows a protective ceramic top coat on the ceramic coating, RexZr1-xOy with Z dissolved in.
Briefly, this invention provides a thermal barrier ceramic coating for application to a metallic article, with the ceramic coating having a formula of RexZr1-xOy with Z dissolved in, wherein Re is a rare earth element selected from the group consisting of Ce, Pr, Nd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb and Lu, where O<x<0.5 and 1.75<y<2 and Z is an oxide of a metal selected from the group consisting of Y, Mg, Ca, Hf and mixtures thereof. A preferred embodiment is wherein Re is Nd and Z is yttrium.
DETAILED DESCRIPTION OF THE INVENTIONThis invention provides a thermal barrier ceramic coating having a formula of RexZr1-xOy with Z dissolved in, where 0<x<0.5, 1.75<y<2, Re is a rare earth element selected from Ce, Pr, Nd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb and Lu and Z is an oxide of a metal selected from the group consisting of Y, Mg, Ca, Hf and mixtures thereof. The ceramic is formed by doping oxides of the selected rare earth elements and the selected metal oxides into a host zirconia ceramic. A preferred embodiment is where Re is Nd with the formula NdxZr1-xOy, where 0<x<0.5 and 1.75<y<2 and Z is yttria. The NdxZr1-xOy ceramic coating with yttria dissolved in can be prepared by doping 4 to 15 mole % of Nd2O3 and 2 to 14 mole %, preferably 2.6 to 5.6 mole %, Y2O3 into ZrO2. One example of NdxZr1-xOy is Nd0.1Zr0.9O1.95 having a non-pyrochlore, cubic crystal structure with 10 mole % of Nd2O3 and 2.6 mole % Y2O3 doped into ZrO2. The ceramic coating of this invention is applied to a metallic article providing a thermal barrier coating with low thermal conductivity and high resistance to cyclic oxidation.
The ceramic coating of this invention has a low thermal conductivity generally within the range of about 0.78 to 1.02 W/mK from 600° C. to 1100° C. This thermal conductivity is around 50% of the measured thermal conductivity of a typical 7YSZ coating (1.65-2.22 W/mK from 600° C. to 1100° C.). In addition the ceramic coating of this invention has a high resistance to cyclic oxidation. Cyclic oxidation testing of the coating Nd0.1Zr0.9O1.95 with 2.6 mole % Y2O3 dissolved in demonstrated a lifetime greater than twice that of 7YSZ.
Techniques for applying the ceramic coatings, RexZr1-xOy with Z dissolved in include air plasma thermal spray (APS), low pressure plasma spray (LPPS), high velocity oxygen fuel (HVOF), sputtering and electron beam physical vapor deposition (EBPVD), etc. In a preferred embodiment the ceramic coating is applied by electron beam physical vapor deposition (EBPVD) due to the columnar microstructure with inter-column gaps produced. The ceramic coating can be deposited as a straight columnar microstructure or a saw tooth microstructure or a layered microstructure or mix of thereof for further reduction in thermal conductivity. Generally, the ceramic coating is applied to a thickness within the range of about 5 to 500 μm, preferably about 25 to 400 μm. In a layered microstructure, the ceramic coating can have at least 2 layers, preferably from 5 to 100 layers, each at least about 1 μm thick, preferably about 5 to 25 μm thick.
The process of applying the ceramic coating by EBPVD is similar to that of applying 7YSZ in production. The evaporating source in a crucible is a solid ingot of the RexZr1-xOy with Z dissolved in, which is sintered zirconia doped with the selected rare earth oxide and selected metal oxide. The layered microstructure of the ceramic coating, RexZr1-xOy with Z dissolved in, is applied by evaporating the solid ingots from two crucibles under controlled gun on/off program of electron beam physical vapor deposition. The ceramic coating, RexZr1-xOy with yttria dissolved in with 6-8 wt % YSZ at the top is deposited by evaporating the solid ingot of RexZr1-xOy with Z dissolved in from one crucible and 6-8 wt % YSZ ingot from another crucible by electron beam physical vapor deposition.
For increased adhesion a metallic bond coat is applied onto the metallic article, such as a nickel or cobalt based superalloys prior to deposition of the ceramic coating. The metallic bond coat can be a MCrAlY alloy, wherein M is Ni, Co or mixtures thereof. Such alloys have a broad composition of 10 to 35% chromium, 5 to 15% aluminum, 0.01 to 1% yttrium, or hafnium, or lanthanum, with M being the balance. Minor amounts of other elements such as Ta or Si may also be present. The MCrAlY bond coat can be applied by EBPVD, though sputtering, low pressure plasma or high velocity oxy fuel spraying or entrapment plating may also be used.
Alternatively, the metallic bond coat can be comprised of an intermetallic aluminide such as nickel aluminide or platinum aluminide. The aluminide bond coating can be applied by standard commercially available aluminide processes whereby aluminum is reacted at the substrate surface to form an aluminum intermetallic compound, which provides a reservoir for the growth of an alumina scale oxidation resistant layer. Thus the aluminide coating is predominately composed of aluminum intermetallic [e.g., NiAl, CoAl and (Ni, Co) Al phase] formed by reacting aluminum vapor species, aluminum rich alloy powder or surface layer with the substrate elements in the outer layer of the superalloy component. This layer is typically well bonded to the substrate. Aluminizing may be accomplished by one of several conventional prior art techniques, such as, the pack cementation process, spraying, chemical vapor deposition, electrophoresis, sputtering, and appropriate diffusion heat treatments. Other beneficial elements can also be incorporated into diffusion aluminide coatings by a variety of processes. Beneficial elements include Pt, Pd, Si, Hf, Y and oxide particles, such as alumina, yttria, hafnia, for enhancement of alumina scale adhesion, Cr and Mn for hot corrosion resistance, Rh, Ta and Cb for diffusional stability and/or oxidation resistance and Ni, Co for increasing ductility or incipient melting limits. In the specific case of platinum modified diffusion aluminide coating layers, the coating phases adjacent to the alumina scale will be platinum aluminide and/or nickel-platinum aluminide phases (on a Ni-base superalloy).
Through oxidation an alumina (i.e., aluminum oxide) layer is formed over the metallic bond coat. This alumina layer provides both oxidation resistance and a bonding surface for a ceramic coating. The alumina layer may be formed before the ceramic coating is applied, during application of the coating or subsequently by heating the coated article in an oxygen containing atmosphere at a temperature consistent with the temperature capability of the superalloy, or by exposure to the turbine environment. The sub-micron thick alumina scale will thicken on the aluminide surface by heating the material to normal turbine exposure conditions. The thickness of the alumina scale is preferably sub-micron (up to about one micron). The alumina layer may also be deposited by chemical vapor deposition following deposition of the metallic bond coat.
Alternatively, the metallic bond coat may be eliminated if the substrate is capable of forming a highly adherent alumina scale or layer. Examples of such substrates are very low sulfur (<1 ppm) single crystal superalloys, such as PWA 1487 and Rene N5, which also contain 0.1% yttrium to enhance adhesion of the thermally grown alumina scale.
FIGS. 3(a) and 3(b) show a protective ceramic top coat 50 which is coated after the columnar ceramic coating RexZr1-xOy with Z dissolved in 40, to provide increased erosion resistance on the top surface which is subject to hot gas impact during turbine engine operation. This protective ceramic top coat can be a dense and/or wide column of ceramic coating RexZr1-xOy with Z dissolved in, or alternatively, layer 50 could also be 6-8 wt % YSZ. This protective ceramic top coat, 50 for erosion resistance, generally has a thickness of about 5 to 50 μm preferably about 10 to 25 μm thick. A protective top coat of 7YSZ with an appropriate thickness for erosion resistance on the ceramic coating, Nd0.1Zr0.9O1.95 with 2.6 mole % Y2O3 dissolved in, provides a thermal conductivity which is equivalent to the ceramic coating without the protective ceramic top coat.
The ceramic coating system of this invention provides many advantages for use in gas turbine engines. The reduction in thermal conductivity of 50 percent can reduce the thickness required for the thermal barrier coating (TBC) by approximately one half for the same degree of thermal insulation. This will lower the cost of the TBC due to the time saved in applying the coating, ingot material savings and energy savings in production. Decreasing in the coating thickness will also lower the weight of the gas turbine component, e.g. blades and vanes, which can provide a significant reduction in the weight of the disk that holds these components. Depositing the same thickness of the ceramic coating will allow an increased operating temperature to be achieved without overheating the metallic parts allowing the engine to operate a higher thrust and efficiency. The increased insulating capabilities of the ceramic coating could also reduce the requirements for air cooling the part.
This invention is generally applicable to any metallic article which uses thermal barrier coating system, and includes various modifications according to the principles of this invention.
EXAMPLEA ceramic coating having a formula Nd0.1Zr0.9O1.95 with yttria dissolved in was applied by EBPVD evaporating ZrO2 ceramic ingots doped with 10 mole % of Nd2O3 and 2.6 mole % Y2O3. The coating displayed a saw tooth columnar structure oriented perpendicularly to the surface of the substrate. The intercolumnar gaps are visible and tend to be gradually wider from bottom to top. Phase identification conducted on as-deposited ceramic coating of Nd0.1Zr0.9O1.95 with yttria dissolved in by XRD showed a ceramic coating of Nd0.1Zr0.9O1.95 having a non-pyrochlore, cubic crystal structure produced on the top layer of the EBPVD thermal barrier coating system.
It is known that the thermal conductivity of material K (W/mK) is determined by its thermal diffusivity a (cm2/s), density p (g/cm3) and specific heat Cp (J/g K), i.e. K=α·ρ·Cp. The specific heat of the ceramic coating Nd0.1Zr0.9O1.95 with yttria dissolved in was tested using Differential Scanning Calorimetry (DSC) on an Omnitherm DSC 1500 in Oak Ridge National Lab. The samples are free standing ceramic coating, i.e. an intact ceramic coating without substrate. The free standing samples of ceramic coating are 180 to 230 μm thick and are machined to 6 mm in diameter to meet the requirements of the testing instrument. The test was run in the temperature range of 100° C. to 1100° C. using sapphire as the baseline standard. The results in
The thermal diffusivity (a) was measured by the laser flash technique at Oak Ridge National Laboratory on a Flashline 5000 Thermal Diffusivity System, see H. Wang, R. B. Dinwiddie and P. S. GAAL, “Multiple Station Thermal Diffusivity Instrument”, THERMAL CONDUCTIOVITY 23, Proceedings of the Twenty-Third International Thermal Conductivity Conference, P119-126. Two or three free standing ceramic samples of each kind were measured at every 100-degree interval from 600° C. to 1100° C. Three measurements of each sample were conducted at every temperature. The time-temperature curves were analyzed by the method of Clark and Taylor, which takes into account radiation losses and uses the heating part of the curve to calculate thermal diffusivity. The average readings of two or three samples with three measurements in each at temperature from 600° C. to 1000° C. are plotted in
The density of the ceramic coating is about 4.7 g/cm3, which is less than that of the typical 7YSZ coating (5.0 g/cm3). This lower density allows the gas turbine component coated with the ceramic coating to have less coating weight than that currently used for typical 7YSZ coated components.
The thermal conductivity of Nd0.1Zr0.9O1.95 ceramic coatings with yttria dissolved in is calculated according to their value of thermal diffusivity, density and specific heat, and then is plotted in
Another significant concern is the thermal conductivity of a thermal barrier coating during operation in a turbine gas engine where the coatings are subject to high temperature for a long period of time. There are two factors that will affect the intrinsic thermal conductivity during engine operation—sintering and radiation. To investigate the effect of sintering on thermal conductivity of the ceramic coatings, samples of the Nd0.1Zr0.9O1.95 ceramic coatings with yttria dissolved in applied by EBPVD were aged heat treat at 1200° C. for 50 hours.
During engine operation at high temperature, a thermal barrier coating is subjected to incident radiation from the hot combustor. Radiation is then absorbed by the soot that is usually covered on the exposed coating due to the combustion environment. A translucent coating, such as typical yttria stabilized zirconia, permits the energy to be transported internally by radiation, thereby increasing the total energy transfer and acting to increase thermal conductivity. However, the Nd0.1Zr0.9O1.95 ceramic coating with yttria dissolved in has a sky-blue color, which can reduce the internal radiation transport. Therefore, the effect of radiation on the insulating ability of the new coatings is expected to be negligible.
Nd0.1Zr0.9O1.95 with 2.6 mole % of yttria dissolved in also demonstrates a high resistance to cyclic oxidation. The cyclic oxidation test was conducted at 2025° F., one hour cycle, i.e. 7 minutes heat up to 2025° F., soak at 2025° F. for 50 minutes, followed by 3 minutes air-forced cooling down to 200° F. 7YSZ coated specimens used as a reference were loaded in the same furnace chamber. The specimens were inspected at specified intervals. Six samples of each coating were tested. The lifetime is measured by the number of cycles to failure (spallation of 30% of the coating area). The results show that the lifetime of the new coating Nd0.1Zr0.9O1.95 with yttria dissolved in is more than twice that of 7YSZ, with the 7YSZ coating sample having an average lifetime of 707 cycles, while the Nd0.1Zr0.9O1.95 with 2.6 mole % of yttria dissolved in had an average lifetime of 1429 cycles with 4 of the 6 samples tested not failing at 1473 cycles when the test was stopped.
The coefficient of thermal expansion (CTE) of the ceramic coating Nd0.1Zr0.9O1.95 with 2.6 mole % of yttria dissolved in is about 6.7 to 9.5×10−6/° C. from room temperature to 1400° C., which is larger than that of typical 7YSZ. Thus, the thermal stress in the new ceramic coating, which is caused by the difference of thermal expansion between the substrate and the ceramic coating, is lower than that of a 7YSZ thermal barrier coating system.
Claims
1. A metallic article comprising a metallic substrate and a thermal barrier ceramic coating on its surface with the ceramic coating having a formula of RexZr1-xOy with Z dissolved in where Re is a rare earth element selected from the group consisting of Ce, Pr, Nd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb and Lu, where 0<x<0.5 and 1.75<y<2 and Z is an oxide of a metal selected from the group consisting of Y, Mg, Ca, Hf and mixtures thereof.
2. Article of claim 1 further comprising a metallic bond coat between the ceramic coating and the metallic substrate.
3. Article of claim 2 wherein the metallic bond coat is a MCrAlY, wherein M is selected from Ni and/or Co.
4. Article of claim 2 wherein the metallic bond coat is intermetallic aluminide.
5. Article of claim 1 wherein the ceramic coating has a columnar microstructure.
6. Article of claim 5 wherein the columnar microstructure is a straight or a saw tooth structure.
7. Article of claim 1 wherein the ceramic coating has a layered microstructure.
8. Article of claim 1 further comprising a protective ceramic top coat overlaying the ceramic coating.
9. Article of claim 8 wherein the protective ceramic top coat is a dense or a wide columnar microstructure.
10. Article of claim 8 wherein the protective ceramic top coat is 6 to 8 wt % YSZ.
11. Article of claim 1 wherein the ceramic coating has a thermal conductivity of from about 0.78 to 1.02 W/mK from 600° C. to 1100° C.
12. Article of claim 1 wherein the ceramic coating has a coefficient of thermal expansion of about 0.67 to 0.95×10−6/° C. from room temperature to 1400° C.
13. Article of claim 1 wherein the ceramic coating has been applied by EBPVD, air plasma spray or HVOF.
14. Article of claim 2 wherein the metallic article is a turbine component.
15. Article of claim 14 wherein the turbine component has a metallic substrate of a nickel or cobalt based superalloy.
16. Article of claim 1 wherein the ceramic coating has a thickness within the range of about 5 to 500 μm.
17. Article of claim 8 wherein the protective ceramic top coat has a thickness of about 5 to 50 μm.
18. A metallic article comprising a metallic substrate and a thermal barrier ceramic coating on its surface, the ceramic coating having a formula of NdxZr1-xOy with yttria dissolved in wherein 0<x<0.5 and 1.75<y<2.
19. Article of claim 18 wherein the ceramic has a cubic crystal structure.
20. Article of claim 18 further comprising a metallic bond coat between the ceramic coating and the metallic substrate.
21. Article of claim 20 wherein the metallic bond coat is a MCrAlY, wherein M is selected from Ni and/or Co.
22. Article of claim 20 wherein the metallic bond coat is intermetallic aluminide.
23. Article of claim 18 wherein the ceramic coating has a columnar microstructure.
24. Article of claim 23 wherein the columnar microstructure is a straight or a saw tooth structure.
25. Article of claim 18 wherein the ceramic coating has a layered microstructure.
26. Article of claim 18 further comprising a protective ceramic top coat overlaying the ceramic coating.
27. Article of claim 25 wherein the protective ceramic top coat is a dense or a wide columnar microstructure.
28. Article of claim 25 wherein the protective ceramic top coat is 6-8 wt % YSZ having a thickness of about 5 to 50 μm.
29. Article of claim 18 wherein the ceramic coating has a thermal conductivity of from about 0.72 to 1.02 W/mK from 600° C. to 1100° C.
30. Article of claim 18 wherein the ceramic coating has been applied by EBPVD, air plasma spray or HVOF.
31. Article of claim 18 wherein the metallic article is a turbine component.
32. Article of claim 31 wherein the turbine component has a metallic substrate of a nickel or cobalt based superalloy.
33. Article of claim 18 wherein the ceramic coating has a density of about 4.7 g/cm3.
34. A method for applying a thermal barrier ceramic coating to a metallic article comprising:
- forming a ceramic having a formula RexZr1-xOy with Z dissolved where Re is a rare earth element selected from the group consisting of Ce, Pr, Nd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb and Lu, wherein 0<x<0.5 and 1.75<y<2 and wherein Z is an oxide of a metal selected from the group consisting of Y, Mg, Ca, Hf and mixtures thereof, by doping Z and oxides of Re into a zirconia ceramic; and
- applying the ceramic as a coating onto the metallic article.
35. Method of claim 34 wherein the ceramic is applied by electron beam physical vapor deposition.
36. Method of claim 34 wherein 4 to 15 mole % of Nd2O3 and 2 to 14 mole % Y2O3 are doped into ZrO2.
37. Method of claim 36 wherein the ceramic is applied by electron beam physical vapor deposition.
38. Method of claim 37 wherein 10 mole % of Nd2O3 and 2.6 mole % Y2O3 is doped into ZrO2 forming Nd0.1Zr0.9O1.95 with yttria dissolved in.
Type: Application
Filed: Jan 12, 2004
Publication Date: Jul 14, 2005
Inventors: Yourong Liu (Hillsdale, NJ), Paul Lawton (Rock Hill, NY)
Application Number: 10/756,209
