Ceramic material for high temperature service

Abstract

Thermal barrier coating made from a thermally sprayable powder that includes yttria stabilized zirconia and hafnia, from 6 to 9 weight percent yttria, and total impurities less than or equal to about 0.1 weight percent. The thermal barrier coating has from about 5 to 250 vertical macro cracks per 25.4 mm length measured along a coating surface and the macro cracks are oriented perpendicular to a surface of a substrate containing said coating.

Claims

1. A thermally sprayed barrier coating comprising: a coating material that includes: a yttria stabilized material comprising at least one of zirconia and hafnia, wherein the yttria is from 6 to 9 weight percent; and a purity of said zirconia or said hafnia being at least 99.5 weight percent; wherein total impurities of said coating material comprises less than or equal to: about 0.015 weight percent alumina, about 0.005 weight percent silica, about 0.002 weight percent titania, and about 0.002 weight percent magnesia; and said barrier coating being a collection of frozen droplets or splats formed from the coating material and comprising vertical cracks arranged at repeating intervals and extending in a thickness direction of said thermal barrier coating.

2. The coating of claim 1, wherein the vertical cracks run from a coating upper surface to a bond coat arranged between a substrate and the thermal barrier coating.

3. The coating of claim 1, further comprising a bond coating arranged between the substrate and said barrier coating.

4. The coating of claim 3, wherein the bond coating comprises a MCrAlY bonding layer.

5. The coating of claim 1, wherein the barrier coating is arranged over a substrate and the substrate a turbine component.

6. The coating of claim 5, wherein the turbine component is a turbine blade.

7. The coating of claim 1, wherein the vertical cracks run through an entire thickness of the thermal barrier coating.

8. The coating of claim 1, wherein the total impurities comprises less than or equal to: about 0.002 weight percent soda, about 0.04 weight percent hematite, and about 0.02 weight percent calcia.

9. The coating of claim 1, wherein the coating material is formed from a thermally sprayed powder.

10. A method of applying the thermal barrier coating of claim 1 on a substrate, the method comprising: thermally spraying the coating material of claim 1 onto the substrate so as to form the barrier coating having vertical cracks extending along a direction of a coating thickness.

11. A high purity thermal barrier coating comprising: a thermally sprayed coating of a high purity zirconia and hafnia coating material, wherein said coating material consists essentially of: a purity of at least 99.5 weight percent and with less than about 0.015 weight percent alumina impurity; from 6 to 9 weight percent yttria; a balance of at least one of zirconia and hafnia stabilized by the yttria; said thermal barrier coating having a structure formed from a collection of frozen droplets or splats of the coating material and having repeating vertical cracks extending in a thickness direction; and said vertical cracks extending between an upper surface of said coating and another material layer arranged on a substrate.

12. The coating of claim 11, wherein the other material layer is a bond coat.

13. The coating of claim 11, wherein the coating material is a powder and comprises impurities less than or equal to: about 0.005 weight percent silica, about 0.002 weight percent titania, and about 0.002 weight percent magnesia.

14. The coating of claim 11, wherein the coating material is formed from a thermally sprayed powder.

15. A high purity thermal barrier coating comprising: a thermally sprayed coating of a high purity yttria stabilized zirconia coating material, wherein said coating material consists essentially of: from 6 to 9 weight percent yttria; zirconia stabilized by said yttria; a purity of at least 99.5 weight percent and with less than about 0.015 weight percent alumina impurity; said thermal barrier coating being made up of frozen splats of said coating material and having cracks repeating at regular intervals and extending along a thickness direction and between an upper surface of said coating and another material layer arranged on a substrate.

16. The coating of claim 15, wherein the total impurities comprises less than or equal to: about 0.005 weight percent silica, about 0.002 weight percent titania, and about 0.002 weight percent magnesia.

17. The coating of claim 15, wherein the coating material is formed from a thermally sprayed powder.

18. A thermal barrier coating comprising: a coating material that includes between 6.5 and 7 weight percent yttria stabilized zirconia; and a purity of said coating material being at least 99.5 weight percent; wherein total impurities of said coating material comprises less than or equal to: about 0.015 weight percent alumina, about 0.005 weight percent silica, about 0.002 weight percent titania, and about 0.002 weight percent magnesia; and said thermal barrier coating having a structure made up of splats of said coating material and repeating vertical cracks extending in a thickness direction of said thermal barrier coating.

19. A method of applying the barrier coating of claim 1, comprising: spraying a powder material that includes the coating material of claim 1: depositing the coating material, formed from the powder material, as a barrier coating on a substrate, wherein the deposited coating material is a collection of frozen droplets or splats formed from the coating material and comprising vertical cracks arranged a repeating intervals and extending in a thickness direction of said barrier coating.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The accompanying drawings are included to provide further understanding of the invention and are incorporated in and constitute a part of this specification. The accompanying drawings illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. In the figures:

(2) FIG. 1 illustrates a perspective view of a turbine blade coated with a thermal barrier of ceramic material;

(3) FIG. 2 provides a graph showing the effect of impurities on the sintering rates;

(4) FIG. 3 provides a phase diagram for ZrO.sub.2;

(5) FIG. 4 provides a standard phase diagram for stabilized ZrO.sub.2 showing the general alloying trends for various stabilizers [Ceramic Phase Diagram, Volume 4, Fig 05241];

(6) FIG. 5 provides a phase diagram for ZrO.sub.2 with stabilizer;

(7) FIG. 6 provides a diagram showing a lamellar thermal barrier coating structure containing porosity and microcracks;

(8) FIG. 7 provides a diagram showing a thermal barrier coating containing porosity, microcracks and macrocracks;

(9) FIG. 8 provides a diagram showing a thermal barrier coating deposited from the vapor phase;

(10) FIG. 9 provides a diagram showing the Thornton model for predicting structure of a coating formed from the gas phase; and

(11) FIG. 10 provides a diagram showing a thermal barrier coating deposited from both vapor and liquid phase.

DETAILED DESCRIPTION OF THE INVENTION

(12) Reference will now be made in detail to the preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings.

(13) In an exemplary use of a material of the invention, FIG. 1 shows one component of a turbine. Turbine blade 100 has a leading edge 102 and an airfoil section 104, against which hot combustion gases are directed during operation of the turbine, and which undergoes severe thermal stresses, oxidation and corrosion. A root end 106 of the blade anchors the blade 100. Venting passages 108 may be included through the blade 100 to allow cooling air to transfer heat from the blade 100. The blade 100 can be made from a high temperature resistant material. The surface of the blade 100 is coated with a thermal barrier coating 110 made of ultra-pure zirconia (ZrO.sub.2) and/or hafnia (HfO.sub.2) alloys in accordance with the invention. The thermal barrier coating 110 may be applied on, for example, a MCrAlY bonding layer with an alumina scale (not shown) applied between the blade surface and the coating 110. The coating 110 may be applied onto the bond coating surface through a variety of methods known in the art including thermal spray techniques such as powder flame spray and plasma spray and vapor deposition methods such as electron beam physical vapor deposition (EBPVD), high speed physical vapor deposition and low pressure plasma spraying (LPPS).

(14) When applied, the coating 110 contains a crack network that allows it to survive the stress of numerous thermal cycles. As described in the above background section, the crack network is altered to a less desirable state by sintering and temperature cycling during service. Thus the structure formed upon coating manufacture changes with time, the rate depending on the starting material phases. Decreasing the sintering rate increases the amount of time before the closing of microcracks and creation of massive cracks, increasing coating lifetime.

(15) A dominant factor affecting sintering is the presence of specific impurity phases within the structure made up of oxides which when combined with each other or the zirconia alloy result in melting points hundreds of degrees lower than that of the zirconia alloy itself. These impurity oxides increase the sintering rate. FIG. 2 shows the effect of impurity on the sintering rate.

(16) In one embodiment of the present invention, the material contains zirconia (ZrO.sub.2) and/or hafnia (HfO.sub.2) partially stabilized by a total of 4 to 20 weight percent of one or more rare earth oxides having total impurities less than or equal to 0.15 weight percent, and preferably less than or equal to 0.1 weight percent. For purposes of the invention, rare earth oxides can be defined as any oxide from group IIIB (column 3) of the periodic table of elements, which includes scandia (Sc.sub.2O.sub.3), yttria (Y.sub.2O.sub.3), lanthanide oxides and actinide oxides.

(17) The material of the present invention contains zirconia (ZrO.sub.2) and/or hafnia (HfO.sub.2) partially stabilized by a total of 4 to 20 weight percent of a primary stabilizing oxide such as ytterbia and/or yttria, (and optionally additional stabilizers of one or more rare earth oxides) having total impurities less than or equal to 0.15 weight percent, and preferably less than or equal to 0.1 weight percent. For purposes of the present invention, oxide impurities can be defined as materials which when combined with each other or with zirconia form phases with melting points much lower than that of pure zirconia, especiallybut not limited tosoda (Na.sub.2O), silica (SiO.sub.2), and alumina (Al.sub.2O.sub.3). Other specific concentration ranges of stabilizers are provided herein and in co-pending and commonly assigned U.S. patent application Ser. No. 11/520,041, entitled HIGH PURITY CERAMIC ABRADABLE COATINGS, U.S. patent application Ser. No. 11/520,044, entitled OPTIMIZED HIGH TEMPERATURE THERMAL BARRIER, and U.S. application Ser. No. 11/520,042, entitled OPTIMIZED HIGH PURITY COATING FOR HIGH TEMPERATURE THERMAL CYCLING APPLICATIONS each filed on Sep. 13, 2006 and each incorporated herein by reference.

(18) In accordance with embodiments of the invention, the limits for known impurities in order to achieve a desirable sintering rate and therefore increase service lifetime when used as a coating are about:

(19) TABLE-US-00001 soda (Na.sub.2O) 0.1 weight percent silica (SiO.sub.2) 0.05 weight percent alumina (Al.sub.2O.sub.3) 0.01 weight percent titania (TiO.sub.2) 0.05 weight percent hematite (Fe.sub.2O.sub.3) 0.05 weight percent calcia (CaO) 0.05 weight percent, and magnesia (MgO) 0.05 weight percent.

(20) In a preferred embodiment, the limits for known impurities are about:

(21) TABLE-US-00002 Na.sub.2O 0.01 weight percent SiO.sub.2 0.01 weight percent Al.sub.2O.sub.3 0.01 weight percent TiO.sub.2 0.01 weight percent Fe.sub.2O.sub.3 0.01 weight percent CaO 0.025 weight percent, and MgO 0.025 weight percent.

(22) The impurity limits in the embodiments above are not indicative that any or all of the impurities listed will be included in the material in any amount. The embodiment of the invention may include zero weight percent of one or more of the above-listed impurities.

(23) FIG. 3 provides a phase diagram for pure zirconia. (The diagram can be found, for example, in Ceramic Phase Diagrams vol. 3, figure 04259.) As shown in FIG. 3, pure zirconia exists in three crystal phases at different temperatures. At very high temperatures (>2370.degree. C.) the material has a cubic structure. At intermediate temperatures (1200 to 2372.degree. C.) it has a tetragonal structure. At relatively lower temperatures (below 1200.degree. C.) the material transforms to the monoclinic structure. The transformation from tetragonal to monoclinic is rapid and is accompanied by a 3 to 5 percent volume increase that causes extensive stress in the material. Thus, pure zirconia cannot fulfill the coating requirements for high-temperature cycling. The resulting strain difference between the coating and substrate caused by the phase transformation results in a stress that is greater than the bond strength between them, so the coating will detach.

(24) In accordance with embodiments of the invention, in order to overcome the volume change caused by the undesired phase transformation described above, one or more elements are added to the zirconia to modify the amount of phase transformation that occurs. The stabilizing elements, which are suitable for changing the amount and rate of phase transformation that occurs in the oxide coating, may include the following: scandium, yttrium and the rare earths, particularly the lanthanides, since they have solubility in zirconia. Scandium is not typically used due to its rarity and resulting prohibitive cost. Use of rare earths metals from the actinide group such as uranium and thorium may be limited due to their radioactivity. Thus, yttrium is a preferred stabilizing element.

(25) FIG. 4 provides a standard phase diagram for stabilized zirconia showing the general alloying trends for the zirconia stabilizers. A specific diagram for zirconia with yttria as a stabilizer is given in FIG. 5. (The diagram can be found, for example, in Ceramic Phase Diagram, vol. Zirconia, figure Zr-157.)

(26) Phase transformation in partially stabilized zirconia may possibly cause localized stresses that lead to the formation of micron-sized micro-cracks in the coating upon thermal cycling that cancel out some of the massive stress caused by coating volume shrinkage. Thus, these two phenomena of the coating structureshrinking and crackingwork against each other and finding a balance between them will maximize coating lifetime. This mechanism implies then that the structure of the crack network of the coating is changing with time as the phase of the ceramic material changes. This mechanism is required for a thermal barrier or high temperature abradable coatings to survive thermal cycling.

(27) The addition of a stabilizing element affects two main properties of the zirconia coating system in a positive manner. First, the addition of a stabilizer as illustrated in FIG. 4 generally increases the melting temperature of the zirconia (in the partially stabilized composition ranges). Second, the addition of a stabilizer generally decreases the thermal conductivity. Once the critical composition that has the highest thermal cycling values is found experimentally for a stabilizer, the stabilizers can be compared by the melting point at the critical composition.

(28) Rising fuel cost and other factors continue to drive the need for improved operational efficiency, and thus higher operating temperatures, of gas turbines. While yttria stabilized zirconia is the material of choice for stabilization, greater operational temperatures can be achieved using ytterbia (FIG. 4) for example. Zirconia partially stabilized by ytterbia provides a better composition, since it also has one of the lowest thermal conductivities of the potential stabilizers when alloyed with zirconia. As the need for higher operating temperatures increases, a higher coating material cost may be tolerated, so ytterbia partially stabilized zirconia may become a preferred thermal barrier or high temperature abradable coating system. Given then the trade-offs of cost and performance, a combination of both yttria and ytterbia stabilizers is expected to have optimum performance to cost ratio.

(29) A blend of two or more partially stabilized high-purity material compositions may also be used. For example, in another embodiment, a blended ceramic material for use in high-temperature thermal barriers is provided. The blended materials include a first material with a yttria (Y.sub.2O.sub.3) stabilizer, and a balance of at least one of zirconia (ZrO.sub.2) and hafnia (HfO.sub.2) and combinations thereof, wherein the zirconia (ZrO.sub.2) and/or hafnia (HfO.sub.2) is partially stabilized by the yttria stabilizer, and having total impurities less than or equal to 0.15 weight percent, and preferably less than or equal to 0.1 weight percent. The range of Y.sub.2O.sub.3 stabilizer is about 4-12 weight percent, and preferably 6-9 weight percent. The second material of the blended material may contain a ytterbia (Yb.sub.2O.sub.5) stabilizer and a balance of at least one of zirconia (ZrO.sub.2) and hafnia (HfO.sub.2) and combinations thereof, wherein the zirconia (ZrO.sub.2) and/or hafnia (HfO.sub.2) is partially stabilized by the ytterbia stabilizer, and having total impurities less than or equal to 0.15 weight percent, and preferably less than or equal to 0.1 weight percent. The range of Yb.sub.2O.sub.5 stabilizer is about 4-16 weight percent, and preferably 10-16 weight percent. In the blended material, the ytterbia (Yb.sub.2O.sub.5) stabilized zirconia (ZrO.sub.2) and/or hafnia (HfO.sub.2) fraction may include about 5-50 weight percent of the total and preferably about 15-30 weight percent of the total. The yttria stabilized zirconia (ZrO.sub.2) and/or hafnia (HfO.sub.2) fraction may include about 50-95 weight percent of the total and preferably about 70-85 weight percent of the total blend.

(30) In another embodiment the blended material includes a first material with a ytterbia (Yb.sub.2O.sub.5) stabilizer, and a balance of at least one of zirconia (ZrO.sub.2) and hafnia (HfO.sub.2) and combinations thereof, wherein the zirconia (ZrO.sub.2) and/or hafnia (HfO.sub.2) is partially stabilized by the ytterbia stabilizer, and having total impurities less than or equal to 0.15 weight percent, and preferably less than or equal to 0.1 weight percent. The range of Yb.sub.2O.sub.5 stabilizer is about 4-16 weight percent, and preferably 10-16 weight percent. The second material of the blended material may contain a stabilizer of at least one of neodymium (Nd.sub.2O.sub.3), europia (Eu.sub.2O.sub.5), and combinations thereof and a balance of at least one of zirconia (ZrO.sub.2) and hafnia (HfO.sub.2) and combinations thereof, wherein the balance is partially stabilized by the stabilizer, and having total impurities less than or equal to 0.15 weight percent, and preferably less than or equal to 0.1 weight percent. The range of Nd.sub.2O.sub.3 stabilizer is about 4-20 weight percent, and preferably 8-16 weight percent. The range of Eu.sub.2O.sub.3 stabilizer is about 4-16 weight percent, and preferably 10-16 weight percent. The range of the combined Nd.sub.2O.sub.3 and Nd.sub.2O.sub.3 stabilizer is about 4-16 weight percent. In the blended material, the ytterbia (Y.sub.2O.sub.3) stabilized zirconia (ZrO.sub.2) and/or hafnia (HfO.sub.2) fraction may include about 5-50 weight percent of the total and preferably about 15-30 weight percent of the total. The yttria stabilized zirconia (ZrO.sub.2) and/or hafnia (HfO.sub.2) fraction may include about 50-95 weight percent of the total and preferably about 70-85 weight percent of the total blend.

(31) In another embodiment of the invention the blended material includes a first material with a yttria (Y.sub.2O.sub.3) stabilizer, and a balance of at least one of zirconia (ZrO.sub.2) and hafnia (HfO.sub.2) and combinations thereof, wherein the zirconia (ZrO.sub.2) and/or hafnia (HfO.sub.2) is partially stabilized by the yttria stabilizer, and having total impurities less than or equal to 0.15 weight percent, and preferably less than or equal to 0.1 weight percent. The range of Y.sub.2O.sub.3 stabilizer is about 4-12 weight percent, and preferably 6-9 weight percent. The second material of the blended material may contain a stabilizer of at least one of neodymium (Nd.sub.2O.sub.3), europia (Eu.sub.2O.sub.5), and combinations thereof and a balance of at least one of zirconia (ZrO.sub.2) and hafnia (HfO.sub.2) and combinations thereof, wherein the balance is partially stabilized by the stabilizer, and having total impurities less than or equal to 0.15 weight percent, and preferably less than or equal to 0.1 weight percent. The range of Nd.sub.2O.sub.3 stabilizer is about 4-20 weight percent, and preferably 8-16 weight percent. The range of Eu.sub.2O.sub.3 stabilizer is about 4-16 weight percent, and preferably 10-16 weight percent. The range of the combined Nd.sub.2O.sub.3 and Eu.sub.2O.sub.3 stabilizer is about 4-16 weight percent. In the blended material, the neodymium (Nd.sub.2O.sub.3) and/or europia (Eu.sub.2O.sub.5) stabilized zirconia (ZrO.sub.2) and/or hafnia (HfO.sub.2) fraction may include about 5-50 weight percent of the total and preferably about 15-30 weight percent of the total. The yttria stabilized zirconia (ZrO.sub.2) and/or hafnia (HfO.sub.2) fraction may include about 50-95 weight percent of the total and preferably about 70-85 weight percent of the total blend.

(32) In a further embodiment of the invention the blended material includes a blend of at least three materials. The first material may contain a yttria (Y.sub.2O.sub.3) stabilizer, and a balance of at least one of zirconia (ZrO.sub.2) and hafnia (HfO.sub.2) and combinations thereof, wherein the zirconia (ZrO.sub.2) and/or hafnia (HfO.sub.2) is partially stabilized by the yttria stabilizer, and having total impurities less than or equal to 0.15 weight percent, and preferably less than or equal to 0.1 weight percent. The range of Y.sub.2O.sub.3 stabilizer is about 4-12 weight percent, and preferably 6-9 weight percent. The second material of the blend may contain a ytterbia (Yb.sub.2O.sub.5) stabilizer, and a balance of at least one of zirconia (ZrO.sub.2) and hafnia (HfO.sub.2) and combinations thereof, wherein the zirconia (ZrO.sub.2) and/or hafnia (HfO.sub.2) is partially stabilized by the ytterbia stabilizer, and having total impurities less than or equal to 0.15 weight percent, and preferably less than or equal to 0.1 weight percent. The range of Yb.sub.2O.sub.5 stabilizer is about 4-16 weight percent, and preferably 10-16 weight percent. The third material of the blend may contain a stabilizer of at least one of neodymium (Nd.sub.2O.sub.3), europia (Eu.sub.2O.sub.5), and combinations thereof and a balance of at least one of zirconia (ZrO.sub.2) and hafnia (HfO.sub.2) and combinations thereof, wherein the balance is partially stabilized by the stabilizer, and having total impurities less than or equal to 0.15 weight percent, and preferably less than or equal to 0.1 weight percent. The range of Nd.sub.2O.sub.3 stabilizer is about 4-20 weight percent, and preferably 8-16 weight percent. The range of Eu.sub.2O.sub.3 stabilizer is about 4-16 weight percent, and preferably 10-16 weight percent. The range of the combined Nd.sub.2O.sub.3 and Eu.sub.2O.sub.3 stabilizer is about 4-16 weight percent. In the blended material, the ytterbia (Y.sub.2O.sub.3) stabilized zirconia (ZrO.sub.2) and/or hafnia (HfO.sub.2) fraction may include about 5-45 weight percent of the total, and preferably about 15-30 weight percent of the total. The neodymium (Nd.sub.2O.sub.3) and/or europia (Eu.sub.2O.sub.5) stabilized zirconia (ZrO.sub.2) and/or hafnia (HfO.sub.2) fraction may also include about 5-45 weight percent of the total and preferably about 15-30 weight percent of the total. The yttria stabilized zirconia (ZrO.sub.2) and/or hafnia (HfO.sub.2) fraction may include about 10-90 weight percent of the total, and preferably about 30-60 weight percent of the total blend.

(33) Material of embodiments of the present invention can be provided in a variety of forms for use in thermal spray applications. For example, the material is supplied in the form of a powder, a slurry of powder, or a chemical solution of the constituents. If in powder form, the powder may be in the form of a spray dried powder of the individual constituents and organic binder, spray dried powder of the combined individual constituents and organic binder, fused and crushed powder, agglomerated and sintered powder, plasma densified material or powder from chemical solution routes. Typical particle sizes for the powders may vary but typically range between about 5-150 microns when deposited by various thermal spray equipment, preferably ranging between about 15-125 microns for air plasma spray and ranging between about 5-30 microns for low pressure plasma spray.

(34) Typically for thermal spray applications, a polymer or organic material in powder form can be added to the material blend. Powder may be in the form of a spray dried powder of the individual constituents and an organic binder, spray dried powder of the combined individual constituents and an organic binder, fused and crushed powder, agglomerated and sintered powder, plasma densified material or powder from chemical solution routes. The organic binder may be used to at least partially bond together the placeholder material and the ceramic material. For high temperature abradable coatings, the benefit of adding a fugitive phase is that a higher porosity than is achievable with conventional deposition methods. The increased porosity aids abradability by introducing more surfaces to the crack network of the coating, decreasing the coating elastic modulus and thus promoting controlled material removal. Thus, according to an embodiment of the invention, a coating material may have about 70 to 99 weight percent of an ultra-pure ceramic material as previously described and about 1-30 weight percent (and preferably 2-15 weight percent) of a placeholder material. The placeholder material may be an organic powder material or an inorganic powder material that can be burned out subsequent to deposition of the coating material.

(35) Porosities and cracks provide strain tolerance to TBCs and help to reduce thermal conductivities. Using thermal spray process, such as air plasma spray, flame spray or low pressure plasma spray, a high purity coating structure 120 (shown in FIG. 6) that comprise a ceramic matrix, porosity and microcracks can be achieved. The high purity coating structure is formed by injecting particles of invention high purity materials into a high temperature and high velocity flame. These particles are then heated and accelerated in the flame. Before reaching the substrate, some particles are molten, while some other particles are semi-molten or not melted. Referring to FIG. 6, molten and semi-molten particles strike on the substrate 100 (or optional bond coat 112) and then spread and solidify rapidly to form disk-like deposits 111, which are referred to as splats. Although some unmelted particles are entrapped and incorporated into the coating, most of them bounce off when they hit the substrate. The accumulation of splats and small amount of unmelted particles results in the coating formation. Due to shrinkage occurred during rapid solidification and imperfect packing of splats and unmelted particles, voids and cracks are generated in the coating. Herein, porosity refers to a void with an aspect ratio (length divided by width) of less than about 10. Typical porosity is in the range of about 5.about.20 volume percent, preferably in the range of about 7.about.15 volume percent. Micro cracks refers to a void with an aspect ratio (length divided by width) of larger than about 10 and the length of the void is less than about 100 micrometers. Typical volume percentage of micro cracks is in the range of about 2.about.15 volume percent, preferably in the range of about 5.about.10 volume percent.

(36) In order to enhance the strain tolerance of the aforementioned high purity TBCs, macro cracks that runs normal to the top coat and substrate interface can be introduced into the coating. As a result, another high purity coating structure that comprise a ceramic matrix, porosity, macro cracks and micro cracks (FIG. 7) can be achieved by thermal spray processes, such as air plasma spray, flame spray or low pressure plasma spray. The high purity coating structure 130 of FIG. 7 is formed by injecting particles of inventive materials into a high temperature and high velocity flame. These particles are then heated and accelerated in the flame. Before reaching the substrate, some particles are molten, while some other particles are semi-molten or not melted. Molten and semi-molten particles strike on the substrate 100 (or optional bond coat 112) and then spread and solidify rapidly to form disk-like deposits, which is referred to as splats. Although some unmelted particles are entrapped and incorporated into the coating, most of them bounce off when they hit the substrate. The accumulation of splats 131 and small amount of unmelted particles results in the coating formation 130. Due to shrinkage occurred during rapid solidification and imperfect packing of splats and unmelted particles, voids and cracks are generated in the coating. When coating deposition conditions are controlled to generate large shrinkage stress and improve the packing of splats to reduce voids and gaps between splats, cracks 132 normal to the coating 130 and substrate 100 interface are created. Herein, macro cracks refers to a void with an aspect ratio (length divided by width) of larger than about 10 and the length of the void is longer than about 100 micrometers. More than about 90% of the macro cracks are arranged in the direction normal to the top coat and substrate interface. These macro cracks are referred to as vertical macro cracks, while the macro cracks parallel to the top coat and substrate interface are referred to as horizontal vertical cracks. For this coating structure, typical volume percentage of porosity and micro cracks is less than about 10% and 5%, preferably less than about 5% and 3%, respectively. The average number of vertical macro cracks in a length of 25.4 mm along the top coat and substrate interface is in the range of about 5 to 250, preferably in the range of about 50 to 150.

(37) When coatings are produced using a vapor deposition process, such as electron beam assisted physical vapor deposition process (EB-PVD) or low pressure (lower than ambient) plasma spraying, the resulting coating has a unique columnar structure. The gaps between columns impart excellent strain tolerance to the coating. Accordingly, TBCs produce using vapor deposition process, such as EB-PVD or low pressure (lower than ambient) plasma spraying, usually have a higher durability than TBCs produced using thermal spray processes. As illustrated in FIG. 8, if vapor deposition process was employed, another high purity coating structure 140 that comprises ceramic columns 143 and gaps 141 between them can be achieved. An optional bond coat 112 is shown between the substrate 100 and the coating 140. The high purity coating structure 140 is formed by vaporizing the inventive high purity materials in a form of powder, ingot, target, solution or suspension. The formed vapor then deposited atomically on the substrate. By controlling processing temperature and pressure according to the Thornton's model (FIG. 9), a coating with columnar structure is formed. Herein, ceramic columns 143 are basically a cluster of crystals. More than about 90% of the crystals are at an angle of about 45 to 135 degree to the top coat and substrate interface. Within the cluster of crystals, voids smaller than about 20 micrometers are present. The gaps 141 between the columns have an aspect ratio (length divided by width) of larger than about 10. More than about 90% of the gaps are at an angle of about 45 to 135 degree to the top coat and substrate interface.

(38) In low pressure (lower than ambient) plasma spraying process, if molten droplets are also generated during the vaporization of the invention high purity materials, then the entrapment and incorporation of these droplets into the coating results in the formation of another high purity coating structure. As illustrated in FIG. 9, the high purity coating structure 150 comprises ceramic columns 143, gaps between the columns 141, and nodules 142 distributing randomly in the gaps and columns. An optional bond coat 112 is shown between the substrate 100 and the coating 150. Herein, ceramic columns 143 are basically a cluster of crystals. More than 90% of the crystals are oriented at an angle of 45 to 135 degree to the top coat and substrate interface. Within the cluster of crystals 143, voids smaller than 20 micrometers are present. The gaps 141 between the columns have an aspect ratio (length divided by width) of larger than about 10. More than 90% of the gaps 141 are oriented at an angle of 45 to 135 degree to the top coat and substrate interface. The nodules 142 distributing randomly in the gaps and columns are frozen droplets. The size of these nodules 142 is typically less than about 45 micrometers, preferably less than about 30 micrometers.

(39) While exemplary embodiments of the invention have been shown and described herein, it will be obvious to those skilled in the art that such embodiments are provided by way of example only. Numerous insubstantial variations, changes, and substitutions will now be apparent to those skilled in the art without departing from the scope of the invention disclosed herein by the Applicants. Accordingly, it is intended that the invention be limited only by the spirit and scope of the claims, as they will be allowed.