Prestressed ceramic coatings

(54) NAME Method for applying a ceramic coating to a metal substrate (56) QUOTE PUBLICATIONS: US A 3 091 548 (117: 70), US A 3 721 534 (29: 195),

US A 3 975 165 (29: 182.2), US A 4 109 031 (427: 91),

US A 4,163,071 (427: 34), US A 4,299,865 (427: 243) (57) SUMMARY:

The present invention relates to prestressed ceramic coatings and in particular a means coated with graduated ceramic material and methods of applying coatings. Methods for maintaining low ratios of mechanical stresses and strength across the thickness of the coating are discussed. In a particular member, the coating is applied to a metal support (12), the coating comprising a metal-bonding layer^z14), a first intermediate layer (16 'of metal / ceramic material, a second intermediate layer' 18) of metal / ceramic material with an increased proportion of ceramic material, and a layer of ceramic material only. By modulating the metal substrate temperature during the coating process, a desired pattern of residual stresses is established in the part.

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The numbers in brackets indicate the international identification code. INID code. The letter within the squeeze indicates international island document coo

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The present invention relates to a method of applying a ceramic coating to a metal substrate and to ceramic heat-barrier layers, and in particular to coatings graduated from a metallic to a ceramic composition obtained by applying said method.

The basic tanks were developed in the industry for the production of gas turbine engines, more specifically for the manufacture of air seals outside turbines, but have more extensive utility in both the aforementioned industry and others.

In modern gas turbine engines, working medium gases are expanded which have temperatures exceeding 1093 ° C (2000 ° F ') over rows of turbine blades for extraction of power from the flowing medium. A mantle, referred to as an outside air seal, encloses each row of turbine blades to prevent working media gases from leaking over the blade tips.

The graduated ceramic coatings and methods of their application according to the invention were developed for special use in connection with seals against the air outside turbines. Organics were searched for .. with long life and with the possibility of coping with long-term operation in the stressful turbine environment. Special requirements are the ability to withstand high temperatures and good resistance to thermal shock. In addition, the sealing material must have sufficient surface abrasion resistance so as not to be destroyed in the event that the seal comes into sliding contact with the rotor blades that surround it.

Examples of prior art with regard to seals coated with ceramic material may be mentioned US Patents 5,091,548, 5,817,719, 3,879,800, 3,911,891, 3,918,925, 3,975,165 and 4,109,031.

As stated in some of the aforementioned patents, and in particular in U.S. Patent No. 4,165,071, the metal substrate on which the ceramic coating is applied can be pre-heated, which allows control either by mechanical stresses or by the density of the coating. Such heating has generally taken place at a constant temperature.

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Although many of the materials and methods described in the aforementioned patents are considered to be highly satisfactory, organs made of these materials and by these methods have not yet achieved perfection, especially when used in a stressful environment. Significant research on further improved materials and methods is ongoing.

According to the present invention, graded layers of metal / ceramic material with increasing content of ceramic material are applied in turn on a metal substrate under varying temperature conditions of the substrate, which means that one or more layers are applied at an initial temperature higher than or equal to the final temperature at which the previous layer was applied, and a final temperature, which is lower than or equal to the temperature at which the nearest subsequent layer is to be applied.

According to a more detailed method described, the graduated layers are applied continuously, with variations in the composition of metal and ceramic material being arranged without coating. The coating process is interrupted, and variations in the substrate temperature are arranged in transition zones during the coating process.

A primary feature of the invention is that an adjustment of error adjustments is achieved as a result of heat stresses. By controlling the temperature of the substrate during the coating process, a temperature characteristic is established in the coated part at which the material in the part in question is substantially free of stresses or mechanical stresses. Modulation of the substrate temperature, not only during each layer application, but also during application of the layer itself with uniform composition, results in a preferred distribution of residual stresses through the layers.

An important advantage of the invention is that safe relations between stress (mechanical stress) and strength of the coating are obtained. Safe conditions are maintained throughout the temperature ranges and temperature gradient ranges for which the part is exposed during working cycles. Deficiencies in the coating are also prevented under temperature conditions. The same can vary up to about 1095 ° C (2000 ° Fk). As the part is initially heated in working or operating environments, the pressure conditions are reduced. ceramic material. Further heating induces thermal stresses in the ceramic material but not with as large values ​​as could cause

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The foregoing features and advantages of the invention will become more apparent from the following description of a preferred method of applying the invention and the accompanying drawings, in which Figure 1 illustrates, in simplified form, the process of applying a coating to a ceramic surface seal in a gas turbine engine. FIG. 2 Fig. 5 is a graph showing residual stresses at room temperature indicated as a function of the thickness of the coating in a coating applied by the method according to the invention. FIG. 5 is a graph showing temperature control of a metal substrate on which a graduated ceramic coating is applied by the method of the invention. FIG. 4 is a diagram showing the stress-free temperature characteristic of a ceramic coating applied by the method of the invention; 5 is a diagram showing the relationship between stress and strength of a ceramic coating applied to a sealing of the turbine to the external air by the method of the present invention under the acceleration of the engine to the conditions at sea level start, under conditions at sea level start and below conditions of deceleration to downtime.

The process of the present invention for applying a graduated ceramic coating to a metal support and the resultant means is illustrated with reference to a seal against the external air, which seal is of the type used in a gas turbine engine. The metal substrate 12 is usually made of a nickel alloy, for example the alloy known in the gas turbine industry as INCONEL 713, and has a blend layer 14 of metallic material attached thereto. A common material for this bonding layer is the nickel-chrome aluminum alloy known in the gas turbine industry as METCO 445.

One or more intermediate layers of graduated metal and ceramic material with increasing content of ceramic material in the composition are attached to the blending layer 14. In an embodiment with two intermediate layers, the first intermediate layer 16 can be, for example, a mixture of zirconia (ZrO weight percent and CoCrAlYmaterial in an amount of 60 weight percent. The CoCrAlY material has, in an effective embodiment, the following nominal composition: chromium 25.0%, aluminum 1J, O 2, yttrium 0.65 3 and. residual amount of mainly cobalt. For example, the second intermediate layer 18 may be a mixture of zirconium oxide (ZrOg) in an amount of 85% by weight and CoCrAlY material in an amount of 15% by weight, the CoCrAlY material having the same composition as in the first intermediate layer. In such embodiments, each subsequent layer of metal / ceramic material has a higher proportion of ceramic material than the preceding layer and a smaller proportion of ceramic material than the layer to be applied thereafter.

One to 100 $ ceramic material, zirconia (ZrOg), is applied over the last metal / ceramic intermediate layer. In a preferred form, two layers of ceramic material are used, namely a first layer 20 of dense ceramic material and a second layer 22 of porous ceramic material. In practice, the porous layer has a reduced density, and it is applied as a mixture of zirconia powder (ZrOg 2 and 2.5 to 10% by weight polyester). The polyester is removed after application of the layer leaving a porous ceramic coating. Heating of the seal against the external air during manufacture or in situ during operation is performed to burn away the polyester material from the ceramic material. The polyester can be used in an amount outside the specified weight percent range to achieve the corresponding porosity.

There are two reasons for using a ceramic surface material in a means for sealing against the outer or surrounding air, namely to provide a thermal barrier which protects the substrate from the hot gases which constitute working medium in the turbine and for which the substrate would otherwise be exposed and for to provide a wear-resistant seal that can withstand thermal deviations of the rotor blades that it surrounds without being destroyed.

During the coating process, the substrate temperature is regulated to a predetermined extent for the establishment of patterns of residual stresses and mechanical stresses in the manufactured seal. Heating means 24 for heating the substrate are provided for this purpose. The coatings themselves are applied by plasma spraying methods that are common in the industry. A plasma spray gun 26 in which the powders or mixture of powder is injected is caused to move in a repeated pattern over the substrate as the coating builds up to the desired thickness. A plasma stream 28 transfers the coating powders 10 to the surface of the article to be coated. Coating mixtures are suitably changed during continuous operation. Changes in the substrate temperature are made as quickly as possible in transition zones immediately before, during or

459 976 immediately after each change in the powder composition.

The substrate temperature cycle upon coating with the component described above is illustrated by the diagram in Fig. 2. The following layers were applied at the indicated thickness and at the indicated initial and final temperature of the substrate:

Thickness Initial temperature Final temperature

<td>coating layer</td><td>mm</td><td>inch</td><td>2c_</td><td>° F</td><td>° C</td><td>° F</td>

<td>Metallic bonding layer</td><td>Z 0.06?</td><td> (0,005)</td><td> 558</td><td> (1000)</td><td> 816</td><td> (1500)</td>

<td>First middle layer</td><td>Z0,675</td><td> (0,050)</td><td> 816</td><td> (1500)</td><td> .580</td><td> (1075)</td>

<td>Other intermediate layers</td><td> « 0,675</td><td> (0,050)</td><td> 665</td><td> (1225)</td><td> 607</td><td> (1125)</td>

<td>Dense ceramic material</td><td> «0,675</td><td> (0,050)</td><td> 657</td><td> (1215)</td><td> 545</td><td> (1010)</td>

<td>Porous ceramic rial</td><td> = 0,7875</td><td> (0,055)</td><td> 545</td><td> (1010)</td><td> 482</td><td> ‘(900)</td>

<td colspan="5">As shown both in the above table and in</td><td colspan="2">the diagram in</td>

<td colspan="3">Fig. 2 affixes each intermediate layer</td><td colspan="2">metal / ceramic</td><td colspan="2">material underneath</td>

various conditions as to the temperature of the substrate, namely at an initial temperature higher than the temperature at the final application of the previous layer and at a final temperature lower than or equal to the temperature at which subsequent layers are to be applied. The substrate temperature at the end of the application of each intermediate layer is also lower than the substrate temperature at the initial application of this intermediate layer. Stepwise temperature changes at the transition between layers of different compositions are likely to be desirable. However, in a continuous coating process, it is convenient for practical reasons to have a transition zone that occurs near the change in composition.

The coefficients of heat dissipation of up to 100 $ ceramic material and substrates of up to 100 $ metal per se are disposed of by grading the coatings and by inducing pressure stress during application of the layers. The diagram of increasing stresses shown in Fig. 5 is characteristic of parts manufactured according to the present method. The diagram shows increasing pressure stresses measured at the back of the substrate as the coating thickness is increased. The ever-increasing stresses indicate the absence of significant discontinuities at all coating thicknesses. About stress reversals. would occur, they would appear as peaks or valleys along the curve.

As discussed above, the coating is so designed that it

459 976 £ is voltage-free or stress-free at a pre-selected temperature. . The temperature at which the coating is stress free lies between the cold state and the maximum temperature. Fig. 4 illustrates a substantially stress-free member at a temperature of about 649 ° C (1200 ° F). Stress values ​​at different coating thicknesses for materials with different compositions are highlighted. As the temperature of the body is lowered from the temperature corresponding to freedom from stresses, the metal backing side of the body shows signs of tensile stress while the ceramic side shows signs of stress. As the temperature of the member is raised above the temperature corresponding to freedom from stresses, the metal backing side shows signs of strain, while the ceramic side shows signs of tensile stress.

Fig. 5 is a diagram showing the relationship between stresses and strengths of the cross-section of the coating as a function of the state of the motor. The states illustrated are (A) acceleration to sea level start, (B) steady state sea level start, (C) deceleration from sea level start to stationary. In all conditions, the ratio of stresses and strengths in each layer of material is well below the ratio of 1.0, above which fracture can be expected. Excessive stresses induced by differential stresses between layers are avoided. The effects of controlling the temperature of the substrate and the different coefficients of heat expansion between the materials in successive layers are adjusted to achieve this result.

Although the invention has been illustrated and described with reference to detailed embodiments thereof, it will be apparent to those skilled in the art that various changes may be made within the scope of the invention as claimed.

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