JP2918264B2 - Coated metal products - Google Patents

Description

The present invention relates to a coated article comprising a metal alloy substrate having a glass-ceramic coating selected from the group consisting of barium silicate or strontium silicate. About. The coating on the metal substrate surface serves as an oxygen barrier to prevent oxidative corrosion of the metal at high temperatures and as a thermal barrier to prevent rapid overheating of the metal.

(Prior Art) For example, in a turbine engine and a heat exchanger,
The need for materials that can withstand operating temperatures above 1000 ° C. is well recognized. Various high-temperature materials are known, but in more severe use conditions it is customary to use alloys known as superalloys. Characteristically, these alloys are rich in nickel, cobalt, chromium or iron, with nickel or cobalt being the primary metal substrate. Superalloys differ significantly from other high temperature materials in that they are sufficiently oxidation resistant and can be used without coating in oxidizing environments. Nevertheless, under the harsh conditions encountered by aircraft engine turbine blades, even superalloys tend to degrade unless protected by a barrier coating.

A common method of protecting materials from oxidation at high temperatures is
Applying a continuous integrated glass coating. This completely seals the material from the surrounding (containing oxygen) atmosphere. However, viscous flow of the glass coating can occur when large surface stresses occur during high temperature use. In that case, a thin portion may be formed on the glass barrier coating and may be catastrophically broken.

The high temperature viscosity of the glass coating can be increased by mixing the crystalline material with the glass frit before applying the coating. However, these glassy crystalline mixtures tend to be non-homogeneous in firing, making it very difficult to control the size and homogeneity of the crystals. Thus, some parts of the substrate have no crystals at all and others have too many (or large) crystals, so firing is not successful. It is difficult to obtain a void-free coating with this heterogeneous glass-crystal mixture.

Therefore, in order to protect superalloy components, an insulating layer of stabilized ZrO ₂ is currently applied commercially by a plasma spraying method. U.S. Patent Nos. 4,485,151 and 4,535,033 (Stecul
a) describes such a procedure using an alloy as a binder for stabilized zirconia. U.S. Patent 4,
No. 4,64,994 (Demeray) describes the use of aluminum oxide alloys as an intermediate coating.

These procedures involve several steps that are too slow for commercial production and difficult to control. Also,
Thermal gradients occur during plasma spraying and tend to introduce defects into the finished coating. Also, the coating tends to be porous.
This allows gases, especially O ₂ , SO ₂ and water vapor, to enter, all of which contribute to coating failure. Nevertheless, such methods are widely used to protect jet engine components from corrosive oxidative damage during normal periodic overhauls.

A basic object is to provide a reliable and reproducible oxygen barrier coating for superalloys required to be used above 1000 ° C.

Another object is to provide a coating that is more effective and easier to apply than previously known coatings.

Yet another object is to provide an oxygen barrier coating that is non-porous and continuous and free from defects such as needle holes and cracks.

Yet another object is to provide an oxygen barrier coating that adheres tightly and resists delamination during thermal cycling.

Another object is to provide an oxygen barrier coating that exhibits excellent flow rates of the glass coating when fired in one temperature range and resists flow (due to crystallization) when heated in a higher temperature range. Is to do.

Yet another object is to provide a superalloy metal object having an oxygen barrier coating that adapts the object to service temperatures up to 1200 ° C.

Yet another object is to provide a useful degree of thermal insulation to metal surfaces.

SUMMARY OF THE INVENTION An object of the present invention is a coated article comprising an alloy substrate adapted for use at a temperature of 1000 ° C. or more in an atmosphere containing oxygen, the alloy comprising a nickel-based alloy. , Cobalt-
Base, chromium-base and iron-base alloys, the coating of which forms an oxide barrier over the metal surface and comprises a glass-ceramic having a composition selected from the barium silicate and strontium silicate systems. If the composition is barium silicate based on oxide percentage by weight,
20-65% BaO and 25-65% SiO ₂ , in the case of the strontium silicate system, 20-60% SrO and 30-70% SiO ₂ , each system further comprising up to 15% Al ₂ O ₃ , up to 15% ZrO ₂ , up to 15% Y ₂ O ₃ , up to 25% MnO, up to 25% NiO, up to 30% MgO, up to 30% CoO and oxidation up to 40% Consist essentially of at least one oxide selected from the group consisting of iron and do not exceed 5% of B ₂ O ₃ +
R ₂ O, the total amount of such additives is 50% for barium silicates and 40% for strontium silicates.
% Of the coated article.

The oxide selected is Al ₂ O ₃ or ZrO ₂ , and its composition desirably does not include R ₂ O and B ₂ O ₃ .

Within the preferred range of barium silicate based, the composition is substantially SiO ₂ to BaO ratio is a molar ratio of 2: 1 to 5: 1 (approximately SiO ₂ in weight percent also Izure is 40-65%, the BaO 30 −55
%) And consists of 1-10% of Al ₂ O ₃ by weight. Within the preferred range of similarly strontium silicate-based, substantially in SiO ₂ to BaO ratios are molar ratio of 1.5: 1 to 4: 1 (by weight approximately 45-65% of SiO ₂ and 30-55% of SrO ) And 5-10
Consisting ZrO ₂ or Al ₂ O ₃ mole%.

Attention is directed to the following U.S. patents as prior art references.

U.S. Pat. No. 3,397,076 (Little et al.) Describes a melted crystallizable undercoat and overcoat coating for high temperature alloys where the primary element is cobalt, nickel, chromium, iron or mixtures thereof. I have. Coating of the undercoat is free of lithium 35-65% of SiO ₂ and 12-45% of Ba
Contains O. The examples also show that significant amounts of R ₂ O and B ₂
O ₃ and TiO ₂ are also included.

(By MacDowell) No. 3,467,534, consisting essentially of 20-70% BaO and 30-80% SiO _2, glass having a primary crystal phase of a barium silicate - with ceramic article is published I have. The example is stated to have been considered as a coating for metal.

U.S. Pat. No. 3,531,303 (by Bahat) discloses glass-ceramic articles in the alkaline earth aluminosilicate class, wherein hexagonal or triclinic alkaline earth feldspars constitute the primary crystalline phase. I have. This material has a high fire resistance up to 1700 ° C. operating temperature, substantially 12-53% SiO ₂ , 17-55% RO (where RO is 17-50%
SrO and 20-40% of BaO), consisting of 10-58% of Al ₂ O ₃ and nucleating agents.

U.S. Pat. No. 3,578,470 (by Bahat) includes BaO--Al ₂ O ₃ --Si nucleated with Ta ₂ O ₅ and Nb ₂ O ₅ which are particularly suitable for sealing with tungsten or molybdenum and their alloys.
Glass in the class of O ₂ Composition - ceramic material have been published.

US Patent No. 3,837,978 (by Busdiecker)
Barium aluminosilicate glass-ceramics which are nucleated with tin oxide, have a hexagonal heavy earth feldspar primary crystal phase, and have a coefficient of thermal expansion in the range of 50-170 × 10 ^-7 / ° C.

The present invention provides an extremely effective oxygen barrier on the surface of a metal alloy substrate. This barrier is a coating of barium silicate or strontium silicate material. The coating is continuous and resistant to delamination during temperature cycling without defects such as needle holes, cracks or thin areas of the coating.

The present invention relies, to a considerable extent, on the discovery that certain additives have an unusual effect on the crystallization properties of thermally crystallizable barium and strontium silicate glasses. In particular, these additives allow the glass to be formed into a continuous, mirror-like coating before the glass softens and crystallization proceeds sufficiently to prevent flow. If at least one of these additives is missing, the glass will tend to crystallize and harden before fully coating. The result is a porous, cracked coating. This finding is key to creating an effective oxygen barrier on the superalloy surface.

Another major requirement in creating an effective oxygen barrier coating is that the thermal expansion between the coating and the metal substrate be closely matched. The coefficient of thermal expansion of superalloys is usually 130-160 ×
10 ^-7 / ° C. Therefore, many heat-resistant glasses and ceramics are not considered. As mentioned above, this results in the use of an adhesive layer.

The primary crystalline phase in the glass-ceramics of the present invention is usually barium silicate or strontium silicate.
However, as already seen, there is usually a cristobalite phase, which may be a primary crystal phase. Also, as described above, cristobalite crystals are concentrated in an area adjacent to the boundary with the surface of the superalloy.

We believe that the thermal expansion match is due to this cristobalite crystal concentration rather than solely to the glass-ceramic coefficient of thermal expansion. We further believe that the movement of metal ions from the alloy into the glass as it is heated is due to the cristobalite crystals near the boundary.

Superalloys are well known in metallurgy. Generally, they are highly heat resistant and withstand operating temperatures of 1000 ° C. and above. Its applications are found in equipment such as turbine engines, air heaters and heat exchangers.

There is no fixed composition limit for superalloys. Rather, they are usually classified by base metal. These metal groups include nickel, iron, chromium and cobalt,
Nickel is the most commonly used base metal. A series of known nickel-based superalloys include Ninomic, Ma.
There are stelloy, Hestelloy, Wapaloy & Rene series.
Cobalt superalloys include the Mar-M and AR-series.

It is said that superalloys have sufficient oxidation resistance and can be used in oxidizing environments without a surface coating. This resistance in nickel-based superalloys is provided by the addition of chromium and aluminum. Nevertheless, under severe conditions of use, such as turbine blades for aircraft engines, even superalloys tend to degrade rapidly unless protected by an oxygen barrier coating.

Research on coatings that can withstand higher temperatures than available glass has naturally turned to the field of glass-ceramics. Glass-ceramic materials and methods of making them were first published in (Stookey) U.S. Pat. No. 2,920,971. Briefly, to produce a glass-ceramic coating material, the thermally crystallizable glass is melted and rapidly cooled to avoid crystallization. The quenched glass is then applied to the powder as a ground coating. The powdered glass coating is then heated to precipitate a fine dispersion of nuclei.
These nuclei act as centers for heating the glass to a higher temperature to promote crystallization of the primary crystal phase.

In theory, the glass-ceramic coating should maintain the excellent oxygen barrier properties of the original mirror-like coating. Also, the glass should be hardened to form a crystalline network and should be resistant to flow once the coating has been formed. Thus, the glass-ceramic coating must be a reliable barrier to oxygen up to the temperature at which the crystalline phase begins to melt. However, previous studies have shown that crystallization tends to occur too early. This makes it impossible to take into account the degree of glass flow required to produce a continuous mirror-like coating.

Large ceramic expansion coefficients are usually somewhat large amount of alkali oxide (LiO _2, NaO _2, or K ₂ O) contains.
These alkali ions are extremely mobile at high temperatures in most ceramic structures and easily displace other ions. Therefore, these must be removed as major components in the coating which must work continuously at elevated temperatures. Therefore, only a few remain as candidates for the heat-resistant coating.

The B ₂ O _3, barium, magnesium and calcium magnesium silicate mixture coating was fluxing in U.S. Patent No. 4,256,796, No. 4,358,541 and No. 4,38
No. 5,127. The limiting factor in the high temperature resistance of these coatings lies in the use of B ₂ O ₃ . High residual B ₂ O ₃ glasses (or even borate glasses) tend to have microstructures that can move at temperatures much lower than the solidus of the primary refractory silicate phase. Thus, the present invention prepares a coating containing little or no diboron trioxide.

We small amount of heat-resistant material, the glass-forming oxides Al ₂ O _3, ZrO ₂
And the addition of Y ₂ O ₃ have been found to alter the crystallization behavior of crystallizable barium silicate and strontium silicate glasses. In particular, these additives delay crystallization to such an extent that the glass softens and flows to form a continuous, mirror-like coating before the glass becomes dense and hardened by crystallization.

While these oxides are very effective in slowing crystallization, their use must be limited. Its amount should not exceed about 15% by weight, preferably about 10%
It is desirable not to exceed. Otherwise, they tend to form undesirable crystalline phases, such as aluminosilicates, which may have low coefficients of thermal expansion or other detrimental properties.

The most effective refractory oxide additive is Al ₂ O _3. Therefore, the best barium silicate coatings have 1-10% by weight.
Using a BaO—SiO ₂ binary system to which Al ₂ O ₃ is added, the molar ratio of SiO ₂ to BaO is about 2: 1 to 5: 1.

We have also found that certain transition metal oxides, such as MgO, are effective in creating a continuous, well-flowing glass coating on barium silicate glass before crystallization. Transition metal oxides include MnO, NiO, CoO, FeO and Fe ₂ O ₃ .

Manganese oxide additives have been found to be particularly effective in producing coatings having excellent adhesion, good flow prior to crystallization, and peel resistance. However, significant amounts of MnO
, The crystalline phase tends to dissolve at about 1050 ° C. or higher, which limits the heat resistance of the coating containing this oxide. The crystal phases formed below 1050 ° C. were Ba ₂ MnSi ₂ O ₇ and cristobalite.

Coatings containing FeO, CoO or NiO are not as smooth and less adherent as coatings containing manganese. However, they withstand temperatures up to at least 100 ° C. higher. In these coatings, alpha-BaSi ₂ O ₅ (sunbonite) with a small amount of cristobalite was the primary crystalline phase. Small amounts of additives, usually less than 5%, such as ZrO ₂ , Al ₂ O ₃ , CeO ₂ , TiO ₂ , Nb ₂ O ₅ and B ₂ O ₃ improved the fluidity and appearance of the coating.

BaO or SrO, in addition to SiO ₂ and one or more of the heat-resistant material and the transition metal oxide modifier, the composition of the silicate Sutoronchimu coatings of the present invention, optionally other oxides up to about 25% by weight Can be included. They have 0-25% ZnO,
0-10% TiO ₂ , 0-20% CaO, 0-20% SrO, 0-
A 20% of Nb ₂ O ₅ and 0-10% F.

It is desirable to exclude the alkali metal oxides Na ₂ O, K ₂ O, and Li ₂ O (R ₂ O) and B ₂ O ₃ . Flux oxides often reduce thermal effectiveness and increase the coefficient of expansion. However, in some cases they can be tolerated up to 5% by weight.

In studies of barium silicate based coatings, it has been observed that good adhesion can be obtained if the coating is fired in a dominant helium atmosphere. This was even with as much as 5% oxygen present. However, the adhesion was not as good as when the coating was fired in air (about 21% oxygen).

This led to the study of strontium silicate.
Surprisingly, the strontium silicate coating gave similarly good adhesion coatings whether fired in air or in a helium atmosphere. For this reason, strontium silicate based coatings further enhance their practicality in many applications.

As in the case of the barium silicate system, about 15% by weight
The addition of small amounts of Al ₂ O ₃ , ZrO ₂ , or Y ₂ O ₃ up to% promotes proper glass flow before crystallization and helps to strengthen the coating. A preferred strontium silicate system is 5-10
Mol% Al ₂ O ₃ or ZrO _2, with a ratio of SiO ₂ to SrO of 1.5:
It is in the range of 1 to 4: 1.

The strontium silicate coating is well crystallized, smooth and has good adhesion. These can include SrSiO ₃ as the primary crystal phase together with a small amount of cristobalite. However, cristobalite can also be the primary crystal phase, as in the case of the barium system. Cristobalite appears to be concentrated at the interface between the cladding and the metal. As in the barium silicate system, this withstands thermal cycling delamination and provides a distinctly better expansion match.

Manganese, nickel, tin and magnesium oxide
Combination with approximately the same molar amount of SrO is effective in producing good flow and thus useful coatings. However, optimal results are obtained when at least some Al ₂ O ₃ or ZrO ₂ is present. If transition metal oxides and MgO are present, Sr ₂ Mg
Crystal phases such as Si ₂ O ₇ , Sr ₂ ZnSi ₂ O ₇ and nickel silicate and manganese silicate may be observed as the primary silicate phase.

Like barium silicate, the composition of strontium silicate can include up to 25% added oxide in addition to the conditioning oxide. These include 0-25%
ZnO, 0-20% CaO, 0-10% F, 1-10% Ti
O _2, there is 0-20% of Nb ₂ O ₅ and 0-20% BaO. Also in this case, small amounts of R ₂ O and B ₂ O ₃ are allowed, but it is desirable to remove them.

In practicing the present invention, the surface of the preformed superalloy body may be coated with powdered glass by an ordinary method. We prefer electrostatic spraying, in which case the electrostatically charged glass powder is evenly sprayed onto a superalloy body supported by an electrostatically charged wire mesh of the opposite polarity. Alternatively, the powdered glass can be mixed with a suitable medium, such as water or an organic carrier, spread evenly over the glass surface and dried.

The metal substrate coated with the glass powder coating is then heated to below 1000 ° C. This softens the glass particles and results in a dense, smooth, well-formed, continuous glass coating without substantial crystallization. The glass-coated substrate is then heated to a somewhat higher temperature. This results in a crystalline phase that forms a dense, strong, heat resistant crystalline coating. A feature of this method is that the timing of crystallization can be adjusted, which may lead to reproducibility in the coating process.

The coating of the present invention has been described as applying ground glass to the surface of the superalloy body. It should be understood, however, that fillers and additives for other purposes can be incorporated so that the additives do not prevent the ground glass from flowing into a continuous mirror-like coating.

EXAMPLES The present invention further describes some specific examples of glass-ceramic materials according to the present invention that can be applied as a coating to a superalloy body. These materials can provide a smooth, coherent, non-porous coating that exhibits little or no tendency to peel during temperature cycling. This is a check of the close expansion match near the interface.

Table I lists the compositions in the barium silicate system.
Table II lists the compositions in the strontium silicate system. In each composition, the components are provided on an oxide basis. All compositions are given in weight percent (wt%) and molar ratio (MR).

Glass batches corresponding to each of the compositions in Tables I and II were mixed. Each batch is 16 hours in a platinum crucible for 2 hours
Dissolved at 00 ° C. The glass melt thus obtained was poured into water and quenched to granulate the glass. The granular glass was milled for 4-8 hours in a ball mill with an aluminum cylinder to an average particle size of 10 to 15 microns.

The powdered glass was dried and pressed into a 1/2 inch diameter cylindrical shape. These were heat-treated at a temperature of 800 to 1200 ° C. for 1/2 to 1 hour to determine sintering characteristics and density (non-porous). Also,
Compressed into a 4 "x 1/4" x 1/4 "rod and sintered
^The coefficient of thermal expansion (exp) expressed in ^-7 / ° C was determined. The sintered sample was subjected to X-ray diffraction tracing to determine the crystal phase generated during sintering.

Tables III and IV list properties observed for glass-ceramic samples prepared from the compositions of Tables I and II. The sample numbers in the table are the same as the corresponding ones for comparison.

The prospective glass was dusted and sintered on the Inconel 718 substrate. In the table, the heating temperature is shown in ° C. (Temp), the time is shown in hours (T), and the atmosphere (Atm.) Is shown in helium (He) or air. In the case of barium silicate glass-ceramics, a helium atmosphere has proven to be effective, even with as much as 20% air, for good coverage. In an air atmosphere, oxidation of the metal surface appeared to occur excessively before a continuous glass coating was formed. Some coatings showed slight delamination but were not as severe. Such conditions could be avoided by precisely adjusting the treatment.

Table V contains four selected from those shown in Tables I and II.
4 shows the flame cycle test results for two coating examples. Corresponding sample numbers are given for convenience of cross-reference.

Specimens were prepared by coating the ground glass described above on both sides of an Inconel 718 superalloy substrate. The glass-coated substrate was sintered at 1100 ° C. for 1 hour in a helium atmosphere. The sample thus prepared was taken out of the support and subjected to 600 thermal cycles. In each cycle, (1) 5
(2) Cooled in the air for 5 minutes. During each cycle, the temperature of the sample reached about 1050 ° C. Up to three out of four samples showed no signs of delamination. This result was excellent given the severity of the thermal gradient and the overall thermal shock encountered.

[Brief description of the drawings]

The only drawing attached is a graphical representation of the thermal expansion curve. The thermal expansion, ΔL / L, in parts per million (ppm) is plotted on the vertical axis and the horizontal axis is the temperature in ° C. The solid line shows the values for the glass-ceramic coating of sample 11 in Table I,
The dotted line shows the values for a typical superalloy Inconel 718. There is good agreement between the thermal expansion behavior of the superalloy and that of a typical barium silicate glass-ceramic coating.

────────────────────────────────────────────────── ─── Continuation of front page (56) References JP-A-61-35588 (JP, A) US Patent 3,467,534 (US, A) US Patent 3,397,079 (US, A) (58) Fields investigated (Int. ⁶ , DB name) C23D 5/00 C23C 30/00

Claims (5)

(57) [Claims] 1. A coated article comprising a metal alloy substrate adapted for use at a temperature of 1000 ° C. or higher, the metal alloy comprising a nickel-based, cobalt-based, chromium-based and iron-based alloy. Wherein the protective coating forms a barrier on the surface of the metal alloy, the coating having a primary crystalline phase and a cristobalite phase selected from barium silicate and strontium silicate types. And wherein there are concentrated cristobalite crystals adjacent to the interface between the glass-ceramic and the metal alloy. 2. Article according to claim 1, wherein the base metal of the alloy is selected from nickel, chromium, cobalt and iron. 3. The primary crystal phase is barium silicate,
Up to 15% Al ₂ O ₃ , 15
The article of claim 1 further comprising an oxide of from Y ₂ O ₃ to ZrO ₂ and 15% until at least one of 15% was chosen to%. 4. The article of claim 1 wherein the glass-ceramic is substantially free of an aluminosilicate crystalline phase. 5. The metal alloy surface is coated with barium silicate glass or strontium silicate glass, the glass-coated surface is heated to a high temperature, and the glass is softened to form a smooth continuous coating. A method of protecting a metal alloy, comprising ultimately heating to crystallize to form a glass-ceramic coating, and forming a barrier on the metal alloy surface, the coating comprising a barium silicate type. A method comprising a glass-ceramic having a primary crystal phase selected from the strontium silicate type and a cristobalite phase, and wherein there is concentrated cristobalite crystals adjacent to a boundary between the glass-ceramic and the metal alloy.