WO2002097162A1

WO2002097162A1 - Thermal barrier coatings and fabrication of same using electrochemical methods

Info

Publication number: WO2002097162A1
Application number: PCT/CA2002/000775
Authority: WO
Inventors: Raed Hashaikeh; Jerzy Szpunar
Original assignee: Mcgill University
Priority date: 2001-05-29
Filing date: 2002-05-28
Publication date: 2002-12-05

Abstract

A thermal barrier coating for a substrate, including nickel superalloy, is provided comprising electrophoretically deposited (NiCoCrAlY) particles which act as a bond coat to the surface of the substrate, an intermediate ceramic layer which is electrolytically coated on the bond coated surface, and a thermal insulating layer comprising yttria stabilized zirconia (YSZ) particles, and a process for fabricating same. There is also provided a method of increasing the temperature tolerance of a nickel superalloy substrate comprising the steps of electrophoretically depositing a mutilayered coating comprise (NiCoCrAIY), ceramic, and yttria stabilized zirconia onto the substrate wherein the coating is used as a thermal barrier coating, and a novel method for the electrolytic deposition of MgO on a substrate comprising the steps of electrolytically depositing magnesium hydroxide from a magnesium nitrate aqueous solution, and calcinating the magnesium hydroxide to produce MgO at the surface of the substrate.

Description

THERMAL BARRIER COATINGS AND FABRICATION OF SAME USING ELECTROCHEMICAL METHODS

FIELD OF THE INVENTION

This invention relates to thermal barrier coatings, and more particularly, thermal barrier coatings fabricated by way of electrochemical methods.

BACKGROUND OF THE INVENTION

Thermal barrier coatings are widely applied in many types of industrial applications, including for use as a coating in engine and aircraft turbines for improved temperature tolerance, due to their properties of low thermal conductivity, high coefficient of thermal expansion, and good thermal shock resistance.

At present, improvement of the temperature capability of gas turbine blades is one of the main driving forces for development of thermal barrier coatings. Gas turbine blades and vanes which are made of nickel-based superalloy, operate at elevated temperatures above 1000° C with short-term peaks above 1100° C, close to 90% of the alloy's melting point (1270 - 1340° C). To increase the temperature capability of the engine, and to protect the blades and vanes from high temperatures, air coming from the compressor discharge is used to cool these blades. To increase temperature tolerance, a thin coat of a heat-insulating thermal barrier coating can be applied on the surface of the blades. Both the themial barrier coating and the cooling air provide a temperature gradient across the turbine blade, with a drop of 90-150°C across the ceramic layer. Thermal barrier coatings are able to improve engine performance either by reducing the turbine cooling air requirements or by allowing the combustor temperature to increase without affecting turbine durability. Reductions in cooling air requirements increase overall turbine efficiency and decrease thrust specific fuel consumption. Additionally, thermal barrier coatings can increase component life by damping thermal transients and lowering the component-coating interface temperature.

A typical thermal barrier system consists of an inner metallic bond coat layer, typically NiCoCrAIY alloy, and an outer ceramic insulating layer, typically yttrium stabilized zirconia (YSZ). The bond coat layer protects the substrate from oxidation and provides adhesion between the substrate and the ceramic layer. Thermal barrier coatings are designed to not only increase temperature tolerance of the substrates on which they are coated, but also to avoid mechanical and adherence problems that can be present between the coatings and the substrates.

Stabilized zirconia is permeable to oxygen at high temperatures, either through pore or lattice diffusion. Therefore, to be used in thermal barrier coatings it must be backed by an oxidation resistance bond coat. This bond coat works as an oxidation resistor and as a compliant next to the substrate that provides the mechanical bonding between the substrate and the topcoat. The interdiffusion between the NiCoCrAIY alloy coating and the substrate alloy creates the bonding between the coating and the substrate ("Advances In High Temperature Structural Materials And Protective Coatings", National Research Council Of Canada, Ottawa, Canada, 1994).

At present, the most common deposition techniques for NiCoCrAIY coatings and YSZ coatings are plasma spraying and electron beam physical vapour deposition (K. H Stern, "Metallurgical And Ceramic Protective Coatings", Chapman & Hall, 1996). Lack of reproducibility and insufficient porosity of plasma sprayed coats has been a traditional problem associated with plasma spraying of thermal barrier coatings. Porosity of a thermal barrier coating is critical in determining the heat insulation, thermal shock and erosive resistance of the barrier. Thermal barrier coatings deposited by electron beam physical vapour deposition form columnar outer structures which can result in an increase in heat conductivity by a factor of two (Stern, ibid.).

Other general drawbacks associated with these methods for applying thermal barrier coatings can include high cost, large time-consumption, compatibility restrictions with regard to the selection of suitable substrate/coating systems, and difficulties when applying to objects with complex shapes and straight angles or edges. As well, thermal coating barriers fabricated by these techniques can spall after a certain time in service, mainly because of mismatch between the ceramic layer and the bond coat layer as a result of difference in thermal expansion coefficient. Progressive oxidation of the bond coat layer, such as NiCoCrAIY alloy, of a thermal barrier coating occurs during thermal cycle conditions, leading to a gradual decline in plasticity and mechanical strength, which can contribute to the spallation of the outer ceramic layer, such as yttrium stabilized zirconia (YSZ). The metal reacts to produce oxides that have a low thermal expansion coefficient which, in turn, results in a high expansion mismatch stress and poor mechanical integrity.

More recently, the use of electrochemical methods have been investigated to fabricate metal/ceramic coatings (Barmak et al., "Electrochemial Processing Of Layered Composite Coatings Of Nickel-Alumimum-Alumina- Yttria Stabilized Zirconia", Mat. Res. Soc. Symp. Proc. Vol. 451, 1991; Nicholson et al., " Electrophoretic Deposition And Its Use To Synthesize Zr0₂/Al₂O₃ Micro-Laminate Ceramic/Ceramic Composites, Journal Of Materials Science, 28, 6274-6278, 1993; Ishihara et al., "Electrophoretic Deposition Of Y₂0₃ Stabilized Zr0 Electrolyte Films hi Solid Oxide Fuel Cells ", Journal Of American Ceramic Society, vol. 79(4), 914-19, 1996). These methods include electrophoretic deposition and electrolytic deposition.

SUMMARY OF THE INVENTION

According to the present invention, a process is provided for fabricating a multilayered coating for a substrate, wherein the coating comprises (NiCoCrAIY), ceramic, and Yttria stabilized zirconia. This process comprises the steps of electrophoretically depositing (NiCoCrAIY) metal particles which act as a bond coat to the surface of the substrate; electrolytically coating the bond coated surface with an intermediate ceramic layer; electrophoretically depositing yttria stabilized zirconia (YSZ) particles to the (NiCoCrAIY)/ ceramic coat which act as a thermal insulating ceramic; and sintering the final coated product.

In accordance with a preferred embodiment, the intermediate ceramic layer is magnesium oxide, and an additional heating step is conducted prior to electrophoretic deposition of yttria stabilized zirconia (YSZ) particles, hi a preferred embodiment, the heating step takes place at approximately 850° C.

In accordance with another aspect of the present invention, there is provided a thermal barrier coating comprising (NiCoCrAIY) and metal hydroxide particles as a bond coat, ceramic as an intermediate coat, and Yttria stabilized zirconia and a thermal insulating ceramic. The hydroxide is created tlirough the electrolysis of water and can improve adhesion and facilitate cementation of the deposited materials. hi accordance with another aspect of the invention, a thermal barrier coating for a substrate is provided comprising electrophoretically deposited (NiCoCrAIY) particles which act as a bond coat to the surface of the substrate, an intermediate ceramic layer which is electrolytically coated on the bond coated surface, and a thermal insulating layer comprising yttria stabilized zirconia (YSZ) particles.

In accordance with another aspect of the invention, there is provided a use of electrochemical techniques for the fabrication of multilayered coatings for deposition on a substrate where the mutilayered coating comprise (NiCoCrAIY), ceramic, and Yttria stabilized zirconia.

In accordance with yet another aspect of the invention, there is provided a method of increasing the temperature tolerance of a nickel superalloy substrate comprising the steps of electrochemically depositing a mutilayered coating comprise (NiCoCrAIY), ceramic, and Yttria stabilized zirconia onto the substrate wherein the coating is used as a thermal barrier coating.

In accordance with yet another aspect of the invention, there is provided a coating comprising layers of NiCoCrAIY, MgO, and Yttria stabilized zirconia for use as a thermal barrier coating wherein the coating is applied to the surface of a nickel superalloy substrate.

In accordance with a preferred embodiment, the substrate comprises the blades and/or vanes of a gas turbine engine. hi accordance with yet another aspect of the invention, there is provided a method for the electrolytic deposition of MgO on a substrate comprising the steps of electrolytically depositing magnesium hydroxide from a magnesium nitrate aqueous solution, and calcinating the deposited magnesium hydroxide to produce MgO at the surface of the substrate.

The terms, "electrophoretic deposition", and "electrolytic deposition" are well understood to those of skill in the art. For purposes of clarity, these terms are generally defined. "Electrophoretic deposition" ("EPD") generally refers to the process whereby charged particles are.t deposited from a suspension into an electrode of opposite charge, under the application of a D.C electrical field (Nicholson et al., J Am. Cersm. Soc, 1999, 82, 3031-36). Particles move and coagulate as a dense layer of particles on the electrode. For a successful electrophoretic deposition, it is an essential prerequisite that the particles be electrically charged and they must have a high electrophoretic mobility. Particles must also remain dispersed throughout the medium so that they can move toward the electrode, packed and deposit in an ordered way independently of each other. The kinetics of the electrophoretic deposition process are understood (R. Mreno and B. Ferrari, " Advanced Ceramics Via EPD Of Aqueous Slurries", The American Ceramic Society Bulletin, January 2000, p. 44-48).

Wherein the term "electrolytic deposition" is used, either alone or with other terms, it embraces the technique wherein, if two electrodes are immersed in an aqueous solution of a metal salt and connected to a source of current of a sufficiently high potential, there will be a passage of electric charge through the solution and at the same time various chemical reactions will take place at the electrodes. Formation of ceramics coatings by electrolytic means has been previously demonstrated (N. B. Dahotre et al., "Intermetallic And Ceramic Coatings", Marcel Dekker, Inc., New York, 1999; L. Gal-Or et al., "Electrolytic Zr0 Coatings; II. Microstructural Aspects", J Electrochem. Soc, Vol. 138, No. 7, 1942-1946, July, 1991; L. Aries, "Preparation Of Electrolytic Ceramic Films On Stainless Steel Conversion Coatings", Surface Engineering, V. 114, n.3, 235-240, 1998).

Advantages of the present invention include little restriction with respect to the shape of coating complexes, versatility, simple deposition apparatus required, suitability for mass production, and relative low cost.

As well, electrolytic methods for depositing MgO coatings according to methods of the present invention afford several benefits, including the ability to produce crack-free coatings, i general, the presence of the intermediate layer, in a preferred embodiment MgO, can impart several benefits, including, for example, to protect the substrate and the bond coat (NiCoCrAIY) layer from oxidation during the sintering process; to provide adhesion between the YSZ insulating ceramic and the bond coat; to work as an oxygen barrier and protect the bond coat from oxidation during operation of the thermal barrier coating; and the MgO layer may reduce the interfacial stresses and mismatch problems between the bond coat and the YSZ coat.

With the presence of the intermediate layer, adverse effects from the sintering step are also reduced. During sintering the substrate is heated to a high temperature, which is contrary to the purpose of thermal barrier coating, which is to protect the substrate from high temperatures and oxidation. However, with the process of the present invention, the sintering time is relatively short, the substrate is not under load at the time of coating, and the intermediate ceramic layer protects the metal substrate and the bond coat from oxidation during sintering.

Other and further advantages and features of the invention will be apparent to those skilled in the art from the following detailed description thereof, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

Figure 1 represents a prior art schematic of a jet engine showing the turbines and the turbine blades.

Figure 2 represents a prior art schematic of a typical thermal barrier coating for use with a turbine blade. Approximate relative thicknesses of the bond coat layer and insulating layer are indicated.

Figure 3 is an SEM image of a cross section of the thermal barrier coating fabricated by the electrochemical methods of the present invention.

Figures 4A - 4D are a schematic model of the fabrication stages of a (NiCoCrAlY)ZMgO and a YSZ multilayered thermal barrier coating. Figure4A illustrates the electrophoretic deposition of (NiCoCrAIY) particles; Figure4B illustrates electrochemical coating of Mg(OH) ; Figure 4C illustrates the heat treatment step to calcinate the Mg(OH)₂ to form MgO and sinter it; Figure 4D illustrates the electrophoretic deposition and sintering of YSZ powder.

Figure 5 is an SEM image of electrochemically coated Al₂O₃ layer over electrophoretically deposited (NiCoCrAIY) particles. Figure 6 is an SEM image showing Al₂O₃ deposits covering all areas of the substrate.

Figure 7 is a graph illustrating the relationship between iodine concentration and the amount of YSZ deposited in three minutes at 35 Volts (sample area 6 cm²).

Figure 8 is a graph illustrating the relationship between the applied voltage on the amount of YSZ deposited. Time of deposition was 3 minutes (acetone bath 0. 3 g/L l²).

Figure 9 is a graph illustrating the weight of YSZ deposited, current density and their relationship with time. Applied voltage was 250 V (acetone bath 0. 3 g/L i²).

Figure 10 is a graph illustrating the weight of the Mg(OH) deposited in 3 minutes vs. current density (sample area 6 cm²).

Figure 11 is a graph showing variation in cell voltage with duration of deposition of Mg(OH) at different current densities.

Figure 12 is a graph of weight change of the Mg(OH) coated sample with heat treatment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Embodiments of the invention will now be described with reference to the drawings wherein like numerals refer to like parts.

Referring now to Figure 1, a schematic of jet engine 10 with turbines 12 and turbine blades 14 is shown. Jet engine 10 also comprises low compressor fan 26, high compressor 28, and combustion chamber 30. Thermal barrier coatings comprising oxidation and hot corrosion resistant metallic layer 32 and insulating ceramic layer 34 have been typically employed on blades 14 comprising nickel superalloy as shown in Figure 2. Hot combustion gases flow in the direction of 36 from combustion chamber 30 of Figure 1 towards turbine blade 14 during operation of jet engine 10. Internal air flowing in the direction of arrow 38 affords internal air cooling to turbine blades 14. For gas turbine applications, a thermal barrier coat is typically 75-125 μm metal bond coat and 250-375 μm insulating ceramic layer (such as yttria stabilized zirconia). Referring now to Figure 3, a cross section of thermal barrier coating 16 on substrate 18 of the present invention is shown wherein bond coat 20 comprises (NiCoCrAIY), intermediate layer 22 comprises MgO, and insulating ceramic layer 24 comprises yttrium stabilized zirconia (YSZ). In a preferred embodiment, substrate 18 comprises the vanes and blades of a gas turbine engine. In another preferred embodiment, the thermal barrier coatings of the present invention are utilized in solid fuel cell applications. Thermal barrier coatings of the present invention and methods of fabrication of same are not limited to use with turbine engines. Applications where improvement in the temperature tolerance of the substrate is desired are envisaged for the thermal barrier coatings and methods of fabrication of the present invention.

A schematic of a preferred embodiment for the fabrication of a (NiCoCrAlY)/MgO/YSZ multilayered thermal barrier coating is shown in Figures 4 A through 4D. Figure 4A depicts the electrophoretic deposition of (NiCoCrAIY) particles 40 to substrate 18. Figure 4B depicts the electrochemical coating of Mg(OH)₂ 42 from a solution of magnesium nitrate to form a uniform, crack free surface that covers both substrate 18 and (NiCoCrAIY) particles 40. Figure 4C depicts the heat treatment at low temperature carried out to calcinate, crystallize and produce a stable layer of MgO 44. h Figure 4D, electrophoretic deposition and sintering of deposited YSZ powder 46 yields almost complete densification of the (NiCoCrAlY)/MgO bond coat layer.

In a preferred embodiment, a binder metal hydroxide is also electrophoretically deposited along with the metal alloy particles, upon dissociation of the electrolyte in Figure 4A. Metal cations adsorb onto the particle surfaces, which in turn interact with hydroxide ions generated at the cathode from water electrolysis to form metal hydroxide deposits. The metal hydroxides dehydrate during drying to form oxides which act as cementing material to hold the metal alloy particles and to aid in particle adhesion. In a preferred embodiment, the metal hydroxide comprises Al(OH)₃ or Mg(OH)₂. hi a preferred embodiment, the electrolyte comprises one of A1C13, Al(NO₃)₃ and Mg(NO₃)₂.

In a preferred embodiment, heat treatment of Figure 4C occurs at 850 °C for 2 hrs, whereby both the bond coat and the MgO were sintered. In another preferred embodiment, the MgO deposit was sintrable at lower temperatures, i general, sintering time was found to be strongly effective and dense MgO ceramic was obtained at low temperature. Coatings are thought to densify by viscous sintering (in viscous sintering, solid-vapor interfaces are removed by viscous flow into pores.) After sintering, the MgO forms a composite coat with the sintered metal alloy particles. In a preferred embodiment, the current density during electrolytic deposition of MgO was less than 5 mA cm . In a preferred embodiment the current density was 2.5 mA/cm².

In another preferred embodiment, the intermediate layer is electrolytically deposited Al₂O₃. This layer is deposited on a coated layer of (NiCoCrAIY). Deposited alumina is cracked and forms a composite coating where the (NiCoCrAIY) particles are incorporated, thus providing adhesion and cementation between the particles and the substrate. Figure 5 illustrates the SEM image of electrochemically coated Al₂O₃ layer over electrophoretically deposited (NiCoCrAIY) particles. Figure 6 is an SEM image showing Al₂O₃ deposits covering all areas of the substrate. In a preferred embodiment, the deposit thickness of the Al O₃ is such that all of the (NiCoCrAIY) particles are not completely covered, as probability of cracking increases with deposit thickness of the Al₂O₃ intermediate layer. As well, problems with subsequent electrophoretic deposition of the YSZ layer can be encountered in the event that cracks develop in the bond coat. In a preferred embodiment, the spallation is minimized by working at low current density. In a preferred embodiment, the current density is 2.5 mA/cm².

In a preferred embodiment, NiCoCrAIY particles for electrophoretic deposition are within the range of 200 (75 μm) and 400 (38 μm) mesh size. In a preferred embodiment, the particles deposited on the surface of the substrate are less than 4 μm in diameter, i another preferred embodiment, most of the particles deposited are 1 μm in diameter. In a preferred embodiment, the electrolyte used for electrophoretic deposition of NiCoCrAIY particles is A1C1₃. In another preferred embodiment, the electrolyte is Al(NO₃)₃. In yet another preferred embodiment, the electrolyte is Mg(NO₃)₂. In a preferred embodiment, the electrolyte concentration is low. hi a preferred embodiment, the concentration of A1C1₃ is approximately 7.5 x 10^"3 (g/L). In another preferred embodiment, the concentration of Al(NO₃)₃ is approximately 2.3 x 10^"2 (g/L). In yet another preferred embodiment, the concentration of Mg(NO₃) is approximately 1.5 x 10^"3 (g/L). hi a preferred embodiment, the current density is minimized to reduce the rate of hydrogen evolution at the cathode. The process of covering the substrate by the metal powder is limited by, among other things, the hydrogen evolution. In a preferred embodiment, the current voltage was 150 V using the preferred electrolyte concentrations identified previously. In a preferred embodiment, methanol is present in the electrolyte bath.

In a preferred embodiment, uniform deposits of YSZ powder were obtained with iodine present in the bath at a concentration ranging from 0. 1 to 0.8 g/L in acetone. In a preferred embodiment, an applied voltage of 10-25 ON. In another preferred embodiment, a small amount of water was added to the iodine solution, hi a preferred embodiment, water was present at 5% v/v in acetone, h a preferred embodiment, a cell voltage range of 50-250 N was applied to a acetone bath containing 0.3 g/L I₂ for deposition periods of 3 minutes. The deposited amount of YSZ also observed to increase with the period of deposition. During the deposition process, the thickness of the coat increases. In a preferred embodiment, the deposition period was greater than 640 seconds.

In a preferred embodiment, sintering of the multilayer TBC is performed in air at 1100 °C for one hour to produce a porous YSZ layer, a dense MgO layer and a dense (NiCoCrAIY) bond coat layer. The sintering step is know to bond the YSZ particles together so that they can form a rigid ceramic layer, as well as chemically bonding the thermal barrier coating layers, hi other preferred embodiments the sintering time was less than one hour, hi another preferred embodiment, the sintering was performed at 1200° C for 30 minutes. In a preferred embodiment, the substrate for coating is nickel superalloy. h another preferred embodiment, the substrate for coating is nickel. In a preferred embodiment, YSZ coated nickel is sintered at temperatures of up to 1100° C.

Further details of the preferred embodiments of the invention are illustrated in the following examples that are understood to be non-limiting with respect to the appended claims.

EXAMPLES

Example 1-Preparation of a thermal barrier coating

Nickel superalloy Qnconel 600) was used as the substrate. The substrate specimens had dimensions of 2 x 1. 5 x 0.7cm. Components of the coating and fabrication thereof are set out as below.

Step 1, Electrophoretic deposition of (NiCoCrAIY) particles - NiCoCrAIY powder from TAFA Material Technologies, Inc., with the following composition and particles size, where used:

The particles were suspended in an aqueous media consisting of 80-v% water and 20-v% methanol. Magnesium nitrate was used as an electrolyte with a concentration of 1.5 x 10^"3 g/1. The substrate was the cathode and the anode material was graphite. These electrodes were 2 cm apart. A voltage of 150 V was applied for 5 minute using (E-C Apparatus Corp.) power supply, which allowed the electrophoretic deposition of the metal alloy particles with a current density of 4 mA/cm². The sample was then left to dry in air for 10 minutes.

Step 2, Electrolytic coating of the intermediate layer - The electrochemical bath consisted of the electrolyte which composed of 0.5M magnesium nitrate Mg(N0₃)₂ (to yield MgO as the intermediate layer) dissolved in 50% water and 50% ethanol solution. The coated substrate with the (NiCoCrAIY) metal alloy particles was the cathode and the anode material was graphite. The deposits were obtained at current densities of 2.5 mA/cm². The deposition time was 20 minutes. The sample was left to dry in air for 10 minutes.

Step 3, Heat treatment- The intermediate layer the coated substrate was heated at 850° C for 2 hours. Step 4, Electrophoretic deposition of YSZ particles - The bath consisted of 10 g/1 yttria stabilized zirconia (YSZ) (TZ-3Y, Tosoh Corp., Tokyo, Japan) with a particle size of 0.4 μm suspended in acetone. The electrolyte was iodine with a concentration of 0.6 g/L The coated and heat treated sample from step 3 was the cathode and the anode material was graphite. The electrodes were 2 cm apart. Deposits were obtained using a voltage of 100 N and the deposition time was 15 minutes.

Step 5, Sintering - The sample was sintered at 1100° C for 1 hr.

Example 2-Preparation of a thermal barrier coating

The procedure was the same as in Example 1, except for the following changes. In Step 2, the intermediate layer was comprised of Al O₃. The electrochemical bath consisted of the electrolyte which composed of 0.5 M Al(ΝO)₃ dissolved in 50% water and 50% ethanol. The heat treatment of Step 3 was not carried out.

Example 3-Deposition of YSZ on nickel superalloy

Three mole % yttria stabilized zirconia (YSZ) (TZ-3Y, Tosoh Corp., Tokyo, Japan) with a particle size of 0.4 μm were suspended in acetone bath (250 ml) which was composed of 10 g/L YSZ powder and an iodine concentration of 0.1 - 0.8 g/L Nickel superalloy (fiiconel 600) was used as a substrate. The substrate specimen was of dimensions 2 x 1.5 x 0.6 cm³. The substrate was connected to the cathode and the anode material was graphite. The distance between them was 2 cm. The source of voltage was a rectifier (E-C Apparatus Corp.). Deposits were obtained at a voltage in the range of 10- 250 V and time of deposition changed from 5 to 30 minutes. Cell voltage and current density were measured with AVOmeters. The deposits were dried in air at room temperature. The coating weights were determined by weighing the specimens before and after deposition. Specimens were sintered at 1100° C and 1200° C in air. The microstructure was characterized using optical methods and scanning electron microscopy (SEM). Example 4-Deposition of YSZ on nickel

The same procedure was followed as in Example 3, except that the substrate was nickel.

Example 5-Deposition of YSZ on nickel superalloy

This data was determined using the solution of Example 3. The relationship between the iodine concentration and the amount electrophoretically deposited YSZ is shown in Figure 7. The deposited weight was almost constant with a slight decrease with increasing iodine concentration. Figure 7 also illustrates that adding a small amount of water (5 %) increased the deposited weight tremendously. This increase in weight with adding water may be related to the increase in solution conductivity.

Example 6-Deposition of YSZ on nickel superalloy

This data was determined using the solution of Example 3. Figure 8 is a graph illustrating the relationship between the voltage applied and the amount of YSZ deposited. The weight of deposited YSZ increased linearly with increasing applied voltage.

The deposited amount of YSZ also increased with the period of deposition as shown in Figure 9. During the deposition process, the thickness of the coat increases. As this is a nonconductive deposit, the resistance for current increased, and since the measurements were carried out at a constant cell voltage, the current density decreased with time as can be seen in Figure 9. The decrease in current density during deposition allowed tracking of the deposition process. Figure 9 also related the current density to the rate of deposition. The current density decreased rapidly to a constant value at which the amount of deposited YSZ at a given time became constant.

Example 7-Deposition of Al₂O on nickel superalloy

Nickel superalloy (hiconel 600) was used as a substrate for the electrolytic deposition of an alumina coating. The substrate specimen had dimensions of 2 x 1.5 x 0.6 cm³. The electrolyte was composed of 0.5 M aluminum mtrate (Al (N0₃)₃)dissolved in 50% water and 50% ethanol solution. The substrate was the cathode and the anode material was graphite. The distance between them was 2 cm. The source of current was (EG&G Instruments Potentiostat) rectifier. Deposits were obtained at current densities ranging from 1-20 mA cm . The microstructure of the deposit was characterized using optical and scanning electron microscopy (SEM). The phase composition and the crystallization state were determined by X-ray diffraction (XRD). The obtained deposit was a gel-like transparent viscous deposit. With drying in air, the deposit became solid white coating, and many cracks appeared and the deposit showed poor adhesion to the substrate. Deposit spallation was minimized by working at low current density of 2.5 mA/cm². The cell voltage was found to increase with time.

Example 8-Deposition of Mg(OH) on nickel superalloy

Nickel superalloy (Inconel 600) was used as a substrate. The substrate specimen had dimensions of 2 x 1.5 x 0.6 cm³, h the electrochemical bath, the electrolyte was composed of 0.5M Magnesium Nitrate (Mg(N0₃) ) dissolved in 50%) water and 50% ethanol. The substrate was the cathode and the anode material was graphite with a distance of 2 cm between them. The source of current was (EG&G Instruments Potentiostat) rectifier. All deposits were obtained at current densities ranging from 2.5-20 mA/cm, and duration from 5 to 60 minutes. All experiments were performed without stirring. The cell voltage and current density were measured with ANOmeters. The deposits were dried in air at room temperature. The coating weights were measured before and after the deposition. Specimens were fired at 700° C and 1100° C in air; TGA was used to study the weight change during firing. The microstructure and of the deposit was characterized after different stages of the process, using the optical and scanning electron microscopy (SEM). The phase composition and the crystallization state were determined by X-ray diffraction (XRD). The obtained deposit was a gel-like transparent solid deposit with a homogeneous coverage of the substrate. The color of the deposit depended on the current density. The deposit color was yellow at low current density, yellow-orange at intermediate current density and brown at high current density. Unlike Al₂O₃ electrolytic deposit, no visible change happened when the deposit was removed from the bath, which indicate that the deposit was Mg(OH)₂ and it did not dehydrate with drying. Both the deposit quality and quantity depended on the current density. The amount deposited in a same period of time increased with increasing current density. High deposition rates were obtained at high current densities; but the deposit quality was best at low current density. At a current density greater than 5 mA/cm the deposit formed cracks after drying. However, at low current density, the deposit was homogeneous without any cracks. Figure 10 shows the relationship between the weight deposited of Mg(OH)₂ in 3 minutes vs. current density (sample area 6 cm²).

Example 9-Deposition of Mg(OH)₂ on nickel superalloy

The solution was the same as in Example 8. The weight deposited of Mg(OH)₂ in 3 minutes vs. the current density (sample area 6 cm²) was measured. Figure 11 shows that the amount of Mg(OH)₂ deposited in a same period of time increased with increasing current density. High deposition rates were obtained at high current densities; but the deposit quality was best at low current density. At a current density greater than 5 mA/cm² the deposit was observed to form cracks after drying. At low current density, the deposit was homogeneous without any cracks.

Example 10-Deposition of Mg(OH)₂ on nickel superalloy

The solution was the same as in Example 8. The weight change of the deposited Mg(OH)₂ during heat treatment was measured. Figure 12 shows the weight change of the deposited Mg(OH) during heat treatment. The weight decreased rapidly with time as temperature increased. This decrease in weight was related to the evaporation of the co-deposited solvent, which was about 20%> of the deposit weight. At a temperature of 200° C, the nickel superalloy substrate started to oxidize and the sample weight started to increase, but at around a temperature of 300° C, the weight of the sample decreased as can be seen in Figure 12. This decrease in sample weight was expected to be a result of the calcination reaction of Mg(OH)₂ to form MgO. The morphology of the deposit was also detected by SEM. Both the calcination process to form MgO and crystallization and sintering of the MgO occurred during the same heat treatment. rυn n

Example 11-Deposition of NiCoCrAIY and Al(OH)₃

Nickel superalloy (hiconel 600) and nickel were used as substrate. The substrate specimens had dimensions of 2 x 1. 5 x 0.7cm.

NiCoCrAIY powder from TAFA Material Technologies, Inc., with the following composition and particles size, where used:

The particles were suspended in an aqueous media consisting of 80-v% water and 20-v% methanol. Aluminum chloride was used as an electrolyte with a concentration of 7.5 x 10^"3 g/L The substrate was the cathode and the anode material was graphite. The distance between the electrodes was 2 cm. A voltage of 150 V was applied for 5 minute using (E-C Apparatus Corp.) power supply, which allowed the electrophoretic deposition of the metal alloy particles with a current density of 4 mA/cm². The deposits were dried at room temperature. The microstructure was then characterized using optical and scanning electron microscopy (SEM).

The adhesion of the substrate is related to the electrochemical co-deposition of Al O with metal alloy powder. It is expected that the aluminum chloride dissociates to give aluminum and chloride ions and the aluminum particles adsorb to the surface of the metal alloy particles. Interaction of the adsorbed cations with hydroxide ions generated at the cation from water electrolysis forms aluminum hydroxide deposits at the surface. Dehydration of the hydroxide during drying form alumina (Al O₃). Al₂O₃ acts as a cementing material to hold the metal alloy particles to the surface and to aid in particle adhesion. The formation of this type of binder is advantageous because it enhances the adhesion of the deposited particles. This binder is mainly formed by the electrolysis reaction with water, which is used in this example at 80 volume% in methanol. This experiment can be carried out using different electrolytes, including but not limited to Al(NO₃)₃ and Mg(NO₃)₂. As well, the electrolyte concentration is an important factor in deciding the electrophoretic mobility and deposition of metal particles. Deposition could only be obtained at low electrolyte concentration. The optimum concentration of electrolyte that allows a successful electrophoretic deposition of the metal alloy powder for the conditions of this experiment are given in the table below.

The quality of the deposit does not change with the type of electrolyte used, except that in the case of A1C1 where the amount of binder (A1₂0₃) deposited with the particles was low. SEM images of deposited (NiCoCrAIY) particles using A1C1₃ electrolyte and Al(NO₃)₃ electrolyte show that more Al₂O₃ is deposited on the substrate coated in Al(NO₃)₃ electrolyte, indicating that the ability of AI(NO )₃ electrolyte to deposit Al(OH)₃ is higher than that of A1C1₃ electrolyte since the reduction reaction of nitrates at the cathode works as an additional supplier of OH- ions.

Although preferred embodiments of the invention have been described herein, it will be understood by those skilled in the art that variations may be made thereto without departing from the spirit of the invention or the scope of the appended claims.

Claims

What is claimed is:

1. A process for fabricating a multilayered coating for a substrate, wherein the coating comprises (NiCoCrAIY), ceramic, and Yttria stabilized zirconia, comprising the steps of:

(a) electrophoretically depositing a layer comprising (NiCoCrAIY) metal particles, wherein the (NiCoCrAIY) metal particles act as a bond coat to the surface of the substrate;

(b) electrolytically coating the bond coated surface with an intermediate ceramic layer;

(c) electrophoretically depositing yttria stabilized zirconia (YSZ) particles to the intermediate coat, wherein the YSZ particles act as a thermal insulating ceramic;

(d) sintering the coated substrate produced in step (c).

2. The process of claim 1, wherein step (a) of electrophoretically depositing (NiCoCrAIY) metal particles further includes the step of electrophoretically depositing metal hydroxide particles, wherein the metal hydroxide particles facilitate adhesion of the (NiCoCrAIY) metal particles to the substrate.

3. The process of claim 2, wherein the metal hydroxide particles of step (a) comprise one of Al(OH)₃ and Mg(OH)₂.

4. The process as claimed in claim 3, wherein the step of electrophoretic deposition of (NiCoCrAIY) metal particles utilizes one of Mg(NO₃)₃, Al(NO₃)₃ and A1CL₃, as the electrolyte.

5. The process as claimed in claim 2, wherein the step of electrophoretic deposition of (NiCoCrAIY) metal particles utilizes an aqueous media consisting of 80 volume % water and 20 volume % methanol.

6. The process as claimed in claim 5, wherein the step of electrophoretic deposition of (NiCoCrAIY) metal particles utilizes graphite as the anode.

7. The process as claimed in claim 6, wherein the step of electrophoretic deposition of (NiCoCrAIY) metal particles utilizes a current density of between 2-5 mA/cm².

8. The process as claimed in claim 1, wherein step (a) includes the step of air drying of the bond coat prior to conducting step (b).

9. The process as claimed in claim 1, wherein step (b) of electrolytically coating the bond coated surface with an intermediate ceramic layer utilizes magnesium nitrate as the electrolyte.

10. The process as claimed in claim 1, wherein step (b) of electrolytically coating the bond coated surface with an intermediate ceramic layer utilizes aluminum nitrate (Al(NO)₃) as the electrolyte.

11. The process as claimed in claim 1, wherein step (b) of electrolytically coating the bond coated surface with an intermediate ceramic layer utilizes an aqueous media consisting of 50 volume % water and 50 volume % ethanol.

12. The process as claimed in claim 1, wherein step (b) of electrolytically coating the bond coated surface with an intermediate ceramic layer utilizes graphite as the cathode and the (NiCoCrAIY) coated substrate as the anode.

13. The process as claimed in claim 1, wherein the deposit time of step (b) is 20 minutes.

14. The process as claimed in claim 1, wherein step (b) includes the step of air drying of the bond coat prior to conducting step (c).

15. The process as claimed in claim 9, wherein the process further includes the step of heating the coated substrate prepared by step (b) prior to conducting step (c).

16. The process as claimed in claim 15, wherein the heating step takes place at approximately 850°C.

17. The process as claimed in claim 16, wherein the heating step takes place for approximately 2 hours.

18. The process as claimed in claim 1, wherein step (c) of electrophoretically depositing yttria stabilized zirconia (YSZ) particles utilizes a bath comprising acetone.

19. The process of claim 18, wherein iodine is present in the bath in concentrations of up to 0.8 g/L.

20. The process as claimed 18, wherein the anode is graphite and the cathode is the coated substrate of step (b).

21. The process as claimed in claim 1 wherein sintering of step (d) is conducted for a time period of up to 1 hour.

22. The process of claim 1 wherein the sintering is conducted in air at approximately 1100°C.

23. The process as claimed in claim 1, wherein the coating functions as a thermal barrier coating.

24. The process as claimed in claim 1 wherein the substrate to be coated is metallic.

25. The process as claimed in claim 1 wherein the substrate to be coated is a gas turbine system.

26. A thermal barrier coating applied to a substrate comprising:

(a) electrophoretically deposited (NiCoCrAIY) particles, wherein the particles act as a bond coat to the surface of the substrate;

(b) an intermediate ceramic layer, wherein the intermediate layer is electrolytically coated on the bond coated surface;

(c) a thermal insulating layer comprising yttria stabilized zirconia (YSZ) particles.

27. The thermal barrier coating of claim 26, wherein the bond coat of step (a) further comprises electrophoretically deposited metal hydroxide particles, wherein the metal hydroxide particles facilitate adhesion of the (NiCoCrAIY) metal particles to the substrate.

28. The process of claim 27, wherein the electrophoretically deposited metal hydroxide particles comprise one of Al(OH)₃ and Mg(OH) .

29. The thermal barrier coating of claim 26 wherein the intermediate ceramic layer comprises MgO.

30. The thermal barrier coating of claim 26, wherein the intermediate ceramic layer comprises Al₂O₃ and wherein the deposit thickness of Al O₃ is such that a portion of the (NiCoCrAIY) coated surface is coated by the Al₂O .

31. The thermal barrier coating of claim 26, wherein the substrate is metal.

32. The thermal barrier coating of claim 31, wherein the substrate comprises nickel.

33. The thermal barrier coating of claim 31, wherein the substrate comprises nickel superalloy.

34. The thermal barrier coating of claim 33, wherein the substrate comprises one or more components of a gas turbine system.

35. The multilayered coating when made by the process of claim 1.

36. The use of electrochemical techniques for the fabrication of multilayered coatings for deposition on a substrate, the mutilayered coating comprising (NiCoCrAIY), ceramic, and Yttria stabilized zirconia.

37. The use as claimed in claim 36, wherein the coating acts as a thermal barrier coating.

38. The use as claimed in claim 36, wherein the ceramic comprises MgO.

39. The use as claimed in claim 36, wherein the ceramic comprises Al₂O_3j and wherein the deposit thickness of Al₂O₃ is such that a portion of the (NiCoCrAIY) coated surface is coated by the Al O₃.

40. A method of increasing the temperature tolerance of a nickel superalloy substrate comprising the steps of electrochemically depositing the coating of claim 1 onto the substrate wherein said coating is used as a thermal barrier coating.

41. A coating comprising layers of NiCoCrAIY, MgO, and Yttria stabilized zirconia for use as a thermal barrier coating wherein the coating is applied to the surface of a nickel superalloy substrate.

42. A process of fabricating a multi-layer thermal barrier coating for a nickel superalloy substrate, wherein the multi-layer thermal barrier coating comprises (NiCoCrAlY)/MgO/Yttria stabilized zirconia, comrprising the steps of:

(a) depositing the layer of NiCoCrAIY to the nickel superalloy substrate by way of electrophoretic deposition;

(b) depositing to the deposited layer of NiCoCrAIY, a layer of MgO by way of electrolytic deposition;

(c) heating the coated substrate of step (b);

(d) depositing to the deposited layer of MgO, a layer of yttrium stabilized zirconia by way of electrophoretic deposition; and

(e) sintering the coated substrate.

43. A method for the electrolytic deposition of MgO on a substrate comprising the steps of:

(a) electrolytically depositing magnesium hydroxide from a magnesium nitrate aqueous solution; and

(b) calcinating the magnesium hydroxide to produce MgO at the surface of the substrate.

44. The method as claimed in claim 43, wherein the substrate is nickel superalloy.