FILM OR COATING DEPOSITION ON A SUBSTRATE

Abstract: A method of depositing a material onto a substrate comprises the steps of feeding a material solution to an outlet to provide a stream of droplets of the material solution; applying a potential difference between the outlet and the substrate to electrostatically attract the droplets from the outlet towards the substrate, so that a corona discharge is formed around the outlet; heating the substrate to provide an increase in temperature between the outlet and the substrate; and progressively increasing the temperature of the substrate during deposition of the material.

Description: FILM OR COATING DEPOSITION ON A SUBSTRATE This invention relates to the field of depositing films or coatings on a substrate. Thermal barrier coatings (TBCs) are a type of film that is deposited on a substrate. A variety of techniques have been examined to fabricate TBCs, but to date only two processing routes have been successfully scaled up to produce thick TBCs (about 250 μm). These are the electron beam assisted physical vapour deposition (EB-PVD) and plasma spraying techniques which make extensive use of vacuum and sophisticated deposition equipment. However, the above fabrication routes are expensive for large area units and mass production. Moreover, the EB-PVD is a line-of-sight process which limits the capability of the method to coat 3-D component uniformly. A new technique termed electrostatic spray assisted vapour deposition (ESAVD) is described in PCT/GB96/03105. ESAVD is a novel processing technique which offers a simple and cost effective method to manufacture ceramic films. However, depositing the thick films that are required for TBCs has remained a problem. This invention provides a method of depositing a material onto a substrate, the method comprising the steps of: (a) feeding a material solution to an outlet to provide a stream of droplets of the material solution; (b) applying a potential difference between the outlet and the substrate to electrostatically attract the droplets from the outlet towards the substrate; (c) heating the substrate to provide an increase in temperature between the outlet and the substrate; and (d) progressively increasing the temperature of the substrate during deposition of the material. In the invention, it is recognised that when a thick film, especially an insulative film such as that required for a TBC is deposited, the insulating properties of the film tend to reduce the temperature of the material-receiving surface of the film with respect to that of the original substrate. So, while the substrate may be heated (e.g. by an electric back-heater on the rear face of the substrate so as to provide a temperature gradient from the substrate to the outlet) to a constant temperature, the actual surface of the deposited film, which receives the next layer of deposited material, can become progressively cooler. To address this problem, the invention provides a progressively increasing substrate temperature so as to maintain a substantially constant temperature at the material-receiving surface. This can lead to much more uniform properties through the deposition process, and so can lead to thick films (e.g. TBCs) being deposited with much more uniform, reliable and predictable thermal and mechanical properties. As an example the technique may be used for the fabrication of thermal barrier coating for gas turbine engines in power generation and aerospace applications. In addition TBCs can also be used in automobile industries. Further respective aspects and features of the invention are defined in the appended claims. An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 schematically illustrates a film deposition apparatus; Figure 2(a) shows the surface morphology of a TBC (YSZ) coating deposited at a temperature within 600-650°C for 1 hour; Figure 2(b) shows the cross-section micrograph of the coating of Figure 2(a); Figure 3(a) shows the surface morphology of an approximately 400 μm thick TBC (YSZ) coating deposited at a temperature within 600-650°C for 1 hour; Figure 3(b) shows the cross-section micrograph of the coating of Figure 3(a); and Figure 4 schematically illustrates a further embodiment of a film deposition apparatus. A film deposition apparatus as shown schematically in Figure 1 comprises an outlet (e.g. a nozzle, spray or similar) 5 connected to a high voltage DC source 7 preferably variable in the range 0-30 kV. In operation with a corona field (see below), a voltage of between about 4 and 15 kV (corresponding to a substrate temperature of approximately 650-850°C) would be normally used for the apparatus as shown, although for other applications a voltage between about 5 and 30 kV might be appropriate. A substrate holder 4 is earthed and is heated by a heater 2. The temperature of the shaped substrate holder 104 is controlled by the controller 1 and an attached thermocouple 3. Heating the substrate holder also causes a corresponding heating of the area surrounding the substrate 14 and between the substrate holder and the outlet 5. This heating serves to set up a temperature gradient whereby the ambient temperature increases as the substrate is approached from the direction of the outlet. This increasing temperature facilitates a chemical vapour reaction of the coating solution that enables deposition of the ceramic film. When an electric field of sufficient or appropriate strength is applied to the outlet 5, a corona discharge is produced from the tip of the outlet 5. A liquid precursor for deposition of the ceramic coating is used to form the films and is fed into the outlet 5 in the direction indicated by an arrow 6. The outlet's inner diameter can vary from 1mm (millimeter) to 0.1mm. This relatively large inner diameter reduces the chances of clogging problems when high viscosity solutions are passed through the outlet 5. A substantially constant flow in the range of 0.4 - 60 ml/h (millilitres per hour) is maintained by use of a syringe pump or a constant static pressure feed. Alternatively, the flow rate can be varied - e.g. pulsed - if desired. This can lead to a layer structure being formed in the deposited film. In this way, the electrostatic field set up between the charged outlet 5 and the earthed substrate holder 4 serves to guide charged coating solution droplets to the substrate 14. Droplets of the coating solution are provided with a positive charge by way of the high voltage DC source 7. These positively charged droplets are attracted to the earthed substrate holder 104. (As an alternative, of course, the droplets could be negatively charged with an earthed holder 104, or vice versa). The coating solution, which may be a mixture of Zr(OC 4 H 9 ) 4 , Butanol and Y(O 2 C 3 H 15 ) 3 , is then passed to the outlet 5 and discharged towards the substrate 14. (As an example, the sol precursor solution of the YSZ coating consists of Zirconium(IV) butoxide Zr(OC 4 H 9 ) 4 , Yttrium 2-ethylhexanoate Y(O 2 C 8 H, 5 ) 3 , and a mixture of combustible solvents (e.g. 70-80vol% butanol and 30-20vol% acetylacetone). Ethanoic acid is used as a catalyst.) Preferably, the concentration of the desired coating solution is approximately 0.01-0.5 mol/1 (moles per litre). In addition, the coating solution may have a viscosity in the region of about 0.01 to 50 mPa.S (millipascal seconds) depending on the type of film that it is desired to produce. The principles behind the technique (such as steering the droplets, types of film, etc.) are described in PCT/GB96/03105, which is incorporated herein by reference. Table 1 below (spanning two pages) shows the compositions of coating solutions for various deposited films. (0 c 00 (0 H H C H m 0) x m H m ro Table 1 (0 DO w H H C H rn in _r m m H c I" m t σ> Accordingly, ESAVD involves spraying atomised precursor droplets across an electric field whereby the charged droplets undergo combustion and/or chemical reaction in the vapour phase near the vicinity of the substrate, leading to the formation of a stable solid film with excellent adhesion onto a heating substrate in a single production process. The film tends to build up as an advantageous columnar structure. This new method has the capability to provide molecular tailoring of microstructure and composition to produce stress-strain columnar structures with carefully engineered microporousity/microcracks, multilayer and graded features to improve the coating adhesion, erosion resistance, and lower the thermal conductivity of the thermal barrier coatings. The stoichiometry of thermal barrier coatings with different dopants can be controlled precisely. Higher deposition rates with this method are possible because the charged droplets are attracted towards an antipole substrate. In experiments, deposition rates were found to be between 1 and 5 microns per minute depending upon the processing conditions (e.g. concentration, flow rate of YSZ coating solution, and substrate temperature). The use of a corona discharge around the outlet tip is preferred for the deposition of dense film with well controlled structure. During the deposition, the corona discharge can be observed in the tip of nozzle. This process can also be performed at reduced, pressure to provide a glow discharge around the output tip. The microstructure. grain size, surface morphology and thickness of YSZ thermal barrier coatings are dependant on the process conditions. The experimental results demonstrate both the technical and commercial viability of the ESAVD method in the cost-effective manufacture of TBCs with columnar structure (Figure 3(b)) which has a strain tolerant capability. When a thick film, especially an insulative film such as that required for a TBC is deposited, the insulating properties of the film tend to reduce the temperature of the material-receiving surface of the film with respect to that of the original substrate. So. while the substrate may be heated (e.g. by an electric back-heater on the rear face of the substrate) to a constant temperature, the actual surface of the deposited film, which receives the next layer of deposited material, can become progressively cooler. This can affect the position, relative to the substrate surface, at which the chemical reaction of the precursor solution takes place, and so can affect the at which the chemical reaction of the precursor solution takes place, and so can affect the deposited film's structure and properties. To address this problem, and in order to maintain the chemical reaction at or near the vicinity of the substrate surface, the substrate temperature is gradually increased (either continuously or in steps) as the coating thickness increases so as to maintain a near constant deposition (i.e. material receiving surface) temperature. The material receiving surface temperature can be measured using a thermocouple or similar probe. In other embodiments (not shown), the surface temperature can be continuously monitored using an infra red pyrometer or other optical detector, and the resulting measurement used to control the substrate heater via a feedback loop to maintain a constant receiving surface temperature. The typical substrate temperature is within the range of 600-850°C for example. A temperature range of 400-850°C is also possible. Films as thick as 3 mm have been produced using the method. The thickness of TBCs can be increased easily by reducing the discharge problem (i.e. the charging up of the substrate to act against the applied field) through changing the polarity (e.g. every twenty minutes) and gradually increase the substrate temperature during the deposition process. The polarity change during the deposition process also offers another benefit such that TBCs with engineered columnar structures can be achieved onto gas turbine hardware. The thickness and microstructure of the TBCs can be controlled by varying process parameters (e.g. the substrate temperature, electric field and deposition time.) for improved thermal conductivity and mechanical properties. The amount of porosity and microcrack in the TBCs can also be controlled by varying the processing conditions. TBCs of YSZ and Al 2 O 3 have been successfully deposited onto Ni-based superalloy substrates. It can also be deposited onto metallic, ceramic or polymer substrates. Thermal barrier coating with multilayer structure or graded composition structure can also be achieved by varying the processing conditions (e.g. electric field, deposition temperature, and flow rate, material solution composition and/or concentration during the coating process). There is no restriction on the heating of the substrates. The heating method can be infrared, laser, flame or microwave, etc. The substrate can also be heated locally to enhance deposition in those areas. The ESAVD process can be scaled-up and adapted for the coatings of 3-D components (e.g. gas turbine blades) by using computer controlled multiple nozzles and/or rotating the gas turbine blades. The multiple nozzles can be stationary and/or movable. The ESAVD process can be performed in an open atmosphere or at a reduced pressure. These techniques are not only applicable for the manufacture of thermal barrier coatings but also for other thick coatings and the fabrication of solid oxide fuel cell components, as well as for a variety of structural and functional applications. Figure 4 schematically illustrates another embodiment of a deposition apparatus, particularly suitable for irregularly shaped parts such as gas turbine engine components. A hollow rotating shaft carries a plurality of spaced nozzles (outlets) 22, through which the precursor solution is sprayed. The shaft rotates about its own axis. The units 24 on which the deposition is to take place are arranged on one or more common supports around the shaft. Using these supports, the units can be rocked up and down as shown. The combination of the rocking motion and the rotation of the spaced nozzles can provide a relatively even deposition coverage. The material solution can be varied during the deposition process, for example to build up a layer structure of different materials or to provide a graded layer having a smoothly varying composition. For example, on a nickel-based superalloy substrate, a bond coating layer of MCrAlY (M may be Ni) is first deposited. A layer of alumina Al 2 O 3 is then deposited (or allowed to form). A graded TBC layer is then deposited, starting with substantially 100% alumina but varying its composition as the layer builds up so that the proportion of alumina is gradually decreased and the proportion of YSZ gradually increases. This is performed by simply changing the precursor composition and/or the deposition temperature as it is being delivered. In other embodiments, to decrease the thermal conductivity (advantageous for TBC coatings) a multilayer approach can be used. The layers could be different material compositions or alternatively different structures of the same composition. For example, alternate layers of YSZ can be formed by: (a) a heterogeneous reaction, which occurs near to the substrate surface, giving a dense layer: and (b) a mixture of heterogeneous and homogeneous reactions (a homogeneous reaction occurs in the gas phase well away from the substrate surface), giving a porous layer. The two types of layers have different mechanical and thermal (insulative) properties. The switch between the two types of deposition is as simple as changing the substrate temperature - e.g. by l0 or l5°C. Other variations can be obtained by varying the material solution concentration.

Claims: CLAIMS 1. A method of depositing a material onto a substrate, the method comprising the steps of: (a) feeding a material solution to an outlet to provide a stream of droplets of the material solution; (b) applying a potential difference between the outlet and the substrate to electrostatically attract the droplets from the outlet towards the substrate; (c) heating the substrate to provide an increase in temperature between the outlet and the substrate; and (d) progressively increasing the temperature of the substrate during deposition of the material. 2. A method according to claim 1, comprising the step of relatively rotating and/or translating the outlet and the substrate during coating deposition. 3. A method according to claim 1 or claim 2, comprising the step of varying the material solution composition and/or concentration during the coating process. 4. A method according to any one of the preceding claims, comprising the step of reversing the polarity of the electric field between the outlet and the substrate at intervals during the deposition process. 5. A method according to any one of the preceding claims, comprising the step of locally heating areas of the substrate to enhance material deposition at the heated areas. 6. A method according to any one of the preceding claims, wherein the material is deposited as a film. 7. A method according to any one of the preceding claims, wherein the outlet is charged to approximately 4 - 15 kilovolts with respect to the substrate. 8. A method according to any one of the preceding claims, wherein the temperature increases to a temperature in the approximate range of about 650 degrees celsius to about 850 degrees Celsius. 9. A method according to any one of the preceding claims, comprising the step of depositing layers of material having different thermal and/or mechanical properties by changing the composition of the material solution and/or the deposition conditions during the material deposition. 10. A method according to any one of the preceding claims, comprising periodically varying the flow of the material solution to the outlet during deposition. 11. A method according to any one of the preceding claims, in which the material solution comprises one or more precursor compounds and a solvent. 12. A method according to claim 11, in which the solvent is acetyl acetone and/or butanol. 13. A method according to any one of the preceding claims, in which the material is deposited as a plurality of adjacent columns of material. 14. A method according to any one of the preceding claims, in which the applied potential is such that a corona discharge is formed around the outlet. 15. A method according to any one of the preceding claims, in which the coating solution is a sol solution. 16. A method according to any one of the preceding claims, in which the process parameters are selected to tailor the molecular microstructure. 17. A method according to any one of the preceding claims, in which the method is used for depositing material onto large areas and/or complex shapes. 18. A method according to any one of the preceding claims, in which deposition is controlled using computer controlled multiple nozzles and/or rotating the substrate. 19. A method according to any one of the preceding claims, in which the method forms a thermal barrier and/or thick coating. 20. A method according to any one of the preceding claims, in which the method is used in forming solid oxide fuel cell components. 21. A method according to any one of the preceding claims, in which the method is used in coating gas turbine blades. 22. A method according to any one of the preceding claims, comprising the step of electrostatically and/or magnetically steering the stream of droplets in transit from the outlet to the substrate. 23. A method according to claim 1, wherein the material is deposited as a film. 24. A method according to claim 23, wherein the film is a multicomponent oxide film: a simple oxide film or a doped film. 25. A method according to claim 23 or claim 24, wherein the film is one or more of: a structural film; a functional film; and an electroceramic film. 26. A method according to claim 1, in which the material is deposited as a powder. 27. A method according to any one of the preceding claims, in which the material solution is a polymer solution. 28. Apparatus for depositing a material onto a substrate, the apparatus comprising: an outlet: means for feeding a material solution to the outlet to provide a stream of droplets of the material solution; a voltage source for applying a potential difference between the outlet and the substrate to electrostatically attract the droplets from the outlet towards the substrate; and heating means for heating the substrate to provide an increase in temperature between the outlet and the substrate, the heating means being operable to progressively increase the substrate temperature during the deposition process.

Inventors: CHOY KWANG-LEONG, CHOY, KWANG-LEONG
Applicants: CHOY KWANG LEONG, IMPERIAL COLLEGE, CHOY, KWANG-LEONG, IMPERIAL COLLEGE OF SCIENCE, TECHNOLOGY AND MEDICINE
Application year: 1998
Main classification: C23C4
IPC codes: B05B5, B05D1, C23C4, C23C16, C23C26