Coating processes – Spray coating utilizing flame or plasma heat
Reexamination Certificate
1998-06-30
2002-09-10
Bareford, Katherine A. (Department: 1762)
Coating processes
Spray coating utilizing flame or plasma heat
C427S450000, C427S452000, C427S453000, C427S455000, C427S456000
Reexamination Certificate
active
06447848
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a thermal spray process which uses solution precursors as a feedstock.
2. Description of the Related Art
Coatings are commonly used to provide desirable surface properties of the underlying bulk substrates. Examples of protective coatings include wear-resistant, corrosion-resistant and thermal barrier coatings. In many applications, multiple properties of the coatings are often desirable. However, it is not always possible to have a single material (single phase material, alloy or composite) that has all the required properties. In such a case, multiple materials with different properties can be used in the form of multilayers.
Conventional coatings, including multilayered coatings, are made of coarse-grained materials with grain sizes which are greater than several microns. These coatings can be prepared by solution chemistry, physical or chemical vapor deposition or thermal spraying. For deposition methods that do not involve solution based chemistry, physical vapor methods such as sputtering and beam induced evaporation are commonly used. The vapor of the materials (as atoms or clusters) condense on the substrate to form coatings. The chemical vapor approach generally involves pyrolysis of chemical precursors at the substrate to form desirable reaction product coatings. Vapor techniques are generally suitable for preparing thick films or thin coatings due to the low rate of deposition.
An alternative approach to fabrication of thick coatings is thermal spraying. In thermal spraying powders are generally used as the feedstock and fed into a flame aimed at the surface of substrates. The powders are propelled in the gas flow and are fused to form coatings on the substrate. Thermal spraying includes plasma methods in the ambient atmosphere or vacuum, high velocity oxyfuel spraying or high velocity impact fusion spraying. In all cases, the feedstock are often very coarse agglomerates of powders. The agglomerate size is typically in the tens of microns. The powder agglomerates often form splat microstructures, which are pancake-like structures in the thermally sprayed coatings.
Although thermal spraying is a viable approach to preparing thick coatings, the use of the powder agglomerate feedstock has limitations and problems. First, the sprayable powders often require reprocessing from the parent powders by controlled agglomeration, which adds more cost to the production and often introduces impurities if surface-active precursors are used as binders. Second, the splat boundaries in the as-sprayed coatings are often the initiation sites for flaw propagation that consequently lead to mechanical failure of the coatings. Third, the as-formed splat microstructures present a limitation on the scale of chemical homogeneity and mixing of multiphasic materials when desired because the splat is at least greater than several microns thick, due to the flattening of the molten particles on impact. From commercial experience, sprayable powders need to be of a certain size such as about 30 microns or larger for efficient deposition. As a result, reconstitution of nanoscale powder to 30 micron-sized agglomerates is often required. Unfortunately, these larger diameter agglomerates produce longer splat microstructures in the coating. These large splat particles become a serious problem when multifunctional applications require multilayered, hybrid coatings with fine, continuous interfaces, since the length scale of an interface is limited by the splat microstructure.
To solve this fine gradient coating problem, we proposed to use liquid solutions wherein the composition of the solution is varied as the coating is applied. Although it has been known to use a liquid feedstock in thermal spraying, such disclosures do not relate to the production of nanostructure coatings and the multilayer and gradient coatings of the present invention.
U.S. Pat. No. 5,032,568 to Lau et al uses an atomized aqueous solution containing at least 3 metal salts precursors into an inductively coupled ultra high temperature plasma for coating. There is no discussion of forming nanostrucure coatings nor of how to provide multilayer and gradient coatings on such a small scale.
U.S. Pat. No. 4,982,067 to Marantz et al relates to an apparatus to eliminate the long-standing problems with radial feed plasma spray apparatus by designing a true axial feed in a plasma spray system. While most of this disclosure is to using particles as the feed, at column 5, lines 51-55, the patent states that “alternatively, the feedstock may be in liquid form, such as a solution, a slurry or a sol-gel fluid, such that the liquid carrier will be vaporized or reacted off, leaving a solid material to be deposited.” Again, there is no discussion of forming nanostructure coatings nor of how to provide multilayer and gradient coatings on this small scale. In addition this patent essentially deals with the deposition of solid particles that are formed by conversion of the droplets to solid particles in flight before impacting the substrate.
U.S. Pat. No. 5,413,821 to Ellis et al relates to an inductively coupled plasma to thermally decompose a chromium bearing organometallic compound. Column 2, lines 19-22, states that the organometallic compound can be introduced to the plasma as a vapor or a solid. However, in Example 4 the tetramethylchromium is cryogenically cooled to the liquid state for application to the plasma coating device. The organometallic liquid was introduced into the plasma by bubbling through a carrier gas or in the form of solid powder entrained in the carrier gas. The former may actually exist in the form of chemical vapor. Again, there is no discussion of forming nanostructure coatings nor of how to provide multilayer and gradient coatings on this small scale.
U.S. Pat. No. 5,609,921 to Gitzhofer et al discloses a suspension plasma spray where a suspension of particles of the material to be deposited is in a liquid or semi-liquid carrier substance. An inductively coupled radio-frequency plasma torch is used. The preformed particles are suspended in a liquid carrier. Vaporization of the liquid carrier in the plasma leads to the agglomeration of the particles. The particles become molten and impact the substrate. Suspension of small particles in a liquid and its subsequent spraying into the plasma flame may lead to an additional problem. If the particles are dispersed and are very fine (such as less than 100 nm), they may not have enough momentum to penetrate into the plasma flame and be carried by the plasma flame to the substrate. Again, there is no discussion of forming nanostructure coatings nor of how to provide multilayer and gradient coatings on this small scale.
SUMMARY OF THE INVENTION
It is an object of this invention to use of solution precursors as feedstock in thermal spraying of ceramic, metallic, organic and hybrid (a combination of various classes of materials) coatings in a potentially competitive, single step fabrication process for coatings.
It is a further object of this invention to provide a thermal spray process which eliminates the need to synthesize powders and reprocess these powders for spraying.
It is a further object of this invention to utilize the chemical conversion of droplets in a thermal spray process to form desirable reaction products as coatings on substrates in a single step synthesis process.
It is a further object of this invention to provide a thermal spray process in which solution feedstocks are employed for better homogeneity and mixing of multiphasic materials.
It is a further object of this invention to reduce costs for preparing coatings of high melting temperature materials by replacing the melting of the powder required in conventional thermal spray of powder feedstock with the lower temperature solidification of thermally sprayed droplets at the substrate.
It is a further object of this invention to utilize the molecular design of solution precursors for desirable reaction products in a thermal spray proces
Chow Gan-Moog
Kurihara Lynn K.
Strock Christopher W.
Strutt Peter R.
Xiao T. Danny
Bareford Katherine A.
Kap George A.
Karasek John J.
The United States of America as represented by the Secretary of
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