Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Particulate matter
Reexamination Certificate
2002-03-21
2004-07-27
Kiliman, Ieszek B (Department: 1773)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Particulate matter
C428S328000, C423S263000, C423S592100, C427S453000
Reexamination Certificate
active
06767636
ABSTRACT:
This invention relates to rare earth oxide particles for thermal spraying, a sprayed component having a coating of the rare earth oxide particles, and a corrosion resistant component comprising the sprayed component.
BACKGROUND OF THE INVENTION
It is a common practice in the art to thermally spray metal oxide particles to metal, ceramic and other substrates to form a coating thereof for imparting heat resistance, abrasion resistance and corrosion resistance.
Conventional methods for preparing particles suitable for thermal spray coating include (1) a method of producing a fused and ground powder by melting a starting material in an electric furnace, cooling the melt for solidification, and grinding the solid in a grinding machine into particles, followed by classification for particle size adjustment; (2) a method for producing a sintered and ground powder by firing a raw material, and grinding the sintered material in a grinding machine into particles, followed by classification for particle size adjustment; and (3) a method for producing a granulated powder by adding a raw material powder to an organic binder to form a slurry, atomizing the slurry through a spray drying granulator, and firing the granules, optionally followed by classification for particle size adjustment.
The thermal spraying particles have to meet the requirements that (i) they can be consistently fed at a quantitative rate to the plasma or flame during spraying, (ii) their shape remains undisrupted during the feed and spraying (in plasma or flame), and (iii) they are fully melted during spraying (in plasma or flame). These requirements are quantitatively expressed by more than ten physical parameters of particles.
Since the thermal spraying particles are fed to the spray gun through a narrow flowpath such as a transportation tube, whether they can be consistently fed at a quantitative rate is largely affected by the fluidity thereof among other physical parameters. However, the fused or sintered and ground powder resulting from method (1) or (2) has irregular shapes and a broad particle size distribution so that the friction between particles during transportation entails formation of finer particles. Additionally, the powder has a large angle of repose and poor fluidity so that the transportation tube or spray gun can be clogged, preventing continuous thermal spraying operation.
Developed as a solution to these problems of the ground powders was the granulated powder obtained by method (3), that is, having the advantage of smooth fluidity due to the spherical or nearly spherical shape of particles. The strength of the granulated powder tends to vary over a wide range because it depends on the particle size distribution of a raw material powder and the firing conditions. Particles with a low strength will readily collapse during the feed to the spray gun and during the spraying (in the flame or plasma).
In the thermal spraying of metal oxide particles, the particles must be completely melted in the flame or plasma in order to form a sprayed coating having a high bond strength. Particularly when particles of rare earth oxide are used for thermal spraying, because of their high melting point, they must have a smaller average particle size so that they may be completely melted.
In the event where granulated powder is prepared using a spray drying granulator, however, an average particle diameter of less than 20 &mgr;m is difficult to accomplish. In the event of the fused or sintered and ground powder resulting from method (1) or (2), a spray material having a small average particle diameter is obtainable owing to grinding in a mill, which can cause contamination. When particles are prepared in a conventional way, it is difficult to avoid the introduction of impurities at a level of several ten ppm.
As mentioned above, the fused/ground powder, sintered/ground powder and granulated powder discussed above individually have advantages and disadvantages and are not necessarily optimum as the spray material of rare earth oxide. Additionally, the powders of these three types all suffer contamination from the grinding, granulating and classifying steps, which is deemed problematic from the high purity standpoint.
Specifically, the fused/ground powder, sintered/ground powder or granulated powder having passed the grinding or granulating and classifying steps contains impurities such as iron group elements, alkali metal elements and alkaline earth metal elements, typically in a content of more than 20 ppm. A sprayed component having a coating obtained by spraying any of these powders is susceptible to corrosion at impurity sites in the coating, failing to provide satisfactory durability.
SUMMARY OF THE INVENTION
An object of the invention is to provide thermal spray rare earth oxide particles of high purity which can be thermally sprayed to form an adherent coating despite the high melting point of the rare earth oxide.
Another object of the invention is to provide a sprayed component having the particles spray coated on a substrate surface.
A further object of the invention is to provide a corrosion resistant component using the sprayed component.
The invention addresses rare earth oxide particles for thermal spraying. We have found that by controlling the average particle diameter, dispersion index and aspect ratio to specific ranges, and optionally, controlling the surface area, bulk density, crystallite size and impurity content to specific ranges, the rare earth oxide particles are improved in fluidity and given so high density and strength that the particles are completely melted rather than being collapsed during thermal spraying. A coating obtained by thermally spraying the particles is smooth and pure as compared with conventional sprayed coatings, and offers better adhesion and corrosion resistance.
In a first aspect, the invention provides rare earth oxide particles for thermal spraying having an average particle diameter of 3 to 20 &mgr;m, a dispersion index of up to 0.4, and an aspect ratio of up to 2. Preferably, the particles have a specific surface area of 0.3 to 1.0 m
2
/g, and a bulk density of 30 to 50% of true density. Preferably, crystallites in the particles have a size of at least 25 nm. The total amount of iron group elements, alkali metal elements and alkaline earth metal elements in the particles is preferably limited to 20 ppm or less.
In a second aspect, the invention provides a sprayed component comprising a substrate having a surface and a coating of the rare earth oxide particles thermally sprayed on the substrate surface. The substrate is typically made of a metal material selected from the group consisting of Al, Fe, Si, Cr, Zn, Zr, Ni and alloys thereof, or a ceramic or glass material. The coating preferably has a surface roughness of up to 60 &mgr;m.
A corrosion resistant component comprising the sprayed component is also contemplated herein.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the invention, particles for thermal spraying are formed of a rare earth oxide. As used herein, the term “rare earth” encompasses rare earth elements of Group 3A in the Periodic Table inclusive of yttrium (Y). The rare earth elements may be used alone or in admixture. It is understood that compound oxides of the rare earth combined with at least one metal selected from Al, Si, Zr, In, etc. are also useful for the inventive particles.
The rare earth oxide particles should have an average particle diameter of 3 to 20 &mgr;m, and especially 7 to 16 &mgr;m. If the average particle diameter is less than 3 &mgr;m, fine particles may evaporate or scatter in the plasma flame during spraying, resulting in a corresponding loss. If the average particle diameter exceeds 20 &mgr;m, some particles may remain unmelted (not completely melted) in the plasma flame during the spraying step and thus form non-fused particles, resulting in a low bond strength.
It is noted that particles have a particle size distribution as measured by a laser diffraction analyzer in which a particle diameter D90, D50 and D10 corresponds to
Maeda Takao
Takai Yasushi
Tsukatani Toshihiko
Birch & Stewart Kolasch & Birch, LLP
Kiliman Ieszek B
Shin-Etsu Chemical Co. , Ltd.
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