Stock material or miscellaneous articles – Composite – Of silicon containing
Reexamination Certificate
2002-05-23
2004-06-22
Stein, Stephen (Department: 1775)
Stock material or miscellaneous articles
Composite
Of silicon containing
C336S174000, C336S174000, C336S174000, C336S174000, C501S102000, C501S103000, C501S106000
Reexamination Certificate
active
06753089
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to coatings on refractory materials, in particular, zircon/zirconia coatings on ceramic substrates.
BACKGROUND OF THE INVENTION
High temperature coatings for ceramics, such as silicon carbide (SiC), silicon nitride (Si
3
N
4
), are prone to failure in oxidizing conditions at elevated temperatures due to interactions with a growing silicon dioxide (SiO
2
) glass layer. The most stable candidates for coatings are found to be alumina-(Al
2
O), mullite- (3Al
2
O
3
•2SiO
2
), and zirconia-(ZrO
2
) based formulations due to their refractory nature, similar coefficients of thermal expansion, and chemical stability in contact with silicon carbide, silicon nitride, and silicon dioxide.
At temperatures above 1200° C., porous alumina coatings will dissolve into the growing silicon dioxide layer. The resulting aluminosilicate glass fills the pores of the alumina coating creating an impermeable membrane. This membrane traps gaseous by-products of the oxidation reaction, such as carbon dioxide. The filled pores coalesce causing the coating to blister or flake off. Dense alumina coatings would tend to crack and craze due to the higher coefficient of thermal expansion. When the alumina coating is used in an ink application, the dissolution of the alumina coating bleeds into the glass leading to bleed-out of the white coating along the edges. For thin coatings, this is especially problematic as they will either fully dissolve into the glass layer or otherwise become invisible.
Similarly, mullite coatings on silicon carbide/silicon nitride ceramics are susceptible to blistering and flaking, and also become translucent when a substantial quantity of the glass layer is present. Stabilized zirconia coatings are known to be chemically stable and have limited solubility in glass. However, zirconia, stabilized with calcium or yttria, were found to flake off the silicon carbide parts after one or two high temperature cycles at or about 1100° C.
The prior art discloses a number of coatings for refractory substrates. U.S. Pat. Nos. 4,804,589 and 4,921,721, both to Matsui et al., disclose a coating for metals and silicon carbide substrates consisting essentially of zirconia partially or fully stabilized with yttria, magnesium oxide, or calcium oxide. The coatings are deliberately kept thin to decrease peeling and crazing. A compatibilizing layer may be added between the zirconia layer and the substrate. Matsui et al. further requires a surface pre-treatment for silicon carbide substrates for surface roughening or reactivity enhancement. The practicality of coating refractory substrates with the current coating composition is diminished with the need for surface pre-treatments. Also, thicker coatings are prone to peeling and cracking.
U.S. Pat. No. 4,950,558 to Sarin discloses a graded coating for silicon based substrates, the coating comprising multiple layers: one or more intermediate layers of aluminum nitride or aluminum oxynitride material, and an outer layer of an oxide of aluminum, zirconium, or yttrium. The coatings are prepared using chemical vapor deposition with a mixture of gases in a continuous deposition process in which the reactant gases are changed gradually to provide the graded layers. It would be desirable to provide a coating which does not have the environmental concerns associated with chemical vapor deposition.
U.S. Pat. Nos. 6,165,594, 6,214,250, and 6,251,212, all to Moh et al., disclose a label for metal and ceramic substrates. The label includes a ceramic body as a base layer which contains a glassy phase which wets the substrate and a refractory phase with light or dark particles; a top layer also contains a glassy phase and a refractory phase with contrasting particles. The color contrast between the top layer and the ceramic body allows for an optically discernible labeling pattern. However, the individual labels must be formed and fired prior to being attached to a substrate.
Notwithstanding the state of the art, it is desirable to provide coating compositions for high temperature refractory ceramic substrates which provide ease of use, good adhesion, and stability in an oxidizing atmosphere.
SUMMARY OF THE INVENTION
The present invention is directed to, in a first aspect, a refractory coating composition for coating high temperature substrates, the composition comprising: unstabilized zirconia; and silica. Preferably, the unstabilized zirconia is present in an amount of about 50 to about 90 parts per hundred of the composition. Preferably, the silica is present in an amount of about 10 to about 50 parts per hundred of the composition. The composition may further include zircon which may be present in an amount of up to 100 parts per hundred of the composition. The composition may also include an inorganic filler such as mullite present in an amount of up to about 50 parts per hundred of the composition. In a preferred embodiment of the composition of the first aspect, the unstabilized zirconia and the silica are present in a weight ratio of about 9:1 to about 1:1. The composition is useful for coating substrates at temperatures greater than about 1100° C.
In a second aspect, the present invention is directed to a refractory coating composition comprising: about 15 to about 75 parts per hundred unstabilized zirconia; about 5 to about 25 parts per hundred silica; and up to about 100 parts per hundred zircon. The composition may be applied as a slurry or as a decal on a substrate. Preferably, the composition is applied as a thin film having a thickness of about 20 to about 500 microns.
In a third aspect, the present invention is directed to a ceramic sintered member comprising: a ceramic body; and a refractory coating formed on a surface of the ceramic body, the refractory coating comprising: unstabilized zirconia; silica; and zircon, wherein the refractory coating maintains stability at temperatures in excess of about 1100° C. Preferably, the ceramic body comprises silicon carbide or silicon nitride.
In a fourth aspect, the present invention is directed to a method of making a ceramic sintered body comprising the steps of: providing a ceramic substrate; providing a refractory coating composition comprising: unstabilized zirconia; silica; and zircon; applying the refractory coating composition on the ceramic substrate; and exposing the coated ceramic substrate to sintering conditions, wherein the refractory coating on the ceramic substrate maintains stability at temperatures greater than about 1200° C. Preferably, the ceramic substrate comprises silicon carbide or silicon nitride. The refractory coating composition may be applied to a portion of the ceramic substrate. The refractory coating composition and the ceramic substrate may be different colors and the composition is applied to a portion of the ceramic substrate as a marker. Preferably, the refractory coating composition is painted, spray coated, sponged, brush coated, or screen printed on the ceramic substrate.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention relates to refractory coating compositions for coating high temperature substrates including unstabilized zirconia, silicon dioxide or silica, and optionally, zircon. Unexpectedly, the coating compositions of the present invention maintain good adhesion to refractory substrates at temperatures up to and over about 1100° C. preferably, over 1200° C., and more preferably over 1400° C. These coating compositions are particularly useful as inks for labeling refractory substrates. When used as an ink on refractory substrates, the coating compositions demonstrated clean lines which did not bleed into the substrate and maintained a good contrast with the substrate, even after multiple heat cycles. As used herein, zircon shall mean ZrSiO
4
and/or its decomposition products SiO
2
and ZrO
2
.
The refractory coating compositions of the present invention may be expressed in terms of parts per hundred by weight where the total weight of the composition is equal to a hundred (100) parts and the sum of
McCormick Paulding & Huber LLP
Saint-Gobain Ceramics & Plastics, Inc.
Stein Stephen
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