A ceramic consisting essentially of from 1 to 15 percent of glass and 99 to 85 percent of a mixture of particulate al2 O3 and particulate ZrO2 is disclosed. ZrO2 is present in a sufficient amount, usually from 1/4 to 6 percent based on the weight of the ZrO2 and al2 O3, to strengthen the ceramic significantly, by comparison with an otherwise identical ceramic where the particulate ZrO2 is replaced either by the glass or by particulate al2 O3. The glass constitutes a vitreous phase bonding the particulates into a dense, gas impervious structure, and can be a calcium magnesium silicate glass containing from 45 to 80 percent of SiO2, from 8 to 55 percent of CaO and MgO, and not more than 15 percent of al2 O3.
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1. A ceramic consisting essentially of from 1 5 to 15 percent and 99 95 to 85 percent of a mixture of particulate al2 O3 and particulate ZrO2, the particulates all being finer than 15 microns in ultimate particle size, ZrO2 constituting from 1/4 to 4 percent based upon the weight of ZrO2 and al2 O3 in the ceramic, and said glass constituting a vitreous phase bonding the particulates into a dense, gas impervious structure, and being a calcium magnesium silicate glass containing from 45 to 80 percent of SiO2, from 8 to 55 percent of CaO and MgO, and not more than 15 percent of al2 O3.
4. A ceramic as claimed in
5. A ceramic as claimed in
weight of the ZrO2 and al2 O3 in the ceramic. 6. A ceramic as claimed in claim 3 wherein particulate al2 O3 constitutes more than 99 and not more than 991/2 percent, based upon the weight of the ZrO2 and al2 O3 in the ceramic. |
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This is a continuation in part of application Ser. No. 893,609, filed Aug. 6, 1986, now U.S. Pat. No. 4751,207 4,751,207 granted June 14, 1988.
As used herein, and in the appended claims, the terms "percent" and "parts" refer to percent and parts by weight, unless otherwise indicated; g means gram or grams; cm means centimeter or centimeters; and MPa means 106 Pascals.
All temperatures herein are in degrees C., unless otherwise indicated.
1. Field of the Invention
This invention relates to an alumina ceramic containing a small, strengthening addition of zirconia; the ceramic is one which can be produced by a method that lends itself to mass production.
2. The Prior Art
The subject of alumina-zirconia ceramics produced both by hot pressing and by sintering techniques has received a great deal of attention during recent years. A journal article, Cer. Eng. and Sci. Proc., Vol 1,7-8(B) 1980, is considered to be typical of the prior art relating to such ceramics made by hot pressing. The articles, D. Lewis III and P. F. Becher, "Thermal Shock Behavior in Al2 O3 -based composites", reports test data indicating that aluminazirconia alumina-zirconia composites which were studied are highly resistant to thermal shock. The data relate to alumina ceramics and to ceramics composed of alumina and up to 30 percent by volume of ZrO2 produced by hot pressing at 1500-1600 and 35 MPa (about 5075 pounds per square inch). The data presented indicate the alumina-zirconia ceramics to have outstanding thermal shock properties. Another journal article, J.Am.Cer.Soc., 61 Vol. 61, No. 12, pp. 85, 86, and U.S. Pat. No. 4,218,253, are illustrative of the prior art relating to the production of such ceramics by sintering. The patent discloses (Example 1) the production of an aluminazirconia alumina-zirconiaceramic from aluminum oxide powder (average particle size 5 microns) and monoclinic zirconium oxide powder (average particle size 1 micron). The process involves wet mixing the two powders, drying and granulating the mixture, isostatically pressing a shape from the granules, and sintering the shape at 1600 for one hour. The journal article discloses a similar process, including sintering at 1500 and 1600, but is silent as to particle size, disclosing only that "composites with a very fine and homogeneous dispersion" were achieved "by a wet-chemical method, starting from a zirconium sulfate-aluminum sulfate solution." It will be appreciated that hot pressing alumina-zirconia ceramics at 1500-1600 and 35 MPa is a costly procedure, that even sintering at 1600 is costly, and that alumina produced by a wet chemical method from a zirconium sulfate-aluminum sulfate solution is a costly starting material. Accordingly, as might be expected, the ceramics produced by the method of the subject references, and all other alumina-zirconia ceramics that have been suggested by the known prior art, are costly and have found only limited commercial use, for example as tool bits.
Japanese patent application No. 53-126008, Nov. 2, 1978, discloses the production of what has been translated as an "aluminum ceramic" having improved thermal shock resistance by pressing discs from ceramic batches and firing to 1065°. Batches containing 94 to 96 parts by weight of Al2 O3, 1 to 2 parts by weight of CaO, 1 part by weight of MgO, 2 to 3 parts by weight of SiO2, up to 2 parts by weight of Cr2 O3 and up to 10 parts by weight of ZrO2 are disclosed.
The present inventor discovered a ceramic composed of a mixture of particulate Al2 O3, particulate ZrO2 and glass bonding the Al2 O3 and the ZrO2 into a dense, gas impervious structure, and also found that all or any part of the particulate ZrO2 in ZrO2 in such ceramics can be replaced by particulate HfO2 or by a solid solution of HfO2 and ZrO2, in particulate form, as well as that Y2 O3 can advantageously be present to stabilize at least a part of the ZrO2, the HfO2 or the ZrO2 -HfO2 solid solution in a cubic crystalline structure. The glass constitutes from 1 to 15 percent of such ceramics, while particulate Al2 O3 constitutes from 75 to 85 percent, based upon the weight of the ZrO2, HfO2, Y2 O3 and Al2 O3. "Manning", U.S. Pat. No. 4,552,852, is directed to this discovery.
The instant invention is based upon the discovery of a ceramic which consists essentially of from 1 to 15 percent of glass and from 99 to 85 percent of a mixture of particulate Al2 O3 and particulate ZrO2 in which particulate ZrO2 is present in a sufficient amount, usually from 1/4 to 6 percent based on the weight of the ZrO2 and Al2 O3, to strengthen the ceramic significantly, by comparison with an otherwise identical ceramic where the particulate ZrO2 is replaced either by the glass or by particulate Al2 O3. The particulates should have an ultimate particle size finer than 15 microns. The glass constitutes a vitreous phase bonding the particulates into a dense, gas impervious structure and, preferably, is a calcium magnesium silicate glass containing from 45 to 80 percent of SiO2, from 8 to 55 percent of CaO and MgO, and not more than 15 percent of Al2 O3. Preferably, the glass constitutes from 3 to 12 percent of the ceramic, most desirably from 5 to 12 percent. Available data indicate that a large increase in strength is achieved when larger a small addition of ZrO2, e.g., 1/4 percent based upon the weight of Al2 O3 and ZrO2 in the ceramic, is made, and that little if any additional increase in strength is achieved when A small larger additions, e.g., 1/2 percent on the indicated basis, are made. Since ZrO2 is an expensive constituent of ceramics of the type in question, it is desirable to minimize the quantity used. Accordingly, ZrO2 preferably constitutes from 1/4 to 4 percent, most desirably from 1/2 to 11/2 percent, based upon the weight of the Al2 O3 and ZrO2 therein, in a ceramic according to the instant invention. In fact, the greatest benefit from the expensive batch ingredient is achieved when ZrO2, on the indicated basis, ranges from 1/2 to less than 1 percent.
As is indicated above, Manning discloses that particulate HfO2 and solid solutions of HfO2 and ZrO2 can be substituted for ZrO2 in ceramics of the type in question, and that Y2 O3 can also be used, the purpose of the Y2 O3 being to stabilize the ZrO2 or the like in a cubic crystalline structure. The same is probably true in ceramics according to the instant invention, but there is ordinarily no reason for such a ceramic to contain more than the amount of HfO2 that is introduced thereinto by the 1 to 3 percent thereof that is present as an impurity in ZrO2 as it occurs in nature. Accordingly, a ceramic according to the invention consists essentially of particulate Al2 O3, particulate ZrO2 and glass, but, in accordance with recognized meaning of the recitation, may contain HfO2, Y2 O3 or other incidental ingredients so long as they do not interfere with the strengthening that is achieved because of the presence of ZrO2.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The following examples are presented solely for the purpose of further illustrating and disclosing the invention. They are to be construed as illustrative, and not as limiting. Example 2 constitutes the best mode presently contemplated by the invention.
A ceramic batch composed of 0.84 part of ZrO2, 82.79 parts of Al2 O3, 16.07 parts of clays and other fluxes and 0.30 part sodium lignosulfonate was wet milled for 8 hours, 70 percent solids in water, in a 17-liter mill jar. The resulting batch was spray dried. The spray dried batch was then pressed isostatically at about 5500 psi. (about 38 MPa) in a cylindrical mold; the pressed shape was rotated and ground by contact with a rotating grinding wheel to a diameter of about 3.8 cm.; and the body was placed in a setter and fired in a gas fired tunnel kiln (slightly reducing atmosphere): about seventeen hours from ambient temperature of about 22 to 1545, and about 1 1/5 hours from 1545 to 1550, followed by cooling. The fired insulator which resulted had a diameter of about 2.5 cm. The mean Charpy Impact Strength of machined bars cut from insulators produced as described in this Example was 1.68×10- 3 foot pounds.
The ZrO2 used as described above in Example 1 is commercially available from TAM Ceramics, Inc. under the trade designation "Zirox 360". It consists of ZrO2, 1 to 3 percent of HfO2, assay, ZrO2 at least 98.5 percent, and incidental impurities. A sample from the shipment from which ZrO2 was used as described herein was found to have a median particle diameter of 9.71 microns and a specific surface area of 0.97 m2 /cm3.
The Al2 O3 used as described above in Example 1 is commercially available from Aluminum Company of America under the designation A-10 alumina. Substantially all of the material is minus 10 microns in ultimate particle size, the median ultimate particle size being 8 microns; the material is agglomerated, however, so that it has the following size characteristics relative to screens of the U.S. Sieve Series:
100 mesh 4-15 percent retained
200 mesh 50-75 percent retained
325 mesh 88-98 percent retained and 2-12 percent through.
The material consists of Al2 O3, assay 99.5 percent, and incidental impurities.
The sodium lignosulfonate used as described above in Example 1 is a water soluble dispersing agent which is commercially available from American Can Company under the designation "MARASPERSE".
The clays and other fluxes used as described above in Example 1 contain SiO2, MgO, CaO and Al2 O3 in such proportions that the fired insulator contained 89.44 percent of Al2 O3, 0.86 percent of ZrO2, 7.02 percent of SiO2, 1.56 percent of MgO, 0.71 percent of CaO and 0.41 percent of incidental impurities. A minor amount of the Al2 O3 was dissolved in a glass which also contained the SiO2, the MgO and the CaO. The glass constituted about 10 percent of the fired insulator. The precise amount of Al2 O3 dissolved in the glass was not determined but, on the basis of available phase data, it was estimated that Al2 O3 constituted less than 5 percent of the glass.
The procedure of Example 1 was repeated to produce insulators from different ceramic batch compositions. The compositions of the ceramic batches in parts and the mean Charpy Impact Strengths are given in the following table:
| ______________________________________ |
| Control Example 2 Example 3 |
| ______________________________________ |
| ZrO2 0.00 0.42 1.26 |
| Al2 O3 |
| 83.63 83.21 82.37 |
| Clays and 16.07 16.07 16.07 |
| other fluxes |
| Sodium 0.30 0.30 0.30 |
| lignosulfonate |
| Charpy Impact |
| 1.07 1.82 1.82 |
| Strength, |
| footpounds, × 103 |
| ______________________________________ |
Insulators were also produced from other ceramic batch compositions using substantially the procedure of Example 1, the only differences being that the batches were milled in 7-liter mill jars, 70 percent solids in water, for 10 hours; and 0.39 part ammonium polyelectrolyte dispersant was used in place of the sodium lignosulfonate. The compositions of the ceramic batches in parts and the mean Charpy Impact Strengths are given in the following table:
| ______________________________________ |
| Con- Example Example Example |
| Example |
| trol 4 5 6 7 |
| ______________________________________ |
| ZrO2 0.00 0.84 1.67 2.50 4.18 |
| Al2 O3 |
| 83.55 82.71 81.88 81.05 79.37 |
| Clays and 16.06 16.06 16.06 16.06 16.06 |
| other fluxes |
| Ammonium 0.39 0.39 0.39 0.39 0.39 |
| poly-electrolyte |
| dispersant |
| Charpy Impact |
| 1.824 2.448 2.309 2.040 2.063 |
| Strength, foot- |
| pounds, × 103 |
| ______________________________________ |
Insulators were also produced from other ceramic batch compositions using substantially the procedure of Example 1, the only differences being that the batches were milled in 7-liter mill jars, 72 percent solids in water, for 91/2 hours. Control insulators were produced from a batch composed of 83.24 parts of Al2 O3, 15.98 parts of clays and other fluxes, 0.25 part of sodium lignosulfonate and 0.53 part of ammonium polyelectrolyte dispersant, while test insulators were produced from batches composed of 82.75 parts of Al2 O3, 15.98 parts of clays and other fluxes, 0.25 part of sodium lignosulfonate, 0.53 part of ammonium polyelectrolyte dispersant and 0.49 part of various grades of zirconia. The grades of zirconia tested, the Charpy Impact Strength in footpounds X ×103 of machined bars cut from insulators made from each of the batches, and the fired bulk density in grams per cubic centimeter of the insulators, and the cantilever breaking load in pounds of the terminal ends of spark plug insulators made from each of the batches are set forth in the following table:
| __________________________________________________________________________ |
| Example |
| Example |
| Example |
| Example |
| Example |
| Control |
| 8 9 10 11 12 |
| __________________________________________________________________________ |
| Zirconia Grade |
| -- "Zirox 360" |
| "SC-101" |
| "Zirox Tr" |
| "HSY-3" |
| "DK-1" |
| Charpy Impact |
| 1.140 |
| 1.940 1.750 |
| 2.010 1.760 |
| 1.690 |
| Strength |
| Fired Bulk |
| 3.524 |
| 3.569 3.571 |
| 3.571 3.575 |
| 3.571 |
| Density |
| Cantilever |
| 465 534 505 503 526 543 |
| Breaking Load |
| __________________________________________________________________________ |
Grade "SC-101" of zirconia is commercially available From Magnesium Elektron. It was found to have a median particle diameter of 4.14 microns and a specific surface area of 1.47 m2 /cm3. It consists of ZrO2, 1-3 percent of HfO2, assay, ZrO2 and HfO2 at least 98.5 percent, and incidental impurities.
Grade "Zirox Tr" of zirconia is commercially available from TAM Ceramics, Inc. It was found to have a median particle diameter of 3.43 microns and a specific surface area of 1.49 m2 /cm3. It consists of ZrO2, 1-3 percent of HfO2, assay ZrO2 and HfO2 at least 98.5 percent, and incidental impurities.
Grade "HSY-3" of zirconia is commercially available from Daiichi Kigenso. It was found to have a median particle diameter of 3.37 microns and a specific surface area of 1.67 m2 /cm3. It consists of ZrO2, 1-3 percent of HfO2, assay, ZrO2 and HfO2 at least 93.1 percent, 5.4 percent of Y2 O3, and incidental impurities.
Grade "DK-1" of zirconia is commercially available from Daiichi Kigenso. It was found to have a median particle diameter of 3.37 microns and a specific surface area of 1.76 m2 /cm3. It consists of ZrO2, 1-3 percent of HfO2, assay ZrO2 and HfO2 at least 98.5 percent, and incidental impurities.
It will be observed from a comparison of the foregoing data concerning Examples 1-3 and the associated control with the data concerning Examples 4-7 and the associated control and that for Examples 8-12 and the associated control that the different milling procedures used caused substantial changes in the magnitudes of the Charpy Impact Strengths. The percentage increases, however, relative to the relevant controls, follow the same patterns, indicating that the strengthening caused by the added ZrO2 is independent of the milling procedures. The percentage increases are reported in the following table:
| ______________________________________ |
| Percent of Added ZrO2 |
| 0.5 1.0 1.5 2.0 3.0 5.0 |
| ______________________________________ |
| Example 2 70 |
| Example 8 70 |
| Example 9 54 |
| Example 10 76 |
| Example 11 54 |
| Example 12 48 |
| Example 1 57 |
| Example 4 34 |
| Example 3 70 |
| Example 5 27 |
| Example 6 12 |
| Example 7 13 |
| ______________________________________ |
Insulators having a nominal 92 percent Al2 O3 content, and containing varying amounts of ZrO2 were also produced using substantially the procedure of Example 1, the only differences being that a different grade of Al2 O3 was used and that the batch was milled in 7-liter mill jars, 72 percent solids in water, for 101/2 hours. Typical batch compositions, Charpy Impact Strengths and densities are given in the following table:
| ______________________________________ |
| Con- Example Example Example |
| Example |
| trol 13 14 15 16 |
| ______________________________________ |
| ZrO2 0.00 0.25 0.49 0.73 0.97 |
| Al2 O3 |
| 87.49 87.49 87.49 87.49 87.49 |
| Clays and 11.73 11.48 11.24 11.00 10.76 |
| other fluxes |
| Sodium 0.25 0.25 0.25 0.25 0.25 |
| lignosulfonate |
| Ammonium 0.53 0.53 0.53 0.53 0.53 |
| polyelectrolyte |
| dispersant |
| Charpy Impact |
| 1.32 1.70 1.94 1.96 1.84 |
| Strength, foot- |
| pounds × 103 |
| Fired Bulk |
| 3.616 3.634 3.642 3.644 3.651 |
| Density, |
| g/cm3 |
| ______________________________________ |
The Al2 O3 used in the batches of the previous table was obtained from Aluminum Company of America under the grade designation A-121. Substantially all of the material is minus 5 microns in ultimate particle size; the material is agglomerated, however, so that it has the following size characteristics relative to screens of the U.S. Sieve Series:
100 mesh 4-15 percent retained
200 mesh 50-75 percent retained
325 mesh 88-98 percent retained and 2-12 percent through
The material consists of Al2 O3, assay 99.5 percent, and incidental impurities.
It will be noted from the data in the foregoing table that both the fired bulk densities and the strengths of the specimens produced were increased by the additions of ZrO2 that were made in the procedures of Examples 13-16. This is generally true of ceramics according to the instant invention. Although the invention is in no way to be limited by the following theory, it is believed that the ZrO2 additions cause a decrease of the size of the pores of the ceramic, and that both the increased density and the strengthening are consequences of this decrease.
It will be apparent that various changes and modifications can be made from the specific details of the invention as described above without departing from the spirit and scope thereof as defined in the appended claims.
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