Compositions: ceramic – Ceramic compositions – Yttrium – lanthanide – actinide – or transactinide containing
Reexamination Certificate
1998-09-28
2001-01-16
Koslow, C. Melissa (Department: 1755)
Compositions: ceramic
Ceramic compositions
Yttrium, lanthanide, actinide, or transactinide containing
C501S153000, C501S154000, C501S127000, C501S134000, C501S096100, C501S096300, C501S076000, C501S097100, C501S098400, C501S120000, C501S123000, C501S032000, C501S126000
Reexamination Certificate
active
06174832
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to ceramic compounds which undergo martensitic transformation and methods for producing them, and also to highly-tough composite materials consisting essentially of the compounds. In particular, the invention is to propose ceramic compounds capable of exhibiting high toughness through the phenomenon that follows their thermo-elastic martensitic transformation, and inorganic composite materials.
The ceramic compounds and the composite materials of the invention are effectively used in broad fields of artificial bones, artificial teeth, engine parts, gas turbine blades, parts for gas turbines, parts of corrosion-resistant devices, crucibles, parts for ball mills, electric insulating materials, tools, heat-insulating materials, substrates for electronic circuits, sealants, joints, parts for valves, pumps, nozzles, roller guides, ferrules, bearings, etc.
2. Description of the Related Art
As compared with metals and polymer materials, in general, ceramics have higher hardness, better heat resistance and better corrosion resistance and are characterized by their good electric and magnetic properties.
On the other hand, however, the toughness of ceramics is much inferior to that of metals and polymers, and therefore, ceramics are defective in that their use is limited.
In order to overcome the drawback (low toughness) of ceramics, various proposals have heretofore been made. Of those, one technique for increasing the toughness of ceramics which is based on the martensitic transformation of ceramic materials is most widely noticed, since the method for producing the toughened ceramics according to the technique is easy and the technique itself is greatly effective. For example, known is highly-tough zirconia, for which the technique is based on the non-reversible stress-inducing phase transformation mechanism for non-thermo-elastic martensitic transformation. Specifically, according to the technique, tetragonal crystals of zirconia in a high-temperature mother phase are stabilized at a temperature not higher than room temperature, at which the crystals may be cracked, while being subjected to phase transformation into martensitic monoclinic crystals, whereby the crystals are prevented from being more cracked owing to the volume expansion that results from the phase transformation.
However, as being based on the non-reversible stress-inducing phase transformation, the conventional highly-tough ceramics that undergo martensitic transformation are still problematic in that the toughness in the area around the cracks having been formed during the phase transformation into monoclinic crystals is rather lowered. In addition, if they are much damaged continuously for a long period of time, their toughness is gradually lowered as a whole, resulting in that they could no more have high toughness.
As so mentioned hereinabove, the conventional highly-tough ceramics are based on the phase transformation from the high-temperature phase of tetragonal crystals into the stable martensite phase via the semi-stable phase that is even at temperatures not higher than room temperature. However, the phase transformation of that type will occur even when ceramic compounds react with water, and is especially activated at temperatures falling between 200 and 300° C. Therefore, the conventional highly-tough ceramics are further problematic in that they are unstable.
Moreover, at present, alumina is principally combined with the conventional highly-tough zirconia to produce highly-tough composite ceramic materials, since the combination has been proved effective. Therefore, there is still another problem in that the conventional highly-tough zirconia could not be combined with any other ceramics except alumina to produce highly-tough composite ceramic materials.
SUMMARY OF THE INVENTION
One essential object of the invention is to obtain ceramic compounds having not only high hardness and strength but also high toughness.
Another object of the invention is to obtain ceramic compounds that are stable even at temperatures not higher than room temperature.
Still another object of the invention is to provide ceramic materials capable of being combined with any and every type of ceramics to give highly-tough composite ceramic materials.
Still another object of the invention is to propose a technique for easily producing highly-tough ceramic compounds that undergo thermo-elastic martensitic transformation.
Still another object of the invention is to provide inorganic composite materials having high strength and toughness and suitable for construction materials.
In order to realize the above-mentioned objects, we, the present inventors have assiduously studied and, as a result, have developed novel ceramic compounds which undergo martensitic transformation.
Specifically, the invention provides ceramic compounds which undergo martensitic transformation and which are represented by a rational formula, Ln
1-x
Si
x
AlO
3+0.5x
, where Ln represents at least one selected from Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb, and x=0.01 to 0.3.
In the invention, preferably, the compounds of formula Ln
1-x
Si
x
AlO
3+0.5x
, in which Ln represents at least one selected from Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb, and x=0.01 to 0.3, are obtained by substituting a part of LnO
1.5
in LnAlO
3
-type compounds with SiO
2
.
The invention also provides a method for producing ceramic compounds which undergo martensitic transformation, the method comprising mixing Ln
2
O
3
, SiO
2
and Al
2
O
3
in a ratio of from 0.99 to 0.7 mols of Ln
2
O
3
, from 0.02 to 0.6 mols of SiO
2
and 1 mol of Al
2
O
3
, followed by reacting the thus-mixed compounds in an oxidizing or non-oxidizing atmosphere at a temperature falling between 1200 and 1600° C. for 0.1 to 3 hours to produce Ln
1-x
Si
x
AlO
3+0.5x
, where Ln represents at least one selected from Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb, and x=0.01 to 0.3.
Preferably, the production method comprises reacting an LnAlO
3
-type compound, in which Ln represents at least one selected from Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb, in an oxidizing or non-oxidizing atmosphere at a temperature falling between 1200 and 1600° C. for 0.1 to 3 hours, thereby substituting a part of LnO
1.5
in said compound with SiO
2
to give a substituted product of Ln
1-x
Si
x
AlO
3+0.5x
, in which x=0.01 to 0.3.
Also preferably, the production method comprises putting a starting mixture composed of from 0.99 to 0.7 mols of Ln
2
O
3
, from 0.02 to 0.6 mols of SiO
2
and 1 mol of Al
2
O
3
, or an LnAlO
3
-type compound into a graphite mold, followed by processing it with discharge plasma in vacuum or in an inert gas at a temperature falling between 1200 and 1600° C. for 0.01 to 0.5 hours to obtain a substituted product of Ln
1-x
Si
x
AlO
3+0.5x
, in which x=0.01 to 0.3.
The invention further provides a highly-tough composite material, which comprises a burned composite product of a mixture composed of the ceramic compound noted above and at least one selected from Al
2
O
3
, wollastonite, spodumene, spinel, mullite, glass, silica glass, hydroxyapatite, ABO
3
complex oxide, germanate, phosphate, titanate, nitride, carbide, boride and silicate.
REFERENCES:
patent: 5384293 (1995-01-01), Omori et al.
patent: 5439853 (1995-08-01), Omori et al.
Harai Toshio
Oomori Mamoru
Adams & Wilks
Koslow C. Melissa
Omori Mamoru
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