Cr2O3-Al2O3 sintered body and processes for producing same

Details Cr2O3-Al2O3 sintered body and processes for producing same Cr2O3-Al2O3 sintered body and processes for producing same

1999-12-20

2002-04-16

Marcantoni, Paul (Department: 1755)

Compositions: ceramic

Ceramic compositions

Refractory

C501S134000, C264S674000

Reexamination Certificate

active

06372679

ABSTRACT:

TECHNICAL FIELD
This invention relates to Cr
2
O
3
—Al
2
O
3
ceramics which can be expected to be applicable to high-temperature components subject to severe corrosion by molten salts, such as coatings for the rotor blades of low-quality oil-fired gas turbines, burner nozzles and diffuser cones of crude fuel-fired boilers, and stokers of refuse incinerators, and to processes for producing the same.
BACKGROUND ART
The resistance of Cr
2
O
3
—Al
2
O
3
ceramics to corrosion by molten salts comprising heavy metal oxides and alkali salts depends primarily on the sintering temperature and the content of Cr
2
O
3
—Al
2
O
3
solid solution. In order to achieve high corrosion resistance, it is necessary to form a dense sintered body consisting of a complete solid solution of Cr
2
O
3
and Al
2
O
3
. However, since Cr
2
O
3
has poor sinterability, a hot isostatic pressing machine or the like must be used to yield a dense sintered body.
Consequently, conventional processes have the disadvantage that they require a considerable cost and have a limited throughput owing to the use of a hot isostatic pressing machine or the like. Moreover, the conventional processes also have the disadvantage that the formation of a Cr
2
O
3
—Al
2
O
3
solid solution does not proceed to the fullest extent, causing Cr
2
O
3
and Al
2
O
3
particles to remain in the sintered body.
DISCLOSURE OF THE INVENTION
The present inventors made intensive investigations on processes in which TiO
2
serving as an assistant for accelerating the diffusion of Al
2
O
3
and Cr
2
O
3
is added in order to improve the sintered density and promote the formation of a Cr
2
O
3
—Al
2
O
3
solid solution, and in which finely powdered raw materials are sintered under conditions including a suitable atmosphere and a suitable temperature. As a result, it has been found that, by adding TiO
2
, using finely powdered raw materials, and employing a suitable sintering atmosphere and a suitable sintering temperature, an improvement in sinterability can be achieved to yield a sintered body showing an improvement in compositional homogeneity and resistance to corrosion by molten salts. The constitution of the present invention is as follows.
Embodiment 1
A process for the production of a sintered body having excellent resistance to corrosion by molten salts which comprises the steps of providing a powder mixture A consisting essentially of Cr
2
O
3
powder having an average particle diameter of not greater than 0.5 &mgr;m and Al
2
O
3
powder having an average particle diameter of not greater than 0.5 &mgr;m, mixing the powder mixture A with TiO
2
powder having an average particle diameter of not greater than 0.5 &mgr;m to form a powder mixture B containing the TiO
2
powder in an amount of 1.5 to 15% by weight based on the combined amount of the powder mixture A and the TiO
2
powder, and sintering the powder mixture B at a temperature of 1,350 to 1,600° C. in a vacuum atmosphere of 10
−20
to 10
−5
atm. to yield a sintered body consisting of an intimate solid solution of Cr
2
O
3
and Al
2
O
3
and having a relative density of not less than 95%.
Embodiment 2
A process for the production of a sintered body having excellent resistance to corrosion by molten salts which comprises the steps of providing a powder mixture A consisting essentially of Cr
2
O
3
powder having an average particle diameter of not greater than 0.5 &mgr;m and Al
2
O
3
powder having an average particle diameter of not greater than 0.5 &mgr;m, and sintering the powder mixture A at a temperature of 1,350 to 1,600° C. in a vacuum atmosphere of 10
−20
to 10
−5
atm. to yield a sintered body consisting of an intimate solid solution of Cr
2
O
3
and Al
2
O
3
and having a relative density of not less than 95%.
Embodiment 3
A process for the production of a sintered body as described above in embodiment 2 wherein the sintering temperature is in the range of 1,450 to 1,600° C.
Embodiment 4
A process for the production of a sintered body as described above in any of embodiments 1 to 3 wherein the Cr
2
O
3
powder is used in an amount of 40 to 60% by weight based on the powder mixture of Cr
2
O
3
powder and Al
2
O
3
powder.
Embodiment 5
A sintered body having excellent resistance to corrosion by molten salts which is a sintered body consisting essentially of a solid solution phase composed of Cr
2
O
3
and Al
2
O
3
and having a relative density of not less than 95% and which contains no phase composed of Al
2
O
3
alone.
The above-described processes for the production of a sintered body in accordance with the present invention make it possible to produce a dense sintered body consisting of a homogeneous solid solution of Cr
2
O
3
and Al
2
O
3
and thereby achieve a concomitant improvement in corrosion resistance.


REFERENCES:
patent: 4544643 (1985-10-01), Fraser
patent: 4823359 (1989-04-01), Ault et al.
patent: 54-43909 (1979-04-01), None
patent: 61-10055 (1986-01-01), None
patent: 03208863 (1991-09-01), None
patent: 03287686 (1991-12-01), None
patent: 04132656 (1992-05-01), None
patent: 05270896 (1993-10-01), None

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