Surface-coated sintered body of silicon nitride

Details Surface-coated sintered body of silicon nitride Surface-coated sintered body of silicon nitride

2001-10-31

2003-11-11

Jones, Deborah (Department: 1775)

Stock material or miscellaneous articles

Composite

Of inorganic material

C501S097200, C428S701000

Reexamination Certificate

active

06645649

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface-coated sintered body of silicon nitride having excellent strength at room temperature through up to high temperatures, excellent toughness against breakage and excellent resistance against oxidation. More specifically, the invention relates to a surface-coated sintered body of silicon nitride that can be suitably used as parts of gas turbine engines, such as turbine rotor, turbine blade, nozzle, combustor, scroll, nozzle support, seal ring, spring ring, diffuser, duct and shroud.
2. Description of the Prior Art
Owing it its excellent strength, hardness and thermal and chemical stability, the sintered body of silicon nitride has heretofore been used as engineering ceramics and, particularly, as a structural material for heat engines. The sintered body of silicon nitride has generally been prepared by adding a sintering assistant such as Y
2
O
3
, Al
2
O
3
or MgO to a powder of silicon nitride and firing the mixture. Use of the above sintering assistant helps accomplish such properties as a high density and a high strength. Though the sintered body of silicon nitride is used, for example, as parts of the engines, it is desired to further improve the strength and resistance against the oxidation of the sintered body of silicon nitride at high temperatures since the parts of the engines have been used in ever high temperature conditions.
To meet such a demand, efforts have been made to improve the sintering assistant, grain boundary phases, sintering conditions, and protection film formed on the surfaces of the sintered body.
Japanese Unexamined Patent Publication (Kokai) No. 183676/1997 teaches an art for covering the surfaces of a sintered body comprising chiefly silicon nitride or SIALON with a glass layer comprising chiefly SiO
2
in order to improve mechanical strength and resistance against oxidation at high temperatures.
In recent years, further, attempts have been made to coat the surfaces of a sintered body of silicon nitride with SiC, Al
2
O
3
or ZrO
2
having good resistance against oxidation as a protection film by such a method as CVD or melt-injection in order to improve resistance against oxidation, resistance against erosion and resistance against corrosion.
However, the surface-coating materials such as SiO
2
, SiC, Al
2
O
3
and ZrO
2
have such problems as lack of resistance against corrosion or heat, developing cracks during the use due to a difference in the coefficient of thermal expansion from that of the sintered body of silicon nitride which is a substrate, resulting in the occurrence of peeling.
By forming the glass layer on the surface as taught in the above Japanese Unexamined Patent Publication (Kokai) No. 183676/1997, properties can be improved under static conditions. When exposed to a gas of a high temperature, a high pressure and a high velocity in a practical engine, however, the glass layer is quickly worn out due to the vaporization of SiO
2
arousing a problem in that the glass layer has a short life and fails to serve as a protection film.
The present applicant has previously proposed an art for preventing the coated layer from peeling or cracking by coating the surfaces of the sintered body of silicon nitride with a layer that contains a crystalline phase such as of a disilicate or a monosilicate having excellent properties against oxidation. Though the sintered body of silicon nitride having a coating layer formed on the surface thereof exhibits favorable properties concerning the initial resistance against corrosion, however, there still exists a problem in that the resistance against the corrosion gradually decreases.
SUMMARY OF THE INVENTION
The present inventors have conducted keen study concerning the corrosion resistance that deteriorates with the passage of time and have discovered the following new fact.
That is, when there exists an excess of SiO
2
on the grain boundaries of crystalline phases of a disilicate or a monosilicate forming a coating layer, oxygen diffuses and infiltrates through SiO
2
into the sintered body of silicon nitride which is a substrate and, besides, the SiO
2
phase on the grain boundaries is gasified upon reacting with water. As a result, the coating layer becomes porous permitting the substrate to be easily corroded. In forming the coating layer, further, even if the ratio of SiO
2
and an oxide of a rare earth element is so set that a predetermined crystalline phase will precipitate, the composition of the coating layer changes through the high-temperature firing, precipitating, for example, trace amounts of a nitrogen-containing crystalline phase such as YAM or apatite in addition to the predetermined crystalline phase. With the nitrogen-containing crystalline phase being oxidized, cracks develop in the coating layer and, as a result, the corrosion resistance gradually decreases.
It is therefore an object of the present invention to provide a surface-coated sintered body of silicon nitride having a coating layer which remains highly strongly adhered to the substrate (sintered body of silicon nitride) even when used in a high-temperature region of from about 800 to about 1600° C. and permits the corrosion resistance to change little.
According to the present invention, there is provided a surface-coated sintered body of silicon nitride constituted by a sintered substrate of silicon nitride and a coating layer formed on the surfaces of said sintered substrate, wherein said coating layer comprises a crystalline phase of RE
2
Si
2
O
7
and/or RE
2
SiO
5
(RE=rare earth element), said crystalline phase having an average crystalline grain diameter of not smaller than 0.1 &mgr;m, and the excess amount of SiO
2
contained in the coating layer being not larger than 10 mole %.
That is, the present invention is based on a discovery that excellent properties of the coating layer can be maintained for extended periods of time upon controlling the grain diameter and amount of oxygen in the crystalline phase in the coating layer. By controlling them as described above, the inventors have succeeded in maintaining a large adhesive force of the coating layer even in a high-temperature zone of from about 800 to about 1600° C. and in effectively avoiding a change (deterioration) in the corrosion resistance of the coating layer with the passage of time.
In the present invention, for example, since the silicon oxide exists in a decreased amount in the grain boundaries of the crystalline phase of RE
2
Si
2
O
7
or RE
2
SiO
5
, the coating layer imparts improved corrosion resistance against water. Besides, since the crystalline phase has an average crystalline grain diameter of not smaller than 0.1 &mgr;m, mechanical properties are improved and the coating layer is prevented from being broken down.
In the above-mentioned sintered substrate of silicon nitride having the coating layer on the surfaces thereof, it is desired that a crystalline phase exists on the grain boundaries of the silicon nitride particles. Then, the grain boundaries of the silicon nitride particles are crystallized exhibiting strikingly improved strength at high temperatures and resistance against the creeping as compared to those of which the grain boundaries are amorphous.
It is further desired that the crystalline phase same as that of the coating layer is precipitated on the grain boundaries of the silicon nitride particles. This enhances the intimate adhesion between the coating layer and the sintered body of silicon nitride which is a substrate, and the coating layer becomes less likely to be peeled off the surface of the substrate.
It is further desired that the sintered substrate of silicon nitride contains the silicon nitride in an amount of from 70 to 99 mole %, the rare earth element (RE) in an amount of from 0.5 to 10 mole % calculated as an oxide thereof, and excess of oxygen at a molar ratio expressed by the following formula,
SiO
2
/RE
2
O
3
wherein SiO
2
is an amount (mols) of excess oxygen calculated as SiO
2
, and
RE
2
O
3
is an amou

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