Electric heating – Heating devices – Resistive element: igniter type
Reexamination Certificate
2001-12-10
2003-09-16
Walberg, Teresa (Department: 3742)
Electric heating
Heating devices
Resistive element: igniter type
C051S307000, C501S153000
Reexamination Certificate
active
06621052
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a silicon nitride—tungsten carbide composite sintered material and process for preparing the same, and more particularly to a silicon nitride—tungsten carbide composite sintered material which is endowed with high strength, can attain reduced electrical resistance, exhibits reliable characteristics under wide ranges of firing conditions, and which avoids generation of tungsten silicide, which is a brittle substance. The present invention also relates to a glow plug employing the silicon nitride—tungsten carbide composite sintered material as a ceramic heating resistor.
2. Description of the Related Art
Conventionally, silicon nitride—tungsten carbide composite sintered material is used as a wear-resistant member such as a bearing ball or as a material for a heater of a glow plug, since the sintered material exhibits excellent wear resistance, and electrical resistance of the sintered material can be controlled easily. A typical approach to attain such intended characteristics of the sintered material is to regulate the amount of tungsten carbide to be incorporated into the sintered material. In order to meet the recent demand for enhancement of wear resistance and reduction of electrical resistance for attaining power savings, in some cases, the ratio of tungsten incorporated into the silicon nitride—tungsten carbide composite sintered material must be increased.
However, when the incorporation amount of tungsten carbide is increased in order to attain enhancement of wear resistance and reduction of electrical resistance with an aim toward power savings, sinterability of raw materials may be lowered significantly. As a result, an effective firing temperature range within which reliable characteristics are obtained is narrowed, and due to the non-uniformity of temperature within the firing furnace or changes in furnace temperature over the course of time caused by impairment of consumable parts of the furnace, reliable characteristics cannot be obtained. Meanwhile, conversion of tungsten carbide in sintered material into tungsten silicide, which is a brittle substance, through reaction between the tungsten carbide and silicon nitride causes deterioration of mechanical characteristics and electrical characteristics of the sintered material. This conversion tends to occur particularly in the vicinity of a firing jig formed from graphite. Thus, hitherto, there remains a need for a silicon nitride—tungsten carbide composite sintered material which exhibits excellent wear resistance, attains reduction of electrical resistance, realizes enhancement of sinterability, provides a wide effective firing temperature range within which reliable characteristics are obtained, and is endowed with high strength.
SUMMARY OF THE INVENTION
In view of the foregoing, an object of the present invention is to provide a silicon nitride—tungsten carbide composite sintered material which is endowed with high strength, can attain reduction of electrical resistance, exhibits reliable characteristics under wide ranges of firing conditions, and prevents generation of tungsten silicide, which is a brittle substance.
The present inventors have studied the relationship between components of a silicon nitride—tungsten carbide composite sintered material and characteristics of the sintered material, and have found that, when the amounts of rare earth elements as reduced to certain corresponding oxides thereof, the elements serving as sintering aid components, are regulated, and when the amount of excess oxygen as reduced to silicon dioxide is regulated, the sintered material exhibits high strength, electrical resistance can be reduced, an effective firing temperature range within which reliable characteristics are obtained can be widened, and conversion of tungsten carbide into tungsten silicide can be prevented. The present invention has been accomplished on the basis of this finding.
The present invention provides a silicon nitride—tungsten carbide composite sintered material comprising silicon nitride and tungsten carbide, characterized in that the total in amounts of an entirety of rare earth elements as reduced to certain corresponding oxides thereof, the elements being contained in the sintered material, and excess oxygen as reduced to silicon dioxide is 6-20 mass %; and the ratio, on a mol basis, of (the amounts of rare earth elements as reduced to the certain corresponding oxides thereof)/(the amounts of the rare earth elements as reduced to the corresponding oxides thereof+the amount of excess oxygen as reduced to silicon dioxide) is 0.3-0.7.
The present invention also provides a process for preparing the above-described silicon nitride—tungsten carbide composite sintered material, which comprises sintering a mixture of silicon nitride powder, an oxide of a rare earth element and silicon dioxide powder, the composite sintered material having a flexural strength of at least 800 MPa obtainable by sintering at an effective firing temperature which encompasses a range of at least 100° C.
The present invention also provides a glow plug having a ceramic heater comprising the above-described silicon nitride—tungsten carbide composite sintered material.
In the silicon nitride—tungsten carbide composite sintered material of the present invention, a crystalline phase is present in an intergrain region of the sintered material. In the silicon nitride—tungsten carbide composite sintered material of the present invention, an effective firing temperature range within which a flexural strength of at least 800 MPa is obtained encompasses at least 100 degrees centigrade.
In the silicon nitride—tungsten carbide composite sintered material of the present invention, the aforementioned “rare earth element” is one or more elements selected from among, for example, Y, Sc, La, Ce, Pr, Nd, Gd, Tb, Dy, Er and Yb. The aforementioned “the amounts of rare earth elements as reduced to certain corresponding oxides thereof” refers to the amounts of the rare earth elements as reduced to their oxides (i.e., RE
2
O
3
, RE: rare earth element), the elements being contained in the silicon nitride—tungsten carbide composite sintered material. In the silicon nitride—tungsten carbide composite sintered material of the present invention, the aforementioned “excess oxygen” refers to oxygen contained in silicon nitride and oxygen in silicon dioxide that is added from the outside. The aforementioned “the amount of excess oxygen as reduced to silicon dioxide” refers to the amount of the above-defined “excess oxygen” as reduced to silicon dioxide. The aforementioned “the amounts of rare earth elements as reduced to certain corresponding oxides thereof” is not particularly limited, so long as the total in amounts of an entirety of rare earth elements as reduced to certain corresponding oxides thereof and excess oxygen as reduced to silicon dioxide, and the ratio, on a mol basis, between the amounts of the rare earth elements and the amount of the excess oxygen satisfy the below-described conditions. The phrase “as reduced to” used herein implies “if converted to” or “if present in the form of”, and does not necessarily imply any chemical reduction reaction.
In the silicon nitride—tungsten carbide composite sintered material of the present invention, the total of the amount of the rare earth element as reduced to the oxide thereof and the amount of the excess oxygen as reduced to silicon dioxide is usually 6-20 mass %, preferably 7-15 mass %, more preferably 7-11 mass %. When the total amount is less than 6 mass %, the amount of the sintering aid is insufficient for sintering of tungsten carbide and silicon nitride. Therefore, sinterability is lowered, and the fine structure of the sintered material tends to vary considerably with firing temperature. As a result, an effective firing temperature range within which reliable characteristics are obtained is narrowed, silicon nitride easily decomposes, and the resultant silicon component reacts with tungsten carbid
Ito Masaya
Matsubara Katsura
Watanabe Hiroki
Fastovsky L
NGK Spark Plug Co. Ltd.
Walberg Teresa
LandOfFree
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