Compositions: ceramic – Ceramic compositions – Refractory
Patent
1998-06-10
1999-12-07
Group, Karl
Compositions: ceramic
Ceramic compositions
Refractory
501 32, 501 972, 264683, C04B 35587
Patent
active
059983191
DESCRIPTION:
BRIEF SUMMARY
SPECIFICATION
On account of its excellent material properties, silicon nitride is used in many fields of engineering. However, its use for mass-production purposes involving large numbers of parts has hitherto not been possible, because Si.sub.3 N.sub.4 parts are extremely expensive to manufacture.
This is mainly due to the sintering conditions required to compact the material during the last stage of the manufacturing process. As a rule, silicon nitride is intended for applications involving high temperatures of up to 1200.degree. C. and sometimes even 1400.degree. C. This means, however, that the oxidic binding phase--typically a glass phase--used for sintering must have a transformation point T.sub.g which is above 1000.degree. C. The physical characteristic T.sub.g, which is a measure of the stability of the solidified glass mass, marks the transition temperature at which the viscosity of the glass--usually under pressure--begins to decrease.
Since the sintering step cannot be carried out until the viscosity of the glass phase is sufficiently low, a temperature in the range between 1750 and 2000.degree. C. is needed. An optimal sintering process thus requires a very high energy input. Additional important requirements include use of an inert atmosphere of very high quality, necessitating a hermetically sealed furnace, and encapsulation of the green compact in Si.sub.3 N.sub.4 powder. In many cases it is moreover necessary to use a glass phase which consists of expensive oxides such as those of the rare earth metals. Besides this, the quality of the sintered product is significantly impaired if the temperature gradient in the furnace is not kept as small as possible, and this often necessitates use of a sophisticated control means. All in all, the sintering of silicon nitride materials is thus a very cost-intensive process.
When one looks at the conditions under which different silicon nitride components are actually used, however, it turns out that in many cases maximum temperatures do not exceed 750.degree. C. and in numerous cases are even below 300.degree. C. In spite of this, parts intended for these low temperature ranges have so far also been sintered under the conditions described previously, which can sometimes only be met at great expense.
The object of the invention was thus to provide a simple and cost-effective method of producing sintered silicon nitride for low-temperature applications. The method is intended to allow the production of Si.sub.3 N.sub.4 --based material which is suitable for use at temperatures below 750.degree. C. from silicon nitride powder with a particle size of <2 .mu.m and a glass phase or the components thereof, each with a particle size of <2 .mu.m. The sintered material is obtained without requiring a high energy input or complicated technical equipment, and is accordingly suitable for economical mass production applications.
The object is established by adding 5-20 wt. % or preferably 5-15 wt. % of one or more glass phases to the silicon nitride powder, the transformation point T.sub.g of the particular glass phase being below 750.degree. C., and by sintering the glass phase(s) with the silicon nitride powder at a temperature below 1400.degree. C.
In the method of the invention the glass phases can be added as such to the silicon nitride powder. It is also possible, however, to add the components of the glass phases individually. Typical glass phase components include SiO.sub.2, Al.sub.2 O.sub.3, B.sub.2 O.sub.3, alkali metal oxides such as Li.sub.2 O, Na.sub.2 O or K.sub.2 O and alkaline earth metal oxides such as MgO and CaO. There may, of course, be other components too, provided that the transformation point T.sub.g of the glass phase does not exceed 750.degree. C.
The transformation points T.sub.g of the individual glass components (eg, SiO.sub.2, Al.sub.2 O.sub.3) may by all means be well above 1500.degree. C. What is crucial for the invention is that the transformation point T.sub.g of the phase components in their entirety does not exceed 750.degre
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patent: 5256603 (1993-10-01), Andrus et al.
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Group Karl
SKW Trostberg Aktiengesellschaft
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