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US 4,904,624 AGranted
Silicon nitride with improved high temperature strength
AssigneeNorton Company
InventorRussell L. Yeckley
Life of this patent
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AssigneeNorton Company
InventorRussell L. Yeckley
Estimating the term…
The flexural strength and stress rupture life of isostatically hot pressed silicon nitride containing between 1 and 12 weight percent of rare earth oxide and not more than 0.5 weight percent alumina is substantially increased by treating green bodies in flowing nitrogen at a temperature between 1000.degree. and 1500.degree. C. before degassing for the isostatic hot pressing. The iron content of the bodies is also reduced by this heat treatment, and this is believed to eliminate sources of fracture failure. Silicon nitride bodies with a flexural strength in excess of 525 MPa at 1370.degree. C. and with stress rupture lives reliably in excess of 200 hours at 300 MPa stress at 1370.degree. C. can be prepared in this way. The strain rates of silicon nitride under high temperature stress can also be reduced.
A polycrystalline ceramic material comprising at least three-fifths atomic fraction of silicon nitride, between 1 and 5 weight percent of a rare earth oxide, not more than 1 weight percent silica as determined by X-ray diffraction, and not more than 0.5 weight percent alumina as determined by chemical analysis, said ceramic material having a flexural strength of at least 525 megapascals when measured at 1370 C.
A polycrystalline ceramic material comprising at least three-fifths atomic fraction of silicon nitride and between 1 and 5 weight percent of a rare earth oxide, said ceramic material having a reliable lifetime of at least one hundred hours under a constant stress of 250 MPA at 1370 C.
A polycrystalline ceramic material comprising at least three-fifths atomic fraction of silicon nitride and between 1 and 5 weight percent of a rare earth oxide, said ceramic material having a strain rate of not more than 10.sup.-6.3 /hour when stressed under a constant stress of 250 MPA at 1370 C.
In a process comprising (a) preparing a silicon nitride green body, said green body comprising at least three fifths atomic fraction of silicon nitride, from 1 to 12 weight percent of rare earth metal oxide, and not more than 0.5% alumina, (b) degassing said green body, (c) encapsulating said green body with a flexible fluid-impermeable membrane, and (d) subjecting said fluid-impermeable membrane to uniform fluid pressure while simultaneously maintaining said green body at elevated temperature, whereby said green body is densified, the improvement wherein said green body is heated in an atmosphere of nitrogen at a temperature between 1000 C and 1500 C prior to step (c), for a time sufficient to reduce below the X-ray diffraction detection limit the content of each of the SiO.sub.2 and alpha E.sub.2 Si.sub.2 O.sub.7 phases where E represents any rare earth metal, within the body after step (d).
Dependencies are inferred from the claim text.