Claims
- 1. A method of making a reaction bonded, hot pressed silicon nitride comprising object having resistance to oxidation at high temperatures, comprising:
- (a) forming a compact from a mixture of silicon with from 1 to 3% SiO.sub.2 associated therewith, an oxynitride forming agent and from 0.4 to 5.0 %. alumina, said oxynitride forming agent and alumina being present in amounts to substantially fully react said silicon upon heating in nitrogen to convert the compact to essentially Si.sub.3 N.sub.4 and oxynitride cyrstallites, encapsulated by a glass film;
- (b) heating said compact in a nitriding atmosphere, without the use of pressure normally associated with hot pressing, to produce said converted compact, said converted compact having a size greater than and a density less than the final object to be formed; and
- (c) hot oppressing said body at a temperature equal to or below 1670.degree. C. to produce a silicon nitride comprising object of required dimension and density useful for a cutting tool and consisting essentially of the chemistry in said converted compact.
- 2. The method as in claim 1, in which said converted compact contains no greater than 0.5% by weight free silicon and unreacted agents.
- 3. The method as in claim 2, in which said converted compact comprises 0.2-1% by weight of a protective amorphous silicate serving as said glass film.
- 4. The method as in claim 1, in which said oxynitride forming agent is present in said mixture in a molar percent of silicon in the range of 0.4-2.4.
- 5. The method as in claim 1, in which said oxynitride forming agent in said mixture consists substantially of Y.sub.2 O.sub.3 in an amount comprising 3-19% by weight of said silicon, and said oxynitride crystallite in said converted compact is comprised of yttrium silicon oxynitride.
- 6. The method as in claim 1, in which said oxynitride forming agent is selected from the group consisting of oxides of rare earths.
- 7. The method as in claim 6, in which a portion of said rare earth oxide is replaced by zirconium oxide.
- 8. The method as in claim 6, in which said silicon is at least 98% pure with less than 1.5% by weight metal contaminants and less than 0.05% by weight carbon.
- 9. The method as in claim 1, in which said converted compact contains 2.9-14.4% by weight oxynitride crystallites.
- 10. The method as in claim 1, in which said glass film is 8-10 angstroms thick.
- 11. The method as in claim 1, in which the chemistry of said mixture is proportioned to form an increased amount of the Y.sub.1 SiO.sub.2 N phase crystallite in said converted compact thereby reducing microporosity associated with mixed phases or with the presence of Y.sub.10 phase, improving the diffusion coefficient of said glass film.
- 12. The method as in claim 11, in which said mixture contains a ratio of ingredients of two parts Y.sub.2 O.sub.3, one part SiO.sub.2, and one part silicon.
- 13. The method as in claim 1, in which said mixture consists of 3-19% Y.sub.2 O.sub.3, 0.4-5% Al.sub.2 O.sub.3, 1-3% SiO.sub.2, and the remainder essentially silicon.
- 14. The method as is in claim 13, in which said Y.sub.2 O.sub.3 and Al.sub.2 O.sub.3 have a purity respectively of at least 99.9% and 99.5%.
- 15. The method as in claim 1, in which the oxynitride forming agent is present in an amount to constitute a weight ratio of oxynitride forming agent to SiO.sub.2 of 1.1-6.4.
- 16. The method as in claim 1, in which the heating of step (b) is carried out to maintain a substantially constant nitrogen content in said atmosphere and thereby control the exothermic reaction of said heating.
- 17. The method as in claim 13, in which in the converted compact of step (b) the N-melilite oxynitride phase (Y.sub.2 O.sub.3.Si.sub.3 N.sub.4) is present in an amount no greater than 0.5% by weight of the body.
- 18. A method of making a reaction bonded, hot pressed silicon nitride comprising cutting tool which has increased oxidation resistance at 1000.degree. C., comprising the steps of:
- (a) forming a compact from a mixture of silicon, SiO.sub.2, Y.sub.2 O.sub.3 and a glass forming oxide selected from the class consisting of the oxides of magnesium, iron, calcium and chromium reactive with SiO.sub.2 and Y.sub.2 O.sub.3 to form upon heating a controlled amount of a protective amorphous silicate glass, said Y.sub.2 O.sub.3 being present in said mixture in an amount of 3-19% by weight of said silicon, said glass forming oxide being present in an amount of at least 0.4 and up to 1.0% by weight of said silicon, said SiO.sub.2 being present in an amount of 1-3% by weight of said silicon, and said mixture having a Y.sub.2 O.sub.3 /SiO.sub.2 ratio of 1:1-6:4;
- (b) heating said compact in a nitriding atmosphere, to product a silicon nitride comprising body consisting of Si.sub.3 N.sub.4, at least one yttrium silicon oxynitride present in an amount of 3-15% by weight of said body and dispersed throughout, up to 0.5% free silicon and residual Y.sub.2 O.sub.3, 1% or less silicate glass encapsulating said yttrium silicon oxynitride, at lease 60% of said Si.sub.3 N.sub.4 being in the alpha form, said body having a size greater than and a density less than said tool;
- (c) hot pressing said body at a temperature equal to or below 1670.degree. C. to produce a silicon nitride comprising cutting tool of required dimension and density consisting essentially of the chemistry developed previously during said heating step.
- 19. The method as in claim 18, in which said mixture is proportioned to form yttrium silicon oxynitride comprised of at least 80% by weight of Y.sub.1 SiO.sub.2 N during heating in the temperature range of 1600.degree.-1650.degree. C. for at least six hours.
- 20. The method as in claim 18, in which said hot pressing is carried out at an ultimate temperature of 1200.degree.-1650.degree. C. in an environment uncontrolled as to inertness.
- 21. The method as in claim 18, in which the average particle size for at least 50% of said mixture is about 3.0 microns.
- 22. The method as in claim 18, in which said hot pressing ultimate temperature is in the range of 2500.degree.-3000.degree. F. (1371.degree.-1650.degree. C.) and the ultimate pressure is in the range of 3600-3800 psi.
- 23. The method as in claim 22, in which said ultimate pressure and temperature is maintained for a period of 0.25-3.0 hours.
- 24. The resulting from the practice of the method of claim 18, said object being characterized by the presence of 1-7.5% by weight silicon oxynitride crystallites, a hardness of 88.5-92.0 on the 45-N scale, and a density of 3.2-3.4 gm/cm
- 25. The object as in claim 24, in which said object additionally is characterized by a fracture strength of about 90,000 psi at 1200.degree. C. in air in a 4-point bend test and an oxidation resistance that prevents weight pickup by the tool after 450 hours in air at 1000.degree. C.
- 26. The object as in claim 25, in which the object has no visual mottle microporosity.
- 27. A method of making a silicon nitride comprising body useful as a starting material in the hot pressing of objects, comprising the steps of:
- (a) forming a compact from a mixture of silicon with from 1 to 3% SiO.sub.2 associated therewith, an oxynitride forming agent, and a glass forming agent, said oxynitride and glass forming agents being present in said mixture in an amount to substantially fully react said silicon upon heating in nitrogen to convert the compact to essentially Si.sub.3 N.sub.4, oxynitride crystallites encapsulated by 0.2-1% silicate by weight, with up to 0.5% by weight free silicon and unreacted agents; and
- (b) heating the compact in a nitriding atmosphere, without the use of pressure normally associated with hot pressing, to produce said converted compact consisting of Si.sub.3 N.sub.4, at least one dispersed silicon oxynitride crystallite phase encapsulated by 0.2-1% by weight of the silicate, and up to 0.5% by weight free silicon and unreacted agents.
- 28. The method as in claim 27, in which said oxynitride forming agent consists of Y.sub.2 O.sub.3 in an amount of 3-16% by weight of the silicon, and said glass forming agent consists of Al.sub.2 O.sub.3 in an amount of 0.4-5.0% by weight of the silicon, the ratio of Y.sub.2 O.sub.3 /SiO.sub.2 in said mixture being 1:1-6.4.
- 29. The method as in claim 27, in which heating in nitrogen is carried out with a nitrogen demand cycle to an ultimate temperature of 1090.degree.-1430.degree. C. (2000.degree.-2600.degree. F.).
- 30. The body resulting from the practice of claim 27, in which said Si.sub.3 N.sub.4 has at least 60% by weight in the alpha form and said yttrium silicon oxynitride consists substantially of Y.sub.1 SiO.sub.2 N.
- 31. The body as in claim 30, characterized by a density of at least 3.25 gm/cm.sup.3.
- 32. The method as in claim 18 wherein said heating in step (b) is conducted with the use of pressure normally associated with hot pressing.
- 33. The method as in claim 18 wherein said heating in step (b) is conducted without the use of pressure normally associated with hot pressing.
- 34. A method of making a reaction bonded, hot pressed silicon nitride comprising object having resistance to oxidation at high temperatures, comprising:
- (a) forming a compact from a mixture of silicon with from 1 to 3% SiO.sub.2 associated therewith, an oxynitride forming agent, present in an amount to substantially fully react said silicon upon heating in nitrogen to convert the compact to essentially Si.sub.3 N.sub.4 and oxynitride crystallites, and a glass forming agent selected from the group consisting of MgO, Fe.sub.2 O.sub.3, CaO and Cr.sub.2 O.sub.3 in an amount from 0.4-1.0% sufficient to encapsulate said oxynitride crystallites by a glass film;
- (b) heating said compact in a nitriding atmosphere to produce said converted compact, said converted compact having a size greater than and a density less than the final object to be formed; and
- (c) hot pressing said body at a temperature equal to or below 1670.degree. C. to produce a silicon nitride comprising object of required dimension and a density useful for a cutting tool and consisting essentially of the chemistry in said converted compact.
- 35. A method of making a reaction bonded, hot pressed silicon nitride comprising cutting tool which has increased oxidation resistance at 1000.degree. C., comprising the steps of:
- (a) forming a compact from a mixture of silicon, SiO.sub.2, Y.sub.2 O.sub.3 and Al.sub.2 O.sub.3 reactive with SiO.sub.2 and Y.sub.2 O.sub.3 to form upon heating a controlled amount of a protective amorphous silicate glass, said Y.sub.2 O.sub.3 being present in said mixture in an amount of 3-19% by weight of said silicon, said Al.sub.2 O.sub.3 being present in an amount of at least 0.4 and up to 5% by weight of said silicon, said SiO.sub.2 being present in an amount of 1-3% by weight of said silicon, and said mixture having a Y.sub.2 O.sub.3 /SiO.sub.2 ratio of 1:1-6:4;
- (b) heating said compact in a nitriding atmosphere to produce a silicon nitride comprising body consisting of Si.sub.3 N.sub.4, at least one yttrium silicon oxynitride present in an amount of 3-15% by weight of said body and dispersed throughout, up to 0.5% free silicon and residual Y.sub.2 O.sub.3, 1% or less silicate glass encapsulating said yttrium silicon oxynitride, at lease 60% of said Si.sub.3 N.sub.4 being in the alpha form, said body having a size greater than and a density less than said tool;
- (c) hot pressing said body at an ultimate temperature of 1200.degree.-1650.degree. C. in an environment uncontrolled as to inertness to produce a silicon nitride comprising cutting tool of required dimension and density consisting essentially of the chemistry developed previously during said heating step.
- 36. The method as in claim 35 wherein said heating in step (b) is conducted with the use of pressure normally associated with hot pressing.
- 37. The method as in claim 35 wherein said heating in step (b) is conducted without the use of pressure normally associated with hot pressing.
- 38. The method as in claim 35, in which said mixture is proportioned to form yttrium silicon oxynitride comprised of at least 80% by weight of Y.sub.1 SiO.sub.2 N during heating in the temperature range of 1600.degree.-1650.degree. C. for at least six hours.
- 39. The method as in claim 35, in which the average particle size for at least 50% of said mixture is about 3.0 microns.
- 40. The method as in claim 35, in which said hot pressing ultimate temperature is in the range of 2500.degree.-3000.degree. F. (1371.degree.-1650.degree. C.) and the ultimate pressure is in the range of 3600-3800 psi.
- 41. The method as in claim 40, in which said ultimate pressure and temperature is maintained for a period of 0.25-3.0 hours.
- 42. The object resulting from the practice of the method of claim 36, said object being characterized by the presence of 1-7.5% by weight silicon oxynitride crystallites, a hardness of 88.5-92.0 on the 45-N scale and a density of 3.2-3.4 gm/cm.
- 43. The object as in claim 42, in which said object additionally is characterized by a fracture strength of about 90,000 psi at 1200.degree. C. in air in a 4-point bend test and an oxidation resistance that prevents weight pickup by the tool after 450 hours in air at 1000.degree. C.
- 44. The object as in claim 43, in which the object has no visual mottle microporosity.
RELATED APPLICATIONS
This is a continuation-in-part application of U.S. application Ser. No. 444,251, filed Sept. 30, 1982 and now abandoned.
US Referenced Citations (4)
Number |
Name |
Date |
Kind |
3839540 |
Arrol et al. |
Oct 1974 |
|
4351787 |
Martinengo et al. |
Sep 1982 |
|
4401617 |
Ezis et al. |
Aug 1983 |
|
4440707 |
Shimamori et al. |
Apr 1984 |
|
Continuation in Parts (1)
|
Number |
Date |
Country |
Parent |
444251 |
Sep 1982 |
|