Firmly adhering silicon nitride-containing release layer
The invention relates to a slip for producing a durable, firmly adhering release layer on a substrate, comprising a suspension of solid particles, wherein the solid particles comprise 67-95% by weight of silicon nitride and 5-33% by weight of an SiO 2 -based high-temperature binder and the SiO 2 -based high-temperature binder is derived from SiO 2 precursors and has been pretreated by heat treatment in a temperature range of 300-1300° C. The invention further provides shaped bodies comprising a substrate having a durable, firmly adhering release layer and also processes for producing them. The shaped bodies of the invention are suitable for use in the field of corrosive nonferrous metal melts.
1. A slip, comprising: a suspension of solid particles, wherein the solid particles comprise 67-95% by weight of silicon nitride and 5-33% by weight of an SiO 2 -based high-temperature binder having a lower sintering activity and the SiO 2 -based high-temperature binder is derived from SiO 2 precursors and has been pretreated by heat treatment in a temperature range of 300-1300° C.
2. The slip according to claim 1 , wherein the SiO 2 -based high-temperature binder is derived from organosilicon SiO 2 precursors prepared via a sol-gel process, preferably from a nanocomposite comprising organosilicon SiO 2 precursors prepared via a sol-gel process and likewise containing nanosize solid particles.
3. The slip according to claim 2 , wherein the nanosize solid particles are selected from the group consisting of SiO 2 , TiO 2 , ZrO 2 , Al 2 O 3 , AlOOH, Y 2 O 3 , CeO 2 , carbon, BN and precursors of these nanosize solid particles which are converted into these solid particles by means of the sol-gel process.
4. The slip according to claim 2 , wherein the nanosize solid particles are SiO 2 particles.
5. The slip according to claim 2 , wherein the nanosize solid particles have been modified by a surface-modifying agent which contains one or more selected from the group consisting of an anhydride group, acid amide group, amino group, SiOH group, hydrolysable radicals of silanes and a β-dicarbonyl group.
6. The slip according to claim 1 , wherein the SiO 2 -based high-temperature binder has been obtained by the sol-gel process by reaction of one or more silanes of the general formula (I):
R x SiA (4-x) (I)
where the radicals A are identical or different and are hydroxyl groups or hydrolysable groups, the radicals R are identical or different and are nonhydrolysable groups and x is 0, 1, 2 or 3, with at least 50 mol% of the silanes having x≧1, and heat treatment of the resulting reaction product in a temperature range of 300-1300° C.
7. The slip according to claim 2 , wherein the nanocomposite has been obtained by the sol-gel process by reaction of nanosize solid particles with one or more silanes of the general formula (I):
R x SiA (4-x) (I)
where the radicals A are identical or different and are hydroxyl groups or hydrolysable groups, the radicals R are identical or different and are nonhydrolysable groups and x is 0, 1, 2 or 3, with at least 50 mol % of the silanes having x≧1.
8. The slip according to claim 1 , wherein the SiO 2 -based high-temperature binder has been pretreated by heat treatment in the temperature range of 700-1150° C.
9. The slip according to claim 1 , wherein mixtures of SiO 2 -based high-temperature binders which have been pretreated at different temperatures are present.
10. The slip according to claim 1 , wherein the solid particles comprise 75-90% by weight of silicon nitride and 10-25% by weight of the SiO 2 -based high-temperature binder.
11. The slip according to claim 1 , wherein the SiO 2 -based high-temperature binder is present as granules.
12. The slip according to claim 1 , wherein the silicon nitride and the SiO 2 -based high-temperature binder are present as mixed granules.
13. The slip according to claim 8 , wherein the SiO 2 -based high-temperature binder has been pretreated by heat treatment in the temperature range of 800-1050° C.
14. The slip according to claim 10 , wherein the solid particles comprise 80-88% by weight of silicon nitride and 12-20% by weight of the SiO 2 -based high-temperature binder.
15. A shaped body which comprises a substrate having a durable, firmly adhering release layer, said shaped body obtained by a process comprising:
a) providing a substrate;
b) applying a slip according to claim 1 to the substrate; and
c) hardening the applied slip by firing at an elevated temperature to form the durable, firmly adhering release layer comprising 67-95% by weight of silicon nitride, 5-33% by weight of an SiO 2 -based high-temperature binder and a total oxygen content of 5-21% by weight.
16. The shaped body according to claim 15 , wherein the release layer comprises 75-90% by weight of silicon nitride and 10-25% by weight of the SiO 2 -based high-temperature binder.
17. The shaped body according to claim 15 , wherein the release layer has a total oxygen content of 6-18% by weight.
18. The shaped body according to claim 15 , wherein the release layer is configured as a multiple layer in which the outermost layer facing away from the substrate has a total oxygen content of not more than 21% by weight.
19. The shaped body according claim 15 , wherein the release layer is configured as a double layer comprising a first layer which is applied to the substrate and has a total oxygen content of not more than 27% by weight, and a second layer which is located on top of the first layer and has a total oxygen content of not more than 21% by weight.
20. The shaped body according to claim 15 , wherein the substrate comprises quartz, graphite, ceramic, SiO 2 (fused silica) or an iron alloy.
21. The shaped body according to claim 15 , which comprises a nonmetallic substrate, wherein the hardening of the applied slip is carried out by firing in step c) at a temperature of 700-1300° C.
22. The shaped body according to claim 15 , which comprises a metallic substrate, wherein the hardening of the applied slip is carried out by firing in step c) at a temperature of 500-1000° C.
23. The shaped body according to claim 15 , which is a melting crucible having a substrate composed of quartz, graphite, ceramic or SiO 2 (fused silica) for corrosive nonferrous metal melts.
24. The shaped body according to claim 15 , which is a riser tube having a substrate composed of SiO 2 (fused silica) or an iron alloy for Al metallurgy.
25. The shaped body according to claim 15 , wherein the release layer has a thickness of 80-3000 μm.
26. The shaped body according to claim 16 , wherein the release layer comprises 80-88% by weight of silicon nitride and 12-20% by weight of the SiO 2 -based high-temperature binder.
27. The shaped body according to claim 17 , wherein the release layer has a total oxygen content of 7-15% by weight.
28. The shaped body according to claim 18 , wherein the release layer is configured as a multiple layer in which the outermost layer facing away from the substrate has a total oxygen content of not more than 12% by weight.
29. The shaped body according claim 19 , wherein the release layer is configured as a double layer comprising a first layer which is applied to the substrate and has a total oxygen content of not more than 16% by weight, and a second layer which is located on top of the first layer and has a total oxygen content of not more than 12% by weight.
30. The shaped body according to claim 21 , which comprises a nonmetallic substrate, wherein the hardening of the applied slip is carried out by firing in step c) at a temperature of 1000-1225° C.
31. The shaped body according to claim 21 , which comprises a nonmetallic substrate, wherein the hardening of the applied slip is carried out by firing in step c) at a temperature of 1050-1200° C.
32. The shaped body according to claim 21 , which comprises a nonmetallic substrate, wherein the hardening of the applied slip is carried out by firing in step c) at a temperature of 1075-1175° C.
33. The shaped body according to claim 21 , which comprises a nonmetallic substrate, wherein the hardening of the applied slip is carried out by firing in step c) at a temperature of 1100-1150° C.
34. The shaped body according to claim 22 , which comprises a metallic substrate, wherein the hardening of the applied slip is carried out by firing in step c) at a temperature of 600-900° C.
35. The shaped body according to claim 22 , which comprises a metallic substrate, wherein the hardening of the applied slip is carried out by firing in step c) at a temperature of 700-850° C.
36. The shaped body according to claim 25 , wherein the release layer has a thickness of 150-2000 μm.
37. The shaped body according to claim 25 , wherein the release layer has a thickness of 200-500 μm.
38. A process for producing a shaped body comprising:
a) providing a substrate;
b) applying a slip according to claim 1 to the substrate; and
c) hardening of the applied slip by firing at an elevated temperature to form the durable, firmly adhering release layer comprising 67-95% by weight of silicon nitride, 5-33% by weight of an SiO 2 -based high-temperature binder and a total oxygen content of 5-21% by weight.
39. The process according to claim 38 , wherein a nonmetallic substrate is provided in step a) and the hardening of the applied slip in step c) is carried out by firing at a temperature of 700-1300° C.
40. The process according to claim 38 , wherein a metallic substrate is provided in step a) and the hardening of the applied slip in step c) is carried out by firing at a temperature of 500-1000° C.
41. The process according to claim 39 , wherein a nonmetallic substrate is provided in step a) and the hardening of the applied slip in step c) is carried out by firing at a temperature of 1000-1225° C.
42. The process according to claim 39 , wherein a nonmetallic substrate is provided in step a) and the hardening of the applied slip in step c) is carried out by firing at a temperature of 1050-1200° C.
43. The process according to claim 39 , wherein a nonmetallic substrate is provided in step a) and the hardening of the applied slip in step c) is carried out by firing at a temperature of 1075-1175° C.
44. The process according to claim 39 , wherein a nonmetallic substrate is provided in step a) and the hardening of the applied slip in step c) is carried out by firing at a temperature of 1100-1150° C.
45. The process according to claim 40 , wherein a metallic substrate is provided in step a) and the hardening of the applied slip in step c) is carried out by firing at a temperature of 600-900° C.
46. The process according to claim 40 , wherein a metallic substrate is provided in step a) and the hardening of the applied slip in step c) is carried out by firing at a temperature of 700-850° C.
47. A method of producing corrosive nonferrous metal melts comprising melting a corrosive nonferrous metal in the shaped body of claim 15 .