Sinterable powder for making a dense slip casted pressureless sintered SiC based ceramic product

Abstract

A SiC based sinterable powder mixture comprising, by dried weight of said powder: a) a mineral content comprising—silicon carbide (SiC) particles, —mineral boron compound particles, the powder comprising at least 50% by weight of SiC and the total mineral content of the powder being at least 90% by weight, b) at least a water insoluble carbon-containing source, in particular a carbon containing resin, the powder comprising at least 1% by weight, and preferably less than 10% by weight, of said water insoluble carbon-containing source, wherein the average particle size of said sinterable powder is comprised between 0.5 to 2.0 micrometers.

Claims

1. A co-milled SiC based sinterable powder comprising, by dried weight of said powder: a) a mineral content comprising silicon carbide (SiC) particles, mineral boron compound particles, the powder comprising at least 50% by weight of SiC and the total mineral content of the powder being at least 90% by weight, b) at least a water insoluble carbon-containing source, the powder comprising at least 1% by weight, of said water insoluble carbon-containing source, wherein the silicon carbide particles, the mineral boron compound particles, and the carbon-containing source are co-milled such that the average particle size of said co-milled SiC based sinterable powder is between 0.7 to 1.5 micrometers.

2. The SiC based sinterable powder according to claim 1, wherein said water insoluble carbon-containing source is a water insoluble carbon-containing resin.

3. The sinterable powder according to claim 1, wherein the insoluble resin is chosen among the group of resins which melt at a temperature higher than 80° C.

4. The sinterable powder according to claim 1, wherein the resin exhibits a carbon content which is higher than 30% by weight.

5. The sinterable powder according to claim 1, wherein the insoluble resin is chosen from among the group of phenolic resins.

6. The sinterable powder according to claim 2, comprising an additional elemental carbon source, so that carbon free content is between 1% and 10% by weight based upon SiC content.

7. The sinterable powder according to claim 1, wherein said water insoluble carbon-containing source consists essentially of at least one of amorphous carbon, crystallized carbon, and graphite oxide.

8. The sinterable powder according to claim 1, wherein the mineral boron compound is boron carbide B.sub.4C.

9. The sinterable powder according to claim 1, comprising an acrylic polymer suspension, in a proportion less than 8% by weight of the co-milled powder of SiC grains and mineral compound of boron.

10. The sinterable powder according to claim 1, wherein the powder comprises at least 60% by weight of SiC.

11. The sinterable powder according to claim 1, wherein the total mineral content of the powder is at least 95% by weight.

12. The sinterable powder according to claim 1, wherein the powder comprises at least 2% and less than 8%, by weight, of said water insoluble carbon-containing source.

13. The sinterable powder according to claim 1, wherein the powder comprises between 0.5 and 5.0% by weight of the boron compound.

14. A slurry comprising the SiC-based powder according to claim 1 and water.

15. A manufacturing method of a sinterable SiC-based powder according to claim 1, comprising at least the following steps: co-milling an aqueous mixture of a mineral powders of SiC and of mineral compound of boron, before or during the co-milling, adding a water insoluble carbon containing source, so that the average particle size of the resulting mixture is comprised between 0.7 to 1.5 micrometers.

16. The manufacturing method according to claim 15, wherein after co-milling, the content of the mixture SiC and mineral Boron compound, is higher than 80% of dried weight the sinterable SiC based powder.

17. The manufacturing method according to claim 15, wherein the co-milling of SiC and mineral boron is operated with the carbon containing source, until the d50 value of the resulting powder including the particles of carbon containing source, is between 0.5 and 2 micrometers.

18. The manufacturing method of a SiC pressureless sintered body comprising the following steps: preparation of an aqueous slurry according to claim 14, the solid loading of which being higher than 65% by weight, slip casting to obtain a shaped body drying of the shaped body optionally curing step under air at a temperature higher than 100° C. and below 300° C. pressureless sintering of the above body, under a sintering temperature of between 2000 and 2400° C.

19. The manufacturing method according to claim 15, further comprising, after the co-milling, drying the resulting mixture to obtain a co-milled powder having less than 1% moisture by weight.

20. A silicon carbide based sintered product obtainable by the pressureless sintering of a slip casted body manufactured by the method of claim 18, characterized in that the relative density of the sintered product is higher than 95%.

Description

EXAMPLE 1

According to the Invention

(1) A raw batch of a mixture of 94% by weight of a silicon carbide powder having a D.sub.50 of 10 microns, 1% by weight of a boron carbide powder having a D.sub.50 of 3 microns, and 5% by weight of water-insoluble Novolak resin grains having an average particle size of 20 micrometers is milled in a conventional attrition mill in a water-based slurry. The average diameter D.sub.50 of both starting powders is about 4 micrometers. The co-milling process is continued three days, so that the average particle size D50 of the particles mixture, as measured by the mastersizer 2000 laser diffraction particle size analyzer, is decreased to about 1 micrometer, with a D10 of about 2 micrometer and a D90 of about 0.5 micrometer.

(2) A dried powder with a moisture content of less than 1% by weight is obtained from the resulting composition by spray drying using an atomizer apparatus.

(3) 5% of an acrylic emulsion is added to the dried powder and water to achieve a solid content of 75 percent solids relative to the dry matter. Such slurry is rolled during 24 h to homogenize the powder mixture. Rubber coated steel balls are used as mixing aid. The slip is adjusted to a pH of 8-8.5 by adding a Sodium Hydroxide solution before casting. The viscosity of the obtained slurry is about 200 mPa.Math.s (at 40 s.sup.−1 and at 20° C.)

(4) Three Burner nozzles were shaped having a tronconical shape with the following dimensions: bottom large internal diameter 10 cm, top small internal diameter 4 cm, bottom large external diameter 11 cm, top small external diameter 4 cm, total height 30 cm. The pieces are shaped by casting the slip in plaster moulds. The pieces after air drying during 12 hours are demolded, placed in an oven at 110° C. The green density is 2.0 g/cm3. The pieces are then fired at 2110° C. under Argon during 4 hours.

(5) The apparent density of the pieces was measured according to ISO 5017 standard. Theoretical density is calculated from the composition of the sintered body. In this case theoretical density is 3.21. Relative density is the ratio of the apparent density vs the theoretical density. Results are gathered in the tables 1 and 2 below.

EXAMPLE 2

According to the Invention

(6) The same proceeding as example 1 according to the invention was followed but the water insoluble resin was added after co-milling and with an average grain size of about 1.3 micrometers.

EXAMPLE 3

According to the Invention

(7) The same proceeding as example 1 of the invention was followed but 40% by weight of the carbon containing resin was replaced by the carbon black.

EXAMPLE 4

According to the Invention

(8) The same proceeding as comparative example 5 but with the following changes: a dispersant consisting of modified low Na methylmethacrylate under a liquid form comprising about 40 wt % of active content is added to water at an amount of 0.5 wt % with respect to the raw batch of a mixture. the raw batch mixture is added progressively to the solution with above dispersant and water. Initial PH at about 7.0 is progressively adjusted to 9.1 by adding a dispersant consisting of 2-amino-2 methyl propanol (instead of sodium hydroxide) during raw batch mixture addition.

COMPARATIVE EXAMPLES 1 and 2

(9) The same proceeding was followed but in comparative example 1 and in comparative example 2 but the milling time was respectively increased by 50% and decreased by 30%. The resulting final powder mixture has then an average particle size of 0.5 micrometer for comparative example 1 and 2 micrometers for comparative example 1.

COMPARATIVE EXAMPLE 3

(10) The same proceeding as example 1 according to the invention was followed but the resin was added with water after co-milling and was water soluble. The water soluble Resolic phenolic resin is Bakelite® PF0435 FW01 supplied by Hexion.

COMPARATIVE EXAMPLE 4

(11) The same proceeding as example 2 according to the invention was followed but the water insoluble resin was added after co-milling and has an average grain size of about 20 micrometers.

COMPARATIVE EXAMPLE 5

(12) The same proceeding as example 1 according to the invention was followed but the water insoluble resin was totally replaced by a Carbon black powder at a level of 2% to achieve the same level of carbon addition.

(13) Raw materials, essential technical processing parameters and characterization results are shown in the table 1 and 2 below.

(14) Examples 1 and 2 according to the present invention compared to the comparative examples 1, 2, 3 and 4 show that appropriate selection of the size of the particles SiC, boron mineral compound and resin lead to the obtaining of a slip with high solid loading and then relative density higher than 95%.

(15) Example 3 of the present invention shows that is possible to use a source of carbon like carbon black in addition to the resin grains, provided that the average particle size remains within the claimed range, while keeping high solid loading and high relative density.

(16) The comparison of invention example 3 with comparative example 5 show that the replacement of all the insoluble resin powder by a carbon black powder leads to an equivalent solid loading but a smaller sintering final density, lower than 95%. However, as shown by example 4 according to the invention, by adjusting the dispersant system with a low molecular weight polymer with low sodium content in the slip casting composition to minimize the steric effect as described above in the description, it remains possible to replace all the insoluble resin powder by carbon black powder to achieve a higher relative density.

(17) TABLE-US-00001 TABLEAU 1 Invention invention Invention Invention example 1 example 2 example 3 example 4 Process All compounds SiC + boron Part of the resin All the resin co-milled to 1 compound co- replaced by replaced by micron milled and carbon black carbon black further addition of resin SiC D50 = 10 micron D50 = 10 micron D50 = 10 micron D50 = 10 micron Boron compound B4C powder B4C powder B4C powder B4C powder d50 = 3 micron d50 = 3 micron d50 = 3 micron D50 = 3 micron Carbon- Novolak PF0235 Novolak Novolak 20 micron Carbon black containing DP PF7219 60/40% carbon particles compound solid powder suspension 32% black particles d50 = 20 micron solid content added before co- added before co- d50 = 1.3 milling milling microns added after co- milling Co milling of the 3 days milling time milling time same milling time mixture same as as invention same as invention example 1 invention example 1 example 1 Size after co- SiC + B4C + Novolak d50 = 1.0 micron SiC + B4C + Novolak SiC + B4C + Carbon milling resin resin + Carbon Black average average particle Black average particle size size d50 = 1.0 particle size d50 = 0.79 micron micron d50 = 1.1 micron Drying Spray-drying Mixing of slip for 5% Acrylic 5% Acrylic 5% Acrylic 5% Acrylic slip casting suspension + suspension + suspension + suspension + water + water + water + water + 24 hour rolling 24 hour rolling 24 hour rolling dispersants* + 24 hour rolling Solid loading- 75 wt % 75 wt % 75 wt % 76 wt % relative to water pH/viscosity 8.5/ 9.1/ of slip 0.2 Pa .Math. s at 40 s.sup.−1 0.26 Pa .Math. s 40 s.sup.−1 shaping Slip casting Firing shrinkage 18% linear 19% linear 18% linear 18% linear apparent Density 3.14 g/cm.sup.3 or 98% 3.12 g/cm.sup.3 or 3.09 g/cm.sup.3 or 96% 3.12 g/cm.sup.3 or g/cm3 after of theoretical 97% of of Theoretical 97% of sintering and (3.21)) Theoretical (3.21) Theoretical relative density (3.21) (3.21) *low Namethylmetacrylate + 2-amino-2 methyl propanol

(18) TABLE-US-00002 TABLEAU 2 Comparative Comparative Comparative Comparative Comparative example 1 example 2 example 3 example 4 example 5 Process Same as Same as Water- Coarse resin Co-milled to 1 example 1 but example 1 soluble resin grains added micron co-milled to but co- used instead after co- but only with 0.4 micron milled to of Novolak milling carbon black 2.1 microns SiC D.sub.50 = 10 μm D.sub.50 = 10 μm D.sub.50 = 10 μm D.sub.50 = 10 μm D.sub.50 = 10 μm Boron compound B4C powder B4C B4C powder B4C powder B4C powder D.sub.50 = 3 μm powder D.sub.50 = 3 μm D.sub.50 = 3 μm D.sub.50 = 3 μm D.sub.50 = 3 μm Carbon-containing Novolak Novolak Water Novolak Only carbon compound 20 micron 20 micron soluble resin 20 micron black particles added before added added after added after added before co-milling before co- co-milling co-milling co-milling milling Co milling of the +50% more −30% milling time milling time milling time mixture duration vs duration vs same as same as same as example 1 example 1 invention invention invention example 1 example 1 example 1 Size after co- SiC + B4C + SiC + B4C + SiC + B4C SiC + B4C SiC + B4C + milling Novolak Novolak d50 = 0.9 μm d50 = 1.1 μm Carbon Black d50 = 0.4 μm d50 = 2.1 μm d50 = 1.0 μm Drying Spray-drying Mixing of slip for 5% Acrylic 5% Acrylic 5% water 5% Acrylic 5% Acrylic slip casting suspension + suspension + soluble resin + suspension + suspension + water + water + 5% Acrylics + water + water + 24 hour 24 hour water + 24 hour 24 hour rolling rolling 24 hour rolling rolling rolling Solid loading- 64 wt % 78 wt % 61.3 wt % 75 wt % 75 wt % relative to water pH/viscosity of slip 8.5/ 0.2 Pa .Math. s at 40 s.sup.−1 shaping Slip casting Firing shrinkage 22% linear 12% linear 20% linear 18% linear 17% linear apparent Density 3.04 g/cm3 2.90 g/cm3 2.90 g/cm3 2.96 g/cm3 3.03 g/cm3 or g/cm3 after below 95% or 90% of or 90% of or 92% of 94% of sintering density of Theoretical Theoretical Theoretical Theoretical Theoretical (3.21) (3.21) (3.21) (3.21) (3.21)