Title of Invention

"A PROCESS FOR MAKING SILICON CARBIDE - MULLITE COMPOSITE AND ITS USE COMPONENTS"

Abstract The present invention particularly relates to the development of a process for making consolidated and sintered silicon carbide - mullite composite with a mullite rich coating on the internal and external surfaces of the article produced. The main usages of the silicon carbide - mullite composite with a mullite rich coating on the internal and external surfaces of the article produced is in the field of machines with static and moving parts where the atmosphere is oxidizing or neutral and where the operation takes places at room temperature, below room temperature and elevated temperature upto 1800°C. In the process purification of starting Si C powder is not required. No aluminum powder is required. Firing in inert atmosphere is totally eliminated.
Full Text The present invention relates to a process for making silicon carbide-mullite composite useful for making industrial components and a process for making components thereof.
The present invention particularly relates to the development of a process for making consolidated and sintered silicon carbide - mullite composite with a mullite rich coating on the internal and external surfaces of the article produced.
The main usages of the silicon carbide - mullite composite with a mullite rich coating on the internal and external surfaces of the articje produced is in the field of machines with static and mo\ing parts where the atmosphere is oxidising or neutral and where the operation takes places at room temperature, below room temperature and elevated temperature upto 1800°C, The component may act as component of a machine, as nozzle or in any shape as may be deemed fit. The said article may also be used as special refractory' material where thermal shock, abrasion, erosion, oxidative corrosion etc. are to be countered at room temperature, below room temperature and at elevated temperature uptc 1800°C.
The present day methods of making silicon carbide - mullite composite sintered material essentially consists of using a mixture of aluminium powder, alumina powder and silicon carbide powder as starting material. The powders are mixed by attrition milling in acetone medium and then sieved to improve homogeneity. The powder is then isostaticalry pressed at. room temperature and fired in box type furnace to complete various reactions at 1100-1150°C in air. This product is then post HIPPED in argon at 1550°C and 200 Mpa for
20 minutes . Reference may be made to Reaction Bonding and Mechanical Properties of muffite/sflicon carbide composites - Suxing Wu and Nils Claussen, J. Am. Ceram. Soc., 77 (11) 2898 - 904 (1994). where the product formed contained lesser amount of silicon carbide after it was consumed in large proportion by the reaction with aluminium in the system. Hipping is also a non - commercial method of production for all practical purposes. Similar was the case as referred to in "Grain size effect on creep Deformation of Alumina -silicon carbide composites - Hua - Tay Lin, Kathleen B. Alexander and Paul F. Becher, J. Am. Ceram. Soc. 79 (6) 1530 -36 (1996)", where silicon carbide based composite materials like SiC/ZrO2, SiC/Al2O3 etc are developed by HIPPING at 1650 - 1850°C for 2 hr in Ar , Reference also be made to " SiC - Whisker - Reinforced A12O3 composites by free smtering of coated powder" Hu and Mohamed N. Rahaman. J Am. Ceram. Soc. 76 (10) 2549 - 54 (1993) where high purity helium atmosphere was used for sintering. The main drawbacks of all the above processes are:
1. Silicon carbide powder should be very fine, more than 90 percent in the submicron
range.
2. Silicon carbide powder should be ultra pure.
3. Aluminium powder is required in some process which is costly and require special
protective handling of the material.
4. Afixing process require costly equipment like attrition milling with costly solvent like
acetone as milling medium.
5. Firing in inert atmosphere is a necessary condition which require special type of
expensive furnaces with inert gases like argon and helium.

6. Many of the processes require hot isostatic pressing which is not only a cost inhibitive
process but also have several limitations.
7. Xo processes described so far produces any fired item which is coated by a mullite rich
phase which imparts additional oxidation resistance to the fired components.
The main object of the present invention is to provide a process for making silicon carbide - muliite composite useful for making industrial components with a mullite rich coating on the internal and external surfaces of the article produced which obviates the drawbacks as detailed above.
Another object is to provide a process for making industrial components using the composite of the present invention.
Yet another object of the present invention is to eliminate the process of grinding silicon carbide particles to above 10 m2/g to impart sinterability to the material.
Still another object of the present invention is to eliminate the process of purification of the silicon carbide particles to a ultra pure material to impart sinterability to the material.
Yet another object of the present invention is to eliminate the process of firing the preiired (hither to referred to as green product) in argon, helium or nitrogen environment.
Another object of the present invention is to reduce the firing temperature of the green product.
Still another object of the present invention is to form a mullite-rich material coat on the external and internal surfaces of the material in a single firing.
Yet another object of the present invention is to increase the oxidation resistance of the component produced.
Still another object of the present invention is to provide a process to make consolidated mass of silicon carbide - mullite composites as ceramic engineering material.
Yet another object of the present invention is to provide a process to make consolidated mass of silicon carbide - mullite composites as refractory material with improved quality.
Yet another object of the present invention is to pimide a process to make consolidated mass of silicon carbide - nmllite composites as engineering ceramic material to be used upto 1800°C.
Still another object of the present invention is to provide a process to make consolidated mass of silicon carbide - mullite composites as refractory material to be used upto 1800°C.
Another object is to provide a process for making components using the composite prepared by the process of the present invention.
Accordingly the present invention provides a process for making silicon carbide -mullite composite and its components which comprises,
a) preparing silicon carbide suspension by mixing silicon carbide powder and water
in the ratio 1:2 to 1:6 (g/v),
b) preparing aluminum nitrate solution by mixing aluminum nitrate in water in the
ratio 1:4 to 1:6 (g/v),
c) mixing silicon carbide suspension and aluminiumnitrate solution in the ratio of
1:165 to 1:465 (w/v), stirring for 3 to 5 hours,
d) adding ammonia to the mixture under stirring till the pH is in the range of 10-5 to
11-5 to obtain gel like mass,
e) allowing the gel to age for a period of 8 to 48 hours,
f) washing the aged gel like mass and filtering,
g) drying the washed material at a temperature in the range of 100 ± 5 C for a
period of 6 to 48 hours,
h) heat treating the dried gel like mass at a temperature in the range of 800 to 850°C
for 2 to 4 hours,
i) grinding and sieving to get the composite powder, j) the said powder is formed into the components by cold isostatic pressing as
herein described.
In another embodiment the process for making components using the composite powder is isostatically cold pressing in a mould of desired shape and firing in an air environment at a temperature in the range of 1600 to 1800°C for a period of 1 to 5 hours.
The details of the process of the present invention are given below:
a) Silicon carbide powder is mixed with dispersing agents and water (SiC: water =
1:2 to 1:6 g/v) by constant stirring to prepare a suspension.
b) Aluminum nitrate solution (Aluminum nitrate: water =1:4 to 1:6 g/v) is added in
the silicon carbide slip prepared in step 'a'.
c) The entire mixture is stirred for 3 - 5 hrs.
d) Ammonium hydroxide solution (1:1, 1:1.5, 1:2. 1:2.5, 1:3 vol%) is slowly added or
ammonia gas is passed in the mixture produced by step 'a' - 'c' till pH is 10.5-11.5.
e) The entire mass converts to a semisolid gel like mass which is allowed to age for 8-48
hours.
f) Product formed in step V is filtered and washed by warm water.
g) The washed material is dried at 110 + 5 C for 6 to 48 hrs.
h) The dried material obtained in step 'g' is heat treated at 800 - 850°C for 2 - 4 hrs. in air environment.
I) The heat treated product is ground and sieved through 100 mesh.
j) The sieved powder is formed into green shape by cold isostatic pressing.
k) The green pressed product is fired in air environment at 1600 - 1800°C for 1 - 5 hrs.
The process of the present invention can be used to produce consolidated silicon carbide muffile composite of various green shapes and sizes required for application as engineering and refractory material. Silicon carbide particles are always associated with a surface layer of silica which froms highly reactive silicic acid in the aqua suspension stage. Aluminium nitrate mixed throughly with the silicon carbide suspension, will be homogeneously distributed throughout the entire suspending mass. Ammonia solution will convert aluminium nitrate into aluminium hydroxide gel like mass that will remain intimately in contact with silicic acid perhaps forming alumino silicate hydrogel rich in aluminium. On dning the gel like mass will form a uniform layer of alumino silicate gel over the surface of the silicon carbide grains. Due to extremely high reactivity, this will form mullite and reactive alumina. This reactive alumina will combine with freshly formed silica during firing again forming mullite as the silica alumina system is in the alumina rich region. Since the reaction with oxygen in the air is predominant at the surface of the green compacts, more mullite will form at the surface of the component than inside and eventually the entire compact will be coated with muliite rich layer that blocks the path of further oxygen penetration in the compact system.
The following examples are given by way of illustration of the process of the present invention and should not be construed to limit the scope of the present invention.
EXAMPLE -1
Silicon carbide of 1200 grit 65 gms is mixed with 1087 ml of 25% solution of aluminium nitrate and stirred for 3 hrs. 652 ml of ammonium hydroxide (1:1 v/v ) solution is slowly added into the mixture. The gel like mass that forms is aged for 24 hrs. followed by filtration and washing with warm water. The entire mass is dried at 110°C for 48 hrs. followed by heat treatment in air environment at 850°C for 2 hrs. The heat treated material is ground to pass 100 mesh sieve. Articles of desired shapes are formed from the powder by isostatic pressing at room temperature at 200 Mpa pressure. The compacted articles are fired at 1600°C for 2 hrs. in air environment.
EXAMPLE - 2
Silicon carbide of 1200 grit 60 gms is mixed with 1242 ml of 25% solution of aluminium nitrais and stirred for 3 hrs. 745 ml of ammonium hydroxide solution (1:1 v/v ) is slowly added into the mixture. The gel like mass that forms is aged for 24 hrs. followed by filtration and washing with warm water. The entire mass is dried at 110°C for 48 hrs. followed by heat treatment in air environment at 850°C for 2 hrs. The heat treated material is ground to pass 100 mesh sieve. Articles of desired shapes are formed from the powder by iiostatic pressing at room temperature at 200 Mpa pressure. The compacted articles are tired at 1600°C for 2 hrs. in air environment.
EXAMPLE -3
Silicon carbide of 1200 grit 55 gms is mixed with 1398 ml of 25% solution of aluminium nitrate and stirred for 3 hrs. 839 ml of ammonium hydroxide solution (1:1 v/v ) is slowly added into the mixture. The gel like mass that forms is aged for 24 hrs. followed by filtration and washing with warm water. The entire mass is dried at 110°C for 48 hrs. followed by heat treatment in air environment at 850°C for 2 hrs. The heat treated material is ground to pass 100 mesh sieve. Articles of desired shapes are formed from the powder by isostatic pressing at room temperature at 200 Mpa pressure. The compacted articles are fired at 1600°C for 2 hrs. in air environment.
EXAMPLE - 4
Silicon carbide of 1200 grit 50 gms is mixed with 1553 ml of 25% solution of aluminium nitrate and stirred for 3 lirs. 932 ml of ammonium hydroxide solution (1:1 v/v ) is slowly-added into the mixture. The gel like mass that forms is aged for 24 hrs. followed by filtration and washing with warm water. The entire mass is dried at 110°C for 48 hrs. followed by heat treatment in air emironment at 850°C for 2 hrs. The heat treated material is ground to pass 100 mesh sieve. .Articles of desired shapes are formed from (he powder by isostatic pressing at room temperature at 200 Mpa pressure. The compacted articles are
fired at 1600°C for 2 hrs in air emironment.
EXAMPLE - 5
Silicon carbide of 1200 grit 45 gms is mixed with 1708 ml of 2596 solution of aluminium nitrate and stirred for 3 hrs. 1025 ml of ammonium hydroxide solution (1:1 v/v ) is slowly added into the mixture. The gel like mass that forms is aged for 24 hrs. followed by filtration and washing with warm water. The entire mass is dried at 110°C for 48 hrs. followed by heat treatment in air environment at 850°C for 2 hrs. The heat treated material is ground to pass 100 mesh sieve. Articles of desired shapes are formed from the powder by isostatic pressing at room temperature at 200 Mpa pressure. The compacted articles are fired at 1600°C for 2 hrs. in air environment.
EXAMPLE - 6
Silicon carbide of 1200 grit 40 gms is mixed with 1863.5 ml of 25% solution of aluminium nitrate and stirred for 3 hrs. 1118 ml of ammonium hydroxide solution (1:1 v/v ) is slowly added into the mixture. The gel like mass that forms is aged for 24 hrs. followed by filtration and washing with warm water. The entire mass is dried at 110°C for 48 hrs. followed by heat treatment in air at 850°C for 2 hrs. The heat treated material is ground to pass 100 mesh sieve. Articles of desired shapes are formed from the powder by isostatic pressing at room temperature at 200 Mpa pressure. The compacted articles are fired at 1600°C for 2 hrs. in air environment.
EXAMPLE - 7
Siikon carbide of 1200 grit 65 gms is mixed with 1087 ml of 25% solution of aluminium nitrate and stirred for 3 hrs. 652 ml of ammonium hydroxide solution (1:1 v/v ) is slowly added into the mixture. The gel like mass that forms is aged for 24 hrs. followed by filtration and washing with warm water. The entire mass is dried at 110°C for 48 hrs. followed by heat treatment in air at 850°C for 2 hrs. The heat treated material is ground to pass 100 mesh sieve. Articles of desired shapes are formed from the powder by isostatic pressing at room temperature at 200 Mpa pressure. The compacted articles are fired at 17 00°JC for 2 hrs. in air environment.
EXAMPLE - 8
Silicon carbide of 1200 grit 60 gms is mixed with 1242 ml of 25% solution of aluminium niirare and stirred for 3 hrs. 745 ml of ammonium hydroxide solution (1:1 v/v ) is slowly added into the mixture. The gel like mass that forms is aged for 24 hrs. followed by illration and washing with warm water. The entire mass is dried at 110°C for 48 hrs. followed by heat treatment in air environment at 850°C for 2 hrs. The heat treated material is ground to pass 100 mesh sieve. .Articles of desired shapes are formed from the powder by isostatic pressing at room temperature at 200 Mpa pressure. The compacted articles are fired at 1700°C for 2 hrs. in air environment.
EXAMPLE - 9
Silicon carbide of 1200 grit 55 gms is mixed with 1398 ml of 25% solution of aluminium nitrate and stirred for 3 hrs. 830 ml of ammonium hydroxide solution (1:1 v/v ) is slowly added into the mixture. The gel like mass that forms is aged for 24 hrs. followed by filtration and washing with warm water. The entire mass is dried at 110°C for 48 hrs. followed by heat treatment in air environment at 850°C for 2 hrs. The heat treated material is ground to pass 100 mesh sieve. Articles of desired shapes are fonned from the powder by isostatic pressing at room temperature at 200 Mpa pressure. The compacted articles are fired at 1700°C for 2 hrs. in air environment.
EXAMPLE - 10
Silicon carbide of 1200 grit 50 gms is mixed with 1553 ml of 25% solution of aluminium nitrate and stirred for 3 hrs. 932 ml of ammonium hydroxide solution (1:1 v/v ) is slowly added into the mixture. The gel like mass that forms is aged for 24 hrs. followed by filtration and washing with warm water. The entire mass is dried at 110°C for 48 hrs. followed by heat treatment in air environment at 850°C for 2 hrs. The heat treated material is ground to pass 100 mesh sieve. Articles of desired shapes are formed from the powder by isostatic pressing at room temperature at 200 Mpa pressure. The compacted articles are fired at 1700°C for 2 hrs. in air environment.
EXAMPLE-11
Silicon carbide of 1200 grit 45 gms is mixed with 1708 ml of 25% solution of aluminium nitrate and stirred for 3 hrs. 1025 ml of ammonium hydroxide solution (1:1 v/v ) is slowly added into the mixture. The gel like mass that forms is aged for 24 hrs. followed by filtration and washing with warm water. The entire mass is dried at 110°C for 48 hrs. followed by heat treatment in air environment at 850°C for 2 hrs. The heat treated material is ground to pass 100 mesh sieve. Articles of desired shapes are formed from the powder by isostatic pressing at room temperature at 200 Mpa pressure. The compacted articles are fired at 1700°C for 2 hrs in air environment.
EXAMPLE - 12
Silicon carbide of 1200 grit 40 gms is mixed with 1863.5 ml of 25% solution of aluminium nitrate and stirred for 3 hrs. 1118 ml of ammonium hydroxide solution (1:1 v/v ) is slowly added into the mixture. The gel like mass that forms is aged for 24 hrs. followed by filtration and washing with warm water. The entire mass is dried at 110°C for 48 hrs. followed by heat treatment in air environment at 850°C for 2 hrs. The heat treated material is ground to pass 100 mesh sieve. Articles of desired shapes are formed from the powder by isostatic pressing at room temperature at 200 Mpa pressure. The compacted articles are tired at 1700°C for 2 hrs in air environment.
The results obtained by the process of present invention are given below by way of illustration.

(Table Removed)
The inferences drawn from the above table is that all the products formed in the process mantioned develop a coating varied from blackish white to white depending upon the silicon carbide - additives composition . Samples with white coating were found to be of low porosity.
silicon carbide - additives composition . Samples with white coating were found to be of low porosity.
The main advantages of the process of the present invention are :
1. non-requirement of stringent limitations regarding fineness of silicon carbide powder,
2. purification of starting SiC powder is not required.
3. no aluminium powder is required for this process.
4. mixing process is simpler,
5. firing in inert atmosphere is totally eliminated,
6. an insitu formed coating on the surface of SiC-mullite composite fonns which improves
the product quality.










We Claim:
1. A process for making silicon carbide - mullite composite and its components
which comprises:
a) preparing silicon carbide suspension by mixing silicon carbide powder and water
in the ratio 1:2 to 1:6 (g/v),
b) preparing aluminum nitrate solution by mixing aluminum nitrate in water in the
ratio 1:4 to 1:6 (g/v),
c) mixing silicon carbide suspension and aluminiumnitrate solution in the ratio of
1:165 to 1:465 (w/v), stirring for 3 to 5 hours.
d) adding ammonia to the mixture under stirring till the pH is in the range of 10-5 to
11 -5 to obtain gel like mass
e) allowing the gel to age for a period of 8 to 48 hours,
f) washing the aged gel like mass and filtering,
g) drying the washed material at a temperature in the range of 100 ± 5°C for a period
of 6 to 48 hours,
h) heat treating the dried gel like mass at a temperature in the range of 800 to 850°C
for 2 to 4 hours,
i) grinding and sieving to get"the composite powder, j) the said powder is formed into the components by cold isostatic pressing as
herein described.
2. A process as claimed in claim 1 wherein the silicon carbide powder used is in the
range of 600 - 1200 grit size.
3. A process as claimed in claim 1 & 2 wherein the silicon carbide powder is used in
the submicron range.
4. A process as claimed in claims 1 - 3 wherein the silicon carbide suspension is
prepared using water and dispersing agents.
5. A process as claimed in claim 1 - 4 wherein the addition of ammonium is effected
using ammonium hydroxide solution of concentration 1:1, 1:1.5, 1:2, 1:2.5, 1:3
(v/v) volume percent.
6. A process as claimed in claims 1-5 wherein the washing of the gel like mass is
done using warm water at room temperature,
7. A process as claimed in claims 1-6, wherein the heat treated mass is ground to
pass 100 mesh sieve.
8. A process as claimed in claims 1-7 wherein isostatically cold pressing is done in a
mould of desired shape and firing in an air environment at a temperature in the
range of 1600 to 1800°C for a period of 1 to 5 hours.
9. A process as claimed in claims 1-8 wherein the ground and sieved heat treated
mass is isostatically pressed to shapes at a pressure in the range of 100 - 250
Mpa.
10. A process for making silicon carbide - mullite composite and its components
substantially as herein described with reference to the examples.

Documents:

376-del-1998-abstract.pdf

376-del-1998-claims.pdf

376-del-1998-complete specification (granted).pdf

376-del-1998-correspondence-others.pdf

376-del-1998-correspondence-po.pdf

376-del-1998-description (complete).pdf

376-del-1998-form-1.pdf

376-del-1998-form-19.pdf

376-del-1998-form-2.pdf


Patent Number 194367
Indian Patent Application Number 376/DEL/1998
PG Journal Number 43/2004
Publication Date 23-Oct-2004
Grant Date 10-Feb-2006
Date of Filing 13-Feb-1998
Name of Patentee COUNCIL OF SCIENTIFIC AND INDUSTRIAL RESEARCH
Applicant Address RAFI MARG, NEW DELHI-110 001, INDIA
Inventors:
# Inventor's Name Inventor's Address
1 KAJAL KUMAR DHARGUPTA CENTRAL GLASS & CERAMIC RESERACH INSTITUTE, CALCUTTA, 700 032, INDIA
2 ARUP KUMAR SAMANTA CENTRAL GLASS & CERAMIC RESERACH INSTITUTE, CALCUTTA, 700 032, INDIA
3 SANKAR GHATAK CENTRAL GLASS & CERAMIC RESERACH INSTITUTE, CALCUTTA, 700 032, INDIA
4 CHAITANYAMOY GANGULI CENTRAL GLASS & CERAMIC RESERACH INSTITUTE, CALCUTTA, 700 032, INDIA
PCT International Classification Number C04B 35/52
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 NA