Title of Invention

A PROCESS OF MAKING SIC-AIN SINTERED PRODUCT

Abstract The invention relates to a process of making SiC-AIN sintered product. The main usage of this product is in the field of machines with static and moving parts where temperature may be upto 1800 deg C. The product is prepared without using any sintering additive, hot pressing technique and by maintaining temperature in the range of 1850-2000 deg C. In the present invention the sintering is carried out in closed environment and by applying an additional force resulting to densification at lower temperature with lower volatilisation losses.
Full Text The present invention relates to a process of making SiC-AIN sintered product.
The main usage of the SiC-AIN sintered product is in the field of machines with static and moving parts where the temperature may be upto 1800°C. The component may act as component of a machine, as nozzle or in any shape as may be deemed fit.
For the present day process of making SiC-AIN sintered product reference may be made to US Patent 3259509 (1966) "Refractory material and a method of making the same", wherein A12O3, SiO2 and C were reacted in nitrogen atmosphere to make a solid solution of SiC-AIN containing upto 50% SiC followed by sintering either by hot pressing, a nonversatile and uneconomic process or by liquid phase sintering that introduces undesirable phases in the product. Reference may also be made to US Patent 3492153 (1970) "SiC-AIN refractory composites", wherein A1N was formed in situ within a porous SiC body. The drawback being the residual porosity and non homogeneity of SiC-AIN solid solution. Reference of another process may be made to J. Am. Ceram. Soc. 65(5), 260-65 "Composition and properties of hot pressed SiC-AIN solid solutions", wherein SiC & AIN powder
mixture was hot pressed under vacuum to prepare SiC-AIN sintered composites with the inherent drawback of using a hot press for processing.
The main object of the present invention is to provide a method of making SiC-AIN sintered product which obviates the drawback as detailed above.
Another object of the present invention is to sinter the product without using any sinter able additive.
Still another object of the present invention is to eliminate the use of hot press to sinter the product.
Yet another object of the present invention is to confine the sintering temperature within 1850-2000°C.
Accordingly the present invention provides a process of making Silicon carbide-Aluminium nitride sintered product which comprises; mixing SiC 5-55% and A1N 45-95% in a non polar solvent by known methods, removing the solvent by drying to obtain a powder, forming the said dried powder into shapes by uniaxial or isostatic pressing at 150-300 MPa, drying the pressed shapes at a temperature in the range of 80 - 150°C, packing the dried pressed shapes in threaded lid graphite crucible with packing mixture containing powders of same composition as that of the pressed material, closing the graphite crucible with threaded graphite lid, characterised in that heating the pressed material with the crucible at a temperature in the range of 1850-2000°C for a time in the range of 15 minutes to 120 minutes keeping an inert environment inside the chamber,
maintaining a gas pressure in the range of 4-10 atmosphere to obtain silicon carbide-Aluminium nitride sintered product.
In an embodiment of the present invention the amount of A1N in SiC may be varied in the range of 5 to 55 wt %.
In an embodiment of the present invention the non polar solvent used may be such as acetone, hexane, benzene or mixture thereof.
In still another embodiment of the present invention mixing may be carried out by processes such as ball milling & attrition milling.
In still another embodiment of the present invention the shaping may be effected using uniaxial, isostatic pressing at 150-300 MPa.
In yet another embodiment of the present invention the inert atmosphere may be provided by gases such as Ar, NI, Helium.
In still another embodiment of the present invention the gas pressure may preferably in the range of 4-10 atmospheres.
The details of the process of the present invention are given below:
a) 5-55% of A1N powder is mixed with 95-45% of SiC powder in non polar solvents
such as acetone, hexane, and benzene.
b) After thorough mixing the nonpolar solvents is removed by drying.
c) The dried powder mixture so obtained are cold compacted by uniaxial or
isostatic pressing under a pressure of 150-300 MPa.
d) The cold compacted shapes are dried at a temperature in the range of 80°-
150°C.
e) The dried cold compacted shapes are packed in a threaded graphite
crucible under a packing mixture of composition of the compacted shapes
and the crucible closed with a threaded graphite lid.
f) Compacted shapes are heated with the crucible at a temperature in the
range of 1850-2000°C for a time in the range of 15 minutes to 120 minutes
keeping an inert environment inside the heating chamber with argon or
nitrogen maintaining a gas pressure inside the heating chamber at higher than
1 atmosphere upto 10 atmosphere.
SiC exists in two forms, p-SiC(a cubic form) and a-SiC ( a hexagonal form). Normally (3-SiC contains 96% 3C and 4% 4H. Similarly a-SiC contains 94% 6H and 6% 15R AIN, a 2H polytype reacts with SiC to convert it to hexagonal structure and then forms a 2H solid solution. The amount of transformation depends on the amount of AIN in the powder mixture and

time and temperature of heat treatment. At equilibrium following reactions takes place.
1850°C 1950°C
a-SiC → 6H, 4H+ & 2Hss → 6H, 2HSS 1
P-SiC → 3C, 6H & 2HSS →6H, 4H+ & 2Hss 2
Suffix (*) indicates small amount and HSs is 2H phase composed of SiC-AIN solid solution.
Simultaneously several reaction takes place resulting into the formation of a number of volatile phases. Those reactions depend on the nature & amount of reacting species, e.g. SiC is always associated with SiO2 formed by surface oxidation of SiC particles during exposure to air. Similarly A1N is always associated with A12O3. Volatile oxidation products from those impurities will be SiO and A12O respectively. In carbon containing environment, another set of reactions will be operative forming SiC, Al2OC, Al4O4C etc. Those series of reactions impedes solid solution formation as well as densification. In conventional processes the above difficulties are overcome by using hot-pressing and liquid phase sintering (by using sintering additives). The disadvantages of two techniques have already been discussed. By applying La Chatellier's principle, the dissociation reaction may be supressed by making two changes. Instead of keeping the system open (in flowing gas) as followed in conventional processes, here the system is closed (stagnant gas)
and an excess pressure is applied (an additional force towards backward
reactions). The resulting process leads to densification at lower temperature
with lower volatilisation losses.
The novelty of the present invention is that nia sintering additives are
required for densification. The present process also does not require hot
pressing and the sintering temperature is below 2000°C.
The inventive steps resides in using a closed environment and applying gas
pressure over the reactive system. This results in economically viable process
and a defect-free end product.
The following examples are given by way of illustration and therefore should
not be construed to limit the scope of the present invention.
Example 1
95 g of SiC is mixed with 5g. of AIN in acetone medium in an attrition mill. The mixed material is dried. The powder mixture is pressed isostatically under 150 MPa pressure to make compact shapes. The compacted shape is dried at 80°C. The compacted & dried shape is placed in a threaded graphite crucible embedded inside a powder mixture consisting of 95g of SiC & 5g of AIN per lOOg mixture. The crucible is closed by a threaded lid and placed
inside a furnace. The furnace is filled with N2 and an over-pressure of 5
atmosphere is provided. The compacted shapes were heated at 1950°C and
kept at that temperature for 30 minu
Results: %T.D = 95.32 %A.P =1.78
Example 2
75 g of SiC is mixed with 25g. of AIN in acetone medium in an attrition mill.
The mixed material is dried. The powder mixture is pressed isostatically
under 300 MPa pressure to make compact shapes. The compacted shape is
dried at 100°C. The compacted & dried shape is placed in a threaded graphite
crucible embedded inside a powder mixture consisting of 75g of SiC & 25g
of AIN per lOOg mixture. The crucible is closed by a threaded lid and placed
inside a furnace. The furnace is filled with Ar and an over-pressure of 6
atmosphere is provided. The compacted shapes were heated at 1950°C and
kept at that temperature for 60 minutes.
Results: %T.D.= 94.70 %A.P=1.48
Sample 3
85 g of SiC is mixed with 15g. of AIN in acetone medium in an attrition mill.
The mixed material is dried. The powder mixture is pressed isostatically
under 250 MPa pressure to make compact shapes. The compacted shape is
dried at 120°C. The compacted & dried shape is placed in a threaded graphite
crucible embedded inside a powder mixture consisting of 85g of SiC & 15g
of AIN per lOOg mixture. The crucible is closed by a threaded lid and placed
inside a furnace. The furnace is filled with N2 and an over-pressure of 5
atmosphere is provided. The compacted shapes were heated at 2000°C and
kept at that temperature for 40 minutes.
Results: %T.D = 96.57 %A.P.= 0.82
Example 4
80 g of SiC is mixed with 20g. of AIN in acetone medium in an attrition mill. The mixed material is dried. The powder mixture is pressed isostatically under 250 MPa pressure to make compact shapes. The compacted shape is dried at 100°C. The compacted & dried shape is placed in a threaded graphite crucible embedded inside a powder mixture consisting of 80g of SiC & 20g of AIN per lOOg mixture. The crucible is closed by a threaded lid and placed
inside a furnace. The furnace is filled with Ar and an over-pressure of 6
atmosphere is provided. The compacted shapes were heated at 2000°C and
kept at that temperature for 60 minutes.
Results: %T.D.=95.63 %A.P.=0.72
Example 5
75 g of SiC is mixed with 25g. of AIN in acetone medium in an attrition mill.
The mixed material is dried. The powder mixture is pressed isostatically
under 300 MPa pressure to make compact shapes. The compacted shape is
dried at 120°C. The compacted & dried shape is placed in a threaded graphite
crucible embedded inside a powder mixture consisting of 75g of SiC & 25g
of AIN per 100g mixture. The crucible is closed by a threaded lid and placed
inside a furnace. The furnace is filled with N2 and an over-pressure of 6
atmosphere is provided. The compacted shapes were heated at 2000°C and
kept at that temperature for 60 minutes.
Results: %T.D.= 98.13 % A.P.=0.21
Example 6
70 g of SiC is mixed with 30 g of A1N in acetone medium in an attrition mill.
The mixed material is dried. The powder mixture is pressed isostatically
under 250 MPa pressure to make compact shapes. The compacted shape is
dried at 130°C. The compacted & dried shape is placed in a threaded graphite
crucible embedded inside a powder mixture consisting of 70g of SiC & 30g
of A1N per 100g mixture. The crucible is closed by a threaded lid and placed
inside a furnace. The furnace is filled with N2 and an over-pressure of 5
atmosphere is provided. The compacted shapes were heated at 2000°C and
kept at that temperature for 60 minutes.
Results: %T.D.= 98.20 %A.P.= 0.21
Example 7
65 g of SiC is mixed with 35g. of A1N in acetone medium in an attrition mill. The mixed material is dried. The powder mixture is pressed isostatically under 300 MPa pressure to make compact shapes. The compacted shape is dried at 110°C. The compacted & dried shape is placed in a threaded graphite crucible embedded inside a powder mixture consisting of 65g of SiC & 35g of A1N per 100g mixture. The crucible is closed by a threaded lid and placed
inside a furnace. The furnace is filled with N2 and an over-pressure of 7 atmosphere is provided. The compacted shapes were heated at 2000°C and kept at that temperature for 60 minutes.

Results: %T.D.=3.20

%A.P.=0.00

Results of various experiments are summarised in table I (as referred in
the examples):
Table I

(Table Removed)
The main advantages of the present invention are:
1. No sintering additives are required for densification.
2. Hot pressing are not required.
3. Sintering temperature is 4. Evaporation loss is low.









We Claim:
1. A process of making Silicon carbide-Aluminium nitride sintered product which
comprises; mixing SiC 5-55% and A1N 45-95% in a non polar solvent by known
methods, removing the solvent by drying to obtain a powder, forming the said
dried powder into shapes by uniaxial or isostatic pressing at 150-300 MPa, drying
the pressed shapes at a temperature in the range of 80 - 150°C, packing the dried
pressed shapes in threaded lid graphite crucible with packing mixture containing
powders of same composition as that of the pressed material, closing the graphite
crucible with threaded graphite lid, characterised in that heating the pressed
material with the crucible at a temperature in the range of 1850-2000°C for a time
in the range of 15 minutes to 120 minutes keeping an inert environment inside the
chamber, maintaining a gas pressure in the range of 4-10 atmosphere to obtain
silicon carbide-Aluminium nitride sintered product.
2. A process as claimed in claim 1 wherein the non polar solvent is selected from
acetone, hexane, benzene or mixture thereof.
3. A process as claimed in claim 1 wherein mixing is carried out by processes such
as ball milling & attrition milling.
4. A process as claimed in claim 1 wherein the inert atmosphere are provided by
gases such as Ar, N2, Helium.
5. A process of making silicon carbide- Aluminium nitride sintered product
substantially as herein described with reference to the examples.
L V Gomkale) Scientist IPMD, CSIR

Documents:

155-del-2000-abstract.pdf

155-del-2000-claims.pdf

155-del-2000-correspondence-others.pdf

155-del-2000-correspondence-po.pdf

155-del-2000-description (complete).pdf

155-del-2000-form-1.pdf

155-del-2000-form-19.pdf

155-del-2000-form-2.pdf

155-del-2000-form-3.pdf


Patent Number 213229
Indian Patent Application Number 155/DEL/2000
PG Journal Number 01/2008
Publication Date 04-Jan-2008
Grant Date 24-Dec-2007
Date of Filing 25-Feb-2000
Name of Patentee COUNCIL OF SCIENTIFIC AND INDUSTRIAL RESEARCH
Applicant Address RAFI MARG NEW DEHLI -110 001,INDIA.
Inventors:
# Inventor's Name Inventor's Address
1 SANKAR GHATAK CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, CALCUTTA 700 032, INDIA.
2 SANTANU MANDAL CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, CALCUTTA 700 032, INDIA.
3 KAJAL KUMAR DHARGUPTA CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, CALCUTTA 700 032, INDIA.
PCT International Classification Number C22C 29/00
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 NA