Title of Invention

"A PROCESS FOR THE PREPARATION OF DENSE SAMARIUM STABILISED α-SIALON"

Abstract The present invention relates to a process for the preparation of dense Samarium stabilised α-SiAION from the system Si3N4 - Al2O3.AIN -Sm2O3.9AIN - Si02 resulting into dense product of the order of > 98% theoretical density with the advantages such as cost effectiveness, high hardness and high fracture toughness. The produced dense a-SiAION will be useful for low temperature applications as wear parts like bearing and roller materials and particularly for grinding and milling operations like grinding balls. 10
Full Text This invention relates to a process for the preparation of dense Samarium stabilised a-SiAION. Dense a-SiAION is useful for low temperature applications as wear parts like bearing and roller materials and particularly for grinding and milling operations like grinding balls.
The present day method consists of hot pressing the green mixtures of AIM, A^Oa and Srr^Oa in a temperature range of 1550°-1750°C under a pressure of 20 MPa for which the reference may be made to Wang et al. in Mater. Res. Soc. Symp. Proc., Vol. 287, 1993, pp. 387-392 entitled "Formation and densification of R-a' SiAIONs (R= Nd,Sm,Gd,Dy,Er,Yb)".
Reference may also be made to Shen et al. in J. Eur. Ceram. Soc., Vol. 16, No. 1, 1996, pp. 43-53 entitled "Temperature stability of samarium doped a SiAION ceramics" where hot pressing was also used as the fabrication method of the material as stated above.
Reference may still be made to O'Reilly et al. in Mater. Res. Soc. Symp. Proc., Vol. 287, 1993, pp. 393-398 entitled "Parameters affecting pressureless sintering of a' SiAIONs with lanthanide modifying cations" wherein the green mixture containing similar starting materials as above were pressureless sintered which could yield only 50% a-SiAION in the sintered product.
Reference may further be made to Mandal et al. in J. Eur. Ceram. Soc., Vol. 12, No. 6, 1993, pp. 421-429 entitled "Reversible a to p sialon transformation in heat treated sialon ceramics" wherein the green mixtures were either hot pressed or pressureless sintered at 1800°C. Although the hot pressing could produce fully sintered material, the pressureless sintering could only produce around 96% of theoretical density.
Another reference may be made to Cheng et al. in J. Eur. Ceram. Soc., Vol. 14, 1994, pp. 343- 349 entitled "Pressureless sintering and phase relationship of samarium a-sialons" wherein the single phase a-sialon material could not be sintered under pressureless sintering condition with greater than 90% densification.
The major drawback of the above processes is that these involve selection of a composition that require hot pressing for full densification which is evidently a very costly process and by which a complex- shaped material is difficult to be manufactured or that failed to produce high densification under pressure less sintering method.
The main object of the present invention is to provide a process for the preparation of dense Samarium stabilised a-SiAION which obviates the above disadvantages.
Another object of the present invention is to provide a process of making products consisting of a composition as described and claimed in our copending patent application no. 0234del2002, wherein the composition of a-SiAION in the system Si3N4 - AI2O3.AIN - Sm2O3.9AIN - SiO2 is used to obtain dense sintered a-SiAION, using Si3N4, AI2O3, AIM, SiO2 and Sm2O3 as starting materials.
Yet another object of the present invention is to provide a process wherein a cheaper additive oxide such as samarium oxide in comparison to other oxides such as Dysprosium oxide or Ytterbium oxide is required thus making the process economic.
The present invention relates to a process for the preparation of dense Samarium stabilised a-SiAION from the system Si3N4 - AI2O3.AIN -Sm2O3.9AIN - SiO2 resulting into dense product of the order of > 98% theoretical density with the advantages such as cost effectiveness, high hardness and high fracture toughness. The produced dense a-SiAION will be useful for low temperature applications as wear parts like bearing and roller materials and particularly for grinding and milling operations like grinding balls.
Accordingly, the present invention provides a process for the preparation of dense Samarium stabilised a-SiAION which comprises preparing a

homogenous mixture by conventional methods of a powdered composition essentially consisting of : 50-55 mole% Si3N4, 2.0-3.5 mole% AI2O3, 33-42 mole% AIM, 1.5-2.5 mole% SiO2 and 3-4.5 mole% Sm2O3 , passing the powder through 100 mesh, pressing the powder by conventional methods to form green compacts, sintering the green compacts at a temperature in the range of 1750° to 1900°C in nitrogen atmosphere.
In an embodiment of the present invention, Si3N4 may contain oxygen In another embodiment of the present invention, AI2O3 may be of purity >98%.
In still another embodiment of the present invention, AIM may contain oxygen In still yet another embodiment of the present invention, SiO2 may be of purity >98%
In yet another embodiment of the present invention, Sm203 may be of purity >98%.
In a further embodiment of the present invention the pressing may be effected isostatically at a pressure in the range of 65 to 350 Mpa.
The detailed steps of the process of the present invention for the preparation of dense Samarium stabilised a-SiAION from the synergistic composition as described and claimed in our copending patent application no. NF-54/02 is given below:
1. Pure and powdered of a-Si3N4, AI203, AIM, Si02 and Sm2O3 were taken as
starting materials.
2. Accurately weighed appropriate proportions of starting materials were taken
in Si3N4 pot in an attrition mill along with Si3N4 balls (size around 2 to 3 mm)
for attrition milling wherein the ball: powder ratio were kept in the range of 6:1
to 9:1, preferably around 7:1 and wherein the milling was done in a liquid medium of acetone for which the water content was 0.2%. The milling time was for a period ranging between 2 to 8 hours.
3. After milling the powder was separated from the balls through sieving and
was dried.
4. The milled powder was taken in a rubber mould and was isostatically
pressed with pressure ranging from 65 to 350 MPa.
5. The pressed green billets were taken in a graphite resistance heating
furnace and were fired in nitrogen gas atmosphere.
The sintering is found to be enhanced in a-SiAION compositions when selected from the system Si3N4 - AlzOa.AIN - Sm2O3.9AIN - SiO2. It is believed that the mechanism is as follows: In general, the sintering of the a-SiAION materials is difficult primarily due to the presence of some secondary intermediate crystalline phases. In cases of both yttrium- as well as some rare earth- doped compositions, the melilite phase, M203.Si3N4 (M= Y, Yb, Dy, Sm, Nd, etc.) often containing aluminium in solid solution, occur frequently together with a-SiAION in the intermediate sintering temperature range. The phase absorbs large amount of the doping element and becomes competitive for the volume fraction of the liquid phase present thereby hindering densification and the precipitation of a-SiAION as well. The final densification of the material therefore becomes dependent on the dissociation temperatures of the melilite which promotes the amount of the liquid phase once again at high temperature so that the sintering proceeds. The extent of the melilite phase formation is favoured when the starting composition is taken in the nitrogen rich side of the compositional zone. It may be believed that the introduction of Si02 in the starting composition disfavours the formation of the nitrogen rich crystalline phases like melilite etc. and also favours the formation of a larger amount of liquid during sintering thereby promoting an improved densification at comparatively lower temperature with respect to the compositions without Si02.
The novelty of the process of the present invention is that the product obtained using the selected compositional zone from the system SJ3N4 -
AI2O3.AIN - Sm203.9AIN - SiO2 exhibits a-SiAION as single crystalline phase with excellent sinterability and possesses a final density value of not less than 98% of theoretical in the temperature range >1750°C. The novel process provides a cost effective method of preparing dense samarium stabilised a-SiAION having features such as high hardness and high fracture toughness.
The following examples are given by way of illustration of the present invention and should not be construed to limit the scope of the invention:
Example 1
A composition containing Si3N4- 53.22 mole%, AI2O3 2.47 mole%, AIN 37.98 mole%, Si02- 2.38 mole% and Sm2O3 3.95 mole % was attrition milled for 3 h, dried, cold pressed under isostatic pressure and was fired at 1750°C for 2 h in a nitrogen gas atmosphere under a gas pressure of 1 MPa. The linear shrinkage was 15.39%, the firing weight loss was 1.66%. The fired density was 98.1% of the theoretical value.
Example 2
A composition containing Si3N4- 53.22 mole%, AI2O3 2.47 mole%, AIN 37.98 mole%, SiO2- 2.38 mole% and Sm203 3.95 mole % was attrition milled for 3 h, dried, cold pressed under isostatic pressure and was fired at 1800°C for 2 h in a nitrogen gas atmosphere under a gas pressure of 1 MPa. The linear shrinkage was 15.59%, the firing weight loss was 1.67%. The ficed density was 98.2% of the theoretical value.
Example 3
A composition containing Si3N4- 53.22 mole%, AI2O3 2.47 mole%, AIN 37.98 mole%, SiO2- 2.38 mole% and Sm203 3.95 mole % was attrition milled for 3 h, dried, cold pressed under isostatic pressure and was fired at 1825°C for 2 h in a nitrogen gas atmosphere under a gas pressure of 1 MPa. The linear shrinkage was 16.14%, the firing weight loss was 1.73%. The fired density was 98.21% of the theoretical value.
Example 4
A composition containing SJ3N4- 53.22 mole%, AI2C>3 2.47 mole%, AIM 37.98 mole%, SiO2- 2.38 mole% and Sm2O3 3.95 mole % was attrition milled for 3 h, dried, cold pressed under isostatic pressure and was fired at 1850°C for 2 h in a nitrogen gas atmosphere under a gas pressure of 1 MPa. The linear shrinkage was 16.21%, the firing weight loss was 1.74%. The fired density was 98.23% of the theoretical value. The hardness of the final product is 18.7 GPa. The fracture toughness of the final product is 4.5 MPa.m1'2.
Example 5
A composition containing Si3N4- 53.22 mole%, AI2O3 2.47 mole%, AIN 37.98 mole%, SiO2- 2.38 mole% and Sm2O3 3.95 mole % was attrition milled for 3 h, dried, cold pressed under isostatic pressure and was fired at 1900°C for 2 h in a nitrogen gas atmosphere under a gas pressure of 1 MPa. The linear shrinkage was 17.51%, the firing weight loss was 2.30%. The fired density was 98.39% of the theoretical value.
Example 6
A composition containing Si3N4-51.34 mole%, AI2O3 2.45 mole%, AIN 39.77 mole%, Si02- 2.29 mole% and Sm2O3 4.15 mole % was attrition milled for 3 h, dried, cold pressed under isostatic pressure and was fired at 1750°C for 2 h in a nitrogen gas atmosphere under a gas pressure of 1 MPa. The linear shrinkage was 15.42%, the firing weight loss was 1.57%. The fired density was 98.21% of the theoretical value.
Example 7
A composition containing Si3N4- 53.22 mole%, AI203 2.47 mole%, AIN 37.98 mole%, SiO2- 2.38 mole% and Sm2O3 3.95 mole % was attrition milled for 3 h, dried, cold pressed under isostatic pressure and was fired at 1800°C for 2 h in a nitrogen gas atmosphere under a gas pressure of 1 MPa. The linear shrinkage was 15.63%, the firing weight loss was 1.58%. The fired density was 98.4% of the theoretical value.
Example 8
A composition containing Si3N4- 53.22 mole%, AI2O3 2.47 mole%, AIN 37.98 mole%, SiO2- 2.38 mole% and Sm2O3 3.95 mole % was attrition milled for 3 h, dried, cold pressed under isostatic pressure and was fired at 1850°C for 2 h in a nitrogen gas atmosphere under a gas pressure of 1 MPa. The linear shrinkage was 16.33%, the firing weight loss was 1.63%. The fired density was 98.42% of the theoretical value.
Example 9
A composition containing Si3N4- 53.22 mole%, AI2O3 2.47 mole%, AIN 37.98 mole%, SiO2- 2.38 mole% and Sm203 3.95 mole % was attrition milled for 3 h, dried, cold pressed under isostatic pressure and was fired at 1900°C for 2 h in a nitrogen gas atmosphere under a gas pressure of 1 MPa. The linear shrinkage was 17.64%, the firing weight loss was 1.77%. The fired density was 98.43% of the theoretical value.
The main advantages of the present invention are :
1) The process does not require hot isostatic pressing thereby
providing a cost effective method for the preparation of a-SiAION
material.
2) The sintered material prepared display high hardness which makes
it ideal for use as engineering components in areas where abrasive
wear is dominant.
3) The sintered materials prepared possess other important
mechanical property like fracture toughness which is acceptable for
the use as engineering components.
4) The process allows the use of a cheaper additive oxide such as
samarium oxide in comparison to other oxides such as Dysprosium
oxide and Ytterbium oxide, thereby making the products cost
effective.





We claim:
1. A process for the preparation of dense Samarium stabilised a-SiAION
which comprises preparing a homogenous mixture by conventional methods
of a powdered composition essentially consisting of: 50-55 mole% Si3N4> 2.0-
3.5 mole% AI2O3, 33-42 mole% AIN, 1.5-2.5 mole% SiO2 and 3-4.5 mole%
Sm2O3 , passing the powder through 100 mesh, pressing the powder by
conventional methods at a pressure in the range of 65 to 350Mpa to form
green compacts, sintering the green compacts for a period of 1 to 3 hrs at a
temperature in the range of 1750° to 1900°C in nitrogen atmosphere to obtain
dense samarium stabilized a-SiAION.
2. A process as claimed in claim 1 wherein Si3N4 contains oxygen weight%.
3. A process as claimed in claims 1-2 wherein AI2O3 is of purity >98%.
4. A process as claimed in claims 1-3 wherein AIN contains oxygen weight%.
5. A process as claimed in claims 1-4 wherein SiO2 is of purity >98%.
6. A process as claimed in claims 1-5 wherein Sm2O3 is of purity >98%.
7. A process for the preparation of dense Samarium stabilised a-SiAION
substantially as herein described with reference to the examples.


Documents:

235-del-2002-abstract.pdf

235-DEL-2002-Claims-(05-03-2008).pdf

235-del-2002-claims.pdf

235-DEL-2002-Correspondence-Others-(05-03-2008).pdf

235-del-2002-correspondence-others.pdf

235-DEL-2002-Description (Complete)-(05-03-2008).pdf

235-del-2002-description (complete).pdf

235-del-2002-form-1.pdf

235-del-2002-form-18.pdf

235-del-2002-form-2.pdf

235-del-2002-form-3.pdf


Patent Number 233579
Indian Patent Application Number 235/DEL/2002
PG Journal Number 13/2009
Publication Date 27-Mar-2009
Grant Date 30-Mar-2009
Date of Filing 14-Mar-2002
Name of Patentee COUNCIL OF SCIENTIFIC AND INDUSTRIAL RESEARCH
Applicant Address RAFI MARG, NEW DELHI-110001, INDIA.
Inventors:
# Inventor's Name Inventor's Address
1 SIDDHARTHA BANDYOPADHYAY CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, KOLKATA 700032, INDIA.
2 HIMADRI SEKHAR MAITI CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, KOLKATA 700032, INDIA.
PCT International Classification Number C21D 3/02
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 NA