Title of Invention

"A SYNERGISTIC COMPOSITION FOR THE PREPARATION OF DENSE SAMARIUM STABLISED α-SiAION"

Abstract

The present invention provides a synergistic composition for the preparation of dense Samarium stabilised α-SiAION by inventive steps of selecting compositions from the system Si3N4 - AI2O3.AIN - Sm203.9AIN - Si02 which will provide 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 α -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.

Full Text

This invention relates to a synergistic composition 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 Si3N4, AIM, AI203 and Sm2O3 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 pressurless sintering method. The main object of the present invention is to provide a synergistic composition for the preparation of dense Samarium stabilised α-SiAION which obviates the above disadvantages. Another object of the present invention is to provide a synergistic composition wherein the composition of α-SiAION in the system Si3N4 -AI2O3AIN - Sm203.9AIN - Si02 is used to obtain dense sintered α-SiAION, using Si3N4, AI2O3, AIN, Si02 and Sm203 as starting materials. The present invention relates to a synergistic composition for the preparation of dense Samarium stabilised α-SiAION by inventive steps of selecting compositions from the system Si3N4 - AI203.AIN - Sm203.9AIN - Si02 which will provide 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 α-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 synergistic composition for the preparation of dense Samarium stabilised α-SiAION which comprises : 50-55 mole% Si3N4, 2.0-3.5 mole% Al203, 33-42 mole% AIN, 1.5-2.5 mole% Si02 and 3-4.5 mole% Sm203., mixed in proportion to make a total of 100 mole and characterised in that the said α-SiAION exihibit as single crystalline phase with excellent sinterability and possesses a final density value of not less than 98% of theoretical in the temperature more than 1750°C. In an embodiment of the present invention, Si3N4 may contain oxygen weight%. In another embodiment of the present invention, Al203 may be of purity >98%. In still another embodiment of the present invention, AIN 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, Sm2O3 may be of purity >98%. The composition of the present invention is not a mere admixture but is a synergistic mixture and the property of the final product is not the mere aggregation of properties of the individual ingredients. In our copending patent application no. 235del2002, we have described and claimed a process for the preparation of dense Samarium stabilised α-SiAION products which comprises preparing a homogeneous mixture by conventional methods of the composition of the present invention, 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. The details of the present invention for providing a composition for the preparation of dense Samarium stabilised α-SiAION and a process of making products thereof as described and claimed in our co-pending patent application no. 235del2002 is given below : 1. Pure and powdered of α-Si3N4 , AI2O3, AIN, Si02 and Sm203 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 - AI2O3.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, M2O3.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 SiO2 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 SiO2. The novelty of the present invention is that the selected compositional zone from the system Si3N4 - AI2O3.AIN - Sm2O3.9AIN - 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 inventive step lies in the selection of a range of novel synergistic compositions, different from the prior art, that gives the desired product after specified processing. 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%, 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 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- 51.34 mole%, AI2O3 2.45 mole%, AIN 39.77 mole%, SiO2- 2.29 mole% and Sm2O3 4.15 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 fired density was 98.2% of the theoretical value. The main advantages of the synergistic composition of the present invention are: 1) The composition displays easier densification under normal sintering conditions. 2) The composition does not require hot isostatic pressing thereby providing a cost effective method for the preparation of a-SiAION material. 3) The sintered material prepared using the composition display high hardness which makes it ideal for use as engineering components in areas where abrasive wear is dominant. 4) The sintered materials prepared using the composition possess other important mechanical property like fracture toughness which is acceptable for the use as engineering components. We claim: 1. A synergistic composition for the preparation of dense Samarium stabilised α-SiAION which comprises: 50-55 mole% Si3N4, 2.0-3.5 mole% Al203, 33-42 mole% AIN, 1.5-2.5 mole%Si02 and 3-4.5 mole% Sm203 , mixed in proportion to make a total of 100 mole and characterised in that the said a-SiAION exihibits single crystalline phase with excellent sinterability and possesses a final density value of not less than 98% of theoretical in the temperature more than 1750°C. 2. A composition as claimed in claim 1 wherein, Al203 is of purity >98%. 3. A composition as claimed in claims 1-2 wherein, Si02 is of purity >98%. 4. A composition as claimed in claims 1-3 wherein, Sm203 is of purity >98%. 5. A synergistic composition for the preparation of dense Samarium stabilised α-SiAION substantially as herein described with reference to the examples.

Documents:

234-DEL-2002-Abstract-(30-07-2008).pdf

234-del-2002-abstract.pdf

234-DEL-2002-Claims-(30-07-2008).pdf

234-del-2002-claims.pdf

234-DEL-2002-Correspondence-Others-(30-07-2008).pdf

234-del-2002-correspondence-others.pdf

234-del-2002-description (complete)-30-07-2008.pdf

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

234-del-2002-form-1.pdf

234-del-2002-form-18.pdf

234-del-2002-form-2.pdf

234-del-2002-form-3.pdf