Title of Invention

A SYNERGISTIC COMPOSITION USEFUL FOR THE PREPARATION OF DENSE NEWDYMIUM STABILISED β-SILICON NITRIDE-α-SIALON COMPOSITE.

Abstract

A synergistic composition useful for the preparation of dense neodymium stabilised ß-Si3N4 - α-SiAION composite The invention relates to a synergistic composition useful for the preparation of dense neodymium stabilised ß-Si3N4 - α-SiAION composite The composition consists of a synergistic mixture of Si3N4, Al2O3, AIN, SiO2 and Nd2O3The cost effective synergistic composition is useful for the preparation of dense neodymium stabilised ß-Si3N4 - α-SiAION composite of the order of > 98% theoretical density, having high hardness and high fracture toughness. The dense ß-Si3N4 - α-SiAION composite 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 useful for the preparation of dense neodymium stabilised ß-Si3N4 - α-SiAION composite. The dense ß-Si3N4 - a-SiAION composite finds usage in low temperature applications such 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 green mixtures of Si3N4, AIN, AI2O3 and Nd2O3, at a temperature in the range of 1550° to 1750°C, and at a pressure of about 20 Mpa. 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. Am. Ceram. Soc., Vol. 79, No. 3, 1996, pp. 721-32 entitled "Homogeneity region and thermal stability of neodymium- 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 Kail et al. in J. Eur. Ceram. Soc., Vol. 6, 1990, pp. 191-27, entitled "Sialon ceramics made with mixtures of Y2O3- Nd2O3 as sintering aids", wherein the green mixtures were pressureless sintered above 1825°C. Although the high temperature firing could produce fully sintered material, the pressureless sintering at 1750°C could only produce up to 96% of theoretical density even when a-SiAION is completely absent. The major drawbacks of the above noted hitherto known processes are that these involve selection of a composition that requires 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. Hence, there is a definite need for providing a composition useful for the preparation of dense neodymium stabilised ß-Si3N4 - a-SiAION composite, which will take care of the above disadvantages. The main object of the present invention is to provide a synergistic composition useful for the preparation of dense neodymium stabilised (3-Si3N4 - a-SiAION composite, which obviates the drawbacks of the above noted hitherto known prior art. Another object of the present invention is to provide a synergistic composition, useful for the preparation of dense neodymium stabilised ß-Si3N4 - a-SiAION composite, wherein the composition consists of a synergistic mixture of Si3N4, AI203, AIN, SiO2 and Nd2O3. Still another object of the present invention is to provide a synergistic composition, useful for the preparation of dense neodymium stabilised (3-Si3N4 - a-SiAION composite, wherein the composition displays easier densification under normal sintering conditions. Yet another object of the present invention is to provide a synergistic composition, wherein a cheaper additive oxide such as neodymium oxide in comparison to other oxides such as dysprosium oxide or ytterbium oxide, makes the composition economic. The present invention provides a synergistic composition useful for the preparation of dense neodymium stabilised ß-Si3N4 - a-SiAION composite, wherein the composition consists of a synergistic mixture of Si3N4, AI2O3, AIN, SiO2 and Nd2O3. Accordingly, the present invention provides a synergistic composition useful for the preparation of dense neodymium stabilised ß-Si3N4 - a-SiAION composite, which comprises: Si3N4 : 49 to 63 mole%, AI2O3 : 2.5 to 3.5 mole%, AIM : 29 to 43 mole%, Si02 : 2.1 to 2.9 mole%, and Nd203 :3.1 to 4.9 mole%. In an embodiment of the present invention, the Si3N4 contains oxygen less than 1 weight%. In another embodiment of the present invention, the AI2O3 is of purity greater than 98%. In still another embodiment of the present invention, the AIM contains oxygen less than 2.5 weight%. In yet another embodiment of the present invention, the SiO2 is of purity greater than 98% In still yet another embodiment of the present invention, the Nd2O3 is of purity greater than 98%. The composition of the present invention is not a mere admixture but a synergistic mixture having properties which are distinct and different from the mere aggregation of the properties of the individual ingredients. There is no chemical reaction in the said synergistic mixture. The novelty of the present invention resides in providing a synergistic composition useful for the preparation of dense neodymium stabilised ß-Si3N4 - a-SiAION composite, wherein the product obtained using the selected compositional zone from the system Si3N4 - AI2O3.AIN - Nd2O3.9AIN - 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 achievement of the novelty of the present invention has been made possible by the non-obvious inventive step of providing a composition which consists of a synergistic mixture of Si3N4, Al2O3, AIN, SiO2 and Nd2O3, In our co-pending patent application no. 0514del2003, we have described and claimed a process for the manufacture of dense neodymium stabilised ß-Si3N4 - α-SiAION composite from the synergistic composition of the present invention, which comprises preparing a homogeneous mixture of the composition, passing the powder through 100 mesh, pressing the powder to form green compacts, sintering the green compacts at a temperature in the range of 1700° to 1900°C in nitrogen atmosphere. The sintering is found to be enhanced in ß-Si3N4 - α-SiAION compositions when selected from the system Si3N4 - AI2O3.AIN - Nd2O3.9AIN - SiO2. It is believed that the mechanism is as follows: In general, the sintering of the a-SiAION materials are 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 following examples are given by way of illustration of the practical usage of the synergistic composition of the present invention and therefore, should not be construed to limit the scope of the invention: Example 1 A composition containing Si3N4- 53.3 mole%, AI2O3- 2.6 mole%, AIN- 37.7 mole%, Nd2O3- 4 mole% and SiO2- 2.4 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 15.72%, the firing weight loss was 2.14%. The fired density was 98.06% of the theoretical value. The p-Si3N4 : a-SiAION phase ratio is lesser than 1:5. Example 2 A composition containing Si3N4- 53.3 mole%, AI2O3- 2.6 mole%, AIN-, 37.7 mole%, Nd2O3- 4 mole% and Si02- 2.4 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 16.11%, the firing weight loss was 2.2%. The fired density was 98.64% of the theoretical value. The p-Si3N4 : a-SiAION phase ratio is lesser than 1:5. Example 3 A composition containing Si3N4- 53.3 mole%, AI2O3- 2.6 moie%, AIN- 37.7 mole%, Nd2O3- 4 mole% and SiO2- 2.4 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 16.13%, the firing weight loss was 2.28%. The fired density was 98.76% of the theoretical value. The p-Si3N4 : a-SiAION phase ratio is lesser than 1:5. Example 4 A composition containing Si3N4- 53.3 mole%, AI2O3- 2.6 mole%, AIN- 37.7 mole%, Nd2O3- 4 mole% and SiO2- 2.4 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 16.32%, the firing weight loss was 2.29%. The fired density was 98.84% of the theoretical value. The hardness of the final product is 18.6 GPa. The fracture toughness of the final product is 4.7 MPa.m1'2. The p-Si3N4 : a-SiAION phase ratio is lesser than 1:5. Example 5 A composition containing SJ3N4- 53.3 mole%, AI2O3- 2.6 mole%, AIN- 37.7 mole%, Nd2O3- 4 mole% and SiO2- 2.4 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 16.19%, the firing weight loss was 2.71%. The fired density was 98.81% of the theoretical value. The ß-Si3N4 : a-SiAION phase ratio is lesser than 1:5. Example 6 A composition containing Si3N4- 49.06 mole%, AI2O3- 2.75 mole%, AIN- 41.50 mole%, Nd2O3- 4.50 mole% and SiO2- 2.19 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 16.14%, the firing weight loss was 1.98%. The fired density was 98.22% of the theoretical value. The p-Si3N4 : a-SiAION phase ratio is greater than 3:1. Example 7 A composition containing Si3N4- 49.06 mole%, AI2O3- 2.75 mole%, AIM- 41.50 mole%, Nd2O3- 4.50 mole% and SiO2- 2.19 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 16.29%, the firing weight loss was 198%. The fired density was 98.39% of the theoretical value. The |3-Si3N4 : a-SiAION phase ratio is greater than 3:1. Example 8 A composition containing Si3N4- 49.06 mole%, AI2O3- 2.75 mole%, AIN- 41.50 mole%, Nd2O3- 4.50 mole% and SiO2- 2.19 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 16.38%, the firing weight loss was 2.09%. The fired density was 98.44% of the theoretical value. The p-Si3N4 : a-SiAION phase ratio is greater than 3:1. Example 9 A composition containing Si3N4- 49.06 mole%, AI203- 2.75 mole%, AIN- 41.50 mole%, Nd2O3- 4.50 mole% and Si02- 2.19 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 16.18%, the firing weight loss was 2.22%. The fired density was 98.41% of the theoretical value. The ß-Si3N4 : a-SiAION phase ratio is greater than 3:1. The main advantages of the present invention are : 1 Provides a synergistic composition, useful for the preparation of dense neodymium stabilised ß-Si3N4 - α-SiAION composite. 2. Provides a synergistic composition, useful for the preparation of dense neodymium stabilised ß-Si3N4 - α-SiAION composite, wherein the composition displays easier densification under normal sintering conditions. 3. Provides a synergistic composition, wherein a cheaper additive oxide such as neodymium oxide in comparison to other oxides such as dysprosium oxide or ytterbium oxide, makes the composition economic. 4. Provides a synergistic composition which does not require hot pressing thereby providing a cost effective method for the preparation of (3-SisN4 - a- SiAION material. We claim: 1. A synergistic composition useful for the preparation of dense neodymium stabilised ß-Si3N4 - α-SiAION composite, which comprises: Si3N4 : 49 to 63mole%, Al2O3 : 2.5 to 3.5 mole%, AIN : 29 to 43 mole%, SiO2 : 2.1 to 2.9 mole%, and Nd2O3 : 3.1 to4.9mole%. 2. A synergistic composition as claimed in claim 1, wherein the Si3N4 contains oxygen less than 1 weight%. 3. A synergistic composition as claimed in claims 1-2, wherein the AIN contains oxygen less than 2.5 weight%. 4. A synergistic composition useful for the preparation of dense neodymium stabilised ß-Si3N4 - α-SiAION composite, substantially as herein described with reference to the examples.

Documents:

512-DEL-2003-Abstract-(19-02-2009).pdf

512-del-2003-abstract.pdf

512-DEL-2003-Claims-(09-03-2009).pdf

512-DEL-2003-Claims-(19-02-2009).pdf

512-del-2003-claims.pdf

512-DEL-2003-Correspondence-Others-(09-03-2009).pdf

512-DEL-2003-Correspondence-Others-(19-02-2009).pdf

512-DEL-2003-Correspondence-Others-(24-03-2009).pdf

512-del-2003-correspondence-others.pdf

512-del-2003-correspondence-po.pdf

512-DEL-2003-Description (Complete)-(19-02-2009).pdf

512-del-2003-description (complete).pdf

512-DEL-2003-Form-1-(09-03-2009).pdf

512-DEL-2003-Form-1-(24-03-2009).pdf

512-del-2003-form-1.pdf

512-del-2003-form-13-(25-05-2009).pdf

512-del-2003-form-18.pdf

512-del-2003-form-2.pdf

512-DEL-2003-Form-3-(19-02-2009).pdf

512-del-2003-form-3.pdf

512-DEL-2003-Petition-137-(24-03-2009).pdf