Title of Invention	A PROCESS FOR THE MANUFACTURE OF DENSE NEODYMIUM STABILISED β-SILICON NITRIDE-α-SIAION COMPOSITE.
Abstract	The present invention provides a process for the manufacture of dense neodymium stabilised ß-Si3N4 - α-SiAION composite, wherein a synergistic composition essentially consisting of Si3N4, AI2O3, AIN, SO2 and Nd2O3 as starting materials is mixed in proportion to make a total of 100 mole in the mixed batch, 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 process of the present invention provides neodymium stabilised ß-Si3N4 - α-SiAION composites by processing a composition from the system Si3N4 - AI2O3.AIN - Nd2O3.9AIN - 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 ß-Si3N4- α-SiAION will be useful for low temperature applications such as wear parts like bearing and roller materials and particularly for grinding and milling operations like grinding balls.

Title of Invention

A PROCESS FOR THE MANUFACTURE OF DENSE NEODYMIUM STABILISED β-SILICON NITRIDE-α-SIAION COMPOSITE.

Abstract

The present invention provides a process for the manufacture of dense neodymium stabilised ß-Si3N4 - α-SiAION composite, wherein a synergistic composition essentially consisting of Si3N4, AI2O3, AIN, SO2 and Nd2O3 as starting materials is mixed in proportion to make a total of 100 mole in the mixed batch, 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 process of the present invention provides neodymium stabilised ß-Si3N4 - α-SiAION composites by processing a composition from the system Si3N4 - AI2O3.AIN - Nd2O3.9AIN - 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 ß-Si3N4- α-SiAION will be useful for low temperature applications such 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 process for the manufactura of dense neodymium stabilised (3-Si3N4 - a-SiAION composite. The dense (3-Si3N4 - a-SiAION composite manufactured by the process of the present invention will find 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 hoţ pressing the green mixtures of Si3N4, AIN, AI203 and Nd2O3, at a temperature in the range of 1550° to 1750°C, at a pressure of about 20 Mpa. Reference may be made to Wang et al. in Mater. Res. Soc. Symp. Proc., Voi. 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., Voi. 79, No. 3, 1996, pp. 721-32 entitled "Homogeneity region and thermal stability of neodymium- doped a SiAION ceramics" where hoţ 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., Voi. 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 Kall et al. in J. Eur. Ceram. Soc., Voi. 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: (i) These processes require hoţ pressing for full densification, which is evidently a very costly process and by which a complex-shaped material is difficult to be manufactured. (ii) These processes fail to produce high densification under pressure less sintering method when a-SiAION is present, the absence of which phase brings down the hardness of the material. From the above prior art references it is clear that there is a definite need for providing a process for the making of dense neodymium stabilised p-Si3N4 - a-SiAION composite, which removes the said disadvantages. The main object of the present invention is to provide a process for the manufacture of dense neodymium stabilised (3-Si3N4 - a-SiAION composite, which obviates the drawbacks of the hitherto known prior art. Another object of the present invention is to provide a process of making dense Neodymium stabilised p-SiaN4 - a-SiAION composite, wherein the sintered material prepared will display high hardness which will make it ideal for use as engineering components in areas where abrasive wear is dominant. Yet another object of the present invention is to provide a process of making dense Neodymium stabilised p-Si3N4 - a-SiAION composite, wherein the sintered material prepared will display important mechanical property like fracture toughness which is acceptable for the use as engineering components. Still another object of the present invention is to provide a process of manufacture of dense neodymium stabilised p-Si3N4 - a-SiAION composite, wherein the composition of p-Si3N4 - a-SiAION in the system Si3N4 - AfeOs.AIN -Nd2O3.9AIN - SiO2 is used to obtain dense sintered p-Si3N4 - a-SiAION, using Si3N4, AbOs, AIN, SiOa and Nd203 as starting materials which form a synergistic composition as described and claimed in our co-pending patent application no. 0512del2003. Still yet another object of the present invention is to provide a process of manufacture of dense neodymium stabilised β-Si3N4 - α-SiAION composite, wherein the composition of α-SiAION is taken from the system Si3N4 - AI2O3.AIN -Nd2O3.9AIN - SiO2 to obtain dense sintered β-Si3N4- α-SiAION, using Si3N4, AI2O3, AIN, SiO2 and Nd2O3 as starting materials. The present invention provides a process for the manufacture of dense neodymium stabilised β-Si3N4 - α-SiAION composite, wherein a synergistic composition essentially consisting of SisN4, AI2O3, AIN, SiO2 and Nd2O3 as starting materials is mixed in proportion to make a total of 100 mole in the mixed batch, 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 process of the present invention provides neodymium stabilised β-Si3N4 - α-SiAION composites by processing a composition from the system Si3N4 - AI2O3.AIN - Nd2O3.9AIN - 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. Accordingly, the present invention provides a process for the manufacture of dense neodymium stabilised β-Si3N4 - α-SiAION composite, which comprises preparing a homogeneous powdered mixture of composition essentially consisting of: 49 to 63 mole% Si3N4, 2.5 to 3.5 mole% Al2O3, 29 to 43 mole% AIN, 2.1 to 2.9 mole% Si02 and 3.1 to 4.9 mole% Nd2O3, mixed in proportion to make a total of 100 mole in a Si3N4 pot in an attrition mill along with Si3N4 balls of size around 2 to 3 mm, wherein the ball: powder ratio is in the range of 6 :1 to 9 :1 for a time period in the range of 2 to 8 hours to obtain a mixed powdered batch, passing the said powdered batch through 100 mesh followed by drying, pressing the sieved and dried powder to form green compacts, subjecting the green compacts so obtained to sintering at a temperature in the range of 1700° to 1900°C, in nitrogen atmosphere. In an embodiment of the present invention, the starting materials used are pure and powdered. In another embodiment of the present invention, the homogeneous powdered mixture is prepared in a SJ3N4 pot in an attrition mill along with SisN4 balls of size around 2 to 3 mm, wherein the ball : powder ratio is in the range of 6 :1 to 9 :1, and wherein the milling is done in a liquid medium of acetone for which the water content is 0.2%. In yet another embodiment of the present invention, the ball to powder ratio during milling is preferably around 7:1. In still another embodiment of the present invention, the attrition milling is done for a time period in the range of 2 to 8 hours. In still yet another embodiment of the present invention, the green compacts are formed by isostatically pressing in a rubber mould at a pressure in the range of 65 to 350 MPa. In a further embodiment of the present invention, the sintering of the green compacts is carried out in a graphite resistance heating furnace. In the process of the present invention the sintering is found to be enhanced in p-Si3N4 - a-SiAION compositions when selected from the system SisN4 - AI203.AIN - Nd2O3.9AIN - SiO2 The starting materials of the present invention constitutes a synergistic composition consistîng of SisN4, A^Oa, AIN, SiOa and NdaOs, as described and claimed in our co-pending patent application no. NF-78 / 03. It is believed that the mechanism is as foilows: in general, the sintering of the a-3 SiAION materials is difficult prirnarily 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 SiOa 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 product obtained has high hardness and high fracture toughness. This has been made possible by the inventive step of selecting the compositional zone of the present process from the system Si3N4 - AI2O3.AIN - Nd203.9AIN - SiO2, resulting in (3-Si3N4 - 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. Thus the present invention relates to a novei process of making dense neodymium stabilised P-SÎ3N4 - a-SiAION composites by the inventive step of selection of a range of new synergistic compositions different from other processes 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 process of the present invention for making dense neodymium stabilised P-Si3N4 - a-SiAION composite is described below in detail: 1. Pure and powdered a-Si3N4 , AI2O3, AIN and Nd2O3 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 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 compacts were taken in a graphite resistance heating furnace and were fired at a temperature in the range of 1700° to 1900°C, in nitrogen gas atmosphere. 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- 53.3 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 \|3-Si3N4 : ot-SiAION phase ratio is lesser than 1:5. Example 2 A composition containing SÎ3N4- 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 (3-Si3N4 : ct-SiAION phase ratio is lesser than 1:5. Example 3 A composition containing Si3N4- 53.3 mole%, AI203- 2.6 mole%, AIN- 37.7 mole%, Nd203- 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 3-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%, Nd203- 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 lesserthan 1:5. Example 5 A composition containing Si3N4- 53.3 mole%, AI2O3- 2.6 mole%, AIN- 37.7 mole%, Nd2C>3- 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 (3-Si3N4: a-SiAION phase ratio is lesser than 1:5. Example 6 A composition containing Si3N4- 49.06 mole%, AI2C>3- 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%, 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 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 1.98%. 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 Si02- 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%, 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 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 p-Si3N4: a-SiAION phase ratio is greater than 3:1. The main advantages of the present invention are : 1) The sintered material prepared using this process displays high hardness which makes it ideal for use as engineering components in areas where abrasive wear is dominant. 2) The sintered material prepared using this process possesses other important mechanical property like fracture toughness which is acceptable for the use as engineering components. We claim: 1. A process for the manufacture of dense neodymium stabilised ß-Si3N4 - α-SiAION composite, which comprises preparing a homogeneous powdered mixture of composition essentially consisting of: 49 to 63 mole% Si3N4, 2.5 to 3.5 mole% Al2O3, 29 to 43 mole% AIN, 2.1 to 2.9 mole% Si02 and 3.1 to 4.9 rnole% Nd2O3, mixed in proportion to make a total of 100 mole in a Si3N4 pot in an attrition mill along with Si3N4 balls of size around 2 to 3 mm, wherein the ball : powder ratio is in the range of 6 :1 to 9 :1 for a time period in the range of 2 to 8 hours to obtain a mixed powdered batch, passing the said powdered batch through 100 mesh followed by drying, pressing the sieved and dried powder to form green compacts, subjecting the green compacts so obtained to sintering at a temperature in the range of 1700° to 1900°C, in nitrogen atmosphere. 2. A process as claimed in claim 1, wherein the starting materials used are pure and powdered. 3. A process as claimed in claims 1-2, wherein the milling is done in a liquid medium of acetone for which the water content is 0.2%. 4. A process as claimed in claims 1-3, wherein the ball to powder ratio during milling is preferably 7:1. 5. A process as claimed in claim 1-4, wherein the green compacts are formed by isostatically pressing in a rubber mould at a pressure in the range of 65 to 350 MPa. 6. A process as claimed in claims 1-5, wherein the sintering of the green compacts is carried out in a graphite resistance heating furnace. 7. A process for the manufacture of dense neodymium stabilised ß-Si3N4 - α-SiAION composite, substantially as herein described with reference to the examples.

Full Text

This invention relates to a process for the manufactura of dense neodymium stabilised (3-Si3N4 - a-SiAION composite.
The dense (3-Si3N4 - a-SiAION composite manufactured by the process of the present invention will find 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 hoţ pressing the green mixtures of Si3N4, AIN, AI203 and Nd2O3, at a temperature in the range of 1550° to 1750°C, at a pressure of about 20 Mpa. Reference may be made to Wang et al. in Mater. Res. Soc. Symp. Proc., Voi. 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., Voi. 79, No. 3, 1996, pp. 721-32 entitled "Homogeneity region and thermal stability of neodymium- doped a SiAION ceramics" where hoţ 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., Voi. 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 Kall et al. in J. Eur. Ceram. Soc., Voi. 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: (i) These processes require hoţ pressing for full densification, which is
evidently a very costly process and by which a complex-shaped
material is difficult to be manufactured. (ii) These processes fail to produce high densification under pressure less
sintering method when a-SiAION is present, the absence of which
phase brings down the hardness of the material.
From the above prior art references it is clear that there is a definite need for providing a process for the making of dense neodymium stabilised p-Si3N4 - a-SiAION composite, which removes the said disadvantages.
The main object of the present invention is to provide a process for the manufacture of dense neodymium stabilised (3-Si3N4 - a-SiAION composite, which obviates the drawbacks of the hitherto known prior art.
Another object of the present invention is to provide a process of making dense Neodymium stabilised p-SiaN4 - a-SiAION composite, wherein the sintered material prepared will display high hardness which will make it ideal for use as engineering components in areas where abrasive wear is dominant.
Yet another object of the present invention is to provide a process of making dense Neodymium stabilised p-Si3N4 - a-SiAION composite, wherein the sintered material prepared will display important mechanical property like fracture toughness which is acceptable for the use as engineering components.
Still another object of the present invention is to provide a process of manufacture of dense neodymium stabilised p-Si3N4 - a-SiAION composite, wherein the composition of p-Si3N4 - a-SiAION in the system Si3N4 - AfeOs.AIN -Nd2O3.9AIN - SiO2 is used to obtain dense sintered p-Si3N4 - a-SiAION, using Si3N4, AbOs, AIN, SiOa and Nd203 as starting materials which form a synergistic
composition as described and claimed in our co-pending patent application no. 0512del2003.
Still yet another object of the present invention is to provide a process of manufacture of dense neodymium stabilised β-Si3N4 - α-SiAION composite, wherein the composition of α-SiAION is taken from the system Si3N4 - AI2O3.AIN -Nd2O3.9AIN - SiO2 to obtain dense sintered β-Si3N4- α-SiAION, using Si3N4, AI2O3, AIN, SiO2 and Nd2O3 as starting materials.
The present invention provides a process for the manufacture of dense neodymium stabilised β-Si3N4 - α-SiAION composite, wherein a synergistic composition essentially consisting of SisN4, AI2O3, AIN, SiO2 and Nd2O3 as starting materials is mixed in proportion to make a total of 100 mole in the mixed batch, 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 process of the present invention provides neodymium stabilised β-Si3N4 - α-SiAION composites by processing a composition from the system Si3N4 - AI2O3.AIN - Nd2O3.9AIN - 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.
Accordingly, the present invention provides a process for the manufacture of dense neodymium stabilised β-Si3N4 - α-SiAION composite, which comprises preparing a homogeneous powdered mixture of composition essentially consisting of: 49 to 63 mole% Si3N4, 2.5 to 3.5 mole% Al2O3, 29 to 43 mole% AIN, 2.1 to 2.9 mole% Si02 and 3.1 to 4.9 mole% Nd2O3, mixed in proportion to make a total of 100 mole in a Si3N4 pot in an attrition mill along with Si3N4 balls of size around 2 to 3 mm, wherein the ball: powder ratio is in the range of 6 :1 to 9 :1 for a time period in the range of 2 to 8 hours to obtain a mixed powdered batch, passing the said powdered batch through 100 mesh followed by drying, pressing the sieved and dried powder to form green compacts, subjecting the green compacts so obtained to sintering at a temperature in the range of 1700° to 1900°C, in nitrogen atmosphere.

In an embodiment of the present invention, the starting materials used are pure and powdered.
In another embodiment of the present invention, the homogeneous powdered mixture is prepared in a SJ3N4 pot in an attrition mill along with SisN4 balls of size around 2 to 3 mm, wherein the ball : powder ratio is in the range of 6 :1 to 9 :1, and wherein the milling is done in a liquid medium of acetone for which the water content is 0.2%.
In yet another embodiment of the present invention, the ball to powder ratio during milling is preferably around 7:1.
In still another embodiment of the present invention, the attrition milling is done for a time period in the range of 2 to 8 hours.
In still yet another embodiment of the present invention, the green compacts are formed by isostatically pressing in a rubber mould at a pressure in the range of 65 to 350 MPa.
In a further embodiment of the present invention, the sintering of the green compacts is carried out in a graphite resistance heating furnace.
In the process of the present invention the sintering is found to be enhanced in p-Si3N4 - a-SiAION compositions when selected from the system SisN4 - AI203.AIN - Nd2O3.9AIN - SiO2 The starting materials of the present invention constitutes a synergistic composition consistîng of SisN4, A^Oa, AIN, SiOa and NdaOs, as described and claimed in our co-pending patent application no. NF-78 / 03. It is believed that the mechanism is as foilows: in general, the sintering of the a-3 SiAION materials is difficult prirnarily 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 SiOa 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 product obtained has high hardness and high fracture toughness. This has been made possible by the inventive step of selecting the compositional zone of the present process from the system Si3N4 - AI2O3.AIN - Nd203.9AIN - SiO2, resulting in (3-Si3N4 - 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.
Thus the present invention relates to a novei process of making dense neodymium stabilised P-SÎ3N4 - a-SiAION composites by the inventive step of selection of a range of new synergistic compositions different from other processes 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 process of the present invention for making dense neodymium stabilised P-Si3N4 - a-SiAION composite is described below in detail:
1. Pure and powdered a-Si3N4 , AI2O3, AIN and Nd2O3 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 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 compacts were taken in a graphite resistance heating
furnace and were fired at a temperature in the range of 1700° to 1900°C, in
nitrogen gas atmosphere.
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- 53.3 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 |3-Si3N4 : ot-SiAION phase ratio is lesser than 1:5.
Example 2
A composition containing SÎ3N4- 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 (3-Si3N4 : ct-SiAION phase ratio is lesser than 1:5.
Example 3
A composition containing Si3N4- 53.3 mole%, AI203- 2.6 mole%, AIN- 37.7 mole%, Nd203- 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 3-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%, Nd203- 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 lesserthan 1:5.
Example 5
A composition containing Si3N4- 53.3 mole%, AI2O3- 2.6 mole%, AIN- 37.7 mole%, Nd2C>3- 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 (3-Si3N4: a-SiAION phase ratio is lesser than 1:5.
Example 6
A composition containing Si3N4- 49.06 mole%, AI2C>3- 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%, 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 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 1.98%. 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 Si02- 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%, 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 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 p-Si3N4: a-SiAION phase ratio is greater than 3:1.
The main advantages of the present invention are :
1) The sintered material prepared using this process displays high
hardness which makes it ideal for use as engineering components in areas
where abrasive wear is dominant.
2) The sintered material prepared using this process possesses other
important mechanical property like fracture toughness which is acceptable
for the use as engineering components.

We claim:
1. A process for the manufacture of dense neodymium stabilised ß-Si3N4 - α-SiAION composite, which comprises preparing a homogeneous powdered mixture of composition essentially consisting of: 49 to 63 mole% Si3N4, 2.5 to 3.5 mole% Al2O3, 29 to 43 mole% AIN, 2.1 to 2.9 mole% Si02 and 3.1 to 4.9 rnole% Nd2O3, mixed in proportion to make a total of 100 mole in a Si3N4 pot in an attrition mill along with Si3N4 balls of size around 2 to 3 mm, wherein the ball : powder ratio is in the range of 6 :1 to 9 :1 for a time period in the range of 2 to 8 hours to obtain a mixed powdered batch, passing the said powdered batch through 100 mesh followed by drying, pressing the sieved and dried powder to form green compacts, subjecting the green compacts so obtained to sintering at a temperature in the range of 1700° to 1900°C, in nitrogen atmosphere.
2. A process as claimed in claim 1, wherein the starting materials used are pure and powdered.
3. A process as claimed in claims 1-2, wherein the milling is done in a liquid
medium of acetone for which the water content is 0.2%.
4. A process as claimed in claims 1-3, wherein the ball to powder ratio during milling is preferably 7:1.
5. A process as claimed in claim 1-4, wherein the green compacts are formed by isostatically pressing in a rubber mould at a pressure in the range of 65 to 350 MPa.
6. A process as claimed in claims 1-5, wherein the sintering of the green compacts is carried out in a graphite resistance heating furnace.
7. A process for the manufacture of dense neodymium stabilised ß-Si3N4 - α-SiAION composite, substantially as herein described with reference to the examples.

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Patent Number

234988

Indian Patent Application Number

514/DEL/2003

PG Journal Number

28/2009

Publication Date

10-Jul-2009

Grant Date

23-Jun-2009

Date of Filing

28-Mar-2003

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	SIDHARTHA BANDYOPADHYAY	CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE,KOLKATA 700 032,INDIA.
2	HIMADRI SEKHAR MAITI	CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, KOLKATA 700 032, INDIA.

PCT International Classification Number

C04B 35/14

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA