Title of Invention

A PROCESS FOR THE PREPARATION OF DENSE DYSPROSIUM STABILLISED α SIALON

Abstract

A process for the preparation of dense dysprosium stabilised α-SiAION by mixing of 42-54 mole% Si3N4, 1.5-3.5 mole% AI203, 37-47 mole% AIN, 1.5-6 mole% SiO2 and 3.5-5.5 mole% Dy2O3 by known methods passing the powder through 100 mesh and pressing the powder by known methods to form green compacts at a temperature in the range of 1650 to 1900°C sintering in nitrogen atomosphere to obtain dysprosium stabilized α-SiAION.

Full Text

This invention relates to a process for the preparation of dense dysprosium stabilised α-SiAION. Dense α-SiAION is useful for low temperature applications as wear parts Nike 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, AIN and Dy2O3 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-α-SiAIONs (R=Nd, Sm.Gd, Dy,Er,Yb)". Reference may also be made to O'Reilly et. Al. in Mater. Res. Soc. Symp. Proc., 298, 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% α-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 α-ß sialon transformation I heat treated sialon ceramics" wherein an additional starting material α-AI2O3 was used in the green mixture along with those as mentioned in the above prior art. These were either hot pressure 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. Reference may still further be made to Bandyopadhyay et.al. in J. Amer. Ceram. Soc., Vol.79, No.6, 1996, pp. 1537-45, entitled "Densification behaviour and properties of Y2O3-containing a-SiAION bassed composites" wherein the similar starting materials as in the previous work were used but with the composition which could produce a higher densification in pressureless sintering at a temperature >1750°C. The main 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 require a high sintering temperature of >1750°C in the pressurless sintering method. The main object of the present invention is to provide a synergistic composition for the preparation of dense Dysprosium stabilised α-SiAION which obviates the above disadvantages. Another object of the present invention is to provide a synergistic composition wherein the SiO2 added composition of α-SiAION in the system -Si3N4 -AI203.AIN - Dy203.9AIN is used to obtain dense sintered α-SiAION, using Si3N4, AI203, AIN, SiO2 and Dy2O3 as starting materials. Still another object of the present invention is to provide a process of making the products consisting of a composition wherein the SiO2 added composition of α-SiAION in the system Si3N4 - AI203.AIN - Dy2O3.9AIN is used to obtain dense sintered α-SiAION, using Si3N4, AI2O3, AIN, SiO2 and Dy2O3 as starting materials. Yet another object of the present invention is to provide a process wherein a lower sintering temperature is required thus making the process economic. Accordingly, the present invention provides a process for the preparation of dense dysprosium stabilised α-SiAION which comprises mixing of 42-54 mole% Si3N4, 1.5-3.5 mole%AI2O3, 37-47 mole% AIM, 1.5-6 mole% Si02and 3.5-5.5 mole% Dy203 by known methods passing the powder through 100 mesh and pressing the powder by known methods to form green compacts at a temperature in the range of 1650 to 1900°C sintering in nitrogen atomosphere to obtain dysprosium stabilized a-SiAION. 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, AIN may contain oxygen upto 2.5 weight%. In still yet another embodiemtn of the present invention SiO2 may be of purity >98%. In yet another embodiment of the present invention, Dy2O3 may be of purity >98%. The composition is a synergistic mixtrure and the property of the final product is not the mere aggregation of properties of the individual ingredients. Accordingly, the present inveiton also provides a process for the preparation of dense Dyspropsium stabilized α-SiAION products which comprises preparing a homogeneous mixture by known methods of the composition of the present invention, passing the powder through 100 mesh, pressing the powder by known methods to form green compacts, sintering the green compacts at a temperature in the range of 1650 to 19000C in nitrogen atmosphere. The process of the present invention for providing a composition for the preparation of dense Dysprosium stabilised α-SiAION and a process of making the products thereof is described below in detail: 1. Pure and powdered of a-Si3N4 , AI2O3> AIN and Dy2O3 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 200 to 650 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 SiO2 added compositions. 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 mellilite phase, M2O3.Si3N4, often containing aluminium in solid solution, occur frequently together with α-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 α-SiAION as well. The final densification of the material therefore becomes dependent on the dissociation temperatures of the mellilite which promotes the amount of the liquid phase once again at high temperature so that the sintering proceeds. The extent of the mellilite 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 mellilite 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. Thus the inventive steps of the present invention is that the product obtained using the Si02 added compositional zone exhibits α-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 >1700°C. The novelty lies in the new synergistic composition 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 76.14 mole%, AI3O3N- 9.17 mole%, DyAI3N4-11.29 mole% and Si02- 3.4 mole% was attrition milled for 3 h, dried, cold pressed under isostatic pressure and was fired at 1650°C for 2 h in a nitrogen gas atmosphere under a gas pressure of 1 MPa. The shrinkage perpendicular to the cold pressing direction was 15.98%, the firing weight loss was 0.99%. The fired density was 98.9% of the theoretical value. Example 2 A composition containing Si3N4- 76.14 mole%, AI303N- 9.17 mole%, DyAI3N4-11.29 mole% and SiO2- 3.4 mole% was attrition milled for 3 h, dried, cold pressed under isostatic pressure and was fired at 1700°C for 2 h in a nitrogen gas atmosphere under a gas pressure of 1 MPa. The shrinkage perpendicular to the cold pressing direction was 16.87%, the firing weight loss was 1.03%. The fired density was 99.1% of the theoretical value. Example 3 A composition containing Si3N4- 76.14 mole%, AI3O3N- 9.17 mole%, DyAI3N4-11.29 mole% and SiO2- 3.4 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 shrinkage perpendicular to the cold pressing direction was 17.02%, the firing weight loss was 0.99%. The fired density was 99.3% of the theoretical value. The hardness of the final product is 20.3 GPa. The fracture toughness of the final product is 4.1 MPa.m1'2. Example 4 A composition containing Si3N4- 66.4 mole%, AI303N- 11.54 mole%, DyAI3N4-15.39 mole% and SiO2- 6.67 mole% was attrition milled for 3 h, dried, cold pressed under isostatic pressure and was fired at 1650°C for 2 h in a nitrogen gas atmosphere under a gas pressure of 1 MPa. The shrinkage perpendicular to the cold pressing direction was 16.32%, the firing weight loss was 0.98%. The fired density was 99.0% of the theoretical value. Example 5 A composition containing Si3N4- 66.4 mole%, AI3O3N- 11.54 mole%, DyAI3N4-15.39 mole% and Si02- 6.67 mole% was attrition milled for 3 h, dried, cold pressed under isostatic pressure and was fired at 1700°C for 2 h in a nitrogen gas atmosphere under a gas pressure of 1 MPa. The shrinkage perpendicular to the cold pressing direction was 17.05%, the firing weight loss was 1.07%. The fired density was 99.1% of the theoretical value. Example 6 A composition containing Si3N4- 66.4 mole%, AI3O3N- 11.54 mole%, DyAI3N4-15.39 mole% and SiO2- 6.67 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 shrinkage perpendicular to the cold pressing direction was 17.13%, the firing weight loss was 1.11%. The fired density was 99.3% of the theoretical value. The hardness of the final product is 20.1 GPa. The fracture toughness of the final product is 4.2 MPa.rn1'2. The main advantages are : 1) The newer compositions display 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 -SiAION material. 3) The composition does not require hot pressing thereby providing a cost effective method for the preparation of -SiAION material. 4) The sintered material prepared using these compositions display very high hardness which makes it ideal for use as engineering components in areas where abrasive wear is dominant. 5) The sintered materials prepared using these compositions possess other important mechanical property like fracture toughness which is acceptable for the use as engineering components. We claim : 1. A process for the preparation of dense dysprosium stabillised α-SiAION which comprises mixing of 42-54 mole% Si3N4, 1.5-3.5 mole%AI2O3, 37-47 mole% AIN, 1.5-6 mole%SiO2and 3.5-5.5 mole% Dy2O3 by known methods passing the powder through 100 mesh and pressing the powder by known methods to form green compacts at a temperature in the range of 1650 to 1900°C sintering in nitrogen atomosphere to obtain dysprosium stabilized α-SiAION. 2. A process for the preparation of dense dysprosium stabillised α-SiAION substantially as herein described with reference to the examples.

Documents:

1058-del-2000-abstract.pdf

1058-del-2000-claims.pdf

1058-del-2000-correspondence-others.pdf

1058-del-2000-correspondence-po.pdf

1058-del-2000-description (complete).pdf

1058-del-2000-form-1.pdf

1058-del-2000-form-19.pdf

1058-del-2000-form-2.pdf