GB2268190A

GB2268190A - Sintering silicon nitride

Info

Publication number: GB2268190A
Application number: GB9311007A
Authority: GB
Inventors: John Woodthorpe; Carol Jayne Pindar
Original assignee: T&N Technology Ltd
Current assignee: Federal Mogul Technology Ltd
Priority date: 1992-06-27
Filing date: 1993-05-28
Publication date: 1994-01-05
Anticipated expiration: 2013-05-28
Also published as: GB9311007D0; GB2268190B

Abstract

A pre-shaped body made from powder comprising silicon nitride, yttria and silica is sintered to form a shaped article with at least 95% of theoretical density. The sintering is carried out at a temperature of at least 1750 DEG C and at a pressure below 20 Mpa. The oxygen content of the powder is selected to be between 4.1 and 5.2% by weight enabling use of this low pressure.

Description

SINTERING SILICON NITRIDE This invention is concerned with sintering silicon nitride.

It is well known to manufacture articles requiring high flexural strength at high temperatures, e.g. turbine blades, from a sinterable ceramic powder. Such articles need to have a high relative density, e.g at least 95%, and preferably 99%, of theoretical density. However, such powders are difficult to sinter to this high relative density and this is normally achieved by a process known as hot-isostatic pressing (hipping). In hipping, a pre-shaped body of the sinterable powder is prepared and encapsulated in a gas-impermeable substance, e.g. a high-melting point glass. The encapsulated body is then subjected to a temperature of about 17500C in a nitrogen atmosphere under high pressure (about 160 Mpa (1600 atmospheres)). At this temperature, the encapsulating substance becomes plastic allowing the pressure from the nitrogen to squeeze the preshaped body.Because of the very high pressures involved, hipping involves very expensive equipment.

The discussion of hipping given above, is generally applicable to a range of sinterable ceramic powders of which silicon nitride is one. Sinterable powder of silicon nitride (Si3N4) is normally combined with minor proportions of sintering aids such as yttria (Y203) and silica (SiO2).

For example, between 3 and 11% by weight of the yttria may be added and between 0 and 4% by weight of silica. With silicon nitride, it is necessary, if an article with sufficient strength which will not deteriorate over time is.

to be achieved, for the sintered article not to contain a significant amount of the apatitephase of silicon nitride and also not to contain a significant proportion of silicon oxynitride.

It is an object of the present invention to provide a method of making a shaped article from sinterable silicon nitride which does not involve such high pressures as hipping and which avoids the formation of significant proportions of the apatite phase of silicon nitride and the formation of a significant proportion of silicon oxynitride.

The invention provides a method of making, from a sinterable powder, a shaped article with at least 95% of theoretical density, the method comprising preparing a preshaped body made of a powder comprising silicon nitride, yttria and silica, wherein the oxygen content of the powder is between 4.1 and 5.2% by weight, and sintering the powder by applying a pressure thereto below 20 Mpa at a temperature of at least 17500C.

The invention is based on the discovery that, if the oxygen content of the powder is maintained in the specified range, the powder can be successfully sintered at relatively low pressures without the occurrence of the apatite phase or the formation of silicon oxynitride.

Furthermore, it is not necessary to encapsulate the preshaped body, although this can be done if desired. Because the pressures involved are much lower than in hipping, much less expense is necessary for equipment.

In determining the oxygen content of the powder, it is necessary to take account of oxygen present in the silicon nitride powder, this oxygen being present as an impurity in the form of silicon dioxide, and also to take account of oxygen present in the yttria and any added silica.

As in hipping, the pressure can be applied by nitrogen gas.

Preferably, the pressure is applied in two stages, a first stage at a lower pressure and temperature, e.g. 15 bar and 1800"C, (to compress the powder before the highest pressure is applied so that the highest pressure gas does not enter pores in the body and prevent further compression thereof) and a second stage at a higher pressure and temperature, e.g. 80 bar and 19000C.

The yttria may be present in between 6.0 and 9.0 weight per cent and the silica may be present (in addition to any present in the silicon nitride) in between 2.0 and 4.0 weight per cent.

Table 1 gives examples of seven powder formulations which were used in a method according to the invention.

These formulations were made into a pre-shaped body which was sintered by pressure applied by nitrogen gas without encapsulation. In a first stage, a gas pressure of 15 bar was maintained for 2 hours and a temperature of 18000C and, in a second stage, the pressure was raised to 80 bar which was maintained for 2 hours at a temperature of 19000C. The resulting articles were then examined to detect the structure. ' In all cases, the structure was found to be primarily the beta phase of silicon nitride. The table describes other substances detected. The structure was found to contain large and small grain sizes, instead of the all small grain structure produced by hipping, which is considered to be advantageous in avoiding creep.

TABLE 1

EXAMPLE Si3N4 Y2O3 SiO2 Oxygen Content Detected Substances 1 91.0 6.0 3.0 4.12 alpha and beta Y2Si2O7 2 90.0 7.5 2.5 4.16 beta Y2SiO7, traces of alpha Y2Si2O7 and apatite 3 89.5 7.5 3.0 4.42 beta Y2Si2O7 4 89.0 7.5 3.5 4.68 beta Y2Si2O7 5 88.5 7.5 4.0 4.93 beta Y2Si2O7, traces of alpha Y2Si2O7 6 88.5 8.5 3.0 4.61 beta Y2Si2O7 7 87.5 8.5 4.0 5.13 beta Y2Si2O7 Table 2 shows, for comparison purposes, examples falling outside the terms of the invention. These examples were prepared in the same way as those of Table 1.

TABLE 2

Example Si3N4 Y2O3 SiO2 Oxygen Content Detected Substances 8 92.5 7.5 0 2.87 alpha/beta - Y2Si2O7, apatite 9 91.5 7.5 1.0 3.39 Apatite, trace of alpha Y2Si2O7 10 86.5 7.5 6.0 5.45 beta Y2Si2O7 Si2ON2

Claims

CLAIMS 1 A method of making, from a sinterable powder, a shaped article with at least 95% of theoretical density, the method comprising preparing a pre-shaped body made of a powder comprising silicon nitride, yttria and silica, wherein the oxygen content of the powder is between 4.1 and 5.2% by weight, and sintering the powder by applying a pressure thereto below 20 Mpa at a temperature of at least 17500C.
2 A method according to Claim 1, wherein the pressure is applied by means of nitrogen gas.
3 A method according to either one of Claims 1 and 2, wherein the yttria content is between 6 and 9% by weight.
4 A method according to any one of Claims 1 to 3, wherein the silica content is between 2 and 4% by weight.
5 A method according to any one of Claims 1 to 4, wherein the pressure is applied in two stages, a first stage at a lower pressure and a lower temperature followed by a second stage at a higher temperature and a higher pressure.
6 A method of making, from a sinterable powder, a shaped article substantially as hereinbefore described with reference to any one of the examples 1 to 6.
7 A shaped article made by a method according to any one of Claims 1 to 6.