GB1574761A - Method of manufacturing bodies of silicon nitride - Google Patents

Description

(54) METHOD OF MANUFACTURING BODIES OF SILICON NITRIDE (71) We, ASEA AKTIEBOLAG, a Swedish Company of Västerås, Sweden, do hereby declare the invention, for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following Statement: Silicon nitride, chemical formula Si3N4, is a ceramic material which has met with considerable interest during recent years as a possible material for the construction of components which are intended to operate at high temperatures and/or in corrosive atmospheres. Unlike most other high strength ceramic materials, the ability of silicon nitride to withstand thermal shock is excellent, this having to do with its low coefficient of thermal expansion.

Applications which are of particular interest for silicon nitride include material for turbine wheels, rotor blades and other dynamically stressed parts of gas turbines, especially gas turbines used for vehicle operation, material for Wankel engine parts, and bearing material.

Silicon nitride products are manufactured by two well known methods, namely by hot pressing or by reaction bonding.

Hot-pressed products are usually manufactured by compressing silicon nitride powder together with magnesium oxide as a sintering-promoting additive in a graphite tool at a temperature of from 1700 to 1800"C and a pressure which may reach 20 to 30 MPa (IMPa = 10 bar). The quantity of magnesium oxide added is from 0.8 to 5 per cent of the weight of the silicon nitride. For this hot-pressing there is used a silicon nitride which at least substantially consists of silicon nitride of a-phase type and which thus only contains a small quantity of silicon nitride of p-phase type, silicon nitride of a-phase type contains small quantities of oxygen in the molecules. During the hot-pressing, the magnesium oxide reacts to form magnesium silicate, and silicon nitride of a-phase type is converted into silicon nitride of p-phase type. The formation of magnesium silicate is deemed to occur due to the action of silicon dioxide which exist as a thin coating on the silicon nitride grains, and also due to the action of the oxygen present in the silicon nitride of phase type. Generally the silicate also contains a certain amount of calcium. The magnesium silicate forms a glasslike binding phase between the silicon nitride particles. The magnesium oxide is of decisive importance for the hot-pressing process and contributes to the formation of a dense, sintered product having very good strength. It is known, however, that the magnesium silicate phase in the sintered product causes a reduction in the strength of the product at high temperatures, such as temperatures of 900"C and above, a reduction which does not occur with reaction-bonded silicon nitride. It is further known to use yttrium oxide, instead of magnesium oxide, as sintering-promoting additive, and this leads to an improvement of the strength at high temperatures. The addition of yttrium oxide may, in the known case, amount to from 1.0 to 3.5 per cent and even higher percentages, of the weight of the silicon nitride, and the compression is performed in a graphite tool at a temperature of from 17500 to 18000C and a pressure of 40 to 50 MPa.

Reaction-bonded silicon nitride is manufactured by producing a porous body of silicon powder which is then nitrided with nitrogen gas. The body can easily be shaped prior to nitridisation, and the shape changes very little during the process. The porosity of the finished product exceeds 15%. Unlike products made from hotpressed silicon nitride, the strength of products made from reaction bonded silicon nitride is practically independent of temperature, at least up to temperatures of 1600"C, but the strength is considerably less than for hot pressed silicon nitride. Reaction-bonded silicon nitride cannot therefore be used for the manufacture of components which are subjected to high stress in use.

The present invention aims to provide a method of manufacturing a silicon nitride body of considerably higher strength, both at room temperature and at higher tempera tures, than when using previously known -methods for manufacturing products of silicon nitride containing yttrium oxide.

According to the invention a method of manufacturing a body of silicon nitride by compressing, at a temperature of at least 1600"C, either a powder mixture of silicon nitride and a material chosen from the group consisting of yttrium oxide under heating, or a preformed body of the powder mixture, is characterised in that a silicon nitride contain ing, at the most, 1 per cent by weight of impurities in the form of oxides of elements other than silicon and yttrium is used, and in that the compression is performed by isostatic pressing at a pressure of at least 100 MPa with a gas as the pressure medium and with the powder or the preformed body enclosed in a casing.

When yttrium oxide is mixed with the sili con nitride it is preferably used in an amount of from 0.2 to 20 per cent, and advantageously from 0.5 to 10 per cent, of the total weight of silicon nitride and yttrium oxide.

When using an yttrium compound other than yttrium oxide, an amount of the yttrium compound is used which is stoichiometric with that of the yttrium oxide. Examples of such yttrium compounds are yttrium for mate, yttrium oxalate, yttrium carbonate, yttrium acetate, yttrium propionate, yttrium butyrate and yttrium nitrate.

The casing is preferably evacuated prior to the isostatic compression at a temperature of at'least-6000C. If the yttrium compound consists of an yttrium compound other than the oxide, the yttrium compound is then transformed into yttrium oxide with the evolution of gaseous products.

The isostatic pressing may be performed at a pressure of from 150 to 1500 MPa, and preferably at a pressure of from 200 to 300 "MPa. In view of the strength properties of the product and other properties, it is no disadvantage to use pressures higher than 1500 MPa, but the practical difficulties of maintaining such pressures for a sufficient period of time at the required temperature are very great.

The preferred temperature during the isostatic pressing is from 1600 to 1900"C.

particularly preferred range being from 1700 to 18000C.

The pressing time can vary from half an hour to several hours.

Because the compression is achieved by 'isostatic pressing, the products produced will have approximately the same strength in all directions, which is not the case with hotpressed products manufactured in graphite tools, where the strength in the direction of pressing and the strength perpendicular to the direction of pressing show differences, probably due to a certain orientation of the particles during the pressing. Another property os isostatic pressing is that products having complicated shapes may be manufactured directly by the pressing without, or substantially without, subsequent machining by tools, for example by grinding. Because the silicon nitride has a very considerable hardness, this is most important. One further important property of isostatic pressing is that the use of pressing tools is avoided, and thus the very great material problems connected therewith, which are caused by the highe pressures and temperatures requited.

The powder is preferably preformed into a preformed product by subjecting the pwoder to a compaction, suitably arranged in- a sealed capsule of yieldable material. This compaction can be performed advantageously without a-temporary binder at a pressure of at least 100 MPa, preferably from '100 to 1500 MPa, -and at room temperature or a temperature which is considerably below the temperature employed during the subsequent isostatic -pressing. There-after the preformed product can be given its desired shape by machining. Alternatively, conventional techniques for producing ceramic mouldings may be used, for example slip casting, injection moulding or compression moulding. In those cases the powder is usually mixed prior to the compaction with a temporary binder, for example methyl cellulose, cellulose nitrate or an acrylic binder.

After the moulding, the binder can be driven off by heating the preformed product.

After this compaction at low temperature, the preformed product is enclosed in a gastight casing, preferably of a glass such as Vycor Trade Mark glass or quartz glass, or some other glass having a sufficiently high melting point for it not to penetrate into the pores of the body to be subjected to the compression. Instead of high melting point glass, other materials can be used for the casing which are yieldable at the temperatures used for sintering. The casing is preferably evacuated and sealed before the powder body is-subjected to the pressure and temperature conditions required for the sintering.

It is of advantage if, after the casing has been sealed but before the sintering, the casing with its contents is heated to a temperature at which the material of the casing becomes easily mouldable so that the casing is in contact with and closely follows the shape of the preformed body.

The casing may consist of a preformed capsule, for example of glass of the above mentioned kind. However, it is also possible to have the casing formed on the spot by dipping the preformed body in a suspension of particles of glass or other yieldable material, or in some other way surrounding the body with a layer of particles of glass or other yieldable material, and then heating the body, preferably under vacuum, to such a temperature that the particles form a dense casing around it.

It is, however, possible to perform the compression without preforming the powder, the powder then being filled into a glass capsule with the same shape as, but with greater dimensions than, required in the finished compressed silicon nitride body.

According to an advantageous embodiment of the invention, the gas-tight casing consists of glass and the glass casing with its contents is cooled after the isostatic pressing at a rate of 1000"C per hour at the most, and preferably at a rate of 700"C per hour at the most. In this way it has proved possible to avoid completely, or almost completely, the risk of cracks forming in the produced silicon nitride body during the cooling. Such a risk is greatest if the silicon nitride body is of complicated shape, and is primarily due to the different coefficients of thermal expansion of silicon nitride and glass. Particularly if the glass casing has relatively thick walls it is also essential for obtaining a good result that the pressure during cooling is kept at a maximum of 10 MPa, preferably not more than 1 MPa.

It has been found that these measures influence the mechanical properties of the glass so that the casing cracks more easily during the cooling process than if the cooling is not regulated and the pressure not reduced. Because the glass is weakened the risk of unfavourable influence on the silicon nitride body is reduced.

The silicon nitride powder used preferably has a powder grain size less than 100 microns with a crystal grain size less than 5 microns and may, at least for the main part, consist of silicon nitride of a-phase type. Any silicon nitride of this type is converted during the sintering to silicon nitride of p-phase type.

The content of impurities in the form of oxides of elements other than silicon and yttrium in the silicon nitride is, as previously mentioned, 1 per cent by weight at most, and preferably is less than 1 per cent by weight.

Particularly preferred is a silicon nitride in which the content of impurities amounts to 0.6 per cent by weight at the most.

Preferably, the yttrium oxide or other yttrium compound is added in the form of a powder having a powder grain size less than 100 microns and a crystal size less than 5 microns. The silicon nitride and yttrium oxide powders are suitably mixed by wet milling in a suitable liquid, for example methanol. Alternatively, the yttrium compound may possibly be added in the form of a coating on the silicon nitride grains.

The invention will now be described, by way of example, with reference to the accompanying drawings, which which Figure 1 is a side view of a press in which a preformed product of silicon nitride powder can be manufactured by compaction at room temperature, this Figure showing the stand of the press separated from the pressure chamber, Figure 2 is a sectional view of the press of Figure 1, but showing the press stand around the pressure chamber, Figures 3a and 3b are sectional views of two different preformed products each arranged in a casing in the form of a glass capsule, and Figure 4 is a partly sectioned side view of a high-pressure furnace in which the final compression and sintering can be carried out.

The press shown in Figures 1 and 2, comprises a frame 1 which supports a pressure chamber in the form of a high-pressure cylinder 2. The frame 1 is provided with two lugs 3 through which a rod 4 is inserted. The cylinder 2 is provided with two lugs 5 having bearing surfaces dimensioned to fit the rod 4.

The distance between the axially outer surfaces of the lugs 5 is less than the distance between the axially inner surfaces of the lugs 3. This means that the cylinder 2 is axially displaceable through a distance equal to the difference in distance between said surfaces.

The cylinder normally rests on the lower lug 3, a gap 6 thus being formed between the upper lugs 3 and 5. Cylinder 2, which is built up of an inner tube 10 and a surrounding prestressed strip sheath 11, is sealed by an upper end closure 7 and a lower end closure 8 each projecting somewhat into the cylinder.

These end closures are sealed against the cylinder by sealing rings 12 and 13, respectively. The upper end closure is provided with a venting channel 9. Through a channel 16 in the end closure 8 the cylinder communicates with a pressure medium source (not shown). The press comprises a displaceable press stand 22 which is supported by wheels 23 running on raisl (not shown) on the floor 25. The press stand is of the type consisting of an upper yoke 26, a lower yoke 27, and a pair of spacers 28, these items being held together by a prestressed strip sheath 29. The press stand is displaced between the position shown in Figure 1 and Figure 2 by means of an operating cylinder 30. The vertical height of the opening in the press stand is somewhat greater than the distance between the end surfaces of the end closures 7 and 8 when completely inserted in the cylinder 2.

The cylinder 2 is held at such a vertical position on the frame 1 that the clearances 33 and 34 between the yokes 26, 27 of the press stand and the end closures 7, 8 respectively, of the cylinder are approximately equal.

The press of Figures 1 and 2 is employed to compact a preformed product made from a powder mixture consisting of 2 per cent by weight of yttrium oxide powder, having a powder grain size less than 5 microns, and 98 per cent by weight of silicon nitride powder having a powder grain size less than 7 microns and containing 0.5 per cent by weight of impurities in the form of oxides of elements other than silicon and yttrium. The powder mixture is placed in a capsule of plastics material, for example plasticised polyvinyl chloride, or of rubber and the capsule is sealed. The capsule containing the silicon nitride powder is placed in the cylinder 2, the upper end closure 7 is inserted, and the press stand is displaced from the position shown in Figure 1 to the position shown in Figure 2. The cylinder 2 is filled with pressure medium, preferably a liquid such as oil or an oil-water emulsion, through the channel 16, the cylinder being vented through the channel 9 and a venting valve (not shown). When the cylinder 2 has been filled, the venting valve is closed and the pressure raised to a pressure of about 600 MPa. This causes the end closures 7 and 8 to become pressed outwardly against the yokes 26 and 27 which take up the pressure forces acting on the end closures. The capsule was compacted at this pressure (at room temperature) for a time of 5 minutes.

After completion of the compaction and decompression, the capsule with the preformed product is removed from the cylinder 2. The preformed product, which has a density of 65 per cent of the theoretical density, is enclosed, possibly after a certain amount of machining, in a "Vycor" or quartz glass capsule 35 as shown in Figure 3a or 3b in which the preformed product is designated by the numeral 36. The previously mentioned capsule used for the compaction at room temperature may have a shape which is the same as the shape of the body 36. The capsule 35 is larger than the product 36 so that there is a gap between the product and the wall of the capsule. Of course. when the product 36 is placed in the capsule. the latter has no indentation in the region 38. The capsule 35 is then placed in a furnace at a temperature of 1000"C and is there degassed to a pressure of 0. l Pa for a period of 8 hours by a vacuum pump connected to the open end 37 of the capsule. The capsule is then sealed at this pressure by fusing the material of the capsule in the region 38. The sealed capsule containing the preformed product 36 is then preheated in a furnace to a temperature of 1250 C, so that the material of the capsule becomes sufficiently soft to be easily mouldable, and is placed in the high-pressure chamber of the high-pressure furnace shown in Figure 4.

In Figure 4 the numeral 22 designates a press stand which is of the same kind as the press stand shown in Figures 1 and 2 and which is movably arranged on raisl 24 between the position shown in the Figure and a position in which the stand surrounds the high-pressure chamber 42. The highpressure chamber 42 is supported by a frame 49 and includes a high-pressure cylinder, which is built up from an inner tube 50, a surrounding prestressed strip sheath 51 and end rings 52 which hold together the strip sheath axially and constitute suspension means by which the high-pressure chamber is attached to the frame 49. The chamber 42 has a lower end closure 53 projecting into the tube 50. The end closure has a circumferential groove in which a sealing ring 54 is inserted, a channel 55 for supplying a gaseous pressure medium, suitably argon or helium; to the furnace space 66 and a channel 56 for cables for feeding heating elements 57 for heating the furnace. The elements 57 are supported by a cylinder 58, resting on an insulating base 59, which projects into an insulating sheath 60. The chamber 42 has an upper end closure which includes an annular part 61 provided with a sealing ring 62 sealing against the inner wall of the tube 50. The sheath 60 is suspended from the part 61 and connected thereto in a gas-tight manner. The upper end closure also comprises a cover 63 for closing the opening of the part 61. the latter usually being permanently mounted in the high-pressure cylinder. The cover 63 is provided with a sealing ring 64 which seals against the inner wall of the part 61 and with an insulating member 65 which, when the high-pressure chamber is closed, projects into the cylinder 60 and- constitutes part of the insulating shell which surrounds the furnace space 66. The cover 63 is fixed to a bracket 67 supported by a vertically movable and rotatable operating rod 68. As described in relation to the press according to Figures 1 and 2, kthe yokes 27 and 26 take up the pressure forces acting on the end closure 53 and the cover 63-, respectively, when pressure is applied in the furnace space.

When heating the capsule 35 according to Figure 3a or 3b in the high-pressure furnace according to Figure 4, the furnace is first heated to at least the same temperature as that to which the capsule 35 has been preheated. After the capsule has been placed in the furnace space 66, the cover 63 first being raised and then lowered for closing the furnace space, the pressure and the temperature are increased successively to 200 MPa and 1740"C, respectively, and are maintained at these values for 4 hours when the desired density and sintering are obtained. The finished product is then cooled off. Particularly if the preformed product 36 has the shape shown in Figure 3b, it is suitable first to lower the pressure at maintained temperature to around 0.5 MPa. Thereafter the power supply to the heating elements 57 is reduced so that the temperature is decreased at a rate of around 500"C an hour until the product has cooled to room temperature or a manageable higher temperature. When the product has been removed from the highpressure furnace the glass capsule can be removed, for example by blasting. If it should be necessary, the finished product may possibly be subjected to grinding or polishing by diamond tools. The density of the finished product exceeds 99.5 per cent of its theoretical density.

The method is, of course, practicable for manufacturing bodies of any shape and is particularly suitable for use in the case of bodies having very complicated shapes.

WHAT WE CLAIM IS: 1. A method of manufacturing a body of silicon nitride by compressing, at a temperature of at least 16000C, either a powder mixture of silicon nitride and material chosen from the group consisting of yttrium oxide and other yttrium compounds which form yttrium oxide under heating, or a preformed body of the powder mixture, characterised in that silicon nitride containing, at the most, 1 per cent by weight of impurities in the form of oxides of elements other than silicon and yttrium is used, and in that the compression is performed by isostatic pressing at a pressure of at least 100 MPa with a gas as the pressure medium and with the powder or the preformed body enclosed in a casing.

2. A method according to claim 1, in which the weight of yttrium oxide in the powder mixture is from 0.2 to 20 per cent of the total weight of silicon nitride and yttrium oxide.

3. A method according to claim 1, in which the weight of yttrium compound, calculated as yttrium oxide, in the powder mixture is from 0.2 to 20 per cent of the total weight of silicon nitride and yttrium compound, calculated as oxide.

4. A method according to any of claims 1 to 3, in which the compression is performed at a pressure of from 150 to 1500 MPa.

5. A method according to any of claims 1 to 4. in which the compression is performed at a pressure of from 200 to 300 MPa.

6. A method according to any of claims 1 to 5. in which the compression is performed at a temperature of from 1600 to 1900"C.

7. A method according to any of claims l to 6. in which the compression is performed at a temperature of from 1700 to 1800"C.

8. A method of manufacturing a body of silicon nitride substantially as herein described with reference to the accompanying drawings.

9. A body of silicon nitride when made by the method claimed in any of the preced

Claims (1)

1. ing claims.