GB2189507A

GB2189507A - Silicon nitride materials

Info

Publication number: GB2189507A
Application number: GB08708788A
Authority: GB
Inventors: Professor Richard John Brook; Helen Pickup
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1986-04-14
Filing date: 1987-04-13
Publication date: 1987-10-28
Also published as: GB8708788D0; GB2189507B; GB8609067D0

Abstract

A silicon nitride body is prepared by sintering a silicon nitride powder and a mixture of barium oxide, aluminium oxide and silicon dioxide. The silicon dioxide may be or may include silicon dioxide present in the silicon nitride. The additive gives rise to a liquid (glassy) phase to assist sintering, which liquid phase crystallises to form a refractory crystalline phase at the grain boundaries thereby endowing the body with good high temperature strength.The refractory phase may, for example be celsian (BaOAl2O32SiO2).

Description

SPECIFICATION Silicon nitride materials This invention relates to the preparation of sintered silicon nitride.

Silicon nitride powder is difficult to sinterto produce a body of good high temperature strength because the diffusion rate in the solid state is low.

The use of additives as sintering aids has therefore been proposed in the art; the additives provide a grain boundary phase which then assists densification. However, such a phase can be detrimental to the high temperature mechanical properties of the body produced since the presence of fluid phase at the grain boundaries can allow itto deform and subsequentlyfracture.

This invention is concerned with the use of additives for providing a liquid during sintering but which subsequently crystallises to give a sintered product of good high temperature strength. Thus, in the invention, a method of preparing a sintered silicon nitride body comprises the steps of (i) preparing a mixture of silicon nitride powder and an additive comprising barium oxide, aluminium oxide and silicon dioxide, the additive being capable of forming a crystallisable liquid phase at elevated temperature; and (ii) heat treating the mixture under conditions to form the liquid phase and sinterthe silicon nitride, andtocrystallisetheliquid phase.

It has been found that the crystalline phase produced by crystallising the liquid (glassy) phase endows the silicon nitride body produced with good high temperature mechanical properties as illustrated in the examples herein.

The molar ratio of BaOto A1203 and that of BaO and Awl203 combined to SiO2 may be varied as discussed below. An example ofthe crystalline phase produced, which is obtained in the grain boundaries, is celsian, which has the composition BaO.Al2O3.2SiO2, and it has been found that, ifthe molar ratio ofthe BaO to A1203 is 1 :1,thesilicon nitride body produced has superior high temperature mechanical strength. Celsian, having a melting point of 1 760'C, imparts high temperature strength to the body.In general, if the molar ratio of Al203 to BaO exceeds 1 the boundary phase is more refractory and, if the molar ratio of BaO to Al203 exceeds 1 :1,the boundary phase is less refractory.

The molar ratio of BaO plusAI203to SiO2 mayalso be varied and, generally, if an excess of BaO and Al203 above that of celsian is used, a greater quantity of boundary phase and faster densification is achieved. In contrast, use of a molar ratio of BaO and Al203 belowthatofcelsian gives rise to slower densification and a smaller quantity of boundary phase. The quantity of the boundary phase can therefore be optimised to meet specific requirements such as ease offabrication or improved high temperature strength.

It is known that silicon nitride powder usually contains a quantity of SiO2,forexamplefrom 2-10% by weight, and this quantity is to be included when calculating desired molar ratios as discussed above.

Thus, when carrying out the invention, it may not be necessary to provide additional SiO2 in step (i) if sufficient SiO2 to provide a desired molar ratio is already present in the silicon nitride. The term "additive" therefore includes SiO2 present in the silicon nitride.

Step (i) may be carried out in a number ofways.

For example, a slurry of Al203 powder, a BaO precursor powder such as Ba(NO3)2, and Si3N4 powder may be evaporated to dryness to produce BaO. Also, the total proportion of the additive may be varied in accordance with particular requirements.

Step (ii) may be carried out as a single processing stage, i.e. sothat crystallisation takes place immediately after sintering. The step may, however, be carried out in two separate stages, for example by firstly sintering and then annealing to crystallise the liquid phase. Step (ii) may conveniently be carried outwith orwithoutapplying pressure.

Several ways of carrying out the invention will now be described in the following examples.

Example 1 Aluminium oxide powder and barium nitrate powderwere added to silicon nitride powder (surface area 14.8 m2/g) in a slurry with propan-2-ol and the mixture micronised. The proportions ofthe mixture were 73.6 mol % silicon nitride, 6.4 mol % SiO2 (contained in the Si3N4), 10 mol % BaO and 10 mol % Al203. The slurry was evaporated to dryness and any soft agglomerates broken down using light miliing.

The resulting mixture was hot pressed in a graphite die at 1700'Cfor60 minutes art a pressure of 10 MPa. This gave a sintered silicon nitride body of density 3.2 g/cm3. X-ray diffraction indicatedthatthe body contained crystalline celsian and a three point bending method showed its strength to be 254 MPa at 1400 C.

Example 2 Asintered silicon nitride body was prepared as described in Example 1 and then annealed at 1550"C for 60 minutes under a pressure of 5 MPa ion a graphite die. The product, which had a density of 3.2 g/cm3, was found to have a strength of 278 MPa at 1400"C measured by a three point bending method. It was also found to have a greater quantity of ceisian than the product of Example 1.

Example 3 A mixture prepared as in Example 1 was sintered at 1800 Cfor8 hours under an atmosphere of 0.7 MPa of nitrogen, without the application of pressure.

The product had a density of 2.88 g/cm3.

Example 4 The procedure of Example 3 was repeated with the exception that the temperature was 1 700"C. The density of the product was 2.95 g/cm3.

1. A method of preparing a sintered silicon nitride body comprising the steps of

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Silicon nitride materials This invention relates to the preparation of sintered silicon nitride. Silicon nitride powder is difficult to sinterto produce a body of good high temperature strength because the diffusion rate in the solid state is low. The use of additives as sintering aids has therefore been proposed in the art; the additives provide a grain boundary phase which then assists densification. However, such a phase can be detrimental to the high temperature mechanical properties of the body produced since the presence of fluid phase at the grain boundaries can allow itto deform and subsequentlyfracture. This invention is concerned with the use of additives for providing a liquid during sintering but which subsequently crystallises to give a sintered product of good high temperature strength. Thus, in the invention, a method of preparing a sintered silicon nitride body comprises the steps of (i) preparing a mixture of silicon nitride powder and an additive comprising barium oxide, aluminium oxide and silicon dioxide, the additive being capable of forming a crystallisable liquid phase at elevated temperature; and (ii) heat treating the mixture under conditions to form the liquid phase and sinterthe silicon nitride, andtocrystallisetheliquid phase. It has been found that the crystalline phase produced by crystallising the liquid (glassy) phase endows the silicon nitride body produced with good high temperature mechanical properties as illustrated in the examples herein. The molar ratio of BaOto A1203 and that of BaO and Awl203 combined to SiO2 may be varied as discussed below. An example ofthe crystalline phase produced, which is obtained in the grain boundaries, is celsian, which has the composition BaO.Al2O3.2SiO2, and it has been found that, ifthe molar ratio ofthe BaO to A1203 is 1 :1,thesilicon nitride body produced has superior high temperature mechanical strength. Celsian, having a melting point of 1 760'C, imparts high temperature strength to the body.In general, if the molar ratio of Al203 to BaO exceeds 1 the boundary phase is more refractory and, if the molar ratio of BaO to Al203 exceeds 1 :1,the boundary phase is less refractory. The molar ratio of BaO plusAI203to SiO2 mayalso be varied and, generally, if an excess of BaO and Al203 above that of celsian is used, a greater quantity of boundary phase and faster densification is achieved. In contrast, use of a molar ratio of BaO and Al203 belowthatofcelsian gives rise to slower densification and a smaller quantity of boundary phase. The quantity of the boundary phase can therefore be optimised to meet specific requirements such as ease offabrication or improved high temperature strength. It is known that silicon nitride powder usually contains a quantity of SiO2,forexamplefrom 2-10% by weight, and this quantity is to be included when calculating desired molar ratios as discussed above. Thus, when carrying out the invention, it may not be necessary to provide additional SiO2 in step (i) if sufficient SiO2 to provide a desired molar ratio is already present in the silicon nitride. The term "additive" therefore includes SiO2 present in the silicon nitride. Step (i) may be carried out in a number ofways. For example, a slurry of Al203 powder, a BaO precursor powder such as Ba(NO3)2, and Si3N4 powder may be evaporated to dryness to produce BaO. Also, the total proportion of the additive may be varied in accordance with particular requirements. Step (ii) may be carried out as a single processing stage, i.e. sothat crystallisation takes place immediately after sintering. The step may, however, be carried out in two separate stages, for example by firstly sintering and then annealing to crystallise the liquid phase. Step (ii) may conveniently be carried outwith orwithoutapplying pressure. Several ways of carrying out the invention will now be described in the following examples. Example 1 Aluminium oxide powder and barium nitrate powderwere added to silicon nitride powder (surface area 14.8 m2/g) in a slurry with propan-2-ol and the mixture micronised. The proportions ofthe mixture were 73.6 mol % silicon nitride, 6.4 mol % SiO2 (contained in the Si3N4), 10 mol % BaO and 10 mol % Al203. The slurry was evaporated to dryness and any soft agglomerates broken down using light miliing. The resulting mixture was hot pressed in a graphite die at 1700'Cfor60 minutes art a pressure of 10 MPa. This gave a sintered silicon nitride body of density 3.2 g/cm3. X-ray diffraction indicatedthatthe body contained crystalline celsian and a three point bending method showed its strength to be 254 MPa at 1400 C. Example 2 Asintered silicon nitride body was prepared as described in Example 1 and then annealed at 1550"C for 60 minutes under a pressure of 5 MPa ion a graphite die. The product, which had a density of 3.2 g/cm3, was found to have a strength of 278 MPa at 1400"C measured by a three point bending method. It was also found to have a greater quantity of ceisian than the product of Example 1. Example 3 A mixture prepared as in Example 1 was sintered at 1800 Cfor8 hours under an atmosphere of 0.7 MPa of nitrogen, without the application of pressure. The product had a density of 2.88 g/cm3. Example 4 The procedure of Example 3 was repeated with the exception that the temperature was 1 700"C. The density of the product was 2.95 g/cm3. CLAIMS

1. A method of preparing a sintered silicon nitride body comprising the steps of (i) preparing a mixture of silicon nitride powder and an additive comprising barium oxide, aluminium oxide and silicon dioxide,theadditive being capable of forming a crystallisable liquid phase at elevated temperature; (ii) heat treating the mixture under conditions to form the liquid phase and sinterthe silicon nitride, and to crystallisethe liquid phase.

2. A method according to claim 1 wherein the composition ofthe additive is such that the phase formed by crystallising the liquid phase comprises celsian.

3. A method according to either ofthe preceding claim wherein the molar ratio of barium oxide to aluminium oxide in the additive is 1:1.

4. A method according to any ofthe preceding claims wherein step (i) is carried out by preparing a slurry in a liquid medium of silicon nitride powder containing silicon dioxide, aluminium oxide powder and a barium saltconvertibleto barium oxide on heating, and evaporating the scurry to dryness two convert the barium salt to barium oxide.

5. A method according to any ofthe preceding claims wherein step (ii) is carried out by firstly sinteringthesilicon nitride and then annealing to crystallisethe liquid phase.

6. Amethodofpreparing asinteredsilicon nitride bodysubstantiallyas described herein with reference to anyofthe examples.

7. Asinteredsilicon nitride body made by a method according to any of the preceding claims.