GB2195661A - Sintered silicon nitride based ceramic - Google Patents

Abstract

Ceramic material based on silicon nitride is described employing an oxide mixture as a sintering aid to provide densification at normal temperatures by the pressureless sintering technique. The composition of the sintered material comprises 6 to 12 wt% yttria, 1 to 4 wt% alumina, 1 to 4 wt% chromia and balance silicon nitride. The material may be produced by the sintering of a powder mixture containing silicon nitride or by the reaction bonding process as an intermediate step.

Description

SPECIFICATION Ceramics The present invention relates to the production of silicon nitride ceramics and particularly to additions which improve the sintering behaviour of silicon nitride.

In order to achieve sintering of bodies produced from silicon nitride powders other oxides are usually added as sintering aids. Mixtures of yttria and alumina are well known in this regard and have been used to produce almost fully dense silicon nitride. One technique used for sintering silicon nitride powders is often referred to as pressureless sintering where the gas atmosphere in the sintering furnace is at approximately ambient or one atmosphere. To achieve high density an alumina content in the range 4 to 6 wt% is typically required. Such high levels of alumina are disadvantageous in that the high temperature properties of the resulting sintered silicon nitride are adversely affected. To lessen the adverse effect of the alumina on the high temperature properties the level of alumina additions may be reduced.However, at lower levels of alumina addition it is necessary to increase the sintering temperature to achieve a comparable final density. Because of the increased sintering temperature it then becomes necessary to increase the pressure of the gas atmosphere in which the silicon nitride is sintered to prevent or minimise dissociation of the material. This requires much more costly equipment capable of maintaining relatively high gas pressures at very high sintering temperatures.

An alternative known sintering aid used for pressureless sintering is based on the mixture 10% yttria-2% magnesia-2% chromia. This produces high density silicon nitride. The magnesia, however, tends to be lost from the material due to the high temperature during sintering. Loss of magnesia may be partly compensated for by embedding the ceramic part being sintered in a magnesia containing powder bed. In practice this method is inconvenient and messy and eventually leads to costly furnace degradation.

It has now been found that the above disadvantages may be overcome by the use of a new composition of oxides used as a sintering aid.

According to the present invention a ceramic composition comprises 6 to 12 wt% yttria, 1 to 4 wt% alumina, 1 to 4 wt% chromia and balance silicon nitride.

It has been found that silicon nitride having good high temperature properties and a density greater than 98% theoretical may be produced by pressureless sintering without the need of sintering aids in the powder bed surrounding the ceramic during sintering. The basis of a powder bed used when sintering silicon nitride may typically include 50% boron nitride, 45% silicon nitride and 5% silica.

In order that the present invention may be more fully understood an example will now be given by way of illustration only.

Silicon nitride powder of typically 2-3 micron particle size was mixed with 10 wt% yttria, 2 wt% alumina and 2 wt% chromia by ball milling in PROPAN-2-OL alcohol. The mixture was then dried, sieved and compacted by isostatic pressing at 25,000 p.s.i. The compact was then sintered in 1 atmosphere of nitrogen at 1800 C for 5 hours.

The resulting sintered material had a density of approximately 3,25 gm/cc.

Instead of starting with silicon nitride powder as described above the well-known reaction bonding method may be employed. Silicon powder may be mixed with the sintering aids and prior to final sintering an intermediate stage of heating the compacted powder mixture in nitrogen at temperatures up to 1420"C for many hours to convert the silicon to silicon nitride is used.

1. A ceramic material composition comprising 6 to 12 wt% yttria, 1 to 4 wt% alumina, 1 to 4 wt% chromia and balance silicon nitride.

2. A ceramic material composition according to Claim 1 comprising 10 wt% yttria, 2 wt% alumina, 2 wt% chromia and balance silicon nitride.

3. A process for the production of a ceramic material comprising the steps of mixing a powder having the composition 6 to 12 wt% yttria, 1 to 4 wt% alumina, 1 to 4 wt% chromia and balance silicon nitride, pressing the mixed powder to form a compact and then sintering the compact to densify the material.

4. A process for the production of a ceramic material comprising the steps of mixing a powder having the composition 6 to 12 wt% yttria, 1 to 4 wt% alumina, 1 to 4 wt% chromia and balance silicon, pressing the mixed composition to form a compact, performing a reaction bonding treatment on the pressed compact and then sintering to densify the material.

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

Claims (4)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Ceramics The present invention relates to the production of silicon nitride ceramics and particularly to additions which improve the sintering behaviour of silicon nitride. In order to achieve sintering of bodies produced from silicon nitride powders other oxides are usually added as sintering aids. Mixtures of yttria and alumina are well known in this regard and have been used to produce almost fully dense silicon nitride. One technique used for sintering silicon nitride powders is often referred to as pressureless sintering where the gas atmosphere in the sintering furnace is at approximately ambient or one atmosphere. To achieve high density an alumina content in the range 4 to 6 wt% is typically required. Such high levels of alumina are disadvantageous in that the high temperature properties of the resulting sintered silicon nitride are adversely affected. To lessen the adverse effect of the alumina on the high temperature properties the level of alumina additions may be reduced.However, at lower levels of alumina addition it is necessary to increase the sintering temperature to achieve a comparable final density. Because of the increased sintering temperature it then becomes necessary to increase the pressure of the gas atmosphere in which the silicon nitride is sintered to prevent or minimise dissociation of the material. This requires much more costly equipment capable of maintaining relatively high gas pressures at very high sintering temperatures. An alternative known sintering aid used for pressureless sintering is based on the mixture 10% yttria-2% magnesia-2% chromia. This produces high density silicon nitride. The magnesia, however, tends to be lost from the material due to the high temperature during sintering. Loss of magnesia may be partly compensated for by embedding the ceramic part being sintered in a magnesia containing powder bed. In practice this method is inconvenient and messy and eventually leads to costly furnace degradation. It has now been found that the above disadvantages may be overcome by the use of a new composition of oxides used as a sintering aid. According to the present invention a ceramic composition comprises 6 to 12 wt% yttria, 1 to 4 wt% alumina, 1 to 4 wt% chromia and balance silicon nitride. It has been found that silicon nitride having good high temperature properties and a density greater than 98% theoretical may be produced by pressureless sintering without the need of sintering aids in the powder bed surrounding the ceramic during sintering. The basis of a powder bed used when sintering silicon nitride may typically include 50% boron nitride, 45% silicon nitride and 5% silica. In order that the present invention may be more fully understood an example will now be given by way of illustration only. Silicon nitride powder of typically 2-3 micron particle size was mixed with 10 wt% yttria, 2 wt% alumina and 2 wt% chromia by ball milling in PROPAN-2-OL alcohol. The mixture was then dried, sieved and compacted by isostatic pressing at 25,000 p.s.i. The compact was then sintered in 1 atmosphere of nitrogen at 1800 C for 5 hours. The resulting sintered material had a density of approximately 3,25 gm/cc. Instead of starting with silicon nitride powder as described above the well-known reaction bonding method may be employed. Silicon powder may be mixed with the sintering aids and prior to final sintering an intermediate stage of heating the compacted powder mixture in nitrogen at temperatures up to 1420"C for many hours to convert the silicon to silicon nitride is used. CLAIMS

1. A ceramic material composition comprising 6 to 12 wt% yttria, 1 to 4 wt% alumina, 1 to 4 wt% chromia and balance silicon nitride.

2. A ceramic material composition according to Claim 1 comprising 10 wt% yttria, 2 wt% alumina, 2 wt% chromia and balance silicon nitride.

3. A process for the production of a ceramic material comprising the steps of mixing a powder having the composition 6 to 12 wt% yttria, 1 to 4 wt% alumina, 1 to 4 wt% chromia and balance silicon nitride, pressing the mixed powder to form a compact and then sintering the compact to densify the material.

4. A process for the production of a ceramic material comprising the steps of mixing a powder having the composition 6 to 12 wt% yttria, 1 to 4 wt% alumina, 1 to 4 wt% chromia and balance silicon, pressing the mixed composition to form a compact, performing a reaction bonding treatment on the pressed compact and then sintering to densify the material.