GB2155005A

GB2155005A - Silicon nitride refractories

Info

Publication number: GB2155005A
Application number: GB08405595A
Authority: GB
Inventors: Robert William Gardiner; Brian William Viney; Garyth Hughes
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1984-03-02
Filing date: 1984-03-02
Publication date: 1985-09-18
Also published as: GB8405595D0

Abstract

Refractories comprising particles of silicon nitride in a cement matrix are described. The term cement encompasses Ordinary Portland Cement, polymer containing cements, aluminous cement and any other cements used in civil engineering. The refractory may comprise cement in the range from 5 to 30% by weight. Shaped components produced by a simple moulding operation are also described. The refractories are suitable for use in general applications where high abrasion resistance is required and also in applications with some molten metals, e.g. aluminium.

Description

SPECIFICATION Improvements in or relating to refractories This invention relates to refractories for use with molten metals.

Silicon nitride is used as a refractory material in conjunction with molten metal. Monolithic silicon nitride has, however, some disadvantages which render more widespread use uneconomic. Production processes are lengthy and therefore, expensive. Machining of the monolithic is difficult and the scrap rate an be high.

It has now been found that for some applications sufficient of the beneficial properties of silicon nitride may be retained whilst removing most of the disadvantages referred to above.

According to one aspect of the present invention a refractory material comprises particles of silicon nitride in a cement matrix.

The term 'cement' in this specification is intended to encompass Ordinary Portland Cement, polymer containing cements, aluminous cements and any other cements used in civil engineering.

It has been found that relatively thin sections, having acceptable strength, may be produced from this material. It is known to combine particles of one refractory within a matrix of a second refractory the latter refractory usually being in powder form and made into a paste with water for moulding into the desired shape. A major problem with these known refractory mixtures, however, is their low strength in the dried but unfired condition (hereinafter called the green condition or green strength etc).

According to a second aspect of the present invention a method of making a refractory material comprises mixing particles of silicon nitride with cement, adding water and further mixing, moulding into a shape, allowing to harden to a green condition and then firing at elevated temperature.

Refractories according to the present invention have been found to possess adequate green strength to allow safe handling even at matrix proportions of cement as low as 5%.

Furthermore it has been found that the refractory may be cut with conventional diamond saws more easily than monolithic silicon nitride.

Because the refractories of the present invention may use virtually any silicon nitride, whether in the form of ground-up monolithic scrap or silicon nitride powder, the cost thereof may be very much reduced.

Refractories according to the present invention have been employed in apparatus which is used for the production of aluminium alloys by deposition from the vapour phase. The refractories in such apparatus operate under conditions of fairly high vacuum, for example, 10-4 torr and below and are either totally or partially immersed in molten aluminium alloy at temperatures of between 1 200 and in excess of 1600"C. They have been found to have good resistance to attack by the molten aluminium alloy and also to be unaffected by the temperature gradient across the metal line.

In order that the invention may be more fully understood two embodiments will now be described by way of example only.

Example 1 A chimney-shaped, three-sided box with sloping sides was constructed to direct the vapour flux in the evaporation apparatus mentioned above. The refractory mixture was as described below: Silicon nitrode powder 270 g - 8 + 1 8 mesh 130g- 18+30 mesh 400 g - 52 mesh Cement 107 g The powders were thoroughly mixed in the dry state and then mixed with 200 ml of water. The wet mix was then poured into a wooden mould and vibrated at a frequency of 1 20 Hz for about 10 minutes to settle the mixture and exclude air bubbles. The mould and contents were covered with wet rags and the contents allowed to set. Once the refractory had set it was removed from its mould and dried out at 110'C for 24 hours.Firing was accomplished by raising to a temperature of 1330"C over a period of about 8 hours and subsequently allowing to cool slowly.

Wall thickness of the chimney was 12 mm.

Example 2 This component was generally picture-frame shaped. The refractory was as described below Silicon nitride powder 1 60g - 1 8 + 1 8 mesh 75918 18+30 mesh 59g - 30 + 52 mesh 1 76g - 52 mesh Cement 639 The powders were mixed, moulded, dried and fired as described in Example 1.

Wall section of the frame was 22 mm wide x 32 mm deep.

In use the frame of Example 2 was positioned in a horizontal plane within a rebated recess in the top of the metal containing crucible. The molten metal line was maintained within the depth of the frame section.

The chimney of Example 1 sat on top the frame of Example 2. No significant attack of the refractory was noted, even at the metal line.

In the examples given the proportion of portland cement was 13% by weight of the initial dry mixed powders. Other refractory compositions have been produced ranging from about 5% of cement to over 30%.

It will be appreciated that the invention provides material and a process whereby refractories having sufficient of the thermal properties and resistance to corrosion of monolithic silicon nitride may be produced in almost any desired shape by a simple, easily controlled moulding process.

Although in the examples given the refractory was used in apparatus for the evaporation of aluminium alloys there is no reason why the invention cannot be used for other metals in other molten metal containment applications or indeed in more general applications where refractory materials are used. Such applications may be those where the mechanical properties of silicon nitride such as, for example, good abrasion resistance, may be desirable but the cost of monolithic material would be prohibitive. Applications envisaged are furnace hearths operating up to about 1350"C in air and components such as pump impellers in liquid metal feeding apparatus.

It will be appreciated that a very considerable advantage of the present invention is the ability to produce near nett-shaped components easily and quickly. The material may initially be cast into almost any shape by utilising simple, inexpensive moulding tools constructed of wood, rubber etc.

Claims

1. A refractory material comprising particles of silicon nitride in a cement matrix.

2. A refractory material according to claim 1 and wherein the matrix comprises from 5 to 30% by weight of cement.

3. A method of making a refractory material comprising the steps of of mixing particles of silicon nitride with cement adding water and further mixing, moulding into a shape, allowing to harden to a green condition and then firing at elevated temperature.

4. A refractory according to either claim 1 or claim 2 used in a process for the deposition of metal from the vapour phase.

5. A refractory according to either claim 1 or claim 2 for use with molten metal.

6. A refractory according either claim 1 or claim 2 for use as a furnance hearth material.

7. A refractory as hereinbefore described with reference to either of examples 1 or 2 of the specification.