GB2284376A

GB2284376A - A method of bonding ceramic pieces

Info

Publication number: GB2284376A
Application number: GB9423898A
Authority: GB
Inventors: Alexandre Zivkovic; Jean-Pierre Meynckens
Original assignee: Glaverbel Belgium SA
Current assignee: AGC Glass Europe SA
Priority date: 1993-12-06
Filing date: 1994-11-25
Publication date: 1995-06-07
Also published as: GB9423898D0; GB9324991D0

Abstract

Ceramic pieces formed by the sintering of a fine refractory powder having an average particle size below 500 mu m, preferably below 200 mu m are bonded by bringing the pieces, in the presence of an oxygen-containing gas, into contact with a bonding powder mixture. The bonding powder mixture comprises particles of refractory material and particles of at least one material which is exothermically oxidisable. The oxidisable material is oxidised by the oxygen-containing gas to form a bonding mass of refractory material when initiated by heating. The gas may be used to entrain a flow of the bonding powder for projection onto the joint. The invention enables a large scale ceramic body to be formed by forming a plurality of ceramic pieces by sintering a fine refractory powder and bonding the ceramic pieces together as aforesaid.

Description

A METHOD OF BONDING CERAMIC PIECES The present invention relates to a method of bonding ceramic pieces and also to a ceramic body formed by such a method.

In particular the present invention is concerned with pieces and bodies formed of "neo-ceramics".

"Neo-ceramics", are otherwise known as technical ceramics, fine ceramics, advanced ceramics or high-density ceramics.

The term "neo-ceramic" relates to materials having a much finer structure than traditional ceramics such as may be used for forming bricks, sanitary ware or household articles. Neo-ceramics are used in a number of technical applications, particularly in high temperature, mechanical, biological, magnetic and nuclear applications. Such materials have a particularly high mechanical resistance at elevated temperatures which allows their use, in place of metals and alloys, in certain applications such as turbines, space rocket engines, precision instruments etc.

Neo-ceramics may be good heat conductors and may be impermeable to gases. They are obtained from pure, very fine, synthetic powders which are compacted, sintered and worked under strictly controlled conditions. While these materials may differ from previously known ceramics in a number of characteristics, in the context of the present invention we are concerned with ceramic pieces formed by the sintering of a refractory powder having a particle size below 500 Fm, preferably below 200 Clam. This is in contrast to the traditional refractory bodies obtained from granular natural materials.

The preparation of ceramic pieces by sintering of fine refractory particles is discussed in "Preparations et faconage de matriaux SiC, Si3N4, SiAlON et Silcomps" by J L Chermant and F Osterstock, published in Rev. int. hautes Tempter. Refract., Fr., 1980, 17, pp 295-315.

While these ceramic pieces have many advantageous properties, they tend to suffer from their manufacturing process. The possible range of manufactured pieces is limited by a difficulty in working the material and the dimensions of the firing kiln which do not allow large pieces to be produced. It has not therefore been found possible to make large scale bodies without assembling pieces together. To date this assembly has been achieved by gluing or clamping, which does not allow one to obtain all the desirable high temperature benefits. There is therefore a need to find an assembly process capable of remedying these defects.

We have now found a method by which such ceramic pieces may be bonded together, thereby to enable larger scale bodies to be formed.

According to the invention, there is provided a method of bonding ceramic pieces formed by the sintering of a refractory powder having a particle size below 500 pm, preferably below 200 pm, which method comprises bringing the pieces to be bonded, in the presence of an oxygen-containing gas, into contact with a bonding powder mixture comprising particles of refractory material and particles of at least one material which is exothermically oxidisable and causing said oxidisable material to be oxidised by said oxygen-containing gas to form a bonding mass of refractory material.

The bonding mixture comprising refractory particles and particles of oxidisable material are oxidised by the oxygen-containing gas present to release sufficient heat to form a coherent bonding mass on the surfaces of the ceramic pieces to be joined, which we have found provides a joint between the pieces of considerable strength. While not wishing to be bound by theory, we believe that the strength of such a joint may be due to diffusion of molten material of the bonding mass into the ceramic material, possibly together with chemical bonding.

It is surprising that such a mechanically strong joint can be formed without necessarily melting the ceramic material and between materials which are not necessarily of the same composition. It is also surprising that such a mechanically strong joint can be formed from refractory particles having a size which is greater than the fine texture of the ceramic.

By "particle size" as used herein, we mean that the material concerned has a particle size distribution such that at least 90% by weight of particles conform to the given limits. "Average dimension", as used herein, designates a dimension such that 50% by weight of the particles have a smaller dimension than this average.

The ceramic pieces are preferably brought into contact with said bonding powder at an elevated temperature. An elevated temperature below the melting point of the ceramic material may be used.

Preferably, the bonding powder mixture is projected, together with a carrier gas, at the ceramic materials to be bonded, while the latter are at an elevated temperature.

When a projection method is used, the bonding powder mixture is projected in a carrier gas which is preferably selected from oxygen and mixtures thereof with an inert gas, such as nitrogen, argon, carbon dioxide and mixtures thereof. The carrier gas will preferably be free of gaseous combustible substances. Projection is conveniently achieved by the use of a lance. A suitable lance for use in the process of the invention comprises one or more outlets for the discharge of the powder stream, optionally together with one or more outlets for supplementary gas. In some preferred embodiments of the invention, such as when working in a high temperature environment, the gas streams are discharged from a lance which is cooled by fluid circulating through it.

Such cooling may easily be achieved by providing the lance with a water jacket.

The refractory material in the bonding powder mixture may be comprise an oxide, nitride or carbide of silicon or aluminium, or a mixture thereof, and need not be identical to that used for forming the ceramic material piece.

Indeed, we have obtained good results by bonding alumina ceramic pieces with a bonding mass comprising silica, and silicon carbide ceramics with a bonding mass comprising magnesia, alumina and silica.

Preferably, the particles of refractory material in the bonding powder mixture have an average particle size above 200 Fm, such as above 500 pm, but with substantially no particles having a size above 4 mm. It is surprising that one can successfully use such relatively coarse refractory powders for forming a bond between ceramic pieces formed from fine powders. The use of magnesia as a refractory material is particularly useful together with silica, and/or with silicon as the oxidisable material, in that the magnesia transforms the silica of the refractory material or that formed from the oxidation of silicon in the oxidisable material, into a crystalline phase.

The oxidisable material is a material which will oxidise to form a refractory material, which may be the same as, or different to, the refractory material contained in the bonding powder mixture. The particles of oxidisable material may be selected from aluminium, magnesium, silicon and mixtures thereof. Preferably, the particles of oxidisable material in the bonding powder mixture have an average particle size below 100 pm.

Preferably, the bonding powder mixture comprises from 65% to 95% by weight, most preferably from 75% to 90% of refractory particles and from 5% to 20%, most preferably from 10% to 20% of particles of oxidisable material, all based on the total weight of the powder mixture.

Preferably, the ceramic material is selected from alumina, silicon carbide, silicon nitride, sialons and mixtures thereof. Sialons are solid state solutions of oxinitrides of silicon and aluminium, and typical sialons have the formula Si 6xAlxOxN (8-x) where x is between 0 and 6, preferably less than 4.4.

The ceramic pieces are preferably prepared by compacting and sintering powder materials having a high controlled purity.

They are not generally prepared from natural materials. The minor components or impurities, such as alkali metal and alkaline earth metal oxides which may be found in some naturally derived refractory materials may have a negative effect upon the properties of the ceramic pieces formed therefrom. The neo-ceramics may contain added amounts of other constituents, in controlled minor proportions, for example, sodium oxide in alumina, magnesium oxide in silicon nitride and boron, carbon and silicon in silicon carbide.

The presence of such additives facilitates the sintering conditions.

Sintering is achieved by the application of heat and/or pressure. For example, the refractory powder, optionally together with an organic binding agent, is packed into a mould. The mould is then immersed in a tank of liquid, such as water, oil or molten glass according to the temperature being used, to which pressure is applied. After being released from the mould, the so-formed block of compacted powder material can be cut to the required shape. Cutting is easier if the compaction takes place at room temperature, but brittleness is higher. After sintering at high temperatures, the brittleness falls, but cutting becomes more difficult. The required shape of the ceramic material piece may alternatively be achieved by extrusion. Any organic binder present is driven off by heating the sintered ceramic piece. The ceramic pieces prepared as described have relatively small dimensions.

The ceramic material pieces may be in the form of pipes or tiles. Preferably, adjacent edges of the ceramic material pieces to be bonded are formed with a chamfer or is otherwise cut out to receive the bonding powder mixture.

The invention also provides a ceramic body comprising a plurality of ceramic pieces formed by sintering a refractory powder having an average particle size below 500 Clam, preferably below 200 pm, said ceramic pieces being bonded together by bringing the pieces, in the presence of an oxygen-containing gas, into contact with a bonding powder mixture comprising particles of refractory material and particles of at least one material which is exothermically oxidisable and causing said oxidisable material to be oxidised by said oxygen-containing gas to form a bonding mass of refractory material.

The invention will now be further described with reference to the following non-limiting examples.

EXAMPLE 1 Pieces of sintered material comprising silicon carbide impregnated with elemental silicon, were prepared from silicon carbide particles having a size of 100 pm or less.

The pieces were formed by extrusion or pressing and had the shape of tiles with a thickness of 1 cm, provided with chamfered edges. The pieces were disposed, chamfered edge-to-chamfered edge, in an oven where their temperature was raised to 1300"C. A bonding powder was projected in an oxygen carrier gas into the grooves formed between the chamfered edges of adjacent tiles.

The bonding powder mixture had the following composition Refractory material: SiC 79% MgO 8% Oxidisable material Si 8% Al 5% The silicon particles had a dimension below 45 pm and a specific surface area comprised between 2,500 and 8,000 cm2/g. The aluminium particles had a dimension below 45 pm and a specific surface area comprised between 3,500 and 6,000 cam2/9 The dimension of the silicon carbide particles was less than 1.47 mm with 60% by weight from 1 to 1.47 mm, 20% from 0.5 to 1 mm, and 20% below 0.125 mm. The MgO particles had an average dimension of approximately 300 pm.

The joint which was so formed had mechanical and high temperature stability.

As an alternative, instead of being placed in an oven, the pieces may be disposed on a hot surface.

EXAMPLE 2 Pieces of sintered material were prepared from SYALON (Trade Mark) ex Vesuvius International Corp., a commercially available sialon material having a particle size of below 10 Clam. The pieces were formed into the shape of pipes with a diameter of about 30 mm and a wall thickness of 4 mm. The pipe ends were provided with chamfered edges. The pipes were disposed in line, chamfered end edge-to-chamfered end edge, in an oven where their temperature was raised to 1200"C. A bonding powder was projected in an oxygen carrier gas into the annular groove formed between the chamfered edges of adjacent pipes.

The bonding powder mixture had the same composition as in Example 1.

The bonded assembly which was formed in this manner exhibited mechanical resistance and the joint between the two pipes was found to be perfectly gas tight.

EXAMPLE 3 Pieces of sintered material were prepared from alumina particles (99% pure) having a size of less than 100 pm.

The pieces were disposed in an oven where their temperature was raised to over 10000C. A bonding powder was projected having the following composition Refractory material: 12 3 80% CaO 8% Oxidisable material Si 6% Al 6% The silicon particles and the aluminium particles were the same as used in Example 1. The alumina particles were sieved to a particle size of less than 2 mm. The CaO particles had an average dimension of approximately 300 pm.

Claims

1. A method of bonding ceramic pieces formed by the sintering of a refractory powder having a particle size below 500 Am, preferably below 200 pm, which method comprises bringing the pieces to be bonded, in the presence of an oxygen-containing gas, into contact with a bonding powder mixture comprising particles of refractory material and particles of at least one material which is exothermically oxidisable and causing said oxidisable material to be oxidised by said oxygen-containing gas to form a bonding mass of refractory material.

2. A method according to any preceding claim, wherein the ceramic pieces are brought into contact with said bonding powder at an elevated temperature.

3. A method according to claim 2, wherein the bonding powder mixture is projected, together with a carrier gas, at the ceramic pieces to be bonded, while the latter are at an elevated temperature.

4. A method according to claim 3, wherein the carrier gas is selected from oxygen and mixtures thereof with an inert gas.

5. A method according to any preceding claim, wherein the refractory material in the bonding powder mixture comprises an oxide, nitride or carbide of silicon or aluminium, or a mixture thereof.

6. A method according to any preceding claim, wherein the particles of oxidisable material are selected from aluminium, magnesium, silicon and mixtures thereof.

7. A method according to any preceding claim, wherein the particles of refractory material in the bonding powder mixture have an average particle size above 200 pm, preferably above 500 pm.

8. A method according to any preceding claim, wherein the particles of oxidisable material in the bonding powder mixture have an average particle size below 100 pm.

9. A method according to any preceding claim, wherein the bonding powder mixture comprises from 65% to 95% by weight of refractory particles and from 5% to 20% oxidisable material particles.

10. A method according to any preceding claim, wherein the ceramic material comprises one or more materials selected from alumina, silicon carbide, silicon nitride, sialons and mixtures thereof.

11. A method according to any preceding claim, wherein the ceramic material pieces are in the form of pipes or tiles.

12. A method according to any preceding claim, in which adjacent edges of the ceramic material pieces to be bonded are formed with a chamfer to receive the bonding powder mixture.

13. A ceramic body comprising a plurality of ceramic pieces formed by sintering a refractory powder having an average particle size below 500 pm, preferably below 200 pm, said ceramic pieces being bonded together by bringing the pieces, in the presence of an oxygen-containing gas, into contact with a bonding powder mixture comprising particles of refractory material and particles of at least one material which is exothermically oxidisable and causing said oxidisable material to be oxidised by said oxygen-containing gas to form a bonding mass of refractory material.