IE65999B1 - Process for producing moldings from silicon-infiltrated silicon carbide - Google Patents

Abstract

In order to siliconise porous mouldings of silicon carbide/carbon, a mixture of silicon carbide powder, organic binder and optionally carbon is moulded to give a green compact, the binder of the green compact is removed in a nonoxidising atmosphere by carbonisation at about 1000 DEG C and the blank obtained is siliconised by the action of molten silicon, the blank obtained resting on a porous SiSiC support whose lower part is in contact with the liquid silicon, and the arrangement of SiSiC support and resulting moulding being cooled after the end of siliconisation. The support used is a dense packing of porous SiSiC rings which are arranged parallel to one another and at right angles on a graphite combustion plate which is charged with silicon and is impermeable to liquid silicon.

Description

The invention relates to a process for silicizing porous moldings of silicon carbide/carbon, using a dose packing of porous silicon-infiltrated silicon carbide rings, as an infiltration aid.

In the infiltration of silicon carbide moldings with silicon, zones of particularly high silicon content, socalled silicized tracks, and in addition not fully silicized components frequently form. Both these phenomena cause waste.

According to the process of BP-A 0,134,254, the infiltration of the silicon carbide/carbon molding takes place via a porous silicon carbide plate which is provided with a coating of boron nitride, silicon carbide and carbon. Underneath the silicon carbide plate and, if appropriate, to the side thereof, there is (before the furnace is heated up) lumpy elemental silicon. It has been found, however, that the use of plate-shaped SiSiC infiltration aids has disadvantages. When the silicon, which is located between the firing plates and the silicon carbide plates used as infiltration aids, has melted, the plates sink due to gravity through the silicon (which has a lower density) and displace the latter. Due to the surface tension, silicon facings of up to 5 mm in height can form on the firing plate. If, after the silicon carbide plates used as infiltration aids have sunk through, a higher facing ware to form, this would cause the silicon to run off the plate (Figures la and b) . Since the quantity of silicon provided on the firing plate is such that it is just sufficient to infiltrate completely the components located thereon and also to compensate for evaporation losses, the run-off of silicon leads to incomplete infiltration of at least soma components. A precautionary extra provision of silicon on the firing plate is ruled out, since otherwise the components are joined after cooling so firmly to the infiltration aid that detachment without damage is impossible. If the silicon runs from one firing plate to a plate located below, it will cause sticking on the latter.

As mentioned above, the silicon level rises due to sinking of the SiSiC plates into the molten silicon. This can have the result that, when too low plates are used, the components come into direct contact with the molten silicon. This leads to inhomogeneous infiltration and to the occurrence of stresses in the component, which frequently manifests itself by formation of cracks. The resulting cracks are filled with silicon and are visible as silicized tracks in the component. It might be possible to prevent this effect by a reduction in the quan15 tity of silicon per firing plate or by increasing the thickness of the silicon carbide plates. However, both possibilities adversely affect the economics of the process.

The process described, using SiSiC plates, is also used for silicizing blanks which have at least one plane supporting surface.

Because of the small thickness of the porous carrier plate used, and the small distance from the molten silicon, however, it was difficult to achieve homogeneous silicization. This problem, however, can be solved at least partially by arranging the carrier plate on an SiSiC container with molten silicon (DE-A-3,719,606), so that it is no longer in direct contact with the molten silicon.

Furthermore, a plate-shaped substrate also has the disadvantage that the silicized molding sticks to it by its plane supporting surface and is therefore difficult to remove.

DE-A-2,910,628 has disclosed a process, in which a first annular blank of 70 to 95% of silicon carbide, the - 3 remainder being carbon, is silicized in contact with a second annular pressing of 87 to 97% of silicon, the remainder being carbon. On heating, the second pressing is converted into a highly porous, fragile silicon carbide matrix which can readily transport the excess silicon into the first silicon carbide/carbon blank, λ disadvantage is that each fragile matrix can be used only once and must then be removed. Xt is an advantage, however, that substrate and molding are mutually matched geometrically.

It was therefore the object to provide a process in which, after the infiltration process, ready detachment of the moldings is possible, without the latter being damaged by flaking, and the infiltration aid being usable repeatedly. The invention is based on the realization that the contact surfaces between the blank to be silicized and the carrier, i.e. the infiltration aid, should be as small as possible.

A process for silicizing porous moldings of silicon carbide/carbon has now been found, in which a mixture of silicon carbide powder, organic binder and, if appropriate, carbon is molded to give a green compact, the binder of the green compact is removed by carbonization at about 1000 °C in a non-oxidizing atmosphere and the resulting blank is silicized by the action of molten silicon, while the resulting blank rests on a porous SiSiC carrier, whose lower part is in contact with the molten silicon, the assembly of SiSiC carrier and resulting molding being cooled after completion of the silicization. The carrier used here is a close packing of porous SiSiC rings which are arranged mutually parallel and perpendicularly on a graphite firing plate which is charged with silicon and is impermeable to molten silicon.

A plurality of rings can also carry a single blank. If the molding has at least one plane outer surface, with which it rests on the rings, it is advantageous if the rings have the same height. The common contact surface between carrier rings and silicized blank placed thereon (and hence the difficulties in removal of the blank after the silicization) can be reduced by rounding the top surfaces of the rings.

It is possible for the rings to be closed at the bottom and that the interior of the containers thus formed is likewise filled with silicon. If the blank has a spherical shape, the sphere diameter should be greater than the internal diameter of the ring, and one spherical blank should in each case be in contact with one carrier ring. In order to accelerate the infiltration of the spherical blanks, rings can be used which are chamfered on the inside, so that the spheres are in areal contact with the carrier rings. Figures 2 and 3 show two close packings of individual tubes (1) which are set up on a graphite plate (15).

The rate at which the silicon migrates through the SiSiC rings can be controlled by the number of pores and the magnitude of the pore diameter in the aid. This effect has already been described (for the silicization of a green compact) in Special Ceramics 5, 1970, in connection with Figure 6.

A reduction in the pore diameter brakes the flow of silicon, and an increase accelerates the latter. The magnitude of the pore radii depends on the silicon carbide grain sizes employed in the preparation of the firing aids (rings). Coarse grain sizes give large pores, and fine grain sizes give small pores. By means of a suitable selection of the silicon carbide grain sizes, the pore radii distributions can thus be adjusted within a wide range, if required.

Claims (8)

1. A process for silicizing a porous molding of silicon carbide/carbon, in which a mixture of silicon carbide powder, organic binder and, if appropriate, carbon is molded to give a green compact, the binder of the green compact is removed by carbonization at about 1000°C in a non-oxidizing atmosphere and the resulting blank is silicized by the action of molten silicon, while the resulting blank rests on a porous SiSiC carrier, whose lower part is in contact with the molten silicon, the assembly of SiSiC carrier and resulting molding being cooled after completion of the silicization, wherein the carrier used is a close packing of porous SiSiC rings (1) whose axes of symmetry are arranged mutually parallel and perpendicularly on a graphite firing plate (15) which is charged with silicon and is impermeable to molten silicon.

2. The process as claimed in claim 1, wherein a plurality of rings carry one blank, the rings have the same height and the molding has at least one plane outer surface, with which it rests on the rings.

3. The process as claimed in claim 1, wherein the ring surfaces are rounded at the top.

4. The process as claimed in claim 1, wherein the rings are closed at the bottom and the interior of the containers thus formed is likewise filled with silicon.

5. The process as claimed in claim 1, wherein the blank has a spherical shape, the sphere diameter is greater than the internal diameter of the ring, and each spherical blank is in contact with only one carrier ring. -

6. 6. The process as claimed in claim 5, wherein the rings are chamfered on the inside, so that the spheres are in areal contact with the carrier rings.

7. A process as claimed in claim 1, substantially as hereinbefore described with reference to the accompanying drawings.

8. A porous molding of silicon carbide/carbon, whenever treated by a process claimed in a preceding claim.