IE912808A1

IE912808A1 - Process for producing moldings from silicon-infiltrated¹silicon carbide

Info

Publication number: IE912808A1
Application number: IE280891A
Authority: IE
Original assignee: Hoechst Ceram Tec Ag
Priority date: 1990-08-09
Filing date: 1991-08-08
Publication date: 1992-02-12
Also published as: DE4025239C1; JPH072586A; FI913756A; FI913756A0; NO913096D0; EP0470621A2; EP0470621A3; NO913096L

Abstract

In order to siliconise porous mouldings of silicon carbide or silicon carbide/carbon which have at least one flat outer surface, 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 resulting blank is siliconised by the action of liquid silicon at temperatures of at least 1400 DEG C. The blank obtained rests with its flat outer surface on a porous SiSiC support whose lower part is in contact with the liquid silicon. The contact surface of the support is flat and has several recesses.

Description

HOECHST CERAMTEC AG HOE 90/C 010 Dr.SP/sch Description Process for producing moldings from silicon-infiltrated silicon carbide The invention relates to a process for silicizing porous 5 moldings of silicon carbide/carbon, having a plane outer surface, using a porous, plane carrier which is composed of silicon-infiltrated silicon carbide and has recesses.

In the infiltration of silicon carbide moldings with silicon, zones of particularly high silicon content, so10 called silicized tracks, and in addition not fully silicized components frequently form. Both these phenomena cause waste.

According to the process of EP 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 were 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 - 2 infiltration of at least some 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, on the one hand, by a reduction in the quantity 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.

In the case of a small thickness of the porous carrier plate used, i.e. at a smaller distance from the molten silicon, however, it is difficult to achieve homogeneous silicization.

By means of the process indicated in German Offenlegungsschrift 3,719,606, wherein the porous SiC plate is arranged on a container for molten silicon, some of the disadvantages described can be overcome. However, the disadvantage remains that blanks having plane supporting surfaces can in many cases be removed only with difficulty from a plane porous SiC plate, after the - 3 silicization has been completed.

It was therefore the object to provide a process, by means of which moldings having at least one plane outer surface can be silicized and in which the silicized articles can easily be removed from the substrate without damage.

A process for silicizing porous moldings of silicon carbide/carbon, having at least one plane outer surface, has now been found, in which process 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 at temperatures of at least 1400 °C, while the resulting blank rests with a plane outer surface on a porous SiSiC carrier, whose lower part is in contact with molten silicon, the assembly of SiSiC carrier and resulting molding being cooled after completion of the silicization, wherein the supporting surface of the carrier is plane and has a plurality of recesses.

According to one embodiment of the process according to the invention, the porous SiSiC carrier is in the form of a grate which, in addition to outer walls, also has at least one inner wall, the outer walls and inner walls ending at the top at the same height (i.e. they are suitable for depositing an article having a plane outer surface) and at least the outer walls being in contact with the molten silicon.

The carrier can also assume the form of a container, i.e. it can be suitable for receiving molten silicon at the bottom. It is, however, also possible to deposit the grate on a plate known per se, on which lumpy silicon will also be fused later. To increase the silicon flow, the inner walls can also be brought so far down that they - 4 are (like the outer walls) in contact with the molten silicon.

The grate forming the deposition surface has preferably a plurality of parallel inner walls. It is also possible for two systems of parallel inner walls to be present, which intersect and, in particular, are arranged at right angles to one another.

If the outer surface of the blank to be silicized is completely plane, i.e. has neither ribs nor recesses, the common contact surface can be reduced if the inner walls of the grate-shaped carrier have recesses at the upper edge. Preferably, the recesses at the upper edge are rectangular, so that the upper side of the inner walls is given the shape of a crenellated wall. It is also possible at the same time to shape the outer walls correspondingly.

The common contact surface between the carrier and the deposited silicized blank (and hence the difficulties in removing the blank after silicizing) can also be reduced by rounding the upper ends of the grate walls.

Figures 2 to 5 show SiSiC carriers 11 in the form of a grate (1) which, in addition to outer walls 2, also contains at least one inner wall 3. The inner walls of Figures 2 to 4 show a plurality of parallel inner walls.

The inner walls of Figure 3 show two systems of parallel inner walls (3 and 3') which are arranged at right angles to one another. In Figure 4a, the inner walls and some of the outer walls have rectangular recesses (4) at the upper edge.

According to another embodiment of the process according to the invention, the porous SiSiC carrier is in the form of a can which stands on its side walls, the side walls being in contact with the molten silicon at the bottom, and the outer bottom of the can pointing upwards and - 5 being provided with upward-protruding ribs.

Preferably, the can is circular and the ribs are arranged concentrically. However, the bottom can also be annular, i.e. it can have a concentrically arranged recess. Here again, the ribs should be arranged concentrically. Using such an SiSiC carrier, blanks can preferably be silicized which have a T-shaped cross-section. In this case, the central part of the blank is received by the central perforation of the carrier and the blank rests on its bar with the T-shaped cross-section.

The rate at which the silicon migrates through the infiltration aid 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 the 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. Coarse grain sizes (e.g. F 230) give large pores, and fine grain sizes (e.g. F 1200) 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.

In Figure 6, an SiSiC carrier 12 is shown which has the shape of a circular can which stands on its circular side wall 2, the bottom of the can (5) being annular and having a concentrically arranged recess 6. The can also has, in the upward-pointing outer bottom, a plurality of ribs 7 which are likewise arranged concentrically. The side wall of the can is (not shown) at the bottom in contact with the molten silicon. In the Figure, a blank 8 having an approximately T-shaped cross-section is deposited on the carrier. The central part (10) of the - 6 blank is received by the central recess 6 in the carrier The bars of the blank (8) having the T-shaped cross section are supported on the ribs 9.

Claims

1. A process for silicizing a porous molding of silicon carbide or silicon carbide/carbon, having at least one plane outer surface, in which process a mixture 5 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 e C in a non-oxidizing atmosphere and the resulting blank is silicized by 10 the action of molten silicon at temperatures of at least 1400°C, while the resulting blank rests with a plane outer surface on a porous SiSiC carrier, whose lower part is in contact with the molten silicon, wherein the supporting surface of the 15 carrier is plane and has a plurality of recesses.

2. The process as claimed in claim 1, wherein the carrier surface is in the form of a grate which, in addition to outer walls, also has at least one inner wall, the outer walls and inner walls end at the top 20 at the same height and at least the outer walls are in contact with the molten silicon.

3. The process as claimed in claim 2, wherein the grate has a plurality of parallel inner walls.

4. The process as claimed in claim 3, wherein there are 25 two systems of parallel inner walls, which are arranged at right angles to one another.

5. The process as claimed in claim 2, wherein the inner walls have recesses at the upper edge.

6. The process as claimed in claim 5, wherein the 30 recesses at the upper edge are rectangular, so that the upper side of the inner walls is given the shape of a crenellated wall. HOE 90/C 010

7. The process as claimed in claim 1, wherein the upper ends of the walls are rounded.

8. The process as claimed In claim 1, wherein the SiSiC carrier is in the form of a can which stands on Its side walls, the side walls are In contact with the molten silicon at the bottom, and the outer bottom of the can points upwards and Is provided with ribs.

9. The process as claimed in claim 8, wherein the can is circular and the ribs are arranged concentrically.

10. The process as claimed in claim 8, wherein the bottom of the can is annular and has a concentrically arranged perforation, and the ribs are arranged concentrically.

11. The process as claimed In claim 10, wherein a blank is silicized which has a T-shaped cross-section, with the central part located in the central perforation of the carrier.

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

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