MANUFACTURE OF SILICON CARBIDE ARTICLES

Abstract: In a method of making a siliconised silicon carbide article by (i) injection-moulding a mixture which comprises a major proportion of silicon carbide particles and a minor proportion of organic binder material, the mixture being flowable under heat and pressure, (ii) heating the moulded article thus formed so as to remove the organic binder material from it and obtain a porous silicon carbide body containing elemental carbon, and (iii)impregnating the porous body with molten silicon to convert said carbon to silicon carbide which binds together the original silicon carbide particles, in which step (iii) is carried out by utilising as a conduit for said molten silicon the sprue that is formed integrally with the injection-moulded article in step (i). The method avoids excessive contact between the surface of the porous intermediate shaped body and molten silicon employed for siliconisation, and so reduces the possibility of damage to that body while it is in the weakly bonded state.

Description: This invention relates to the manufacture of silicon carbide articles, and in particular to manufacture by injection moulding followed by siliconisation. Articles resistant to thermal shock and with good mechanical properties can be made from particles of the highly refractory substance silicon carbide by a variety of methods. In the injection-moulding/siliconisation method, a mixture flowable under heat and pressure which comprises a major proportion of silicon carbide particles and a minor proportion of organic binder material is injected through a narrow orifice to fill a shaped cavity in a mould at lower temperature. The article thus formed is then removed from the mould, and the sprue (ie the waste piece corresponding to the passage in the mould tool between the moulding orifice and the mould cavity) is machined off. The moulded article is then heated to remove the organic binder material from it and leave a porous silicon carbide body containing elemental carbon which (at this stage) weakly binds together the highly refractory silicon carbides particles. The elemental carbon may be present as a result of generation by pyrolysis of the original organic binder material in the mixture submitted to moulding (as in GB-1,454,622) or as a result of its inclusion per se in that mixture. The porous body thus produced is then impregnated with molten silicon to convert the carbon present to silicon carbide. In more detail, the carbon present dissolves in and reacts with the molten silicon which is drawn up into the pores of the body, and the silicon carbide thus formed bonds together the original silicon carbide particles into a unitary structure of great strength. This step is conventionally carried out in a furnace at high temperature, eg 1500-1600°C, and low pressure, eg 0.5mm Hg, as mentioned in the UK Patent No 1,454,622. The present invention is concerned with improving the method just described so as to avoid excessive contact between the surface of the weakly bonded porous shaped body obtained as intermediate product and the molten silicon employed for siliconisation, and thus reduce the possibility of damage to that body. According to the invention there is provided a method for making a siliconised silicon carbide article by (i) injection-moulding a mixture which comprises a major proportion of silicon carbide particles and a minor proportion of organic binder material, the mixture being flowable under heat and pressure, (ii) heating the moulded article thus formed so as to remove the organic binder material from it and obtain a porous silicon carbide body containing elemental carbon, and (iii) impregnating the porous body with molten silicon to convert said carbon to silicon carbide which binds together the original silicon carbide particles, in which step (iii) is carried out by utilising as a conduit for said molten silicon the sprue that is formed integrally with the injection-moulded article in step (i). Preferably elemental carbon, for example in the form of finely divided graphite or carbon black, is included in the mixture in step (i) above. Step (iii) is carried out under the usual conditions, eg in a furnace under very low pressure. Preferably in step (iii) the sprue is used as a wick to pick up molten silicon from a block of porous carbon. The sprue may be inserted in a suitable sized hole in the block of porous carbon, which is supplied with molten silicon, for example either by locating the block in direct contact with molten silicon in a reservoir, or by using the block itself as a reservoir. The porous carbon block may conveniently be a block of porous carbon insulation material, which we have found to function effectively as a medium for mounting components by means of their sprues during siliconisation and for transferring molten silicon into the sprues, which rest in holes in the block and are arranged to be a snug fit to facilitate transfer of silicon from block to sprue. The siliconised sprue that results from operation of the method is of course very strong, and can if desired be left as it is. It may, however, be cut down flush with the surface of the article, or only partly, so as to leave a stub, of maximum length no more than a few mm, say up to 5-10mm, to serve as a location or alignment means, as explained later in this specification. The invention has particular application to the manufacture of sealing rings, but may be used for the manufacture of other, more complex, components, for example turbocharger rotors. The invention will now be further described with reference to the accompanying drawings, in which: Fig. 1 is a plan of an injection-moulded silicon carbide article, with the sprue still present, shaped as a sealing ring; Fig. 2 is a section on line AA in Fig. 1; Fig. 3 is a diagram showing an assembly of four of the articles of Fig. 1 which have been made porous and are mounted vertically for siliconisation. Figure 4 is a plan view of a sealing ring with an anti-rotation lug; and Figure 5 is a section on line BB in Figure 4. In Figs 1 and 2 the ring-shaped injection-moulded silicon carbide article 1 has an interior shoulder 2 and an exterior shoulder 3, and has the sprue 4 formed during injection moulding still integral with it. The article 1 can be made as follows. A mixture of conventional composition (suitably 61% by weight silicon carbide, particle size below 20 µm; 18% by weight graphite, particle agglomerate size below 20 µm; remainder a thermoplastic binder material) is prepared and granulated, and the granules are injection-moulded in a standard injection moulding machine, to form rings of height 6.5mm, external diameter 72.4mm and internal diameter 61mm, with a sprue length of about 40mm. The ring is removed from the mould and, to avoid distortion, is supported while it cools to room temperature. The article is then heated, while still under support, in an air oven. Once the temperature has reached a value at which the binder is soft, the rate of increase is limited to 2°C per hour until a temperature of about 300°C is attained. This heating first oxidises, and then pyrolyses, the thermoplastic binder while leaving the original elemental carbon in the resulting porous structure as a weak binder for the original silicon carbide particles. The density of the porous body formed is about 2.2g/cm³. After the porous ring has cooled, half the length of the sprue is sawn off so as to shorten the travel path of molten silicon subsequently, and the ring is supported vertically with several others by insertion (see Fig.3) of the sprue 4 in recess 11 in a block 10 of porous carbon situated on the surface of a reservoir (not shown) of silicon flakes in a conventional furnace. Operation of the furnace at high temperature and very low pressure in an entirely conventional manner leads to melting of the silicon flakes and transport of liquid silicon from the reservoir into the porous block 10, and through the conduit formed by porous sprue 4 of the porous ring into the ring itself. There it reacts with the elemental carbon present to form silicon carbide, which 'reaction bonds' the particles of silicon carbide originally included in the mix subjected to injection moulding. The density of the finished product is about 3.1 g/cm³. The sprue can be severed by grinding. If this is done so as to leave a stub, say about 3mm high, protruding above the surface of ring 1 (the upper surface in Fig.2), the stub can serve as a location or alignment means for the ring in a metal or other housing of which it is to form a sealing component. A further alignment means derived from a sprue is illustrated in Figures 4 and 5, Figure 5 being a cross-sectional view of the sealing ring of Figure 4. The sealing ring 12 shown in Figures 4 and 5 has two internal shoulders 15, 16 and the two external shoulders 17, 18 and has been produced as described above with a moulding sprue 13 (shown in dotted lines) having been used for siliconisation. However, the moulding sprue in this embodiment has been ground down to form an anti-rotation lug 14 on the sealing ring, extending between the two external shoulders 17 and 18.

Claims: A method of making a siliconised silicon carbide article by (i) injection-moulding a mixture which comprises a major proportion of silicon carbide particles and a minor proportion of organic binder material, the mixture being flowable under heat and pressure, (ii) heating the moulded article thus formed so as to remove the organic binder material from it and obtain a porous silicon carbide body containing elemental carbon, and (iii)impregnating the porous body with molten silicon to convert said carbon to silicon carbide which binds together the original silicon carbide particles, characterised in that method step (iii) is carried out by utilising as a conduit for said molten silicon the sprue (4,13) that is formed integrally with the injection- moulded article in step (i). A method according to claim 1 characterised in that the mixture used in step (i) also comprises elemental carbon. A method according to claim 1 or 2 characterised in that in step (iii) the sprue is used as a wick to pick up molten silicon from a block of porous carbon. A method according to claim 3 characterised in that the sprue is inserted in a suitably sized hole in the block of porous carbon, which is supplied with molten silicon. A method according to any preceding claim characterised in that the siliconised sprue that results is cut down so as to leave a stub, of maximum length no more than a few mm, protruding from the article. A method according to any preceding claim characterised by being applied to the manufacture of a sealing ring.

Inventors: FARTHING, LESLIE JOHN, HOLME, JOHN DAVID, WORDSWORTH, ROBERT ALAN
Applicants: T&N TECHNOLOGY LIMITED
Application year: 1990
Main classification: C04B35
IPC codes: C04B35