GB2081240A - Silicon Carbide Bodies - Google Patents

Abstract

A self-bonded silicon carbide body produced by siliconising a preformed mixture of particles (shaped by means other than slip-casting) of carbon and silicon carbide in the beta form has a mean grain size in the range of 0.1-5 microns. Such a body may be produced using silicon carbide particles having a mean surface area in the range 0.5-20 square metres per gram. The silicon carbide particles may be produced by heating a mixture of silica and silicon to generate silicon monoxide vapour and passing the vapour through a bed of particulate carbon.

Description

SPECIFICATION Silicon Carbide Bodies This invention relates to silicon carbide bodies and, in particular, to the production of bodies of self-bonded silicon carbide by reaction-sintering of a coherent mixture of particles of silicon carbide and carbon in the presence of molten silicon. Such reaction-sintering is hereinafter referred to as "siliconising" and one method of siliconising is described in UK Patent Specification No. 1,1 80,918.

The coherent mixture of particles of silicon carbide and carbon may be shaped prior to siliconising by any convenient method such as extrusion, injection moulding, slip-casting or pressing and in our copending application Serial No. 10253/77 there is disclosed and claimed the slip-casting of a coherent mixture in which the silicon carbide particles are in the beta form.

The present invention consists in a method of producing a self-bonded silicon carbide body by shaping a coherent mixture of particles of carbon and silicon carbide in the beta form by means other than slip-casting, the silicon carbide particles having a mean surface area in the range 0.5-20 square metres per gram, and siliconising the coherent mixture.

The present invention also consists in a selfbonded silicon carbide body so produced.

A self-bonded silicon carbide body in accordance with the invention, when compared with a self-bonded silicon carbide body produced using particles of alpha silicon carbide, has improved properties, in particular, in the extent and nature of deformation and microcracking around indentations. For example in 500 g load Knoop indentation tests cracking was much more localised and damage far less extensive. Also there is a greater dependence of hardness on load and may be higher hardness at low loads. These results indicate that bodies in accordance with the invention will behave in general in a more plastic manner, have less tendency to crack catastrophically and show greater wear resistance and surface toughness.

The fine silicon carbide particles in beta form are preferably produced by passing silicon monoxide through a bed of particulate carbon which is converted to silicon carbide powder, the silicon monoxide vapour being generated by heating a mixture of silica and silicon separately from the bed of particulate carbon.

The coherent mixture of silicon carbide and carbon is conveniently shaped prior to siliconising by extrusion, injection moulding, or pressing. The following are examples of ways of carrying the invention into effect.

Example 1 A mix containing carbon and beta-silicon carbide powders in the ratio 0.5:1 by weight, and sufficient polymeric binder to provide 42% porosity in the fully-consolidated body on removal of the binder, was formed into a cylindrical pellet by pressing at about 50 MN/m2 with the exclusion of air. The silicon carbide powder had a surface area of 3.7 m2/g and the carbon powder consisted of crystallites which formed agglomerates with a surface area of about 6 m2/g. The pellet was extruded through a profiled die to form components of uniform cross-section and the extrudate was cut and heated to 4000C to volatilise the binder. The 'green' material was then fired at 1 6500C in the presence of molten silicon to convert it to a 90% dense silicon carbide containing 10% free silicon.

Example 2 A mix containing carbon and beta-silicon carbide powders, of the same size as in Example 1 but in the ratio 0.25:1 by weight, and sufficient polymeric binder to form a hard, rigid body on compaction, was pressed isostatically at about 100 MN/m2 to form a component which was subsequently 'green machined', using a diamond tool. The 'green' material was heated to 4000C to volatilise the binder and was then fired at 16500C in the presence of molten silicon to convert it to a 90% dense silicon carbide containing 10(46 free silicon.

Example 3 A beta silicon carbide powder, surface area 0.8 m2/g, was mixed with graphite powder, surface area 60 m2/g, in the ratio 1:0.4 by weight. Binder and lubricants were mixed in and rods 4 mm in diameter were extruded. After removal of the binder the rods were siliconised at 16500C for 2 hours in a vacuum of 1 torr. Density of the rod was 3.12 g/m2 (10% by volume free silicon) the mean grain size was about 5 microns and the Knoop hardness at 50g load was 3,650 hg/mm2.

Claims

1. A method of producing a self-bonded silicon carbide body by shaping a coherent mixture of particles of carbon and silicon carbide in the beta form by means other than slip-casting, the silicon carbide particles having a mean surface area in the range of 0.5-20 square metres per gram, and siliconising the coherent mixture.

2. A method of producing a self-bonded silicon carbide body as claimed in claim 1 wherein the silicon carbide particles have a mean surface area less then 5 square metres per gram.

3. A method of producing a self-bonded silicon carbide body as claimed in claim 1 or claim 2 wherein the silicon carbide particles in the mixture are produced by passing through a bed of particulate carbon silicon monoxide vapour generated separately by heating a mixture of silicon and silica.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Silicon Carbide Bodies This invention relates to silicon carbide bodies and, in particular, to the production of bodies of self-bonded silicon carbide by reaction-sintering of a coherent mixture of particles of silicon carbide and carbon in the presence of molten silicon. Such reaction-sintering is hereinafter referred to as "siliconising" and one method of siliconising is described in UK Patent Specification No. 1,1 80,918. The coherent mixture of particles of silicon carbide and carbon may be shaped prior to siliconising by any convenient method such as extrusion, injection moulding, slip-casting or pressing and in our copending application Serial No. 10253/77 there is disclosed and claimed the slip-casting of a coherent mixture in which the silicon carbide particles are in the beta form. The present invention consists in a method of producing a self-bonded silicon carbide body by shaping a coherent mixture of particles of carbon and silicon carbide in the beta form by means other than slip-casting, the silicon carbide particles having a mean surface area in the range 0.5-20 square metres per gram, and siliconising the coherent mixture. The present invention also consists in a selfbonded silicon carbide body so produced. A self-bonded silicon carbide body in accordance with the invention, when compared with a self-bonded silicon carbide body produced using particles of alpha silicon carbide, has improved properties, in particular, in the extent and nature of deformation and microcracking around indentations. For example in 500 g load Knoop indentation tests cracking was much more localised and damage far less extensive. Also there is a greater dependence of hardness on load and may be higher hardness at low loads. These results indicate that bodies in accordance with the invention will behave in general in a more plastic manner, have less tendency to crack catastrophically and show greater wear resistance and surface toughness. The fine silicon carbide particles in beta form are preferably produced by passing silicon monoxide through a bed of particulate carbon which is converted to silicon carbide powder, the silicon monoxide vapour being generated by heating a mixture of silica and silicon separately from the bed of particulate carbon. The coherent mixture of silicon carbide and carbon is conveniently shaped prior to siliconising by extrusion, injection moulding, or pressing. The following are examples of ways of carrying the invention into effect. Example 1 A mix containing carbon and beta-silicon carbide powders in the ratio 0.5:1 by weight, and sufficient polymeric binder to provide 42% porosity in the fully-consolidated body on removal of the binder, was formed into a cylindrical pellet by pressing at about 50 MN/m2 with the exclusion of air. The silicon carbide powder had a surface area of 3.7 m2/g and the carbon powder consisted of crystallites which formed agglomerates with a surface area of about 6 m2/g. The pellet was extruded through a profiled die to form components of uniform cross-section and the extrudate was cut and heated to 4000C to volatilise the binder. The 'green' material was then fired at 1 6500C in the presence of molten silicon to convert it to a 90% dense silicon carbide containing 10% free silicon. Example 2 A mix containing carbon and beta-silicon carbide powders, of the same size as in Example 1 but in the ratio 0.25:1 by weight, and sufficient polymeric binder to form a hard, rigid body on compaction, was pressed isostatically at about 100 MN/m2 to form a component which was subsequently 'green machined', using a diamond tool. The 'green' material was heated to 4000C to volatilise the binder and was then fired at 16500C in the presence of molten silicon to convert it to a 90% dense silicon carbide containing 10(46 free silicon. Example 3 A beta silicon carbide powder, surface area 0.8 m2/g, was mixed with graphite powder, surface area 60 m2/g, in the ratio 1:0.4 by weight. Binder and lubricants were mixed in and rods 4 mm in diameter were extruded. After removal of the binder the rods were siliconised at 16500C for 2 hours in a vacuum of 1 torr. Density of the rod was 3.12 g/m2 (10% by volume free silicon) the mean grain size was about 5 microns and the Knoop hardness at 50g load was 3,650 hg/mm2. Claims

1. A method of producing a self-bonded silicon carbide body by shaping a coherent mixture of particles of carbon and silicon carbide in the beta form by means other than slip-casting, the silicon carbide particles having a mean surface area in the range of 0.5-20 square metres per gram, and siliconising the coherent mixture.

2. A method of producing a self-bonded silicon carbide body as claimed in claim 1 wherein the silicon carbide particles have a mean surface area less then 5 square metres per gram.

3. A method of producing a self-bonded silicon carbide body as claimed in claim 1 or claim 2 wherein the silicon carbide particles in the mixture are produced by passing through a bed of particulate carbon silicon monoxide vapour generated separately by heating a mixture of silicon and silica.

4. A method of producing a self-bonded silicon carbide body substantially as herein before described in any one of the foregoing Examples.

5. A self-bonded silicon carbide body produced by the method claimed in any preceding claim.