SILICON CARBIDE BODIES

Abstract: A self-bonded silicon carbide body produced by siliconising a preformed mixture of particles of carbon and silicon carbide in the beta form has a mean grain size in the range 0.1-5 microns. Such a silicon carbide body may be produced using silicon carbide particles having a mean surface area in the range 05-20 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: Silicon Carbide 3odies. ' This invention relates to silicon carbide bodies a in particular, to the production of bodies of self-bond silicon carbide by reaction sintering of a preformed mixture of particles of silicon carbide and carbon in t presence of molten silicon. Such reaction sintering is hereinafter referred to as "siliconising" and one metho of siliconising is described in UK patent Specification 1,180,918. The present invention consists in a self-bonded sil carbide body produced by siliconising a preformed mixtu of particles of carbon and silicon carbide wherein the silicon carbide in the mixture is in the beta form and t silicon carbide in the body has a mean grain size in the range 0.1-5 microns. Tb.e present invention also consists in a method of producing a self-bonded silicon carbide body by siliconi sing a preformed mixture of particles of carbon and sili carbide in the beta form, the silicon carbide particles having a mean surface area in the range 0.5-20 square ■ metres per gram, and in a self-bonded silicon carbide bo so produced. A self-bonded silicon carbide body in accordance wi the invention, when compared with a self-bonded silicon carbide body produced using particles of aloha silicon carbide has improved properties, in particular in the extent and nature of deformation an microc ac ing* aroun indentations. For example in 500g load Snoop in ntation tests cracking was much more localised and damage far les extensive. Also there is a greater dependence of hardnes 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 coherent mixture of silicon carbide and carbon may be formed prior to siliconising by any convenient method such as extrusion, injection moulding, slip- casting or pressing. 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 following are examples of ways of carrying the invention into effect. Ξxample 1. A mix containing carbon and beta-silicon carbide powders in the ratio 0.5:1 ' ay weight, and sufficient polymeric binder to provide 42- porosity in the fully-consolidated body on removal of the binder, was formed into a cylinc— rical pellet by pressing at about _>0 I-UT/rn wi the exclu sion of air. The silicon carbide Oc er had a surface area of 3.7 m /g and the carbon powder consisted oi crystallites which formed agglomerates with a surface ar of about β m /g. The pellet was extruded through a profiled die to form components of uniform cross-sectio and the extrudate was cut and heated to 4OO°C to volatil the binder. The *green' material was then fired at 1650 in the presence of molten silicon to convert it to a 9Oi dense silicon carbide containing I Cra 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 j and sufficient polymeric binder to form a hard rigid body on compaction, was pressed isostatically at 2 about 100 IT/m to form a component which was subsequent 'green machined', using a diamond tool. The 'green' material was heated to 4OO°C to volatilise the binder an was then fired at 155O°O in the presence of molten silic to convert it to a 90 dense silicon carbide containing free silicon. Example 3 Beta silicon carbide powder, surface area 2 2/g, was mi with carbon black, surface area 5 m /g, in the ratio 1:0 by weight. An aqueous slip was made up and a slip-cast slab was made. The slab was dried and siliconised at 1650 O for 2 hours in a vacuum of 1 torr. After cooling excess silicon was removed from the surface by abrasive blasting and the density of the slab was found to be 3.O4g/ci_r that is, it contained 1 by volume free silicon. The mean grain size in the slab was approximately 0.7 micro Example 4 2 Beta silicon carbide powder, surface area 4.4 in /g ss mixe 2 with carbon black, surface area 6 /g, in the ratio of 1:O A slab was formed as in Example 3 a nd siliconised at 16OO C for 30 minutes. The density was 2.92 g/car (33?= by volume . free silicon) and the mean grain size was 0.5 microns. Example 5 A beta silicon carbide powder, surface area O.δ m /g was • mixed with graphite powder, surface area 60 /g in the ratio 1:0.4 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 165O°C for 2 hours in a vacuum of 1 torr. Density of the rod was 3.12 " g (10?- by volume free silicon) the eangrain size was about 5 microns and the Knoop hardness at 5θg load was 3,650 hg/m y --. A

Claims: CLAIMS 1. A self-bonded silicon carbide body produced by siliconising a preformed mixture of particles of carbon and silicon carbide wherein the silicon carbide in the preforme mixture is in the beta form and the silicon carbide in the self-bonded silicon carbide body has a mean grain size? in the range O.1—5 microns. 2. A method of producing a self-bonded silicon carbide body by siliconising a preformed mixture of particles of carbon and silicon carbide in the beta form, the silicon carbide particles having a mean surface area in the range of O.5-2O square- metres per gram, 3. A method of-producing a self—bonded silicon carbide body as claimed in claim 2 wherein the silicon carbide particles have a mean surface area less than 5 square. metres per gram. 4. A method of producing a self-bonded silicon carbide body as claimed in claim 2 or claim 3 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.

Inventors: KENNEDY PETER, NORTH BERNARD
Applicants: ATOMIC ENERGY AUTHORITY UK, KENNEDY PETER, NORTH BERNARD
Application year: 1980
Main classification: C04B35
IPC codes: C01B31, C04B35