Title of Invention

A PROCESS FOR PREPARATION OF RECTION BONDED SILICON CARBIDE COMPONENTS

Abstract

This invention relates to a process for preparation of reaction bonded silicon carbide components. The process comprises the steps of mixing SiC, carbon black and phenolic resin to get a slurry, drying the slurry, crushing into dry powder and sieving. The powder is compacted to shape close to component. The compact is then carbonised and silicided by heating in contact with silicon lumps in a graphite crucible in vacuum followed by cooling in inert positive pressure to obtain the reaction bonded silicon carbide component.

Full Text

This invention relates to a process for preparation of reaction bonded silicon carbide components, Silicon carbide has significant potential as high wear resistant structural material for pump seal applications and also for heat exchanger applications. In chemical industry corrosion-erosion of conventional mechanical seals leads to frequent break down of acid pumps. Improvement in performance and efficiency of these acid pumps and automobile radiator water pump depends on the characteristics of pump seal materials. Sintered silicon carbide (SSC) and reaction bonded silicon carbide possess very high hardness, high temperature strength, low density, high flexural strength, low coeffecient of thermal expansion (CTE) and god thermal conductivity. Sintered silicon carbide (SSC) is made by sintering of high purity fine SiC with the help of additives (Fe, B and C). However, several disadvantages are associated with sintered silicon carbide. Sintered silicon carbide is very expensive due to high processing temperature ie. above 2000°C. In case of SSC shrinkage during sintering is quite high. SSC also needs very high purity fine SiC powders. According to a process described by Pollak et al (1986), the SiC and C powder mix is made by vacuum evaporation of the solvent from the slurry of SiC, C, phenolic resin and alcohol. Reaction bonded silicon carbide (RBSC) components are prepared by carbonizing the moulded bodies of SiC and C, at 1000 C, and placing the same above a RBSC support plate and a graphite plate with silicon lumps sandwiched in between and heating to temperature above 1400 C. The RBSC support plate is coated with SiC+C+BN slurry for easy remc^val of the component after siliciding. The above process has the disadvantage that the preparation of powder having d then used for siliciding. The two step process (carbonisation and infiltration separately) takes more time and is uneconomical. Another disadvantage is that of separate preparation of RBSC support plate each time using similar process leads to increased process cost. A further disskdvantage is tha^t for longer heights Si may not climb to infiltrate the sample to full de-?nsity-Another disadvantage of this process is low furnace height to floor rat io, leading to poor economicts. According to another process developed by-Forest el at (1970), molten Si pool is maintained at the bottom of the crucible into which carbonized SiC~C preform is hanged from top and infiltration occurs by surface wetting of silicon. The disadvantage of this process is Si accumulation at the bottom leads to sticking and cracking of the component at the bottom. Another drawback of this process is that sample length should be more than required for facilitating hanging and discarding the bottom cracked portion. Hence, process is not economical. According to another process proposed by Chakrabarti et al (1995) RBSC was made by siliconizing the pressed SiC and C bodies. However the processing details are not revealed- The disadvantage of the said method is that the process results in low flexural strength (270 Mpa) and Fracture toughness (4MF"ay ro). Another disadvantage of the process is that without addition of binders to the powder may give small samples but not large integral components Ger man o ffenlegungs sc h r i f t No.2910&2B disclos€5S the infiltration of pre sintered porous bodies of SiC-C mixture with silicon feeders containing Si and C- Infiltration occurs by capillary and gravitational forces. This proce)ss has the disadvantage of special preparation of Si and C for feeders and the same can not be re-used. Another disadvantage of this process is the formation of spongy SiC feeders which adheres to the component firmly. Hence the process is not economica,. Yet another process disclosed by German offenlegungs schrift No.2644503 the use of SiO containing SiC as the starting material. Dual gas treatment (N and H ) is used to purify SiC before inf i ltration with silicon. The major disadvantage in Ci this; process is the formation of Si N from 1200 C onwards and cumbersome gas treatment process escalate the process cost. Basically all of the processes know from literature for making reaction bonded silicon carbide have disadvantages like s 1. The perform is carbonized separately ar to siliconization; 2. Sticking of component to the crucibles and as a conseqi ; components are damaged while separating 3. Separate preparation of either feeders or RBSC support plates, 4. Formation of hard lumps during powder preparation. A primary object of the present invention is to propose a process for making reaction bonded silicon carbide (RBSC) compacts using cheaply available raw materials. Another object of this invention is to propose a process for making reaction bonded silicon carbide (RBSC) compacts by a single step process where carbonization and siliciding are carried out together in a single heating cycle. According to this invention, there is provided a process for preparation of reaction bonded silicon carbide components comprising steps of (a) preparing the precursor powder comprising the steps of (i) mixing of 70-80% by weight of commercially available a-SiC to 7 to 15% by weight of carbon black and 10-20% by weight of phenolic resin in alcohol medium using a mill selected from planetary ball mill or rubber roller mill with tungsten balls for 6-10 hours to obtain a slurry, (ii) heating the said slurry to 50-100°C while continuously stirring and crushing lumps in the said slurry to obtain dried mass, (iii) crushing the said dried mass to fine powder and sieving through 40 to 100 mesh sieve to obtain the right sized powder, (b) compacting the said fine powder into shapes close to the component shape to obtain a green compact, (c) carbonising and siliciding the said green compact comprising the steps of (i) placing green compact in a fine grained graphite crucible; placing silicon lumps of 1 to 5 mm size in contact with the said compact and covering the crucible with a tight lid, (ii) heating the said crucible at the temperature of 900-1100°C with heating rate of 4-5°C per minute, {Hi) further heating the said crucible at the temperature from 900-1100°C to 1450-1600°C for 1 to 3 hours at a heating rate of 10-20°C/minute, at a vacuum of 10-2 to 1 torr, (iv) cooling the compact in a positive atmosphere comprising inert gas such as argon to obtain the reaction bonded silicon carbide component. Further according to this invention there is provided a process for carbonizing and siliciding the green compacts comprising the step of: a. placing silicon lumps in contact with said green compact in a graphite crucible; b. heating the crucible under vacuum to 900-1100°C with heating rates 4-5°C/min. to convert the resin to carbon residue, c. further heating the crucible from 900-1100°C to 1450-1600°C for 1-3 hrs. at the heating rate of 10-20°C/min. and thereby infiltrate the copact with silicon; and d. thereafter cooling the compacts from the infiltration temperatures under positive pressure of inert atmosphere to avoid excess surface silicon Brief Description of the Drawings The invention will be described in detail with reference to the accompanying drawings whereini: Fig. 1 shows a schematic cross sectional view of crucible set up for making RBSC rings having large L/D ratios. Fig. 2 shows & schematic cross sectional view of crucible set up for making RBSC rings having low L/D ratios. Fig. 3 shows a schematic cross sectional view of crucible set up for making large number of sma 11 er RBSC components,. Commer c i a11y ava i1ab1e powder conta i ns C and SiO as impurities,, typically to the €9xtant of 0.75% and 2.00% respectively. This and carbon black powders along with phenolic resin are? wet blended in alcohol medium using a planetary ball mill/rubber roller mill with tungsten balls for 6-10 hrs for optimum mixing. The slurry obtained after mixing, is heated to 50-100 C- while the bath is kept continously stirred/crushed manually or mechanically to avoid formation of hard lumps. When the mass is crushed to fine powder it is sieved through 40 to 100 mesh sieve to get the right sized powder for making green compact. The powder is compacted into the shapes close to th€= components shape having controlled porosity.. Due to presence of resin there is no need of adding any lubricant during compaction to get good surface finish of the compact. The green compact its placed in a suitable fine grained graphite crucible; and silicon lumps are placed in contact with green compact and the crucible is covered with a tight lid. The crucible for making RBSC seal rings is designed in such a way that the height of the liquid silicon pool formed at high temperratures is the maximum possible. To avoid sticking of the compact (1.4) with crucible (1.1) a graphite ring (1.2) having inner diameter .just 1-5 mm Less than the outer diameter of compact is placed at the bottom of the graphite crucible as shown in Fig, 1 . A 100-300 um thick graphite foil (1.3) coated with mixture of PVAy Boron Nitride and water is provided over the ring before placing compact. Si lumps CI.5) having 1-5 mm size are placed above the graphite foil in contact with the component. As a variant, when the components" L/D raio is low, fine grained graphite discs (2.6> are placed at the centre of the component as shown in Fig. 2 to maintain the height of Si pool. However, graphite crucible |, grsphite support foil (2.3!", green compact (2:,4) and silicon lumps C2.6) are placed as ishown in fig« 2, As another variant, when large numtacjr of small components are to be made, several graphitej holder plates are placed in between each of the graphite holder plates as shown in Fig. 3. F"r6?ssed compacts of SiC and C C3.4> are placed in a separate Born Nitride coated graphite foil boat (3.6) and fine Si lumps C3.5) BTB placed on the component as shown in Fig» 3. The filled graphite foil boat is placed in the holder plates above the separating graphite foil. The stack is placed in the graphite crucible (3.1). In all these crasc-JS the assembly is placed in a vacuum furnace and heated at the o rate of 4-5 C/min up to 900-"1100 C and maintained at such a temperature for a period of 1 to 3 hours. The assembly is then further heated at the rate 10-20 o C/min to temperature in the range of 1400 C~1S00 C at the vacuum levels of 10 --1 torr. This temperature is maintained for 1-3 hours to complete the process. The Si melts and infiltrates into porous compact due to capillary and gravitational forces. Si reacts with carbon and forms SiC and the leftover porosity is filled with molten Si, Then the system is furnace cooled under the positive pressure of inert atmosphere such as argon gas. The SiO present as impurity in the? silicon carbide raw material reacts with molten silicon and forms silicon nicmoxide. This reacts with adjacent carbon and forms SiC and Co (Forrest et al ,, 1970). Hence the presence of SiO in the raw material may not deleterious to this process. Moreover,, on chemical analysis no SiO traces are detected. EXAMPLE i. The powder required for making green compacts of plate and pump seal ring is prepared by mixing 7"5""« by weight of silicon carbide supplied by M/s. Carborundum Universal having average particle size of 5-10 μm and having typical composition of 97—98 "/. silicon carbide, 1-2% SiO and,0.5-0.7% C and lO% by weight of carbon black powder supplied by M/s. Phillips Carbon having particle size less than a micron and 15% by weight of phenolic resin with a coking residues of 35--40"/. in a planetary mill/rubber roller mill using alcohol medium. The milled slurry is granulated by simultaneous heating and crushing and granules* less than O.i mm are sieved. Components of dimensions 120 mm (ID), 140 mm(OD"!y 125 mm(Ht) are? compacted using a die under a pressure of 40-80 MPa. The pressed density of the compact is about 1.90 "2.05 g/cc. This green compact is placed on the graphite foil which is supported by a graphite ring fixed at the bottom of the crucible as shown in fig, 1. Silicon lumps ars placed over the graphite foil in contact with the green compact, e The set-up is heated to 900-1100 C with heating rates up to 4-5 C/min,, for 1-3 hours to convert the resin to carbon residue under vacuum. At this stage the compact has density of 1.80 - 1.95 g/cc and contains ns SiC 81-837. by weight, carbon 16-17% by weight and SiO 1™ 2% by weight. Further, the set-up is heated from 900-1100 C to 1450 ■"1600°C at the rate of 10-20 /min. After soaking for 1-3 hours at this temperature, the; set-u.p is cooled under positive pressure of inert gas. By this process, the product will have a density of about 3.06 g/cc and the free silicon is about 10-~15 Vol"i, EXAMPLE 2 Components of dimensions 250mm (OD) 200mm(lD;)| 26mm(ht.) have been compacted using the powder made according to Example 1. This compact is placed on the graphite ring fixed at the bottom of the graphite crucible and the ring is covered with BN coated graphite foil. As the components L/D ratio is low, to maximise the silicon melt height a fine grained graphite disc is placed at the centre as shown in fig. 2. This crucible set up is infiltrated according to the heating and cool ling cycle described in Example 1. EXAMPLE 3. Components typically of dimensions 50mm COD); 20(lD)j 5mm(Ht.) are pressed from the powder mix processed according to Example I. These greent compacts are placed in a graphite foil boats and silicon lumps are added. The filled graphite foil boats are placed in the holes of the holder plates above the . separating graphite foil. The stacks are placed in a graphite crucible and heating and cooling cycle required for carbonisation and silicon infiltration is followed as described in Example 1. This material shows hardness of 2500 kg/mm and fle;";ural strength of 380-420 MPa and fracture toughness 4.8-5,. 1 MPam RESC made by this process does not leave much excess silicon on the surface of the compacts and use of graphite foil results in non sticking of compact with crucible. The process is much easier and the mould can be reused repeatedly. Use of resin results in stronger green compacts which will withstand the reaction pressures. Use of low cost raw materials make the process economical and thereby product is much cheaper. WE CLAIM « 1. A process for preparation of reaction bonded silicon carbide components comprising steps of: (a) preparing the precursor powder comprising the steps of: (i) mixing of 70-80% by weight of commercially available a-SiC to 7 to 15% by weight of carbon black and 10-20% by weight of phenolic resin in alcohol medium using a mill selected from planetary ball mill or rubber roller mill with tungsten balls for 6-10 hours to obtain a slurry, (ii) heating the said slurry to 50-100°C while continuously stirring and crushing lumps in the said slurry to obtain dried mass, (iii) crushing the said dried mass to fine powder and sieving through 40 to 100 mesh sieve to obtain the right sized powder, (b) compacting the said fine powder into shapes close to the component shape to obtain a green compact, (c) carbonising and siliciding the said green compact comprising the steps of: (i) placing green compact in a fine grained graphite crucible; placing silicon lumps of 1 to 5 mm size in contact with the said compact and covering the crucible with a tight lid, (ii) Heating the said crucible at the temperature of 900-1100°C with heating rate of 4 to 5°C per minute, (iii) Further heating the said crucible at the temperature from 900-1100°C to 1450-1600°C for 1 to 3 hours at a heating rate of 10-20°C/minute, at a vaccum of IO-2 to 1 torr, (iv) Cooling the compact in a positive atmosphere comprising inert gas such as argon to obtain the reaction bonded silicon carbide component. 2. A process as claimed in claim 1, wherein boron nitride coated graphite ring covered with a boron nitride slurry coated graphite foil are placed in the said crucible under the said compact. 3. A process as claimed in claim 2, wherein the said boron nitride slurry comprises a mixture of boron nitride, PVA and water. 4. A process for preparation of reaction bonded silicon carbide components substantially as herein described and illustrated.

Documents:

1886-mas-1996 abstract.pdf

1886-mas-1996 claims.pdf

1886-mas-1996 correspondence others.pdf

1886-mas-1996 correspondence po.pdf

1886-mas-1996 description (complete).pdf

1886-mas-1996 drawings.pdf

1886-mas-1996 form-1.pdf

1886-mas-1996 form-26.pdf

1886-mas-1996 form-4.pdf

1886-mas-1996 petition.pdf