SILICON CARBIDE CERAMICS BONDING COMPOSITIONS

Abstract: A bonding composition for silicon carbide ceramics, comprising powder silicon carbide having a particle size of 1 to 50 µm, powder carbon having a particle size of up to 15 µm, powder silicon having a particle size of up to 10 µm, a thermosetting resin, and a catalytically curable resin and a curing agent therefor is effective in bonding sintered silicon carbide ceramic pieces.

Description: FIELD OF THE INVENTION This invention relates to compositions for bonding silicon carbide ceramics. BACKGROUND OF THE INVENTION In recent years, silicon carbide (SiC) ceramics are used in a variety of industrial fields including semiconductor, automobile, chemical plant, and machine tool because of heat resistance, oxidation resistance, abrasion resistance, hardness, and many other excellent properties. In order that silicon carbide find further widespread application, not only its manufacture, but also bonding of silicon carbide ceramics pieces to each other or to metal pieces are key technology. A simple economical bonding technique is desired particularly from the industrial standpoint. Prior art well-known ceramics bonding methods typically include (1) mechanical bond, (2) brazing or metallizing, (3) solid phase or fusion bonding using metal foils or bands, and (4) bonding utilizing chemical reaction. The former three bonding methods are not regarded simple or inexpensive because mechanical bonding (1) requires a high precision of operation, and metallizing (2) and solid phase or fusion bonding (3) require special equipment. Method (4), bonding utilizing chemical reaction is most simple. Bonding of silicon carbide utilizing chemical reaction, more particularly bonding of reactively sintered silicon carbide is reported by Izeki et al, Yougyou Kyoukai Si (Journal of Japanese Ceramic Society), Vol. 91, 259-264 (1983). Izeki's bonding method involves applying a bonding composition of powder silicon carbide, powder carbon, and a binder (a mixture of phenol resin and furan resin in a solvent) to a surface of a ceramic piece to be bonded, joining the applied piece to another piece, pressing the assembly under pressure, and introducing silicon to the bonding interface in vacuum at 1450°C for bonding. This method, however, has a problem that expansion occurs due to reaction between the introduced silicon and the the carbon in the bonding composition and silicon is further introduced into the voids created by expansion to form a layer consisting solely of silicon, which becomes a main cause of breakage. To avoid the expansion, the pieces to be bonded together should be held in close contact during heat bonding. Holding the pieces together in intimate contact under such elevated temperatures needs an advanced level of technology, imposing limitations on the manufacture of bonding equipment. This often invites a situation that it is only the manufacturer of ceramic pieces who can carry out bonding of ceramic pieces. A further difficulty arises in bonding of complex shaped pieces. In order to produce an article of complex configuration by reactive sintering, it is generally adopted to bond pieces in calcined state with a bonding agent having the same composition as the calcined pieces before reactive sintering is carried out. This method has disadvantages that the resulting bond strength is low and bonding can be practiced only at the ceramics manufacturer. SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a novel and improved bonding composition adapted for bonding of silicon carbide ceramics through chemical reaction, which accomplishes a high bond strength, which is devoid of expansion at the bonding interface due to the reaction, which needs no special tool like press holding jig, which is applicable to ordinary (that is, commercially available) sintered bodies of silicon carbide ceramics instead of calcined bodies, and which enables simple and economical bonding. According to the present invention, there is provided a bonding composition for silicon carbide ceramics, comprising powder silicon carbide having an average particle size of 1 to 50 µm, powder carbon having an average particle size of up to 15 µm, powder silicon having an average particle size of up to 10 µm, a thermosetting resin, a catalytically curable resin, and a curing agent for the catalytically curable resin, wherein 0.4 to 2.2 parts by weight of said powder silicon carbide and 1.9 to 4.2 parts by weight of said powder silicon are blended per part by weight of the total carbon given by said powder carbon plus carbon values resulting from thermal decomposition of said thermosetting resin, said catalytically curable resin, and said curing agent. More detailedly, apart from the concept of external introduction of silicon as proposed by Izeki's method, the present inventors have found that commercially available sintered ceramic pieces can be firmly bonded to an increased bond strength while controlling formation of a silicon layer which would lower the bond strength of the bonding interface, when powder silicon having an average particle size of up to 10 µm is contained in advance in a bonding composition comprising powder silicon carbide having an average particle size of 1 to 50 µm and powder carbon having an average particle size of up to 15 µm in the specific amounts per the weight of the total carbon value, and a thermosetting resin, a catalytically curable resin and a curing agent for the catalytically curable resin are further added to the bonding composition. The bonding mechanism of Izeki's method is based on external introduction of Si to the bonding interface consisting of a powder mixture of SiC and C. Introduction of Si causes the chemical reaction: Si + C → SiC to take place to form a bond. As previously pointed out, this reaction is accompanied by a volume increase or expansion to create voids into which Si further penetrates, eventually forming a layer consisting solely of silicon. The silicon layer expands again upon cooling, causing splitting of the once bonded assembly into respective parts. Rather than the external introduction of silicon, the present inventors propose a bonding method using the above-described bonding composition having a controlled amount and size of powder silicon previously included therein. This is based on the finding that the expansion in volume due to the chemical reaction: Si + C → SiC can be fully accommodated and the controlled amount of co-present silicon avoids formation of a silicon layer. The bonding composition may be applied to commercially available silicon carbide ceramic sintered parts at room temperature and only requires simple heating as by the use of a propane burner for bonding such ceramic sintered parts into an assembly having an increased bond strength without the need for holding together the ceramic parts under pressure during the bonding process. DETAILED DESCRIPTION OF THE INVENTION The present invention provides a bonding composition for silicon carbide ceramics, comprising powder silicon carbide, powder carbon, powder silicon, a thermosetting resin, a catalytically curable resin, and a curing agent for the catalytically curable resin. The bonding composition may preferably take the two part form, that is, a primary pack composed of powder silicon carbide, powder carbon, powder silicon, a thermosetting resin, and a catalytically curable resin, and a secondary pack composed of a curing agent for the catalytically curable resin in the primary pack, the two packs being mixed upon use. The powder silicon carbide (SiC), a first component of the primary pack from which the bonding composition of the invention is constructed may be either alpha-SiC or beta-SiC. For economic reasons, the preferred silicon carbide is commercially available finely divided alpha-SiC by Acheson method. The average particle size of SiC should be in the range of from 1 to 50 µm, preferably in the range of from 3 to 35 µm although the exact size depends on the particle size of the remaining components of the composition. SiC particles with a size of less than 1 µm are too small to accommodate the expansion in volume due to the reaction, allowing cracks to develop during heating. SiC particle sizes of more than 50 µm result in a too thick bonding layer, which will also crack during heating. The powder carbon (C), a second component is an important component in the bonding composition of the invention. The powder carbon should preferably be as fine as possible or have a larger specific surface area in order that it evenly react with the powder silicon (Si), a third component to form homogeneous silicon carbide (SiC). Finer particles are unlikely to create voids or interstices due to expansion at the bonding interface upon reaction, inducing no additional stresses and thus giving a higher bond strength. For this reason, the powder carbon should have an average particle size of up to 15 µm, preferably up to 10 µm. Its specific surface area (BET method) is preferably at least 70 m²/g, especially at least 100 m²/g. Coarse carbon particles of more than 15 µm in diameter cause cracks during heating. With a smaller specific surface area, sometimes the reaction would not proceed evenly, lowering the bond strength. The lower limit of the average particle size may preferably be 0.5 µm and the upper limit of the specific surface area may preferably 500 m²/g. The powder silicon (Si), a third component which reacts with the above-described powder carbon should also preferably be as fine as possible like the powder carbon. The powder silicon should have an average particle size of up to 10 µm, preferably up to 7 µm. Silicon particles with a size of more than 10 µm cause localized reaction and crack formation during heating, lowering the bond strength. The lower limit of the average particle size may preferably be 0.1 µm. A fourth component is the thermosetting resin, which is not particularly limited insofar as it can attach together the ceramic pieces to be bonded until the temperature at which silicon reacts with carbon is reached. Examples of the thermosetting resin include phenol resins, silicon varnishes, melamine resins, phenol-melamine resins and alkyd resins. The catalytically curable resin, a fifth component of the bonding composition of the invention is added for the purpose of curing in the presence of a catalyst to join the ceramic pieces together into an assembly which can be handled at room temperature. Any desired catalytically curable resins may be used for the purpose. Preferably, those resins which can cure in the presence of a catalyst at room temperature are used, for example, furan resins, epoxy resins, unsaturated polyester resins, and urea resins. The foregoing first to fifth components, powder silicon carbide, powder carbon, powder silicon, thermosetting resin, and catalytically curable resin may be combined together to form the primary pack of the composition. The primary pack may contain any desired additional components such as polyisocyanate and polyol binders, N-methyl pyrrolidone, and formamide. It is desirable to add a solvent to the foregoing components to form a paste which is readily applicable to a surface of a ceramic piece to be bonded. The solvent may be added before application or previously included in the primary pack. The useful solvent is a volatile solvent such as acetone, chloroform, dimethyl eter, benzene, ethanol, acetonitrile and the like, and those solvents having a boiling point of 100°C or higher are preferred for non-volatility during shelf storage. Examples of the solvent include dimethyl sulfoxide, dimethylformamide, furfuryl alcohol, cyclohexanol, dimethylacetoamide, N-methyl pyrrolidone, and a mixture thereof. The bonding composition of the invention further includes the curing agent or catalyst for curing the catalytically curable resin in the primary pack. The curing agent may preferably constitute the secondary pack which is separate from the primary pack and is mixed with the primary pack upon use. The identity of curing agent depends on the type of catalytically curable resin used. Useful are those curing agents which are commonly used in condensation reaction, for example, sulfonate ammonium salts, amine salts, sulfonium chlorides, amide inorganic acid salts, sulfamic acid, and trichloroacetic acid. More specifically, n-butylamine, triethanol amine, ethylenediamine, diethylaminopropylamine, diaminodiphenyl sulfone, piperidine, aminoethylpiperidine, dicyandiamide, phthalic anhydride, hexahydrophthalic anhydride, guanidine, trichloroacetic acid, p-toluenesulfonic acid, 2-aminosulfonic acid, aminometasulfonic acid, magnesium carbonate, magnesium chloride, sodium hydroxide, potassium hydroxide and the like are preferred. Since sets of catalytically curable resin and curing agent are commercially available, the resin and the curing agent are advantageously selected therefrom. As a matter of course, the catalyst also causes partial condensation of the thermosetting resin, contributing to an improvement in room temperature strength. The secondary pack composed of the curing agent may further contain the same solvent as added to the primary pack. It is rather preferred to dilute the curing agent with the solvent. The components of the bonding composition disclosed herein are combined in such amounts and proportions as to form a joint having an approximately equal density to the ceramic pieces to be bonded therewith. More specifically, the powder silicon carbide is 0.4 to 2.2 parts by weight, preferably 0.6 to 2 parts by weight and the powder silicon is 1.9 to 4.2 parts by weight, preferably 2 to 4 parts by weight, per part by weight of the total carbon given by the powder carbon plus carbon values resulting from thermal decomposition of the thermosetting resin, the catalytically curable resin and the curing agent. Less than 0.4 parts of silicon carbide per part by weight of carbon is insufficient to accommodate the expansion in volume due to the reaction between C and Si, causing cracks during heating. Compositions containing more than 2.2 parts of silicon carbide per part by weight of carbon lose the bond strength because of pore formation. Cracks occur during heating with less than 1.9 parts of silicon per part by weight of carbon whereas pore formation detracts from bond strength with more than 4.2 parts of silicon per part by weight of carbon. It should be noted that the amount of carbon referred to herein is calculated as including any carbon values in addition to the powder carbon, that is , thermosetting resin, catalytically curable resin, and curing agent being thermally decomposed to leave C at the bonding interface. The carbon formed by decomposition of such components is generally amorphous and moderately reactive with silicon, also contributing to the control of expansion due to reaction at the bonding interface. The catalytically curable resin is preferably blended in an amount of 5 to 20%, especially 7 to 18% by weight based on the total weight of the powder silicon carbide, powder carbon, and powder silicon components. Less than 5% by weight of the catalytically curable resin sometimes fails to achieve a sufficient strength to withstand handling at room temperature. More than 20% by weight of the catalytically curable resin will increase the thickness of the bonding layer or the spacing between the bonding surfaces, losing bond strength. The thermosetting resin is preferably blended in amounts of about one-half to one-third of the weight of the catalytically curable resin. Lesser proportions of the thermosetting resin will sometimes fail to maintain the bonding surfaces in close contact during heating. Excess proportions of the thermosetting resin mean relatively short proportions of the catalytically curable resin, which might fail to achieve a sufficient strength during the initial bonding of ceramic pieces. The amount of the solvent added to the primary pack to form a paste is an amount necessary to maintain the paste. The amount of the solvent used generally ranges from an equivalent to the thermosetting resin to one-half thereof. The amount of the curing agent used varies with its type relative to the catalytically curable resin, but generally ranges from 2 to 5% by weight of the amount of the catalytically curable resin. The method of bonding silicon carbide ceramic pieces with the bonding composition of the invention is not particularly limited. In general, a mixture of powder silicon carbide, powder carbon, powder silicon, thermosetting resin and catalytically curable resin is tumbled with a solvent such as furfuryl alcohol in a ball mill or any other mixing means until a paste is formed. The order of adding these components to the mill is not critical. The paste is combined with the curing agent in the diluent solvent just before use. The resulting composition is applied to a surface of a ceramic piece to be bonded, which is mated to another ceramic piece under pressure to form an assembly where the catalytically curable resin cures, resulting in the assembly which can be handled at room temperature because of the bond between the mating surfaces. The assembly is heated at a temperature of about 100 to 400°C, for example about 150°C, to drive off the solvent and set the thermosetting resin. Then the temperature is gradually increased to carbonize the resins. The assembly is finally heated to a temperature of 1,400°C or higher, preferably 1,450 to 1,600°C for reaction between Si and C. The atmosphere under which the assembly is heated to temperatures of 1,400°C or higher is preferably vacuum or inert gas although heating in air is permissible. Heating may be carried out by means of a propane burner if desired. The heating time varies with the configuration of the joined pieces. The bonding composition of the invention may advantageously be used in bonding between pieces of silicon carbide ceramic material formed by reactive sintering, recrystallization, atmospheric sintering or the like or between such ceramic pieces and pieces of metal or any other materials. As described above, the bonding composition of the invention can bond together silicon carbide ceramic pieces typically in the commercially available final form rather than in the calcined form only available intermediate the manufacturing process. A firm bond can be accomplished by relatively simple heating as by the use of a propane burner if desired. With the use of the present composition, such silicon carbide ceramic pieces can be bonded without a need for any specially designed expensive equipment such as clamping equipment which can withstand high temperatures, vacuum equipment, and high temperature heating equipment. The invention ensures simple and economical bonding of large sized articles and configured articles as well as repair of broken articles. EXAMPLE Examples of the present invention are given below by way of illustration and not by way of limitation. Examples 1 - 14 A primary formulation was prepared by tumbling the following ingredients in a ball mill. A secondary formulation was prepared by mixing the following ingredients. The primary formulation, 0.3 grams was mixed with the secondary formulation, 0.3 grams. The bonding composition was applied to a surface of one of sample pieces. The sample pieces were semi-circular halves of a disk of reactively sintered SiC having a diameter of 30 mm and a thickness of 5 mm. The sample pieces were joined together through the bonding composition layer under pressure. The assembly was heated at 120°C for 30 minutes to cure the composition. At this point, the assembly showed a bond strength of 2 kg/mm² at the bonding interface. Thereafter, the assembly was further heated to 1,500°C for 5 minutes in an argon atmosphere under atmospheric pressure, completing a bond. At this point, the assembly showed a bond strength of 30 kg/mm² at the bonding interface. Similar tests were carried out by the same procedure as above using various bonding compositions of the formulations shown in Table 1. The results are shown in Table 1. It will be understood that Examples 6 through 14 are outside the scope of the invention. Example 15 The procedure of Example 1 was repeated by mixing the same primary and secondary formulations in the same amounts, applying the bonding composition to the same pieces of reactively sintered SiC, mating the pieces under pressure, and heating the assembly at 120°C for 30 minutes to cure the composition. Thereafter, the assembly was heated to a temperature of higher than 1,500°C in air using a propane burner, maintained at the temperature for 30 minutes, and then gradually cooled down. The assembly showed a bond strength of 20 kg/mm² at the bonding interface. Although some preferred embodiments have been described, many modifications and variations may be made thereto in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

Claims: 1. A bonding composition for silicon carbide ceramics, comprising powder silicon carbide having an average particle size of 1 to 50 µm, powder carbon having an average particle size of up to 15 µm, powder silicon having an average particle size of up to 10 µm, a thermosetting resin, a catalytically curable resin, and a curing agent for the catalytically curable resin, wherein 0.4 to 2.2 parts by weight of said powder silicon carbide and 1.9 to 4.2 parts by weight of said powder silicon are blended per part by weight of the total carbon given by said powder carbon plus carbon values resulting from thermal decomposition of said thermosetting resin, said catalytically curable resin, and said curing agent. 2. The bonding composition of claim 1 wherein the powder silicon carbide has an average particle size of 3 to 35 µm. 3. The bonding composition of claim 1 wherein the powder carbon has an average particle size of up to 10 µm. 4. The bonding composition of claim 1 wherein the powder silicon has an average particle size of up to 7 µm. 5. The bonding composition of claim 1 wherein the thermosetting resin is selected from the group consisting of phenol resins and silicone varnish. 6. The bonding composition of claim 1 wherein the catalytically curable resin is selected from the group consisting of furan resins, epoxy resin, unsaturated polyester resins, and urea resins. 7. The bonding composition of claim 1 which further comprises a sufficient amount of volatile solvent to form a paste. 8. The bonding composition of claim 1 wherein 0.6 to 2 parts by weight of the powder silicon carbide is present per part by weight of the carbon. 9. The bonding composition of claim 1 wherein 2 to 4 parts by weight of the powder silicon is present per part by weight of the carbon. 10. The bonding composition of claim 1 wherein the catalytically curable resin is present in an amount of 5 to 20% by weight based on the total weight of the powder silicon carbide, powder carbon, and powder silicon. 11. The bonding composition of claim 1 or 10 wherein the thermosetting resin is present in an amount of 1/2 to 1/3 of the weight of the catalytically curable resin. 12. The bonding composition of claim 1 or 10 wherein the curing agent is present in an amount of 2 to 5% by weight of the weight of the catalytically curable resin.

Inventors: HONGU, TATSUHIKO
Applicants: SHIN-ETSU CHEMICAL CO., LTD.
Application year: 1989
Main classification: C04B37
IPC codes: C04B37