Title of Invention	"AN IMPROVED PROCESS FOR THE PREPARATION OF MOLYBDEMUM DISILICIDE (MOSI2)-REACTION BONDED SILICON CARBIDE (RBSC) COMPOSITE USEFUL AS A STRUCTURAL CERAMIC MATERIAL"
Abstract	A process for the preparation of MoSi2 - RBSC composite containing alloy species mixing it with the PVA solution and pressing isostatically.The invention relates to a process for the preparation of molybdenum disilicide (MoSi2) - reaction bonded silicon carbide (RBSC) in which melt of alloy molybdenum and silicon are bonded by the bonded silicon carbide forming the duplex non-oxide ceramic useful as a structural ceramic material.

Title of Invention

"AN IMPROVED PROCESS FOR THE PREPARATION OF MOLYBDEMUM DISILICIDE (MOSI2)-REACTION BONDED SILICON CARBIDE (RBSC) COMPOSITE USEFUL AS A STRUCTURAL CERAMIC MATERIAL"

Abstract

A process for the preparation of MoSi2 - RBSC composite containing alloy species mixing it with the PVA solution and pressing isostatically.The invention relates to a process for the preparation of molybdenum disilicide (MoSi2) - reaction bonded silicon carbide (RBSC) in which melt of alloy molybdenum and silicon are bonded by the bonded silicon carbide forming the duplex non-oxide ceramic useful as a structural ceramic material.

Full Text	This invention relates to an improved process for the preparation of molybdemum disilicide (MoSi2) - reaction bonded silicon carbide (RBSC) composite useful as a structural ceramic material. In the process of the present invention, synthesis of this duplex non-oxide ceramic is made using infiltration of a melt of alloyed molybdenum (Mo) and silicon (Si) into a porous compact of commercial silicon carbide (SiC) powder together with carbon at high temperatures, which leads to the reaction of silicon (Si) with both carbon (C) and molybdenum (Mo) resulting in the formation of silicon carbide ( SiC ) and molybdenum disilicide ( MoSi2 ) . The novelty of the present invention is that unlike other alloyed melt infiltration technique, the alloyed melt in the present case is not made of the ceramic compounds, but it is prepared directly from elemental source of raw materials. In the process of present invention of MoSi2 reinforced RBSC composite, elemental Mo and Si are used to prepare the infiltrating alloyed melt. The duplex non-oxide ceramic made of MoSi2 reinforced RBSC, hereinafter designated as MoSi2-RBSC, shows excellent physico-chemical properties like, high oxidation, corrosion and thermal shock resistance, superb high temperature strength and toughness together with other related properties, e.g., Young's modulus, hardness, etc., and therefore, is of great technological importance. . These properties can be exploited during the applications of the material in the fields of kiln furniture particularly for heating sanitary wares, saggers, crucibles, for calcining fluorescent powder in fluorescent lamp industries, high speed and high service life domestic and industrial burners, rocket nozzles, recuperators for waste heat utilisation in indirect furnaces, heat-exchangers, indirectly fired open/combined cycle gas turbines to generate electric power, pump seals for pumps handling corrosive fluids, wear resistant inserts, etc. RBSC is normally prepared by infiltrating molten silicon in inert atmosphere into a porous preform of silicon carbide and carbon powder, when silicon reacts with carbon to form newer silicon carbide which precipitates on the existing silicon carbide particles and bind them together. However, the process suffers from a disadvantage, that this exerts the chances of occurrence of free silicon in the final product. The free silicon phase being less refractory than SiC is known to have deleterious effect on almost all the physico-chemical and thermo-mechanical properties of reaction bonded silicon carbide both at room temperature and high temperatures. This difficulty has led to the development of melt infiltration technique in which the liquid infiltrant is taken in the form of a alloy of silicon normally with transition elements. In the case of MoSi2-RBSC non-oxide ceramic, the alloy infiltrant is normally made from the reaction between silicon and the binary compound MoSi2 [Liquid phase reaction bonding of silicon carbide using alloys of silicon -molybdenum melts - R. P. Messener and Y. M. Chiang, J. American Ceramic Soc. v73(5) 1990, pp.1193-1200]. The process appears to be expensive since it involves use of costly MoSi2 compound. In an other study, molten MoSi2 is infiltrated into a porous RBSC material in which the free silicon phase removed using evacuation at high temperature [Microstructure and mechanical properties of RBSC/MoSi2 composite - C. B. Lim, T. Yano and T. Iseki, J. Materials Science, v24, 1989, pp.4144-51]. The process is also expected to be expensive since it involves molten MoSi2 which is prepared at a very high temperature of around 2050°C. Infiltration of porous RBSC with alloyed melt comprising of Mo, Si and aluminium (Al) at high temperature (around 1950°C) has also been reported [High temperature strength of melt infiltrated SiC-Mo(Al,Si)2 composites - K. Shobu, E. Tani, M. Akiyama and T. Watanabe, J. American Ceramic Soc., v79(2), 1996, pp.544-46] for the preparation of Mo(Al,Si)2-RBSC composite and the process may be expensive because of the involvement of very high temperature. The formation of NbSi2-RBSC composite using infiltration of molten niobium (Nb) alloyed melt into a micro-porous preform of carbon (C) has recently been published [Reaction melt infiltration of silicon - niobium alloys into microporous carbon - M. Singh and D. R. Behrendt, J. Materials Research v9(7), 1994, pp. 1701-08] but the resulting composite has been, reported to contain appreciable amount of detrimental silicon phase. A few work in the related fields has been patented. The process patented by Taylor [Cold moulded dense silicon carbide articles and method of making the shapes, K. M. Taylor, USP 3,205,043, Sept. 7, 1965] involves the infiltration of high-char-yielding organic into a resin bonded compact of granular SiC followed by subsequent siliconisation. This process leads to the presence of appreciable porosity in the intermediate stage which gives appreciable amount of residual silicon to be present in the final product, which is detrimental so far as the thermomechanical and other properties are concerned. The process patented by Fitschmun [Method of making silicon carbide article of less than full density, D. R. Fitschmum, USP 3,947,550 Mar. 30, 1976] involves direct combination of silicon and carbon and thereby increses the chances of increasing amount of porosity to be present in the final product. Hence the properties are deteriorated. In an other work [Manufacture of silicon carbide ceramics free of residual silicon, T. R. Grossman, USP 5,571,758 Nov. 5, 1996], the amount of silicon was reduced by nitriding the phase. The precursor was a composite of carbon particulate with SiC fibre in the form of a wick. The process is still costly as it involves two stage process. Refractory grade reaction bonded SiC was prepared [Secondarily formed silicon carbide and free carbon containing microporous silicon carbide ceramics and their manufacture and use, I. Elstner, et.al, WOP 9633,959 Oct. 31, 1996] using anthracite/ graphite carbon. The product is essensially porous and is of inferior quality. The main object of the present invention is to provide an improved process for the preparation of MoSi2 - reaction bonded silicon carbide composite useful as a structural ceramic material which obviates the drawbacks as mentioned above. Another object is to provide an economical process for the preparation of MoSi2-RBSC composite. Yet another object of the present invention is to prepare MoSi2-RBSC composite using elemental Mo and Si. The present invention provides an improved process for the preparation of a duplex non-oxide ceramic particularly MoSi2 - RBSC sintered composite using alloyed Mo/Si infiltrating melt prepared from elemental combination of powder Mo and Si. The uniqueness of the process is that, it demonstrates the possibility of using elemental Mo and Si as the infiltrating components and thereby making the overall process cheaper because of avoidance of costly MoSi2 compound in the melt formulation. Besides, both the elemental Mo and Si are readily available in very pure form. The process of the present invention involves the alloyed Mo/Si melt infiltration at high temperature into a porous compact of silicon and carbon when a part of silicon of the melt reacts with carbon to form newer silicon carbide (SiC) grains which act as the bond between the existing silicon carbide particles. The remaining silicon on reaction with alloying Mo gets converted to refractory MoSi2 phase. The presence of MoSi2 phase in the resulting composite by virtue of its brittle to ductile transition characteristics at around 10000C is expected to improve the fracture toughness of the final composite particularly at high temperature. The low temperature fracture properties may also excel by the matrix microcracking caused by the differential thermal expansion between matrix SiC and reinforcing MoSi2. Moreover, MoSi2 is known to form a dihedral angle of around 120° and as a result there is a strong possibility of crack deflection along the SiC-MoSi2 interface contributing higher fracture strength. Accordingly the present invention provides an improved process for the preparation of MoSi2 - reaction bonded silicon carbide composite useful as a structural ceramic material, which comprises : (i) mixing elemental Mo and elemental Si powder in the desired ratio, heating the mix so obtained in a non-reacting atmosphere to a temperature in the range of 1600°C to 1800°C, to obtain a homogenous melt, (ii) mixing SiC and C powders in the desired ratio, (iii) preparing a green preform of desired shape and size by conventional ceramic fabrication methods using SiC and C mix obtained in step(ii), (iv) heating the SiC and C green preform so obtained in step (iii) in the presence of the Mo-Si homogenised melt obtained in step (i) to a temperature range of 1650°C to 1850°C in an inert atmosphere to obtain the MoSi2-RBSC composite. The present invention provides an improved process for the preparation of MoSi2 - reaction bonded silicon carbide composite useful as a structural ceramic material which is a new and cheaper process for the preparation of MoSi2-RBSC composite. The steps to carry out the process are as follows : 1. Preparation of infiltrating alloyed melt from elemental source, such as, elemental Mo and elemental Si powders by proper weighing and intimate mixing followed by heating in a non-reacting atmosphere, such as, in pure argon gas atmosphere or vacuum, to a temperature in the range of 1600 to 1800°C forhomogenisation. 2. Preparation of SiC and C compact by proper weighing and mixing followed by preparing the shape of a billet or any other required shape by using conventional methods of ceramic fabrication, such as, uniaxial pressing, isostatic pressing, and extrusion. 3. Heating the SiC and C compact obtained in step (2) above in presence of the melt obained in step (1) above at a temperature in the range of 1650 to 1850°C to obtain the duplex non-oxide ceramic. Particularly for the preparation MoSi2-RBSC composite, the ratio of C:SiC and the volume fraction of SiC in the green compact may be varied according to the ratio of MoSi2, Si and SiC desired in the final product. The weight ratio of C:SiC and volume fraction of SiC in the green compact may be in the ranges of 0.1 to 0.6 and 0.30 to 0.34 respectively. The elemental Mo and Si powders used may be of commercial grade having purity >99% and >98% respectively and may be of -200 BSS size each. The concentration of the alloying Mo in Si/Mo melt may be in the range of 5.13 to 26.8 weight %. The particle size of commercial grade SiC may vary from -1200 grit (1.63 micron mean diameter) to -400 grit (23.65 micron mean diameter). The particle size of carbon used may 4-6 micron. The following examples are given to illustrate the process of the invention and the manner in which it may be carried out in practice; however, this should not be construed to limit the scope of the present invention. Example 1 12.15g commercial grade silicon carbide (SiC) of-1200 grit size (1.63 micron mean diameter) and 7.29g carbon (C) of 4.03 micron mean diameter are weighed and mixed in distilled water for 10 min. The mixture is dried, mixed with/PVA solutiom again mixed and finally pressed isostatically in the shape of a billet. The billet is dried at 120°C and its physical parameters, like, dimensions, weight and green density are measured. In a separate test, 0.95g commercial silicon powder [>98% pure, -200 BSS] and 17.56g molybdenum [>99% pure, -200 BSS] are weighed and mixed in isopropyl alcohol for 10 min. It is dried at 100°C and further mixed. The dry powder is taken in a graphite crucible and heated in pure argon gas atmosphere to 1600°C. It is soaked for 30. min to homogenise the melt and cooled. The billet is then placed on the alloyed mass in the crucible and heated once again in argon gas atmosphere to 1650°C. The liquid alloyed melt is allowed to infiltrate into the billet for 30 min and cooled. The mass as obtained after cooling is tested by X-ray diffraction technique. Silicon carbide (SiC) and silicon (Si) are present in the matrix. The presence of MoSi2 is found to be negligibly small. Example 2 Commercial silicon powder [>98% pure, -200 BSS] and molybdenum powder [>99% pure, 200 BSS] are weighed by varying the weight ratio of Mo in Si and Mo mixture in the range of 5.13 to 26.8% and mixed in isopropyl alcohol for 10 min The mixtures are taken in graphite crucibles and heated in pure argon gas atmosphere at 1600°C. It is soaked for 30 min at that temperature to homogenise the melt. 12.15g commercial grade silicon carbide (SiC) of-1200 grit size (1.63 micron mean diameter) and 7.29g carbon (C) of 4.03 micron mean diameter are weighed and mixed in distilled water for 10 min. The mixture is dried, mixed with PVA solution, again mixed and finally pressed isostatically in the shape of a billet. The billet is dried at 120°C and its physical parameters, like, dimensions, weight and green density are measured. The billet is placed on the alloyed mass in the crucible and heated in argon gas atmosphere to 1650°C. The liquid alloyed melt is allowed to infiltrate into the billet for 30 min and cooled. The cold composite, as formed, is tested by X-ray diffraction technique. Silicon carbide (SiC) and silicon (Si) are present in the matrix. Presence of MoSi2 is found to increase with the increase in the amount of Mo in the alloyed melt. Example 3 17.56g commercial silicon powder [>98% pure, -200 BSS] pure and 0.95g molybdenum [>99.5% pure, -200 BSS] are weighed and mixed in isopropyl alcohol for 10 min. It is dried at 100°C and further mixed. The dry powder is taken in a graphite crucible and heated in pure argon gas atmosphere at varying temperatures between 1600 to 1800°C. It is soaked for 30 min to homogenise the melt. 12.15g commercial grade silicon carbide (SiC) of-1200 grit size (1.63 micron mean diameter) and 7.29g carbon (C) of 4.03 micron mean diameter are weighed and mixed in distilled water for 15 min. It is dried at 100°C and further mixed. The mixture is dried, mixed with PVA solution, again mixed and finally pressed isostatically in the shape of a billet. The billet is dried at 120°C and its physical parameters, like, dimensions, weight and green density are measured. The billet is placed on the alloyed mass taken in a graphite crucible and heated once again in argon gas atmosphere to 1650°C. The liquid alloyed melt is allowed to infiltrate into the billet for 30 min. Example 4 17.56g commercial silicon powder [>98% pure, -200 BSS] and 0.95g molybdenum [>99.5% pure, -200 BSS] are weighed and mixed in isopropyl alcohol for 10 min. The dry powder is taken in a graphite crucible and heated in pure argon gas atmosphere to 1600°C. It is soaked for 30 min to homogenise the melt. 12.15g commercial grade silicon carbide (SiC) of-1200 grit size (1.63 micron mean diameter) and 7.29g carbon (C) of 4.03 micron mean diameter are weighed and mixed in distilled water for 10 min. The mixture is dried, mixed with PVA solution, again mixed and finally pressed isostatically in the shape of a billet. The billet is dried at 120°C and its physical parameters, like, dimensions, weight and green density are measured. The billet is placed on the alloyed mass in a graphite crucible and heated in argon gas atmosphere at varying temperature in the range of 1650 to 1850°C. The liquid alloyed melt is allowed to infiltrate into the billet for 30 min. Example 5 Commercial silicon carbide powder of -1200 grit size (1.63 micron mean diameter) and carbon powder of 4.06 micron mean diameter are weighed and mixed in water for 10 min. by varying the weight ratio of C:SiC in the range from 0.1 to 0.6. The mixture is dried, mixed with PVA solution, again mixed and pressed isostatically in the shape of billets. They are dried at 120°C and their physical parameters, like, dimensions, weight and green density are measured. In a separate test, 0.95g commercial silicon powder [>98% pure, -200 BSS] and 17.56g molybdenum [>99% pure, -200 .BSS] are weighed and mixed in isopropyl alcohol for 10 min. It is air dried at 100°C and further mixed. It is dried and further mixed. The dry powder is taken in a graphite crucible, heated in pure argon gas atmosphere to 1600°C. It is soaked for 30 min to homogenise the melt and cooled. The billets are placed on the alloyed mass in the crucible and heated in argon gas atmosphere to 1650°C. The liquid alloyed melt is allowed to infiltrate into the billet for 30 min. Example 6 12.15g commercial grade silicon carbide (SiC) of particle size varying between 1200 grit size (1.63 micron mean diameter) to 400 grit size (23.65 micron mean diameter) and 7.29g carbon (C) of 4.06. micron mean diameter are weighed and mixed in distilled water for 10 min. The mixture is dried, mixed with PVA solution, again mixed and pressed isostatically in the shape of billets. They are dried at 120°C and their physical parameters, like, dimensions, weight and green density are measured. In a separate test, 17.56g commercial silicon powder [>98% pure, -200 BSS] and 0.95g molybdenum [>99.5% pure, -200 BSS] are weighed and mixed in isopropyl alcohol for 10 min. It is dried at 100°C and further mixed. The dry powder is taken in a graphite crucible and heated in pure argon gas atmosphere to 1600°C. It is soaked for 30 min to homogenise the melt. The billets are placed on the cooled alloyed mass in the crucible and heated in argon gas atmosphere to 1650°C. The liquid alloyed melt is allowed to infiltrate into the billet for 30 min. The advantages of the process for production of MoSi2-RBSC composite are : 1. The sources of the raw materials (e.g., Mo and Si) in making the infiltrating melt are elemental in nature, instead of binary compounds of the alloying species which are costlier. One of the raw materials (e.g., Si) is of commercial variety. 2. The source of raw materials in making the starting compact (e.g., SiC and C) are of commercial grade and therefore readily available. 3. The raw materials being of commercial variety are cheaper. 4. Upon proper selection of the alloyed melt composition, SiC:C ratio and the volume fractions of SiC in the starting compact, complete elimination of free silicon in the composite matrix is possible. We claim: 1. An improved process for the preparation of MoSi2 - reaction bonded silicon carbide composite useful as a structural ceramic material, which comprises : (i) mixing elemental Mo and elemental Si powder in the ratio of 5.12 to 26.8 wt % heating the mix so obtained in presence of argon gas atmosphere a temperature in the range of 1600°C to 1800°C, to obtain a homogenous melt, (ii) mixing C powder and Sic. in the weight ratio of 0.1 to 0.6 5(1)(6) (iii) preparing a (green perform of desired shape and size) by conventional ceramic fabrication methods using SiC and C mix obtained in step (ii), (iv) heating the SiC and C green perform so obtained in step (iii) in the presence of the Mo-Si homogenized melt obtained in step (i) to a temperature range of 1650°C to 1850°C in presence of organ atmosphere to obtain the MoSi2-RBSC composite. 2. An improved process as claimed in claim (1) wherein the elemental Mo and Si are of commercial grade with purity >99% and >98% respectively, 3. An improved process as claimed in claim wherein the weight ratio of C: SiC is in the range of unit2 /pass 4. An improved process as claimed in claim wherein the particle size of SiC is in the range of-400 grit to -1200 grit (23.65μ to 1.63 micron mean diameter), 5. An improved process as claimed in claim wherein the particle size of carbon (C) is of 4.03 micron (mean diameter), 6. An improved process for the preparation of MoSi2 -reaction bonded silicon carbide composite useful as a structural ceramic material substantially is herein described with reference to the examples.

Full Text

This invention relates to an improved process for the preparation of molybdemum disilicide (MoSi2) - reaction bonded silicon carbide (RBSC) composite useful as a structural ceramic material.
In the process of the present invention, synthesis of this duplex non-oxide ceramic is made using infiltration of a melt of alloyed molybdenum (Mo) and silicon (Si) into a porous compact of commercial silicon carbide (SiC) powder together with carbon at high temperatures, which leads to the reaction of silicon (Si) with both carbon (C) and molybdenum (Mo) resulting in the formation of silicon carbide ( SiC ) and molybdenum disilicide ( MoSi2 ) . The novelty of the present invention is that unlike other alloyed melt infiltration technique, the alloyed melt in the present case is not made of the ceramic compounds, but it is prepared directly from elemental source of raw materials. In the process of present invention of MoSi2 reinforced RBSC composite, elemental Mo and Si are used to prepare the infiltrating alloyed melt.
The duplex non-oxide ceramic made of MoSi2 reinforced RBSC, hereinafter designated as MoSi2-RBSC, shows excellent physico-chemical properties like, high oxidation, corrosion and thermal shock resistance, superb high temperature strength and toughness together with other related properties, e.g., Young's modulus, hardness, etc., and therefore, is of great technological importance. . These properties can be exploited during the applications of the material in the fields of kiln furniture particularly for heating sanitary wares, saggers, crucibles, for calcining fluorescent powder
in fluorescent lamp industries, high speed and high service life domestic and industrial burners, rocket nozzles, recuperators for waste heat utilisation in indirect furnaces, heat-exchangers, indirectly fired open/combined cycle gas turbines to generate electric power, pump seals for pumps handling corrosive fluids, wear resistant inserts, etc.
RBSC is normally prepared by infiltrating molten silicon in inert atmosphere into a porous preform of silicon carbide and carbon powder, when silicon reacts with carbon to form newer silicon carbide which precipitates on the existing silicon carbide particles and bind them together. However, the process suffers from a disadvantage, that this exerts the chances of occurrence of free silicon in the final product. The free silicon phase being less refractory than SiC is known to have deleterious effect on almost all the physico-chemical and thermo-mechanical properties of reaction bonded silicon carbide both at room temperature and high temperatures. This difficulty has led to the development of melt infiltration technique in which the liquid infiltrant is taken in the form of a alloy of silicon normally with transition elements.
In the case of MoSi2-RBSC non-oxide ceramic, the alloy infiltrant is normally made from the reaction between silicon and the binary compound MoSi2 [Liquid phase reaction bonding of silicon carbide using alloys of silicon -molybdenum melts - R. P. Messener and Y. M. Chiang, J. American Ceramic Soc. v73(5) 1990, pp.1193-1200]. The process appears to be expensive since it involves use of costly MoSi2 compound.
In an other study, molten MoSi2 is infiltrated into a porous RBSC material in which the free silicon phase removed using evacuation at high
temperature [Microstructure and mechanical properties of RBSC/MoSi2 composite - C. B. Lim, T. Yano and T. Iseki, J. Materials Science, v24, 1989, pp.4144-51]. The process is also expected to be expensive since it involves molten MoSi2 which is prepared at a very high temperature of around 2050°C.
Infiltration of porous RBSC with alloyed melt comprising of Mo, Si and aluminium (Al) at high temperature (around 1950°C) has also been reported [High temperature strength of melt infiltrated SiC-Mo(Al,Si)2 composites - K. Shobu, E. Tani, M. Akiyama and T. Watanabe, J. American Ceramic Soc., v79(2), 1996, pp.544-46] for the preparation of Mo(Al,Si)2-RBSC composite and the process may be expensive because of the involvement of very high temperature. The formation of NbSi2-RBSC composite using infiltration of molten niobium (Nb) alloyed melt into a micro-porous preform of carbon (C) has recently been published [Reaction melt infiltration of silicon - niobium alloys into microporous carbon - M. Singh and D. R. Behrendt, J. Materials Research v9(7), 1994, pp. 1701-08] but the resulting composite has been, reported to contain appreciable amount of detrimental silicon phase.
A few work in the related fields has been patented. The process patented by Taylor [Cold moulded dense silicon carbide articles and method of making the shapes, K. M. Taylor, USP 3,205,043, Sept. 7, 1965] involves the infiltration of high-char-yielding organic into a resin bonded compact of granular SiC followed by subsequent siliconisation. This process leads to the presence of appreciable porosity in the intermediate stage which gives appreciable amount of residual silicon to be present in the final product,
which is detrimental so far as the thermomechanical and other properties are concerned. The process patented by Fitschmun [Method of making silicon carbide article of less than full density, D. R. Fitschmum, USP 3,947,550 Mar. 30, 1976] involves direct combination of silicon and carbon and thereby increses the chances of increasing amount of porosity to be present in the final product. Hence the properties are deteriorated. In an other work [Manufacture of silicon carbide ceramics free of residual silicon, T. R. Grossman, USP 5,571,758 Nov. 5, 1996], the amount of silicon was reduced by nitriding the phase. The precursor was a composite of carbon particulate with SiC fibre in the form of a wick. The process is still costly as it involves two stage process. Refractory grade reaction bonded SiC was prepared [Secondarily formed silicon carbide and free carbon containing microporous silicon carbide ceramics and their manufacture and use, I. Elstner, et.al, WOP 9633,959 Oct. 31, 1996] using anthracite/ graphite carbon. The product is essensially porous and is of inferior quality.
The main object of the present invention is to provide an improved process for the preparation of MoSi2 - reaction bonded silicon carbide composite useful as a structural ceramic material which obviates the drawbacks as mentioned above.
Another object is to provide an economical process for the preparation of MoSi2-RBSC composite.
Yet another object of the present invention is to prepare MoSi2-RBSC composite using elemental Mo and Si.
The present invention provides an improved process for the preparation of a duplex non-oxide ceramic particularly MoSi2 - RBSC
sintered composite using alloyed Mo/Si infiltrating melt prepared from elemental combination of powder Mo and Si. The uniqueness of the process is that, it demonstrates the possibility of using elemental Mo and Si as the infiltrating components and thereby making the overall process cheaper because of avoidance of costly MoSi2 compound in the melt formulation. Besides, both the elemental Mo and Si are readily available in very pure form.
The process of the present invention involves the alloyed Mo/Si melt infiltration at high temperature into a porous compact of silicon and carbon when a part of silicon of the melt reacts with carbon to form newer silicon carbide (SiC) grains which act as the bond between the existing silicon carbide particles. The remaining silicon on reaction with alloying Mo gets converted to refractory MoSi2 phase. The presence of MoSi2 phase in the resulting composite by virtue of its brittle to ductile transition characteristics at around
10000C is expected to improve the fracture toughness of the final composite particularly at
high temperature. The low temperature fracture properties may also excel by the matrix
microcracking caused by the differential thermal expansion between matrix SiC and reinforcing MoSi2. Moreover, MoSi2 is known to form a dihedral angle of around 120° and as a result there is a strong possibility of crack deflection along the SiC-MoSi2 interface contributing higher fracture strength.
Accordingly the present invention provides an improved process for the preparation of MoSi2 - reaction bonded silicon carbide composite useful as a structural ceramic material, which comprises :
(i) mixing elemental Mo and elemental Si powder in the desired ratio, heating the mix so obtained in a non-reacting atmosphere to a temperature in the range of 1600°C to 1800°C, to obtain a homogenous melt,
(ii) mixing SiC and C powders in the desired ratio,
(iii) preparing a green preform of desired shape and size by conventional ceramic fabrication methods using SiC and C mix obtained in
step(ii),
(iv) heating the SiC and C green preform so obtained in step (iii) in the presence of the Mo-Si homogenised melt obtained in step (i) to a temperature range of 1650°C to 1850°C in an inert atmosphere to obtain the MoSi2-RBSC composite.
The present invention provides an improved process for the
preparation of MoSi2 - reaction bonded silicon carbide composite useful as a
structural ceramic material which is a new and cheaper process for the
preparation of MoSi2-RBSC composite. The steps to carry out the process
are as follows :
1. Preparation of infiltrating alloyed melt from elemental source, such as,
elemental Mo and elemental Si powders by proper weighing and intimate
mixing followed by heating in a non-reacting atmosphere, such as, in
pure argon gas atmosphere or vacuum, to a temperature in the range of
1600 to 1800°C forhomogenisation.
2. Preparation of SiC and C compact by proper weighing and mixing
followed by preparing the shape of a billet or any other required shape by
using conventional methods of ceramic fabrication, such as, uniaxial
pressing, isostatic pressing, and extrusion.
3. Heating the SiC and C compact obtained in step (2) above in presence of
the melt obained in step (1) above at a temperature in the range of 1650
to 1850°C to obtain the duplex non-oxide ceramic.
Particularly for the preparation MoSi2-RBSC composite, the ratio of C:SiC and the volume fraction of SiC in the green compact may be varied according to the ratio of MoSi2, Si and SiC desired in the final product.
The weight ratio of C:SiC and volume fraction of SiC in the green compact may be in the ranges of 0.1 to 0.6 and 0.30 to 0.34 respectively.
The elemental Mo and Si powders used may be of commercial grade having purity >99% and >98% respectively and may be of -200 BSS size each.
The concentration of the alloying Mo in Si/Mo melt may be in the range of 5.13 to 26.8 weight %.
The particle size of commercial grade SiC may vary from -1200 grit (1.63 micron mean diameter) to -400 grit (23.65 micron mean diameter).
The particle size of carbon used may 4-6 micron.
The following examples are given to illustrate the process of the invention and the manner in which it may be carried out in practice; however, this should not be construed to limit the scope of the present invention.
Example 1
12.15g commercial grade silicon carbide (SiC) of-1200 grit size (1.63 micron mean diameter) and 7.29g carbon (C) of 4.03 micron mean diameter
are weighed and mixed in distilled water for 10 min. The mixture is dried,
mixed with/PVA solutiom again mixed and finally pressed isostatically in
the shape of a billet. The billet is dried at 120°C and its physical parameters, like, dimensions, weight and green density are measured.
In a separate test, 0.95g commercial silicon powder [>98% pure, -200 BSS] and 17.56g molybdenum [>99% pure, -200 BSS] are weighed and mixed in isopropyl alcohol for 10 min. It is dried at 100°C and further mixed. The dry powder is taken in a graphite crucible and heated in pure argon gas atmosphere to 1600°C. It is soaked for 30. min to homogenise the melt and cooled.
The billet is then placed on the alloyed mass in the crucible and heated once again in argon gas atmosphere to 1650°C. The liquid alloyed melt is allowed to infiltrate into the billet for 30 min and cooled. The mass as obtained after cooling is tested by X-ray diffraction technique. Silicon carbide (SiC) and silicon (Si) are present in the matrix. The presence of MoSi2 is found to be negligibly small.
Example 2
Commercial silicon powder [>98% pure, -200 BSS] and molybdenum powder [>99% pure, 200 BSS] are weighed by varying the weight ratio of Mo in Si and Mo mixture in the range of 5.13 to 26.8% and mixed in isopropyl alcohol for 10 min The mixtures are taken in graphite crucibles and heated in pure argon gas atmosphere at 1600°C. It is soaked for 30 min at that temperature to homogenise the melt.
12.15g commercial grade silicon carbide (SiC) of-1200 grit size (1.63 micron mean diameter) and 7.29g carbon (C) of 4.03 micron mean diameter are weighed and mixed in distilled water for 10 min. The mixture is dried, mixed with PVA solution, again mixed and finally pressed isostatically in the shape of a billet. The billet is dried at 120°C and its physical parameters, like, dimensions, weight and green density are measured. The billet is placed on the alloyed mass in the crucible and heated in argon gas atmosphere to 1650°C. The liquid alloyed melt is allowed to infiltrate into the billet for 30 min and cooled. The cold composite, as formed, is tested by X-ray diffraction technique. Silicon carbide (SiC) and silicon (Si) are present in the matrix. Presence of MoSi2 is found to increase with the increase in the amount of Mo in the alloyed melt.
Example 3
17.56g commercial silicon powder [>98% pure, -200 BSS] pure and 0.95g molybdenum [>99.5% pure, -200 BSS] are weighed and mixed in isopropyl alcohol for 10 min. It is dried at 100°C and further mixed. The dry powder is taken in a graphite crucible and heated in pure argon gas atmosphere at varying temperatures between 1600 to 1800°C. It is soaked for 30 min to homogenise the melt.
12.15g commercial grade silicon carbide (SiC) of-1200 grit size (1.63 micron mean diameter) and 7.29g carbon (C) of 4.03 micron mean diameter are weighed and mixed in distilled water for 15 min. It is dried at 100°C and further mixed. The mixture is dried, mixed with PVA solution, again mixed and finally pressed isostatically in the shape of a billet. The billet is dried at 120°C and its physical parameters, like, dimensions, weight and green density are measured. The billet is placed on the alloyed mass taken in a graphite crucible and heated once again in argon gas atmosphere to 1650°C. The liquid alloyed melt is allowed to infiltrate into the billet for 30 min.
Example 4
17.56g commercial silicon powder [>98% pure, -200 BSS] and 0.95g molybdenum [>99.5% pure, -200 BSS] are weighed and mixed in isopropyl alcohol for 10 min. The dry powder is taken in a graphite crucible and
heated in pure argon gas atmosphere to 1600°C. It is soaked for 30 min to homogenise the melt.
12.15g commercial grade silicon carbide (SiC) of-1200 grit size (1.63 micron mean diameter) and 7.29g carbon (C) of 4.03 micron mean diameter are weighed and mixed in distilled water for 10 min. The mixture is dried, mixed with PVA solution, again mixed and finally pressed isostatically in the shape of a billet. The billet is dried at 120°C and its physical parameters, like, dimensions, weight and green density are measured. The billet is placed on the alloyed mass in a graphite crucible and heated in argon gas atmosphere at varying temperature in the range of 1650 to 1850°C. The liquid alloyed melt is allowed to infiltrate into the billet for 30 min.
Example 5
Commercial silicon carbide powder of -1200 grit size (1.63 micron mean diameter) and carbon powder of 4.06 micron mean diameter are weighed and mixed in water for 10 min. by varying the weight ratio of C:SiC in the range from 0.1 to 0.6. The mixture is dried, mixed with PVA solution, again mixed and pressed isostatically in the shape of billets. They are dried at 120°C and their physical parameters, like, dimensions, weight and green density are measured.
In a separate test, 0.95g commercial silicon powder [>98% pure, -200 BSS] and 17.56g molybdenum [>99% pure, -200 .BSS] are weighed and mixed in isopropyl alcohol for 10 min. It is air dried at 100°C and further mixed. It is dried and further mixed. The dry powder is taken in a graphite
crucible, heated in pure argon gas atmosphere to 1600°C. It is soaked for 30 min to homogenise the melt and cooled. The billets are placed on the alloyed mass in the crucible and heated in argon gas atmosphere to 1650°C. The liquid alloyed melt is allowed to infiltrate into the billet for 30 min.
Example 6
12.15g commercial grade silicon carbide (SiC) of particle size varying between 1200 grit size (1.63 micron mean diameter) to 400 grit size (23.65 micron mean diameter) and 7.29g carbon (C) of 4.06. micron mean diameter are weighed and mixed in distilled water for 10 min. The mixture is dried, mixed with PVA solution, again mixed and pressed isostatically in the shape of billets. They are dried at 120°C and their physical parameters, like, dimensions, weight and green density are measured.
In a separate test, 17.56g commercial silicon powder [>98% pure, -200 BSS] and 0.95g molybdenum [>99.5% pure, -200 BSS] are weighed and mixed in isopropyl alcohol for 10 min. It is dried at 100°C and further mixed. The dry powder is taken in a graphite crucible and heated in pure argon gas atmosphere to 1600°C. It is soaked for 30 min to homogenise the melt.

The billets are placed on the cooled alloyed mass in the crucible and heated in argon gas atmosphere to 1650°C. The liquid alloyed melt is allowed to infiltrate into the billet for 30 min.
The advantages of the process for production of MoSi2-RBSC composite are :
1. The sources of the raw materials (e.g., Mo and Si) in making the
infiltrating melt are elemental in nature, instead of binary compounds of
the alloying species which are costlier. One of the raw materials (e.g., Si)
is of commercial variety.
2. The source of raw materials in making the starting compact (e.g., SiC
and C) are of commercial grade and therefore readily available.
3. The raw materials being of commercial variety are cheaper.
4. Upon proper selection of the alloyed melt composition, SiC:C ratio and
the volume fractions of SiC in the starting compact, complete elimination
of free silicon in the composite matrix is possible.

We claim:
1. An improved process for the preparation of MoSi2 - reaction bonded silicon carbide composite useful as a structural ceramic material, which comprises :
(i) mixing elemental Mo and elemental Si powder in the ratio of 5.12 to 26.8 wt % heating the mix so obtained in presence of argon gas atmosphere a temperature in the range of 1600°C to 1800°C, to obtain a homogenous melt,
(ii) mixing C powder and Sic. in the weight ratio of 0.1 to 0.6 5(1)(6)
(iii) preparing a (green perform of desired shape and size) by conventional ceramic
fabrication methods using SiC and C mix obtained in step (ii),
(iv) heating the SiC and C green perform so obtained in step (iii) in the presence of the
Mo-Si homogenized melt obtained in step (i) to a temperature range of 1650°C to 1850°C
in presence of organ atmosphere to obtain the MoSi2-RBSC composite.
2. An improved process as claimed in claim (1) wherein the elemental Mo and Si are of commercial grade with purity >99% and >98% respectively,
3. An improved process as claimed in claim wherein the weight ratio of C: SiC is in the range of unit2 /pass
4. An improved process as claimed in claim wherein the particle size of SiC is in the range of-400 grit to -1200 grit (23.65μ to 1.63 micron mean diameter),
5. An improved process as claimed in claim wherein the particle size of carbon (C) is of 4.03 micron (mean diameter),

6. An improved process for the preparation of MoSi2 -reaction bonded silicon carbide composite useful as a structural ceramic material substantially is herein described with reference to the examples.

Documents:

1683-del-1998-abstract.pdf

1683-del-1998-claims.pdf

1683-del-1998-complete specification (granted).pdf

1683-del-1998-correspondence-others.pdf

1683-del-1998-correspondence-po.pdf

1683-del-1998-description (complete).pdf

1683-del-1998-form-1.pdf

1683-del-1998-form-19.pdf

1683-del-1998-form-2.pdf

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Patent Number

226270

Indian Patent Application Number

1683/DEL/1998

PG Journal Number

04/2009

Publication Date

23-Jan-2009

Grant Date

16-Dec-2008

Date of Filing

18-Jun-1998

Name of Patentee

COUNCIL OF SCIENTIFIC AND INDUSTRIAL RESEARCH

Applicant Address

RAFI MARG, NEW DELHI-110001, INDIA.

Inventors:

#	Inventor's Name	Inventor's Address
1	OM PRASH CHAKRABARTI	CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, CALCUTTA 700032,
2	PROBAL KUMAR DAS	CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, CALCUTTA 700032,

PCT International Classification Number

C04B 35/58

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA