Title of Invention	A PROCESS FOR JOINING CERAMICS TO CERAMICS OF ALUMINA MONOLITHS AND CERAMIC COMPOSITES USING SILICATES HAVING HIGH SERVICE TEMPERATURE AND BREAK STRENGTH
Abstract	Alumina/ alumina, mullite/high alumina porcelains, zirconia/ zirconia, silicon carbide/silicon carbide, mullite/ zirconia and alumina/ zirconia ceramic joints prepared by using magnesium alumino silicates (MAS), magnesium silicates (MS), calcium silicates (CS), calcium magnesium silicates (CMS) or calcium magnesium alumino silicates (CMAS) as joining media showed higher bending strength and high temperature stability. These ceramic/ ceramic joints are found to -withstand temperatures ~155()oC. Procedure for the preparation of joints is very simple and straightforward. The method of preparation of the joints is as follows: (i) homogenization of source materials for the joining medium using ball milling in presence of a suitable organic liquid medium, (ii) drying, precalcinations followed by annealing of the homogenized mixtures, (iii) surface treatment of the heat- treated powders with wetting agents and deflocculants, (iv) preparation of the slurry of surface treated powders using organic or inorganic binders, (v) adding antifoaming agents to the slurry to avoid bubble formation,(vi) coating of the slurry on the joining parts (alumina substrates) and curing inside a hot air-oven and heat-treating the green joints at a suitable higher .;(; temperatures. The joining of ceramic parts, particularly alumina takes place by the liquid phase assisted reaction bonding that sets in between the I. substrates and the joining medium.

Title of Invention

A PROCESS FOR JOINING CERAMICS TO CERAMICS OF ALUMINA MONOLITHS AND CERAMIC COMPOSITES USING SILICATES HAVING HIGH SERVICE TEMPERATURE AND BREAK STRENGTH

Abstract

Alumina/ alumina, mullite/high alumina porcelains, zirconia/ zirconia, silicon carbide/silicon carbide, mullite/ zirconia and alumina/ zirconia ceramic joints prepared by using magnesium alumino silicates (MAS), magnesium silicates (MS), calcium silicates (CS), calcium magnesium silicates (CMS) or calcium magnesium alumino silicates (CMAS) as joining media showed higher bending strength and high temperature stability. These ceramic/ ceramic joints are found to -withstand temperatures ~155()oC. Procedure for the preparation of joints is very simple and straightforward. The method of preparation of the joints is as follows: (i) homogenization of source materials for the joining medium using ball milling in presence of a suitable organic liquid medium, (ii) drying, precalcinations followed by annealing of the homogenized mixtures, (iii) surface treatment of the heat- treated powders with wetting agents and deflocculants, (iv) preparation of the slurry of surface treated powders using organic or inorganic binders, (v) adding antifoaming agents to the slurry to avoid bubble formation,(vi) coating of the slurry on the joining parts (alumina substrates) and curing inside a hot air-oven and heat-treating the green joints at a suitable higher .;(; temperatures. The joining of ceramic parts, particularly alumina takes place by the liquid phase assisted reaction bonding that sets in between the I. substrates and the joining medium.

Full Text	The invention relates to a process of Ceramic-Ceramic joining of monoliths and ceramic composites using silicates for high service temperatures and break strengths. Background: Widespread interests in advanced ceramics stems from their potential applications in new technology and are associated with the properties that distinguish ceramics from other materials. In high temperature environments such as energy conversions, gas turbine engines, aerospace explorations, machining and grinding of machine parts and in metallurgical industries, ceramics remain stiff at temperatures not tolerated by metals and super alloys. To achieve technical advances in ceramics appropriate and unique characteristics as well as methods for fabricating or assembling them into structures that will function effectively are required. Methods by which complex-shaped ceramic components may be formed are numerous, but each one has its limitations. Injection molding has shown promises but difficulties with the frequent occurrence of macroscopic defects, removal of binder/lubricants from thick cross sections without cracking, control of warping and lengthy cycles of debinderization limiting the fast throughput have been encountered. Further, slip casting, gel casting or pressure casting methods have their own difficulties with intricate shapes of varying cross sections posing unique problems such as mold release, drying shrinkage cracking and lengthy drying cycles. The uses of ceramic materials have been inhibited by the difficulty and cost of manufacturing components of complex shapes. Limitations in size can potentially be overcome by joining small and simple parts together to form a complex shaped aggregate. Objects made of most of the metals and alloys have the advantage of easy welding, brazing or soldering. However, improved joining techniques are required for ceramics so that the joints are not the performance limiting 'weak-link' in the aggregate body. Most of the scientific and engineering efforts have been devoted to ceramic-metal joining. Whereas ceramic-ceramic joining is not a highly developed technology; particularly, joining of oxide ceramics, without any applied pressure under conventional sintering schedule with air as the surrounding atmosphere. There are a few techniques available for ceramic-ceramic joining. Organic adhesives as well as cement/mortar adhesives have been used as interlayer for ceramic-ceramic joining. Even though joining of ceramics with these adhesives are relatively inexpensive and simple, the joints prepared with these adhesives are not stable at higher temperatures. Strength of these joints is very low. Mechanical joining of ceramics by direct bonding without interlayer materials requires very high temperatures. Breaking strength as well as the service temperature of the mechanical joints is low. In addition, the high temperature mechanical joining requires the application of external pressure on the joining parts. Mechanical fastening is frequently inadequate because ceramic parts are inherently brittle. Metals or metallic alloys have been employed as an interlayer for ceramic-ceramic joining. This involves brazing, soldering or diffusion bonding. Joining of ceramics by metal brazing can only be done for the surface treated (premetallized) substrates. Service temperature of ceramic-ceramic joints with metal or alloy interlayer is limited by their respective melting point. Another drawback of these joints is the large differences in thermal expansion between the ceramic substrates and the joint interlayer, which in turn reduces the stability of the joints even at, lower service temperatures. For joining of ceramics by fusion welding using electron beam or laser process, the substrates must be preheat treated to relatively higher temperatures to avoid the thermal shock in the weld area, which usually leads to severe cracking. This type welding introduces localized stresses, which cannot be accommodated by a stiff material. Positioning of the beam and the beam parameters, as they plays major role in joining process, critically effects joining of ceramics by fusion welding using electron or laser beam. The object of the Invention: An object of the present invention is to establish a simplified processing procedure for ceramic - ceramic joining. Another object of the invention is to prepare ceramic - ceramic joints of high breaking strength. Yet another object of the invention is to provide a ceramic substrates joint at relatively lower temperatures. Still another object of the invention is to provide ceramic - ceramic joints that are stable at higher temperatures. It is a further object of the invention to provide novel materials for ceramic-ceramic joining. It is yet a further object of the invention is to develop a process for joining of alumina ceramics. It is still a further object of the invention is to develop alumina ceramic parts for joining and evolve compatible methods of joining ceramic parts to one another. To achieve the above objectives this invention provides for a process of ceramic-ceramic joining of monoliths and ceramic composites comprising: homogenization of source materials as herein described for the joining medium drying the said homogenized mixture precalcinating the said dried homogenized mixture to get precalcined powders annealing the homogenized mixtures after milling the said precalcined powders, subjecting the said annealed homogenised mixture to surface treatment with wetting agents and deflocculants as herein described to obtain a slurry treating the slurry of said annealed homogenised mixture using binders as herein described, adding antifoaming agents as herein described to the slurry to avoid bubble formation to obtain a joining medium, coating the said joining medium on said ceramic parts and joining them to get joints, curing the said joints inside a hot air-oven and heat-treating to get green joints subjecting the said green joints to temperatures of 1250°C to 1550°C to yield ceramic-ceramic joints of high breaking strength The source materials are homogenized using ball milling in presence of an organic liquid medium as herein described to obtain a homogenized mixture of source materials. The source materials are magnesium silicates (MS), magnesium alumino silicates (MAS), calcium silicates (CS), calcium magnesium silicates (CMS) or calcium magnesium alumino silicate (CMAS) The liquid medium used for homogenizing is acetone or ethanol. The ceramic parts are ceramic or alumina substrates- The homogenised mixture is air dried in an air-oven at 90*^C to 110°C. The homogenised mixture is precalcined upto lOOO^C for 3 hours. The homogenised mixture is annealed at temperature upto 1200 °C for 3 hours. The surfactants are ethoxylated nonylphenol or ethoxylated tridecylalcohol. The deflocuUants are ammonium polycrylate or ammonium citrate. The binders may be organic or inorganic binders. The organic binders are refined starch, dextrine, polyvinyl butyral, methylcellulose, polyvinyl alcohol and polymethyl methacrylate or organic silicate. The organic silicate is ethyl silicate. The antifoaming agents are calcium stearate, aluminium stearate or tributyl phosphate. The ceramic parts are joined during the liquid phase bonding between said ceramic parts and said joining medium. Brief Description of the Drawings: The invention will now be described according to the accompanying drawings. Figure 1 shows the compositional variation of the joining materials that is magnesium silicates (MS) and magnesium alumino silicates (MAS) in the form of schematic ternary phase diagram. Figure 2 gives the schematic ternary phase diagram that indicates the compositional variation of the calcium silicates (CS) joining materials. Figure 3 shows the compositional variation of the calcium magnesium silicates (CMS) in the form of schematic phase diagram. Figure 4 gives the XRD pattern of the magnesium silicate (MgO. SiO2) (a) heat treated at 1200°C (b) heat treated at 1450°C (c) joining material coated on alumina specimen and fired at 1450°C Figure 5 XRD pattern of the magnesium alumino silicate (MgO+Al203 +Si02) (a) heat treated at 1200oC (b) heat treated at 1550 Figure 6 XRD pattern of the calcium magnesium silicate (CaO+MgO+Si02) (a) heat treated at 125(>> Q (b) joining material coated on the alumina substrate and fired at 1300^ C, (c) joining material coated on the alumina substrate and fired at 1350°C Figure 7 XRD pattern of the calcium magnesium alumino silicate (CaO+MgO+Al 203+SiO2) (a) heat treated at 1300° C for Ih and (b) joining material coated on the alumina substrate and fired at 1450° C for Ih, Figure 8 The effect of composition of joining material and duration of joining process [curve (a) Ih (b) 1.5 h and (c) 2h] on flexural strength of the alumina - alumina ceramic joints. Detailed Description of Invention with respect to the accompanying drawings: Materials for ceramic/ceramic joining: Different materials with various compositions are prepared and used for ceramic/ceramic joining. The basic principle involved in preference to the use of a particular material with specific composition is that the material should exhibit partial or complete melting and hence it should form a reaction couple with the ceramic parts to be joined. After melting, the material can be partly crystalline and partly amorphous or fully amorphous. The facts also to be taken into account are that the material and its should be stable at higher temperatures and it should give higher mechanical strength compositions must be such that, after the joining process the ceramic/ceramic joints for wider temperature range i.e. from room temperature to higher service temperature. It is also desirable that the materials chosen as joining media should be easily to prepare and economically viable. Based on these essential requirements, materials preferred as a medium for ceramic/ceramic joining are magnesium silicates (MS), magnesium alumino silicates (MAS), calcium silicates (CS), calcium magnesium silicates (CMS) and calcium magnesium alumino silicate (CMAS). In all the above mentioned materials, different compositions have been optimized for ceramic/ceramic joining. Figure 1 shows the compositional variation of the joining materials: magnesium silicates (MS) and magnesium alumino silicates (MAS) in the form of schematic ternary phase diagram. Figure 2 gives the schematic ternary phase diagram, which indicates the compositional variation of the calcium silicates (CS) joining materials. Figure 3 shows the compositional variation of the calcium magnesium silicates (CMS) in the form of schematic phase diagram. Method and procedure of ceramic/ceramic joining: Source materials for ceramic/ceramic joining are taken in the required quantities and are homogenized by using ball milling in presence of a suitable liquid medium such as acetone or ethanoL Homogenized mixtures are dried in air-oven at 100°C, and precalcined at 1000°C; for 3h. Precalcined powders are then milled again and further annealed at 1200°C for 3h. Heat-treated powders are surface treated with surfactants such as ethoxylated nonylphenol, ethoxylated tridecylalcohol and these surfactants acts as wetting agents. Surface treated powders are mixed with anunonium polyacrylate or ammoruum citrate to hinder the flocculation or agglomeration of the powder particles. Different organic as well as inorganic binders are used for the preparation of the slurry of the powders. Refined starch, dextrine, polyvinyl buiyral, meihylcellulose, polyvinyl alcohol and polymethyl methacrylate are a few of the organic materials that are used as binders. Some of the organic silicates such as ethyl silicates are also used as binding agent. After the binder addition, slurry is treated with anti-foaming agents, calcium stearate, aluminium stearate or tributyl phosphate, which reduces the surface viscosity whereby, prevents the bubble formation in the slurry due to air or any other dissolved gas. The slurry is coated on the porous or nonporous alumina specimens. The coated objects are mounted one over the other. This joint configuration is cured inside the air-oven at 100°C, These green joints are heat treated at suitable temperatures varying from 1250°C to 1550°C for 2h. This results in the reaction between the joining medium and the alumina substrates, which leads to the joining of the two alumina parts. Bending strength of the porous alumina specimens was found to be 220MPa whereas the nonporous specimens show failure at 380MPa, Microstructure of the alumina specimens shows size of the equiaxial grains varies from 1.5 to 3µm, and the pore size in the porous specimens varies between 0.5 to lµm. Evaluation of the ceramic/ceramic joints: The ceramic-ceramic joints are evaluated to determine their mechanical strength and high temperature stability. Flexural strength of the joints is measured in three-point bending mode. Maximum service temperature stability of the unsupported beams is found out by heat-treating the joined specimens at different temperatures and at different durations. Reaction products formed in the joining process are identified using X-ray diffraction. Materials and methods for joining of oxide, non-oxide ceramics and ceramic composites: In the present invention it is found that the different silicates (MS, MAS, CS, CMS and CM AS), which are used as joining media, are not only applicable for the alumina ceramics but also applicable for other oxide ceramics. Experimental determination of the present invention reveals that the aforementioned silicates are applicable as effective joining media for other oxide ceramics mullite, alumina rich silicates and partially or fully stabilized zirconia. It is also found that the objects made of silicon carbide (SiC) (non-oxide ceramics) and silicon carbide/alumina/zirconia composites can successfully joined by using magnesium silicates (MS), magnesium alumino silicates (MAS), calcium silicates (CS), calcium magnesium siUcates (CMS) or calcium magnesium alumino sihcates (CMAS) as joining media. The above mentioned silicates find their application as joirung media for joining of mullite-zirconia and alumina-zirconia composites as well Methods and procedures followed for joining of oxide, non-oxide ceramics and ceramic composites are the same as that followed for joining of alumina ceramics except that inert or reducing atmosphere, mostly hydrogen/nitrogen mixtures is used for joining of silicon carbide (SiC) ceramics to prevent the oxidation of the substrates. Application of the joining materials as sintering aids: In the present invention it is found that, the reaction between the joining medium and the substrates promotes the densification of porous substrates through diffusion process. Hence the materials, which are used as joining media, can be used as sintering aids to prepare non-porous monolithic ceramics. These materials can be used individually or in different combinations as well. Hence the main objective of developing non- porous alumina ceramic parts for joining is extendable to sintering process. These materials can also be used for joining of ceramic greens after curing. It is determined from the present investigation that the joining materials have broader applications as sintering aids in the fabrication of ceramic-matrix composites with controlled porosity. Examples of joining materials: (i) MgO+Si02 To prepare magnesium silicate 916g of magnesium hydroxy carbonate (MgCOs Mg(OH)2) containing 44% MgO is mixed with 601g of aerosol silica (SiO2) by ball milling in presence of liquid a medium (acetone or ethanol) for 4h. Homogenized mixture is dried in air oven at lOO°C. After drying, the powder samples are precalcined at 10(XX] for 3h. Heat treatment of the calcined powders is carried out at 1200°C for 3h. Annealed powders are surface treated with wetting agents (0.5wt%) and surface modified powder is mixed with deflocculant (0.5 tolwt %) to hinder the flocculation or agglomeration of the powder particles. Slurry of the surface-treated powder sample is prepared by mixing it with aqueous solution of organic binder (2wt%). After the binder addition, slurry is treated with anti-foaming agents (0.5wt%), which reduces the surface viscosity whereby preventing the bubble formation in the slurry due to air or other dissolved gases. Thick films of slurry are coated on the porous or non-porous alumina specimens and the coated objects are mounted one over the other or joined laterally. The joined configurations are cured inside the air-oven at 100^. After curing, the joints are strong enough to hold them in position. These green joints are then fired at higher temperature 1450^ -1470O C for 2h. This high temperature firing sets the reaction between the substrate and the joining materials whereby forming crystalline spinel phase (MgAl2O4) as a reaction product along with clinopyroxene (Mg2Si2O6), which leads to joining of the substrates. The reaction that takes place upon heating of the starting material can be given in the form of chemical equation: The percentage of the liquid phase decreases as the duration of the joining stage is extended. Although anticipated, no cordierite phase crystallized from the melt. The reaction between the substrates and the joining material also promotes the densification of porous alumina substrates. These ceramic joints are stable upto 1500°C without any deformation. It shows that the service temperature of these ceramic joints is around the joining temperature. Flexural strengths of the specimens after joining are measured in three-point bending mode. Flexural strengths of the joined specimens are found to be 90-95MPa at room temperature. Figure 4 gives the XRD pattern of the magnesium silicate (MgCSiOz) (a) heat treated at 1200°C (b) heat treated at 1450KZ (c) joining material coated on alumina specimen and fired at 1450°C (ii) MgO+Al2O3+SiO2 To prepare this magnesium aluminosilicate, 1820g of magnesium hydroxy carbonate (Mg(OH)2 .MgCOs) containing 44% MgO and 1020g of y-alumina (Y"Al203) are mixed with 902g of aerosol silica (SiOi) using ball milling in presence of a liquid medium (acetone) for 4h. Homogenized mixture is dried in the air oven at lOO°C. Dried powder sample is precalcined at lOOO° C for 3h then milled again and further annealed at 1200°C for 3h. Annealed powders are surface treated with wetting agents (0.5wt%) and surface modified powder is mixed with deflocculants (0.5 tolwt%) to hinder the flocculation or agglomeration of the powder particles. Slurry of the surface treated powder is prepared by mixing with aqueous solution of organic binder (2wt%). After the binder addition, the slurry is treated with anti foaming agents (0.5wt%), which reduces the surface viscosity whereby preventing the bubble formation in the slurry due to air or other dissolved gases. Thick film of slurry is coated on the porous or non-porous alumina substrates and the coated objects mounted one over the other or joined laterally. These joint configurations are cured inside the air oven at lOOoC. The green joints are then fired at higher temperature (1550oC for 2h). At higher temperatures the coated slurry reacts with the alumina substrate giving rise to form crystalline spinel phase (MgAl2O4) as the reaction product. The reaction that takes place upon heating of the starting material can be given in the form of the chemical equation. With joining trials of extended durations the sapphirine (Mg4Al10O23) crystallizes together with spinel (MgAl2O4). This is indicative the different interaction of alumina substrate with the liquid generation. It is also found that the reaction between the substrate and the joining material enhances the densification process in the porous substrates. Flexural strengths of the joined specimens are measured in three-point bending mode. Flexural strength of the joined specimens attains 85-90MPa at room temperature. These ceramic joints are found to be stable up to 1550oC. Figure 5 gives XRD pattern of the magnesium alumino silicate (MgO+AI2O3 +Si02) (a) heat treated at 1200<: heat treated at isso joining material coated on alumina substrate and fired> (iii) CaO+Si02 lOOOg of calcium carbonate (CaCO3) containing 56% CaO and 601g of aerosol silica (SiC)2) are mixed using ball milling in presence of a liquid medium (acetone) for 4h. Homogenized mixture is dried in air oven at lOOoC. Dried powder sample is pre calcined at l000oC for 3h and further annealed at 1200oC for 3h. Crystalline pseudo-wollastanite powder is surface-treated with the wetting agents (0.5wt%). The treated powder is mixed with deflocculants (0.5 tolwt %) to hinder the flocculation or agglomeration of the powder particles. Slurry of the surface treated powder sample is prepared by mixing it with aqueous solution of organic binder (2wt%). After the binder addition, slurry is treated with anti-foaming agents (0.5wt%), which reduces the surface viscosity whereby, prevents the bubble formation in the slurry due to air or other dissolved gases. Thick films of slurry are coated on the porous or non-porous alumina substrates and the coated objects mounted one over the other or laterally. These joint configurations are cured inside the air-oven at 100oC. The green joints are then fired at higher temperatures >1350o C for Ih. Flexural strengths of the joined specimens are measured in three-point bending mode. It is found that the modulus of rupture of the joined specimens is 55-60MPa. These calcium silicate joints are stable up to 1250oC, (iv) CaO+MgO+Si02 lOOOg of calcium carbonate (CaCO3) containing 56% CaO is mixed with 915g of magnesium hydroxy carbonate (Mg(OH)2 .MgCOs) containing 44% MgO and 1202g of aerosol silica (Si02) by ball milling in presence of an inert liquid medium (e.g. acetone or ethanol) for 4h. After drying the homogenized mixture in the air oven at 100oC, dried powder is precalcined at lOOOo C for 3h, Calcined powder is further annealed at 1200oC for 3h. Crystalline calcium magnesium silicate powder is surface treated with wetting agent (0,5wt%). The treated powder is mixed with deflocculants (0.5 to lwt%) to hinder the flocculation or agglomeration of the powder particles. Slurry of the surface treated powder sample is prepared by mixing it with aqueous solution of organic binder (2wt%). After the binder addition, slurry is treated with anti-foaming agents (0.5wt%), which reduces the surface viscosity thereby preventing the bubble formation in the slurry due to air or other dissolved gases. Thick films of the slurry are coated on the porous or non-porous alumina substrates and the coated objects mounted one over the other. These joint configurations are cured inside the air-oven at 100°C. The cured joints are strong enough to hold them in position. These cured joints are heat treated at higher temperature 1300oC for Ih. High temperature firing of the joint configurations results in reaction between the substrates and the joining material, whereby producing the liquid phase that crystallizes into the crystalline phases anorthite (CaAl2Si2O8) and spinel (MgAl2O4) as reaction products. The reaction that takes place upon heating of the starting material can be given in the form of the chemical equation. These reactions are found to increase the densification of the porous substrates. Flexural strengths of the joined specimens are measured in three-point bending mode. Room temperature flexural strength of the joined specimens is found to be 125-130MPa. Service temperature of these ceramic joints reaches 1250°C Figure 6 shows the XRD pattern of the calcium magnesium silicate (CaO+MgO+SiO2) (a) heat treated at 1250oC (b) joining material coated on the alumina substrate and fired at 1300oC(c) joining material coated on the alumina substrate and fired at 1350oC. lOOOg of calcium carbonate (CaCOa) containing 56% CaO is mixed with 1285g of magnesium hydroxy carbonate (Mg(OH)2 .MgCOs) containing 44% MgO, 1683g of aerosol silica (SiO2) and 200g of y-alumina (y-Al2O3) by using ball milling in presence of an inert liquid medium (acetone or ethanol) for 4h. Homogenized mixture is dried in the air oven (lOOoC). Dry powder samples are precalcined at 1000oC for 3h and then further annealed at 1200oC for 3k Armealed calcium magnesium silicate powder is surface treated with wetting agent (0.5wt%). The treated powder is mixed with deflocculant (0.5 to lwt%) to hinder the flocculation or agglomeration of the powder particles. Slurry of the surface treated powder is prepared by mixing with aqueous solution of an organic binder (2wt%). After the binder addition, slurry is treated with anti-foaming agents (0.5wt%), which reduces the surface viscosity whereby preventing the bubble formation in the slurry The reaction that takes place between the joining material and the substrates during the joining process can be given as The present invention shows that the flexural strengths of the alumina -alumina ceramic joints are dependent on the composition of the joining material as well as the processing parameters at high temperatures. Figure 8 shows the effect of composition and duration of joining process [curve (a) Ih (b) 1.5 h and (c) 2h] on flexural strength of the alumina -alumina ceramic joints. We claim: 1. A process of ceramic-ceramic joining of monoliths and ceramic composites comprising: homogenization of source materials as herein described for the joining medium drying the said homogenized mixture precalcinating the said dried homogenized mixture to get precalcined powders armealing the homogenized mixtures after milling the said precalcined powders, subjecting the said annealed homogenised mixture to surface treatment with wetting agents and deflocculants as herein described to obtain a slurry treating the slurry of said annealed homogenised mixture using binders as herein described, adding antifoaming agents as herein described to the slurry to avoid bubble formation to obtain a joining medium, coating the said joining medium on said ceramic parts and joining them to get joints, curing the said joints inside a hot air-oven and heat-treating to get green joints subjecting the said green joints to temperatures of 1250°C to 1550°C to yield ceramic-ceramic joints of high breaking strength 2. A process as claimed in claim 1 wherein said source materials are homogenized using ball milling in presence of an organic liquid medium as herein described to obtain a homogenized mixture of source materials. 3. A process as claimed in claim 1 wherein the said source materials are magnesium silicates (MS), magnesium alumino silicates (MAS), calcium silicates (CS), calcium magnesium silicates (CMS) or calcium magnesium alumino silicate (CMAS) 4. A process as claimed in claim 1 wherein the said liquid medium used for homogenizing is acetone or ethanol. 5. A process as claimed in claim 1 wherein the said ceramic parts are ceramic or alumina substrates. 6. A process as claimed in claim 1 wherein the said homogenised mixture is air dried in an air-oven at 90 to 110 degree C. 7. A process as claimed in claim 1 wherein the said homogenised mixture is precalcined upto lOOO^C for 3 hours. 8. A process as claimed in claim 1 wherein the said homogenised mixture is annealed at temperatures upto 1200 degrees C for 3 hours. 9. A process as claimed in claim 1 wherein the said surfactants are ethoxylated nonylphenol or ethoxylated tridecylalcohol. 10. A process as claimed in claim 1 wherein the said deflocuUants are ammonium polycrylate or ammonium citrate. 11. A process as claimed in claim 1 wherein the said binders may be organic or inorganic binders. 12. A process as claimed in claim 11 wherein the said organic binders are refined starch, dextrine, polyvinyl butyral, methylcellulose, polyvinyl alcohol and polymethyl methacrylate or organic silicate. 13. A process as claimed in claim 12 wherein the said organic silicate is ethyl silicate. 14. A process as claimed in claim 1 wherein the said antifoaming agents are calcium stearate, aluminium stearate or tributyl phosphate. 15. A process as claimed in claim 1 wherein the said joints are of coated objects are cured in the said air-oven at temperatures upto 100 degrees C to yield green joints. 16. A process as claimed in claim 15 wherein the said green joints are heat treated at temperatures ranging from 1250 to 1550 degrees C for over 2 hours. 17. A process as claimed in claim 1 wherein the green joints are fired at 1450°C to 1470°C, 18. A process as claimed in claim 1 wherein the said ceramic parts are joined during the liquid phase bonding between said ceramic parts and said joining medium. 19. A process of ceramic-ceramic joining of monoliths and ceramic composites substantially as herein described with reference to the foregoing examples.

Full Text

The invention relates to a process of Ceramic-Ceramic joining of monoliths and ceramic composites using silicates for high service temperatures and break strengths.
Background:
Widespread interests in advanced ceramics stems from their potential applications in new technology and are associated with the properties that distinguish ceramics from other materials. In high temperature environments such as energy conversions, gas turbine engines, aerospace explorations, machining and grinding of machine parts and in metallurgical industries, ceramics remain stiff at temperatures not tolerated by metals and super alloys.
To achieve technical advances in ceramics appropriate and unique characteristics as well as methods for fabricating or assembling them into structures that will function effectively are required. Methods by which complex-shaped ceramic components may be formed are numerous, but each one has its limitations. Injection molding has shown promises but difficulties with the frequent occurrence of macroscopic defects, removal of binder/lubricants from thick cross sections without cracking, control of warping and lengthy cycles of debinderization limiting the fast throughput have been encountered. Further, slip casting, gel casting or pressure casting methods have their own difficulties with intricate shapes of varying cross sections posing unique problems such as mold release, drying shrinkage cracking and lengthy drying cycles.
The uses of ceramic materials have been inhibited by the difficulty and cost of manufacturing components of complex shapes. Limitations in size can

potentially be overcome by joining small and simple parts together to form a complex shaped aggregate. Objects made of most of the metals and alloys have the advantage of easy welding, brazing or soldering. However, improved joining techniques are required for ceramics so that the joints are not the performance limiting 'weak-link' in the aggregate body. Most of the scientific and engineering efforts have been devoted to ceramic-metal joining. Whereas ceramic-ceramic joining is not a highly developed technology; particularly, joining of oxide ceramics, without any applied pressure under conventional sintering schedule with air as the surrounding atmosphere.
There are a few techniques available for ceramic-ceramic joining. Organic adhesives as well as cement/mortar adhesives have been used as interlayer for ceramic-ceramic joining. Even though joining of ceramics with these adhesives are relatively inexpensive and simple, the joints prepared with these adhesives are not stable at higher temperatures. Strength of these joints is very low. Mechanical joining of ceramics by direct bonding without interlayer materials requires very high temperatures. Breaking strength as well as the service temperature of the mechanical joints is low. In addition, the high temperature mechanical joining requires the application of external pressure on the joining parts. Mechanical fastening is frequently inadequate because ceramic parts are inherently brittle.
Metals or metallic alloys have been employed as an interlayer for ceramic-ceramic joining. This involves brazing, soldering or diffusion bonding. Joining of ceramics by metal brazing can only be done for the surface treated (premetallized) substrates. Service temperature of ceramic-ceramic joints with metal or alloy interlayer is limited by their respective melting

point. Another drawback of these joints is the large differences in thermal expansion between the ceramic substrates and the joint interlayer, which in turn reduces the stability of the joints even at, lower service temperatures. For joining of ceramics by fusion welding using electron beam or laser process, the substrates must be preheat treated to relatively higher temperatures to avoid the thermal shock in the weld area, which usually leads to severe cracking. This type welding introduces localized stresses, which cannot be accommodated by a stiff material. Positioning of the beam and the beam parameters, as they plays major role in joining process, critically effects joining of ceramics by fusion welding using electron or laser beam.
The object of the Invention:
An object of the present invention is to establish a simplified processing procedure for ceramic - ceramic joining.
Another object of the invention is to prepare ceramic - ceramic joints of high breaking strength.
Yet another object of the invention is to provide a ceramic substrates joint at relatively lower temperatures.
Still another object of the invention is to provide ceramic - ceramic joints that are stable at higher temperatures.
It is a further object of the invention to provide novel materials for ceramic-ceramic joining.

It is yet a further object of the invention is to develop a process for joining of alumina ceramics.
It is still a further object of the invention is to develop alumina ceramic parts for joining and evolve compatible methods of joining ceramic parts to one another.
To achieve the above objectives this invention provides for a process of ceramic-ceramic joining of monoliths and ceramic composites comprising:
homogenization of source materials as herein described for the
joining medium
drying the said homogenized mixture
precalcinating the said dried homogenized mixture to get
precalcined powders
annealing the homogenized mixtures after milling the said
precalcined powders,
subjecting the said annealed homogenised mixture to surface
treatment with wetting agents and deflocculants as herein
described to obtain a slurry
treating the slurry of said annealed homogenised mixture
using binders as herein described,
adding antifoaming agents as herein described to the slurry to
avoid bubble formation to obtain a joining medium,
coating the said joining medium on said ceramic parts and
joining them to get joints,
curing the said joints inside a hot air-oven and heat-treating to
get green joints

subjecting the said green joints to temperatures of 1250°C to 1550°C to yield ceramic-ceramic joints of high breaking strength
The source materials are homogenized using ball milling in presence of an organic liquid medium as herein described to obtain a homogenized mixture of source materials.
The source materials are magnesium silicates (MS), magnesium alumino silicates (MAS), calcium silicates (CS), calcium magnesium silicates (CMS) or calcium magnesium alumino silicate (CMAS)
The liquid medium used for homogenizing is acetone or ethanol.
The ceramic parts are ceramic or alumina substrates-
The homogenised mixture is air dried in an air-oven at 90*^C to 110°C.
The homogenised mixture is precalcined upto lOOO^C for 3 hours.
The homogenised mixture is annealed at temperature upto 1200 °C for 3 hours.
The surfactants are ethoxylated nonylphenol or ethoxylated tridecylalcohol. The deflocuUants are ammonium polycrylate or ammonium citrate.
The binders may be organic or inorganic binders.

The organic binders are refined starch, dextrine, polyvinyl butyral, methylcellulose, polyvinyl alcohol and polymethyl methacrylate or organic silicate.
The organic silicate is ethyl silicate.
The antifoaming agents are calcium stearate, aluminium stearate or tributyl phosphate.

The ceramic parts are joined during the liquid phase bonding between said ceramic parts and said joining medium.
Brief Description of the Drawings:
The invention will now be described according to the accompanying drawings.
Figure 1 shows the compositional variation of the joining materials that is magnesium silicates (MS) and magnesium alumino silicates (MAS) in the form of schematic ternary phase diagram.
Figure 2 gives the schematic ternary phase diagram that indicates the compositional variation of the calcium silicates (CS) joining materials.

Figure 3 shows the compositional variation of the calcium magnesium silicates (CMS) in the form of schematic phase diagram.
Figure 4 gives the XRD pattern of the magnesium silicate (MgO. SiO2) (a) heat treated at 1200°C (b) heat treated at 1450°C (c) joining material coated on alumina specimen and fired at 1450°C
Figure 5 XRD pattern of the magnesium alumino silicate (MgO+Al203 +Si02) (a) heat treated at 1200oC (b) heat treated at 1550 Figure 6 XRD pattern of the calcium magnesium silicate (CaO+MgO+Si02) (a) heat treated at 125(>> Q (b) joining material coated on the alumina substrate and fired at 1300^ C, (c) joining material coated on the alumina substrate and fired at 1350°C
Figure 7 XRD pattern of the calcium magnesium alumino silicate (CaO+MgO+Al 203+SiO2) (a) heat treated at 1300° C for Ih and (b) joining material coated on the alumina substrate and fired at 1450° C for Ih,
Figure 8 The effect of composition of joining material and duration of joining process [curve (a) Ih (b) 1.5 h and (c) 2h] on flexural strength of the alumina - alumina ceramic joints.
Detailed Description of Invention with respect to the accompanying
drawings:
Materials for ceramic/ceramic joining:
Different materials with various compositions are prepared and used for ceramic/ceramic joining. The basic principle involved in preference to the

use of a particular material with specific composition is that the material should exhibit partial or complete melting and hence it should form a reaction couple with the ceramic parts to be joined. After melting, the material can be partly crystalline and partly amorphous or fully amorphous. The facts also to be taken into account are that the material and its should be stable at higher temperatures and it should give higher mechanical strength compositions must be such that, after the joining process the ceramic/ceramic joints for wider temperature range i.e. from room temperature to higher service temperature. It is also desirable that the materials chosen as joining media should be easily to prepare and economically viable.
Based on these essential requirements, materials preferred as a medium for ceramic/ceramic joining are magnesium silicates (MS), magnesium alumino silicates (MAS), calcium silicates (CS), calcium magnesium silicates (CMS) and calcium magnesium alumino silicate (CMAS). In all the above mentioned materials, different compositions have been optimized for ceramic/ceramic joining.
Figure 1 shows the compositional variation of the joining materials: magnesium silicates (MS) and magnesium alumino silicates (MAS) in the form of schematic ternary phase diagram.
Figure 2 gives the schematic ternary phase diagram, which indicates the compositional variation of the calcium silicates (CS) joining materials.
Figure 3 shows the compositional variation of the calcium magnesium silicates (CMS) in the form of schematic phase diagram.

Method and procedure of ceramic/ceramic joining:
Source materials for ceramic/ceramic joining are taken in the required quantities and are homogenized by using ball milling in presence of a suitable liquid medium such as acetone or ethanoL Homogenized mixtures are dried in air-oven at 100°C, and precalcined at 1000°C; for 3h. Precalcined powders are then milled again and further annealed at 1200°C for 3h. Heat-treated powders are surface treated with surfactants such as ethoxylated nonylphenol, ethoxylated tridecylalcohol and these surfactants acts as wetting agents. Surface treated powders are mixed with anunonium polyacrylate or ammoruum citrate to hinder the flocculation or agglomeration of the powder particles. Different organic as well as inorganic binders are used for the preparation of the slurry of the powders. Refined starch, dextrine, polyvinyl buiyral, meihylcellulose, polyvinyl alcohol and polymethyl methacrylate are a few of the organic materials that are used as binders. Some of the organic silicates such as ethyl silicates are also used as binding agent. After the binder addition, slurry is treated with anti-foaming agents, calcium stearate, aluminium stearate or tributyl phosphate, which reduces the surface viscosity whereby, prevents the bubble formation in the slurry due to air or any other dissolved gas. The slurry is coated on the porous or nonporous alumina specimens. The coated objects are mounted one over the other. This joint configuration is cured inside the air-oven at 100°C, These green joints are heat treated at suitable temperatures varying from 1250°C to 1550°C for 2h. This results in the reaction between the joining medium and the alumina substrates, which leads to the joining of the two alumina parts. Bending strength of the porous alumina specimens was found to be 220MPa whereas the nonporous specimens show failure at 380MPa, Microstructure of the

alumina specimens shows size of the equiaxial grains varies from 1.5 to 3µm, and the pore size in the porous specimens varies between 0.5 to lµm.
Evaluation of the ceramic/ceramic joints:
The ceramic-ceramic joints are evaluated to determine their mechanical strength and high temperature stability. Flexural strength of the joints is measured in three-point bending mode. Maximum service temperature stability of the unsupported beams is found out by heat-treating the joined specimens at different temperatures and at different durations. Reaction products formed in the joining process are identified using X-ray diffraction.
Materials and methods for joining of oxide, non-oxide ceramics and ceramic composites:
In the present invention it is found that the different silicates (MS, MAS, CS, CMS and CM AS), which are used as joining media, are not only applicable for the alumina ceramics but also applicable for other oxide ceramics. Experimental determination of the present invention reveals that the aforementioned silicates are applicable as effective joining media for other oxide ceramics mullite, alumina rich silicates and partially or fully stabilized zirconia. It is also found that the objects made of silicon carbide (SiC) (non-oxide ceramics) and silicon carbide/alumina/zirconia composites can successfully joined by using magnesium silicates (MS), magnesium alumino silicates (MAS), calcium silicates (CS), calcium magnesium siUcates (CMS) or calcium magnesium alumino sihcates (CMAS) as joining media. The above mentioned silicates find their application as joirung media for joining of mullite-zirconia and alumina-zirconia composites as well Methods and procedures followed for joining

of oxide, non-oxide ceramics and ceramic composites are the same as that followed for joining of alumina ceramics except that inert or reducing atmosphere, mostly hydrogen/nitrogen mixtures is used for joining of silicon carbide (SiC) ceramics to prevent the oxidation of the substrates.
Application of the joining materials as sintering aids:
In the present invention it is found that, the reaction between the joining medium and the substrates promotes the densification of porous substrates through diffusion process. Hence the materials, which are used as joining media, can be used as sintering aids to prepare non-porous monolithic ceramics. These materials can be used individually or in different combinations as well. Hence the main objective of developing non- porous alumina ceramic parts for joining is extendable to sintering process. These materials can also be used for joining of ceramic greens after curing. It is determined from the present investigation that the joining materials have broader applications as sintering aids in the fabrication of ceramic-matrix composites with controlled porosity.
Examples of joining materials: (i) MgO+Si02
To prepare magnesium silicate 916g of magnesium hydroxy carbonate (MgCOs Mg(OH)2) containing 44% MgO is mixed with 601g of aerosol silica (SiO2) by ball milling in presence of liquid a medium (acetone or ethanol) for 4h. Homogenized mixture is dried in air oven at lOO°C. After drying, the powder samples are precalcined at 10(XX] for 3h. Heat treatment of the calcined powders is carried out at 1200°C for 3h.
Annealed powders are surface treated with wetting agents (0.5wt%) and surface modified powder is mixed with deflocculant (0.5 tolwt %) to hinder

the flocculation or agglomeration of the powder particles. Slurry of the surface-treated powder sample is prepared by mixing it with aqueous solution of organic binder (2wt%). After the binder addition, slurry is treated with anti-foaming agents (0.5wt%), which reduces the surface viscosity whereby preventing the bubble formation in the slurry due to air or other dissolved gases. Thick films of slurry are coated on the porous or non-porous alumina specimens and the coated objects are mounted one over the other or joined laterally. The joined configurations are cured inside the air-oven at 100^. After curing, the joints are strong enough to hold them in position. These green joints are then fired at higher temperature 1450^ -1470O C for 2h. This high temperature firing sets the reaction between the substrate and the joining materials whereby forming crystalline spinel phase (MgAl2O4) as a reaction product along with clinopyroxene (Mg2Si2O6), which leads to joining of the substrates. The reaction that takes place upon heating of the starting material can be given in the form of chemical equation:

The percentage of the liquid phase decreases as the duration of the joining stage is extended. Although anticipated, no cordierite phase crystallized from the melt. The reaction between the substrates and the joining material also promotes the densification of porous alumina substrates.
These ceramic joints are stable upto 1500°C without any deformation. It shows that the service temperature of these ceramic joints is around the joining temperature. Flexural strengths of the specimens after joining are measured in three-point bending mode. Flexural strengths of the joined specimens are found to be 90-95MPa at room temperature.
Figure 4 gives the XRD pattern of the magnesium silicate (MgCSiOz) (a) heat treated at 1200°C (b) heat treated at 1450KZ (c) joining material coated on alumina specimen and fired at 1450°C
(ii) MgO+Al2O3+SiO2
To prepare this magnesium aluminosilicate, 1820g of magnesium hydroxy
carbonate (Mg(OH)2 .MgCOs) containing 44% MgO and 1020g of y-alumina (Y"Al203) are mixed with 902g of aerosol silica (SiOi) using ball milling in presence of a liquid medium (acetone) for 4h. Homogenized mixture is dried in the air oven at lOO°C. Dried powder sample is precalcined at lOOO° C for 3h then milled again and further annealed at 1200°C for 3h.
Annealed powders are surface treated with wetting agents (0.5wt%) and surface modified powder is mixed with deflocculants (0.5 tolwt%) to hinder the flocculation or agglomeration of the powder particles. Slurry of the surface treated powder is prepared by mixing with aqueous solution of organic binder (2wt%). After the binder addition, the slurry is treated with

anti foaming agents (0.5wt%), which reduces the surface viscosity whereby preventing the bubble formation in the slurry due to air or other dissolved gases. Thick film of slurry is coated on the porous or non-porous alumina substrates and the coated objects mounted one over the other or joined laterally. These joint configurations are cured inside the air oven at lOOoC. The green joints are then fired at higher temperature (1550oC for 2h). At higher temperatures the coated slurry reacts with the alumina substrate giving rise to form crystalline spinel phase (MgAl2O4) as the reaction product. The reaction that takes place upon heating of the starting material can be given in the form of the chemical equation.

With joining trials of extended durations the sapphirine (Mg4Al10O23) crystallizes together with spinel (MgAl2O4). This is indicative the different interaction of alumina substrate with the liquid generation. It is also found that the reaction between the substrate and the joining material enhances the densification process in the porous substrates.

Flexural strengths of the joined specimens are measured in three-point bending mode. Flexural strength of the joined specimens attains 85-90MPa at room temperature. These ceramic joints are found to be stable up to 1550oC.
Figure 5 gives XRD pattern of the magnesium alumino silicate (MgO+AI2O3 +Si02) (a) heat treated at 1200<: heat treated at isso joining material coated on alumina substrate and fired> (iii) CaO+Si02
lOOOg of calcium carbonate (CaCO3) containing 56% CaO and 601g of aerosol silica (SiC)2) are mixed using ball milling in presence of a liquid medium (acetone) for 4h. Homogenized mixture is dried in air oven at lOOoC. Dried powder sample is pre calcined at l000oC for 3h and further annealed at 1200oC for 3h.
Crystalline pseudo-wollastanite powder is surface-treated with the wetting agents (0.5wt%). The treated powder is mixed with deflocculants (0.5 tolwt %) to hinder the flocculation or agglomeration of the powder particles. Slurry of the surface treated powder sample is prepared by mixing it with aqueous solution of organic binder (2wt%). After the binder addition, slurry is treated with anti-foaming agents (0.5wt%), which reduces the surface viscosity whereby, prevents the bubble formation in the slurry due to air or other dissolved gases. Thick films of slurry are coated on the porous or non-porous alumina substrates and the coated objects mounted one over the other or laterally. These joint configurations are cured inside the air-oven at 100oC. The green joints are then fired at higher temperatures >1350o C for Ih.

Flexural strengths of the joined specimens are measured in three-point bending mode. It is found that the modulus of rupture of the joined specimens is 55-60MPa. These calcium silicate joints are stable up to 1250oC,
(iv) CaO+MgO+Si02
lOOOg of calcium carbonate (CaCO3) containing 56% CaO is mixed with 915g of magnesium hydroxy carbonate (Mg(OH)2 .MgCOs) containing 44% MgO and 1202g of aerosol silica (Si02) by ball milling in presence of an inert liquid medium (e.g. acetone or ethanol) for 4h. After drying the homogenized mixture in the air oven at 100oC, dried powder is precalcined at lOOOo C for 3h, Calcined powder is further annealed at 1200oC for 3h.
Crystalline calcium magnesium silicate powder is surface treated with wetting agent (0,5wt%). The treated powder is mixed with deflocculants (0.5 to lwt%) to hinder the flocculation or agglomeration of the powder particles. Slurry of the surface treated powder sample is prepared by mixing it with aqueous solution of organic binder (2wt%). After the binder addition, slurry is treated with anti-foaming agents (0.5wt%), which reduces the surface viscosity thereby preventing the bubble formation in the slurry due to air or other dissolved gases. Thick films of the slurry are coated on the porous or non-porous alumina substrates and the coated objects mounted one over the other. These joint configurations are cured inside the air-oven at 100°C. The cured joints are strong enough to hold them in position. These cured joints are heat treated at higher temperature 1300oC for Ih. High temperature firing of the joint configurations results in reaction between the substrates and the joining material, whereby producing the liquid phase that crystallizes into the crystalline phases anorthite (CaAl2Si2O8) and spinel (MgAl2O4) as reaction products. The

reaction that takes place upon heating of the starting material can be given in the form of the chemical equation.

These reactions are found to increase the densification of the porous substrates.
Flexural strengths of the joined specimens are measured in three-point bending mode. Room temperature flexural strength of the joined specimens is found to be 125-130MPa. Service temperature of these ceramic joints reaches 1250°C
Figure 6 shows the XRD pattern of the calcium magnesium silicate (CaO+MgO+SiO2) (a) heat treated at 1250oC (b) joining material coated on the alumina substrate and fired at 1300oC(c) joining material coated on the alumina substrate and fired at 1350oC.

lOOOg of calcium carbonate (CaCOa) containing 56% CaO is mixed with 1285g of magnesium hydroxy carbonate (Mg(OH)2 .MgCOs) containing 44% MgO, 1683g of aerosol silica (SiO2) and 200g of y-alumina (y-Al2O3) by using

ball milling in presence of an inert liquid medium (acetone or ethanol) for 4h. Homogenized mixture is dried in the air oven (lOOoC). Dry powder samples are precalcined at 1000oC for 3h and then further annealed at 1200oC for 3k
Armealed calcium magnesium silicate powder is surface treated with wetting agent (0.5wt%). The treated powder is mixed with deflocculant (0.5 to lwt%) to hinder the flocculation or agglomeration of the powder particles. Slurry of the surface treated powder is prepared by mixing with aqueous solution of an organic binder (2wt%). After the binder addition, slurry is treated with anti-foaming agents (0.5wt%), which reduces the surface viscosity whereby preventing the bubble formation in the slurry

The reaction that takes place between the joining material and the substrates during the joining process can be given as

The present invention shows that the flexural strengths of the alumina -alumina ceramic joints are dependent on the composition of the joining material as well as the processing parameters at high temperatures.
Figure 8 shows the effect of composition and duration of joining process [curve (a) Ih (b) 1.5 h and (c) 2h] on flexural strength of the alumina -alumina ceramic joints.

We claim:
1. A process of ceramic-ceramic joining of monoliths and ceramic
composites comprising:
homogenization of source materials as herein described for the
joining medium
drying the said homogenized mixture
precalcinating the said dried homogenized mixture to get
precalcined powders
armealing the homogenized mixtures after milling the said
precalcined powders,
subjecting the said annealed homogenised mixture to surface
treatment with wetting agents and deflocculants as herein
described to obtain a slurry
treating the slurry of said annealed homogenised mixture
using binders as herein described,
adding antifoaming agents as herein described to the slurry to
avoid bubble formation to obtain a joining medium,
coating the said joining medium on said ceramic parts and
joining them to get joints,
curing the said joints inside a hot air-oven and heat-treating to
get green joints
subjecting the said green joints to temperatures of 1250°C to
1550°C to yield ceramic-ceramic joints of high breaking
strength
2. A process as claimed in claim 1 wherein said source materials are homogenized using ball milling in presence of an organic liquid medium as herein described to obtain a homogenized mixture of source materials.
3. A process as claimed in claim 1 wherein the said source materials are magnesium silicates (MS), magnesium alumino silicates (MAS), calcium silicates (CS), calcium magnesium silicates (CMS) or calcium magnesium alumino silicate (CMAS)
4. A process as claimed in claim 1 wherein the said liquid medium used for homogenizing is acetone or ethanol.

5. A process as claimed in claim 1 wherein the said ceramic parts are ceramic or alumina substrates.
6. A process as claimed in claim 1 wherein the said homogenised mixture is air dried in an air-oven at 90 to 110 degree C.
7. A process as claimed in claim 1 wherein the said homogenised mixture is precalcined upto lOOO^C for 3 hours.
8. A process as claimed in claim 1 wherein the said homogenised mixture is annealed at temperatures upto 1200 degrees C for 3 hours.
9. A process as claimed in claim 1 wherein the said surfactants are ethoxylated nonylphenol or ethoxylated tridecylalcohol.
10. A process as claimed in claim 1 wherein the said deflocuUants are ammonium polycrylate or ammonium citrate.
11. A process as claimed in claim 1 wherein the said binders may be organic or inorganic binders.
12. A process as claimed in claim 11 wherein the said organic binders are refined starch, dextrine, polyvinyl butyral, methylcellulose, polyvinyl alcohol and polymethyl methacrylate or organic silicate.
13. A process as claimed in claim 12 wherein the said organic silicate is ethyl silicate.
14. A process as claimed in claim 1 wherein the said antifoaming agents are calcium stearate, aluminium stearate or tributyl phosphate.
15. A process as claimed in claim 1 wherein the said joints are of coated objects are cured in the said air-oven at temperatures upto 100 degrees C to yield green joints.

16. A process as claimed in claim 15 wherein the said green joints are
heat treated at temperatures ranging from 1250 to 1550 degrees C for
over 2 hours.
17. A process as claimed in claim 1 wherein the green joints are fired at
1450°C to 1470°C,
18. A process as claimed in claim 1 wherein the said ceramic parts are
joined during the liquid phase bonding between said ceramic parts
and said joining medium.
19. A process of ceramic-ceramic joining of monoliths and ceramic
composites substantially as herein described with reference to the
foregoing examples.

Documents:

527-mas-2002-abstract.pdf

527-mas-2002-claims filed.pdf

527-mas-2002-claims granted.pdf

527-mas-2002-correspondnece-others.pdf

527-mas-2002-correspondnece-po.pdf

527-mas-2002-description(complete) filed.pdf

527-mas-2002-description(complete) granted.pdf

527-mas-2002-description(provisional).pdf

527-mas-2002-drawings.pdf

527-mas-2002-form 1.pdf

527-mas-2002-form 19.pdf

527-mas-2002-form 26.pdf

527-mas-2002-form 3.pdf

527-mas-2002-form 5.pdf

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Patent Number

208665

Indian Patent Application Number

527/MAS/2002

PG Journal Number

35/2007

Publication Date

31-Aug-2007

Grant Date

06-Aug-2007

Date of Filing

15-Jul-2002

Name of Patentee

INDIAN INSTITUTE OF SCIENCE

Applicant Address

BANGALORE 560 012.

Inventors:

#	Inventor's Name	Inventor's Address
1	KUTTY T.R.N	BANGALORE 560 012.

PCT International Classification Number

B 28 B 01/52

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA