Ceramics and process for producing

Abstract

The present invention provides a process for producing alumina matrix carbide and boride reinforced ceramic composites wherein for any particular composite, the relative density is about 97% or more of the theoretical density. The composites are prepared in a container wherein the interior surfaces of the container are graphite and have a protective coating consisting of a first layer comprising silicon carbide and boron carbide with a binder and a second layer comprising silicon carbide particles, wherein the protective coating prevents carbon bleed-through and the protective coating maintains a boride-containing equilibrium atmosphere during the process. The present invention further provides an alumina-based ceramic composite which comprises a metal carbide preferably selected from the group consisting of silicon carbide, titanium carbide and zirconium carbide, and mixtures thereof, and a boride preferably selected from the group consisting of boron carbide, titanium boride, or zirconium boride, and mixtures thereof. Finally, the present invention provides a protective coating for a surface comprising a first layer of a silicon carbide and a boron carbide in a binder and a second layer comprising a silicon carbide wherein the protective coating is able to withstand repeated exposure to high temperature.

I claim:  
     
         1 - A process for preparation of a dense alumina-based ceramic composition which comprises: 
 (a) providing a container with a removable closure, wherein inside surfaces of the container and closure are graphite and have been first coated with a mixture of metal carbide particles, boride particles, and an organic binder in water to form a first layer which is then coated with a second layer of silicon carbide particles to form a coating which is then dried;    (b) introducing a dried green preform made from a mixture of alumina and a metal carbide powder and a boride powder, wherein the mixture has been milled together, into the container;    (c) firing the preform at a temperature sufficient to produce the ceramic composition which has a density of at least 97 percent of a theoretical density for the ceramic composition.    
     
     
         2 - A process for preparation of a dense alumina-based ceramic composition which comprises: 
 (a) providing a container with a removable closure, wherein inside surfaces of the container and closure are graphite and have been first coated with a mixture of silicon carbide powder, boron carbide powder, and an organic binder in water to form a first layer which is then coated with a second layer of silicon carbide particles to form a coating which is then dried;    (b) introducing into the container a dried green preform made from a mixture of alumina and metal carbide and boride powders, wherein the mixture has been milled together;    (c) firing the preform at a temperature sufficient to produce the ceramic composition which has a density of at least 97 percent of a theoretical density for the ceramic composition.    
     
     
         3 - The process of  claim 2  wherein the metal carbide is selected from the group consisting of silicon carbide, titanium carbide, zirconium carbide, and mixtures thereof and the boride is selected from. the group consisting of boron carbide, titanium boride, zirconium boride, and mixtures thereof.  
     
     
         4 - A process for preparation of a dense alumina-based ceramic composition which comprises: 
 (a) providing a container with a removable closure, wherein inside surfaces of the container and closure are graphite and have been first coated with a mixture of silicon carbide powder, boron carbide powder, and an organic binder in water to form a first layer which is then coated with a second layer of silicon carbide particles to form a coating which is then dried;    (b) introducing into the container a dried green preform made from a mixture of aluminum oxide, silicon carbide, and boron carbide powders, wherein the mixture has been milled together;    (c) firing the preform at a temperature sufficient to produce the ceramic composition which has a density of at least 97 percent of a theoretical density for the ceramic composition.    
     
     
         5 - The process of  claim 2  or  4  wherein the firing is performed by ramping the temperature to about 500° C. at a rate of between about 1° to 5° C. per minute, then to about 1250° C. to 160° C. at a rate of 1° to 10° C. per minute, then to a temperature of between about 1600° to 1900° C. at a rate of about 1 to 20° C. per minute, and then maintaining the preform at the temperature of 1600° to 1900° C. for a time sufficient to achieve 97% or more of the theoretical density.  
     
     
         6 - The process of  claim 4  wherein the firing is performed by ramping the temperature to about 500° C. at a rate of between about 1° to 5° C. per minute, then to about 1600° C. at a rate of 5 to 20° C. per minute, and then maintaining the preform at the temperature of 1600° to 1900° C. for a time sufficient to achieve 97% or more of the theoretical density.  
     
     
         7 - The process of  claim 2  wherein the green preform comprises 65 to 85 wt % of the alumina with a median particle size in the size range of 0.4 to 1.5 d 50  μm, 0.5 to 20 wt % of the boride with a median particle size not more than 30 d 50  μm, and 2 to 21.4 wt % of the metal carbide with a median particle size in the size range of 2 to 10 d 50  μm.  
     
     
         8 - The process of  claim 7  wherein the green preform comprises a sintering aid selected from the group consisting of yttria, rare earths, lanthanides, magnesia, and calcia.  
     
     
         9 - The process of  claim 7  wherein either the metal carbide or the boride is provided as whiskers.  
     
     
         10 - The process of  claim 7  wherein the alumina is 80 wt % with a median particle size in the size range of 0.4 to 1.5 d 50  μm, the boride is boron carbide and is 5.4 wt % with a median particle size in the size range of 3 to 11 d 50  μm, the metal carbide is silicon carbide and is 2 wt % with a median particle size in the size range of 2 to 10 d 50  μm and 12.6 wt % with a median particle size in the size range of 2 to 10 d 50  μm.  
     
     
         11 - The process of  claim 7  wherein the alumina is 80 wt % with a median particle size in the size range of 0.4 to 1.5 d 50  μm, the boride is boron carbide and is 5.4 wt % with a median particle size in the size range of 3 to 11 d 50  μm, and the metal carbide is silicon carbide and is 14.6 wt % with a median particle size in the size range of 0.5 to 1.0 d 50  μm.  
     
     
         12 - The process of  claim 7  wherein the alumina is 78.1 wt % with a median particle size in the size range of 0.4 to 1.5 d 50  μm, the boride is boron carbide and is 0.5 wt % with a median particle size not more than 1.2 d 50  μm, and the metal carbide is silicon carbide and is 21.4 wt % with a median particle size in the size range of 0.5 to 1.0 d 50  μm.  
     
     
         13 - The process of  claim 7  wherein the alumina is 60 to 85 wt % with a median particle size in the size range of 0.4 to 1.5 d 50  μm, the boride is boron carbide and is 1 to 20 wt % with a median particle size in the size range of 1 to 30 d 50  μm, and the metal carbide is silicon carbide and is 5 to 15 wt % with a median particle size in the size range of 0.5 to 20 d 50  μm.  
     
     
         14 - The process of  claim 7  wherein the alumina is 60 to 85 wt % with a median particle size in the size range of 0.4 to 1.5 d 50  μm, the boride is titanium boride and is 1 to 20 wt % with a median particle size in the size range of 1 to 30 d 50  μm, and the silicon carbide is 5 to 15 wt % with a median particle size in the size range of 0.5 to 20 d 50  μm.  
     
     
         15 - The process of  claim 2  or  4  wherein the first layer comprises 98 wt % of the silicon carbide powder, 1 wt % of the boron carbide powder, and 2 wt % of the organic binder.  
     
     
         16 - The process of  claim 2  or  4  wherein the silicon carbide particles are of about 70 to 120 mesh.  
     
     
         17 - The process of  claim 1 ,  2  or  4  wherein the green preform is made into a shape by pill casting, slip casting, isostatic pressing, or dry bag pressing.  
     
     
         18 - The process of  claim 1 ,  2  or  3  wherein the firing is performed in an atmosphere consisting of an inert gas wherein the gas is selected from the group consisting of argon, helium, nitrogen, and mixtures thereof.  
     
     
         19 - A green composite for producing a ceramic comprising 65 to 85 wt % of an alumina with a median particle size in the size range of 0.4 to 1.5 d 50  μm, 0.5 to 20 wt % of a boride with a median particle size of not more than 30 d 50  μm, and 2 to 21.4 wt % of a metal carbide with a median particle size in the size range of 2 to 10 d 50  μm.  
     
     
         20 - The green composite of  claim 19  wherein the alumina is 80 wt % with a median particle size in the size range of 0.4 to 1.5 d 50  μm, the boride is boron carbide and is 5.4 wt % with a median particle size in the size range of 3 to 11 d 50  μm, the metal carbide is silicon carbide and is 2 wt % with a median particle size in the size range of 2 to 10 d 50  μm and 12.6 wt % with a median particle size in the size range of 2 to 10 d 50  μm.  
     
     
         21 - The green composite of  claim 19  wherein the alumina is 80 wt % with a median particle size in the size range of 0.4 to 1.5 d 50  μm, the boride is boron carbide and is 5.4 wt % with a median particle size in the size range of 3 to 11 d 50  μm, and the metal carbide is silicon carbide and is 14.6 wt % with a median particle size in the size range of 0.5 to 1.0 d 50  μm.  
     
     
         22 - The green composite of  claim 19  wherein the alumina is 78.1 wt % with a median particle size in the size range of 0.4 to 1.5 d 50  μm, the boride is boron carbide and is 0.5 wt % with a median particle size not more than 12 d 50  μm, and the metal carbide is silicon carbide and is 21.4 wt % with a median particle size in the size range of 0.5 to 1.0 d 50  μm.  
     
     
         23 - The green composite of  claim 19  wherein the alumina is 60 to 85 wt % with a median particle size in the size range of 0.4 to 1.5 d 50  μm, the boride is boron carbide and is 1 to 20 wt % with a median particle size in the size range of 1 to 30 d 50  μm, and the metal carbide is silicon carbide and is 5 to 15 wt % with a median particle size in the size range of 0.5 to 20 d 50  μm.  
     
     
         24 - The green composite of  claim 19  wherein the alumina is 60 to 85 wt % with a median particle size-in the size range of 0.4 to 1.5 d 50  μm, the boride is titanium boride and is 1 to 20 wt % with a median particle size in the size range of 1 to 30 d 50  μm, and the metal carbide is silicon carbide and is 5 to 15 wt % with a median particle size in the size range of 0.5 to 20 d 50  μm.  
     
     
         25 - The green composite of  claim 19  further comprising a sintering aid selected from the group consisting of yttria, rare earths, lanthanides, magnesia, and calcia.  
     
     
         26 - The process of  claim 19  wherein either the metal carbide or the boride is provided as whiskers.  
     
     
         27 - A fired ceramic composite comprising 65 to 85 wt % of an alumina with a median particle size in the size range of 0.4 to 1.5 d 50  μm, 0.5 to 20 wt % of a boride with a median particle size of not more than 30 d 50  μm, and 2 to 21.4 wt % of a metal carbide with a median particle size in the size range of 2 to 10 d 50  μm and which has a density of at least 97% of a theoretical density for the ceramic.  
     
     
         28 - The fired ceramic composite of  claim 27  wherein the alumina is 80 wt % with a median particle size in the size range of 0.4 to 1.5 d 50  μm, the boride is boron carbide and is 5.4 wt % with a median particle size in the size range of 3 to 11 d 50  μm, the metal carbide is silicon carbide and is 2 wt % with a median particle size in the size range of 2 to 10 d 50  μm and 12.6 wt % with a median particle size in the size range of 2 to 10 d 50  μm.  
     
     
         29 - The fired ceramic composite of  claim 27  wherein the alumina is 80 wt % with a median particle size in the size range of 0.4 to 1.5 d 50  μm, the boride is boron carbide and is 5.4 wt % with a median particle size in the size range of 3 to 11 d 50  μm, and the metal carbide is silicon carbide and is 14.6 wt % with a median particle size in the size range of 0.5 to 1.0 d 50  μm.  
     
     
         30 - The fired ceramic composite of  claim 27  wherein the alumina is 78.1 wt % with a median particle size in the size range of 0.4 to 1.5 d 50  μm, the boride is boron carbide and is 0.5 wt % with a median particle size not more than 12 d 50  μm, and the metal carbide is silicon carbide and is 21.4 wt % with a median particle size in the size range of 0.5 to 1.0 d 50  μm.  
     
     
         31 - The fired ceramic composite of  claim 27  wherein the alumina is 60 to 85 wt % with a median particle size in the size range of 0.4 to 1.5 d 50  μm, the boride is boron carbide and is 1 to 20 wt % with a median particle size in the size range of 1 to 30 d 50  μm, and the metal carbide is silicon carbide and is 5 to 15 wt % with a median particle size in the size range of 0.5 to 20 d 50  μm.  
     
     
         32 - The fired ceramic composite of  claim 27  wherein the alumina is 60 to 85 wt % with a median particle size in the size range of 0.4 to 1.5 d 50  μm, the boride is titanium boride and is 1 to 20 wt % with a median particle size in the size range of 1 to 30 d 50  μm, and the metal carbide is silicon carbide and is 5 to 15 wt % with a median particle size in the size range of 0.5 to 20 d 50  μm.  
     
     
         33 - A protective coating for a surface that is exposed to high temperatures wherein the surface is graphite prepared by a process comprising providing a mixture of metal carbide particles, boride particles, and an organic binder in water to form a first layer on the surface which is then coated with a second layer comprising metal carbide particles which is then dried to form the protective coating.  
     
     
         34 - The container of  claim 33  wherein the metal carbide is silicon carbide and the boride is boron carbide.  
     
     
         35 - The protective coating of  claim 34  wherein the first layer comprises 98 wt % of the silicon carbide powder, 1 wt % of the boron carbide powder, and 2 wt % of the organic binder.  
     
     
         36 - The protective coating of  claim 33  wherein the second layer is silicon carbide particles of 70 to 120 mesh.  
     
     
         37 - A container with a removable closure for firing ceramics wherein inside surfaces of the container and closure are graphite and have been first coated with a mixture of metal carbide particles, boride particles, and an organic binder in water to form a first layer which is then coated with a second layer of metal carbide particles to form a coating which is then dried.  
     
     
         38 - The container of  claim 37  wherein metal carbide is silicon carbide and the boride is boron carbide.  
     
     
         39 - The container of  claim 38  wherein the first layer comprises 98 wt % of the silicon carbide powder, 1 wt % of the boron carbide powder, and 2 wt % of the organic binder.  
     
     
         40 - The container of  claim 37  wherein the second layer is silicon carbide particles of 70 to 120 mesh.  
     
     
         41 - An industrial blast nozzle assembly comprising: 
 (a) a ceramic composite liner having a bore extending therethrough to provide an inlet opening and an outlet opening wherein the ceramic composite liner which comprises 65 to 85 wt % of an alumina, 0.5 to 20 wt % of a boride, and 2 to 21.4 wt % of a metal carbide has a density of at least 97% of a theoretical density for the ceramic; and    (b) a metal casing having a bore extending therethrough, wherein the liner is mounted in the bore of the metal casing    
     
     
         42 - The nozzle assembly of  claim 41  wherein the metal casing is a metal selected from the group consisting of brass and aluminum.  
     
     
         43 - The nozzle assembly of  claim 41  wherein the metal casing has a threaded end and the liner is mounted in the bore of the metal casing such that the threaded end of the metal casing and the inlet end of the liner form an end which is substantially flush.  
     
     
         44 - The nozzle assembly of  claim 41  wherein a protective coating binds together the liner and metal casing to provide the industrial blast nozzle assembly.  
     
     
         45 - The nozzle assembly of  claim 44  wherein the protective coating is polyurethane.  
     
     
         46 - The nozzle assembly of  claim 41  wherein the inlet opening has a wider diameter than the outlet opening and there is a venturi shape in the bore between the inlet and the outlet openings.  
     
     
         47 - The nozzle assembly of  claim 41  wherein the alumina has a median particle size in the size range of 0.4 to 1.5 d 50  μm, the boride has a median particle size of not more than 30 d 50  μm, and the metal carbide has a median particle size in the size range of 2 to 10 d 50  μm.  
     
     
         48 - The nozzle assembly of  claim 41  wherein the metal carbide is selected from the group consisting of silicon carbide, titanium carbide, zirconium carbide, and mixtures thereof and the boride is selected from the group consisting of boron carbide, titanium boride, zirconium boride, and mixtures thereof.  
     
     
         49 - A liner for an industrial blast nozzle assembly comprising a ceramic composite having a bore extending therethrough to provide an inlet opening and an outlet opening wherein the ceramic composite which comprises 65 to 85 wt % of an alumina, 0.5 to 20 wt % of a boride, and 2 to 21.4 wt % of a metal carbide has a density of at least 97% of a theoretical density for the ceramic.  
     
     
         50 - The liner of  claim 49  wherein the inlet opening has a wider diameter than the outlet opening and there is a venturi shape in the bore between the inlet and the outlet openings.  
     
     
         51 - The liner of  claim 49  wherein the alumina has a median particle size in the size range of 0.4 to 1.5 d 50  μm, the boride has a median particle size of not more than 30 d 50  μm, and the metal carbide has a median particle size in the size range of 2 to 10 d 50  μm.  
     
     
         52 - The liner of  claim 49  wherein the metal carbide is selected from the group consisting of silicon carbide, titanium carbide, zirconium carbide, and mixtures thereof and the boride is selected from the group consisting of boron carbide, titanium boride, zirconium boride, and mixtures thereof.