Abstract:
The present invention includes a method for producing high-alumina bodies with superior chemical properties at reduced sintering temperatures. One form of the method includes the steps of providing an alumina powder precursor, adding about 2 wt. % magnesia powder precursor and about 2 wt. % titania powder precursor, mixing the resultant green powder precursor, pressing a green body from the green powder precursor, removing residual moisture and organic material from the green body, and firing the green body to about cone  13.    
     The resulting high-alumina body has a substantially uniformly sized grain structure, is resistant to dissolution in molten aluminum, and has superior resistance to chemical attack over substantially the entire pH range.

Description:
REFERENCE TO RELATED APPLICATION  
       [0001]    The present application is a utility application based upon Provisional application Ser. No. 60/188,506, filed Mar. 10, 2000. 
     
    
     
       TECHNICAL FIELD OF THE INVENTION  
         [0002]    The present invention relates generally to a method for forming high-alumina bodies, and, more particularly, to a method for sintering high alumina bodies having superior properties and at reduced temperatures.  
         BACKGROUND OF THE INVENTION  
         [0003]    Alumina (also known as Al 2 O 3  or corundum) is a useful and ubiquitous ceramic material. Alumina is a very hard crystalline material. It has a structure that may be described as a hexagonal close-pack array of oxygen atoms with metal atoms in two-thirds of the octahedrally coordinated interstices. Each metal atom is thus coordinated by six oxygen atoms, each of which has four metal neighbors (6:4 coordination). Alumina products include abrasives, insulators, structural members, refractory bricks, electronic substrates, and tools. Alumina is stable, hard, lightweight, and wear resistant, making it attractive for such applications as seal rings, air bearings, electrical insulators, valves, thread guides, and the like, as well as the ceramic reinforcing component in metal matrix composites.  
           [0004]    Alumina is produced on an industrial scale using the Bayer Process to separate ferric oxide, silica and aluminum oxides. Bauxite ore is ground finely then treated with sodium hydroxide (NaOH) in an iron autoclave at an elevated temperature. The alumina dissolves as sodium aluminate via the equation:  
           Al 2 O 3 +2NaOH→2NaAlO 2 +H 2 O  
           [0005]    The silica dissolves to form sodium silicate but the ferric oxide, being insoluble, is filtered off. Carbon dioxide is then passed through the solution, decomposing the sodium aluminate (NaAlO 2 ) to form aluminum hydroxide and sodium carbonate:  
           2NaAlO 2 +CO 2 →Na 2 CO 3 +⇓2Al(OH) 3    
           [0006]    The aluminum hydroxide is separated by filtration and calcined at 1000° C. or higher, when it loses its water of constitution, yielding alumina:  
           2Al(OH) 3 →Al 2 O 3 +3H 2 O  
           [0007]    Pure crystalline alumina is a very inert substance and resists most aqueous acids and alkalis. It is more practical to use either alkaline (NaOH) or acidic (KHSO 4 , KHF 2 , etc) melts. Concentrated boiling sulfuric acid also can be used as an etchant.  
           [0008]    In order to produce useful bodies, alumina must be densified or sintered. Sintering is the process in which a compact of a crystalline powder is heat treated to form a single coherent solid. The driving force for sintering is the reduction in the free surface energy of the system. This is accomplished by a combination of two processes, the conversion of small particles into fewer larger ones (particle and grain growth) and coarsening, or the replacement of the gas\solid interface by a lower energy solid\solid interface (densification). This process is modeled in three stages:  
           [0009]    Initial—the individual particles are bonded together by the growth of necks between the particles and a grain boundary forms at the junction of the two particles.  
           [0010]    Intermediate—characterized by interconnected networks of particles and pores.  
           [0011]    Final—the structure consists of space-filling polyhedra and isolated pores.  
           [0012]    The kinetics of sintering tend to be temperature sensitive, such that an increase in sintering temperature generally substantially accelerates the sintering process. In industrial applications, while an increase in sintering temperature decreases sintering time and increases throughput, the economic gains therefrom are offset by increased fuel costs and decreased furnace life (since higher firing temperatures result in more rapid degradation of both the furnace refractory structure and heating elements.) Therefore, an economically optimum sintering temperature is one that best balances gains from throughput with losses from fuel and furnace wear and tear.  
           [0013]    The sintering of alumina at temperatures above 1600° C. is required to achieve a high density, and alumina is commonly sintered in the temperature range of 1700-1800° C., since higher temperatures promote more rapid sintering. Sintered alumina bodies reflect the properties of the constituent alumina crystallites or grains, such that they are hard, tough, substantially inert, and resistant to chemical attack. Mechanical and/or chemical failure of sintered alumina bodies usually occurs as a grain boundary phenomenon. Since the grain boundaries usually contain porosity and a glassy phase, a sintered alumina body is not as hard, tough, inert, and/or chemically resistant as a comparable single crystal alumina body.  
           [0014]    There is therefore a need for a technique for decreasing the sintering temperature of alumina without sacrificing throughput (increasing the sintering time) or quality. There is also a need for producing sintered alumina bodies having bulk physical characteristics closer to those of single-crystal alumina. The present invention addresses these needs.  
         SUMMARY OF THE INVENTION  
         [0015]    One form of the present invention relates to a process for the low-temperature sintering of high-alumina bodies. The high-alumina bodies so produced have a substantial resistance to dissolution in molten aluminum. The effective sintering temperature for a given sintering time required to achieve substantially full densification were decreased through the addition of quantities a of magnesia-titania mixture to the alumina precursor powders. The resulting substantially fully dense high-alumina bodies exhibited superior resistance to chemical attack over a broad range of pH and temperature conditions.  
           [0016]    One object of the present invention is to provide a method for producing substantially dense high-alumina bodies at lower sintering temperatures when sintered for comparable times. Related objects and advantages will become apparent from the following description.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 is a flow chart schematically representing the processing steps of the present invention.  
         [0018]    [0018]FIG. 2 is a table illustrating some of the physical properties of high-alumina material made by the process of FIG. 1.  
         [0019]    [0019]FIG. 3 is a table presenting the results of exposure of the high-alumina material made by the process of FIG. 1 to various hostile chemical environments.  
         [0020]    [0020]FIG. 4 is a table presenting some properties of thermal spray coatings of the high-alumina material of FIG. 1.  
         [0021]    [0021]FIG. 5 is a table presenting some properties of metal matrix composites made from the low-fired high-alumina material of FIG. 1  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0022]    For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiment illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated device, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates.  
       Known Methods of Decreasing the Firing Temperature of Alumina  
       [0023]    To promote the rapid densification of Al 2 O 3 , additives such as CaO, MgO and TiO 2 , as well as titanates of baria and strontia, have frequently been used. The effect of these additives is sensitive to certain experimental or production procedures, including the fabrication history of the Al 2 O 3  powder, the amount of additives (especially MgO), the sintering temperature, type and concentration of impurities, and so on. The effectiveness of known methods of densification is also a function of the purity alumina and additives. Densification generally increases as a function of purity.  
         [0024]    MgO is known in the art to act to retard grain growth or, more precisely, to restrict the mobility of grain boundaries. Two categories of grain boundaries can be distinguished: those that intersect pores and are therefore active in densification (type A) and those that are entirely connected to other grain boundaries (type B). The existence of type B boundaries is due to inhomogeneties in the original powder compaction of the green body. Densely packed regions of the green compact undergo local densification, leading to the development of dense, pore-free regions upon firing in an otherwise porous microstructure. These dense regions will be better able to support grain growth because of the drag exerted by porosity is absent.  
         [0025]    The effect of MgO is limited to the increase in the grain boundary area. MgO alone has no effect on the pore surface area. The raising of the grain boundary area can be interpreted as being due to the inhibition of grain growth in the densified regions, i.e. the grains remain small. The function of the additive is to restrain grain growth in the densely packed regions until the less densely packed regions have an opportunity to densify. MgO can be thought of as acting as a homogenizer of the microstructure, in that MgO smoothes out the consequences of inhomogeneity.  
         [0026]    The mechanism by which MgO aids in densification has been a source of contention in the known art. Generally, two mechanisms have been considered: pore mobility and grain boundary mobility. The contention arises from (including but not limited to) the nature and amount of impurities, experimental regimen and sintering atmosphere. One possible densification mechanism is that MgO increases the surface diffusion kinetics and thus increases pore mobility. The resultant high pore mobility keeps pores on the migrating grain boundaries during the final stages of sintering. Other mechanisms such as solute segregation at the grain boundaries and second phase pinning of grain boundaries may have been proven untenable, but the data may not be conclusive and they are being mentioned because they describe interesting sintering phenomenon.  
         [0027]    Another sintering aid known to be effective in the densification of alumina is TiO 2 . Additions of TiO 2  to alumina have resulted in more rapid sintering relative to pure alumina. For additions of titania as the only sintering additive, the rate of initial sintering generally increases approximately exponentially with titania concentration up to a percentage beyond which the rate of sintering remained constant or decreased slightly. The concentration, which produces the maximum rate of sintering, is thought to be the solubility of TiO 2  in Al 2 O 3 . For alumina particles larger than 2 μm in initial stage sintering experiments with temperatures of 1520° C. and 1582° C., the kinetic process was mainly grain boundary diffusion. For smaller particles (less than 1 μm) in initial stage sintering experiments with temperatures ranging from 1150° C. to above 1400° C., volume diffusion dominated. For particles with sizes between the two, sintering occurred by a combination of the two kinetic mechanisms. It should be noted that the above details are for initial stage sintering wherein a maximum density of about 85% was achieved.  
         [0028]    Fine-grained alumina bodies of about 95% theoretical density were achieved by sintering green alumina bodies with 2 wt. % additions of low melting point additives at 1400° C. Silicate additions were used because they form a liquid phase during the firing cycle. Silicate fluxes were prepared using MgO and CaO and in long firing regimens (15-17 hours) under an argon atmosphere with temperatures ranging from 1320° C. to 1430° C., theoretical densities of 93-96% were achieved with the MgO flux.  
       Forming Dense High-alumina Bodies  
       [0029]    The present invention relates to a method for producing dense bodies having a high-alumina content from powder alumina precursors. More particularly, the present invention relates to a technique for the sintering of high-alumina bodies at lower temperatures to form dense high-alumina bodies having superior physical properties, as shown schematically in FIG. 1. In general, the first step in the low-temperature production of high-alumina bodies is to blend a high-alumina green powder. The high-alumina green powder is blended from calcined alumina powder, with additions of about 1-10 wt. magnesia (or a magnesia-former standardized to about 1-10 wt. % magnesia) and about 1-10 wt. % titania. The magnesia addition may be conveniently achieved through the addition of a magnesia-former, such as MgCO 3 , the firing of which readily forms magnesia upon heating according to the relation:  
         MgCO 3 .MgO+CO 2    
         [0030]    For the convenience of the reader, “magnesia” hereinbelow will be taken to refer to both MgO and any MgO forming material standardized to produce MgO. Likewise, “titania” will refer to TiO 2  and any TiO 2  forming material standardized to produce TiO 2 . Preferably, about 4 wt. % additive mixture is added to the calcined alumina powder to constitute the green precursor. Also preferably, the ratio of magnesia to titania in the additive portion is about 50:50, and more preferably the ratio is about 42:58. The precursor powders are preferably mixed by wet ball milling with alumina media, although any convenient ceramic powder mixing technique may be chosen. Also, binder phase such as carboxymethylcellulose (CMC), may be added to the green powder, depending upon the requirements of the pressing and firing parameters necessary to produce the desired high-alumina body.  
         [0031]    The dried green powder is then sieved and formed into a green body having the desired shape. The green body is then baked to remove excess moisture and the binder phase (if any) and then fired. Preferably, the alumina body is fired in air to about cone  13  to achieve full sintering and densification. It should be noted that the cone system of measurement combines firing time and temperature to achieve what is essentially a measure of a system&#39;s energy state, i.e. the energy at which a cone of a specific composition softens and deforms. Cone  13  is roughly analogous to firing to about 1250° C. for about 2 hours. The baking and firing phases may be performed separately, or as part of one continuous process.  
         [0032]    One alternative to the firing step is passing the green particles through a heat source, such as a flame or laser. If the green particles are rapidly passed through a sufficiently intense hot zone, rapid sintering may be induced. Moreover, if the green particles are passed through the hot zone under weightless or quasi-weightless conditions (such as aspiration), surface tension effects from the molten binder phase will cause the heated particles to take on a substantially spherical shape as they sinter.  
         [0033]    Preferentially, CMC in a 3% aqueous solution is used as the binder. In other contemplated embodiments, other convenient organic binders may be used. Likewise, while the preferred concentration of CMC is 3% in aqueous solution, any convenient concentration of CMC capable of producing a crushable solid residue may be chosen.  
         [0034]    The purity of the green powder precursor materials are not critical to the present invention, although if the production of a highly pure high-alumina body is desired, the use of high purity green powder precursor materials may likewise be desirable. If the purity of the resultant high-alumina bodies is not a consideration, green powder precursors of any desired purity level may be selected.  
         [0035]    In the preferred embodiment, the calcined alumina precursors were chosen from powders having a particle size of about 1 micron or less, but precursor particles of any convenient size may be selected. The low-temperature high-alumina sintering process of the present invention is not especially sensitive to precursor particle size, with the size of the precursor particles primarily influencing slurry mixing conditions and green body pressing/forming parameters. However, it is generally preferable for the mean particle size of the additives to be about equal to or smaller than that of the calcined alumina.  
       Properties of Low-fired High-alumina Bodies  
       [0036]    [0036]FIG. 2 is a table illustrating the basic physical properties of low-fired high-alumina material made by the above process, while FIG. 3 is a table showing the effects of various hostile chemical environments of the same low-fired high-alumina material. In addition, high-alumina bodies produced by the above process have a number of interesting properties, including: substantially full density; increased resistance to chemical attack over a very broad pH range; the substantial absence of a secondary glassy phase (i.e., they are non-vitreous); substantially uniform and linear thermal expansion; optical translucence; high-temperature corrosion resistance; and substantially uniform grain size.  
         [0037]    Of particular interest is the pH range over which the low-fired high-alumina material is resistant to chemical attack, as illustrated in FIG. 3. Bodies made of the low-fired high-alumina material have been subjected to pH conditions ranging from extremely alkaline (concentrated hot NaOH) to extremely acidic (hot concentrated HF, H 2 SO 4  and hot H 2  gas) with minimal corrosive effects. The high-alumina bodies are even resistant to dissolution and/or corrosion from prolonged immersion in molten aluminum.  
         [0038]    Moreover, the above process produces high-alumina pieces having a very low rate of defect, allowing net shape formability through conventional green body forming and firing means. Further, the high-alumina pieces formed by the above process consistently exhibit a superior surface finish of about 8 rms. The savings (both in energy costs and increased furnace life), the uniform and linear thermal expansion, substantially uniform grain size, low defect rate, and superior surface finish make the formation of low-fired high-alumina material by the above process very attractive from a manufacturing standpoint. Low-fired high-alumina bodies of the do not require kiln furniture or spacers for separation and may be stacked directly in contact with one another for firing without risking fusing or other firing defects.  
       Low-fire High-alumina Spray Coatings  
       [0039]    Low-fired high-alumina material made by the above process may also be applied as a thermal spray material coating via techniques such as subsonic plasma coating or high velocity oxygenated fuel (HVOF) means. Thermal spray coatings of a low-fired high-alumina material of the present invention provide a tough ceramic wear resistant and corrosion resistant coating layer suitable for mechanical or electronic applications without sensitivity to the application technique. FIG. 4 tabulates some of the properties of low-fired high-alumina thermal spray coatings.  
       Metal Matrix Composites Containing Low-fire High-alumina Materials  
       [0040]    [0040]FIG. 5 presents some properties of metal matrix composite (MMC) materials made using the low-fired high-alumina material of the present invention (in spherical form) as a reinforcement phase. In this embodiment, the metal matrix was aluminum, although any convenient metal matrix may be reinforced using the present low-fired high-alumina material. The high resistance to dissolution in molten aluminum exhibited by the present low-fired high-alumina material allows MMCs made therefrom to be made by a casting process instead of the more expensive cold pressed powder metallurgical process. In addition, MMCs made with the present low-fired high-alumina material enjoy the advantages of enhanced welded joint integrity, an expanded heat treatment range and a higher manufacturing throughput.  
         [0041]    While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the spirit of the invention are to be desired to be protected.