13 Political And Economic Factors Impacting Development And Application Of Advanced Ceramics

A major change reducing opportunities for development and application of ceramics (and other advanced materials) was the end of the cold war's reduction of military-aerospace funding. It probably also reduced opportunities for advanced processing and materials development, such as of ceramics with potential for extremes of performance, by shifting the balance from more impetus on performance to more on availability affordability. This made such funding decisions driven even less by technology...

1100 1150 1200 1250 1300 1300

Figure 3.1 Effects of additives on the y to a-transformation temperature of A1203 powders after Xue and Chen 57 and Ozawa and coworkers 58 . Note that (1) since the transition temperatures with no additives (NA) differed between the two studies, the two temperature scales have been shifted so these two transition temperatures align vertically (2) oxide additions are shown by the metal only (the two fluoride additives are fully designated), and (3) data of Xue and Chen is for 1 m o additions and...

M

H high, M medium, L low, none or very low. Note also some limited fabrication of particulate or platelet (e.g., crystallized glasses) and fiber (e.g., eutectic) composites, as well as some fabrication of glass matrix fiber composites using hot glass flow injection (Fig. 7.10) H high, M medium, L low, none or very low. Note also some limited fabrication of particulate or platelet (e.g., crystallized glasses) and fiber (e.g., eutectic) composites, as well as some fabrication of glass matrix fiber...

12 Why Ceramics And Which Ones

The first decision to be made in selecting material candidates for an application is to determine which types of materials to consider. This commonly entails both fabrication and cost issues discussed below, especially where ready availability is desired or required, and significant development is not realistic. However, a basic question for many needs, especially longer term ones, is, What material candidates have the best intrinsic property potential to meet the requirements of the...

Melt Derived Fibers and Filaments

Consider now production of filaments by CVD, where several ceramics have been demonstrated by such processing. The focus here is on B 67-38 and SiC 36-40 filaments which are commercially produced by this process. Though there are variations for both materials, both use a core on which the fiber material is deposited. Originally, both apparently used W wires as cores, but the W resulted in high-temperature performance limitations for SiC fibers. Generally a 25-(jm-diameter W wire is used for B...

662 Single Crystals

Consider next single-crystal growth, which is mainly via directional solidification from the melt, and can be divided into techniques for growing one single crystal at a time versus those that grow many crystals at a time, the latter typically via growing a large ingot of large grains. Consider the latter first. This is typically done via skull melting the melting of almost all of the material in a suitably designed water-cooled crucible, leaving a thin layer of unmelted, and hence...

822 Effects of Alternate Heating Methods

Alternate methods of heating ceramics such as various types of plasma and especially microwave heating have received substantial attention in recent years 16-23 . There are a variety of uses of such heating, especially microwave heating, ranging from drying (long used in industry), calcining, joining, ignition of reactions (briefly noted below), and especially for heating for pressureless sintering. There is also some application to hot pressing as discussed in Section 6.2.2 and below. The...

73 Fabrication Of Porous Bodies 731 Introduction

Fabrication of porous ceramic bodies is an important and growing area of both research and application, as recently reviewed, since, while porosity decreases may properties, it can provide important functions that are best done with a variety of controlled porosity 56,68-71 , These include large and diverse applications in catalysis and thermal insulation, as well as a diversity of less extensive applications for lightweight materials for mechanical functions, and various existing and...

21 Introduction And Background

Ceramic powders are the basic starting materials for the majority of fabrication processes for producing components and samples of both monolithic and composite ceramics. Thus, sintering processes, whether with or without pressure, are the most used processes for fabrication of monolithic and composite ceramics. These processes not only start with powders, but generally depend critically on the nature of the powder for both component shape fabrication and subsequent sintering to be successful...

83 Manufacturing Factors 831 Machining and Surface Finishing

Many ceramic components require machining to achieve the shape needed, e.g., many single-crystal components. Further, while some ceramic components are used as-fired, many require some surface finishing for meeting dimensional or surface finish or both requirements, some have coating requirements (Sec. 7.5), and some both coating and finishing requirements. Though there are some surface finish operations such as grit blasting or tumbling, and thermal or chemical polishing, most finishing...

23 Production Of Other Single And Mixedoxide Powders Via Salt Precursor Decomposition

Aqueous precipitation of precursor salts, followed by calcining them to produce oxide ceramic powders, has important applications as illustrated above for some important ceramics. It also has application to a substantial number of other single oxides, mixtures of oxides, and mixed-oxide compounds however, it also has substantial limitations. Melting on some salts, especially hydrated ones, as for some nitrates, is one limitation, which was noted earlier. Another, much broader limitation is in...

61 Introduction

While powder consolidation and pressureless sintering dominate the production of many ceramic products, there are other densification and fabrication methods that are important. These include several fabrication methods that generally have broad applicability, or the potential for it, and those that are more specialized, such as fabrication of fibers or designed porosity. Though the division between these two areas is sometimes uncertain, those processes deemed falling more in the latter...

663 Eutectic Ceramics and Directional Crystallization of Glasses

Consider now primarily melt processing of eutectic materials that entails processing of bodies of both single and polycrystalline character. Many combinations of ceramic compounds and some combinations of a ceramic compound with a refractory metal can be grown by directional solidification to produce an array of eutectic composites. Such composites often consist of single-crystal lamelli of rods or strips of one phase in a given crystallographic orientation in a single-crystal matrix of the...

1990

Ceramic fracture mode-intergranular vs. transgranular fracture. In J.R. Varner, V.D. Frechette, G.D. Quinn, eds. Ceramic Transactions. 64. Fractography of Glasses and Ceramics III. Westerville, OH Am. Cer. Soc., 1-53, 1996. 295. R.W. Rice. Mechanical properties of ceramics and composites, grain and particle effects. New York Marcel Dekker, Inc., 1999. 296. R.W. Rice, K.R. McKinney, C.Cm. Wu, S.W. Freiman, W.J. McDonough. Fracture energy of Si3N4. J. Mat. Sci. 20 1392-1406, 1985....

16 Summary And Conclusions

In summary, the large and growing families of ceramics have many known individual, as well as combinations of, properties of great technological importance. While the ending of the cold war substantially reduced the driving force for application of ceramics, especially more performance-driven development, and tax and regulation driving forces may come, change, and go, there is still substantial need and opportunity for ceramic applications, and this is likely to increase in the future. However,...

Mh

Using organic adhesives is another basically simple and usually low-cost method of joining or attaching ceramics, where environmental and other factors allow. This may be used to bond ceramic components to themselves or other materials directly via their adhesive capabilities. Organic adhesives may also be combined with mechanical attachment for example, some radomes have been attached to missile structures by bonding a layer of plastic composite to the inside of the dome base such that this...

541 Aluminates

Consider first magnesium alum nate, i.e., magnesia alumina spinel, MgAl204, which is of interest for both structural and especially IR window applications. One of the earlier successes in obtaining translucent to transparent MgAl,04 was work of Rhodes and coworkers 136 they used various, mostly fine MgAl204 powders or mixtures of MgO and A1,03. Dense bodies were obtained by hot pressing without additives at 1300-1400 C with 70-100 MPa pressure with or without vacuum, or by press forging at...

1996

A practical approach to sintering. Am. Cer. Soc. Bui. 76(2) 72-76, 1997. 13. H. Iwasaki, S Shikanai, T. Takahashi, A. Tsuge, H. Ohkuma, T. Ishii. Studies of Elastically Structural Anisotropies in Hot Pressed Ceramic Materials. Toshiba Research and Development Center, Report, March 1975. 15. G.J. Oudemans. Continuous hot-pressing technique. Proc. Brit. Cer. Soc. No. 12, Fabrication Science 2, 3 83-98, 1969. 16. W. Rhodes, J. Thomson, Jr. Continuous hot pressing of...

421 Die Pressing

Powder can be consolidated by applying mechanical pressure, which is most simply done under uniaxial compression of powder in a die, hence the term die pressing, also referred to as cold pressing to distinguish it from hot pressing. Such pressing can be done with no or limited, e.g., < 12 , binder lubricant content, though some is generally used, for example, water in simple cases, leading to terms such as dry or damp pressing with respectively < 4 and > 4 water. Often better (i.e.,...

76 Coatings

Application of ceramic and some metallic surface coatings on ceramic and metal components is briefly summarized here. Traditional use of ceramic glass-based coatings has been for decorative, surface sealing, or both purposes, such as glaze coatings on ceramic dinnerware. Some of these coatings (and sometimes just quenching surfaces from high temperatures) also may contribute to mechanical reliability by providing useful levels of surface compressive stresses 107 . Use of ceramic coatings on...

References

Industrial Ceramics. London Chapman and Hall, 1984. 2. W. Ryan. Properties of Ceramic Raw Materials. New York Pergamon Press, 1968. 3. J.S. Reed. Introduction to the Principles of Ceramic Processing. New York John Wiley & Sons, 1988. 4. D.W. Richerson. Modern Ceramic Engineering. New York Marcel Dekker, Inc., 1992. 5. P.E.D. Morgan. Chemical processing for ceramics (and polymers). In H. Palmour III, R.F. Davis, T.M. Hare, eds. Processing of Crystalline Ceramics,...

34 Use Of Additives In The Growth Of Ceramic And Related Whiskers And Platelets

Though generally constrained in their use by health issues, ceramic whiskers are of interest for use in ceramic composites, which have found some industrial use as speciality wear parts and especially in cutting tools, and are also of interest for some metal and possibly other composites. Whiskers, which are small filamentary single crystals, are also of importance as a tool in understanding both the crystal physics of their properties and the crystal chemistry of their growth. Platelets, which...

44 Colloidal Processing 441 Slip Tape and Pressure Casting

Forming ceramic bodies by slip casting is a long standing process consisting of making a slurry of the green body constituents, then casting this in a porous mold, typically of (naturally porous) plaster, and more recently porous plastics. As much of the slurry liquid at or near the mold surface is absorbed in the pores in the mold, a layer of solid the cast layer is formed by the interlocking solid particles in the region near the mold surface. As the process continues, this solid layer...

Fracture Surfaces

A STRENGTHS 20-40 LOWER THAN B STRENGTHS FIGURE 4.5 Fracture initiation from large laminar pores in a commercial die-pressed PZT sonar transducer ring showing the orientation of lens-shaped laminar pores normal to the pressing direction and resultant strength anisotropy, i.e., lower strengths of A versus B specimens. (From Ref. 17, published with permission of the American Ceramic Soc.) tion. that is, better lubrication, greater green strength (e.g., stronger binders), and less ram travel...

52 Additives For Densification Of Aluminum Oxide

From both a historical and economical standpoint, liquid-phase sintering of oxides, especially A1203 via use of Si02-based glass phases (e.g., to produce commercial bodies of. nominally, 85 , 90 , 94 , 96 , and 99 alumina), is most important. While A1203 can be sintered with Si02 (and other additives to form a glass phase at lower temperature) 1 , industrial practice is to use natural minerals, mainly clays and talcs, as the sources of the desired in situ formed glasses. Although these...

84 Fabrication Overview And Opportunites To Improve Manufacturing Processes

Having surveyed the various routes and steps to fabricating ceramics, it is useful to summarize and compare them as well as to address some opportunities to improve manufacturing. Overall powder-based processing via pressureless sintering is, and will remain, the dominant fabrication route for polycrystalline ceramics and some ceramic, mainly particulate, composites. However, there will be continued changes in specific steps in the fabrication, especially in green forming, and possibly some in...

48 Discussion And Summary

The substantial and growing diversity of powder-based fabrication options not only allow a broad range of components to be fabricated, but also often provides different fabrication options and thus the opportunity and need to make fabrication processing choices. Some choices are often clearly based on component size, shape, or dimensional requirements. Thus, long straight cylinders or tubes with homogeneous character and a substantial range of cross-sectional shapes will often be made by...

64 Hot Isostatic Pressing Hiping

Extending hot consolidation from the typical uniaxial densification of hot pressing to triaxial hot pressing hot isostatic pressing is a logical step, as is the extension of uniaxial die cold pressing to cold isopressing (CIPing), but is more costly. Some earlier HIPing was conducted in a CIP by selecting suitable metal sheets of it to be formed and welded to form a larger metal bag (with power feed-throughs) that contained a pressure transmitting and electrically and thermally insulating...

25 Processing Of Nonoxide Powders

The preparation of nonoxide ceramic powders, though having some similarities to those for oxide ceramics, has both some different preparations or combinations of processing as well as different emphasis of methods, all of which reflect the diversity of their chemical character, relatively more limited development of their processing, or both. There is much less use of salt precursors and more of processes directly producing the nonoxide powder, as well as other processes. An important example...

55 Nonoxides

Consider nonoxides, for which additive development and use is primarily for binary borides, carbides, and nitrides, which are covered in the order listed. Within each of these families, individual compounds are generally taken in alphabetical order, except where some modification of this order is advantageous because of related technical or editorial factors. The focus is on additives whose key function is to aid densification whether they aid or diminish some other aspects of performance other...