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Processing technology for high performance ceramics
Materials Science and Engineering, 71 (1985) 305-312
305
Processing Technology for High Performance Ceramics* R. J. BROOK
Department of Ceramics, University of Leeds, Leeds LS2 9JT (Gt. Britain) (Received January ] 1, 1985)
ABSTRACT The emphasis that has been placed on ceramic c o m p o n e n t reproducibility and consistency has underlined the importance o f processing technology and has indicated the progress that can be made for a wide range o f systems by striving for microstructural quality and homogeneity. This has had consequences in drawing attention not only to p o w d e r preparation and to refined forming procedures but also to mechanisms by which inhomogeneity, once introduced, can be reduced or even eliminated. The implications o f these developments for processing technology and science are considered in terms o f a sequence o f options that are open to the processor. 1. INTRODUCTION Processing and fabrication developments are n o w widely recognized as being key factors in the progress of engineering ceramics towards full exploitation [ 1]. This recognition has been responsible for the emphasis on processing technology that can be f ound in national research programmes [2] and that has resulted in a series of recent conferences on the topic [ 3 - 5 ] . The subject is both wide ranging and complex. Systems of interest include binary oxides such as A120 3 and ZrO2, more complex oxides such as the perovskites and spinels, non-oxides such as Si3N 4 and SiC and mixed systems within these groups and, as with the S i - A 1 - O - N materials, between groups. Levels o f purity vary greatly. For each system, substantial choice exists as to specific fabrication *Paper presented at the International Symposium on Engineering Ceramics, Jerusalem, Israel, December 16 20, 1984. 0025-5416/85/$3.30
route with each rout e consisting of a sequence of process steps within which a wide range of process variables must be considered. A difficulty with the subject therefore is that, while discussion of the full range of processing technology must inevitably be somewhat general, the problems e n c o u n t e r e d in processing are often very specific. It must also be recognized that the solutions to such specific problems are oft en proprietary in view of their commercial significance. Progress is publicly measured more c o m m o n l y in terms of claimed p r o p e r t y enhancements in the p r o d u c t that in terms of fully underst ood and openly described processing improvements. Faced with this characteristic of the subject, the plan of the following discussion is to review briefly some of the microstructural targets which have been established for processing and then to consider a series of choices which c o n f r o n t the processor and which have been t hrow n into relief by recent work.
2. TARGETS Ideal microstructures vary widely corresponding to the wide range of applications (electrical, magnetic, thermal and mechanical) to which ceramics are put. If, however, emphasis is given to the mechanical engineering applications that are central to the conference reported in this volume, then control of creep and the at t ai nm ent of high reproducible strength have received most attention. Other properties such as wear resistance while of comparable importance have at this stage been less confidently related to microstructural parameters and this has resulted in their receiving less emphasis. Creep resistance is most clearly linked to the elimination of viscous grain b o u n d a r y © Elsevier Sequoia/Printed in The Netherlands
306 phases from the microstructure. Such phases are very commonly present in polycrystalline ceramics as a consequence of additives or impurities; the formation of boundary liquid phases at the firing temperature greatly assists densification but correspondingly degrades load-bearing performance in subsequent use of the material at high temperatures. A common objective, and one treated elsewhere in this volume, is the removal of such boundary phases subsequent to processing: possible routes include vacuum evaporation (LiF from MgO [6]), devitrification (Y203-SiO2 from Si3N4 [7]), reaction with the grain material to form solid solutions ( S i - A 1 - O - N systems [8]), and localization at solid second phases (A1203 additions to ZrO 2 [9]). High reproducible strength as a target has drawn attention both to the elimination or minimization of the processing flaws capable of acting as fracture origins and to the design of microstructural toughening mechanisms with higher energy-absorbing capacity as fracture takes place. Such design includes a number of topics such as transformation toughening [10, 11 ], microcrack toughening [12] and the use of fibre composites [13] which are considered in other papers in this volume. From the viewpoint of processing technology, the question of flaw elimination (Fig. 1) probably remains as the central consideration; it will be taken as the major influence in the choices discussed in later sections. A final point in respect of targets relates to cost. Many of the applications proposed for large-scale use of ceramics involve industries such as that of motor manufacture where cost considerations are notoriously closely
argued; this places importance on the avoidance of high cost steps in processing (machining is a notable example) and on the development of forming methods capable of giving high shape definition. The cost factor is central to process selection; making the right decisions at a time when many of the eventual markets remain ill defined has been the requirement; the general response has commonly been that of initial design to reach desired property targets followed by subsequent refinement to trim costs. Many of the responses to the processing options discussed in the following paragraphs are influenced by the wish to allow this approach.
3. PROCESS DESIGN Although the problems posed by the classical ceramics processing route (Fig. 2) have been sufficient to bring a b o u t close consideration of radically different methods [ 5 ], the weight of attention has been given to the refinement and development of the conventional pattern. On this basis it is possible to identify both changes in emphasis that have occurred over recent years as the study of ceramic fabrication has intensified and specific options that have opened up at different stages in the processing sequence. Anyone designing a processing route can therefore select the overall pattern of emphasis that fits the problem at hand; there will also be the need for choice or compromise in selecting between the options. These are discussed in
Mixing
I
~3
o
°
°°~ ~o 5
Shaped powder form green bodyl
o
o 2
Deb!nding I
o
o
Shaping
o
I
Firing I
Fig. 1. Flaw types occurring in ceramic components a s a consequence of fabrication difficulties: 1, machining damage; 2, porosity; 3, inclusion; 4, thermal expansion anisotropy; 5, packing inhomogeneity.
Dense produtf
I
I
Machining
Fig. 2. Standard processing sequence for the fabrication of monolithic ceramics.
307 the following sections with some emphasis on powder quality and on firing procedure.
4. PLACING OF EMPHASIS (CHOICE I) In considering the overall processing flow diagram (Fig. 2) it is convenient to distinguish between the firing stage and the stages of processing before firing, a distinction which makes it possible to identify two extreme approaches (Fig. 3). The first approach aims at sufficient control of the pre-firing processing that the pre-fired component is itself free of structural faults, the subsequent firing process simply serving to consolidate the component. The current emphasis on ideal powders [14] capable of taking up perfect ordered arrays in green components is an example of such an approach. In the second, control of the firing conditions is seen as providing the best opportunity for the reliable elimination of flaws; in its most developed form this viewpoint leads to processes such as hot isostatic pressing in which faults from, say, variations in powder packing density can be eliminated during heat treatment. Thus, choice I, the point of emphasis for the attainment of product reliability, is represented by prefiring processing, ( ) firing processing, powder quality, additives, ordered powder arrays hot isostatic pressing Without necessarily taking development to the extent of ordered powder arrays, there is
no doubt that the attention given to powder quality and component forming has yielded clear benefits in terms of eventual properties. The nature and origin of the strength-determining faults vary according to the quality of processing being used, and different manufacturers are c o m m o n l y at different levels of sophistication; it is therefore likely that, in most cases, strength is still determined by accidents in pre-firing processing (contamination, variable raw material quality) and the attention to the earlier steps in the processing sequence has therefore been rewarding in helping to eliminate these faults. Once such accidents have been removed, the questions intrinsic to powder processing can be more clearly put, namely the degree of perfection required in the initial powder array to bring about flaw-free fired products and the nature of the forming methods capable of yielding the required quality of green microstructure. As examples, it is not yet clear that ordered arrays offer eventual advantage in comparison, say, with dense random packed arrays; it is also not yet clear whether arrays free from large faulting can be prepared without the use of imposed forces (as in centrifugal casting). Much basic work is now in progress on such questions and the results will be influential for process design. The trend therefore has been to move the emphasis from the firing stage to earlier stages in processing. The results of this change in emphasis in terms of property improvements and the promise of further benefits from concern with powder quality and forming refinements are such that the trend may be expected to continue. 5. OPTIONS FOR POWDERS (CHOICE II)
(a)
I
(b) Fig. 3. Approaches to flaw elimination can emphasize (a) powder quality and forming control or (b) corrective measures during firing.
Much attention has been given to the development of powders suitable for ceramic fabrication and the range of "user-friendly" powders available for A1203, ZrO 2 and Si3N4 materials has grown rapidly in recent years. Emphasis has been given to those preparation methods which appear to offer the best opportunities for the close control of powder design; choices remain, however, in respect of the best design target. For some characteristics of the powders, substantial agreement exists. Thus freedom from contamination is important; this can in-
308
clude both avoidance of such inclusions as ball mill debris (a widely occurring problem [15]) and avoidance or control of more general chemical impurities resulting from earlier powder processing. Examples of the latter are soda in A120 3 powders [16] and oxide in Si3N4 powders [17]. Similarly, avoidance of powder clusters or agglomerates incapable of being broken down during the forming stage is seen as a critical requirement. Such agglomerates are recognized as a source of packing inhomogeneity and are consequently identified as sites for flaw development [18]. Much work has centred on agglomerate control and the benefits of avoiding such steps as drying from aqueous suspensions at elevated temperatures, e.g. by freeze drying [19] or by using organic suspensions [20], have been widely discussed. An additional point of agreement is powder particle shape; with a view to packing ease and the attainment of a homogeneous packing density, equiaxed grains are preferred with sphericity being seen as the target. The factors where some debate remains are those of powder particle size and size distribution. The arguments relating to size distribution have assessed the relative merits of wide distributions {capable of giving a high green density and consequently a small firing shrinkage) and those of narrow distributions (resistant to abnormal grain growth and structural coarsening and in the limit capable of providing ordered arrays). The trend has been to narrow distributions over recent years with recognition of the benefits of grain growth suppression and packing homogeneity. The fact that these benefits are real can be seen from comparison of a range of Si3N4 powders (Fig. 4): those powders with a high green density show a tendency to rapid grain growth with resulting fall in sinterability; conversely, those with a narrow size distribution yield a low green density but provide a relatively high sustained densification rate throughout the heat treatment. Nonetheless the limitations of monosize powders in terms of cost, in terms of low green density and indeed in terms of green structure quality have been raised [ 21], and it seems attractive to explore the possible benefits of a wider range of distribution designs. The question of ideal powder size remains open. The benefits of fine powders are widely
log (densificafion rate)
2 ,~
~.
\ \
Density (g cm-3) '
'
210
. . . .
3.LO
'
Fig. 4. Kinetics of hot pressing for a range of different commercial Si3N4 powders (all data are for 10 MPa and 1650 °C using identical additives); A, ©, (~, starting density; A, e, 0, final density. (By courtesy of H. Pickup.)
log(rate) ",!~ ~af lice diffusion
grain bounda diffusion ~ , ~ , +
e- c
"\i" [oc,](parficte size) Fig. 5. The particle size dependence of microstructural change shows the ability of small particles to enhance the desired densifying transport mechanisms (lattice diffusion and grain boundary diffusion) at the expense of the evaporation-condensation (e-c) coarsening mechanism. Surface diffusion mechanisms are also enhanced at fine sizes but this is self-correcting as growth of the particle size (coarsening) occurs. Surface diffusion (were assumed to be lower than grain boundary diffusion) is parallel to it on the plot.
recognized both in terms of the acceleration of the microstructural changes (the promise of lower firing temperatures) and in terms of relative avoidance of coarsening by evaporation-condensation mechanisms (Fig. 5), but these are to some extent offset by the increased handling problems posed by fine powder systems. The choice then lies between emphasizing handleability with a view, say, to the formation of structured arrays (powder
309 size, about 0.5 gm) and emphasizing reactivity by pushing for progressively smaller sizes. The merits of fine grain sizes in product microstructures, together with the possibility of bestowing a degree of handleability onto very fine systems by methods such as controlled agglomeration (the formation of soft agglomerates), has given impetus to the ultrafine powder approach. It is n o t e w o r t h y that many of the sinterable powders that have become commercially available in recent years seem to have accepted the target of ultrafine sizes even at the expense of relatively wide distributions. The available ZrO2 powders follow this pattern with sizes in the 100 nm range being not atypical. Thus, choice II, powder quality, is given by ultrafine powders, < ) ideal powders, smallest possible size, monosize spheres, agglomerate control ordered arrays As with other choices, that of powder design criteria offers good possibility for combined approaches. As experience with novel powder preparation routes is accumulated, a tendency to retain the ideal powder characteristics of sphericity and distribution control down to progressively finer sizes is likely to be continued in view of the attractions of fine-grain product properties and low processing temperatures. The eventual property benefits emerging from ordered arrays will be an important factor in deciding the finally accepted point of compromise. 6. OPTIONS FOR FORMING (CHOICE III) The selection of forming method is influenced by the cost and scale of production arguments and by the need for compatibility with other steps in the processing sequence. No specific preferred m e t h o d has won dominance; cost and convenience have established a degree of manufacturer loyalty to diepressing methods but the need for fluid processing routes with ideal powder arrays and the need for good as-fired tolerances on fired pieces have given emphasis respectively to casting and to injection-moulding methods. Therefore, choice III, forming, can be described by dry methods, < ~ die pressing, isostatic pressing
fluid methods, casting, injection moulding
With the overall target of achieving structural homogeneity prior to firing it seems likely that all methods will continue to be refined, the specific choice being then dictated by shape, scale, cost and property considerations. Increased attention to the fluid processing routes will be needed in order to bring them to the level of the more established methods.
7. OPTIONS FOR FIRING (CHOICE IV) As with powder design, much consensus has now been achieved with respect to the selection of firing conditions. A clearer picture has been attained by regarding microstructure development during heat treatment as the combination of the densification and grain growth processes which offer alternative means for the reduction of the system's surface energy [22]. This approach has for example been beneficial in suggesting criteria for the selection of firing conditions (temperature, pressure) just as it was earlier for the selection of powder quality (particle size). On the basis that structural targets commonly involve a small grain size and high density, the firing objective is so to choose conditions that the ratio of densification rate to coarsening rate is enhanced. Under conditions where the mechanisms for the two processes are different and where different temperature dependences are found (different activation enthalpies), short high temperature firings or long low temperature firings are suggested as favourable depending on whether the densification activation enthalpy is greater or less than that for coarsening {Fig. 6). Such concepts have formed the grounds for fast firing in its several forms [23-25]. Similar ratio enhancement explains the merits of hot pressing, once a sufficiently high density has been reached for cross-boundary atom jump processes to be responsible for grain growth (Fig. 7). They also apply to powder particle size selection. For the remaining question of chemical composition (nonstoichiometry, additives), the picture is more complex. Where additions are such as to provide a second phase at the boundaries (probably the most common occurrence), the ability of the
310
log (rate)
~
grain growth rate 15 .densificafion rate
icafion undoped/ maferiai~ / /
10
:
\
highT
\
lowT
maferiat
1/RT Fig. 6. Temperature dependence of microstructural change; for the behaviour shown, high temperature firing cycles with rapid heating would be advantageous.
log(rate)
densificahon~
/"
graingrowth
(coarsenin)g
Iog(ap~ied pressure) Fig. 7. Pressure dependence of microstructural change ; at lower densities where pore drag controls boundary movement, grain growth is also accelerated by applied pressure as the pores are more rapidly reduced in size.
second phase to act as a high diffusivity path results in e n h a n c e m e n t of the ratio. Si3N 4 and associated additive systems provide the best example [7]. Where additives in solid solution have a clearly demonstrable effect on process diffusion coefficients, similar results can arise. This is perhaps best evidenced in systems such as ZrO2 [26] where substantial solubility occurs and where as a consequence the influence of trace impurity can be neglected. Therefore, choice I V, optimization of firing, can be represented by ratio control, < > rapid densification, slow coarsening
homogeneity, b o u n d a r y pinning
F or additives at low levels (less than 1 wt.%) in solid solution, alternative explana-
relative density 0
0.6
0.7 '
0.8 '
Fig. 8. Hot-pressing kinetics suggest that the ratio of coarsening to densification is relatively little affected in A1203by the presence of MgO additive [27 ] (1450 °C; 10 MPa).
tions arise. Work on the classical MgO addition to A120 3 has for example shown that the ratio is relatively little affected (albeit favourably) over a wide span o f densities (Fig. 8). Where an additional effect is found is in respect of the ability of this dopant to restrict grain b o u n d a r y m o t i o n in A120 3 [28]. The i m p o r t a n t consequence of this ability relates to the firing of less than perfectly homogeneous compacts [29]; in such compacts, different regions densify at different rates and the prevention of the development of large grains from rapid b o u n d a r y m o v e m e n t in those regions which attain a high density at an early stage is necessary to prevent abnormal grain growth and to preserve the possibility of attaining structural homogeneity. For low concentration additives, this correction of structural i nhom ogenei t y in the green component may emerge as the dom i nant function; it emphasizes the great lack of reliable information in respect of the effect of solid solution additives on grain b o u n d a r y mobilities. This is a subject where work is needed since, for those cases where t hey have been f o u n d to be effective, additives emerge as one of the lowest cost routes to a given microstructural objective.
8. METHODOLOGY (CHOICE V) In addition to questions relating to specific steps in processing as outlined above, the
311 general issue of the a p p r o p r i a t e m e t h o d o l o g y to a d o p t for ceramics f a b r i c a t i o n has also aroused debate. F o r a long p e r i o d o f developm e n t , the emphasis in e x t e n d i n g the understanding o f processing and its e x p l o i t a t i o n lay in the identification o f controlling a t o m i c mechanisms and in the use o f such k n o w l e d g e to i m p r o v e and refine process m e t h o d s . T h e p r e s e n t r e c o g n i t i o n o f the e x t r e m e c o m p l e x i t y o f the events occurring during processing could cause a r e a c t i o n against mechanistic int e r p r e t a t i o n s , the a m b i t i o n to achieve a s o u n d p h e n o m e n o l o g i c a l u n d e r s t a n d i n g in terms o f an a d e q u a t e m a t h e m a t i c a l description o f processing being seen as a m o r e realistic and helpful target. Choice V, m e t h o d o l o g y , can be described by phenomenology, process c o n t r o l
( ~
mechanisms, process d e v e l o p m e n t
In general, b o t h o f these a p p r o a c h e s should be seen as helpful. With greater raw material r e p r o d u c i b i l i t y , with greater r e q u i r e m e n t for p r o d u c t reliability and with a growing range o f m o n i t o r i n g and quality assurance m e t h o d s , the merits o f the p h e n o m e n o l o g i c a l a p p r o a c h for process design and c o n t r o l are apparent. Ceramic p r o d u c t i o n is still for the m o s t part a linked s e q u e n c e o f individual process steps b u t the a t t a i n m e n t o f a c o n t i n u o u s process flow p a t t e r n has been a long-recognized target for which this a p p r o a c h offers the greatest promise. H o w e v e r , w h e n it c o m e s to specific process d e v e l o p m e n t as in questions o f particle size d i s t r i b u t i o n choice or additive selection, the benefits o f mechanistic arguments can be seen. As in the o t h e r choices b u t perhaps here in a m o r e striking f o r m , the benefits o f the dual a p p r o a c h are apparent.
9. THE PERENNIAL OPTION (CHOICE VI) A final q u e s t i o n r e t u r n s to the issue o f the t o t a l process design. As each step in the classical flow sheet grows m o r e sophisticated, the end result can b e c o m e s o m e w h a t daunting. If very refined p o w d e r s m u s t be used which in t u r n require e l a b o r a t e and c o m p l e x f o r m i n g m e t h o d s and if these s u b s e q u e n t l y require firing with additional h o t isostatic pressing and m a c h i n i n g steps, the t o t a l picture can be-
c o m e b o t h c u m b e r s o m e and p o t e n t i a l l y very expensive. Thus, choice VI is given by classical processing, f o r m , fire, finish
< >
Under these c o n d i t i o n s the possibilities o f radically alternative a p p r o a c h e s to fabrication [30] can b e c o m e attractive. Thus far, striking advance has been m a d e by r e n e w e d a t t e n t i o n to p o w d e r quality and to such issues as process cleanliness, and this has o f f e r e d the promise o f avoiding the m o s t e c o n o m i c a l l y i n c o n g r u o u s e l e m e n t s f r o m the classical process route. Nonetheless, particularly where m i n i a t u r i z a t i o n is involved, it seems t h a t c o r r e s p o n d i n g progress with non-classical r o u t e s will be m a d e and, as with the o t h e r choices, t h e r e will be merit in keeping the full range o f possibilities in review.
ACKNOWLEDGMENTS Discussion with Dr. A. I. Kingon and Dr. A. J. Moulson has been helpful in the p r e p a r a t i o n o f this p a p e r and is gratefully acknowledged.
REFERENCES 1 A. G. Evans, J. Am. Ceram. Soc., 65 (3) (1982) 127-137. 2 E. M. Lenoe, Am. Ceram. Soc., Bull., 63 (3) (1984)422. 3 J. A. Mangels (ed.), Forming o f Ceramics, Advances in Ceramics, Vol. 9, American Ceramic Society, Columbus, OH. 1984. 4 P. Vincenzini (ed.), Ceramic Powders, Elsevier, Amsterdam, 1983. 5 R. F. Davis, H. Palmour III and R. L. Porter (eds.), Emergent Process Methods for Hightechnology Ceramics, Mater. Sci. Res., 1 7 (1984). 6 E. W. Rice, Proc. Br. Ceram. Soc., 12 (1969) 99. 7 F. F. Lange, Am. Ceram. Soc., Bull., 62 (12) (1983) 1369-1374. 8 M. H. Lewis, B. D. Powell, P. Drew, R. J. Lumby, B. North and A. J. Taylor, J. Mater. Sci., 12 (1977) 61. 9 E. P. Butler and J. Drennan, J. Am. Ceram. Soc., 65 (10) (1982) 474-478. 10 A. H. Heuer, N. Claussen, W. M. Kriven and M. Ri~hle, J. Am. Ceram. Soc., 65 (12) (1982) 642650. 11 N. Claussen, in H. Kr6ckel, M. Merz and O. van der Biest (eds.), Ceramics in Advanced Energy
312
12 13
14 15
16 17 18 19
20
Technologies, Reidel, Dordrecht, 1984, pp. 5186. A. G. Evans and K. T. Faber, J. Am. Ceram. Soc., 67 (4) (1984) 255-260. R. W. Davidge (ed.), Survey of the technological requirements for high temperature materials research and development, Section 3, Ceramic composites for high temperature engineering applications, Rep. EUR 9565 EN, 1985 (Commission of the European Communities). E. A. Barringer and H. K. Bowen, J. Am. Ceram. Soc., 65 (12) (1982) C-199-C-201. T. L. Francis, G. MacZura, J. E. Marhanka and H. J. Barker, Am. Ceram. Soc., Bull., 51 (6) (1972) 535-538. L. Berrin, D. W. Johnson and D. J. Nitti, Am. Ceram. Soc., Bull., 51 (11) (1972) 840-844. G. WStting and G. Ziegler, Ceram. Int., 10 (1) (1984) 18-22. F. F. Lange, J. Am. Ceram. Soc., 67 (2) (1984) 83-89. A. Roosen and H. Hausner, in P. Vincenzini (ed.), Ceramic Powders, Elsevier, Amsterdam, 1983, pp. 773-782. C. E. Scott and J. S. Reed, Am. Ceram. Soc., Bull., 58 (6) (1979) 587-590.
21 I. A. Aksay, in J. A. Margels (ed.), F o r m i n g o f Ceramics, Advances in Ceramics, Vol. 9, American Ceramic Society, Columbus, OH, 1984, pp. 94-104. 22 C. A. Handwerker, R. M. Cannon and R. L. Coble, Adv. Ceram., 10 (1984) 619-643. 23 A. Morell and A. Hermosin, Am. Ceram. Soc., Bull., 59 (6) (1980) 626-629. 24 M. P. Harmer, E. W. Roberts and R. J. Brook, Trans. Br. Ceram. Soc., 78 (1) (1979) 22. 25 D. L. Johnson, V. A. Kramb and D. G. Lynch, in R. F. Davis, H. Palmour III and R. L. Porter (eds.), Emergent Process Methods for Hightechnology Ceramics, Mater. Sci. Res., 1 7 (1984) 207 -211. 26 Suxing Wu and R. J. Brook, Solid State Ionics, 14 (1984) 123-130. 27 R.J. Brook, E. Gilbart, N. J. Shaw and U. Eisele, Metals Society 1984 Powder Metallurgy Group Meet., Harrogate, 1984, Preprint 24, pp. 1-3. 28 S. J. Bennison and M. P. Harmer, J. Am. Ceram. Soc., 66 (5) (1983) C-90-C-92. 29 A. G. Evans, J. Am. Ceram. Soc., 65 (1982) 497501. 30 J. D. Birchall, Trans. Br. Ceram. Soc., 83 (6) (1984) 158-165.
305
Processing Technology for High Performance Ceramics* R. J. BROOK
Department of Ceramics, University of Leeds, Leeds LS2 9JT (Gt. Britain) (Received January ] 1, 1985)
ABSTRACT The emphasis that has been placed on ceramic c o m p o n e n t reproducibility and consistency has underlined the importance o f processing technology and has indicated the progress that can be made for a wide range o f systems by striving for microstructural quality and homogeneity. This has had consequences in drawing attention not only to p o w d e r preparation and to refined forming procedures but also to mechanisms by which inhomogeneity, once introduced, can be reduced or even eliminated. The implications o f these developments for processing technology and science are considered in terms o f a sequence o f options that are open to the processor. 1. INTRODUCTION Processing and fabrication developments are n o w widely recognized as being key factors in the progress of engineering ceramics towards full exploitation [ 1]. This recognition has been responsible for the emphasis on processing technology that can be f ound in national research programmes [2] and that has resulted in a series of recent conferences on the topic [ 3 - 5 ] . The subject is both wide ranging and complex. Systems of interest include binary oxides such as A120 3 and ZrO2, more complex oxides such as the perovskites and spinels, non-oxides such as Si3N 4 and SiC and mixed systems within these groups and, as with the S i - A 1 - O - N materials, between groups. Levels o f purity vary greatly. For each system, substantial choice exists as to specific fabrication *Paper presented at the International Symposium on Engineering Ceramics, Jerusalem, Israel, December 16 20, 1984. 0025-5416/85/$3.30
route with each rout e consisting of a sequence of process steps within which a wide range of process variables must be considered. A difficulty with the subject therefore is that, while discussion of the full range of processing technology must inevitably be somewhat general, the problems e n c o u n t e r e d in processing are often very specific. It must also be recognized that the solutions to such specific problems are oft en proprietary in view of their commercial significance. Progress is publicly measured more c o m m o n l y in terms of claimed p r o p e r t y enhancements in the p r o d u c t that in terms of fully underst ood and openly described processing improvements. Faced with this characteristic of the subject, the plan of the following discussion is to review briefly some of the microstructural targets which have been established for processing and then to consider a series of choices which c o n f r o n t the processor and which have been t hrow n into relief by recent work.
2. TARGETS Ideal microstructures vary widely corresponding to the wide range of applications (electrical, magnetic, thermal and mechanical) to which ceramics are put. If, however, emphasis is given to the mechanical engineering applications that are central to the conference reported in this volume, then control of creep and the at t ai nm ent of high reproducible strength have received most attention. Other properties such as wear resistance while of comparable importance have at this stage been less confidently related to microstructural parameters and this has resulted in their receiving less emphasis. Creep resistance is most clearly linked to the elimination of viscous grain b o u n d a r y © Elsevier Sequoia/Printed in The Netherlands
306 phases from the microstructure. Such phases are very commonly present in polycrystalline ceramics as a consequence of additives or impurities; the formation of boundary liquid phases at the firing temperature greatly assists densification but correspondingly degrades load-bearing performance in subsequent use of the material at high temperatures. A common objective, and one treated elsewhere in this volume, is the removal of such boundary phases subsequent to processing: possible routes include vacuum evaporation (LiF from MgO [6]), devitrification (Y203-SiO2 from Si3N4 [7]), reaction with the grain material to form solid solutions ( S i - A 1 - O - N systems [8]), and localization at solid second phases (A1203 additions to ZrO 2 [9]). High reproducible strength as a target has drawn attention both to the elimination or minimization of the processing flaws capable of acting as fracture origins and to the design of microstructural toughening mechanisms with higher energy-absorbing capacity as fracture takes place. Such design includes a number of topics such as transformation toughening [10, 11 ], microcrack toughening [12] and the use of fibre composites [13] which are considered in other papers in this volume. From the viewpoint of processing technology, the question of flaw elimination (Fig. 1) probably remains as the central consideration; it will be taken as the major influence in the choices discussed in later sections. A final point in respect of targets relates to cost. Many of the applications proposed for large-scale use of ceramics involve industries such as that of motor manufacture where cost considerations are notoriously closely
argued; this places importance on the avoidance of high cost steps in processing (machining is a notable example) and on the development of forming methods capable of giving high shape definition. The cost factor is central to process selection; making the right decisions at a time when many of the eventual markets remain ill defined has been the requirement; the general response has commonly been that of initial design to reach desired property targets followed by subsequent refinement to trim costs. Many of the responses to the processing options discussed in the following paragraphs are influenced by the wish to allow this approach.
3. PROCESS DESIGN Although the problems posed by the classical ceramics processing route (Fig. 2) have been sufficient to bring a b o u t close consideration of radically different methods [ 5 ], the weight of attention has been given to the refinement and development of the conventional pattern. On this basis it is possible to identify both changes in emphasis that have occurred over recent years as the study of ceramic fabrication has intensified and specific options that have opened up at different stages in the processing sequence. Anyone designing a processing route can therefore select the overall pattern of emphasis that fits the problem at hand; there will also be the need for choice or compromise in selecting between the options. These are discussed in
Mixing
I
~3
o
°
°°~ ~o 5
Shaped powder form green bodyl
o
o 2
Deb!nding I
o
o
Shaping
o
I
Firing I
Fig. 1. Flaw types occurring in ceramic components a s a consequence of fabrication difficulties: 1, machining damage; 2, porosity; 3, inclusion; 4, thermal expansion anisotropy; 5, packing inhomogeneity.
Dense produtf
I
I
Machining
Fig. 2. Standard processing sequence for the fabrication of monolithic ceramics.
307 the following sections with some emphasis on powder quality and on firing procedure.
4. PLACING OF EMPHASIS (CHOICE I) In considering the overall processing flow diagram (Fig. 2) it is convenient to distinguish between the firing stage and the stages of processing before firing, a distinction which makes it possible to identify two extreme approaches (Fig. 3). The first approach aims at sufficient control of the pre-firing processing that the pre-fired component is itself free of structural faults, the subsequent firing process simply serving to consolidate the component. The current emphasis on ideal powders [14] capable of taking up perfect ordered arrays in green components is an example of such an approach. In the second, control of the firing conditions is seen as providing the best opportunity for the reliable elimination of flaws; in its most developed form this viewpoint leads to processes such as hot isostatic pressing in which faults from, say, variations in powder packing density can be eliminated during heat treatment. Thus, choice I, the point of emphasis for the attainment of product reliability, is represented by prefiring processing, ( ) firing processing, powder quality, additives, ordered powder arrays hot isostatic pressing Without necessarily taking development to the extent of ordered powder arrays, there is
no doubt that the attention given to powder quality and component forming has yielded clear benefits in terms of eventual properties. The nature and origin of the strength-determining faults vary according to the quality of processing being used, and different manufacturers are c o m m o n l y at different levels of sophistication; it is therefore likely that, in most cases, strength is still determined by accidents in pre-firing processing (contamination, variable raw material quality) and the attention to the earlier steps in the processing sequence has therefore been rewarding in helping to eliminate these faults. Once such accidents have been removed, the questions intrinsic to powder processing can be more clearly put, namely the degree of perfection required in the initial powder array to bring about flaw-free fired products and the nature of the forming methods capable of yielding the required quality of green microstructure. As examples, it is not yet clear that ordered arrays offer eventual advantage in comparison, say, with dense random packed arrays; it is also not yet clear whether arrays free from large faulting can be prepared without the use of imposed forces (as in centrifugal casting). Much basic work is now in progress on such questions and the results will be influential for process design. The trend therefore has been to move the emphasis from the firing stage to earlier stages in processing. The results of this change in emphasis in terms of property improvements and the promise of further benefits from concern with powder quality and forming refinements are such that the trend may be expected to continue. 5. OPTIONS FOR POWDERS (CHOICE II)
(a)
I
(b) Fig. 3. Approaches to flaw elimination can emphasize (a) powder quality and forming control or (b) corrective measures during firing.
Much attention has been given to the development of powders suitable for ceramic fabrication and the range of "user-friendly" powders available for A1203, ZrO 2 and Si3N4 materials has grown rapidly in recent years. Emphasis has been given to those preparation methods which appear to offer the best opportunities for the close control of powder design; choices remain, however, in respect of the best design target. For some characteristics of the powders, substantial agreement exists. Thus freedom from contamination is important; this can in-
308
clude both avoidance of such inclusions as ball mill debris (a widely occurring problem [15]) and avoidance or control of more general chemical impurities resulting from earlier powder processing. Examples of the latter are soda in A120 3 powders [16] and oxide in Si3N4 powders [17]. Similarly, avoidance of powder clusters or agglomerates incapable of being broken down during the forming stage is seen as a critical requirement. Such agglomerates are recognized as a source of packing inhomogeneity and are consequently identified as sites for flaw development [18]. Much work has centred on agglomerate control and the benefits of avoiding such steps as drying from aqueous suspensions at elevated temperatures, e.g. by freeze drying [19] or by using organic suspensions [20], have been widely discussed. An additional point of agreement is powder particle shape; with a view to packing ease and the attainment of a homogeneous packing density, equiaxed grains are preferred with sphericity being seen as the target. The factors where some debate remains are those of powder particle size and size distribution. The arguments relating to size distribution have assessed the relative merits of wide distributions {capable of giving a high green density and consequently a small firing shrinkage) and those of narrow distributions (resistant to abnormal grain growth and structural coarsening and in the limit capable of providing ordered arrays). The trend has been to narrow distributions over recent years with recognition of the benefits of grain growth suppression and packing homogeneity. The fact that these benefits are real can be seen from comparison of a range of Si3N4 powders (Fig. 4): those powders with a high green density show a tendency to rapid grain growth with resulting fall in sinterability; conversely, those with a narrow size distribution yield a low green density but provide a relatively high sustained densification rate throughout the heat treatment. Nonetheless the limitations of monosize powders in terms of cost, in terms of low green density and indeed in terms of green structure quality have been raised [ 21], and it seems attractive to explore the possible benefits of a wider range of distribution designs. The question of ideal powder size remains open. The benefits of fine powders are widely
log (densificafion rate)
2 ,~
~.
\ \
Density (g cm-3) '
'
210
. . . .
3.LO
'
Fig. 4. Kinetics of hot pressing for a range of different commercial Si3N4 powders (all data are for 10 MPa and 1650 °C using identical additives); A, ©, (~, starting density; A, e, 0, final density. (By courtesy of H. Pickup.)
log(rate) ",!~ ~af lice diffusion
grain bounda diffusion ~ , ~ , +
e- c
"\i" [oc,](parficte size) Fig. 5. The particle size dependence of microstructural change shows the ability of small particles to enhance the desired densifying transport mechanisms (lattice diffusion and grain boundary diffusion) at the expense of the evaporation-condensation (e-c) coarsening mechanism. Surface diffusion mechanisms are also enhanced at fine sizes but this is self-correcting as growth of the particle size (coarsening) occurs. Surface diffusion (were assumed to be lower than grain boundary diffusion) is parallel to it on the plot.
recognized both in terms of the acceleration of the microstructural changes (the promise of lower firing temperatures) and in terms of relative avoidance of coarsening by evaporation-condensation mechanisms (Fig. 5), but these are to some extent offset by the increased handling problems posed by fine powder systems. The choice then lies between emphasizing handleability with a view, say, to the formation of structured arrays (powder
309 size, about 0.5 gm) and emphasizing reactivity by pushing for progressively smaller sizes. The merits of fine grain sizes in product microstructures, together with the possibility of bestowing a degree of handleability onto very fine systems by methods such as controlled agglomeration (the formation of soft agglomerates), has given impetus to the ultrafine powder approach. It is n o t e w o r t h y that many of the sinterable powders that have become commercially available in recent years seem to have accepted the target of ultrafine sizes even at the expense of relatively wide distributions. The available ZrO2 powders follow this pattern with sizes in the 100 nm range being not atypical. Thus, choice II, powder quality, is given by ultrafine powders, < ) ideal powders, smallest possible size, monosize spheres, agglomerate control ordered arrays As with other choices, that of powder design criteria offers good possibility for combined approaches. As experience with novel powder preparation routes is accumulated, a tendency to retain the ideal powder characteristics of sphericity and distribution control down to progressively finer sizes is likely to be continued in view of the attractions of fine-grain product properties and low processing temperatures. The eventual property benefits emerging from ordered arrays will be an important factor in deciding the finally accepted point of compromise. 6. OPTIONS FOR FORMING (CHOICE III) The selection of forming method is influenced by the cost and scale of production arguments and by the need for compatibility with other steps in the processing sequence. No specific preferred m e t h o d has won dominance; cost and convenience have established a degree of manufacturer loyalty to diepressing methods but the need for fluid processing routes with ideal powder arrays and the need for good as-fired tolerances on fired pieces have given emphasis respectively to casting and to injection-moulding methods. Therefore, choice III, forming, can be described by dry methods, < ~ die pressing, isostatic pressing
fluid methods, casting, injection moulding
With the overall target of achieving structural homogeneity prior to firing it seems likely that all methods will continue to be refined, the specific choice being then dictated by shape, scale, cost and property considerations. Increased attention to the fluid processing routes will be needed in order to bring them to the level of the more established methods.
7. OPTIONS FOR FIRING (CHOICE IV) As with powder design, much consensus has now been achieved with respect to the selection of firing conditions. A clearer picture has been attained by regarding microstructure development during heat treatment as the combination of the densification and grain growth processes which offer alternative means for the reduction of the system's surface energy [22]. This approach has for example been beneficial in suggesting criteria for the selection of firing conditions (temperature, pressure) just as it was earlier for the selection of powder quality (particle size). On the basis that structural targets commonly involve a small grain size and high density, the firing objective is so to choose conditions that the ratio of densification rate to coarsening rate is enhanced. Under conditions where the mechanisms for the two processes are different and where different temperature dependences are found (different activation enthalpies), short high temperature firings or long low temperature firings are suggested as favourable depending on whether the densification activation enthalpy is greater or less than that for coarsening {Fig. 6). Such concepts have formed the grounds for fast firing in its several forms [23-25]. Similar ratio enhancement explains the merits of hot pressing, once a sufficiently high density has been reached for cross-boundary atom jump processes to be responsible for grain growth (Fig. 7). They also apply to powder particle size selection. For the remaining question of chemical composition (nonstoichiometry, additives), the picture is more complex. Where additions are such as to provide a second phase at the boundaries (probably the most common occurrence), the ability of the
310
log (rate)
~
grain growth rate 15 .densificafion rate
icafion undoped/ maferiai~ / /
10
:
\
highT
\
lowT
maferiat
1/RT Fig. 6. Temperature dependence of microstructural change; for the behaviour shown, high temperature firing cycles with rapid heating would be advantageous.
log(rate)
densificahon~
/"
graingrowth
(coarsenin)g
Iog(ap~ied pressure) Fig. 7. Pressure dependence of microstructural change ; at lower densities where pore drag controls boundary movement, grain growth is also accelerated by applied pressure as the pores are more rapidly reduced in size.
second phase to act as a high diffusivity path results in e n h a n c e m e n t of the ratio. Si3N 4 and associated additive systems provide the best example [7]. Where additives in solid solution have a clearly demonstrable effect on process diffusion coefficients, similar results can arise. This is perhaps best evidenced in systems such as ZrO2 [26] where substantial solubility occurs and where as a consequence the influence of trace impurity can be neglected. Therefore, choice I V, optimization of firing, can be represented by ratio control, < > rapid densification, slow coarsening
homogeneity, b o u n d a r y pinning
F or additives at low levels (less than 1 wt.%) in solid solution, alternative explana-
relative density 0
0.6
0.7 '
0.8 '
Fig. 8. Hot-pressing kinetics suggest that the ratio of coarsening to densification is relatively little affected in A1203by the presence of MgO additive [27 ] (1450 °C; 10 MPa).
tions arise. Work on the classical MgO addition to A120 3 has for example shown that the ratio is relatively little affected (albeit favourably) over a wide span o f densities (Fig. 8). Where an additional effect is found is in respect of the ability of this dopant to restrict grain b o u n d a r y m o t i o n in A120 3 [28]. The i m p o r t a n t consequence of this ability relates to the firing of less than perfectly homogeneous compacts [29]; in such compacts, different regions densify at different rates and the prevention of the development of large grains from rapid b o u n d a r y m o v e m e n t in those regions which attain a high density at an early stage is necessary to prevent abnormal grain growth and to preserve the possibility of attaining structural homogeneity. For low concentration additives, this correction of structural i nhom ogenei t y in the green component may emerge as the dom i nant function; it emphasizes the great lack of reliable information in respect of the effect of solid solution additives on grain b o u n d a r y mobilities. This is a subject where work is needed since, for those cases where t hey have been f o u n d to be effective, additives emerge as one of the lowest cost routes to a given microstructural objective.
8. METHODOLOGY (CHOICE V) In addition to questions relating to specific steps in processing as outlined above, the
311 general issue of the a p p r o p r i a t e m e t h o d o l o g y to a d o p t for ceramics f a b r i c a t i o n has also aroused debate. F o r a long p e r i o d o f developm e n t , the emphasis in e x t e n d i n g the understanding o f processing and its e x p l o i t a t i o n lay in the identification o f controlling a t o m i c mechanisms and in the use o f such k n o w l e d g e to i m p r o v e and refine process m e t h o d s . T h e p r e s e n t r e c o g n i t i o n o f the e x t r e m e c o m p l e x i t y o f the events occurring during processing could cause a r e a c t i o n against mechanistic int e r p r e t a t i o n s , the a m b i t i o n to achieve a s o u n d p h e n o m e n o l o g i c a l u n d e r s t a n d i n g in terms o f an a d e q u a t e m a t h e m a t i c a l description o f processing being seen as a m o r e realistic and helpful target. Choice V, m e t h o d o l o g y , can be described by phenomenology, process c o n t r o l
( ~
mechanisms, process d e v e l o p m e n t
In general, b o t h o f these a p p r o a c h e s should be seen as helpful. With greater raw material r e p r o d u c i b i l i t y , with greater r e q u i r e m e n t for p r o d u c t reliability and with a growing range o f m o n i t o r i n g and quality assurance m e t h o d s , the merits o f the p h e n o m e n o l o g i c a l a p p r o a c h for process design and c o n t r o l are apparent. Ceramic p r o d u c t i o n is still for the m o s t part a linked s e q u e n c e o f individual process steps b u t the a t t a i n m e n t o f a c o n t i n u o u s process flow p a t t e r n has been a long-recognized target for which this a p p r o a c h offers the greatest promise. H o w e v e r , w h e n it c o m e s to specific process d e v e l o p m e n t as in questions o f particle size d i s t r i b u t i o n choice or additive selection, the benefits o f mechanistic arguments can be seen. As in the o t h e r choices b u t perhaps here in a m o r e striking f o r m , the benefits o f the dual a p p r o a c h are apparent.
9. THE PERENNIAL OPTION (CHOICE VI) A final q u e s t i o n r e t u r n s to the issue o f the t o t a l process design. As each step in the classical flow sheet grows m o r e sophisticated, the end result can b e c o m e s o m e w h a t daunting. If very refined p o w d e r s m u s t be used which in t u r n require e l a b o r a t e and c o m p l e x f o r m i n g m e t h o d s and if these s u b s e q u e n t l y require firing with additional h o t isostatic pressing and m a c h i n i n g steps, the t o t a l picture can be-
c o m e b o t h c u m b e r s o m e and p o t e n t i a l l y very expensive. Thus, choice VI is given by classical processing, f o r m , fire, finish
< >
Under these c o n d i t i o n s the possibilities o f radically alternative a p p r o a c h e s to fabrication [30] can b e c o m e attractive. Thus far, striking advance has been m a d e by r e n e w e d a t t e n t i o n to p o w d e r quality and to such issues as process cleanliness, and this has o f f e r e d the promise o f avoiding the m o s t e c o n o m i c a l l y i n c o n g r u o u s e l e m e n t s f r o m the classical process route. Nonetheless, particularly where m i n i a t u r i z a t i o n is involved, it seems t h a t c o r r e s p o n d i n g progress with non-classical r o u t e s will be m a d e and, as with the o t h e r choices, t h e r e will be merit in keeping the full range o f possibilities in review.
ACKNOWLEDGMENTS Discussion with Dr. A. I. Kingon and Dr. A. J. Moulson has been helpful in the p r e p a r a t i o n o f this p a p e r and is gratefully acknowledged.
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