Silicon Nitride-FromPowder Synthesis to Ceramic Materials By Horst Lange,” Gerhard Wotting, and Gerhard Winter Dedicated to Professor Karl Heinz Biichel on the occasion of his 60th birthday Silicon nitride is a ceramic material of great interest to advanced engine construction and mechanical engineering owing to an outstanding combination of favorable properties like high mechanical strength at high temperatures, corrosion and wear resistance, great hardness, and low density. The material is based on high-quality Si,N, powders, which are shaped and sintered to the ceramic component. This overview outlines the properties required for Si,N, powders suitable for advanced ceramics. Processes in commerical use and those under development for the production of high quality Si,N, powders are discussed as well as material manufacturing processes and material properties. By steadily improving powder quality, material properties, and the economy of powder and component production, chemistry and chemical technology play a major role in recent efforts to create a solid fundament for broad applications of silicon nitride ceramics. “If one would surrender to geological phantasies, one could imagine that during the formation of our planet, when elements combined to the compounds making up its crust and mountain ranges, silicon reacted with nitrogen, and the still red-hot nitrogen-silicon, on contact with water, may have decomposed to silicic acid and ammonia. Thus, ammonia may have been formed originally and nitrogen thereby introduced into the forming organic compounds when living nature first started to appear.” H. Sainte-Claire Deville and F. Wohler[**’ 1. Introduction Following speculations by Deville and Wohler,“’ silicon nitride attracted attention for the first time as a possible raw material for the technical synthesis of ammonia. A series of patents from the period 1909- 1918describes the synthesis of ammonia by heating Si,N, with solutions or dispersions of basic oxides, hydroxides, or salts, by treatment with hot water vapor, or by heating it in hydrogen Because of the concurrent development of the Haber-Bosch process, these methods never gained any importance. It is not surprising, therefore, that this early interest in silicon nitride soon declined. However, silicon nitride saw a remarkable renaissance after the discovery that it can be converted into a ceramic [‘I Dr. H. Lange PK-F, Bayer AG W-4150 Krefeld-Uerdingen (FRG) Dr. G. Wotting Cremer Forschungsinstitut W-8633 Rodental (FRG) Prof. Dr. G. Winter Hermann C. Starck Berlin GmbH Co KG W-3380 Goslar (FRG) “Ueber die directe Bildung des Stickstoffsiliciums” in: Ann. Chem. Phurm. 34 (1 859) 248: “If one would surrender to geological phantasies, one could imagine that during the formation of our planet, when elements combined to the compounds making up its crust and mountain ranges, silicon reacted with nitrogen, and the still red-hot nitrogen-silicon, on contact with water, may have decomposed to silicic acid and ammonia. Thus, ammonia may have been formed originally and nitrogen thereby introduced into the forming organic compounds when living nature first started to appear. + material. Silicon nitride ceramics significantly extend the limits of metallic materials under combined thermal and mechanical stress in corrosive environments. 2. Silicon Nitride as Ceramic Material Silicon nitride is of great interest as a heavy-duty material for applications in chemical engineering, wear technology, metal working, energy technology, and especially in engine and turbine construction.[’ 2-241 This broad interest arises from a favorable combination of great hardness, wear resistance, chemical stability, low density, and high mechanical strength at temperatures up to 1300 “C. The application of Si,N, ceramic parts in combustion engines and gas turbines promises advantages like higher efiiciency and better fuel use due to higher combustion temperatures, faster response of oscillating or rotating engine components (valves, pistons, turbocharger, and turbine rotors) due to low density, and longer service intervals due to a generally improved wear behavior. Silicon nitride ceramics are produced either by nitriding silicon powder compacts or by sintering Si,N, powder comp a c t ~ . [ ~ ~ *first method produces a porous, reactionThe ~ ~ l bonded material (RBSN, reaction-bonded silicon nitride) with almost negligible shrinkage; this material is used either directly or may be densified by a following sintering step (SRBSN, sintered RBSN).12’- 301 The second method, which is covered comprehensively in this article, leads to Si,N, ceramics of superior quality. These are of special interest to 0570-0833/91/1212-iS79 S3.50-k ,2510 [*‘I Angew. Chem. Int. Ed. Engl. 30 (199i) 1579-1597 0 VCH Verlugsgesellschuft mbH. W-6940 Weinheim, 1991 1519 specification pressure. To demonstrate their technological benefits reliably. Nonuniformity of the starting powder-for example. To avoid environmental contamination by. Since 1986 he has taught at the Universitat-GesamthochschuleSiegen.[33. and sintering into clean rooms has been discussed. sintering. Manufacturing process from powder to component manufacturing touch many problems of solid-state chemistry. Engl. 1580 Angew.IZ6* some of which are fundamental. In 1967 he began work at Bayer AG. powder.51 in Markredwitz (Oberfranken) and studied Materials Science from 1969 to 1977 at the Fachhochschule SelblRegensburg and the Technische Universitat Berlin with emphasis on glass and ceramics.which are built up from a three-dimensional Horst Lange was born in 19. and Strength Crystalline silicon nitride exists in two hexagonal modifications.57 in Enkhausen (Sauerland) and studied chemistry from 1975 to 1981 at the Universitat Dortmund.D. In 1983 he received his Ph. Since 1985 he has worked at Bayer AG (Uerdingen) f and since 1989 he has been leader o the engineering ceramics group. temperature I J. fibers. say.[3 321 Repeatedly. He currently heads research and development at Hermann C. which cannot be remedied in the following manufacturing steps. Ceramic Component I testing. and finishing. 1). defects also arise during shaping. Since t987 he has been involved in developmental work in the area o materials technologylhigh-performance ceramics at the Cremer Forschungsinstitut. tribochemistry.und Versuchsanstaltfur Luftund Raumfahrt in Cologne. surface chemistry. or whiskers. Products of inferior quality and reliability are the result. its processing. chemistry plays an important role. more or less agglomerated lumps.341 Furthermore. These nowise trivial problems have to be solved convincingly to allow an economical. as well as extraneous particles or impurities dragged in through powder processing-induce fracture-causing structural defects. Shreeve at the University o Idaho. the powder producer has a special responsibility to supply extremely well-defined. from the synthesis of the Si. He earned his doctorate in 1966 with a dissertation on graphite inclusion compounds under Prof. shaping. Ed. Hausnerfor work on the sintering behavior o silicon nitridepowders.D. f Gerhard Wotting was born in 19. high-quality powders. this measure has been described to improve fracture strength and reliability.advanced engine construction and advanced mechanical engineering. 1. Starck Berlin in Goslar and is the author o the book “Industrielle f Anorganische Chemie ”. Nauman for work on perfluoroalkylzinc and -cadmium compounds and then worked for one year as a postdoc with Prof. even just a few extraneous particles. application Fig. a and /?. Riidorff. ill-defined particle clusters. The Structure of Silicon Nitrideorigin of Hardness. sintering to the precise finishing of the structural component (Fig. components require a perfect and economical control of the entire manufacturing process.N. forming. dust particles. Int. Here. 3. 30 (1991) 1579-1597 . under Prof. and chemical engi351 neering. f Gerhard Winter was born in 1937 in Klein-Mohrau (Sudetenland) and studied chemistry from 1957 to 1963 at the Universitat Tubingen. From 1981 to 1987 he worked as a researcher f at the Institut fur Werkstoff-Forschung der Deutschen Forschungs. Rodental. the transfer of powder processing. or particle morphologies such as a mass of disordered. Si. large-scale production of silicon nitride components. since powder synthesis and component ‘9 Powder synthesis.N. In 1982 he received his Ph. under Prof. Chem. Since powder-inherent sources of defects can hardly be eliminated by subsequent ceramic processing. Durability. N.t62-661 silicon compounds [Eqs.N.NR].N. view approximately along c axis. the formation of the a phase required significantly less oxygen than presumed.5 (actually an oxinitride phase).] ~ ~ .[36. ti) Early work on the conditions leading to formation of either the a or P phase indicated that the a phase. this method is less interesting for powder synthesis than for the manufacturing of Si..[58-611 carbothermal reduction of SiO.721 SKI. (Fig. channels with a diameter of about 0. [ ' ~ . + 4NH. In contrast to direct nitridation or carbothermal reduction.5N1500.N.1.N. Inr.O Si. Furthermore..[73-751 (i). - + 6H. is built from a Si.N. Today a. directly.N.2 to 4.N. 1 100. Several other reactions [Eq. (f) and (g)]-t67-69. 25-1400°C Si. NH. ammonolysis does not yield crystalline Si. diffusion is much more difficult than in P-Si. There is also some fundamental interest in the use of silazanes as binders or plasticizers for silicon nitride powder molding by dry pressing or injection molding. even large ones.N.~ ~ and p-Si. tetrahedrons. 3SiC + 4NH. . is the stable low-temperature modification.N.N. Required Powder Properties A Si. Ed.N. a p -+ c( transformation has not yet been observed. + 6NH. 3SiC1. + 3Si0.N. + 6C+ 4NH. 30 (1991) 1579-1597 .N. Probably for kinetic reasons. 800-1400"C. (b)] or ammonia. 3). 3SiS. such as too great expense.t431By chemical vapor deposition it was possible to produce a-Si. -RH. 3 Si(NH). assumed to be an oxygen-stabilized low-temperature modification with a narrow range of existence. to a-Si.N. Engl. content of a powder increases as the temperature of synthesis decreases and since CI -+ transformation occurs only at temperatures exceeding 1650 "C. Si. (h). + 4NH.[56* 7 1 5 Angebi. + 4NH.N. (c) and (d)] .N.40-421 However..N. Polymeric silazanes of suitable structure may be processed by spinning to produce fibers or by melt or solution coating.N. in the presence of nitrogen and ammonolysis of reactive [Eq. Si. (amorphous) ca.1400°C Si. subsequent pyrolysis usually gives a 60-80 YOceramic yield..S Si. powders the three following methods are important (see Sections 4.N.4-11.371 Rigid structure and strong covalent bonds are the cause of the extraordinary hardness. too slow reaction kinetics. 3Si0. (a)]. unit cell.N. The C I / ratio of commercially available powders is usually ~ determined by X-ray 491 but also IRtsO. + 3CH... 3SiH. 4. -H2 tively complicated and costly production and handling of silazanes. (k)] or ~ i l o x a n e s .[241 since no channels occur in this structure. 25 "C Si(NH).2 1 and 5 29Si MAS NMR spectroscopy may be applied. are regarded as polymorphic modifications. 3 Si. a-Si. fibers or coatings.. [Eqs. (4 Fig 2 Section of the crystal structure of p-Si.network of SiN.1400°C ca.3-0. since the a-Si.N.1600°C + 6CO (b) (C) 600. has the empirical formula Si11. + 6H. (amorphous) + 2NH. Section of the crystal structure of a-Si.Is41 This method is also suitable for the study of sintering phenomena.7 1 1 3 Si + 2N.136. powder qualified for applications in advanced ceramics has to meet numerous demands regarding chemical 1581 Chem. (e)]. single crystals extremely low in o ~ y g e n .N.4): nitridation of silicon powder [Eq. by pyrolysis or carbothermical reduction from silazanes [Eq. durability. since pyrolysis leaves nothing but Si.N.1200°C Si.N. + 4NH. a-Si.15 nm play a certain role in the diffusion of atoms. via amorphous Si.N. which is built from a Si6N. unit Along the crystallographic c m 4. there have been numerous attempts to synthesize Si.Cl (e) 900 . or the development of undesirable particle morphologies.[761(j)[77* 781] have not gained importance for technological or economical reasons. Figure 2 shows the crystal structure of fi-Si. + 12HCI + 12H. (f) 1300-1500°C a-Si.N. (a) + 2N. Si. 3.N. view approximately along c. The Synthesis of Si. and mechanical strength of this material.[53-55] 29Si MAS NMR spectroscopy is particularly useful since it can be employed to distinguish and quantify not only the crystalline phases but also amorphous Si3N4. The reaction leads initially to extremely moisture-sensitive silicon diimide [Eq.N. Fig. [Because of the compara~~] 1 -[SIR. which is then transformed by pyrolysis.N. 1450. Si. Powders For the technical synthesis of high-quality Si. t91 moves with a speed of up to 0. The spectrum of all these properties influences the general sintering activity of a powder. A variation of the direct nitridation described here is the so-called SHS synthesis (self-propagating high-temperature synthesis[” .N. crystallinity. In order to minimize the efforts spent on powder processing. powder shaping has to aim at particle packing densities as high and as homogeneous as possible.901). and exclusion of coarse or hard agglomerated particles. Some powder properties. charge size. Process for Si. which requires the usual processing by milling and acid leaching. high sintering shrinkage) as particle sizes decrease. 4. powder.. exclusion of extraneous particles.and physical properties. Engl. To minimize shrinkage and to avoid the development of density gradients during sintering. Besides commercially available. particle size distribution.[”] For the actual synthesis. are important characteristics of powders with good sintering behavior and favor the development of a homogeneous grain structure during sintering.N4. it is possible to use semiconductorgrade silicon as starting material of extremely high purity.61.[60.N4 but also strongly sintered agglomerates of Si. U Si Powder NRridation E 5 a-Si3N4 powder Drying Fig. 4. These factors. Here the reaction proceeds very fast (“thermite-like”) after local ignition under increased N. further investi971 gation is required. primary particles. h i . possible metallic impurities due to milling wear are removed by subsequent acid leaching. may be influenced subsequently by suitable powder processing (e. Whether SHS synthesis will ”3 Angew. which will not crumble into dust particles but which is easy to densify. However. that is. synthesis by direct nitridation.g. -”I Und er these conditions the reaction zone pressure. like particle size. The selection of silicon powders of different purities strongly influences the impurity level of Si.[60.[60. screening. mean particle sizes less than 1 pm. [ ~ ~ . milling. A free-flowing granulated Si. others.[951Other open questions in the further development of SHS synthesis. milling. particle shape.1 msec-’ through the powder bed with temperatures often exceeding 1700 “C.[60.main ]processing steps are nitridation of silThe ~ ~ icon powder. This is achieved by defined particle size distributions. is favorable for charging automated dry presses and may illustrate this point. 831 Especially traces of Fe catalyze the nitridation. Roughly. particle size distribution. powder packing density.[96.N.61.[^^-^^^ 941 concern the control of the cr/P ratio and the mastery of costefficient.61. and type of furnace. and a high crib ratio. spherical particle shapes.It may also be linited by a suitable. low-cost. To what extent this approach is technologically feasible and economically appropriate depends on the synthetic method chosen.N. degree of particle agglomeration. the search for universally applicable powders often requires a compromise between opposing demands. in turn.The evolution of heat can be 82] reduced by lowering the concentration of nitrogen in the nitriding atmosphere after onset of the reaction (“exothermal control”r861). offer possibilities for special “powder designs”.N. Generally. powder is suitable for manufacturing advanced silicon nitride ceramics has been answered positively only sporadically. 30 (1991) 1579-1597 . Synthesis by Direct Nitridation of Elemental Silicon Direct nitridation of elemental silicon-the process used more than 80 years ago to show the correct stoichiometry of silicon nitride to be Si. which may also be used for the production of other refractory corn pound^. These circumstances.[82. Si. Depending on the powder processing.750 kJ mol. Careful control of the exothermic reaction is also necessary to avoid an uncontrolled temperature rise exceeding the melting point of silicon in the powder bed. as well as technological properties like good processing behavior in different powder shaping techniques and good sintering activity. depth of powder bed. or C and 0 content. it is advantageous to adjust as many of the required properties as possible during the synthesis. metallurgical-grade silicon of different levels of purity. tailor-made powders with special advantages in certain manufacturing processes. have to be adjusted during the Si. and cr/P ratio.[801-is still the dominant procedure for the industrial synthesis of silicon nitride powd e r ~ . often empirically determined temperature control depending on nitridation kinetics. large-scale production.N. sintering activity increases with decreasing particle size. Important criteria for powder quality include purity. and finally purification of the crude product (Fig.The product is a high-/3-phase material (/3 > 95%). piles of silicon powder are allowed to react with nitrogen either discontinuously in chamber furnaces or continuously in conveyor-type pusher fur1582 naces according to Equation (a) (AH = . bad densification behavior in dry pressing. or annealing).N4 synthesis.2. which finally require a more intensive milling process. 4). Reaction rates strongly depend on particle size and chemical purity of the silicon powder.841 Ow’ to its ing strong exothermal character the reaction has to be controlled carefully. 821 After synthesis the sintered product is crushed and finally milled to the desired particle size. low temperatures and the presence of hydrogen in the nitriding gas favor the formation of cr-phase material. Therefore.’). The reaction proceeds at a reasonable rate only above 1100 “C. Chem.61* 851 Too high temperatures cause not only the formation of undesired P-Si. like the c r / j ratio. Ed. powder processing becomes increasingly difficult (dust generation. together with defined C and 0 contents.[951 far the So question whether high-P-phase Si.N. 345 atm.N4 formation generally increases with increasing temperature. reaction mixtures of quartz sand. Eqs. where the CO partial pressure is kept low by flowing N./C ratios of 1 :2 to 1 :I0 are employed. 8- I LI 0- +C SiO + CO SiO + C Si + CO 3Si + ZN.99. . and C powders from. 6).[99.SIClSI + Clsl (1) 05 (m) 06 07 08 1000~ T-'[K-'I - 09 Fig. may be removed by H F leaching. incompletely reacted SiO. atmosphere. are formed at temperatures exceeding 1450 "C. Synthesis by Carbothermal Reduction This method uses the reaction of SiO. . Si. Thermal cracking yields an extremely homogeneous SiO.15. or chlorine-containing atmospheres['". 12- 1800 11001200 - TIOCI 1000 800 I ii Cl dl -. Engl. 3Si0. furfurol) yield easy-to-handle polymeric granules containing Si. --* - . c) 0. + 6CO SiO.N. By tempering in air.4. when P e r ~ o z [isolated a white precip'~~~ 1583 . coarse. natural products like quartz sand[981 and clay[1oL1 may also be used. 6.01 atm. synthesis by carbothermal reduction. Ed. Int. 0 S O p Powder QI I Mixing Carbothermal Reduction 1450 . Chem.l l . although at the expense of purity.N4 particles or Si. and NH.N.[62-66.N4 whiskers as nuclei to the reaction Since carbon acts also as a nucleation center for Si. Si.N4 formation.allow direct manufacturing of high-quality sintered components is another interesting issue.[661 NH. SiO. loS1at temperatures in the range of 600-800 "C excess carbon is removed. Si. N4 Synthesis by Liquid-Phase Reaction o SiCI. powder characteristics as Angen. 30 (1991) 1579-1597 well. synthetic materials from pyrolysis[621 sol-gel or reactions.[632 I'' 99. increasing amounts of Sic. lg PN. A very good alternative is provided by sol-gel-type methods. low-surface powders favor whisker formation.1 atm. dates back to 1830.1550 "C. "] There was no concern about chemical purity or particle morphology.[' 0 5 ] Because carbothermal reduction may also proceed reversibly. Usually.. 0./C under varying CO partial pressures (1061. (I). b) 0. and NH.. d) 0.N. Though stoichiometry requires a SiO.1031Very good results yield extremely fine SiO. + 6 C + 2 N 2 Si. 3 4 Particle size and particle morphology may be controlled by adding either crystalline spheroidal Si.N. Ammonolysis of Reactive Silicon Compounds 4.N4 as starting material for industrial-scale ammonia synthesis. f The work on the liquid-phase reaction of SiCI. carbothermal reduction attracted early interest during the first attempts to synthesize Si. Since cheap and readily available silicate raw materials may be used. carbon. (a). If required. depending on CO partial pressure. its particle size influences Si3N. 4. 1061 (Fig. CO formed in the course of the reaction has to be removed. in which polycondensation of silicon halides with alcohols prone to polymerization (e.4 for a complete reaction. powder with carbon at temperatures of about 1500 "C in a N. Nz 1 HCllHF Acid Leaching Drying I ~~ a-Si3N4 Powder ~ I Fig.3. All compounds are present as solids./C mixture." Modern procedures ensure chemical purity by careful control of the purity levels of the starting materials. producing Si3N4via a series of reactions [e. Besides high-purity. In practice. and C in ultradisperse distribution.1. Purification of the product from metallic impurities and nonreacted carbon was first achieved by acid leaching and oxidative treatment. silicates./SiO.N4 particles. 5. which undergoes ready carbothermal reduction. a considerable excess of carbon is required in practice.. high-surface carbon powders favor the formation of spheroidal Si.4. Very fine. S I ~ N ~ I+S ) Clsl 4. 1oz.l . and (m)].1021 After synthesis the reaction product is milled to adjust the desired particle size and the particle size distribution (Fig.[62*o 3 . and coke were even compounded with metal oxides to decrease the reaction temperature. Process for Si. for example. the reaction is carried out either continuously in conveyor-type pusher furnaces or in chamber kilns. the S i c is practically inseparable from the Si N C65. Stability diagram for the system SiC/Si./C ratio by weight of just 1 :0. l An intensive homogenization of the powders is necessary to ensure that the reaction is homogeneous and proceeds to completion. The overall reaction is given by Equation (b).g.N.g. 5). Though Si. pco: a) 1 atm. gas-phase pyrolysis. 7).N.]. or SiH. At temperatures exceeding 1500 "C mainly pSi. however.N4 + 12NH4CI (0) 4. Amorphous S y b i Crystallization Fig. 30 (1991) 1579-1597 1584 . accompanied by particle coarsening. is formed. moisture-sensitive.N.C1 still present from the Sic]. is the formation Angew. + 4NH4CI (e) process. generated according to Equation (0). with increasing temperature. though. powder from easily purified and readily available starting materials. . processable a-Si3N4via several intermediates and amorphous Si.:::./NH. 8)./NH. Int. particles cluster to aggregates and agglomerates.[' z31 After formation and growth of nuclei in the reaction zone.4. or N. Subsequently. Because of spontaneous ignition of SiH. this reaction may be described in its most simple form by Equation (e). 8. Engl..N3H.[67-69s112-1171 The transformation of amorphous silicon nitride to crystalline a-Si. precipitate is filtered and washed with liquid ammonia to remove residual NH.CI by-product through either hot-gas filtration. The Si(NH)../NH. the use of expensive SiH.N.]. with NH. However.[' 15] In the absence of NH. Si(NH).itate. 7. .t - Si.::::zone o reaction and f i. which he considered to be silicon tetraamide.CI by-product./ NH. reaction [Eq.[' '*O] Since SiCI./H. (n)]. proceeds above 1200 "C in a diffusion-controlled manner with an activation energy of 306 kJ mol-l. liquid-phase reaction allows the synthesis of a very pure and fine Si. extraction.CI. powder showing primary particle sizes of about 10-30 nm and BET surfaces greater than 100 mz per g. + 3 H. 1 2 1 1 or by reacting SiCl. is formed. vapor with 11 thus dissolving the by-product excess liquid NH. Particle formation in gas-phase reactions [123]. However.2. The SiCI. ..N.***.[l 1 8 ] -[Si(NH). until finally amorphous Si. synthesis by SiCI.CI separation by sublimation.Cl and finally calcined and crystallized to give a-Si.. 3 1200-1400°C a-Si. For economical and technical reasons gas-phase reactions of SiCI. From a technological point of view the use of cheap SiCl. at temperatures below 1200 K leads to the formation of an extremely fine. Si(NH). j **Pi ) nuclei formation 2 : ) nuclei growth . Chem. this method (often referred to as the diimide process) has been developed on a pilot scale (Fig. gasphase reactions have a great potential for the production of extremely tine powders. reactants tube reactor :=::t:. this procedure offers the technological advantage that no corrosive NH3/HCI vapors are generated. . Process for Si. Ed. is stable to 200 "C.CI Separation NH4CI I t I Si(NH)Z 1 Fig. in contact with air. In contrast to thermal NH. :.CI.CI. . + 6NH. are of special interest. Si. A technological advantage. it* *.:t*. retention time. requires extensive safety precautions. + 16NH. An economically favorable and ecologically safe application or disposal has yet to be found for this by-product. 1 1 7 . requires the separation of the corrosive NH.[' "1 Subsequent high-temperature pyrolysis leads to stable. and impurities. further investigation showed a very complex SiCI. liquid-phase reaction.: organic solvent -30 to 0 "C Filtration NH. it gradually loses NH. reaction is carried out in a two-phase system consisting of an organic solvent and liquid NH3[72* 6 . *. liberation [Eq. Prolonged heating at temperatures of 1200 to 1400 "C induces crystallization of or-Si.CI results in reaction of Si(NH). this precipitate was iden. Owing mainly to homogeneous nuclei formation. however. with NH. accompanied by NH.N4 Synthesis by Gas-Phase Reactions of SiC14 or SiH. which led to differently polymerized.69* '' Th ough tified as silicon diimide.N. thermal decomposition in the presence of NH.[691 Particle morphology. 193 It is remarkable that NH. or sublimation.t - SI(NH). particle size. upscaling has to take into consideration the large amount of NH. a/p ratio are influenced by temperature.). pyrolysis. (e)] influences the Si(NH).N. which may be separated as fine dust (Fig.N.N. amorphous Si. which decomposes to amorphous silicon nitride above 800 "C. and HCI to give a compound of the composition Si.**. + N. Si(NH.. Pyrolysis of Si(NH).N. ..N. . extremely moisture-sensitive [Si(NH).*. and 3SiC1.I' NH. and retention time is important for the generation of very fine. Engl./NH.N. ilar to industrial-scale synthesis of SiO. Figures 12 and 13 show experimental setups for laser-induced and plasmachemical Si. thus SiCl.N.1000 "C NHpCl Separation Si(NHh. uniform. As liquid-phase reactions. which are based on chemical reactions already described above. the BET surface may be adjusted in the range of 2-20 m2g-1. (p)]./NH. these reactions represent a good technological basis for a continuous. and NH. particles by crystallization of amorphous Si. Process for Si. are extremely fast heating and cooling rates of about 10' to lo6 K s-' and fast reaction rates on the order of about s.of hydrogen as gaseous by-product. The powders from SiCl.N. Angew.N. or SiCl. high-a-phase Si.. The reaction of SiH. gas-phase reactions at temperatures of 300-1700 "C showed the formation of extremely ~~ Fig. rity./SiCI. a-Si. SiH. Chem.291 and plasmachemical reactions" 3 0 . which is easily separated. reaction parameters may be adjusted to approach the stoichiometry of the reaction [Eq. an extremely high degree of particle fineness and uniformity is guaranteed. stoichiometric ratio of reactants. with NH. 10. + 2NH. + 4HCI (P) keeping the amount of solid NH./ NH. i n t . gas-phase reaction. Characteristic properties of both processes.N. powders.CI by-product low compared to the corresponding liquid-phase reaction. crystallinity.-NH. gas-phase reaction. synthesis from reactions of SiH. and high sintering activity. and TiO. 9. Synthesis by Laser-Induced and Plasmachemical Reactions The work on laser-induced" 26. Si. spheroidal and amorphous particles with BET surfaces up to 300 m2g12'] Crystallization of these precursors induces particle coarsening. and NH. If SiCI. which allows only product formation rates on the order ofjust a few grams per hour. thus.[70. good manufacturing properties. Ed. - Si(NH).341 as alternatives to "usual" thermal gasphase reactions tries to avoid any heterogeneous nuclei formation due to contact of the reacting gases with hot reactor walls.N. amorphous Si3N4 filtering system for separation of solid products Amorphous Sit$ Fig. precursors because of the short retention times of gas and particle streams in the hot reaction zone.N. 9). (Fig. temperature. synthesis by SiCIJNH. from SiCI. 4. a / p ratio. Because of the extreme temperatures of several 1585 ' Fig. These amorphous precursors have to be crystallized separately in a subsequent step.N. '11 Our investigations on SiCI. 11. gasphase reactions require significantly less processing (Fig.. 11) than products from direct nitridation or carbothermal reduction because of their inherent fine particle size and weak agglomeration. in the temperature range of 500-900 "C yields amorphous powders with particle sizes of 30-200 nm and BET surfaces up to 26 m2g. Sim- fine.1600 "C Sublimation500 .5. gas-phase reactions also yield amorphous Si. 30 (1991) 1579-1597 .N. Instead of using expensive pressure vessels. large-scale production of fine powders.. pressure. Particle size. thus. powder by crystallization of amorphous Si. Setup of a gas-phase reactor for the synthesis of Si. and laser or plasmagenerator power output. and stoichiometric composition of the reaction product are influenced by mass flow. They are characterized by high chemical pu- ' Gas-phase Reaction electrical heaters 300 . gas-phase reactions are run at atmospheric pressure in heated tube reactors (Fig. from SiCl. 12'] Strict control of impurities. is used. 10).[". A major problem of laser-induced synthesis is the small reaction diameter of the laser beam. 30 (1991) 1579-1597 1586 . a stream of inert carrier gas is used to guide the Si vapor into a reaction zone in the upper part of the apparatus where it reacts with N. Ed. the same methods as in the synthesis of spheroidal Si. Both laser-induced After the discovery that crystal whiskers show unusual mechanical strength!' 351 interest in the properties. Silicon is evaporated from a crucible at the bottom. powder particles are used for the production of Si. Si.N. Chem. In!. the reactants are broken apart to different radicals. from SiCl. However. Angew. __c gas outlet 1 n. despite intense research.N. whiskers precipitate onto graphite baffles from where they may be collected. 12. whiskers. the products often show incorrect stoichiometric composition. Whiskers from the Reaction of Si and N. Reactor for laser-induced Si. 14. which may recombine in various ways in the temperature gradients of the reaction zone. n v h pon for temperature measuring fquenching * gas ' quenching zone +filter reaction and nucleation zone Fig.N. In direct nitridation of elemental silicon.N. 13. powders.window water-cooled "--ilowing reaction gas inlet (SiH.N. SiCI.1. NH3) II gas inlet Fig. there are still problems in producing pure and uniform whiskers on a large scale in an economical way. production.N. gas-phase reaction [140].N. . Since fluid dynamics is also very important. Figure 14 shows a simple experimental setup for this production method. are amorphous. Engl. Furthermore. thousand K in plasmachemical reactions. synthesis from SiH. laser-induced and plas- Fig. Reactor for Si. N. not exactly reproducible a//3 ratios. whisker synthesis from a Si/N.N. and NH. carrier gas ii sedimentation and plasmachemical synthesis allow the generation of extremely fine powders with particle sizes of just 10-30 nm and BET surfaces greater than 100 m'g-'. H20 H20 c plasma gas 5. Silicon Nitride Whiskers laser *." 38. inlet =+ stabilization gas 3 + cooling gas induction coil . they may contain free silicon as an undesired by-product from decomposition reactions. [133].beam \."36313']The use of whiskers to reinforce metallic or ceramic materials aims at improving fracture toughness or. 5. more generally.filter n da :le machemical reactions are not yet of importance to largescale synthesis of Si. . and applications of ceramic whiskers grew. additional measures are required in order to apply general techniques like vaporphase transport or crystallization by vapor-liquid-solid (VLS) mechanisms to favor whisker formation. However. [126]. and NH. or NH. The Si. mechanical properties of the matrix material. or possess changing. Reactor for plasmachemical synthesis of Si. Because of these difficulties. it is very difficult to control the reaction precisely. In spite of the strong general interest in ceramic whiskers. whiskers are formed by reaction of silicon vapor and N.. 1391 Chemically. The development of whisker production and processing methods especially has to take into account health precautions in order to exclude a potential hazard like that arising from asbestos fibers. Si. yields whiskers from the gas-phase reaction of SiO and N. polymer. 900 .1500 "C (Si-S-N-H)wiyme.N. with retention of morphology..CI. spirals by vapor-phase deposition from a mixture of Si. and packing density of the reaction mixture of silica and carbon raw materials. 15). 15. NH. which thermally decomposes under NH to amorphous silicon nitride [Eqs. Inr. ca.or silicate-containing eutec1431 tic melt droplets[72* (Fig.['431 Fig. [-' 1431 By adjusting stoichiometry. If small amounts of Fe or SiO. SiS. leads to SiS..2. Si + 2H. with NH. Whiskers from the Reaction of SiS. whisker formation is optimized. whiskers by crystallization of amorphous Si. the reaction of SiS.S 900 "C SiS. Reaction of SiS.N. (amorphous. The reaction of S i c with NH. [ ~ In -the~ I step. Originally intended to produce high-purity Si. which may be run continuously in conveyortype pusher furnaces..5. ~~ homogeneity.N.. (n) The high vapor pressure of SiS. This process. Right: a-Si. Whiskers from the Reaction of SICI. or NH. Recently. to 1440 "C followed by a short-term temperature rise to 1480 "C. onto a Fe-coated graphite substrate [144].S at high temperature [Eq. which yields amorphous Si. which forms on dissociation of SiS. with NH. with NH. 5. Some of these spirals could be seen with the naked eye. whisker growth by a VLS mechanism from eutectic droplets of SiS and sulfur vapor. and control of the reaction temperature. r-Si. and H. Glemser und Horn1761 obtained spiral-shaped a-Si.N4 particles of unusual morphology. SiS. 25-9OO'C (Si-S-N-H).. with NH. + 2H. 30 (1991) 1579-1597 1587 . [72].N. (q)]. is remarkable since it leads to Si. Ed.N. similar observations have been made in the investigation of vapor-phase deposition of Si3N4from Si.CI. Engl.N4 ~ h i s k e r s . is synthesized by ~ ~ first reaction of Si powder with H. Amorphous Si. addition of silicon or metal oxides.. left). in CO-containing atmos p h e r e ~ . Whisker formation is also favored if Si3N4 whiskers are added to the reaction mixture as growth nuclei. consequently.N. the reaction of SiCl. Si. leads to Si(NH). (r) and (s)].N. with NH. polymers [74]. in presence of Fe [76].N4 on calcination.5. 16. 5. crystalline) (s) 5. As described above. whiskers (Fig.. "spaghetti" from thermal decomposition of (Si-S-N-H). the amount of SiO generation and. The amorphous silicon nitride is obtained in "spaghetti-like'' morphology (Fig. Angen. Chem. 17 Left: a-Si. By heating a mixture of S i c and Fe powder in NH. particle size.4. 16) and may be crystallized to cr-Si.. . Si... NH.N.. offers a good way to produce Fig.3. Si3N4whiskers are formed by a vaporliquid-solid mechanism from Fe. NH. Whiskers from Carbothermal Reduction of SiO. spirals from the reaction of Sic and NH. affords a (Si-S-N-H).N4 powder. 17. Whiskers from the Reaction of SIC and NH. Fig.N. are added to this material and the resulting mixture is heated to 1200-1500 "C in the absence of oxygen. and particle surface properties. 5000 ppm max. For technological and economical reasons. H. may be carried out after Si. Toshiba Ceramics. [c] GP = SiCI. the originally intended toughening of the matrix actually turns out to be a ~ e a k e n i n g . a spiral diameter of 5-10 pm.N. several precautions have to be taken in order to prevent excessive decomposition during sintering of Si. since heaps of needlelike particles are much more difficult to compact by. Properties of commercially available Si.N. Si.18g~m-~ 4000. Physical and Chemical Aspects of Si. The production of Si. Ini.7-2. The Manufacture of Sintered Si3N4 Ceramics 7. Starck. Ind. Tokyo).200 pm 0 3 . (solid) -- 3 Si (liquid) + 2N.1. Often variations of particle sizes.4 pm 5-20 pm 10-200 3. Direct nitridation and SiCl.N.N. 1 ’ ~ ~ . Ube Ind. Because of their inherent fine particle size and only weak particle agglomeration. fracture-causing structural defects are often induced during material processing.. Berlin).2 <O./NH. 5. [fl Not determined. H. since Si. Berlin). [el CT = carbothermal nitridation (A 200. Product ~~~~~~ Tateho SNW [a] ~ ~ ~ Ube SN-W [b] - Elemental analysis [wt %] N 38. Starck. directly.03 <0. high-purity Si. dry pressing than heaps of spheroidal particles. particle size distribution. U Mean particle size d. the reaction products require extensive subsequent processing. whiskers. [a] Tateho Chem. [d] FP = SiCI.001 Fe <0.0 0.NH.1.9 Synthetic method Crystal phase Diameter d Length I Aspect ratio / / d Density Fe A1 Ca Mg carbothermal reduction U from amorphous Si.N.N. Properties of Si. Because of technological dif- Table 2.N.6 0.8 0. 1000 ppm rnax.1.8-2. Japan [72]. carbothermal reduction.005 <0. spirals are formed only in a narrow fiber-diameter range of about 0. whiskers are shown in Table 1. However. say. Sintering “Dry” sintering techniques used in densification of oxide ceramics cannot be applied to the sintering of Si.N.. synthesis by suitable powder processing. Fundamentals of Sintering and Structure Development 6. a-Si.1-0. the extent of subsequent powder processing is considerably reduced compared with direct nitridation and carbothermal reduction..6.N.5-1. [b] Ube Ind.008 A1 <0. They show a quite uniform morphology with a regular pitch of 3-5 pm.01 <0.N.06 tO.1 C <0. at high temperatures.N.2 CI <0. 17.5 138.5000 ppm < 100 ppm < 100 ppm not specified 2-3% [a] DN 1 =direct nitridation (LC 12 . and ammonolysis of reactive silicon compounds are principally able to furnish very fine. H.002 <0. Tokyo)./NH. especially carbothermal reduction and whisker growth from amorphous Si. C. gas-phase reaction (Grade GP. Powder type DN 1 [a] DN 2 [b] GP [c] FP [dl CT [el Table 1. ammonolysis of reactive silicon compounds yields moisture-sensitive products which require further thermal treatment for crystallization to yield 1588 7.. On the other hand. Chem.005 Ca t0. Whiskers Some properties of commercially available Si. whiskers in a matrix material. 2000 ppm max. powder compacts.6 <2. C.0 1.r144. > 92 Y o ~ ~ 38.004 <0.2000 3.6 pm 5 . whisker-reinforced materials have not found broad applications because of high costs and a lack of large-scale production methods for reliable components./NH3 liquid-phase reaction (SN E 10.SX. slightly contaminated by carbon and oxygen. Engl.01 <0. (91. Starck.5 37.05 <0.1. Most important is the addition of comAngew.01 98 Y o 0. powders for applications in advanced ceramics (Table 2). C. should be suited best to meet an eventually increasing demand.N. at 1200 “C onto a graphite substrate coated with Fe-containing compounds. whiskers is not yet commercially important.N. Obviously.01 <0.0 pm thick and consist of a-phase Si...1-1.2 1. 30 (1991) 1579-1597 .2 0 1. and H. [b] DN 2 = direct nitridation (LC 10.~ max.1 -5 pm.1 . [pm] 0.N.N. ficulties in producing a homogeneous distribution of Si.N.N. Japan [143]. powders from different synthetic methods.007 0. gas-phase reactions seem to offer the best chances to furnish economically high-quality powders for the manufacturing of broadly applicable silicon nitride ceramics.2 0. Berlin). right) are about 0.0 <0.OOi < 0.Fundamental problems in whisker processing are often connected with shaping and sintering. and a length of 50. 1451 The spiral-shaped whiskers (Fig.5 0. Strong covalent Si-N bonds hinder useful mass transport through grain-boundary or lattice diffusion. Direct nitridation and carbothermal reduction use cheap raw materials and yield crystalline Si.5 2. Comparison of high-purity Si.N.002 a-Si. The activation of diffusion by simply raising the sintering temperature is limited by ever-increasing thermal decomposition [Eq. 7. Ed. The principal problems in whisker production are the requirements of high chemical purity and uniform whisker morphology.9 [fl 0.0 < 0.2 <0.N.1 <0.100 pm. 2000 ppm not specified 0.O1 > 94 Y@ ~ 38.18 g ~ m . (gas) 3Si (gas) Since a N. in these cases.005 >90% 295% 0. Remarks on the Procedures for Synthesis of Si3N4 Powders A comparison of chemical and physical characteristics of powders from commercially used synthetic methods shows that direct nitridation. which are important to ceramic processing. partial pressure of 1 bar is reached at about 1900 “C. or Si. 18./Al. powder [158]. The efficiency of these mechanisms is illustrated by a model experiment using a presynthesized. Int.N. This process allows grain growth with minimization of boundary surface energy Fig.N. Additives: 1 5 w t % Y . sintering.[148-152] Above 1300 "C sintering additives react with SiO. In the case of sufficient wetting and solubility of Si.O.N.N.N. Silicon vapor pressure in equilibrium with silicon nitride as a function of N.N.N. Engl. The hatched region represents the range where sintering of Si.O always present in Si. nitrogen-saturated Mg-AI silicate glass as sintering additive (Fig.2.. 30 (1991) 1579-1597 1589 . If P-Si.[155.4 1 0 . starting powder. At pN2 Iatml1 0 . 7. dissolution/reprecipitation occurs almost in equilibrium.. then 1820 'C (2 h).N. which increases the packing density almost without influencing particle shape and particle size distribution. since nitrogen saturation of the silicate glass does not allow further dissolution of N. Ed.N. The surface energy difference between small and large particles causes a continuous dissolution of small particles and their reprecipitation on large grains. solubility increases and densification via dissolution/reprecipitation is observed. bold lines show temperature interThe vals of 100 K from 1300 "C to 2200 "C.~ 1 0 . Fig. Middle: 1800 "C (2 h). this grain morphology is desired. C'hrm.-doped Si. Starting from a-Si. Figure 19 shows that this technique significantly extends the temperature range of Si. [ ' ~ ~ ] a fine-grained.N.pounds that form molten silicate phases and allow densification by liquid-phase sintering mechanisms. Structure Development t [min] ----. takes place. 19. Well-suited additives include alkaline-earth oxides and rare-earth oxides either alone or in combination with AI. Angew. Right: SO sintering cycles (see middle for conditions). O . powders to yield molten silicate phases.N4 grains far from the energetic mini m ~ m . sintering in the absence (1) and dS/df presence ( 2 ) of a nitrogen-saturated glass [153].O. or A1N. Temperature -shrinkage dS/df in [%min-']. from almost saturated or slightly oversaturated solutions leads to crystallization of needle.or rodlike P-Si. + 3.. 15@. these silicate melts allow densification by capillaryforce-induced particle rearrangements and dissolution/reprecipitation processes. 20. With increasing temperature Si. sample without sintering additives shows a completely unsatisfactory densification. true sintering by dissolution/reprecipitation occurs at higher temperatures. Left: 1800 "C (10 min).O. Another important precaution is sintering under high N.N. reprecipitation of dissolved cc-Si. After particle rearrangement at low temperatures. At present. pressure to suppress decomposition according to Equation (t).O. 1300 "C dilatometric monitoring of the process shows densification by capillary-force-induced particle rearrangement.N. structure development is thought to depend on the kind of Si. powder is used. needlelike structure has a Since positive influence on fracture-mechanical properties ("insitu whisker reinforcement") for numerous applications.~ 1 0 -1 ~ 10-2 10-3 101 102 l o 3 3 2 PSi [atml 1 I 10-4 1 0 10-6 -1 lg Psi lo-' 10-8 -2 -3 2000 10-10 10 1 2 3 4 5 6 7 8 I 1800] lg PN2I SXlO [%] dSldt Fig. A Si. The pressures were measured in Pa. Dilatometric investigation of Si.N.4 w t % AI. pressure and temperature (1491.N. Structure development in sintering of a Y.1. or Si. 18).. A a grain triple point. and. which sin- Besides the kind and amount of additives. is shown in Figure 20. it is necessary to break up particle agglomerates and to produce a homogeneous distribution of sintering additives. ceramic proper- Different additive systems may form compounds of the type a‘-Me(Si. favors grain growth by minimization of boundary energy (“Ostwald ripening”) also in needle-grained structures and leads to grain coarsening with growth of spheroidal grains.N. structural development leads to the formation of globular grains.N. Influence of type of additives on sintering behavior. B a grain boundary.-doped The influence of typical additives on sintering behavior is shown in Figure 22.O. Chem. 23.-doped Si. + 20001 1800 r20 s [%. in particular.O.O). no matter whether a.S = shrinkage. may proceed at the low temperature level of (MgO Al.---YY. the densification of Y. are of importance to the course of sintering. Figure 23 shows a comparison of these materials with a material sintered with MgO/AI.4. is added. or complete elimination of this phenomenon is of major importance to the optimization of Si. The black lines show the directiop (lOTO). far beyond the moment when densification is complete.-MgO + + Fig.N.1631 The expectation that Si.N. with newly precipitated Si. ceramics free of secondary grain-boundary phases can be produced by using this principle has only been fulfilled for Be-doped 1601 The amount and kind of additives also influence structure development and thus material properties. 7. 22.01Ns-. powder. Relevant powder properties can be divid- Fig. systems doped with rare-earth oxides). Transmission electron microscopy (TEM) of amorphous secondary phase along grain boundaries in sintered Si. The exact reasons for this behavior are not yet completely understood.O. A1.I { 20 ] ] ] 40 60 t [min] - 80 100 Fig. has been used as starting material. Int. since they determine sintering kinetics.g. 21. 30 (1991) 1579-1597 . Of further importance are the powder properties resulting from processing prior to shaping and sintering. which developed a needlelike structure..N. The influence of additives is especially clear in the structure development of /3’-SiAION materials. which is formed by sintering additives. right: needle-type structure in a MgO/AI>O.)-doped materials if AI..[162.---YY. Fine-grained Si.O.O. Influence of Sintering Additives 7.. finally. ture.O.. For instance. material properties depend strongly on the powder characteristics of the starting Si. For example.3. or /3’-Si6-xAI. material properties at high temperatures are greatly influenced by the secondary phase. In this case.. grains.O.O. [159].~ -temperaA1. all these materials contain an amorphous or semicrystalline secondary phase in between grain boundaries or in grain triple points (Fig. the sintering behavior. After sintering. 21).(N.N. the reduction. 1590 Angew.-doped material. Influence of Powder Characteristics The amount and chemistry of the molten silicate phase. where globular grains are favored.1. whereas the rate of diffusion is crucial in Y.N..or P-Si. A study on structure development on prolonged heat treatment of a-phase Si. Left: Globular structure in a F-SiAI-0-N material. .N.N.O. modification. structure development. the kinetics of MgO-doped systems are governed by dissolution. Engl.leading to the development of approximately globular Prolonged thermal treatment..AI). structures are favored in systems where the molten silicate phase shows a relatively high viscosity at sintering temperatures (e. Since especially ters at relatively high temperatures. Ed.1. matrix to form silicides or nitrides. which results in enormous shrinkage during sintering. particle morphology. chemical. matrix. particles cause similar effects. and Ti react with the Si. ratio has already been mentioned and is still a subject of intensive discus~ i 0 n .N. since it reduces the oxygen content of the powder with formation of volatile SiO and CO. 2 * 16’] A high a-phase content enhances sintering activity and favors the development of rodlike PSi. structure development. amorphous Si. inhomogeneities due to chemical reactions are larger” and thus generally detrimental to mechanical properties like fracture strength (Fig. its large-scale use is complicated owing to its sensitivity towards hydrolysis and its low powder-packing density. it is a compromise between beneficial effects on sintering behavior and detrimental effects on high-temperature material properties. since their size and the lack of sintering additives in their cores would induce very large defects and thus limit strength and reliability of the sintered component. it is directly available for silicate phase formation by reaction with sintering additives. Thus. which allow fine-tuning of the amount of sintering additives required for complete densification and thereby result in improved material properties. Fig. Since sintering activity increases with increasing surface energy. In this way.or fiberlike Si. The coating of a Si. The amount of “coarse grains” attracts special attention. Also the distribution of oxygen in the particle itself is of importance.N. the oxygen content has to be adjusted to the amount of sintering additives required (or vice versa). Although a great amount of low-viscosity silicate melt phase enhances sintering. grains. Metal particles like Fe.[’ 5391 6 9 1 Chemically bonded carbon in the form of SIC is much less critical.[’66316’] Therefore. are broken apart. 24. particle with an amorphous layer rich in oxygen is shown in Figure 24. particle with an amorphous layer rich in oxygen [168]. this material may be considered as inert 1 filler. lowering its viscosity with the already discussed effects on sintering behavior. 1 300200100- a[MPa] & y pm scattering range surtacecrack I I I I I I I 200 400 600 800 Size of extraneous particles [pm] - I I Fig. finally poor Angrn.N. Usually.N. Free carbon has a negative influence on sintering behavior. Because of its inherent stability. Mn. The influence of ct/P-Si. they may dissolve in the liquid phase with incorporation in the silicate structure or they may remain as particle inclusions with or without reaction with the Si.ed into physical. “Coarse grains” greater than 1 pm. undesired structure development. Beam direction (T2TO). 25. which are principally present. Furthermore. CIwm. Whisker. and particle agglomeration. the size and hardness of these agglomerates determine dispersibility and homogeneity of the sintering admixture. Therefore.N.N. Metal oxides and metal halides either evaporate during sintering or dissolve in the liquid phase. since it influences the softening behavior and the amount and viscosity of the silicate phase formed on reaction with sintering additives. 25).[’701Compared to original particle sizes. surface oxygen improves sintering activity considerably. Engl. However. which together determine processing behavior and sintering activity in a complex way. If oxygen is concentrated on particle surfaces. as well as deviant particle morphologies. since these particles will dissolve in the silicate phase comparatively slowly.s 5 . 30 (I9911 1579-1597 Granulometric powder properties. Dependence of fracture stress u on size of extraneous particles in Si.N. With respect to sintering activity. glassy grain-boundary phase. Ed. the oxygen content is of special importance.t108*4 7 1 The effects of other impurities have to be judged by their behavior in sintering. commercially available powders possess an oxygen content in the range of 1 -2 %. sintering is slowed down and some particles may survive more or less unchanged to form structure inhomogeneities. additionally making powder shaping more difficult. I1~4~1.N. TEM picture of the coaling of a Si. powders usually possess oxygen-enriched particle surfaces. A qualitative and quantitative description of agglomeration mechanisms and agglom- 1591 . which involve particle size distribution. The arrows indicate Schottky partial dislocations. Among chemical powder characteristics. l n l . Powder processing has to guarantee that agglomerates of primary crystallites. it has a negative effect on the high-temperature properties of the material if the silicate phase remains as an amorphous. influence processing behavior and the extent of powder processing required to produce an admixture of high sintering activity. The resulting ill-defined variations in silicate phase chemistry may cause incomplete sintering. and.N. sintered ceramics (1711. high-quality Si. “sub-pm” powders with a high content of particles less than 1 pm are advantageous. and material properties.[’68’ material properties. and technical characteristics. would be of even greater advantage. these properties are very important quality criteria. usually are not destroyed during powder processing to generate the sintering admixture.N. Inr. Binders ensure the stability of powder compacts in dry pressing.O. The processing of a Si. Engl. this step is especially critical in injection molding. Here. and technological powder properties have a complex influence on sintering behavior. if required. plates..N. All shaping techniques require certain. these tensions would otherwise lead to cracks and fissures during temperature treatment. cracks. plastic deformation of the softening material of the capsule results in uniform applicaAngew. Chem. stable slips for slip casting. Axial hot pressing in graphite matrices is used almost only for components of relatively simple geometry like rods. Since especially injection molding promises an economical. Oxygen contamination due to partial hydrolysis of Si. and final material properties. tubes. Wet processing in mills of high-energy density is well-suited for this purpose. rub&r She. 1751 Figure 26 shows that this approach leads to extremely homogeneous additive distributions. usually organic. its development has attracted worldwide attention. Right: By precipitation of Y(OH). which may be decomposed either chemically or thermally after powder shaping. or injection molding is used for powder shaping (Fig.2.N4 powder to a highly sintering active admixture aims at breaking up particle agglomerates and distributing homogeneously the sintering additives. 1592 . sinteringpowder admixture. codrying of salt solutions and Si. rigorous controls are necessary to ensure the highest possible uniformity of processing from lot to lot. large-scale production of complex components. allows sintering of complex component~. 30 (1991) 1579-1597 Extremely homogeneous distribution of sintering additives may be achieved far better by precipitation from solution than by intensive mechanical dispersion.1.2. slip casting. Sinrering The sintering techniques used for the manufacture of Si.2.l 'hydraulic liquid Fig. is of fundamental importance.N. Physical. the desired powder shaping and sintering techniques. Ed. 7. Slip casting and injection molding are favored to produce net-shape components of complex geometry on a large scale. and the material property profile of the component to be manufactured. Ceramic powder shaping methods. and plasticizers allow injection molding. The relative influence of a single powder property may be estimated by 711 means of statistical ~a1culations. the densities of the powder-shaped bodies prior to sintering) must be as high as possible to reduce the danger of critical tensions due to high shrinkage. To improve this situation the development of new organic plasticizer systems.2. or fissures.N.erate properties is steadily gaining importance as a goal of research. on economics and.. Although shaping is finished in a few seconds. or cylinders.e.'' 74. p = pressure components with simple geometry like plates.2." 721 The shape-sharp-edged to spheroidal/globular-and the size distribution of primary crystallites determine also the limits of densification and the extent of sintering shrinkage.. 27). Dry pressing is used mainly for the production of Powder shaping methods I I I I I axial molding product 7. These procedures are well-known to the ceramic industry. structure development. The extent of processing depends on starting powder quality. Green densities (i. Y distribution in Y. Technological Aspects powder 7. usually about 1025 weight percent. Left: By mechanical dispersal of Y. or cylinders. 27. Hot isostatic pressing (HIP). extrusion. on the feasibility of large-scale production. chemical. Because of the high amount of plasticizers. Powder Shaping Axial or isostatic dry pressing.O. attention has to be paid to the kind and amount of wear debris from these machines.3. 26. Selection criteria depend on component complexity. However.-doped Si.N4 and carbon contamination are important if processing is done in either water. ceramics are compiled in Figure 28 together with process characteristics. Since powder characteristics may vary considerably during processing.["~] Powder compacts are sealed gas-tight in metals or glasses of high melting point. slips['531has been investigated as well as precipitation of hydroxides from salt solutions[' 731 and precipitation by hydrolysis of metal alcoholates. processing additives.or organic-solvent-containing media. however. Fig. Powder Processing powder injection powder Droduct gypsum mold p .l'~~~ 7.r'76-'78]All organic additives have to be burned out quickly and quantitatively prior to sintering without inducing defects like pores. plasticizer burnout requires several hours to several days depending on component thickness and kind of plasticizer. dispersion agents allow the production of highly particle-loaded. This method shortens the overall time required for complete densification .1841 ~ ~ ~'~ Ultrasonic erosion is of special interest. mech. grinding.N. Laser cutting and spark erosion are under d e v e l ~ p m e n t .N. Temperature-pressure-time profile during gas-pressure sintering of Si. -20 8. Ed. as well as high-quality surface finishing. = mechanical. "hipping" allows the manufacture of Si. Cost-efficient mass production of near-finished Si. components. th.t'491 -100 7. finishing has to be done extremely carefully in order to avoid strength and reliability deterioration by fracture-causing surface defects like cracks. 1593 . only height very extensive less extensive less extensive extensive Fig. sintered materials with very good high-temperature properties due to the only very small amount-or even absence-of an amorphous grain-boundary phase. sintered components because of some defect healing. sintering at increased N. which is effective only if the stage of closed porosity above 93 % theoretical density was reached by normal pressure sintering. Engl. 28. tion of the external gas pressure on the powder compact. 30).g.) 1 < 182OOC i7 near nel-shape component presintering (to 293% th. A < 2100°C <5h 0. 28). Mainly cutting. Densification occurs with very precise retention of shape. corrections of shape with adjustments of tolerances.1 MPa N2 <5h 2 10 MPa N1 product process characteristics linear shrinkage finishing dense body 1 HIP < 2000°C <2h 2 200 MPa (Nt + Ar) + encasing LJ 218% G near net-shape component 5 18% HIP < 2000°C c4h 5 200 MPa Ar decasing J.N. that is. fissures. this socalled capsule-hipping is an expensive and slow procedure not suited for low-cost mass production of ceramic components.N4 components is possible by N. it even has a (limited) ability to increase strength and reliability of Si. However.N. 1149). gas-pressure sintering. The course of temperature and pressure has to be adjusted to furnace geometry. Sintering under normal pressure limits the maximum temperature to about 1820 "C owing to the decomposition of Si. and ultrasonic erosion are used for finishing.. gas turbine rotors) from simple monolithic shapes (Fig. Thus.N. and BN. D ) . I I * . '"I These developments. the capsule is removed mechanically (sandblasting) or chemically (etching). materials. Sintering processes for the manufacturing of dense Si. sintering additives. Finishing -Oo -60 t lbarl h 2 -40 P i 8. may gain importance for the manufacturing of Si. A typical course of temperature/pressure versus time in gas pressure sintering is shown in Figure 29. Usually. it is applied for the production of components which are used under extreme conditions and would otherwise require an even greater effort for However. 29. pressure allows a further temperature increase and results in a pressure-assisted densification. honing. 5 1880°C 21 h 2 50 MPa mech. lapping. Chem. components showing long-term mechanical strength at high temperatures. are required.4. and powder compact geometry (characteristic data in Fig. 30 (1991) 1579-1597 Since sintered components are rarely used as fired. industrially employed. D.N.['491In this process a powder compact is heated to sintering temperature in a powder bed consisting of Si.2. Because of high hardness and comparatively high toughness of Si. to date on laboratory scale. I n i . = theoretical density. 120 I 20 40 60 t bin1 - I I 80 100 Fig.N. since it allows the production of very complex components (e.["'. these steps are time-consuming and expensive.Method: Material abbreviation ~ ~~~ Hot Pressing HPSN S$t$ sintering admixtures hot pressing Sintering normal gas pressure pressure SSN (GP-)SSN Si& powder compacts Hot Isostatic Pressing sinter-HIP capsule-HIP HIP-SSN Si& HIP-SN powder compacts starting material processing steps conditions T t p(gas) + sintering J. After sintering is complete. Generally. which was developed during the seventies and which is meanwhile Angew. especially for applications as engine or turbine components. without precise finishing. and pits.N.. Improving fracture toughness Kjc by in situ whisker reinforcement (1911. 8.6 10-6K-1 700J Kg-lK-I Rcm 1400-1700 MNm-’ 600-1000 MPa 5-8 MPam”’ 280-320GPa 15-30 Wm -’K-’ 600-800 K forts to improve Si. 3I.202 gcm-’ 95-100% th. usually it is considered as a key criterion of quality. material (K.1891 By controlled structure development (in situ whisker reinforcement) it is possible to increase the fracture toughness of Si.168-3. materials with fracture strengths exceeding 1000 MPa have been described. monolith [ISS].r1901 a conventional material is compared to an “in situ whisker reinforced” material in Fig.Fig. The amount and kind of these phases determine creep and long-time fracture strength. Today. the main criteria of quality include not only fracture strength. pressure. Property ~~~ Value - Decomposition temperature Theoretical density (th. D. materials from a level of about 7 MPam1j2 to almost 10 MPam1/2.N. Chem. Physical and Chemical Properties 8.[188.= = 6.1000 MPa at room temperature. Inr. 31. A general comparison of fracture strength is difficult.N. however. Engl. fracture toughness. material.[1871 However. which are influenced by starting powder characteristics.~ a-phase: 3. Some years ago. isostatically hot-pressed Si. Fracture strength is surely the charac- Table 3. Right: Turbine wheels made of Si. the state of the art is a strength level of 800. (GP-SSN) > 15. powder shaping.19-3. (HIP-SSN).[l9l1 The toughness level attained here could formerly be achieved only by reinforcement with extraneous particles (like whiskers or l9. 2. Ed. powder quality and production methods. but also the strength level at temperatures greater than 1000 “C. for applications in automotive gas turbines [186].1. Right: Structure of an in situ whisker-reinforced material (K. The (limited) ability of a particular sintering process to anneal defects like pores. which is used to characterize the scattering of fracture strength and thus the reliability of the material. Physical properties of dense Si.N. > 20. > 10.<= 9.N.5 MPam’’z). Here. and soaking time during sintering.N. bubbles. The influence of sintering techniques on material properties is reflected in the so-called Weibull modulus (a statistical parameter to characterize a property distribution over a manifold of samples). The scattering range of the data is mainly due to structural differences. 30.9-3.) Material density Thermal expansion coefficient (293 -1473K) Specific heat Electrical resistance [a] Microhardness (Vickers) Bending strength [a] [b] Fracture toughness [a] Elasticity modulus [a] Thermal conductivity [a] Critical temperature difference in thermal shock ~ 2173 K a-phase: 3.N. and sintering techniques as well as temperature. a significant dependence of material properties on the method of producing the starting powder was observed.N. Left: Manufacturing study for ultrasonic erosion of a turbine wheel from a hotpressed Si. [b] Four-point bending teristic property determined most often.N. 1594 Angew. The following values are typical for Weibull moduli of commercial materials: pressureless sintered Si.1 At temperatures exceeding 1000 “C a decrease of strength in many materials is caused by slowly softening amorphous grain-boundary glass phases. and fissures is thereby quantified to a certain extent.5 MPam”*).N. since materials for specialized applications have been developed.118 g ~ m . gas-pressure-sintered Si. these differences almost vanished as a result of constant ef- Fig. 30 (1991) 1579-1597 . D. amount and kind of sintering additives.N. and hardness or wear resistance. internal cracks. Physical Properties Table 3 shows a compilation of important properties of completely densified Si. All modifications of materials or technological processes to im- [a] RT = room temperature. Left: Structure of a conventional Si. ceramics. N. Chemical and physical characteristics of the starting material. the required high-quality Si.N. Ed. gas turbines) the oxidation behavior of sintered Si. bearings. N. Chem. Al. the use of high-purity starting powders. The work on economical methods to produce high-quality Si." 891 An evaluation of a commercially available MgO-doped material is given in Figure 32. and Cr.N. materials are attacked by hot strong caustic solutions or melts with formation of NH. gas-phase reaction seems to offer the best potential to furnish economically high-purity. MgO-doped Si. This phenomenon has been observed primarily in some ternary Y-Si-0-N and Ce-Si-0-N phases. that the sintered component is completely destroyed in a short time.. kind and amount of sintering additives. and sintering technique determine material properties. which induce enhanced corrosion accompanied by N.g. to a lesser extent also by SiCI. however. bearing gaskets.N. mechanical engineering. corrosion and wear resistance. wear and chemical technology.['991A suitable selection of sintering additives.['95 . In contract with molten salts. high-quality. at the surface results in the formation of oxidic glass phases of low viscosity. materials. mill cladding.prove the dependence of fracture strength on temperature and to improve the strength level at high temperatures aim at reducing the amounts of amorphous grain-boundary glass phases or at generating higher refractory. Ni. highly sintering-active Si. Injection molding.rn.[lg4]Here. Mainly porous Si. Pressureless or low-pressure sintering is of great interest for the low-cost production of near-finished components. powders. A flawless manufacturing process from powder synthesis to finished component is required to produce high-quality. and corrosion resistance especially at high temperatures. and low density are desired. 30 (19911 1579-1597 . Co. and a reduction of additive and oxygen content are important in the development of high-temperature oxidation-resistant sintered Si. Among processes under development. Summary Sintered silicon nitride is a promising material for applications in engine construction. and glasses. powders are commercially produced mainly by direct nitridation. Recently. In the use of engine components made of Si. turbocharger. its chemical properties have been investigated with special regard to corrosive behavior. Cutting tools. ceramics are resistant to mineral acids. where high mechanical strength at high temperatures.lg8] Silicon nitride is inert to numerous molten metals (e. it reacts readily to form metal silicides and N. Cu. Their interdependence influences structure development as well as the kind and composition of silicate phases formed at grain boundaries. liquid-phase reaction (diimide process). Sn. u = flexural strength. fast fracture 8006000 [MPa] 400200I 1 9.['961 The first mechanism is based on the diffusion of sintering additives and impurities along grain boundaries owing to the higher oxygen potential at the surface of the material. turbocharger rotors. Chemical Properties Owing to the broad possible applications of sintered Si. Inr. A second mechanism will lead to complete destruction if4epending on sintering additives-crystalline phases are formed during sintering. a successful realization of these concepts has been reported.N. Crystallization and reduction of the amount of these silicate phases are important to further improve material properties like fracture strength.N. crystallized grainboundary phases together with optimization of structure.N. the SiCIJNH. and Cd). 32. Fracture strength of hot-pressed. easily processible Si. the failure probability resulting from Weibull analysis in the range of linear elastic stress up to 900 "C and probable reasons for positive or negative deviations caused by defect annealing or defect induction is shown. ReacAngew. in contact with molten transition metals like Fe.y tion with SiO.2. a good oxidation resistance is shown up to 1400 "C. formation. In high-temperature oxidation two fundamentally different mechanisms of damage are observed. has a great potential for the large-scale manufacturing of very complex components. powders. is especially important. To avoid 12001000. oxidic slags. reliable ceramic components. hardness.. Engl.. To date. which are readily oxidized with concomitant volume expansion. V. valves.N. only slow corrosion occurs. With the exception of hydrofluoric acid. powders on a large scale. and gas turbines are prominent examples where this material is already used routinely or where it is being tested as a basic material.. 8. premature failure at a given temperature. Internal stress may build up so strongly.. (e../NH. Ag.. to improve steadily powder and material 1595 I I I I 200 400 600 800 1000 1200 7 ["C] - 1 I I I 1400 1600 Fig.N.. as a powder-shaping technology using cheap. is coated with a thin protective layer of SiO. on heating in air. Since Si. and engine components like valves.N. Pb. creep. pores and modification changes of grain-boundary phases leading to different thermal expansion coefficients between glass phase and matrix material induce defects which lower the fracture strength of the material. The development of economical methods for finishing sintered components of this very hard material is still necessary to decrease the very high finishing costs.N. the material should bear only considerably lower mechanical stresses than shorttime fracture strengths would indicate. valve guides.g. Si. as a function of temperature and time [164].N. Figure 32 also shows the life expectancy of the material below short-time fracture load. Zn. Sci. Chem. K. Morgan. [29] M. [67] M. [I161 US 4196178 (1980). [I021 S. W Campbell. 59 ff. E. Desmaison. Priest. Muter. E. 2 (1990) 398. T. M. S . Boden. A. [94] S . Electrochem. Engl. J. H. [73] P. J. Bandyopadhyay. L. Blegen. J. R. G. Franz. K. Spec.. [76] 0. W. [70] S. 298 (1959) 134.): Combustion and Plasma Synthesis o High Temperature Materials.. Chem. Anorg. H. Am. 22 (1987) 710. Ube Industries. Br. G. Muter. Ceram. [71] W. 26 (1987) 371. Muter. R. J. [84] M. [41] K. Priest. E. [68] 0. D. Nitomo. M. Billy. Chem. Cooke. 3rd Int. Peuckert. Horn. 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