《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_fibrous molithic-56

Materials Science and Engineering A 500(2009)130-149 Contents lists available at Science Direct Materials Science and Engineering A ELSEVIER journalhomepagewww.elsevier.com/locate/msea Dense and near-net-shape fabrication of Si3 N4 ceramics M.H. Bocanegra-Bernala, *, B Matovic b to de Fisica de materials, Miguel de Cervantes 120 Complejo industrial Chihuahua, 31109 Chihuahua, Chihuahua, Mexico b Vinca Institute of Nuclear Sciences, Materials Science Laboratory, Belgrado, Serbia ARTICLE INFO A BSTRACT With silicon nitride significant progress has been made in order to search for fully dense, strong, reli- 1 July 2008 able structural ceramics to find wide use in applications at high temperatures which are allowing new and innovative solutions to component design problems. Taking into account that more and more September 2008 ceramic components based on Si3N4 are being used in the aerospace and automobile industries, it is a great challenge to fabricate such complex-shaped components with high reliability and with defect Keywords: ilicon nitride free microstructures such as pores, inclusions or any other inhomogeneity at acceptable costs On the other side, the high hardness of Si3 Na ceramics is almost always cost prohibitive to shape component jection molding by hard machining. It is therefore great effort exhibited in the development of near-net-shape fabrication boasting Aqueous slurries imize the number and size of microstructural defects within design limits. In this review, the fabrication of near-net-shape Si3N4 ceramics is given in detail. All kinds of these techniques(injection molding, gel- casting, robocasting, mold shape deposition, rapid prototyping)and their advantages and disadvantages re explained. O 2008 Elsevier B.V. All rights reserved 1. Introduction the high temperature properties of the ceramics such as creep and high temperature strength[17, 32, 33]. Considering this, it is very Structural ceramics based Si3 Na have been explored since the important to stress that the recent advances in improving prop- late 1960s [1]emphasizing Si3 NA based materials primarily for use erties are mainly attributed to improved processing techniques. in high temperature, structural applications such as heat engines. purer raw materials and the use of gas pressure sintering or HIP Taking into account their unique combination of properties, silicon techniques in order to reduce critical flaw size [33 nitride and related materials have probably become the most thor- It is a common practice to densify Si3N4 by alternative tech- oughly characterized non-oxide ceramics with wide applications niques and or supplementary means such as nitridation of silicor cluding heat exchangers, turbine and automotive engine com- powder or with the application of pressure in order to assist the ponents, valves and cam roller followers for gasoline and diesel sintering process. These techniques can be summarized: i) Reac ngines and radomes on missiles as well as insulators, electronic tion Bonding Silicon Nitride( rBsn), ii Hot Pressing Silicon substrates, high Tc superconductors, tool bits, wear surfaces, to(HPSN), iii) Sintering Silicon Nitride(ssn), iv) Sintering Re ame a few [2, 3-17 The market for these applications is very high; Bonding Silicon Nitride(SrBSn), v) Hot Isostatic Pressing there are still several difficulties that must be overcome before the Nitride(hipSn), vi) Hot isostatic Pressing Reaction Bonding Sili full potential of structural ceramics based silicon nitride is real- con Nitride(HIPrBSn). vii) Hot Isostatic Pressing Sintered Silicon ized The sintering of silicon nitride is very difficult because of the Nitride(hipssn)and viii) Hot Isostatic Pressing Sintered Reaction low self-diffusivity of this covalent material [13-20, 21-30 Doping Bonded Silicon Nitride(HiPsrBsn)[20, 22-34. It is very difficult ure Si3 Na with of some oxides provides the formation ofintergran- to produce pure dense silicon nitride ceramics by means of con- lar liquid phase which aids the further densification of the silicon ventional sintering(simple heating of powder compacts) due to nitride during different sintering routes [3-7, 13-31. These oxides, the high degree of covalent bonding between silicon and nitroger however, remain as grain boundary glassy phase, which deteriorate [35]. The principal reason for this is that the diffusion of sil on(at1400°CDs≈05×10-19m2s-l) and nitrogen(at1400° DN≈6.8×10-10m2s-l) in the volu 526144394801;fax+526144394823. tremely slow [36]. Taking into account that the den- sification by sintering requires mass transport via volume or grain 0921-5093/s-see front matter o 2008 Elsevier B.V. All rights reserved. doi:10.016/msea2008

Materials Science and Engineering A 500 (2009) 130–149 Contents lists available at ScienceDirect Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea Dense and near-net-shape fabrication of Si3N4 ceramics M.H. Bocanegra-Bernal a,∗, B. Matovic b a Centro de Investigación en Materiales Avanzados, CIMAV S.C., Departamento de Física de Materiales, Miguel de Cervantes # 120 Complejo Industrial Chihuahua, 31109 Chihuahua, Chihuahua, Mexico b Vinca Institute of Nuclear Sciences, Materials Science Laboratory, Belgrado, Serbia article info Article history: Received 11 July 2008 Received in revised form 4 September 2008 Accepted 8 September 2008 Keywords: Silicon nitride Gelcasting Injection molding Robocasting Aqueous slurries abstract With silicon nitride significant progress has been made in order to search for fully dense, strong, reli￾able structural ceramics to find wide use in applications at high temperatures which are allowing new and innovative solutions to component design problems. Taking into account that more and more ceramic components based on Si3N4 are being used in the aerospace and automobile industries, it is a great challenge to fabricate such complex-shaped components with high reliability and with defect￾free microstructures such as pores, inclusions or any other inhomogeneity at acceptable costs. On the other side, the high hardness of Si3N4 ceramics is almost always cost prohibitive to shape components by hard machining. It is therefore great effort exhibited in the development of near-net-shape fabrication processes that can produce complex-shaped components with a minimum of machining as well as to min￾imize the number and size of microstructural defects within design limits. In this review, the fabrication of near-net-shape Si3N4 ceramics is given in detail. All kinds of these techniques (injection molding, gel￾casting, robocasting, mold shape deposition, rapid prototyping) and their advantages and disadvantages are explained. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Structural ceramics based Si3N4 have been explored since the late 1960s [1] emphasizing Si3N4 based materials primarily for use in high temperature, structural applications such as heat engines. Taking into account their unique combination of properties, silicon nitride and related materials have probably become the most thor￾oughly characterized non-oxide ceramics with wide applications including heat exchangers, turbine and automotive engine com￾ponents, valves and cam roller followers for gasoline and diesel engines and radomes on missiles as well as insulators, electronic substrates, high Tc superconductors, tool bits, wear surfaces, to name a few [2,3–17]. The market for these applications is very high; there are still several difficulties that must be overcome before the full potential of structural ceramics based silicon nitride is real￾ized. The sintering of silicon nitride is very difficult because of the low self-diffusivity of this covalent material [13–20,21–30]. Doping pure Si3N4 with of some oxides provides the formation of intergran￾ular liquid phase which aids the further densification of the silicon nitride during different sintering routes [3–7,13–31]. These oxides, however, remain as grain boundary glassy phase, which deteriorate ∗ Corresponding author. Tel.: +52 614 4394801; fax: +52 614 439 4823. E-mail addresses: miguel.bocanegra@cimav.edu.mx (M.H. Bocanegra-Bernal), mato@vin.bg.ac.yu (B. Matovic). the high temperature properties of the ceramics such as creep and high temperature strength [17,32,33]. Considering this, it is very important to stress that the recent advances in improving prop￾erties are mainly attributed to improved processing techniques, purer raw materials and the use of gas pressure sintering or HIP techniques in order to reduce critical flaw size [33]. It is a common practice to densify Si3N4 by alternative tech￾niques and/or supplementary means such as nitridation of silicon powder or with the application of pressure in order to assist the sintering process. These techniques can be summarized: i) Reac￾tion Bonding Silicon Nitride (RBSN), ii) Hot Pressing Silicon Nitride (HPSN), iii) Sintering Silicon Nitride (SSN), iv) Sintering Reaction Bonding Silicon Nitride (SRBSN), v) Hot Isostatic Pressing Silicon Nitride (HIPSN), vi) Hot Isostatic Pressing Reaction Bonding Sili￾con Nitride (HIPRBSN), vii) Hot Isostatic Pressing Sintered Silicon Nitride (HIPSSN) and viii) Hot Isostatic Pressing Sintered Reaction Bonded Silicon Nitride (HIPSRBSN) [20,22–34]. It is very difficult to produce pure dense silicon nitride ceramics by means of con￾ventional sintering (simple heating of powder compacts) due to the high degree of covalent bonding between silicon and nitrogen [35]. The principal reason for this is that the diffusion of sili￾con (at 1400 ◦C DSi ≈ 0.5 × 10−19 m2 s−1) and nitrogen (at 1400 ◦C DN ≈ 6.8 × 10−10 m2 s−1) in the volume or at the grain boundaries of Si3N4 is extremely slow [36]. Taking into account that the den￾sification by sintering requires mass transport via volume or grain boundary diffusion and since such diffusion is a thermally activated 0921-5093/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2008.09.015

M.H. Bocanegra-Bermal B Matovic/ Materials Science and Enginee process, a higher sintering temperature would result in a highly sintering times and temperatures, reduction of grain size and dis- lense material but at high temperatures Si3 N4 starts to dissociate tribution, higher sintered densities with the use of low pressures and reduction or sintering aids, etc, all of which lead to obtain Many different sintering techniques have been developed since excellent properties [47-51. There has been considerable interest the material properties strongly depend on the fabrication method in developing ceramic materials for use in advanced heat engines and the silicon nitride cannot be considered as a single material Therefore, possessing low density and excellent thermomechanical [38]. As was previously outlined, the most common sintering meth- properties, ceramics materials provide a means for producing heat ods used to consolidate Si3 N4 based ceramics [20]are: i) Reaction engines with very efficiency ceiling far above what is presently pos- bonding(RBSN), ii) Hot pressing(HPSN), iii) Hot isostatic press- sible with today's super alloys. Silicon nitride(si3 N4), has received considerable attention due to its high decomposition tempera and vi) Sintering reaction bonding(SRBSN) On the other hand, ture(approximately 1880 C)as well as excellent creep properties in order to manufacture Si3 N4 ceramics for application in heat 3, 52, 53. Today, commercially available Si3 N4 powders are pre- engines, development of near-net-shape fabrication methods is pared by means of various routes, already in use for production critical [39]. Recognizing this need, several companies in the United on a technical scale because these powders are the starting point States, Europe and japan have invested significant resources to for dense materials, namely develop injection molding, gelcasting of silicon metal, and slip cast- [40]. Among the shapemaking techniques, slip casting is belie parts 2.1. Silicon nitridation be a method appropriate for prototypes, whereas injection mold- Chemically pure Si powder (particle diameter <10 um)is ing is ideally suited for high-volume, cost effective production of nitrided in an atmosphere of NH3, N2/H2 or N 2 as follows: complex parts [28, 41 Nowadays, robocasting is a new freeform brication technique for dense silicon nitride ceramics [42). This 3Si+ 2N2 colloidal method has shown its potential to improve the strength nd reliability of high-performance ceramics. Ceramic components under controlled furnace conditions such as bed-depth[54].The with simple or complex shapes can be rapidly produced from nitriding process results in Si3 N4 lumps which are crushed and a computer aided-design(CAD) drawing directly to a finished milled. The reaction(1)is the high temperature reaction of silicon omponent that requires little or no machining after fabrication powder with a nitrogen and it is a reaction strongly exothermic with 4243] AH-733 k] mol-I. It is important to stress that the density of sil- More complex techniques for manufacturing silicon nitride icon is 2329 m- and that of silicon nitride is -3185kgm-3,so ceramics have been used to produce reliable parts Injection mold- that a volume expansion of 21.7% occurs during nitride formation. ing slip casting, robocasting aqueous, gelcasting, are some of them At normal nitriding temperatures (1200-1450 C) silicon nitride [42, 44, 45. The fabrication techniques to obtain near-net-shape shows no noticeable plasticity, and as the overall compact volume Si3Na ceramics as well as their advantages and disadvantages are change during nitridation of silicon powders is essentially zero, it given in detail. is clear that considerable internal rearrangement of product mate- rial must occur within the pre-existing void spaces of the compact 2. Manufacture of silicon nitride powders The nitridation method has proven to be flexible for Some studies on the sintering of silicon nitride powders to duction of very different powder qualities. The raw silicon nitride dense bodies have shown the great importance of surface com- formed in the nitridation process already consists of morphology position: the nature and amount of sintering aids(such as Al2O3. such as whiskers, elongated particles, and equiaxed particles [6] a Y203, Yb2O3. ZrO2 etc. )at high temperatures(1750-2000 C)[3-9] well as spherical after the milling process [56]. Additionally, with depended on the surface oxygen content of the powders 6]. How- wet milling, very fine powders of up to 25m2g-l or more with ever, the sintering of silicon nitride ceramics without additives is narrow particle size distributions can be produced. n important approach to reducing impurity phases in the densi Important attention is drawn to quality-determining steps of ed bodies achieving Si3 N4 ceramics with the intrinsic properties of powder production, which are assumed to be responsible for the the materials. Ceramics free of aids sintered under high pressures sinter-active behavior of the finest powders. A high a-phase(95%) exhibited improved mechanical properties at high temperatures content is desirable in order to ensure beneficial transformation mpared to those sintered with additives [8, 9 nto the B-form during sintering, leading to densification and the These materials tend to be expensive due to the high cost of formation of an interlocked needle structure with high strength. In the silicon nitride powder used to produce them. Therefore, the silicon powder, unless the silica layer is disrupted either physically reduction of cost has been recognized as a major factor for the or chemically, the nitridation reaction does not start [ 57.Afterinit introduction of silicon nitride ceramics into the marketplace with ation, the nitridation reaction proceeds and is controlled by factors as mean particle size and size distribution, the nature and distri- alloys, tungsten carbides and some ceramics such as Al2O3 and zro ution of impurities in the starting silicon powder, size and size in automotive aerospace, metal processing and forming, mineral distribution of open porosity in the silicon compacts, dimensions processing, machining, oil field services, petrochemical, semicon- of the silicon compacts, and nitriding conditions ductor processing industries, etc. [10-12 As indicated a number years ago [3], there is a great advan- 22. Vapor phase reaction tage to processing ceramics from powders with an idealized set of physical and chemical characteristics [46 as follows: i)small By means of this method, a fine amorphous silicon nitride pow- size less than 1 um, i) narrow size distribution, ii equiaxed mor- der is obtained from the gas phase reaction of silicon tetrachloride, phology tending towards spherical, no agglomeration, or very weak SiCl4, and ammonia at temperature of 1546 C according to the agglomerate bonds which can be broken during processing and reaction: iv) high degree of chemical and crystal purity. With these char- acteristics, it is possible to obtain advantages such as reduction of 3SiCla (g)+ 4NH3()- Si3N4+ 12HCI(g)

M.H. Bocanegra-Bernal, B. Matovic / Materials Science and Engineering A 500 (2009) 130–149 131 process, a higher sintering temperature would result in a highly dense material but at high temperatures Si3N4 starts to dissociate [35,37]. Many different sintering techniques have been developed since the material properties strongly depend on the fabrication method and the silicon nitride cannot be considered as a single material [38]. As was previously outlined, the most common sintering meth￾ods used to consolidate Si3N4 based ceramics [20] are: i) Reaction bonding (RBSN), ii) Hot pressing (HPSN), iii) Hot isostatic press￾ing (HIPSN), iv) Sintering (SSN), v) Gas pressure sintering (GPSN) and vi) Sintering reaction bonding (SRBSN). On the other hand, in order to manufacture Si3N4 ceramics for application in heat engines, development of near-net-shape fabrication methods is critical[39]. Recognizing this need, several companies in the United States, Europe and Japan have invested significant resources to develop injection molding, gelcasting of silicon metal, and slip cast￾ing for fabrication of complex cross-section structural ceramic parts [40]. Among the shapemaking techniques, slip casting is believed to be a method appropriate for prototypes, whereas injection mold￾ing is ideally suited for high-volume, cost effective production of complex parts [28,41]. Nowadays, robocasting is a new freeform fabrication technique for dense silicon nitride ceramics [42]. This colloidal method has shown its potential to improve the strength and reliability of high-performance ceramics. Ceramic components with simple or complex shapes can be rapidly produced from a computer aided-design (CAD) drawing directly to a finished component that requires little or no machining after fabrication [42,43]. More complex techniques for manufacturing silicon nitride ceramics have been used to produce reliable parts. Injection mold￾ing, slip casting, robocasting aqueous, gelcasting, are some of them [42,44,45]. The fabrication techniques to obtain near-net-shape Si3N4 ceramics as well as their advantages and disadvantages are given in detail. 2. Manufacture of silicon nitride powders Some studies on the sintering of silicon nitride powders to dense bodies have shown the great importance of surface com￾position: the nature and amount of sintering aids (such as Al2O3, Y2O3, Yb2O3, ZrO2, etc.) at high temperatures (1750–2000 ◦C) [3–9] depended on the surface oxygen content of the powders [6]. How￾ever, the sintering of silicon nitride ceramics without additives is an important approach to reducing impurity phases in the densi- fied bodies achieving Si3N4 ceramics with the intrinsic properties of the materials. Ceramics free of aids sintered under high pressures exhibited improved mechanical properties at high temperatures compared to those sintered with additives [8,9]. These materials tend to be expensive due to the high cost of the silicon nitride powder used to produce them. Therefore, the reduction of cost has been recognized as a major factor for the introduction of silicon nitride ceramics into the marketplace with a broad range of properties to replace the stainless steels, super alloys, tungsten carbides and some ceramics such as Al2O3 and ZrO2 in automotive, aerospace, metal processing and forming, mineral processing, machining, oil field services, petrochemical, semicon￾ductor processing industries, etc. [10–12]. As indicated a number years ago [3], there is a great advan￾tage to processing ceramics from powders with an idealized set of physical and chemical characteristics [46] as follows: i) small size less than 1 m, ii) narrow size distribution, iii) equiaxed mor￾phology tending towards spherical, no agglomeration, or very weak agglomerate bonds which can be broken during processing and iv) high degree of chemical and crystal purity. With these char￾acteristics, it is possible to obtain advantages such as reduction of sintering times and temperatures, reduction of grain size and dis￾tribution, higher sintered densities with the use of low pressures and reduction or sintering aids, etc., all of which lead to obtain excellent properties [47–51]. There has been considerable interest in developing ceramic materials for use in advanced heat engines. Therefore, possessing low density and excellent thermomechanical properties, ceramics materials provide a means for producing heat engines with very efficiency ceiling far above what is presently pos￾sible with today’s super alloys. Silicon nitride (Si3N4), has received considerable attention due to its high decomposition tempera￾ture (approximately 1880 ◦C) as well as excellent creep properties [3,52,53]. Today, commercially available Si3N4 powders are pre￾pared by means of various routes, already in use for production on a technical scale because these powders are the starting point for dense materials, namely: 2.1. Silicon nitridation Chemically pure Si powder (particle diameter <10 m) is nitrided in an atmosphere of NH3, N2/H2 or N2 as follows: 3Si + 2N2 −→ 1746−1996◦C Si3N4 (1) under controlled furnace conditions such as bed-depth [54]. The nitriding process results in Si3N4 lumps which are crushed and milled. The reaction (1) is the high temperature reaction of silicon powder with a nitrogen and it is a reaction strongly exothermic with H ∼ 733 kJ mol−1. It is important to stress that the density of sil￾icon is 2329 kg m−3 and that of silicon nitride is ∼3185 kg m−3, so that a volume expansion of 21.7% occurs during nitride formation. At normal nitriding temperatures (1200–1450 ◦C) silicon nitride shows no noticeable plasticity, and as the overall compact volume change during nitridation of silicon powders is essentially zero, it is clear that considerable internal rearrangement of product mate￾rial must occur within the pre-existing void spaces of the compact [55] The nitridation method has proven to be flexible for the pro￾duction of very different powder qualities. The raw silicon nitride formed in the nitridation process already consists of morphology such as whiskers, elongated particles, and equiaxed particles [6] as well as spherical after the milling process [56]. Additionally, with wet milling, very fine powders of up to 25 m2 g−1 or more with narrow particle size distributions can be produced. Important attention is drawn to quality-determining steps of powder production, which are assumed to be responsible for the sinter-active behavior of the finest powders. A high -phase (>95%) content is desirable in order to ensure beneficial transformation into the -form during sintering, leading to densification and the formation of an interlocked needle structure with high strength. In silicon powder, unless the silica layer is disrupted either physically or chemically, the nitridation reaction does not start[57]. After initi￾ation, the nitridation reaction proceeds and is controlled by factors as mean particle size and size distribution, the nature and distri￾bution of impurities in the starting silicon powder, size and size distribution of open porosity in the silicon compacts, dimensions of the silicon compacts, and nitriding conditions. 2.2. Vapor phase reaction By means of this method, a fine amorphous silicon nitride pow￾der is obtained from the gas phase reaction of silicon tetrachloride, SiCl4, and ammonia at temperature of 1546 ◦C according to the reaction: 3SiCl4(g) + 4NH3(g) → Si3N4 + 12HCl(g) (2)

M.H. Bocanegra-BemaL B Matovic/ Materials Science and Engineering A 500(2009)130-149 Although a-Si3N4 crystallizes between 1673 and 2046C, these or, on the other hand, silazanes compounds containing Si-N-Si silicon nitride powders have interesting properties including high bonds as follows: hemical purity; amorphous microstructure and me in the manometers scale [18, 58] as well as are suitable raw materi- 2(CH3 )3 SiCl 3NH3-[(CH3)3SiJ2NH +2NH4CI als of advanced silicon nitride ceramics. However, it is interesting It is very important to note that the stability of sily to note that silicon nitride powders with particle size nanomet- with respect to silazane formation increases with functional group ric range, can be densified and sintered without additives under size and structures of representative silazanes [54 Si3 NA pow ultrahigh pressure(1.0-5.0 GPa) between room temperature and ders by means of conversion of silazanes have been achieved 1600°C[59] in various physical forms. Very thin films(<1 um) have been Although these powders are generally present in amorphous deposited from gas mixtures of hexamethyldisilazane/ NH3 and form, obtaining crystallized ceramics requires sintering at least hexamethylclotrisilizane/NH3 using chemical vapor depositi oC). On the(CVD)technology, as well as fiber bundles of a-Si3 Na with diam- other hand, sintering of amorphous powders bellow the crystal- eter of approximately 1. um at 1400 C by means of pyrolisis of lization temperature may generate bulk amorphous ceramics. LiLi hexaphenylcyclotrisilizane in nitrogen [64, 65. et al.[59 reported two typical pressing results, together with nose sintered at high temperature, where by means of XRD was 3. Fabrication of near-net-shape Si3 Na ceramics identified that sintered specimens obtained below 1000-1100C emained amorphous. The high relative density obtained indicates The articles manufactured by near-net-shape forming tech- that the amorphous nano-size powders can be almost fully den- niques involve generally little, if any post-densification machining, sified below the crystallization temperature under sufficient high surface preparation or cleaning prior to use. There is a great pressure. Therefore, bulk Si3 N4 amorphous ceramics can be formed challenge to fabricate complex-shaped components with high reli- at sintering temperatures slightly below that the onset of crystal- ability and with defect-free microstructures at acceptable costs. zation. Moreover, the sintering of amorphous nano-size powder The high hardness of Si3 NA ceramics is almost always cost pro- vithout additives is an important approach to reducing impurities hibitive to shape components by hard machining. It is therefore phases in the sintered bodies and hence achieving Si3N4 ceram- great effort exhibited in the development of near-net-shape fab- ics with the intrinsic properties of the materials and improved rication processes that can produce complex-shaped components mechanical and high temperature compared to those sintered with with a minimum of machining as well as to minimize the number dditives [60 and size of microstructural defects within design limits. Injection molding, gelcasting, robocasting, mold shape deposition, rapid pro- 3. Imide dec totting of them It is considered as a liquid phase reaction method [54 It is 3.1. Injection molding of SiaN4 ceramics interesting to note that reactions attracting attention in the 1980s vere first investigated as 1830. In that year a white precipitate was Silicon nitride, when properly prepared, is a superlatively tough obtained from the interaction of Sicl and ammonia gas 61) in ceramic whose high temperature stability; low weight; and wear, an inert solvent(benzene)at approximately 273 K In later stud- erosion and corrosion resistance have put it high up on the wish ies[55] the product of this reaction was considered to be silicon list of turbine engine designers. Injection molding of ceramics tetramide, Si(NH2)4. However the precipitate was unstable and lost was initially demonstrated over 50 years ago [66-68] and it is an NH3(g) at ambient temperature to give silicon diimide, Si(NH)2 attractive method among the processes for near-net-shape produc hich is heated at high temperature in N2 or NH3 atmosphere after tion of ceramic parts, requiring little subsequent grinding and no eparating ammonium halide[ 62]. From the different methods for need of machining 169). The use of polycrystalline high tempera- manufacturing silicon nitride, the thermal decomposition method ture ceramics in different applications such as turbochargers and of Si(NH)2 is considered to be very suitable for use in the mass pro- gas turbine vanes, blades and rotors [70-72], reciprocating [73, 74] uction of Si3N4 powder with high quality. because the starting and turbine engines [75, 76] has been possible by considerable materials can be easily and highly purified and the productivity is developments in the fabrication of fine powders [77, 78]. Success high[55]. However, a-Si3 N4 powders synthesized by diimide route of the injection molding process of Si3N4 is critically depending produce powders with a high area and fine particle size(10-30 nm), on starting powder, binder, and the process parameters such as but they are prohibitively expensive [63] molding and binder removal conditions and subsequent densifi- It is very important to control the crystallization a to B ratio cation 39]. The development of injection molding technology for and grain morphology of the product, because th e better control sintered silicon nitride was initiated at gte labs under a sub- of these characteristics of Si3N4 powder is considered to be the contract to the Detroit Diesel Allison Division( DDA)of General portant key point in the production of high-performance Si3N4 Motors as a part of the Ceramic Applications in Turbine Engines ceramics [61] (CATE)[28 Successful development of a injection molding process fornet-shape thick-cross-section(1 cm) 2.4. Silazanes as precursor of Si3N4 components is expected to have a strong impact on the commer- cial development of automotive gas turbines and other related It is known that the chlorosilanes react with NH. primary or heat engines applications. The aim of the injection molding tech- econdary amines to form silymines as follows [63] nology is therefore to produce an unsintered pai which will shrink isotropically to yield a shape slightly over (C2H5)3SiCl 2NH3-(C2H5 B3SINH2+NH4CI (3) size for final machining. Distortion of the ceramic body during molding, binder removal or sintering may render the component useless (CH3)3SiCl 2NH(C2H5 )2-(CH3)3SiN(C2H5)2+(C2H5)2NH2CI The injection molding of Si3 N4 ceramics normally consists of five steps as follows: i) powder processing, ii)powder binder (4) compounding, iii) injection molding, iv) binder burnout and v)

132 M.H. Bocanegra-Bernal, B. Matovic / Materials Science and Engineering A 500 (2009) 130–149 Although -Si3N4 crystallizes between 1673 and 2046 ◦C, these silicon nitride powders have interesting properties including high chemical purity; amorphous microstructure and mean particle size in the manometers scale [18,58] as well as are suitable raw materi￾als of advanced silicon nitride ceramics. However, it is interesting to note that silicon nitride powders with particle size nanomet￾ric range, can be densified and sintered without additives under ultrahigh pressure (1.0–5.0 GPa) between room temperature and 1600 ◦C [59]. Although these powders are generally present in amorphous form, obtaining crystallized ceramics requires sintering at least above the crystallization temperature (>1000–1300 ◦C). On the other hand, sintering of amorphous powders bellow the crystal￾lization temperature may generate bulk amorphous ceramics. LiLi et al. [59] reported two typical pressing results, together with those sintered at high temperature, where by means of XRD was identified that sintered specimens obtained below 1000–1100 ◦C remained amorphous. The high relative density obtained indicates that the amorphous nano-size powders can be almost fully den￾sified below the crystallization temperature under sufficient high pressure. Therefore, bulk Si3N4 amorphous ceramics can be formed at sintering temperatures slightly below that the onset of crystal￾lization. Moreover, the sintering of amorphous nano-size powder without additives is an important approach to reducing impurities phases in the sintered bodies and hence achieving Si3N4 ceram￾ics with the intrinsic properties of the materials and improved mechanical and high temperature compared to those sintered with additives [60]. 2.3. Imide decomposition method It is considered as a liquid phase reaction method [54]. It is interesting to note that reactions attracting attention in the 1980s were first investigated as 1830. In that year a white precipitate was obtained from the interaction of SiCl4 and ammonia gas [61] in an inert solvent (benzene) at approximately 273 K. In later stud￾ies [55], the product of this reaction was considered to be silicon tetramide, Si(NH2)4. However the precipitate was unstable and lost NH3 (g) at ambient temperature to give silicon diimide, Si(NH)2 which is heated at high temperature in N2 or NH3 atmosphere after separating ammonium halide [62]. From the different methods for manufacturing silicon nitride, the thermal decomposition method of Si(NH)2 is considered to be very suitable for use in the mass pro￾duction of Si3N4 powder with high quality, because the starting materials can be easily and highly purified and the productivity is high [55]. However, -Si3N4 powders synthesized by diimide route produce powders with a high area and fine particle size (10–30 nm), but they are prohibitively expensive [63]. It is very important to control the crystallization  to  ratio and grain morphology of the product, because the better control of these characteristics of Si3N4 powder is considered to be the important key point in the production of high-performance Si3N4 ceramics [61]. 2.4. Silazanes as precursor of Si3N4 It is known that the chlorosilanes react with NH3, primary or secondary amines to form silymines as follows [63]: (C2H5)3SiCl + 2NH3 → (C2H5)3SiNH2 + NH4Cl (3) (CH3)3SiCl + 2NH(C2H5)2 → (CH3)3SiN(C2H5)2 + (C2H5)2NH2Cl (4) or, on the other hand, silazanes compounds containing Si–N–Si bonds as follows: 2(CH3)3SiCl + 3NH3 → [(CH3)3Si]2NH + 2NH4Cl (5) It is very important to note that the stability of silylamines with respect to silazane formation increases with functional group size and structures of representative silazanes [54]. Si3N4 pow￾ders by means of conversion of silazanes have been achieved in various physical forms. Very thin films (<1 m) have been deposited from gas mixtures of hexamethyldisilazane/NH3 and hexamethylclotrisilizane/NH3 using chemical vapor deposition (CVD) technology, as well as fiber bundles of -Si3N4 with diam￾eter of approximately 1.3 m at 1400 ◦C by means of pyrolisis of hexaphenylcyclotrisilizane in nitrogen [64,65]. 3. Fabrication of near-net-shape Si3N4 ceramics The articles manufactured by near-net-shape forming tech￾niques involve generally little, if any, post-densification machining, surface preparation or cleaning prior to use. There is a great challenge to fabricate complex-shaped components with high reli￾ability and with defect-free microstructures at acceptable costs. The high hardness of Si3N4 ceramics is almost always cost pro￾hibitive to shape components by hard machining. It is therefore great effort exhibited in the development of near-net-shape fab￾rication processes that can produce complex-shaped components with a minimum of machining as well as to minimize the number and size of microstructural defects within design limits. Injection molding, gelcasting, robocasting, mold shape deposition, rapid pro￾totyping are some of them. 3.1. Injection molding of Si3N4 ceramics Silicon nitride, when properly prepared, is a superlatively tough ceramic whose high temperature stability; low weight; and wear, erosion and corrosion resistance have put it high up on the wish list of turbine engine designers. Injection molding of ceramics was initially demonstrated over 50 years ago [66–68] and it is an attractive method among the processes for near-net-shape produc￾tion of ceramic parts, requiring little subsequent grinding and no need of machining [69]. The use of polycrystalline high tempera￾ture ceramics in different applications such as turbochargers and gas turbine vanes, blades and rotors [70–72], reciprocating [73,74] and turbine engines [75,76] has been possible by considerable developments in the fabrication of fine powders [77,78]. Success of the injection molding process of Si3N4 is critically depending on starting powder, binder, and the process parameters such as molding and binder removal conditions and subsequent densifi- cation [39]. The development of injection molding technology for sintered silicon nitride was initiated at GTE Labs under a sub￾contract to the Detroit Diesel Allison Division (DDA) of General Motors as a part of the Ceramic Applications in Turbine Engines (CATE) [28]. Successful development of a cost-effective ceramic injectionmolding process for net-shape thick-cross-section (>1 cm) components is expected to have a strong impact on the commer￾cial development of automotive gas turbines and other related heat engines applications. The aim of the injection molding tech￾nology is therefore to produce an unsintered particle assembly which will shrink isotropically to yield a shape slightly over￾size for final machining. Distortion of the ceramic body during molding, binder removal or sintering may render the component useless. The injection molding of Si3N4 ceramics normally consists of five steps as follows: i) powder processing, ii) powder binder compounding, iii) injection molding, iv) binder burnout and v)

M.H. Bocanegra-Bermal B Matovic/ Materials Science and Engineering A 500(2009)130-149 densification by sintering and or Hot Isostatic Pressing(HIPing) duced by injection molding with sufficient dimensional control for 28.79-81 The control of each of these process steps and appropri- turbine engine applications. ate selection of the starting material (powder and organic/ aqueou In the fabrication of Si3Na radial power turbine wheel, three binder)are critically important for the overall process success 70 forming methods have been used: i) injection molding, ii slip cast The optimum selection of a binder system is one of the most crit- ing and iii) cold isostatic pressing. Considering that one of the ical factors in silicon nitride part fabrication by injection molding must difficult components to form by conventional ceramic form- [39]. The binder system used for injection molding of Si N4 parts ing processing has been the radial turbine wheel, the development contains a paraffin wax(90 w/), a liquid epoxy (5 w/o), and a sur- of the forming technique for a large, complex-shaped compo- factant(5 w/o)[79). Very care must be taken during binder removal nent as well as the development of a high strength and refractory in order to avoid delamination and cracks as well as part bloat- material to achieve a turbine inlet temperature(Tit)of 1350.Cis ing. During this stage the organic binder filling the spaces between required [1, 25, 39, 82-84, 86]. Takatori et al. [1 used for fabrication ceramic grains must be removed without disrupting the part. The of radial turbine wheel Si3N4 with additions of 5 wt% Y203 and binder removal is accomplished by a thermal cycle which results 5 wt% MgAl2 O4(spinel) as additives. Using this composition, sev- in controlled binder distillation and degradation. When molding eral small components for reciprocating engines were fabricated a submicron ceramic powder such as silicon nitride it was also by injection molding. However, the ceramics obtained had good discovered that an extremely long thermal cycle can be equally mechanical properties at moderate temperatures, but the hightem- detrimental. As the liquid binder is depleted in long cycles the fine perature strength of the ceramics was no satisfactory for gas turbine N4 particles tend to rearrange into denser packing configurations components which would operate at 1350 C In order to overcome under capillary forces, leading to shrinkage. The part exterior of the this problem, two methods for injection molding the wheel were ceramic body is at a more advance stage of binder removal where attempted: one body forming(the whole body of the wheel is injec- no rearrangement of particles can occur and the internal shrinkage tion molded at one time)and two-piece forming(the wheel was produces interior crack formation observable only by radiography divided in two pieces that are injection molded separately. In the 288283 case of one body forming, the sintered bodies presented internal In the works concerning to injection molding of Si3 N4 and car- cracks as well as others were damaged by surface crack genera ried out by different authors, the major objective was to identify tion and broke down during removal binder Takatori et al. [1 and a Si3 N4 powder-based formulation and binder combination with Shimizu et al. [25] concluded that one body forming of the large proved resistance to the stresses generated during solidification radial wheel was not practical using the current injection mold and cooling of injection molded components in the mold Specif- ing method. Therefore the processes of fabrication of radial wheel ically, several key variables on the properties of injection molded were attempted dividing the hub of the wheel the thickest sec- parts have been studied such as particle size distribution of the tion )in two axially symmetrical parts to reduce the thickness about starting powder, binder composition, powder solid loading, and in half [ 87]. The resulting parts were injection molded separately compounding shear level [1, 39, 82-84. and the survival probability of the wheel after binder removal step A variety of turbine components and related parts have been increased markedly. These parts were joined by CIP treatment after produced, supporting several turbine development programs. binder removal. It is noteworthy that even for the divided two-piece Bandyopadhyay and Neil [70 reported two compositions used process; prolonged heat schedule was required with the purpose for fabrication of components: Si3 Na containing 6 w/o Y2O3 and to obtain a sound binderless body labeled as PY6 and Si3 N4 with additions of 6 w/o y203 and 1.5 to Althou injection molding technique is highly viable for fab- 2.0 w/o Al2O3 and labeled as AY6 [85 which in turn is the mate- rication of complex-shaped ceramic components and numerous rial with superior strength properties from room temperature to prototype silicon nitride components have been fabricated by 1200.C meanwhile, the PY6 composition was designed to provide different laboratories in the world being tested successfully in heat- strength maintenance to temperatures at or above 1200C due to engine environments, specific challenges are required [88 such a grain boundary phase wh nore refractory than that of AY6. as: i) achieving raw and palletized feedstocks with consistent and Glass encapsulated HIPing process was carried out at 1750.C or stable flow characteristics, ii) developing mold designs that allow higher in order to obtain full density. These compositions have also feedstock to fill completely while minimizing defects during han- een reported by Neil et al. [40 and although the compositions dling and subsequent processing, iii) attaining enough control over have remained the same, substantial improvements in the material the entire process to ensure high, reliable yields of uniform parts properties of these systems have been realized through improve- and iv)reducing the manufacture cycle time so that the result- nents in process control and microstructure engineering. However, ing silicon nitride parts are competitive with all-metal components Neil et al. [40] identified that for powder/ binder formulations with that they are intended to replace. The application of powder injec- similar binder systems, powders with higher ages of coarser tion moulding enables the production of intricate features and particle agglomerates measured higher material viscosity at the unusual geometries, offering and economic solution to difficult injection molding temperature. Increases in the powder solids production problems when part complexity goes beyond of more lations also increased the material viscosity for all powder formu- basic forming technologies such as dry pressing. This technique offers excellent batch to batch repeatability and process capabil Neil et al.(28]reported difficulties with the large cross-section ities achieving tolerances of +<0.3% for applications in markets rotors in both the molding and binder removal steps within the as aerospace communications, automotive, electronic, chemical, rogram of Ceramics Applications in Turbine Engines. The slight medical, etc. rinkage in the binder system during the solidification was magni- fied due to the much larger cross-section of the rotor hub. with this 3. 2. Gelcasting of si3 Na ceramics problem at hand, a new binder system was developed which com bined low shrinkage during the solidification in the die with greatly Gelcasting is a molding technique for ceramic and metallic mate- improved binder removal characteristics. Therefore, radial turbine rials [89-93] which offers distinct res as an alternative tors were molded using this binder system which was visually to the more conventional ceramic nethods such as dry flaw free after molding and binder removal. The same authors have pressing, slip casting and injection [89, 94-97 Principal reported that liquid phase sintering of Si3 N4 ceramics can be pro- advantages include near-net-shape forming, high green density

M.H. Bocanegra-Bernal, B. Matovic / Materials Science and Engineering A 500 (2009) 130–149 133 densification by sintering and/or Hot Isostatic Pressing (HIPing) [28,79–81]. The control of each of these process steps and appropri￾ate selection of the starting material (powder and organic/aqueous binder) are critically important for the overall process success [70]. The optimum selection of a binder system is one of the most crit￾ical factors in silicon nitride part fabrication by injection molding [39]. The binder system used for injection molding of Si3N4 parts contains a paraffin wax (90 w/o), a liquid epoxy (5 w/o), and a sur￾factant (5 w/o)[79]. Very care must be taken during binder removal in order to avoid delamination and cracks as well as part bloat￾ing. During this stage, the organic binder filling the spaces between ceramic grains must be removed without disrupting the part. The binder removal is accomplished by a thermal cycle which results in controlled binder distillation and degradation. When molding a submicron ceramic powder such as silicon nitride it was also discovered that an extremely long thermal cycle can be equally detrimental. As the liquid binder is depleted in long cycles the fine Si3N4 particles tend to rearrange into denser packing configurations under capillary forces, leading to shrinkage. The part exterior of the ceramic body is at a more advance stage of binder removal where no rearrangement of particles can occur and the internal shrinkage produces interior crack formation observable only by radiography [1,28,82,83]. In the works concerning to injection molding of Si3N4 and car￾ried out by different authors, the major objective was to identify a Si3N4 powder-based formulation and binder combination with improved resistance to the stresses generated during solidification and cooling of injection molded components in the mold. Specif￾ically, several key variables on the properties of injection molded parts have been studied such as particle size distribution of the starting powder, binder composition, powder solid loading, and compounding shear level [1,39,82–84]. A variety of turbine components and related parts have been produced, supporting several turbine development programs. Bandyopadhyay and Neil [70] reported two compositions used for fabrication of components: Si3N4 containing 6 w/o Y2O3 and labeled as PY6 and Si3N4 with additions of 6 w/o Y2O3 and 1.5 to 2.0 w/o Al2O3 and labeled as AY6 [85] which in turn is the mate￾rial with superior strength properties from room temperature to 1200 ◦C meanwhile, the PY6 composition was designed to provide strength maintenance to temperatures at or above 1200 ◦C due to a grain boundary phase which is more refractory than that of AY6. Glass encapsulated HIPing process was carried out at 1750 ◦C or higher in order to obtain full density. These compositions have also been reported by Neil et al. [40] and although the compositions have remained the same, substantial improvements in the material properties of these systems have been realized through improve￾ments in process control and microstructure engineering. However, Neil et al. [40] identified that for powder/binder formulations with similar binder systems, powders with higher percentages of coarser particle agglomerates measured higher material viscosity at the injection molding temperature. Increases in the powder solids loading also increased the material viscosity for all powder formu￾lations. Neil et al. [28] reported difficulties with the large cross-section rotors in both the molding and binder removal steps within the program of Ceramics Applications in Turbine Engines. The slight shrinkage in the binder system during the solidification was magni- fied due to the much larger cross-section of the rotor hub. With this problem at hand, a new binder system was developed which com￾bined low shrinkage during the solidification in the die with greatly improved binder removal characteristics. Therefore, radial turbine rotors were molded using this binder system which was visually flaw free after molding and binder removal. The same authors have reported that liquid phase sintering of Si3N4 ceramics can be pro￾duced by injection molding with sufficient dimensional control for turbine engine applications. In the fabrication of Si3N4 radial power turbine wheel, three forming methods have been used: i) injection molding, ii) slip cast￾ing and iii) cold isostatic pressing. Considering that one of the must difficult components to form by conventional ceramic form￾ing processing has been the radial turbine wheel, the development of the forming technique for a large, complex-shaped compo￾nent as well as the development of a high strength and refractory material to achieve a turbine inlet temperature (TIT) of 1350 ◦C is required [1,25,39,82–84,86]. Takatori et al. [1] used for fabrication of radial turbine wheel Si3N4 with additions of 5 wt% Y2O3 and 5 wt% MgAl2O4 (spinel) as additives. Using this composition, sev￾eral small components for reciprocating engines were fabricated by injection molding. However, the ceramics obtained had good mechanical properties atmoderate temperatures, but the high tem￾perature strength of the ceramics was no satisfactory for gas turbine components which would operate at 1350 ◦C. In order to overcome this problem, two methods for injection molding the wheel were attempted: one body forming (the whole body of the wheel is injec￾tion molded at one time) and two-piece forming (the wheel was divided in two pieces that are injection molded separately. In the case of one body forming, the sintered bodies presented internal cracks as well as others were damaged by surface crack genera￾tion and broke down during removal binder. Takatori et al. [1] and Shimizu et al. [25] concluded that one body forming of the large radial wheel was not practical using the current injection mold￾ing method. Therefore, the processes of fabrication of radial wheel were attempted dividing the hub of the wheel (the thickest sec￾tion) in two axially symmetrical parts to reduce the thickness about in half [87]. The resulting parts were injection molded separately and the survival probability of the wheel after binder removal step increased markedly. These parts were joined by CIP treatment after binder removal. It is noteworthy that even for the divided two-piece process; prolonged heat schedule was required with the purpose to obtain a sound binderless body. Although injection molding technique is highly viable for fab￾rication of complex-shaped ceramic components and numerous prototype silicon nitride components have been fabricated by different laboratories in the world being tested successfully in heat￾engine environments, specific challenges are required [88] such as: i) achieving raw and palletized feedstocks with consistent and stable flow characteristics, ii) developing mold designs that allow feedstock to fill completely while minimizing defects during han￾dling and subsequent processing, iii) attaining enough control over the entire process to ensure high, reliable yields of uniform parts and iv) reducing the manufacture cycle time so that the result￾ing silicon nitride parts are competitive with all-metal components that they are intended to replace. The application of powder injec￾tion moulding enables the production of intricate features and unusual geometries, offering and economic solution to difficult production problems when part complexity goes beyond of more basic forming technologies such as dry pressing. This technique offers excellent batch to batch repeatability and process capabil￾ities achieving tolerances of ±<0.3% for applications in markets as aerospace, communications, automotive, electronic, chemical, medical, etc. 3.2. Gelcasting of Si3N4 ceramics Gelcasting is amolding technique for ceramic andmetallicmate￾rials [89–93] which offers distinct advantages as an alternative to the more conventional ceramic forming methods such as dry pressing, slip casting and injection molding [89,94–97]. Principal advantages include near-net-shape forming, high green density

and low organic levels in the dried green ceramics. Therefore, a final mechanical properties of Si3 N4 parts: i)the surface quality wide variety of ceramic materials have been prepared using gel of the mold that is transferred into the final part. Therefore, the casting process including Si3 N4, SiC, AlO3 and zro,, among others. higher this surface roughness, leads to lower the final mechanical In gel casting, slurry made from ceramic powder and a water-based strength. The defects in the mold surface can cause notches which monomer solution is poured into a mold, polymerized in situ to lead to stress concentration in the final part and ii)with perfectly mobilize the particles in a gelled part, removed from the mold smooth mold surface, the difference between bulk microstructure while still wet, then dried and fired. If the solvent for the monomers and surface microstructure has to be considered when describing is organic, it is no aqueous gel casting: if is water, it is aqueous gel- the mechanical properties of silicon nitride parts. The maximum casting [94, 96, 98 The development of an aqueous process using strength is achieved with polished samples. Similarly, Stampfl et acrylamide as monomer was completed in 1988[95, 99]. However, al. [93 obtained strength values of 414, 950, and 983 MPa in ncerns regarding health, safety and disposal of acrylamide caused Si3N4 unpolished, Si3N4 polished and Si3 N4 GPS and polished industrial rejection of the process because the acrylamide is a neu- respectively, where all samples were sintered at 1750 C in nitro- toxin. Therefore, the development of a low toxicity process was gen atmosphere. On the other hand, materials such as aluminum, initiated to deal with the lack of acceptance and it was fully demon anodized aluminum, brass, glass, graphite indium alloys, neoprene trated in 1990 90 rubber, plaster and polyethylene are commonly used in construct- eramic parts from different ceramics such as aluminum oxide ing gelcasting molds. Al203, and high-performance silicon nitride Si3 N4, have been pro- injection molding, before a ceramic body can be sintered, luced by gelcasting ranging in size from <l g to >6 kg with thin the binder added must be removed. One advantage of gelcasting sections as small as 0. 2 mm and solids loading as high as 55-60 vol% is the small amount of polymer that remains in the green bod in alumina slurries and 45-57 vol% in silicon nitride suspensions after drying [101]. The dried gelcast ceramic contains only about [99 Although gelcast bodies typically shrink -23% during densifi- 2-6wt% polymer, which depends on the solids loading of the slurry, tion and resulting sometimes in distortion in the densified parts, the concentration of monomers in the premix, and the density this technique starts to be used not only for manufacturing of com- of powder. For comparison, a 45 vol% solids silicon nitride with plicated shaped dense products such as Si3N4 parts of turbines but the composition Si3N4+5 wt% Al203 +5 wt% Y203 made using a also for manufacturing of porous ceramic[ 90, 100). The most 15 wt% 4: 1 Methacrylamide-N, N'-Methylene bisacrylamide(MAM- dvanced works in this field are already in the phase or commer- MBAM) premix contains about 5.5 wto polymer in the dried part, alization and, Allied Signal Ceramics Components (torrance CA. meanwhile an injection molded silicon nitride with the same com- USA)working together with ORNL (Oak Ridge, TN, USA)has devel- position and solid loading would contain about 27 wt% polymer, ped and automated gelcasting fabrication process of production nearly five times as much. During burn out binder process, a lower Si3 N4 ceramic turbine rotors [100 temperature is required to remove the polymer carefully or else To optimize gelcasting of silicon nitride, Omatete[98 reported the final product may have defects and cracks. Heating rates on the he optimal gelcasting condition for the AlliedSignal Ceramic com- order to 0.5-1oCmin- to temperatures as high as 650C have been onents GN-10 silicon nitride formulation in a near-production used successfully for both silicon nitride and alumina gelcast parts. nvironment. The principal criterion used to determine optimum On the other side the AlliedSignal engineers [98, 99, 103, 104] noted design was the green strength. The investigation predicted 80% that gelcasting did not have two problems that plague injection increase in green strength(4.3 MPa versus 2.4 MPa )but the results molding such as the binder that can be as high as 20% of the weight howed only 60%increase(3.8 MPa). It is preferred that gelcasting of the ceramic versus to 4% in gelcasting and the other problem is slurries be at least 50 vol% solids: higher solids loadings are desir- related to the burning out the binder in injection molding that may ables. In most of the cases, solids loadings above 501 ompared to less than a day for a gelcast ceramic Addi- achieved and some cases, solids loading above 60 vol% are attain- tionally, defects and cracks can also develop in other stages of the ble. Notwithstanding, sometimes these requirements cannot be injection molding process such as drying. Such problems are rarely and Si3N4(Ube E10)is an example of a ceramic powder in which it seen in gelcast ceramics if they are properly dried [101] is nearly impossible to achieve a solids loading above 45 vol% in a In the manufacture of Si3 Na gelcast gelcast slurry due to that this powder is very difficult to disperse for turbine engines, the reproducibility of the parts is a critical any application inasmuch as possess a high surface area. In this con- tor. Janney et al. [101]reported the fabrication of a series of silicon text and with the purpose to obtain Si3 N4 ceramics with excellent nitride batches prepared under identical conditions and gelcast properties, a series of commercial dispersants was evaluated for Eleven batches were prepared for the repeatability study and each heir efficacy in dispersing silicon nitride in water[ 90]. The follow- batch was prepared on a different day over a period of 18 days. The ng conditions should be obtained for a good dispersant in silicon results were favorable and the uniformity of the castings is shown nitride: i)The 24-h sedimentation height should be high: i. e the clearly by the standard deviation values which indicate a varia- powder should not have settled out of a suspension in a short time tion of only 0. 1-0.3% about the average value for the dimensions should pack very well when is does finally settle out of suspension, bodies show that gelcasting process is reproducible. A standard and iii) the 3-week cloudy /clear interface height should be high; deviation of only 0.02 mm or about 0. 1% was obtained i.e. the finest of the particles should stay in suspension for a very As outlined above, gelcasting provides an excellent alternative long time and should not reagglomerate and settle. Therefore, the to manufacturing large, complex-shaped components such as tur- evelopment of excellent dispersant system is critical. As example, bine rotors, valves and cam followers for gasoline and dieseleng the Table 3 of [101 summarizes the dispersants cor In reason of this, the high degree of homogeneity required to for gelcasting ceramics including Si3N produce excellent parts can be retained. Janney et al. [101 taking The mold selection. mold fabrication and mold use are the crit- data from Pollinger [105 ]illustrated this point wherein after drying. ical aspects of successful gelcasting 93, 101. Proper selection of the green density of several sections of the rotor was determined mold material, fabrication method, filling method and mold release by the Archimedes immersion method. They observed that the In make the difference between producing excellent parts and variation in green density was extremely low and all the sections producing"also rans"Stampfl et al. [93, 102] have concluded that measured except one were within 0. 2% of the average green density two parameters in addition to the bulk properties determine the of 57.77% of the theoretical density. This is an especially significant

134 M.H. Bocanegra-Bernal, B. Matovic / Materials Science and Engineering A 500 (2009) 130–149 and low organic levels in the dried green ceramics. Therefore, a wide variety of ceramic materials have been prepared using gel casting process including Si3N4, SiC, Al2O3 and ZrO2, among others. In gel casting, slurry made from ceramic powder and a water-based monomer solution is poured into a mold, polymerized in situ to immobilize the particles in a gelled part, removed from the mold while still wet, then dried and fired. If the solvent for the monomers is organic, it is no aqueous gel casting; if is water, it is aqueous gel￾casting [94,96,98]. The development of an aqueous process using acrylamide as monomer was completed in 1988 [95,99]. However, concerns regarding health, safety and disposal of acrylamide caused industrial rejection of the process because the acrylamide is a neu￾rotoxin. Therefore, the development of a low toxicity process was initiated to deal with the lack of acceptance, and it was fully demon￾strated in 1990 [90]. Ceramic parts from different ceramics such as aluminum oxide Al2O3, and high-performance silicon nitride Si3N4, have been pro￾duced by gelcasting ranging in size from <1 g to >6 kg with thin sections as small as 0.2 mm and solids loading as high as 55–60 vol% in alumina slurries and 45–57 vol% in silicon nitride suspensions [99]. Although gelcast bodies typically shrink ∼23% during densifi- cation and resulting sometimes in distortion in the densified parts, this technique starts to be used not only for manufacturing of com￾plicated shaped dense products such as Si3N4 parts of turbines but also for manufacturing of porous ceramic objects [90,100]. The most advanced works in this field are already in the phase or commer￾cialization and, Allied Signal Ceramics Components (Torrance, CA, USA) working together with ORNL (Oak Ridge, TN, USA) has devel￾oped and automated gelcasting fabrication process of production Si3N4 ceramic turbine rotors [100]. To optimize gelcasting of silicon nitride, Omatete [98] reported the optimal gelcasting condition for the AlliedSignal Ceramic com￾ponents’ GN-10 silicon nitride formulation in a near-production environment. The principal criterion used to determine optimum design was the green strength. The investigation predicted 80% increase in green strength (4.3 MPa versus 2.4 MPa) but the results showed only 60% increase (∼3.8 MPa). It is preferred that gelcasting slurries be at least 50 vol% solids; higher solids loadings are desir￾ables. In most of the cases, solids loadings above 50 vol% can be achieved and some cases, solids loading above 60 vol% are attain￾able. Notwithstanding, sometimes these requirements cannot be and Si3N4 (Ube E10) is an example of a ceramic powder in which it is nearly impossible to achieve a solids loading above 45 vol% in a gelcast slurry due to that this powder is very difficult to disperse for any application inasmuch as possess a high surface area. In this con￾text and with the purpose to obtain Si3N4 ceramics with excellent properties, a series of commercial dispersants was evaluated for their efficacy in dispersing silicon nitride in water [90]. The follow￾ing conditions should be obtained for a good dispersant in silicon nitride: i) The 24-h sedimentation height should be high: i.e. the powder should not have settled out of a suspension in a short time, ii) the 3-week sedimentation height should be low; i.e. the powder should pack very well when is does finally settle out of suspension, and iii) the 3-week cloudy/clear interface height should be high; i.e. the finest of the particles should stay in suspension for a very long time and should not reagglomerate and settle. Therefore, the development of excellent dispersant system is critical. As example, the Table 3 of [101] summarizes the dispersants commonly used for gelcasting ceramics including Si3N4. The mold selection, mold fabrication and mold use are the crit￾ical aspects of successful gelcasting [93,101]. Proper selection of mold material, fabrication method, filling method and mold release can make the difference between producing excellent parts and producing “also rans”. Stampfl et al. [93,102] have concluded that two parameters in addition to the bulk properties determine the final mechanical properties of Si3N4 parts: i) the surface quality of the mold that is transferred into the final part. Therefore, the higher this surface roughness, leads to lower the final mechanical strength. The defects in the mold surface can cause notches which lead to stress concentration in the final part and ii) with perfectly smooth mold surface, the difference between bulk microstructure and surface microstructure has to be considered when describing the mechanical properties of silicon nitride parts. The maximum strength is achieved with polished samples. Similarly, Stampfl et al. [93] obtained strength values of 414, 950, and 983 MPa in Si3N4 unpolished, Si3N4 polished and Si3N4 GPS and polished, respectively, where all samples were sintered at 1750 ◦C in nitro￾gen atmosphere. On the other hand, materials such as aluminum, anodized aluminum, brass, glass, graphite, indium alloys, neoprene rubber, plaster and polyethylene are commonly used in construct￾ing gelcasting molds. As injection molding, before a ceramic body can be sintered, the binder added must be removed. One advantage of gelcasting is the small amount of polymer that remains in the green body after drying [101]. The dried gelcast ceramic contains only about 2–6 wt% polymer, which depends on the solids loading of the slurry, the concentration of monomers in the premix, and the density of powder. For comparison, a 45 vol% solids silicon nitride with the composition Si3N4 + 5 wt% Al2O3 + 5 wt% Y2O3 made using a 15 wt% 4:1 Methacrylamide-N,N’-Methylene bisacrylamide (MAM￾MBAM) premix contains about 5.5 wt% polymer in the dried part, meanwhile an injection molded silicon nitride with the same com￾position and solid loading would contain about 27 wt% polymer, nearly five times as much. During burn out binder process, a lower temperature is required to remove the polymer carefully or else the final product may have defects and cracks. Heating rates on the order to 0.5–1 ◦C min−1 to temperatures as high as 650 ◦C have been used successfully for both silicon nitride and alumina gelcast parts. On the other side, the AlliedSignal engineers [98,99,103,104] noted that gelcasting did not have two problems that plague injection molding such as the binder that can be as high as 20% of the weight of the ceramic versus to 4% in gelcasting and the other problem is related to the burning out the binder in injection molding that may take a week compared to less than a day for a gelcast ceramic. Addi￾tionally, defects and cracks can also develop in other stages of the injection molding process such as drying. Such problems are rarely seen in gelcast ceramics if they are properly dried [101]. In the manufacture of Si3N4 gelcast ceramic components for turbine engines, the reproducibility of the parts is a critical fac￾tor. Janney et al. [101] reported the fabrication of a series of silicon nitride batches prepared under identical conditions and gelcast. Eleven batches were prepared for the repeatability study and each batch was prepared on a different day over a period of 18 days. The results were favorable and the uniformity of the castings is shown clearly by the standard deviation values which indicate a varia￾tion of only 0.1–0.3% about the average value for the dimensions measured. The measurement of diameter both green and sintered bodies show that gelcasting process is reproducible. A standard deviation of only 0.02 mm or about 0.1% was obtained. As outlined above, gelcasting provides an excellent alternative to manufacturing large, complex-shaped components such as tur￾bine rotors, valves and cam followers for gasoline and diesel engines [104]. In reason of this, the high degree of homogeneity required to produce excellent parts can be retained. Janney et al. [101] taking data from Pollinger [105]illustrated this point wherein after drying, the green density of several sections of the rotor was determined by the Archimedes immersion method. They observed that the variation in green density was extremely low and all the sections measured except one were within 0.2% of the average green density of 57.77% of the theoretical density. This is an especially significant