135 accomplishment given the large vari e purpose ness in the rotor about 50 m due to der form silico of low cost 20% of the c preforms silicon nitride a series of tests of the complex shapes mus or [119]. Assembly Mold SDM Shape
M.H. Bocanegra-Bernal, B. Matovic / Materials Science and Engineering A 500 (2009) 130–149 135 accomplishment given the large variation in cross-sectional thickness in the rotor taking in to account that the hub of the rotor is about 50 mm in diameter and the blade tips are only 1.5 mm tick. Variations in green density >2.5 th which caused cracking of the parts during consolidation at high temperature, were obtained with slipcast rotor of the same ceramic composition used with gelcasting. Although gelcasting was developed as a near-net-shape forming process, green machining of gelcast parts can be particularly useful for producing prototypes, for custom manufacturing or for adding features to a cast part that would be too difficult or too costly to include in the mold [106]. The high strength of the green body is of great advantage for handling of the parts before sintering and for being able to produce large castings. This is achieved with a uniform distribution of the binder throughout the casting and with inherent strength of the crosslinked polymer [90,97,98,101,103,104]. Some studies have shown that adding a plasticizer such as glycerine or poly (ethylene glycol) to the gelcasting formulation markedly improves the machinability of green gelcast parts. In Addition, with the proper preparation gelcasting formulation (binder, plasticizer, and dispersant, for example) and the addition of 3–8% sintering aids (usually a combination of Y2O3, La2O3, and Al2O3) allows densification and predictable shrinkage in near-net-shape moderate-pressure gas sintering (3 h at 1800–1900 ◦C in N2 at 1–2 MPa) [107]. Especially in the case of silicon nitride and advanced ceramics, the control of the microstructure, grain growth and acceptable final properties is crucial for the quality of final part. In order to achieve this, dispersed submicron powders are used in the suspensions [93,103]. With high quality silicon nitride powders (oxygen below 0.2%), the parts routinely achieve bending rupture strength over 750 MPa at temperatures up to 827 ◦C, very high fracture toughness and Weibull modulus above 15 [108]. The highest Weibull modulus has been obtained using La2O3 as the primary sintering aid in the formulations [109]. With the proper sintering cycle, a bimodal microstructure is obtained which enhances toughness and high temperature creep strength [110]. Using of Lu2O3 (which forms a Lu2Si2O7 phase) for most or all of the sintering aids results in slightly reduced RT strength, Weibull modulus, and shrinkage accuracy. Since for applications in gelcasting process most surfaces of the sintered part cannot be further machined or ground to obtain the near-shape, the surface microstructure has to be considered in addition to the bulk microstructure. Therefore, the roughness of the sintered part is determined by two factors as follows: i) the original particle size and ii) the amount of grain growth of the -needless which build up during the --Si3N4 phase transformation. Stampfl et al. [93] have reported the typical -needles in a glassy matrix in the bulk material of sintered silicon nitride, while due to the inhibited grain growth at the surface, the individual powder particles sinter together to form a fairly smooth surface with surface roughness of the final part between 0.5 and 1.8 m. It is well known that Si3N4 based ceramics tend to be prohibitively expensive due to the high cost or silicon nitride powders used to produce them. Therefore, nowadays the reduction of cost has been recognized as a major need for the successful introduction of silicon nitride ceramics in the wide marketplace [111,112]. With this expectative at hand, Sintered Reaction Bonded Silicon Nitride (SRBSN) is an attractive alternative to sintered silicon nitride which is formed by reacting silicon powder with nitrogen gas in order to form silicon nitride taking into account that silicon is a raw material of low cost [92,97,109,113] (high purity silicon powder is only about 20% of the cost of silicon nitride powder). On the other hand, silicon preforms undergo less sintering shrinkage than performs made of silicon nitride powders. However, silicon metal performs of very complex shapes must be made by expensive cumbersome forming process such as injection molding. Nonetheless, with the purpose to make these silicon performs, gelcasting is a simple, inexpensive process which has been developed as a method for forming ceramic greenware. In gelcasting of silicon metal compositions wherein the typical slurry is aqueous and basic (having pH of about 8.4), two difficulties have been observed: i) the poor dispersion characteristics of the silicon powder in the slurries and ii) the generation of gas bubbles in the slurry caused by reaction of the siliconmetal with water. Several experimental investigations showed that these processing difficulties could be overcome by reducing the pH of the aqueous slurry or by using isopropyl alcohol as the solvent system in place of water. Therefore, both an acidic aqueous system (with 35–40 vol% solids) and an alcohol-based system (with 50 vol% solids) were developed and successfully adapted for SRBSN gelcasting. With these parameters, Nunn et al. [97] reported that the green density of the as-cast samples showed a distinct difference between the two gelcastings systems, obtaining green densities of about 43% in aqueous slurries while the alcohol-based bodies had green densities of 51–55%. High green density can be detrimental conventional nitriding processes due to the tendency of the nitriding reaction to start at the surface of the sample and progress inward. Likewise, the volume expansion can close off the pore structure and prevent the nitrogen gas from reaching the unreacted silicon in the interior of the sample, especially in thick bodies. On the other hand, Kiggans et al. [114] and Kiggans and Tiegs [115] have shown that microwave heating results in improved mechanical properties in the final SRBSN product and their study was based in the composition 67 wt% Si-metal (Elkem Metallurgical grade) + 13 wt% Y2O3 (Molycorp-5600) + 4 wt% Al2O3 (RCHP-DBM) + 14 wt% Si3N4 (Stark LCION) + 2 wt% SiO2 (U.S. Silica-5 micron) which was turbomilled for ∼2 h with 4 mm Si3N4 media in isopropanol and Darvan-C and PVP as dispersants. With this composition the weight gain obtained (after the single-step nitridation and sintering treatment) was about 60% and is considered as near complete nitridation since weight losses occur during the sintering step [92,116]. The authors in their experimental study found that appears to be no relationship between the final sintered densities and the green densities of the materials calculating as theoretical density 3.3 g cm−3. For the fabrication of Si3N4 ceramic components for micro gas turbine engines, Assembly Mold SDM Shape Deposition Manufacturing (SDM) has been used in combination with gelcasting [117]. Assembly Mold Shape Deposition Manufacturing is a derivation of Mold SDM, an additive-subtractive layered manufacturing process developed at Stanford University [118]. With this technique, once a fugitive assembly mold has been made, the gelcasting process is applied to build a monolithic ceramic part. However, the major drawback of the SDM process is the possibility of geometrical inaccuracy during the mold assembly process. In order to combine both Assembly Mold SDM and gelcasting processes, the Rapid Prototyping Laboratory (RPL) at Stanford University developed a miniature Si3N4 ceramic gas turbine with its industrial partners [117]. These two techniques have allowed the fabrication of the rotor group as well as inlet nozzle to spin at 800,000 rpm to generate 100W where due to the complexity of the geometry, the casting mold is decomposed into five parts: a cap, a turbine, an interconnect, a compressor and a shaft. Nonetheless, Liu et al. [117] reported that after the rotor group was sintered, it is found that the geometry features have shifted from the concentric center (an eccentricity of 0.5 mm). However, after removing the errors from the fabrication and the mold assembly and provide better geometric support, the functionality of the rotor group using these processes has been demonstrated by a series of tests of the turbine and the compressor [119]. Assembly Mold SDM Shape
M.H. Bocanegra-BemaL B Matovic/ Materials Science and Engineering A 500(2009)130-149 Deposition Manufacturing process will be analyzed in more detail imentations, the optimal conditions for robocast found by He et al. later on. Compared to other colloidal processes(for example, slip [120 were 52 vol% Si3 N4 with 1 wt% Darvan 821A as dispersant casting and tape casting) the outstanding advantage of gelcasting and 0. 4 wt% aluminum nitrate at pH 7.8-8.5 based on rheologi is that its slurry can be in situ consolidated which in turn results cal studies of Si3 Na slurries. Likewise, no meshing, warping and in near-net-shape forming. This promising technique has been cracking were observed during forming and drying and the green employed with submicrometer or micrometer ceramic materials and sintered densities were about 56 and 99.3% of theoretical den- such as alumina, zirconia, silicon nitride, etc For Si3N4, however, sity, respectively. On the other hand, the sintered ceramics showed it is difficult to form a dense structure because of gas-discharging regular hexagonal shapes of cross-section of B-Si3N4 fibers which reactions. In this process, parts shrink and also experience warpage are characteristics of typical dense silicon nitride ceramics and are as a direct result of drying of the part. There are several factors believed to be responsible for the superior mechanical properties of which contribute to this as humidity, temperature and geometry. Si3N4 ceramics. Robocasting technology can also fabricate parts of large thicknesses that are unobtainable using slip casting. Likewise, (HA)that show promise as load-bearing scaffolds for bonxyapatite this technique has been used to develop lattices of hy he repal The Robocasting is a relatively new freeform fabrication tech- The advantages of robocasting technique are many. The aqueous nique for dense ceramics wherein to control deposition of ceramic systems are binderless and have very low toxicity. a densified part urries through an orifice the use of robotics are required [43, 120). can be made in less of 24 h. This technique is also amenable to Ceramic components with simple or complex shapes can be rapidly multi-material fabrication[121, 122). roduced form a computer aided-design drawing directly to a finished component that requires little or no machining after fab- 3. 4. Si3 Na ceramic components by mold shape deposition rication. The highlighted of this novel technique is moldless and manufacturing(MOLD SDM) inderless, and ceramics parts can be formed, dried, and sin ered within 24 h. This procedure uses the deposition of highly As outlined above gelcasting is a versatile method for making concentrated colloidal slurries with low organic content(<1 wt%) quality ceramic parts, however due to the poor interlayer bonding to construct complex, three-dimensional (3-D)components in it was not possible to build parts incrementally in layers. However, layer-by-layer built sequence [121, 122]. Robocasting has great by using the waxes and soldermasks, as part and support materi- romise for the rapid manufacture of complex, multiphase assem- als, respectively, it would be possible to build complex fugitive wax plage devices, such as piezoelectric ceramic-polymer composites, molds using SDM. These molds could then be used for gelcasting. ohotonic-band-gap lattices as well as Si3N4 ceramics in advanced This combination of process and materials was the starting point engines and gas turbines. for Mold SDM [117, 118. Based on the limitations of the existing pro- It is well known that Si3 Na is a non-oxide material and gener- cesses, the purpose for SDM and then mold SDM of Si is quite difficult to process via colloidal procedures. Hence, it is to improve the green part fabrication process and to develop is important to understand aspects about the dispersion and rheol- method to produce high quality green parts with high shape com ogy of Si3 N4 in order to obtain the optimal robocasting conditions. plexity and low cost Mold SDM makes molded parts using fugitive tion formed by a-Si3N4 with particle size of 0.77 um, surface area tion of this interesting technique is described elsewhere 18/ As example, He et al. [120]used in their investigations a composi- wax molds built using SDM techniques. A more complete descrip of 7.7mg. As dispersant was used Darvan 821A(with 40 wt% of Since mold SDM is based on SDM it shares many of the same ammonium polyacrylate of molecular weight about 3500). Finally, advantages and disadvantages over ot the ph was adjusted nalytical grade nitric acid(IN)and processes. The principal advantage of Mold SDM over other layered ammonium hydroxide solutions(40%). Robocast bars were formed manufacturing processes is that the final part is cast monolithi- and dried overnight at room temperature and sintered by pressure cally. This is beneficial for two reasons: i) the finished part will sintering in a N2 atmosphere from 1600 to 1800C for 1-2 h. The not contain any layer boundaries which in turn can be a source of final results indicated that the 1 wt% dispersant added is the most weakness due to incomplete bonding or the presence of foreig efficient concentration for dispersing Si3 N4 powder in aqueous sus- particles, ii) the finished part will not contain any of the residual pensions. This result implies that further addition of the dispersant stresses that typically result from layered manufacturing. The mold beyond a certain level had no more measurable contribution to may contain residual stresses, but these will not be transferred to he interfacial charge of the powder because the adsorbent dis- the cast part. Therefore, the lack of residual stresses in the finished persant layers on powder surfaces are saturated. Other authors, part will reduce the tendency for distortion. The elimination of the for example Albano and garrido [ 123] and Liu and Malghan [124 need for interlayer bonding in the part material allows Mold SDM who used other experimental methods obtained similar results to to use materials which cannot be used in SDM. The ACR(Advanced the obtained by He et al. [120] who concluded that the minimum Ceramics Research)gelcasting formulations are one example[ 126]. viscosity (greatest degree of dispersion)occurred at 1 wt% Darvan Mold SDM has three disadvantages over SDM. i)a third compatible lize a solids content of less than 47 vol%. When the volume percent of materials that can be used, i) there are extra casting an i material, the mold material, is required. The additional materials increases, the viscosity also increases and at low solids loading, removal steps which increase process time and iii)mold filling Newtonian. At 35 vol% solids, the slurries begin to show pseudo- vents can be added to ensure complete mold fillin e es sprues and dispersed slurries exhibit very low viscosity and are rheologically issues may limit part geometry, although in many cas plastic behavior and the viscosity is even relatively low while solids The materials selection process for Mold SDM is an important content around 50 vol%, interparticle interactions and interparticle factor in order to obtain a successful implementation of the process collisions become dominant, the viscosity begins to increase appre There are two categories of materials used in mold SDM: those that ciably and the rheological behavior becomes highly shear-thinning. are used to build molds and those that are cast into the finished With these conditions to the hand, for optimal robocasting, it is molds to produce parts. The two categories have different property desirable to robocast with slurries that have solids loading close to requirements because of their different uses. Property require- he dilatant transition (about 47 vol%)[43, 125 After several exper- ments can also be roughly divided into two groups: those that are
136 M.H. Bocanegra-Bernal, B. Matovic / Materials Science and Engineering A 500 (2009) 130–149 Deposition Manufacturing process will be analyzed in more detail later on. Compared to other colloidal processes (for example, slip casting and tape casting) the outstanding advantage of gelcasting is that its slurry can be in situ consolidated, which in turn results in near-net-shape forming. This promising technique has been employed with submicrometer or micrometer ceramic materials such as alumina, zirconia, silicon nitride, etc. For Si3N4, however, it is difficult to form a dense structure because of gas-discharging reactions. In this process, parts shrink and also experience warpage as a direct result of drying of the part. There are several factors which contribute to this as humidity, temperature, and geometry. 3.3. Robocasting of Si3N4 aqueous slurries The Robocasting is a relatively new freeform fabrication technique for dense ceramics wherein to control deposition of ceramic slurries through an orifice the use of robotics are required [43,120]. Ceramic components with simple or complex shapes can be rapidly produced form a computer aided-design drawing directly to a finished component that requires little or no machining after fabrication. The highlighted of this novel technique is moldless and binderless, and ceramics parts can be formed, dried, and sintered within 24 h. This procedure uses the deposition of highly concentrated colloidal slurries with low organic content (<1 wt%) to construct complex, three-dimensional (3-D) components in layer-by-layer built sequence [121,122]. Robocasting has great promise for the rapid manufacture of complex, multiphase assemblage devices, such as piezoelectric ceramic–polymer composites, photonic-band-gap lattices as well as Si3N4 ceramics in advanced engines and gas turbines. It is well known that Si3N4 is a non-oxide material and generally is quite difficult to process via colloidal procedures. Hence, it is important to understand aspects about the dispersion and rheology of Si3N4 in order to obtain the optimal robocasting conditions. As example, He et al. [120] used in their investigations a composition formed by -Si3N4 with particle size of 0.77 m, surface area of 7.7 m2 g−1. As dispersant was used Darvan 821A (with 40 wt% of ammonium polyacrylate of molecular weight about 3500). Finally, the pH was adjusted with analytical grade nitric acid (1N) and ammonium hydroxide solutions (40%). Robocast bars were formed and dried overnight at room temperature and sintered by pressure sintering in a N2 atmosphere from 1600 to 1800 ◦C for 1–2 h. The final results indicated that the 1 wt% dispersant added is the most efficient concentration for dispersing Si3N4 powder in aqueous suspensions. This result implies that further addition of the dispersant beyond a certain level had no more measurable contribution to the interfacial charge of the powder because the adsorbent dispersant layers on powder surfaces are saturated. Other authors, for example Albano and Garrido [123] and Liu and Malghan [124] who used other experimental methods obtained similar results to the obtained by He et al. [120] who concluded that the minimum viscosity (greatest degree of dispersion) occurred at 1 wt% Darvan dirpersant. Conventional silicon nitride slurries for slip casting typically utilize a solids content of less than 47 vol%. When the volume percent increases, the viscosity also increases and at low solids loading, dispersed slurries exhibit very low viscosity and are rheologically Newtonian. At 35 vol% solids, the slurries begin to show pseudoplastic behavior and the viscosity is even relatively low while solids content around 50 vol%, interparticle interactions and interparticle collisions become dominant, the viscosity begins to increase appreciably and the rheological behavior becomes highly shear-thinning. With these conditions to the hand, for optimal robocasting, it is desirable to robocast with slurries that have solids loading close to the dilatant transition (about 47 vol%) [43,125]. After several experimentations, the optimal conditions for robocast found by He et al. [120] were 52 vol% Si3N4 with 1 wt% Darvan 821A as dispersant and 0.4 wt% aluminum nitrate at pH 7.8–8.5 based on rheological studies of Si3N4 slurries. Likewise, no meshing, warping and cracking were observed during forming and drying and the green and sintered densities were about 56 and 99.3% of theoretical density, respectively. On the other hand, the sintered ceramics showed regular hexagonal shapes of cross-section of -Si3N4 fibers which are characteristics of typical dense silicon nitride ceramics and are believed to be responsible for the superior mechanical properties of Si3N4 ceramics. Robocasting technology can also fabricate parts of large thicknesses that are unobtainable using slip casting. Likewise, this technique has been used to develop lattices of hydrixyapatite (HA) that show promise as load-bearing scaffolds for bone repair. The advantages of robocasting technique are many. The aqueous systems are binderless and have very low toxicity. A densified part can be made in less of 24 h. This technique is also amenable to multi-material fabrication [121,122]. 3.4. Si3N4 ceramic components by mold shape deposition manufacturing (MOLD SDM) As outlined above, gelcasting is a versatile method for making quality ceramic parts, however due to the poor interlayer bonding it was not possible to build parts incrementally in layers. However, by using the waxes and soldermasks, as part and support materials, respectively, it would be possible to build complex fugitive wax molds using SDM. These molds could then be used for gelcasting. This combination of process and materials was the starting point for Mold SDM [117,118]. Based on the limitations of the existing processes, the purpose for SDM and then Mold SDM of Si3N4 ceramics is to improve the green part fabrication process and to develop a method to produce high quality green parts with high shape complexity and low cost. Mold SDM makes molded parts using fugitive wax molds built using SDM techniques. A more complete description of this interesting technique is described elsewhere [118]. Since Mold SDM is based on SDM it shares many of the same advantages and disadvantages over other Rapid Prototyping (RP) processes. The principal advantage of Mold SDM over other layered manufacturing processes is that the final part is cast monolithically. This is beneficial for two reasons: i) the finished part will not contain any layer boundaries which in turn can be a source of weakness due to incomplete bonding or the presence of foreign particles, ii) the finished part will not contain any of the residual stresses that typically result from layered manufacturing. The mold may contain residual stresses, but these will not be transferred to the cast part. Therefore, the lack of residual stresses in the finished part will reduce the tendency for distortion. The elimination of the need for interlayer bonding in the part material allows Mold SDM to use materials which cannot be used in SDM. The ACR (Advanced Ceramics Research) gelcasting formulations are one example [126]. Mold SDM has three disadvantages over SDM. i) a third compatible material, the mold material, is required. The additional materials compatibility and processing requirements may restrict the range of materials that can be used, ii) there are extra casting and mold removal steps which increase process time and iii) mold filling issues may limit part geometry, although in many cases sprues and vents can be added to ensure complete mold filling. The materials selection process for Mold SDM is an important factor in order to obtain a successful implementation of the process. There are two categories of materials used in Mold SDM: those that are used to build molds and those that are cast into the finished molds to produce parts. The two categories have different property requirements because of their different uses. Property requirements can also be roughly divided into two groups: those that are