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 ShapeM.H. Bocanegra-Bernal, B. Matovic / Materials Science and Engineering A 500 (2009) 130–149 135 accomplishment given the large variation in cross-sectional thick￾ness 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 gelcast￾ing. 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, plas￾ticizer, 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 tough￾ness and Weibull modulus above 15 [108]. The highest Weibull modulus has been obtained using La2O3 as the primary sinter￾ing 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 transfor￾mation. 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 pow￾der 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 pro￾hibitively 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 sil￾icon 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 difficul￾ties 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 param￾eters, 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 expan￾sion 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 com￾position 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 Manufac￾turing (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