M.H. Bocanegra-Bermal B Matovic/ Materials Science and Engineering A 500(2009)130-14 137 necessaries to make the Mold SDM process work, and those that is ceramic components for use in gas turbine engines. Ideally Mold are desirable because they improve the process by making it better, SDM can be used as a production process, rather than a proto- faster or cheaper. The material properties related to deposition are: typing process. Prototyping processes are useful during design but low shrinkage, low viscosity, good wetting for the replication of fine there is always the issue of how the production parts will be made details, strong interlayer bonding curing temperatures compatible [119 In large turbines, both vanes and blades are large enough to vith other materials On the other side the properties related to be manufactures with internal and film cooling which allows high casting and curing are: i)Viscosity: lower viscosity materials are temperature metals to be used in most cases. However, in the case easier to cast because they flow through the small passages in the of micro-scale turbines(diameter <100 mm), the limits of scale pre- molds more easily. Lower viscosity materials are also easier to deair clude active blade cooling, and thus the blades must be solid which because bubbles can move through the liquid and up to the surface makes ceramics desirable, if they can be produced [ 128. more easily, ii) Wetting: mold filling is also dependent on how well On the other hand. the reliability and the lack of the shap- the part material wets the mold material. If wetting is poor the ing techniques have been the major issues for the application of part material will have difficulty flowing into fine features in the Si3 N4 ceramics to gas turbines and rotors in particular. However nold Applying pressure during casting will help but this compli- the second issue has been overcome by using Mold SDM process cates the process, iii) Working time: longer working times make in combination with gelcasting. Taking into account the high rotat ting easier because there is more time to fill the mold and deair ing speeds(designed to rotate at 800.000 rpm with turbine inlet it Longer working times usually correlate with longer cure time temperature above 1000C), the straightness of the shaft and the but this is not as much of an issue with part materials because there shape accuracy is crucial for a functioning device. Therefore, with is only one casting operation per part, instead of one per mold layer the proper development of high-speed bearing technology and the as there is with mold and support materials, iv)Cure conditions: application of Mold SDM process to manufacture of the hot rotating whatever conditions are required to cure the part material must be elements in silicon nitride, micro-scale gas turbine engine appears compatible with the mold material. Materials are often cured by to be a possibility. In addition to ceramic parts, Mold SDM pro- heating them, in which case the cure temperature must be suffi- cess can be used to make parts from a variety of castable materials ciently low that it does not cause the mold to soften or melt. The including polymers such as polyurethane, epoxy and silicone. The cure exotherm must also be taken into account, v )Cure exotherm: Mold SdM process has been automated by the addition of depo- any materials cure exothermically and since the molds are made sition and curing hardware to a commercially available milling of wax this can be an issue if enough heat is generated that the mold machine Parts have been made without manual intervention using softens or melts. Faster curing materials generally exhibit higher this machine. Two main disadvantage of this process are i) the cure exotherms, vi) Chemical compatibility: part materials must be imperfect deposition of wax generating micro bubbles and dis- chemically compatible with the mold material and the mold mate- tortion, and ii) to preserve the straightness during the sintering rial must not chemically inhibit the curing of the part material or [118, 128]. affect the properties of the materials, and vii). Shrinkage on cur part materials must shrink as little as possible on cure for two rea-.5. Rapid prototyping of si3 Na ceramic parts sons. First, if there is significant shrinkage then the cured part may not accurately duplicate the geometry of the mold cavity Section a wide variety of commercially available systems for rapid pro- of the part might shrink away from the mold surface and cause sink totyping enables the user to fabricate prototypes with almost any holes on the part surface. Second, if the part shrinks it may break shape in a large range of different sizes 93. Most RP techniques put itself or the mold due to the stress created [118 less emphasis on materials issues, and if they do 129, 130 it is not All ceramic parts made using Mold SDM have been made by easy to switch between different materials. Rapid Prototyping pro- elcasting. The initial material was proprietary non-aqueous alu- cesses, also known as Solid Freeform Fabrication(SFF) processes. mina gelcasting slurry developed by ACr Silicon nitride slurry has build parts in a layerwise fashion. There are a number of reasons also been developed, based on the alumina formulation. However, for adopting this approach: i) shape complexity, ii)elimination of due to differences in the surface chemistries of alumina and silicon tooling, iii) simplified process planning, iv)automated fabrication, nitride the organic components are slightly different in each slurry. v)material limitations, vi) surface quality and vii) material quality. The silicon nitride slurries are more difficult to make and do not Some RP processes have been used in smaller scale in order urrently have as high solids loadings as the alumina slurries[ 127. to manufacturing Si3N4 ceramic components, such as: a)Stere- For example, the solids loading for Al2O3 slurry is about 50% and lithography( SLA)[131 For this process, the sintered Si3Na was cure conditions of 30 min at 55C compared to solids loading of only 90% dense which account for the low strength of 412 MPa 52% and cure of 30 min at 55C required for Si3 N4 slurries. reported by Zimbeck et al. [132], b)Fused Deposition Model- Cooper [118] reported the processing conditions for the silicon ing(FDM)[133-135. The strength obtained with silicon nitride nitride gelcasting slurries used in Mold SDM as follows: 1)Cur- ceramic components by using this technique was 824+ 110 MPa ing at 50-60 C for 2 h where the parts are hermetically sealed [135. c) Three-Dimensional Printing(3DP)[136 Si3 N4 ceramics and evacuated to exclude air, ii) Drying performed in air 4 h at produced by this technique have obtained strength values about 6°c.1.5hat96°,2hat155°,15hat165°,2hat186°,i)570MPa137] Burnout performed in air using the following time-temperature Most of these techniques were developed to prepare plastic, profile:80°hlto160°c,10hlto300°c2hat300°c6°ch-1wax, or paper parts[138] However, although the individual pro to 400C, 2 h at 400 C, 12Ch-I to 500 C, 18Ch- to 600 C, 1h cess differs, they all produce a solid part directly from a 3D CAD at 600C, iv) Sintering carried out in nitrogen atmosphere at tem- drawing, without the need of dies or molds. The initial steps for eratures between 1700 and 1750 C. It is very important to control each of the different flexible manufacturing techniques are simi- the atmosphere to obtain the best results. lar. An integration of rapid prototyping technologies into standard The goal in developing Mold SDM is to develop a production ceramic shaping processes has been studied by Loschau [139)and manufacturing process to enable the fabrication of complex func- Knitter et al. [140]. After debinding and sintering, there are complex tional ceramic parts, particularly in the hot sections of the engines functional ceramic components that can be immediately used. It where highly stressed turbines and other components must sur- also possible than after the correction of the shrinkage which was vive in contact with very hot gasses[128 ]. The primary application estimated during an interactive step, must be started in order toM.H. Bocanegra-Bernal, B. Matovic / Materials Science and Engineering A 500 (2009) 130–149 137 necessaries to make the Mold SDM process work, and those that are desirable because they improve the process by making it better, faster or cheaper. The material properties related to deposition are: low shrinkage, low viscosity, good wetting for the replication of fine details, strong interlayer bonding curing temperatures compatible with other materials. On the other side, the properties related to casting and curing are: i) Viscosity: lower viscosity materials are easier to cast because they flow through the small passages in the molds more easily. Lower viscosity materials are also easier to deair because bubbles can move through the liquid and up to the surface more easily, ii) Wetting: mold filling is also dependent on how well the part material wets the mold material. If wetting is poor the part material will have difficulty flowing into fine features in the mold. Applying pressure during casting will help but this compli￾cates the process, iii) Working time: longer working times make casting easier because there is more time to fill the mold and deair it. Longer working times usually correlate with longer cure times, but this is not as much of an issue with part materials because there is only one casting operation per part, instead of one per mold layer as there is with mold and support materials, iv) Cure conditions: whatever conditions are required to cure the part material must be compatible with the mold material. Materials are often cured by heating them, in which case the cure temperature must be suffi- ciently low that it does not cause the mold to soften or melt. The cure exotherm must also be taken into account, v) Cure exotherm: many materials cure exothermically and since the molds are made of wax this can be an issue if enough heat is generated that the mold softens or melts. Faster curing materials generally exhibit higher cure exotherms, vi) Chemical compatibility: part materials must be chemically compatible with the mold material and the mold mate￾rial must not chemically inhibit the curing of the part material or affect the properties of the materials, and vii). Shrinkage on cure: part materials must shrink as little as possible on cure for two rea￾sons. First, if there is significant shrinkage then the cured part may not accurately duplicate the geometry of the mold cavity. Sections of the part might shrink away from the mold surface and cause sink holes on the part surface. Second, if the part shrinks it may break itself or the mold due to the stress created [118]. All ceramic parts made using Mold SDM have been made by gelcasting. The initial material was proprietary non-aqueous alu￾mina gelcasting slurry developed by ACR. Silicon nitride slurry has also been developed, based on the alumina formulation. However, due to differences in the surface chemistries of alumina and silicon nitride the organic components are slightly different in each slurry. The silicon nitride slurries are more difficult to make and do not currently have as high solids loadings as the alumina slurries [127]. For example, the solids loading for Al2O3 slurry is about 50% and cure conditions of 30 min at 55 ◦C compared to solids loading of 52% and cure of 30 min at 55 ◦C required for Si3N4 slurries. Cooper [118] reported the processing conditions for the silicon nitride gelcasting slurries used in Mold SDM as follows: i) Cur￾ing at 50–60 ◦C for 2 h where the parts are hermetically sealed and evacuated to exclude air, ii) Drying performed in air 4 h at 86 ◦C, 1.5 h at 96 ◦C, 2 h at 155 ◦C, 15 h at 165 ◦C, 2 h at 186 ◦C, iii) Burnout performed in air using the following time-temperature profile: 80 ◦C h−1 to 160 ◦C, 10 ◦C h−1 to 300 ◦C, 2 h at 300 ◦C, 6 ◦C h−1 to 400 ◦C, 2 h at 400 ◦C, 12 ◦C h−1 to 500 ◦C, 18 ◦C h−1 to 600 ◦C, 1 h at 600 ◦C, iv) Sintering carried out in nitrogen atmosphere at tem￾peratures between 1700 and 1750 ◦C. It is very important to control the atmosphere to obtain the best results. The goal in developing Mold SDM is to develop a production manufacturing process to enable the fabrication of complex func￾tional ceramic parts, particularly in the hot sections of the engines where highly stressed turbines and other components must sur￾vive in contact with very hot gasses [128]. The primary application is ceramic components for use in gas turbine engines. Ideally Mold SDM can be used as a production process, rather than a proto￾typing process. Prototyping processes are useful during design but there is always the issue of how the production parts will be made [119]. In large turbines, both vanes and blades are large enough to be manufactures with internal and film cooling which allows high temperature metals to be used in most cases. However, in the case of micro-scale turbines (diameter <100 mm), the limits of scale pre￾clude active blade cooling, and thus the blades must be solid which makes ceramics desirable, if they can be produced [128]. On the other hand, the reliability and the lack of the shap￾ing techniques have been the major issues for the application of Si3N4 ceramics to gas turbines and rotors in particular. However, the second issue has been overcome by using Mold SDM process in combination with gelcasting. Taking into account the high rotat￾ing speeds (designed to rotate at 800.000 rpm with turbine inlet temperature above 1000 ◦C), the straightness of the shaft and the shape accuracy is crucial for a functioning device. Therefore, with the proper development of high-speed bearing technology and the application of Mold SDM process to manufacture of the hot rotating elements in silicon nitride, micro-scale gas turbine engine appears to be a possibility. In addition to ceramic parts, Mold SDM pro￾cess can be used to make parts from a variety of castable materials including polymers such as polyurethane, epoxy and silicone. The Mold SDM process has been automated by the addition of depo￾sition and curing hardware to a commercially available milling machine. Parts have been made without manual intervention using this machine. Two main disadvantage of this process are i) the imperfect deposition of wax generating micro bubbles and dis￾tortion, and ii) to preserve the straightness during the sintering [118,128]. 3.5. Rapid prototyping of Si3N4 ceramic parts A wide variety of commercially available systems for rapid pro￾totyping enables the user to fabricate prototypes with almost any shape in a large range of different sizes [93]. Most RP techniques put less emphasis on materials issues, and if they do [129,130] it is not easy to switch between different materials. Rapid Prototyping pro￾cesses, also known as Solid Freeform Fabrication (SFF) processes, build parts in a layerwise fashion. There are a number of reasons for adopting this approach: i) shape complexity, ii) elimination of tooling, iii) simplified process planning, iv) automated fabrication, v) material limitations, vi) surface quality and vii) material quality. Some RP processes have been used in smaller scale in order to manufacturing Si3N4 ceramic components, such as: a) Stere￾olithography (SLA) [131]. For this process, the sintered Si3N4 was only 90% dense which account for the low strength of 412 MPa reported by Zimbeck et al. [132], b) Fused Deposition Model￾ing (FDM) [133–135]. The strength obtained with silicon nitride ceramic components by using this technique was 824 ± 110 MPa [135], c) Three-Dimensional Printing (3DP) [136]. Si3N4 ceramics produced by this technique have obtained strength values about 570 MPa [137]. Most of these techniques were developed to prepare plastic, wax, or paper parts [138]. However, although the individual pro￾cess differs, they all produce a solid part directly from a 3D CAD drawing, without the need of dies or molds. The initial steps for each of the different flexible manufacturing techniques are simi￾lar. An integration of rapid prototyping technologies into standard ceramic shaping processes has been studied by Loschau [139] and Knitter et al.[140]. After debinding and sintering, there are complex functional ceramic components that can be immediately used. It is also possible than after the correction of the shrinkage, which was estimated during an interactive step, must be started in order to