Rapid manufacture of net-shape SiC components

Transcription

1 Int J Adv Manuf Technol (2010) 46: DOI /s ORIGINAL ARTICLE Rapid manufacture of net-shape SiC components Xiaoyong Tian & Dichen Li Received: 26 November 2008 /Accepted: 11 May 2009 /Published online: 30 May 2009 # Springer-Verlag London Limited 2009 Abstract Stereolithography was introduced into the netshape SiC components fabrication process to produce the molds for the preparation of porous carbon preforms. The mixed resin was cast into the molds and then pyrolyzed to produce the porous carbon preforms which had a high porosity of 40.93% and were infiltrated and reacted with molten silicon to get reaction-formed silicon carbide (RFSC) components. To realize the complete infiltration of the thick wall parts, pipelines as the channels for the molten silicon were added into the components. The hierarchical structures of the porous carbon had been attained to realize the controllability of the microstructure and properties of RFSC. The samples had a high linear and volume shrinkage, 24.7% and 57.3%, respectively, during the pyrolysis process. Phase composition had been investigated by optical microscopy and X-ray diffraction analysis. The results indicate that no residual carbon was available in the final RFSC. The net-shape fabrication process for the RFSC components could promote the industry applications where SiC components with complex surface and inner structure were needed. Keywords SiC. Rapid manufacture. Reaction infiltration. Ceramic design. Pipelines X. Tian : D. Li State Key Laboratory of Mechanical Manufacture System Engineering, Xi an Jiaotong University, Xi an , China X. Tian (*) Institute of Nonmetallic Materials, Clausthal University of Technology, Clausthal-Zellerfeld 38678, Germany xiaoyong.tian@tu-clausthal.de 1 Introduction Silicon carbide ceramic has attracted the attention of many researchers as an advanced ceramic because of its superior mechanical properties, wear resistance, and corrosion resistance in the high-temperature conditions. At the same time, just because of these properties, the fabrication processes of silicon carbide components are very difficult and expensive. The lack of the net-shaping capability and high process temperature are the bottleneck problems for the low-cost engineering applications of silicon carbide. Preparation of reaction-formed silicon carbide (RFSC) by using the molten silicon infiltration technique is a lowertemperature fabrication process, which has been extensively studied by many researchers. A method had been proposed to produce silicon carbide by pyrolyzing the mixture of furfural resin and furfural alcohol and being infiltrated with molten silicon [1, 2]. RFSC components had also been fabricated via infiltrating the molten silicon into the machined porous oak charcoal [3 7]. The reactive kinetics of the molten silicon infiltration into the porous medium had been already studied in some literatures [8 10]. In these conventional fabrication processes, the porous carbon preforms have been produced by machining natural wood or casting the liquid mixtures into a mold previously machined. If components with complex curved surfaces and inner structures need to be fabricated using the RFSC process, the molds or carbon preforms should be previously machined using the conventional fabrication processes. But the molds or the preforms with complex outer and inner fine structures could not be machined or could be machined only with high cost and low efficiency. So these RFSC processes still have a bottleneck problem mold or preform fabrication.

2 580 Int J Adv Manuf Technol (2010) 46: Rapid prototyping is a new fabrication method which can build three-dimensional parts with complex outer and inner structures directly from a computer-aided design (CAD) representation according to material addition layer by layer instead of removing materials in the conventional fabrication process. Friedel and Travitzky [11] firstly combined the selective laser sintering (SLS, one of the rapid prototyping processes) with the molten silicon infiltration process to fabricate the ceramic parts. The components manufactured by this process have a rough surface because of the low fabrication resolution of the SLS process. Stereolithography (SL) is another rapid prototyping process which has higher fabrication accuracy than SLS and specially fit for the fabrication of molds with complex structures. It had been used to rapidly manufacture polymer mold which was used for the fabrication of the porous artificial bone with complex inner structures for the cell culture [12, 13]. In the present work, SL technology was coupled with molten silicon infiltration to produce RFSC composite parts with complex outer and inner structures. In order to realize the full infiltration of the parts with thick walls during the molten silicon infiltration process, a sort of microstructure, pipelines, was added into the parts as main channels for the molten silicon infiltration. The apparent porosity of porous carbon can be controlled by altering the composition of the mixture resin. After the infiltration process, the residual channels might be used as inner cooling system for the high-temperature applications or lubrication system for the abrasion application. Being compared with laser direct sintering of ceramic parts, the advantages of this process are the controllability of the microstructures and properties by altering the morphologic characteristic of porous carbon and pipelines, and the microstructure of the final SiC components is denser than that of the laser direct sintered parts. This process mainly included the following three steps: (1) structure design of the molds with complex and controllable pipelines, (2) fabrication of resin molds by using SL technology, and (3) casting the mixed resins into the molds followed by pyrolyzing and reaction infiltration with molten silicon. Rapid tooling Casting Mixed resin Polymerization T<100ºC, Air Pyrolysis T<1,000ºC, Ar Molten silicon infiltration T= ºC, Vacuum<10-3 Time=5min The fabrication process of the silicon carbide parts is schematically shown in Fig. 1. The mold was designed using 3D CAD software and exported in STL format and then fabricated on a stereolithography machine. The processes for the porous carbon preparation and molten silicon infiltration were after a process originated by Hucke [14] and discussed in other literature [1, 2]. The composition of mixed resin had been investigated according to the microstructures of produced porous carbon in the present work. 2.1 Material preparation Mould design Mould prototyping Polymer precursor Amorphous carbon preform SiC part Fig. 1 Scheme of the fabrication process for RFSC components Liquid resin mixture of furfural resin and furfuryl alcohol was used as the precursor, with paratoluenesulfonic acid (PTSA) as the catalyst. Tri-ethylene glycol was added into the liquid resin mixture as a porosity agent. Furfural resin and furfuryl alcohol were mixed with tri-ethylene by stirring mechanically for 10 min at room temperature. The mixed resin was put into a drying cabinet at 100 C for another 6 h in order to get a homogenous mixture. After the stirring and heating, it could be observed under 2 Fabrication process Fig. 2 Mold with complex inner scaffolds structure fabricated by the SL process

3 Int J Adv Manuf Technol (2010) 46: Fig. 3 CAD models for the mold of a blade wheel: a inner pipelines; b outer contour Fig. 4 Carbon preform of a standard sample with pipelines Fig. 5 Carbon preform of a blade wheel with pipelines: a top view; b bottom view

4 582 Int J Adv Manuf Technol (2010) 46: an optical microscopy that the two kinds of resin were homogenously mixed. The PTSA solid powder as a catalyst could not be distributed uniformly in the liquid resin in a shorter time if the PTSA was directly added to the mixture. Some polymerized blocks were found in the bottom of the resin container after casting the resin into the mold. So, in the following experiments, ethanol solution with 70 wt.% PTSA was added into the mixture as a liquid catalyst to realize its homogenous distribution in a shorter time. In order to decrease the polymerization Fig. 6 a h SEM micrographs of the fractured cross sections in cubic standard samples with different morphological structures by varying the ratios of the compositions in the mixed resin (SEM, CamScan CS4 Cambridge, UK)

5 Int J Adv Manuf Technol (2010) 46: rate, the resin was cooled down to 80 C. The catalyst was added. After stirring, the new mixed resin with catalyst was poured into the mold. Polymerization between furfural resin and furfuryl alcohol took place in the SL molds. The miscible resin underwent phase separation at a temperature lower than 80 C accompanying polymerization. The viscosity of the furfuryl-rich phase increased, eventually forming a rigid skeleton. The glycol-rich phase remained fluid and volatile and could be readily removed through the open pore structure. After pyrolysis, the furfuryl-rich phase transformed to amorphous carbon, forming a microporous carbon with reticulate structure. The morphological structure of the porous carbon could be controlled by changing the ratio of the resins in the mixture. 2.2 Mold design and rapid manufacture To validate the feasibility of the fabrication process for these complex pipeline structure, a simplified mold with complex scaffolds (Fig. 2) was fabricated by using the SL process. The outline dimension of the mold was 60 mm 8 mm 6 mm and the wall thickness was 1.5 mm. The diameter of the scaffolds in the mold was 0.5 mm. The molds with a high accuracy were fabricated with a UVcurable resin on the SL machine (SPS450B, Hengtong Intelligent Machine Co. Ltd, XJTU, China). The UVcurable resin was provided by the DSM Company, USA. In the present work, pure UV-curable resin was pyrolyzed in an argon atmosphere-protected furnace to measure the mass of residual matter (carbon). The mass percentage of the residual carbon was calculated by the ratio of the weights of residual carbon and UV-curable resin, which was less than 3%. A blade wheel with complex inner pipelines was fabricated in the present work. The geometric models of the blade wheel mold are demonstrated in Fig. 3. The scaffolds in the mold shown in Fig. 3a were designed to be interconnected with each other. The preform of the blade wheel was prepared by casting the mixed resin into the mold. When the pyrolysis process was finished, the mold with scaffolds was almost burned out, producing the interconnected pipelines. 2.3 Preform fabrication The preforms were prepared in simple cube and complex blade wheel shapes by casting the initial liquid mixture resin into the molds. Subsequent temperature cycle of polymerization and pyrolysis was carefully designed on the basis of the thermogravimetric analysis (TGA) results, microstructure observations, and measurements of density and porosity. When the preforms were polymerized with the molds in the drying oven, the shrinkage and vitrification of the resin molds should be taken into consideration. The UV-curable resin vitrified and lost its mechanical properties when the temperature was raised to around 60 C. To avoid the preforms distortion, the polymerization temperature should be kept lower than 60 C as long as the partially polymerized preforms had enough strength to keep the shapes before the resin molds totally vitrified and lost their strength. The cast preforms were first polymerized at 40 C for 6 h in the drying oven. And then the temperature was raised to 60 C and held for the following 12 h. Finally, the temperature was raised to 100 C for another 12 h to complete the polymerization and make the preforms dry out. The polymerized preforms with the resin molds were put into the quartz tube furnace with a programmed temperature controller. The preforms and the resin molds were heated to a maximum temperature of 800 C and pyrolyzed according to the temperature curve in a flowing argon atmosphere. As the UV-curable resin used for the molds has a lower carbon production of about 3% when a Apparent porosity (%) b Preform density (g.cm -3 ) % Tri-ethylene glycols 40% Tri-ethylene glycols The content of furfuryl Alcohol in the mixture resin 50% Tri-ethylene glycols 40% Tri-ethylene glycols The content of furfuryl Alcohol in the mixture resin Fig. 7 a, b Properties of the porous carboninthecubicstandardsamples

6 584 Int J Adv Manuf Technol (2010) 46: Fig. 8 a d Hierarchical structures of the cubic standard samples with pipelines and porous carbon b c pyrolyzed, the mold almost disappeared and the polymer precursor was pyrolyzed into the porous carbon with pipelines after the heating process. The total weight loss of the polymer precursor during the thermal cycle of pyrolysis was on average 64% and preform volume shrinkage was about 50%. The carbon preforms of the cube sample and the blade wheel are shown in Figs. 4 and 5, respectively. The cube sample with pipelines basically held their original shape. It indicated that this process was appropriate for the preparation of the porous carbon preforms used for the RFSC. Some defects were found in the bottom of the blade wheel as shown in Fig. 5b. It was because of the appearance of air bubbles during casting the liquid resin into the mold. In a future work, casting operation should be conducted under the vacuum condition. 2.4 Reactive infiltration Porous carbon preforms were mounted on a graphite holder and covered by the silicon powder, contained in a graphite crucible in a vacuum furnace at Torr. Furnace temperature should be high enough to melt silicon powder into pure liquid phase and realize the spontaneous infiltration process. Infiltration time must be sufficient to complete the reactions. In the case of pure silicon powder, the temperature of 1,500 C just above the melting point (1,410 C) and a short reaction time of 5 min were enough for the complete infiltration reaction. After the infiltration reaction, furnace temperature was raised up to 1,550 C to evaporate the residual silicon in the pipelines and on the sample surfaces, and then cooled down to room temperature. Table 1 Dimensional retention and density of the samples during the pyrolysis and infiltration processes Furfuryl alcohol content (%) Pyrolysis Linear ratio (%) Volume ratio (%) Preform density (g/ cm 3 ) Infiltration Linear ratio (%) Weight ratio (%) RFSC ceramic density (g/cm 3 ) Tri-ethylene glycol content=50wt.%

7 Int J Adv Manuf Technol (2010) 46: Fig. 9 Optical micrographs of the polished cross section in a RFSC component: a marginal area; b central area of the specimen a Outer b SiC Si defects Inner 3 Results and discussion 3.1 Properties of porous carbon preform The porous carbons in Fig. 6a d had 40 wt.% tri-ethylene glycol and 50 wt.% in Fig. 6e h in the mixed resin before polymerization. The content of tri-ethylene glycol in the mixed resin had great influence on the morphological characteristics of the porous carbon. The morphological structure of the porous carbon changed obviously from isolated pores to interconnected microstructures in every row of Fig. 6 when the content of tri-ethylene glycol increased from 40 to 50 wt.%, and the content of furfuryl alcohol kept unchanged. When the tri-ethylene glycol content kept invariant, the content of furfuryl alcohol in the mixed resin increased from a to d and e to h in Fig. 6, and the apparent porosities of porous carbon rose slightly. The maximum diameter of the pores decreased from about 10 to 3 µm, and the mean thickness of the wall between the pores reduced from 5 to 1.5 µm in the right column of the Fig. 6. Density and porosity of the preforms were measured by using Archimedes technique in dimethyl benzene solution. The results are plotted in Fig. 7. The mean apparent porosity jumped from 7.75% to 40.93% with the increase of tri-ethylene glycol content as shown in Fig. 7a. However, tri-ethylene glycol content had little influence on the densities of performs, as shown in Fig. 7b. The densities decreased from 1.00 to 0.88 g/cm 3 with the increase of the furfuryl alcohol content (Fig. 7b). The designability of the SiC ceramic properties could be achieved by realizing the controllability of the hierarchical microstructure in the porous carbon, as shown in Fig. 8. All the regular pipelines (Fig. 8a, b) in the preform were produced by the pyrolysis of scaffolds in the resin mold which were fabricated using the SL process. This was the macrolevel of the structure in the porous carbon preform. The interconnected irregular microstructure (Fig. 8d) in the porous carbon preform was at the bottom level in this hierarchical structure, which could be prepared by the resin phase separation, polymerization, and pyrolysis. Morphological difference in the microstructures of porous carbon preform would directly affect the properties of the RFSC. If the regular patterns of this affection were available to the designer, controllability and designability of the properties of the RFSC parts would be realized. In the present work, the idea and the basic method were only proposed and validated. Some quantitative analysis will be conducted in the future work. 3.2 SiC components Dimensional retention The dimensional retention during the pyrolysis and infiltration process had been studied on the standard cubic samples, which had an original outline size of 50 mm 5 mm 3 mm before pyrolysis. The linear dimensional retention was measured in the length direction of the standard sample. The linear dimensional retention for other two directions had been measured but not shown in this paper. The typical results of dimensional retention are shown in Table 1. With the increase of the furfuryl alcohol content, the linear ratio and volume ratio increased. On the Intensity (cps) Si SiC SiC Si Theta(deg.) Fig. 10 X-ray diffractogram of the polished cross section in a RFSC component Si SiC SiC

8 586 Int J Adv Manuf Technol (2010) 46: contrary, the densities of the preforms decreased. The linear shrinkage ratios L/L between the polymer precursors and carbon preforms were from 21.8% to 24.7%, and the volume shrinkage ratios were from 51.9% to 57.3%. The shrinkage values of the samples in the infiltration process were very small and could be ignored. The density of RFSC (2.82 g/cm 3 ) was smaller than the theoretical density of SiC (3.21 g/cm 3 ). The appearance of pores and Si phase in the RFSC might be the reasons for the decrease of the density. The shrinkage deformation for the blade wheel which had complex structures was uniform. But for the standard samples in a strip shape, there was a warp in the length direction to some of the samples. Some additives, activated carbon powder and expanded graphite powder, had been added into the mixture resin in order to decrease the shrinkage of the samples during the pyrolysis process. The microstructures of the porous carbon and properties of the final SiC components would be dramatically affected by these additives. Details about these experiments will be published future literature Phase composition After the melt silicon infiltration of a standard cubic sample, the microstructures of its polished cross section had been observed under the optical microscopy (KP-50 color digital, Hitachi, Japan). It was a typical SiC Si composite structure, as shown in Fig. 9. The darker phase was SiC and the brighter phase was the residual Si. No residual carbon was found in the cross section of the RFSC. Some pores appeared in the central area of the cross section. These would be the defects and decrease the mechanical strength of the RFSC. There were some differences in the phases between marginal and the central area in the cross section of the specimen. The residual Si phase increased from the outer side to the inner side of the cross section (Fig. 9a). There were lots of big silicon phase lakes in the central area of cross section (Fig. 9b). The appearance of big Si phase lakes and its heterogeneity would reduce the high-temperature mechanical properties of the SiC ceramic parts. The X-ray diffraction (XRD) analysis was conducted on the polished cross section of the infiltrated sample (Siemens diffractometer D 5000, Siemens AG, Germany). XRD result is shown in Fig. 10. The phases of β-sic and Si were the primary compositions being detected in the specimen. The phase of α-sic was the minor in the specimen and not marked out in the Fig. 10. The micrographs and XRD results indicate that porous carbon completely reacted with molten silicon, and no carbon phase was available in the finial composite components. 4 Conclusions A new fabrication methodology of net-shape SiC ceramic components was proposed and a blade wheel with complex surface and pipelines was fabricated to validate the state of the art in the present work. The molds with scaffolds used for the preparation of preforms were fabricated by using the stereolithography method. The scaffolds transformed into pipelines after the pyrolysis. The pipelines were designed as main channels for the molten silicon to realize the complete reaction infiltration. The microstructures of the porous carbon were investigated. A hierarchical and designable porous structure was achieved by the pipelines and the mixed resin with changeable compositions. Phase compositions were examined by optical microscopy and XRD analysis. Results indicate that porous carbon completely reacted with molten silicon because no elemental carbon was found in SiC samples. In the future work, the influence of properties of pipelines and porous carbon on the mechanical strength of final SiC components should be carefully investigated. Composites of SiC and refractory Si metal compounds can be produced by infiltrating Si alloys into the porous carbon, which will increase the high-temperature properties of the final components. Dimensional accuracy might be improved by adding some additives into the mixture resin to decrease the shrinkage of the samples in the pyrolysis process. The surface roughness of the final components could be improved by increasing the surface quality of the SL mold. Relative experiments have been already conducted. The results will be published in other publications. Acknowledgement The authors really appreciate the financial support from the National Natural Science Foundation of China (Project no.: ). References 1. Hozer L, Lee JR, Chiang YM (1995) Reaction-infiltrated, netshape SiC composites. Mater Sci Eng A 195: doi: / (94) Wang YX, Tan SH, Jiang DL, Zhang XY (2003) Preparation of porous carbon derived from mixtures of furfuryl resin and glycol with controlled pore size distribution. 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