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2001:
Preforming and housing techniques for cost-effective MMC processing were addressed (EMPA-project 860055); the goals were to develop recoverable or lost (and cheap) housings allowing easy unmoulding and to find a reliable method of preform design for near net shape parts, in order to minimize machining time after processing. Polymer precursor processing for ceramic preforms, selective laser sintering processes, development of pressure tight soluble cores and rapid prototyping techniques were studied. It was concluded that the introduction of Rapid Prototyping for housing or preform making is mandatory for the production of near net shape MMC parts by squeeze casting. The feasibility of different methods involving miscellaneous materials was evaluated. New processing routes were proposed and their advantages and problems listed in the related final report.
From the results of this first project a second project focussing on the development of low cost tooling technologies for near net shape squeeze casting was derived (EMPA-project 860068). It was shown that methods concerning cement and glass are promising routes, which can be improved and need further investigation. Also salt compaction and squeeze casting of molten salt are suitable for housing manufacturing, particularly due to the compression resistance, impermeability, solubility in water and low cost (1 CHF/Kg). For more details, refer to the related project report.
Moreover, the topic of a diploma thesis on 3DP processing technology was outlined; this thesis is planned to be started in march 2002 at EPFL, provided a suitable candidate can be selected.
An interinstitutional doctoral thesis between EMPA and EPFL on "Solid free-form fabrication of porous ceramic parts from ceramic powders and preceramic polymers" has been initiated and is scheduled to start in early 2002.
2002:
The preforming technique based on the use of pre-ceramic polymer precursors was assessed. Two types of precursors were investigated: polysiloxanes transforming into SiCxOy and phenolic resins transforming into C. The precursors were mixed with SiC powders by means of mechanical dry and wet mixing techniques, varying the polymer content between 5 and 40 vol-%. The resulting powder mixtures were then submitted to curing and subsequent pyrolisis treatments (under Ar-atmosphere) to convert the precursor into a ceramic phase, the aim being to minimise the amount of binder in view of the required green body strength for further processing by liquid metal infiltration. On the one hand the powder mixtures were first hot pressed, than "fired" in conventional furnaces. On the other hand, the same powder mixtures were submitted to laser
treatment for polymerization and pyrolysis. It was found in both cases that the techniques described were viable to produce porous SiC-preforms. The best results were obtained with the polysiloxane precursors, particularly with the solid resin MK from Wacker. However the distribution, dispersion and wettability of the polymer within the SiC-particles obtained using the selected wet mixing method coupled with a rotavapor to evaporate the solvent showed deficiencies and it is quite obvious that a homogeneous that a homogeneous coating of the ceramic particles with the precursor is a prerequiste for the desired preform quality and binder content minimization; this is expected to be achieved by using the spray drying technique in the ongoing studies. The consequent developments and planned optimization steps are described hereafter.
2003:
The achievements of the year 2002 have been compiled and a related paper has been submitted for publication to Adv. Eng. Mat. in Oct. 2003.
The spray drying technique has been assessed for coating the SiC particles with the pre-ceramic precursor (solid polysiloxane dissolved in ethanol). The obtained spray dried powders were processed to SiC preforms by warm pressing at EMPA and by selective laser sintering (SLS) at EPFL (PhD-thesis Alexandre), after having studied the laser polymer interaction (laser absorption) and adapted the laser parameters (pulsation, power) for polymer curing and pyrolisis.
The warm pressed and pyrolised SiC preforms were characterized and compared with those manufactured in 2002 from powders mixed by the rotavap-technique. The properties (e.g. strength) of the new preforms made from spray dried powders were found to be clearly superior compared to the "old ones", owing to an increased homogeneity of the precursor distribution. In this case it was also shown, that when using spray drying, the amount of precursor can be reduced, thus enhancing the cost effectiveness of the technique.
The preforms made by warm pressing and SLS have been infiltrated with Al using the squeeze casting technique at EMPA. The MMCs resulting from the warm pressed preforms are currently being characterised. The investigation of the MMCs obtained from SLS-preforms is just about to start.
2004:
spray drying & hot pressing of SiC particles with pre-ceramic polymer binders
The up-scaling of the hot pressing technique has been completed. Preforms with varying binder characteristics and SiC powder mixtures & volume fractions have been produced in the dimension 100x100x10 mm. The fabrication of nano-particle preforms has not been successful so far due to difficulties in dispersing particles finer than 500 nm in the binder solvant. Process and pyrolisis of the preforms have been performed according to previously established procedures [1]. A preform selection has been infiltrated with Al. The resulting MMCs are now being characterised; the aim is to study the effect of the parameters mentioned on the mechanical behaviour of the composites. Eventually, the mechanical properties obtained will be compared with those of composites obtained from "commercial" preforms to assess the application potential of the new materials.
selective laser sintering of SiC preforms
The spray drying process has been shown to be the most effective method to apply the polymer coating on the SiC particles. To establish optimum processing conditions a fundamental investigation of the effects of the selective laser processing on the preceramic polymer-ceramic powder mixtures was performed. The polymer decomposition in conventional pyrolysis has been characterized and the results have been compared with the kinetics of the non-equilibrium laser decomposition. Based on the results obtained and coupled to heat flow simulation, models of the laser/powder interaction as well as of the chemical decomposition and non-equilibrium pyrolysis kinetics of a preceramic polymer are now being developed to predict local chemical reactions induced by laser heating; the current achievements have been recently presented [2].
[1] Porous SiC-preforms by intergranular binding with preceramic polymers
M. Thünemann, A. Herzog, U. Vogt, O. Beffort
Advanced Engineering Materials, Vol. 6, No. 3, March 2004, pp. 167-172
[2] Layered manufacturing of porous ceramic parts from SiC powders and preceramic polymers
T. Alexandre, J. Giovanola, S. Vaucher, O. Beffort, U. Vogt
4th Int. Conf. on Laser Assisted Net Shape Engineering, LANE, 21-24.09. 2004, Erlangen, DE
2001
After an initial bibliographic research to establish the state of the art on rapid prototyping, a study of feasibility of miscellaneous techniques will be performed, with special emphasis in Selective Laser Sintering and 3D printing which are suitable routes to produce porous ceramics (months 0-3). This will allow to select the most suitable techniques and materials to process preforms, and to define the acquisition of a rapid prototyping machine (months 3-6). Preforms will be elaborated by layer manufacturing. A careful selection of ceramic reinforcements and binders will be performed, and systematic experiments will be conducted to optimize the processing parameters (months 6-12).
