Syntactic foams are cellular materials in which porosity is introduced via hollow filler particles. Producing such materials based on polymer matrices is state of the art, with large-scale applications e. g. in the offshore industry. Metal matrix syntactic foams are a more recent object of study. Processes are typically either based on melt infiltration or powder metallurgy approaches. An example of the latter type are iron and steel matrix syntactic foams synthesized by means of the Metal powder Injection Moulding (MIM) process, which require microscale filler particles. These materials have been realized based on a significant variety of matrices ranging from pure iron (Fe99.7) over stainless steels like 304L or 316L to Invar (FeNi36) in combination with hollow glass microspheres and/or cenospheres. The resulting syntactic foams show attractive combinations of properties, including higher strength to weight ratios than conventional, non-syntactic foams based on the identical matrices. Despite this, initial studies have shown a considerable potential for further optimization, and for new material variants. Metal coating of hollow microspheres is one example in this respect: This approach could
(a) extend the processing window (temperature range) for usage of glass microspheres as filler, as it would hinder the infiltration with glass, in early stages of sintering, of the metal powder particle structure through capillary forces, which is typically observed at sintering temperatures in excess of 1000°C,
(b) improve mechanical performance specifically under tensile load through improved microsphere-matrix interface properties,
(c) facilitate a completely new type of syntactic metal foams which is based on the direct sintering of metal-coated hollow microspheres.
The present publication will present first results along these lines of research.
Besides, a major handle for improving syntactic foam properties is raising the mechanical properties of the filler particle walls above those of the matrix material. In terms of the Young’s modulus, this can practically be achieved through replacing glass and aluminosilicate microspheres with alumina or silicon carbide microspheres. Currently, however, these are not available in a size range that would fit powder metallurgical production processes like MIM. The present publication will provide details and first results on a process for production of such micro-scale ceramic microspheres.