CMCs have already been used in demanding high-temperature space applications, and the manufacturing, transport and energy sectors are obvious potential beneficiaries. However, these advanced materials are currently difficult and expensive to produce, requiring long processing times and high energy consumption. Overcoming these challenges to open the door to new material technologies was the impetus behind the EU-funded project HELM (High-frequency electro-magnetic technologies for advanced processing of ceramic matrix composites and graphite expansion). Scientists investigated innovative MW heating technologies integrated with standard thermal processing routes. The latter include chemical vapour infiltration (CVI), liquid silicon infiltration (LSI), graphite expansion (GE), and polymer impregnation and pyrolysis (PIP). An MW-CVI furnace was produced entirely of graphite for the first time to avoid contamination of the material produced. Infiltration tests performed on three different materials showed that CMC manufacturing time was reduced to one third compared to that of conventional isothermal CVI. Tests on an existing small lab-scale MW-LSI furnace produced very promising results. Liquefaction of silicon occurred in only a few minutes compared to several hours for a conventional industrial furnace. Project partners also produced LSI and GE pilot-scale furnaces for producing braking systems and antiballistic plates, cutting CMC processing time by 50 %. Building on success with the MW-CVI furnace chamber that did not utilise quartz, MW heating of the PIP process was done without a quartz cavity. Tests on a small-scale system showed considerable reductions in processing time and energy consumption as well as improvements in CMC mechanical properties such as fracture toughness or tensile strength. Work on producing a larger-scale PIP furnace that should be able to process a preform of a brake disc is ongoing even after the project's end, as there is still room for significant improvements. HELM scientists delivered novel MW heating technology that significantly reduces processing time and energy consumption compared to standard thermal processes alone. The processing facilitates more cost-effective and high-quality CMCs and expanded graphite, enabling achievement of new microstructures currently not accessible with conventional technology.
Ceramic matrix composites, microwave, HELM, graphite expansion, heating technologies