Viable route to the manufacture of short carbon fiber-rich UHTC complex shapes with enhanced toughness

ZrB2 - based pasted, containing 5, 20, 45, 60 vol% short carbon fibers (Cf), were prepared in order to obtain complex shapes via direct ink writing, which were then sintered by hot pressing. The rheological behavior of the pastes indicated that at low Cf concentrations, the fibers act as a lubricant decreasing the paste viscosity. Above 20 vol% Cf, they act also as a binder creating an interconnected network that dominates over the lubrication effect, resulting in a viscosity increase. Complex shapes in the form of a hollow cylinder and a winglet with a functionally graded composition were then extruded and hot pressed, using a versatile powder bed method that enabled full densification with no need to use ad-hoc machined graphite inserts. Bars obtained from the direct ink writing process resulted in high alignment of the short fiber and displayed fracture toughness increase of 54% over that of the unreinforced matrix, while demonstrating a 42% increase with respect to conventionally hot pressed dry mixtures of ZrB2 and short fibers. This work shows the great potential of manufacturing complex shapes in ultra-high temperature ceramics matrices reinforced with short fibers for a plethora of design possibilities, including functional grading with control over layer composition, thickness and even alignment of short fiber.