Thixotropic casting of ceramic-metal functionally gradient materials

Functionally gradient materials (FGM), a recent development in composite materials, consist of a continuously graded interface between two component phases. Previously reported FGM fabrication methods are reviewed, including vapour deposition, plasma spraying, electrophoretic deposition, controlled powder mixing, slipcasting, sedimentation forming, centrifugal forming, laser cladding, metal infiltration, controlled volatilization, and self propagating high-temperature synthesis. A novel approach to FGM fabrication is presented involving thixotropic casting — vibratory casting of highly concentrated highly thixotropic suspensions castable only under vibration. The hydroxyapatite/316L stainless steel system (a biocompatible ceramic/metal system) was used due to appropriate matches in true densities, thermal expansion, and sintering temperatures. Solids loadings from 76.08–82.67 wt% were trialled. Solids loading was found to be critical. At 78.17 wt%, an ideal continuous FGM formed, showing a gradual transition from pure ceramic to pure metal across 60 mm. At the nearest increments trialled (±0.3 wt%) gradation was far from optimal: 77.87 wt% gave a sharp ceramic/metal interface with negligible grading; 78.47 wt% gave a relatively homogeneous sample with only a small degree of grading, from a slightly metal-rich end to a slightly metal-poor end.