THIXOTROPIC CASTING OF CERAMIC-METAL FUNCTIONALLY GRADIENT MATERIALS

Functionally gradient materials (FGM), a recent development in composi te materials, consist of a continuously graded interface between two c omponent phases. Previously reported FGM fabrication methods are revie wed, including vapour deposition, plasma spraying, electrophoretic dep osition, controlled powder mixing, slipcasting, sedimentation forming, centrifugal forming, laser cladding, metal infiltration, controlled v olatilization, and self propagating high-temperature synthesis. A nove l approach to FGM fabrication is presented involving thixotropic casti ng - vibratory casting of highly concentrated highly thixotropic suspe nsions castable only under vibration. The hydroxyapatite/316L stainles s steel system (a biocompatible ceramic/metal system) was used due to appropriate matches in true densities, thermal expansion, and sinterin g temperatures. Solids loadings from 76.08-82.67 wt % were trialled. S olids loading was found to be critical. At 78.17 wt %, an ideal contin uous FGM formed, showing a gradual transition from pure-ceramic to pur e 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 homog eneous sample with only a small degree of grading, from a slightly met al-rich end to a slightly metal-poor end.