Influence of alumina particles in the production of biomimetic composites

The aim of the work was to evaluate the influence of the filler size and shape in the final architecture of biomimetic composite produced through the freeze casting technique.

Different samples of ceramic infiltrated with resin, composed by a different ratio between RonaFlair White Sapphire aluminium oxide powder - EMD Chemicals/Rona Gibbstown, USA (platelet-like morphology and average particle size <16 µm) and Almatis alumina filler ‑ CT3000SG, Almatis, USA (average particle size of 0.8 µm) have been produced. The samples have been then characterized through thermal analysis and calorimetry (Model TG/DT A 6300, Seiko Instruments USA Inc. Torrance, CA, USA), scanning electron microscopy (Phenom-World BV, Eindhoven, The Netherlands), and optical microscope (Nikon Eclipse E600 microscope). Moreover the percentage of shrinkage after the ceramic sintering have been measured through a digital calibre (Mitutoyo USA, Aurora, IL, USA). Four different groups have been considered: Group 1 (10R): 10% RonaFlair- 90% Almitas; Group 2 (20R): 20% RonaFlair- 80% Almitas; Group 3 (40R): 40% RonaFlair- 60% Almitas; Group 4 (60R): 60% RonaFlair- 40% Almitas.

Optical microscope and SEM observations have confirmed that all samples were characterized by a biomimetic hierarchic structure. For what concerning the average thickness of lamellae measured at 4 and 8 mm from the cooling plate, samples 10R were characterized by higher thickness at both levels. The measured parameter indeed decreases with the increment of the percentage of RonaFlair in the slurry composition. On the contrary the distance between lamellae (wavelength), was characterized by an opposite trend. The measurement of the diameters of the samples after the sintering process through a digital calibre has shown that 10R and 20R samples were characterized by a constant shrinkage in all the samples. On the contrary, groups 40R and 60R were distorted and were characterized by a higher diameter at the bottom respect the upper portion of the samples. Indeed these samples were characterized in the bottom by a greater amount of particles, both Almatis than RonaFlair ones. We have supposed that gravity force, during the freeze casting process, tended to push down in the lower layers all heaviest particles and then the lightest ones have occupied the spaces they have left. Consequently in upper layers there were only few particles so during the sintering process were characterized by the higher shrinkage. This phenomenon was limited in samples 10R and 20R because the percentage of greatest particle was so much inferior that their position was not influential.

The present study has shown that samples composed by a higher percentage of RonaFlair, were characterized by a higher distortion after sintherization; the cause of this phenomenon could be the different sedimentation process that characterizes particles of different weight and size.