Effect of a Ball milling Treatment on mechanical and in vitro biological properties of Bioactive Glass-Hydroxyapatite composites
ANGIONI D. 1, ORRU' R. 1, CAO G. 1, BELLUCCI D. 2, CANNILLO V. 2
1 Dip. Ing. Meccanica, Chimica e dei Materiali, University of Cagliari, Italy, Cagliari, Italy; 2 Dipartimento di Ingegneria “Enzo Ferrari”, University of Modena and Reggio Emilia, Italy, Modena, Italy
Hydroxyapatite (HA) and a lab-made bioactive glass (BGMS10), with low tendency to crystallize, are combined for the fabrication of bulk composites (25HA-75BGMS10, 50HA-50BGMS10, and 75HA-25BGMS10, wt.%) by Spark Plasma Sintering (SPS). Prior to consolidation, HA and BGMS10 powders are co-ball milled for different time durations (tBM=30, 60 and 120 min) with a charge ratio equal to 2. The Ball milling (BM) treatment modifies powder characteristics (particles size, surface area, HA crystallites size) and promotes the formation of HA/BGMS10 interfaces. In turn, these outcomes are observed to affect, in not a trivial way, the sintering behavior of the processing powders, as well as microstructural, mechanical, and biological characteristics of the bulk products.
Relatively short BM treatments (30 min) are found to inhibit the consolidation of BG-rich composites (25HA-75BGMS10), while no appreciable changes are observed in 50HA-50BGMS10 and 75HA-25BGMS10. In contrast, longer milling times (120 min) are particularly effective in improving relative density of HA-rich products.
The amorphous nature of the glass fraction is generally preserved during SPS for temperatures up to 800 °C, above which glass crystallization occurs in all investigated formulations. However, under such SPS condition, crystalline SiO2 could be formed during consolidation of longer milled powders.
Sintered products from composite mixtures subjected to BM exhibit relatively finer microstructures, with HA crystallites size less than 100 nm, compared to unmilled samples (>200 nm). Improved Young’s modulus and hardness values are also typically obtained from mechanically treated powders.
Biological tests evidence that, under properly controlled conditions, an extraordinary biological response can be produced with BM samples, being their surface completely covered by HA precipitates just after few days immersion in SBF. These findings can be explained by the increased surface area and the significant reduction of HA crystallites size induced by the mechanical treatment.
However, prolonged treatments lead to the nucleation and segregation of the SiO2 phase during SPS which apparently plays a negative role on the biological performances of the resulting composite.