Novel biodegradable zinc-based alloys for medical applications : synthesis ands evaluation of properties

Zinc alloys are a newly proposed family of materials which can be used as biodegradable metals. Their main advantage is the ideal corrosion behavior, together with a good biocompatibility. The main concern regarding zinc is connected to its very poor mechanical properties. The first part of this work consisted in developing twelve candidate alloys, either binary or ternary, with alloying elements Mg and Al. Pure zinc was used as reference. Microstructural observations after casting allowed to see the formation of a eutectic second phase for Mg-bearing alloys, while for Zn-1Al a eutectoid phase segregated at grain boundary. Solution treatment succeeded in reprecipitating the Mg-rich second phase, while it permitted to dissolve Al in the matrix. The alloys were processed in the form of minitubes by hot extrusion. This process led to a fine and nearly equiaxed grain structure. All binary alloys showed a nearly-ideal corrosion rate. Mechanical properties were studied both in compression and in tension, together with mechanical integrity tests. It was concluded that the most promising alloys are Zn-0.15 Mg, Zn-0.5Mg, Zn-0.5Al and Zn-0.5Al-0.15Mg. However, further studies to assess their non-toxicity are required. The second part involved the study of novel Zn nanocomposites, using nanodiamond (ND) and hydroxyapatite (HA) as reinforcements. The powders were compacted both by extrusion and ECAP. The composites with high reinforcement content showed clustering when compacted by hot extrusion, while the compaction by ECAP was not successful. Moreover, Zn-HA composites proved not to be effective in improving mechanical properties; in addition to this, composites compacted by ECAP showed much worse performances than composites compacted by extrusion. ND seems more effective in improving the composite strength. Also in this case, further studies are needed to gather all the data necessary for determining whether such composites may be used as biodegradable metals.