Strengthening and corrosion control of pure magnesium and WE43 magnesium alloy for biomedical applications

Abstract

For the last decade magnesium and its alloys have received more and more attention as candidate materials for biomedical applications owing to their advantages over other metallic materials, especially with regard to their biocompatibility, biodegradability and low density. However these benefits are not matched by their high corrosion rate and low mechanical performance, which is an obvious obstacle to their use in biomedical applications. In this thesis a thermo-mechanical treatment using a deformation process referred to as three-roll planetary milling (PSW) is proposed to improve the strength of magnesium implants. Compared to the well known equal channel angular pressing (ECAP) process this technique would be suitable for the fabrication of implants in industry scale production without promoting softening effect. In order to increase further the strength of magnesium, an ageing treatment was combined with PSW process to induce hardening precipitation. The corrosion rate of magnesium, and therefore its degradation rate, was controlled by coating its surface with a hydroxyapatite (HA) poly-L-lactid acid (PLLA) ceramic polymer composite. Although HA is known to have very good bioactivity due to its similar composition with that of the bone, its brittle behaviour reduces considerably its performance under strain. Therefore a fexible PLLA layer was coated in addition of HA to provide stability and strain resistance. In the first study, PSW was used to trial its potential for enhancing the mechanical properties of magnesium. Pure magnesium was processed via this technique with a first pass conducted at 250°C and followed by 10 others passes with a scaling down of temperature to finally reach room temperature. A total true strain of 96% was obtained and it was shown that pure magnesium reached excellent yield strength of 146 MPa, comparable with the strength levels of magnesium alloys and with that of the cortical bone, allowing therefore its use for bone graft applications. A homogenization of the grain refinement through the sample was also found after 6 passes with a grain size of 5-6 microns, favorable to strengthen the material, together with a change of texture. In the second study the attention was focused on a magnesium alloy, namely WE43 (4% Yttrium 3% Neodymium), which is believed to be suitable for biomedical applications owing to its good corrosion resistance and non-toxicity. The alloy was first solution treated at 525°C for 5h, subsequently deformed at 400°C (1 pass) using the PSW process and water quenched. It was then aged at 210°C for 16h. Although no significant grain refinement occurred in the rod sample after one pass of roll milling, a strength improvement was observed, more pronounced at the rim area of the billet, with maximum strength values of about 220 MPa, owing to the development of (110) texture. Further number of passes did not promote strengthening of the material. The formation of beta' phase precipitates during the first period of ageing resulted in another increase of strength up to 300 MPa. However, a loss of mechanical properties was observed after a long ageing period (16h) associated with a grain growth and the formation of phase precipitates. The third study aimed at fabricating a biocompatible and strain resistant coating for WE43 substrate in order to control its corrosion rate. After a first pre-treatment consisting in annealing the material at 525°C for 5h to reduce the second phase particles size and obtain a homogeneous microstructure for further treatment, HA was coated on WE43 by immersion in an aqueous solution containing calcium and phosphate sources. A 3 m thick layer of needle like shape HA crystals was uniformly formed on the substrate. PLLA layer was then deposed onto HA by dip coating technique using vacuum system. In order to mimic real cases and possible deformation during implantation, a 5% tensile strain was applied to the coated WE43 alloy. C