Surface metal matrix nano-composite of magnesium/hydroxyapatite produced by stir-centrifugal casting

Abstract

The present study aims to investigate a liquid state method of stir and centrifugal casting as an in-situ and cost-attractive processing technology for the production of magnesium/hydroxyapatite surface metal matrix nano-composites (Mg/HA surface metal matrix nano-composite). The main contribution of this study is to design the best condition for achieving a uniform Mg/HA surface nano-composite as a potential bone implant. It was shown how casting parameters and the distribution of hydroxyapatite affect mechanical properties of nano-composites measured using nano-indentation, nano-scratch, and compression tests. Response surface method in Design Expert software was used to predict the best model and the optimum condition of casting based on the experimentally measured data. The surface metal matrix nano-composites, consisting of a magnesium matrix with different amounts of nano-sized hydroxyapatite and silicon-doped hydroxyapatite (0.75-3 wt%) particles, were prepared. Hot isostatic pressing was used to homogenize the nano-composites in terms of particle distribution and to reduce porosity. It was shown that the weight percent of hydroxyapatite reinforcement is the parameter which is best suited to tailor targeted strength values. The target values of maximum compression strength (187 MPa) and elastic modulus (33 GPa) were achieved with a combination of the following parameters: 1.83 wt% hydroxyapatite, 800 rpm mold rotational speed, and a propeller rotational time of 6.3 min. A specimen prepared under these conditions had a homogeneous distribution of nano-hydroxyapatite in magnesium metal matrix after hot isostatic pressing at 450 °C and 100 MPa for a holding time of 120 min. It indicated the best mechanical resistance in terms of hardness and material loss during the nano-scratch testing. Moreover, the XRD results show that there is no considerable chemical reaction between the reinforcement particles of n-HA and Mg metallic matrix during casting at 700 °C and thermo-mechanical treatment of HIP at 450 °C.