In vitro degradation of AZ31 magnesium alloy coated with hydroxyapatite by sol-gel method

In recent years, magnesium and its alloys have been proposed as a new class of metallic bioabsorbable implant material. Unfortunately, the rapid corrosion rate of magnesium alloys could result in the accumulation of the hydrogen gas and the reduction in the mechanical properties before the healing of bone tissue. It is necessary to develop biocompatible coatings to delay the degradation rate. In the present research, hydroxyapatite coatings are successfully deposited on AZ31 magnesium alloy by sol-gel technique to slow down the degradation and improve the bioactivity. The structure, composition and chemical states of the sol-gel coatings are characterised by scanning electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. The bonding strength, corrosion behaviours and hydrogen evolution rate of the samples are also studied. The results show that improving the heat treatment temperature can improve the corrosion resistance and the bonding strength between the coating and the substrate. The degradation rate of the samples reduces with improving heat treatment temperature. However, the lowest hydrogen evolution rate is still much higher than the tolerated level of human body. Because of the low melting point of magnesium alloy, it is difficult to reduce the hydrogen evolution rate to the tolerated level of human body only by improving the heat treatment temperature.