Influence of extrusion direction on hydroxyapatite coating formation, corrosion and mechanical performance of a Mg-Zn-Zr alloy.

Abstract

Extruded Mg-Zn-Zr alloys are promising materials for orthopedic applications due to their bio-safe alloying elements and high strength. However, their rapid corrosion remains a major limitation. Surface treatment using hydroxyapatite (HAp) coatings is an effective approach to enhance both corrosion resistance and biocompatibility of the alloys. However, understanding about formation of the HAp coating on different extrusion directions, and its resulting corrosion and mechanical behavior, is limited. In this study, a HAp coating was formed on both cross-sectional and longitudinal direction of a commercially extruded Mg-Zn-Zr alloy via a chemical conversion method. The cross-sectional surface exhibited various regions of poor coating coverage above coarse second-phase particles, whereas the longitudinal surface showed easily detachable coating regions formed over the elongated former Mg grains. These defective sites then promoted severe pitting corrosion. The coating reduced the corrosion rate of the alloy by up to 36.39% by day 14; however, its protective effect diminished quickly thereafter. Regardless of coating presence, the strength reduction rate doubled during the period from day 14 to 28 due to sudden fracture when pit depth exceeded 0.6-0.8 mm, indicating a critical threshold for mechanical integrity. After immersion, both uncoated and coated samples remained at about 70% of their initial strength, approximately twice that of human cortical bones, highlighting a strong potential for load-bearing implants.