Fatigue property of a bioabsorbable magnesium alloy with a hydroxyapatite coating formed by a chemical solution deposition

A hydroxyapatite (HAp) coating was directly formed on an extruded AZ31 magnesium alloy by a single-step chemical solution deposition. The HAp coating consists of an outer porous HAp layer, an inner continuous HAp layer, and a thin intermediate MgO layer, and the inner HAp and MgO layers are composed of nanocrystals. Tensile and fatigue tests were performed on the HAp-coated AZ31 in air. The HAp coating microscopically showed neither crack nor detachment at 5% static elongation (1.5% residual strain). With further elongation under tensile stress, cracks were formed perpendicularly to the tensile direction, and fragments of the coating detached with a fracture inside the inner continuous HAp layer. The fatigue strengths at 10⁷ cycles (fatigue limit) of HAp-coated and mechanically polished AZ31 were ca. 80MPa and ca. 90MPa, respectively. The slight decrease in the fatigue limit with the HAp coating is attributed to small pits with a depth of ca. 10μm formed on the substrate during the HAp-coating treatment. The HAp coating remained on the specimen without cracks after 10⁷ cycles at the fatigue limit, which provides ca. 3% cyclic elongation.