The role of target-to-substrate distance on the DC magnetron sputtered zirconia thin films' bioactivity

Zirconium dioxide thin films were deposited on 316L-stainless steel type substrates using DC unbalanced magnetron sputtering. The process parameter of this work was the target-to-substrate distance (dt-s), which was varied from 60 to 120 mm. The crystal structure and surface topography of zirconium dioxide thin films were characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM). The results demonstrate that all of the ZrO2 thin films are composed monoclinic phase. The film sputtered at short dt-s (60 mm) shows a rather heterogeneous, uneven surface. The grain size, roughness, and thickness of thin films were decreased by increasing dt-s. The bioactivity was assessed by investigating the formation of hydroxyapatite (Ca10(PO4)6(OH)2) on the thin film surface soaked in simulated body fluids (SBF) for 7 days. XRD and scanning electron microscopy (SEM) were used to verify the formation of apatite layers on the samples. Bone-like apatites were formed on the surface of the ZrO2 thin film in SBF immersion experiments. A nanocrystalline hydroxyapatite (HA) with a particle size of 2-4 μm was deposited. Higher crystallinity of HA on the surface was observed when the distance dt-s increased to more than 80 mm. Therefore, it seems that a dt-s greater than 80 mm is an important sputtering condition for inducing HA on the zirconia film.