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

Abstract

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 120mm. 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 (60mm) 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 7days. 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 2–4μm was deposited. Higher crystallinity of HA on the surface was observed when the distance dt-s increased to more than 80mm. Therefore, it seems that a dt-s greater than 80mm is an important sputtering condition for inducing HA on the zirconia film.