Improvement in crystallinity of apatite coating on titanium with the insertion of CaF₂ buffer layer

Abstract

In the apatite coatings on Ti the heat treatment process is necessary to crystallize the apatite structure for improved chemical stability and biological properties. However, the heat treatment normally degrades the mechanical strength of the coating layer associated with thermally induced stress. In this study, we aimed to improve the crystallization of apatite coating by using calcium fluoride (CaF₂) as a buffer layer. The insertion of a thin layer of CaF₂ (0.2–1 μm) between apatite and Ti significantly improved the crystallization behavior of apatite. Moreover, this crystallization was more enhanced as the thickness of CaF₂ was increased. When a 1 μm-thick CaF₂ was inserted, the crystallization of apatite initiated at a temperature as low as 320 °C, being a dramatic improvement in the crystallization when considering the crystallization initiation temperature of a bare apatite coating on Ti was ∼450 °C. As a result of this crystallization enhancement, the dissolution behavior of CaF₂-inserted apatite coatings was more stable than that of the bare apatite coating, showing much reduced initial-burst effect. Preliminary cellular assay showed the CaF₂-inserted apatite coating provided a substrate for cells to spread and grow favorably, as being similar to the bare apatite coating. This novel way of apatite coating on Ti using CaF₂ buffer layer may be useful in the coating systems particularly requiring low temperature processing and increased crystallinity with high chemical stability.