A protocol to develop crack-free biomimetic coatings on Ti6Al4V substrates
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A. Cuneyt Tas Department of Biomedical Engineering, Yeditepe University, Kayisdagi/Kadikoy, Istanbul 34755, Turkey (Received 22 November 2006; accepted 16 February 2007)
Biomimetic coating of titanium and related alloys with carbonated apatitic calcium phosphate is an important area of research in implantology. While this paper specifically refers to coating Ti6Al4V, the results are valid with other related alloys as well. One step in the protocol involves an intermediate alkali treatment of Ti6Al4V to form a sodium titanate layer on the alloy surface. This pretreatment enhances the formation of the coating from simulated body fluid (SBF) solutions. Many papers in the biomimetic coating literature demonstrate the presence of cracks in coatings, irrespective of the SBF compositions and placement of the substrates. The presence of cracks may result in degradation and delamination of coatings. To the best of our knowledge, this issue remains unresolved. Therefore, the aim of this study was: (i) to examine and understand the reasons for cracking and (ii) based on the results, to develop a protocol for producing crack-free apatitic calcium phosphate coatings on Ti6Al4V substrates. In this study, the authors focused their attention on the alkali treatment procedure and the final drying step. It is hypothesized that these two steps of the process affect the crack formation the most. In the first case, the surfaces of alkali-treated substrates were examined with/without water-soaking treatment before immersing in SBF. This water treatment modifies the sodium titanate surface layer. In the second case, two different drying techniques (after soaking in SBF) were used. In one procedure, the coated substrates were dried rapidly, and in the other they were dried slowly. It was observed that the water treatment, irrespective of the drying method, provides a surface, which on subsequent soaking in SBF forms a crack-free apatitic calcium phosphate coating. Based on these results, the authors suggest a protocol incorporating a water-soaking treatment after the alkali treatment and prior to the SBF soaking treatment to obtain crack-free coatings.
I. INTRODUCTION
Coating titanium and its alloys with carbonated apatitic calcium phosphate layer has been of increasing interest due to its ability to directly bond with bone.1,2 Of the many coating processes available today, the biomimetic coating technique has many advantages including producing carbonated apatitic calcium phosphate coatings, which is otherwise impossible to produce. The coatings are formed by soaking an alkali pretreated titanium (or an alloy) substrate in a simulated body fluid (SBF) solution.3–6 The alkali pretreatment is a crucial step; it
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0193 J. Mater. Res., Vol. 22, No. 6, Jun 2007
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forms a hydrous sodium or potassium titanate layer (depending upon the alkali used) on the substrate.3–6 The as-formed sodium (
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