Comparative study of the radio-frequency magnetron sputter deposited CaP films fabricated onto acid-etched or pulsed electron beam-treated titanium
Introduction
For decades, the application of coatings on the surfaces of metallic implants has improved the bone-binding properties of such implants [1]. Calcium phosphate (CaP) is often used for such coatings, with the most commonly used form of CaP being hydroxyapatite (HA) because it is a mineral component of bones. The study of CaP films deposited onto various modified surfaces of Ti is an active area of research [2]. A recent review noted that long-term clinical studies of films indicate contradictory results [3]. However, the surface modification of metallic implants with CaP will help to improve the clinical outcome, at least for short-term performance, when primary stability is required [1].
Over the last 30 years, a variety of coating techniques have been used to fabricate CaP coatings onto metallic materials with desirable properties. These methods include plasma spraying [4], [5], pulsed laser deposition [6], biomimetic methods [7], [8], electrophoretic deposition [9], sol–gel deposition [10], and radio-frequency (RF) magnetron sputtering [2], [11], [12], [13], [14], [15]. Although plasma spraying has achieved broad application, this method still has a number of limitations. Plasma spraying produces coatings with poor adhesion, high porosity and a lack of uniformity regarding the morphology and crystallinity of the deposited films. In addition, secondary phases of minerals in the coating, such as tricalcium phosphate and calcium oxide, are formed.
RF magnetron sputtering deposition allows for close control over the processing conditions, with the resultant possibility to develop nanostructured CaP coatings of high quality [2], [16]. The general approach to explaining the effects of sputtering and many other elementary processes in plasma-surface interactions in a broad range of elemental composition, structure, and dimensionality were critically reviewed by Ostrikov et al. [17].
This possibility of high-quality CaP coatings makes RF magnetron sputtering a potentially extremely useful technique to obtain wear-resistant coatings; moreover, a good biological response, both in vitro and in vivo, was found for the CaP coatings produced via RF magnetron sputtering [18], [19], [20].
Titanium implants exhibiting microtopography have demonstrated enhanced osteointegration. The common commercial method to generate rough structures in titanium involves the treatment of titanium via acid etching (AE). Currently, biomaterial scientists are making important efforts to improve current materials and provide alternative methods of material production. In the past decades, a number of experimental studies have demonstrated that the structure resulting from electron beam treatment significantly increases the dynamic strength, wear and corrosion resistance of metals [21], [22], [23], [24].
Much of our group's research includes recent work on nanocomposite CaP films deposited onto mechanically polished titanium substrates, NiTi substrates, and silicon plates [16], [25]. These results have led to efforts to prepare titanium substrates using pulsed electron beam (PEB) treatment.
The surface topography, hardness and Young's modulus of materials are the primary factors that affect wear and fatigue performance. The importance of mechanical strength and substrate adhesion, in particular, to ensure stability of HA and other Ca-phosphates in orthopaedic reconstruction applications, was discussed by Xu et al. [26]. Because the film growth mechanism is strongly dependent on the film fabrication method and the substrate, it was expected that the substrate morphology would play an important role in determining the coating properties.
However, the effects of titanium substrate surface treatment and morphology induced by the preparation on the morphology and mechanical features of CaP coatings deposited by RF magnetron sputtering, especially in the case that the substrate is not atomically smooth, have not been investigated experimentally. Moreover, this fundamental research of the dependence of the means of substrate preparation on the properties of films has great potential for applications. The morphological study of CaP films is of great importance for preparing thin films with the tailored morphology necessary to acquire unique biocompatible properties. For example, the morphology of the CaP coating is of critical importance to cell behaviour (adhesion, proliferation and differentiation). In this paper, we perform atomic force microscopy (AFM) and nanoindentation analyses to determine the influence of Ti substrate preparation on the surface morphology, nanohardness, and Young's modulus of the CaP layers deposited via RF magnetron sputtering deposition.
Section snippets
Materials and methods
A commercially available apparatus with an RF (13.56 MHz, COMDEL) magnetron source was used to deposit the CaP coatings. Each coating was deposited at an RF-power level of 500 W in argon atmosphere for 8 h onto a substrate mounted in a grounded substrate holder. A powder of HA (Ca10(PO4)6(OH)2) was prepared by mechanochemical activation and then was used as a precursor-powder to prepare a target for sputtering. The powder was pressed and then sintered in air at 1100 °C for 1 h. The chemical
Results and discussion
The growth rate of the CaP films was equal to 1.43 ± 0.26 nm·min− 1 for the CaP coatings deposited onto Si substrates. The total thickness of the deposited CaP layers was 690 ± 125 nm. The results for the fitted high resolution XPS data obtained for O 1s, Ca 2p and P 2p regions of CaP films deposited by RF magnetron sputtering onto titanium substrates treated by two different preparation methods are identical, as shown in Fig. 1. For the CaP coating, the O 1s (Fig. 1a) envelope was fitted with two
Conclusions
The morphology and mechanical properties of CaP coatings fabricated by means of RF magnetron sputtering onto titanium substrates of different substrate morphologies using different methods of substrate preparation (AE and PEB) were studied. A comparison of the СaP films on titanium substrate surfaces modified by acid etching and pulsed electron beam treatment revealed significant differences in the morphology of the films (roughness, grain size and shape). A smoother layer morphology was
Acknowledgements
The authors thank Prof. M. Chaikina for the synthesis of the HA-powder, Prof. M. Epple and Mrs. E.S. Ivanova for fruitful discussions, A. Sharonova for the measurements of the coating thickness, Mr. V.P. Ignatov for the help with acid etching, and Prof. V. Pichugin for administrative support. This research was supported by an FP7 Marie Curie grant (327701), the Russian Fund for Basic Research (13-08-98082, 14-08-31027 моl-ɑ), the Russian President's Stipend SP-6664.2013.4 and grant MK-485.2014.8
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