Elsevier

Applied Surface Science

Volume 255, Issue 22, 30 August 2009, Pages 9132-9138
Applied Surface Science

Effect of annealing upon the structure and adhesion properties of sputtered bio-glass/titanium coatings

https://doi.org/10.1016/j.apsusc.2009.06.117Get rights and content

Abstract

Bio-glass films were deposited by radio-frequency magnetron sputtering technique onto medical grade Ti6Al7Nb alloy substrates from prepared silica based bio-glass target. A low deposition temperature was used (150 °C) and three different working pressures, followed by annealing in air at 550 and 750 °C. A quasi-stoichiometric target to substrate atomic transfer was found for Si, Ca and P, along with strong enrichment in Na and depletion in K and Mg, as evidenced by the energy dispersive microanalysis. The best results, taking into account stoichiometry and surface roughness, were obtained for the BG layers deposited at 0.3 Pa argon working pressure. The infrared spectroscopy of the as-sputtered and of the annealed films evidenced the characteristic molecular vibrations of silicate, phosphate and carbonate functional groups. The as-deposited films are amorphous and became partly crystalline after annealing at 750 °C, as evidenced by X-ray diffraction. The pull-out measurements, performed with a certified pull-test machine, gave very strong film–substrate adhesion strength values. For the non-crystalline layers, the pull-out strength is higher than 85 MPa, and decreases after annealing at 750 °C to 72.9 ± 7.1 MPa. The main objective of this work was to establish the influence of the working pressure upon the composition and morphology of the as-deposited films, and of the annealing temperature upon structure and film–substrate adhesion.

Introduction

The first bio-glass (BG), known as 45S5 Bioglass®, was reported in 1969 [1], [2], [3], [4], [5]. After this, various kinds of bioactive glasses and glass-ceramics with different characteristics have been developed by melting and glazing methods. Some of them are already commercialized as valuable bone-repairing materials. The prerequisite for glasses and glass-ceramics to bond to living bone is the formation of bone-like apatite layer on their surfaces in the body. This takes place for a SiO2 content smaller than ∼60 wt%, high Na2O and CaO content and a high Ca/P ratio (between 1 and 5) [1]. Unfortunately the coating of titanium substrates with thick BG films by enameling and plasma spray techniques, usually fail due to a weak glass/metal interface and rapid dissolution in body fluids when implanted [6], [7], [8]. That is why in recent years, chemical [9], [10], [11], [12], [13], [14] and laser [15], [16], [17], [18], [19], [20], [21], [22] methods were also investigated in order to prepare adherent micrometric thin films. In comparison with hydroxyapatite films the control of composition and adhesion to metallic substrates seems to be more difficult to accomplish in case of the BG ones. There are very few papers referring to BG coating deposited by magnetron sputtering [23]. It is widely accepted that both mechanical properties and chemical composition are important factors in the preliminary physiological bond of such implants. Low mechanical properties are the major problem that prevented the use of BG/Ti structures for load-bearing applications. The purpose of the present work is to prepare adherent magnetron sputtered (MS) bio-glass films having a structure as much as similar with that of the bio-glass target.

Section snippets

Materials

The magnetron sputtering target was prepared from a bioactive glass ceramic [1], [2] powder with micrometric granular size. The glass ceramic powder had the following composition (wt%): SiO2—55, CaO—15, P2O5—10, K2O—10, MgO—5, Na2O—5. The starting reagents were: SiO2, CaCO3, (NH4)2HPO4, K2CO3, MgO and NaHCO3. The mixture of the desired composition was mechanically homogenized and heat treated at 400 °C for 10 min. The heat-treated mixture was reground and then melted at 1300 °C for 30 min and

Results and discussions

The film's thicknesses decrease as the working pressure increases (Table 1). The decrease of the deposition rate with the Ar pressure is due to the shortening of the mean free path of the sputtered particles. Increasing the sputtering pressure, the sputtered particles suffer more collisions while traveling from the target to substrate, and some of the sputtered particles are back-scattered towards the target. This resulted in a decrease of the deposition rate due to scattering.

The EDS results

Conclusions

Bio-glass films with biomedical destination were deposited by magnetron sputtering onto medical grade titanium alloy substrates. The work evidenced the influence of sputtering pressure on the film stoichiometry, thickness and surface morphology. The best results were obtained for the BG layers deposited at the higher argon working pressure (0.3 Pa). For this optimized BG film the layer-substrate pull-out strength, measured with a certified pull-test machine, exceeded the glue limit (∼85 MPa) for

Acknowledgments

The authors wish to thank Professor Simion Simon for preparing the bio-glass powder and Professor Sorin Ciuca for the helpful advices during the UNIL stage. This work was supported by the Romanian Ministry of Research and Technology through R&D framework programs: CEEX 307/2006 and PN II 71-110/2007.

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