Elsevier

Biomaterials

Volume 23, Issue 16, August 2002, Pages 3329-3340
Biomaterials

Effect of phosphorus-ion implantation on the corrosion resistance and biocompatibility of titanium

https://doi.org/10.1016/S0142-9612(02)00020-0Get rights and content

Abstract

This work presents data on the structure and corrosion resistance of titanium after phosphorus-ion implantation with a dose of 1017P+/cm2. The ion energy was 25 keV. Transmission electron microscopy was used to investigate the microstructure of the implanted layer. The chemical composition of the surface layer was examined by X-ray photoelectron spectroscopy and secondary ion mass spectrometry. The corrosion resistance was examined by electrochemical methods in a simulated body fluid at a temperature of 37°C. Biocompatibility tests in vitro were performed in a culture of human derived bone cells in direct contact with the materials tested. Both, the viability of the cells determined by an XTT assay and activity of the cells evaluated by alkaline phosphatase activity measurements in contact with implanted and non-implanted titanium samples were detected. The morphology of the cells spread on the surface of the materials examined was also observed. The results confirmed the biocompatibility of both phosphorus-ion-implanted and non-implanted titanium under the conditions of the experiment. As shown by transmission electron microscope results, the surface layer formed during phosphorus-ion implantation was amorphous. The results of electrochemical examinations indicate that phosphorus-ion implantation increases the corrosion resistance after short-term as well as long-term exposures.

Introduction

Titanium is a well-known implant material widely and successfully used in medicine. The primary reason for using titanium in medical applications is its biocompatibility. Examinations of titanium in the solutions simulating physiological fluids have shown that, in this environment, calcium phosphate forms spontaneously on its surface [1], [2]. This probably contributes to the good biocompatibility of titanium. The spontaneously formed calcium phosphate layer is very thin: after 30 days of exposure its thickness is of the order of a few nanometers. Hanawa et al. [3], [4], [5], [6], [7] have shown that the process of formation of the phosphates can be accelerated if the titanium surface is first modified by calcium-ion implantation.

The present authors have found that, at potentials higher than 2.5 V, calcium implanted titanium is subject to pitting corrosion [8]. This was the reason why we thought of implanting phosphorus, which is a constituent of phosphates.

There are only few publications concerned with the phosphorus-ion implantation into the surface of titanium. The available papers discuss how phosphorus-ion implantation affects the structure and chemical composition of the surface layer formed during the implantation [9], [10], [11]. Ferdjani et al. [9] examined the distribution of the phosphorus concentration within a surface layer implanted with a dose of 5×1016 P+/cm2 at a beam energy of 130 keV. The phosphorus atoms appeared to occupy the interstitial sites in this layer. The effect of phosphorus-ion implantation into the surface of titanium upon the chemical composition of the implanted layer was discussed by Baunack et al. [10]. They examined samples implanted with doses of 1×1015–3×1017 P+/cm2 at an energy of 20 keV. They found that, at doses below 1016, phosphorus did not enter in chemical bonds with titanium, whereas at doses equal to, and above 1017 P+/cm2, a new TiP phase appeared. The amount of this TiP phase increased with increasing phosphorus dose.

The effects of the phosphorus-ion beam energy and phosphorus dose on the chemical composition and structure of the implanted surface layers were examined by Wieser et al. [11]. The implantation parameters they used were: 3×1017 P+/cm2 at an ion beam energy of 30 keV, and 5×1017 P+/cm2 with a beam energy of 195 keV. The thickness of the implanted layer was 60 nm at an energy of 30 keV, and 250 nm at an energy of 195 keV. XRD examinations identified no titanium/phosphorus chemical compounds, but indicated that the surface layer became partially amorphous.

No data are available in the literature describing how the phosphorus-ion implantation affects the corrosion resistance of titanium and the precipitation of calcium phosphate on the titanium surface. The aim of the present study was to examine whether the corrosion resistance of titanium can be improved by phosphorous ion implantation.

Section snippets

Materials and methods

The material examined was pure commercial titanium (grade 2). The test specimens in the form of discs 6.2 and 14 mm in diameter and 2 mm thick were polished one side to the mirror finish and then their surfaces were implanted with phosphorus ions at a dose of 1×1017 P+/cm2. The ion energy was 25 keV. The conditions under which the implantation was carried out were so selected that the phosphorus concentration was maximum on the surface. During the implantation, the temperature of the samples did

TEM results

The starting material contained dislocations and a high density of subgrains with blurred-boundaries that occurred within the original grains whose size exceeded 1 μm. This is a characteristic microstructure of the non-recrystallized structure of a metal subjected to plastic deformation (Fig. 1). Fig. 2 shows the diffraction image of a surface layer formed as a result of phosphorus implantation. The blurred rings indicate that the surface layer has become amorphous. By measuring the diameter of

Discussion

The phosphorus-ion implantation has appeared to improve the corrosion resistance of titanium. An increased corrosion resistance of phosphorus implanted titanium placed in SBF was observed after both short-term and long-term exposures. This increase can be attributed to the alterations of the structure and of the chemical composition of the surface layer due to the implantation.

TEM examinations of the microstructure show that, after the implantation, the surface layer becomes amorphous, whereas

Conclusion

Based on the results obtained we can conclude that:

Phosphorous ion implantation with a dose of 1×1017 P+/cm2 results in an amorphisation of the surface layer and the formation of TiP.

Phosphorous ion implantation with a dose of 1×1017 P+/cm2 increases the corrosion resistance after short-term as well as long-term exposures.

The phosphate layers formed on the titanium surface during the exposure do not affect the corrosion resistance.

The biocompatibility of phosphorus-ion implanted titanium was

Acknowledgments

The authors acknowledge the support of the State Committee for Scientific Research though the Grant No. 7T08C 024 13.

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