Preparation and characterisation of electrophoretically deposited hydroxyapatite coatings on type 316L stainless steel
Introduction
Metals and alloys are used in restoration of anatomical structures for centuries owing to their superior mechanical properties. However, the degradation of most metals implanted in the human body has narrowed the choice of clinically usable metals and alloys to mainly––stainless steels, cobalt–chromium and titanium and its alloys [1], [2]. These metallic devices are unique that they are exposed to living cells, tissues and biological fluids which are not only dynamic but are also a hostile environment for the survival of the implant [3]. Type 316L stainless steel (SS) are widely used for implantation purposes in orthopaedic surgery owing to their corrosion resistance, mechanical properties and low cost. However, clinical experience has shown that they are susceptible to localised corrosion in the human body causing the release of metal ions into the tissues surrounding the implants. Several incidences of failures [4], [5] of such devices have demanded the application of biocompatible and corrosion resistant coatings and surface modification of the alloys.
There is widespread interest in the basic calcium phosphate mineral, hydroxyapatite (Ca10(PO4)6(OH)2) (HAP) which is the prototype of one of the major constituent of bones and teeth. It is biocompatible and bioactive and is capable of interacting with the surrounding bone. HAP is known to have a simulating effect on bone formation, which is known as osseo-induction. It enhances the osseo-ı̀ntergration, and there are indications that chemical bonding may occur between HAP and bone [6]. But, its poor mechanical properties inhibit its use for implantation purposes. Hence, it is proposed in this work to develop thin layers of HAP on the surface of type 316L SS by electrophoretic deposition and to study their electrochemical properties for applications as orthopaedic devices. The excellent biocompatibility and biostability of HAP coatings have become well established and the use of this material for prosthetic applications are being rapidly popularised in the past few years [7].
The dominant requirements connected with the development of HAP coatings on metallic implants are––preparation of stoichiometric powder material with required chemical and phase composition. This is established by their chemical identity (Ca/P ratio 1.66) and by close crystallographical affinity with bone tissue. Another essential criteria are their deposition as coatings without the presence of non-stoichiometric phases of the powder. A number of novel methods [8] offering the potential for better control of film structure for coating HAP include hot isostatic pressing, flame spraying, ion beam deposition, laser ablation and electrochemical deposition along with plasma spraying which has been widely studied over the decade. The major problems associated with plasma spraying process are that it is a line of sight process that produces a non-uniform coating with heterogeneous structure. The high temperature involved alters the HAP and metal substrate phases [9]. Hence, electrophoretic deposition of HAP on metal substrates was used to overcome the above drawbacks and to achieve the uniform distribution of fine HAP deposits. The advantages of this technique include high purity of layers formed, ease of obtaining the desired thickness and stronger adhesion to the substrate. The present work was undertaken to optimise the applied coating potential required to produce adherent HAP coatings on SS and electrochemically evaluate their corrosion resistance and impedance behaviour in simulated body conditions.
Section snippets
Materials
HAP powder was chemically synthesised by wet chemical method using H3PO4 (0.3 M) and Ca(OH)2 (0.1 M) solutions at the optimum conditions developed in our laboratory. The reaction temperature was maintained at 60 °C and pH of the solution at 10. After the addition was complete, the samples are subjected to ripening treatment which included refluxing for 30 min followed by stirring for 1 h and the solution was aged for 24 h. The precipitate was then filtered and sintered at 900 °C for an hour in
Open circuit potential–time measurements
The OCP–time plots for the uncoated and HAP coated type 316L SS are shown in Fig. 1. The OCP of uncoated sample shifted towards active direction and reached a potential of −0.237 V (vs SCE) after 60 min. This could be due to the dissolution that could occur at the alloy surface [11]. The OCP–time curves of the coated HAP specimens shifted towards noble direction. This indicates the protective nature of the coatings on 316L SS. The noble behaviour of the ceramic HAP coatings could be due to the
Discussion
The coating thickness increases with the increase in applied potential during EPD. At lower coating potentials (<30 V) thinner coatings are obtained whereas at higher potentials (>90 V) the coatings are highly porous and fragile in nature. The coatings obtained in the potential range from 30 to 90 V were uniform and stable after vacuum sintering at 800 °C for one hour and on immersion in the electrolyte. No flaking, cracks or decohesion of the coatings was observed on sintering and after the
Conclusions
The optimum coating parameters for electrophoretic deposition of HAP on type 316L SS was established at 60 V and 3 min. XRD and SIMS studies confirm the presence of stoichiomertic structure and presence of Ca and P on depth profiling of the HAP coatings. The OCP and breakdown potentials of HAP coated samples shifted towards the nobler direction when compared with the uncoated 316L SS. Marginal changes observed in the impedance parameters (|Z|, Rp and C) for the coated samples before and after
Acknowledgements
The authors are thankful to Dr. H.S. Khatak, Head, Corrosion Science and Technology Division (CSTD) and Dr. R.K. Dayal, Head, ACSSS, CSTD, IGCAR, Kalpakkam, for their constant support and encouragement during the course of the present investigation. Thanks are due to Dr. S. Rajagopalan, MSD for help in SIMS experiments and Mr. V.S. Sastry and Mr. K.L.N. Reddy, MSD for X-ray studies. One of the author’s (T.M.S) acknowledge the Council of Scientific and Industrial Research, New Delhi, for
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