Nacre coatings deposited by electrophoresis on Ti6Al4V substrates

Abstract

Crack-free nacre coatings on titanium alloys were produced by electrophoretic deposition in a nacre/ethanol suspension with or without a hydrochloric acid (HCl) additive. The microstructure and morphology of coatings were investigated by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). The deposition yield of nacre powders was determined by weighting the dried deposits. The results show that the HCl additive has no obvious effects on the crystalline phase of nacre coatings. The deposition yield of nacre powders increases with adding acid additives, prolonging deposition time, and improving applied voltage. A uniform and glue-like nacre coating is deposited in a suspension with the HCl additive via a dissolution–precipitation reaction. After the HCl additive is added into a nacre/ethanol suspension, calcium ions are released from nacre powder surfaces, and move toward the cathode within an electric field. The local concentrations of hydroxide ions and carbonate ions increase due to the reduction of hydrogen ions at the cathode surface. Calcium ions react with carbonate ions to form calcium carbonate as the ionic activity product exceeds its thermodynamic solubility product, and re-precipitates on the active sites of nacre powder surfaces.

Introduction

Nacre (mother of pearl) is a natural composite material composed of alternating layers of aragonite (CaCO₃) and a biopolymer [1]. One or more signal molecules in nacre biopolymers, like bone morphogenetic proteins (BMPs), are capable of activating osteogenic bone marrow cells leading to bone formation [2]. Nacre can support human osteoprogenitor cell attachment, migration, growth and differentiation in vitro and in vivo. New bone is directly formed on the surfaces of nacre implant without any soft tissue intervention [3]. Therefore, nacre is considered a promising osteoinductive material for bone graft substitutes and for the correction of bone irregularities. However, the intrinsic shape and size of shell nacre hinder its wide applications in hard tissue replacement materials.

This drawback can be overcome by depositing nacre powders on metallic materials (e.g., titanium alloys, stainless steels, tantalum, and cobalt–chromium–molybdenum alloys), which exhibit excellent mechanical properties under load-bearing conditions [4], [5]. Nacre coatings on inert metallic implants are not only bioactive and biodegradable, but also mechanically robust [6]. Such coatings have been obtained by diverse deposition techniques, including plasma spraying [7], hydrothermal hot-pressing method [8], biological fabrication [6], and electrophoresis technique [9], [10]. Among these techniques, plasma spraying is one of most common methods for coating implant parts with bioceramics, but it is performed usually at near 10,000 °C over the decomposed temperature of nacre [11]. Hydrothermal hot-pressing method is a line-of-sight process; thus it is difficult to apply uniform coatings on implants with complex geometries [12]. Electrophoretic deposition represents an important technological alternative due to rapid coatings production, high reproducibility, low processing cost, and the possibility of forming coatings with complex shape and patterns [9], [13], [14]. A high degree of control of coating deposit thickness and morphology can be obtained by adjusting the deposition conditions and bioceramic powder size and shape. Moreover, the proteins inside nacre powders can be preserved after electrophoretic deposition [15].

However, electrophoretic coatings have the major drawback of poor adhesion as compared with plasma spraying or thermal spraying [16]. These coatings must be posttreated by densification at about 1200 °C [17]. Such high temperatures not only deteriorate the mechanical properties of metal implants [18], but also cause decomposition of nacre coatings. Fortunately, acid pretreatment is an important substituted method to overcome this drawback. A TiO_x layer with a homogeneous roughly microtopography is created on a substrate surface after chemical pretreatment. The bonding strength between substrates and coatings is improved by micromechanical interlocking bonding [19]. Moreover, the TiO_x layer obtained on titanium alloys can improve corrosion resistance and induce bonelike apatite formation [20].

The deposition rate of electrophoresis is influenced by the Zeta potential (ζ) and the conductivity of suspensions. Previous studies [17] have shown that the addition of acid or alkaline (HNO₃ or NH₄OH) can obtain an ideal and stable suspension for deposition of bioceramic particles and improve its conductivity. In our studies, we find that the addition of hydrochloric acid (HCl) not only increases the deposition rate, but also alters the morphology of nacre coatings.

In this work, Ti6Al4V substrates were pretreated by a 1.0 mol/l H₃PO₄–1.5 wt.% HF solution to form a TiO_x layer on substrate surfaces. Nacre coatings were deposited on Ti6Al4V substrates by electrophoretic technique in a nacre/ethanol suspension with or without acid additives. The effect of acid additives on nacre coatings was investigated by using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM).