Electrophoretic deposition and characterization of thin hydroxyapatite coatings formed on the surface of NiTi shape memory alloy

Abstract

The NiTi surface was modified so as to improve its biocompatibility by forming multifunctional protective layers of TiO₂ and hydroxyapatite (HAp). A special focus was placed on obtaining relatively thin (~2.4 µm), crack-free and homogenous HAp coatings using the electrophoretic deposition method (EPD). A well-dispersed and stable colloidal suspension with the predominant particles size of ca 100 nm was used for the deposition process. The coatings were obtained at the following deposition parameters: 40 V /120 s and after vacuum-sintering at 800 °C by 2 h providing to an increase of the surface roughness. It was revealed that the composite coatings had adhesion features of (15.1±1.4) MPa and crack resistance to deformation of 2.48%. Moreover, no layer spalling was found, even at the maximum deformation value ε=3.53%. The newly manufactured coatings were also observed to have no impact on the martensitic transformation responsible for shape memory effect. Additionally, a two-step martensitic transformation below room temperature was observed.

Introduction

NiTi alloys with chemical composition close to that of equiatomic ones display unique mechanical properties, such as shape memory effects (SME) and superelasticity, which are used in the preparation of numerous biomedical devices and implants [1], [2], [3], [4], [5], [6], [7]. However, application of NiTi shape memory alloys (SMA), especially in long-term implants, is limited due to alloy corrosion and potential migration of toxic nickel ions into human organism. In order to improve the corrosion resistance and ensure protection against the harmful influence of Ni ions, the surface ought to be modified by forming different layers based on diamond-like phases, titanium oxides, titanium carbides and nitrides, polymers, apatites or composites [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]. It is widely known that modifying the surface with bioactive calcium phosphate (CaPs) coatings, such as hydroxyapatite (HAp), enhances the material's biocompatibility and osteoconductivity [21], [22], [23], [24].

Most of the deposition techniques require using high -temperature conditions, which may lead to the decomposition of the B2 parent NiTi phase to equilibrium ones, such as Ni₃Ti and/or NiTi₂, limiting at the same time the shape memory effect. SME is related to reversible martensitic transformation occurring between the B2 parent phase (cubic structure) and the B19′ martensite (monoclinic structure) [1], [2], [3], [4], [5], [6], [7]. The martensitic transformation (B2→B19′) is induced by reducing the temperature, whereas the reverse martensitic transformation (B19′→B2) occurs due to alloy heating.

Surface modifications methods, such as electrophoretic deposition (EPD), have to be repeatable, inexpensive and ensure rapidity of the process. These techniques additionally enable controlling the coating thickness, its uniformity and deposition rate. EPD is also particularly recommended for the formation of coatings on substrates with complicated shapes and morphology. It may also be used without the need to ensure elevated temperatures [25], [26], [27], [28], [29]. The desirable thickness of manufactured coatings can be easily obtained by altering the deposition parameters (time or voltage) as well as the concentration of ceramic particles in colloidal suspension. The crucial thing in the process of manufacturing thin and crack-free coatings is to use submicron or nano-sized ceramic particles and to make sure that the prepared colloidal suspension is well stabilized and unagglomerated. It is worth mentioning that according to Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, colloidal stability is closely related to Zeta potential of colloidal particles and pH value. Hence, higher pH values indicate a better dispersion of ceramic particles in the suspension and determine the possibility of applying anodic or cathodic deposition [25], [26], [27], [28].

EPD is one of the most commonly used technique for producing thick hydroxyapatite coatings [30], [31], [32], [33], [34]. It is worth noting that the unique shape memory effect may be limited or completely blocked by too thick and/or too rigid coatings. According to literature, HAp coatings electrophoretically deposited on NiTi alloy had a thickness of about 120 µm but the influence of such thick coatings on the outstanding shape memory effect and its behavior under deformation have not yet been considered [32].

In the present work a new way of improving biocompatibility has been proposed. The surface of NiTi SMA was first modified by autoclaving, which allowed preparing a thin TiO₂ film [14], [35], and, next, electrophoretically covered with hydroxyapatite coatings. Passivation in an autoclave leads to the formation of a thin amorphous TiO₂ layer, which improves the corrosion resistance [35] and strength of adhesion of ceramic particles to the metallic substrate [36]. This morphology and topography, structural characteristics and the strength of hydroxyapatite coatings’ adhesion to the NiTi substrate as well as the ability to deform and the influence of the technology on martensitic transformation related to the shape memory effect have been analyzed in detail.