Hydroxyapatite-carboxymethyl cellulose-graphene composite coating development on AZ31 magnesium alloy: Corrosion behavior and mechanical properties

Highlights

• HA-CMC-Gr biodegradable composite coating was developed on the AZ31 magnesium alloy using the EPD technique.

• The HA-CMC-Gr coating had a dense and uniform microstructure.

• The corrosion resistance performance of the HA coatings containing CMC and Gr was better than that of the HA coating.

• Nanoindentation test showed that Gr enhanced the Young's modulus of the HA coating.

Abstract

In this study, hydroxyapatite (HA) coatings containing carboxymethyl cellulose (CMC) and graphene (Gr) were developed on AZ31 magnesium alloy through two-step electrophoretic deposition method. The morphology and chemical bonding of coatings were characterized and also the phase identification was done using scanning electron microscopy, Fourier transform infrared spectroscopy, and X-ray diffraction, respectively. Moreover, the corrosion behavior of the applied coatings was compared with the bare AZ31 Mg alloy substrate in the simulated body fluid by the means of potentiodynamic polarization test and electrochemical impedance spectroscopy. Obtained results revealed that the novel HA-CMC-Gr coating possesses the highest corrosion resistance compared to the HA, HA-CMC, and HA-Gr coatings due to its uniform and compact structure. To investigate the mechanical properties and to elucidate the effect of CMC on the adhesion of coating-alloy interface, pull-off test was employed, where results demonstrated that the addition of CMC increases the adhesion force from 1.06 MPa to 1.62 MPa. Besides, the modulus of elasticity and the hardness of HA and HA-Gr composite coatings were compared by applying nanoindentation test. Interestingly, it is detected that the presence of Gr has considerably increased the elastic modulus of the coating by approximately 30% in comparison to the pure HA coating.

Introduction

Classical metallic alloys such as stainless steel, titanium, and cobalt-chromium have been utilized for many years as body implants. However, the imposed pains and the high cost for the second surgery to remove the implant after curing encouraged scientists to look for temporary and biodegradable implants. Although bioactive glass and polymeric biomaterials have been highly recommended and used as temporary implants, they do not show enough strength in load-bearing points [[1], [2], [3], [4], [5]].

In the last decades, Mg alloys have emerged as suitable candidates for temporary load-bearing implants for their unique biocompatibility, the ability to degrade naturally under physiological conditions, and promising mechanical properties close to those of the bone tissue [6,7]. Despite the advantages of Mg and its alloys, they suffer from an uncontrollable high corrosion rate and serious hydrogen gas (H₂) evolution during the tissue repair process which could lead to loss of mechanical integrity before sufficient tissue regeneration [8].

There are various strategies to tackle the known negative effects of the Mg-based implants rapid degradation while they are placed in vivo and to improve their clinical applications. Among these strategies, alloying [3], purification [9], surface treatment, and grain reinforcement [10] are commonly used. One key solution to extend the service life of Mg implants is to covering the implant surface with a biocompatible calcium phosphate coating [11]. Comparing different surface coatings developed by calcium phosphate compounds, hydroxyapatite (HA) [Ca₁₀(PO₄)₆(OH)₂] is the most thermodynamically stable member of the family, especially at the human body temperature and pH value [12,13]. HA is a crystalline inorganic composition that bears a striking chemically and crystallinity resemblance to the mineral part of the natural human bone tissue [14]. A range of techniques such as dip-coating [15], electrochemical deposition [16], sputtering [17], magnetron RF-sputtering [18], and micro-arc oxidation [19] have been synthesized to apply protective HA coating on Mg alloys surface. Among them electrophoretic deposition (EPD) method has advantages such as rapid deposition rate, requiring simple and low-cost equipment and process. The ability to provide a coating on complex shapes and porous surfaces [4], the possibility of controlling the thickness, and morphology of the coating by altering some deposition parameters are additional benefits of the EPD [20]. It can also be implied that it is possible to deposit such a bio-ceramic on metallic surfaces at room temperature.

Due to these remarkable characteristics, EPD has received numerous attention for applying thin films of ceramic coatings on the material surfaces [1,17,21,22]. In spite of the mentioned merits of the EPD technique, there are some limitations including the weak adhesion bonding between the coating and the metal substrate, and also water electrolysis that results in generation of H₂ gas on the metal surface causing the initiation of cracks and holes on the coating surface resulting in a poor quality microstructure [21,23].

Being a ceramic material, HA has poor mechanical properties which can be overcome by addition of some additives such as zirconia, silicon carbide, alumina, chitosan, carbon nanotubes, etc. to the coating [22,[24], [25], [26]]. Coating adhesion is also a concern that is related to the durability of the implant in the body solution [27].

A reinforcing material like graphene (Gr), has excellent mechanical properties and can be added to make the HA coating suitable for use under significant mechanical tension in the body physiological environment. It has also been reported that Gr enhances the corrosion resistance of the HA coating. In this regard, Rafiee et al. [28] reported that Gr platelets, single-walled carbon nanotubes (SWCNT), and multi-walled carbon nanotubes (MWCNT) improved the Young's modulus, tensile strength, and fracture toughness of epoxy nanocomposites 2–3 times more than the time when CNTs were used. Composite coatings such as HA-carbon are considered as an innovative approach that can compensate for the limited mechanical properties of HA. For instance, Rashad et al. [29] investigated the effect of Gr platelets on the yield strength and ductility of Mg–10Ti alloys. Their results revealed that the properties were enhanced due to the high specific surface area, adhesion, and the two-dimensional nature of the Gr platelets. According to a survey, a thin layer of Gr on Mg could reduce the corrosion rate in an aqueous solution containing chloride ion [30]. It is also noteworthy that the Gr which is derived from graphite is less cytotoxic than CNTs [31]. Adhesion of the ceramic coating to the alloy surface is an important property to be considered in EPD. Carboxymethyl cellulose (CMC) is an anionic, water-soluble polymer with a chemical structure similar to that of chitosan that can be used to increase the coating adhesion to the surface. In an investigation, the biocompatibility and appropriate stability of CMC have been proved thus, it can be used in composite coatings [32]. It seems that both Gr and CMC can be used as new and innovative reinforcing agents in the HA coating. In best of our knowledge it seems that, no research has yet focused on the mechanical properties and corrosion behavior of modified HA coating by biocompatible additives, such as Gr and CMC on AZ31 Mg alloy.

In this study, coatings consisting of HA, Gr and CMC have been applied on AZ31 Mg alloy by the EPD method. Furthermore, the effect of Gr and CMC additives on the microstructure, corrosion and mechanical behavior of HA coating is comprehensively investigated.