Micro arc oxidized HAp–TiO2 nanostructured hybrid layers-part I: Effect of voltage and growth time
Research highlights
► HA–TiO2 nanostructured porous layers were derived by micro arc oxidation method. ► Effect of the growth time on properties of the layers was studied. ► A correlation between the voltage and the properties of the layers was proposed.
Introduction
Titanium and its alloys coated by bioceramic layers have become a research focus in the field of materials science due to their appropriate biological activity. It is because of increasing demand for a substitute material for the replacement of hard tissue, such as bone and teeth [1], [2], [3], [4], [5], [6]. Their good mechanical properties, high corrosion resistance, and excellent biocompatibility have expressed them as preferred materials for load bearing implants in dentistry, osteosynthesis, and orthopaedics [7], [8]. However, it is difficult to induce the direct bone growth on the surface of titanium implants because of their lower bioactivity. Therefore, modifications of metal surfaces are necessary as a mean of controlling tissue–titanium interactions and shortening the time of bone fixation [9]. One of these modifications is coating the titanium implant by hydroxyapatite (HAp) which exhibits good bioactivity after it is implanted in body. Unfortunately, this bioceramic material suffers from poor mechanical properties and is not suitable for load-bearing conditions [10]. The application of hydroxyapatite HAp coatings on metallic implant devices offers the possibility of combining the strength of the metals with the bioactivity of the ceramics [11]. The use of HAp coatings on titanium alloys leads to a structure that has good mechanical strength and good osteointegration properties at the surface and in the manufacturing of prosthetic devices. The system HAp/Ti is used to improve the surface properties of the device and to induce osteointegration process [12]. Furthermore, it has been demonstrated that the bond between HAp and bone is better than the bond between titanium and bone [13].
HAp layers with different morphologies and structures have been grown on titanium substrates via many surface treatment techniques namely plasma spraying [14], [15], [16], immersion in physiological fluid [17], sol–gel method [18], [19], [20], [21], electrophoretic deposition [22], [23], [24], cathodic deposition [25], [26], laser forming [27], and biomimetic method [28], [29]. HAp layers can be also synthesized employing micro arc oxidation (MAO) technique [30], [31], [7] which is a relatively convenient and effective method to deposit various functional coatings with porous structures on the surfaces of titanium, aluminum, magnesium, zirconium, and their alloys. In this process, the micro discharges rapidly develop and extinguish within 10−4 to 10−5 s on the vicinity of the anode and heat the metal substrate to less than 373–423 K. Simultaneously, the local temperature and pressure inside the discharge channels, which are formed by electrical sparks, reach 103–104 K and 102–103 MPa, respectively. These temperature and pressure are enough to give rise to plasma thermochemical interactions between the substrate and the electrolyte. These inter actions result in formation of melt-quenched high-temperature oxides and complex compounds on the surface, composed of oxides of both the substrate material and electrolyte-borne modifying elements. MAO has been considered to be an effective technique to modify the thickness, structure, composition, and topography of the Ti oxide. Due to strong electric field (106–108 V m−1) between anode and cathode, electrolyte anions are drawn into the structural pores where they can attend electrochemical reactions [32], [33], [34], [35], [36], [37], [38], [39]. For example, utilization of electrolytes consisting of glycerophosphate (GP) and Ca acetate results in the formation of an adhesive porous anodic oxide whose composition includes Ca and P [30], [31], [7], [40]. MAO is a promising technology through which high quality ceramic coatings can be obtained where chemical composition of the layers can be controlled by altering the electrolyte composition [41], [42], [43], [44], [45], [46], [47], [48], [49], [50].
In this research, effect of the applied voltage and growth time on the surface morphology, topography, chemical composition, and phase structure of the HAp–TiO2 nanostructured layers derived by micro arc oxidation technique is investigated.
Section snippets
Sample fabrication
A home-made rectifier with a maximum output of 600 V/30 A, able to supply AC, DC and pulse-DC, was used as current source. Commercially pure (Grade 2) titanium substrates with dimensions of 30 mm × 10 mm × 0.5 mm were connected to the positive pole of the power supply as anode. They were surrounded by an ASTM 316 stainless steel cylindrical cathode during MAO treatment. Typical experimental setup is schematically shown in our other works [49]. Prior to the coatings synthesis, the titanium substrates
Influence of the applied voltage
SEM surface morphology of the HAp–TiO2 layers grown for 3 min under different voltages is depicted in Fig. 1 where a porous structure is observed. As is seen, pores size increases with the applied voltage. It is worthy to emphasize that applying higher voltages causes more energetic electrical sparks due to the higher electrical current passing the electrochemical cell. Such strong electric avalanches result in formation of the wider pores. In addition, surface pore density increases when the
Conclusions
HAp–TiO2 composite layers with a porous structure were grown employing MAO technique and the influence of the applied voltage and growth time on the surface morphology, topography, chemical composition, and phase structure of the layers was studied. The layers revealed a porous structure and rough surface where the pore size and the surface roughness increased with voltage and time. The highest pores density was also achieved under intermediate voltages. The synthesized layers consisted of the
Acknowledgements
The authors would like to express their sincere appreciations to all personnel working in the Ceramics Synthesis Laboratory at Iran University of Science and Technology. Meanwhile, financial support of Iran National Science Foundation (INSF) is highly appreciated.
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