Tribo-Mechanical Properties and Corrosion Behavior ... - MDPI

coatings Article

Tribo-Mechanical Properties and Corrosion Behavior Investigation of Anodized Ti–V Alloy Bingrong Han 1 , Erfan Zal Nezhad 1,2, *, Farayi Musharavati 3, *, Fadi Jaber 4 and Sungchul Bae 5 1 2 3 4 5

*

Department of Mechanical Engineering, Hanyang University, Seoul 04763, Korea; [email protected] Department of Mechanical Engineering, Biomechacin LLC., San Antonio, TX 78251, USA Department of Mechanical and Industrial Engineering, College of Engineering, Qatar University, P.O. Box 2713, Doha 2713, Qatar Department of Biomedical Engineering, Ajman University, Ajman 2758, UAE; [email protected] Department of Architectural Engineering, Hanyang University, Seoul 04763, Korea; [email protected] Correspondence: [email protected] (E.Z.N.); [email protected] (F.M.)

Received: 19 September 2018; Accepted: 26 November 2018; Published: 12 December 2018

Abstract: In the work presented in this manuscript, a self-organized TiO2 nanotube array film was produced by electrochemical anodization of a Ti–V alloy in an electrolyte containing NH4 F/H3 PO4 and then annealed at different temperatures under different atmospheres. The effect of annealing temperature in different atmospheres on the morphology of the film was analyzed, and the tribo-mechanical property and corrosion behavior of TiO2 were investigated. The morphological features and phase compositions were characterized using scanning electron microscopy (SEM) and X-ray diffraction (XRD) respectively. The results indicated that the TiO2 characteristic peaks did not appear after anodization because of the intrinsic amorphous feature. However, highly crystalline TiO2 (anatase and rutile) was produced after annealing from 200 to 600 ◦ C. In addition, there was an improvement in the wear resistance of the Ti–V alloy due to the high hardness and low coefficient of friction of the TiO2 nanotubes’ coating. Moreover, the corrosion behaviors of TiO2 coated and uncoated substrates were evaluated in the synthetic medium, and it was confirmed that the corrosion resistance of the TiO2 -coated Ti–V alloy, annealed at 200 ◦ C in the atmosphere, was significantly higher when compared to the uncoated sample. Keywords: Ti–V alloy; TiO2 nanotube; tribology; corrosion; anodization; nanoindentation; modulus of elasticity

1. Introduction Titanium and titanium alloys are very popular in biomedical applications, especially as substitutes for hard-tissue and as bone-implant materials due to properties such as low elastic modulus and good fatigue strength, corrosion resistance, biocompatibility, formability, and machinability [1]. Nevertheless, they do not meet all the desired clinical requirements (such as biocompatibility, bounding, etc.) and thus, surface modification is often performed in order to enhance their biological, chemical, and mechanical functionality [2]. For example, sand blasting [3], calcium phosphate coating [4], and acid etching and alkali treatment [5] have been used to modify the surface layer of these materials and improve implant bioactivity and bone growth. Another suitable surface modification technique is that of self-organized TiO2 nanotube layers. These are realizable using different methods such as the sol-gel technique [6], electrophoretic deposition [7], and anodization [8]. Comparison of the three aforementioned methods has shown that anodization has superior layer adhesion strength and is, therefore, more suitable for biological applications [9]. Using anodic oxidation, the chemical bond that is formed between the oxide and Ti substrate likely results in enhanced adhesion strength [10]. Coatings 2018, 8, 459; doi:10.3390/coatings8120459

www.mdpi.com/journal/coatings

Coatings 2018, 8, 459

2 of 22

The first generation of TiO2 nanotube arrays was grown in HF electrolytes [11] or acidic HF mixtures [12], however, it has been shown that self-organized nanotube TiO2 layers with thicknesses higher than 2 µm could be grown when using buffered neutral electrolytes containing NaF [13] or NH4 F [14] instead of HF and taking into consideration the importance of the pH gradient within the tube [15]. Materials implanted in vivo come into contact with extracellular body fluids such as blood and interstitial fluids. In terms of corrosion resistance, TiO2 nanotube layers on titanium have demonstrated higher resistance to corrosion compared to smooth Ti when placed in simulated bio-fluid [16]. In fact, the functional specifications of nanotubular arrays may be manipulated by varying the process parameters [17]. For example, a TiO2 nanotube array with a vertical alignment can be produced on the surface of a titanium substrate by anodization in an electrolyte that contains slightly dissolving fluoride [18]. Electrolytes without fluoride, like sulfuric acid [19], are used to create a compact non-porous TiO2 layer at a low potential and porous oxides at high potential due to the oxide’s electrical breakdown [20]. Investigating the wear on mechanical parts is important for assessing the reliability of materials used in medical implants. This is so since an implant, apart from onetime peak stresses, must also withstand the several million load cycles that it typically experiences throughout its lifetime [21]. In this work, the emphasis was given on the relationship between microstructure characterization and the mechanical properties of TiO2 nanotubes on a Ti alloy substrate fabricated under different conditions: Sintering in the air atmosphere and in vacuum chambers in the presence of Ar gas at different annealing temperatures. Nanoindentation was performed to probe the mechanical properties of the different samples such as Young’s modulus and nanohardness. The wear resistance and corrosion behavior of anodized samples annealed at different temperatures in versatile environments were investigated. The structure and morphology of the coated samples were evaluated using scanning electron microscopy (SEM), atomic force microscopy (AFM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) analysis. 2. Materials and Methods 2.1. Materials Ti–V tablets (20 mm × 5 mm, Special Material and Equipment Produce Co., Ltd., Baoji, China) were used in this study with the chemical composition (wt %) tabulated in Table 1. Phosphoric acid (H3 PO4 , ≥85.0%, DaeJung Chemicals & Metals, Shiheung, Korea), ammonium fluoride (NH4 F, ≥99.99%), calcium chloride dehydrate (CaCl2 ·2H2 O, ≥99%), sodium phosphate monobasic monohydrate (NaH2 PO4 ·H2 O, ≥98%), and sodium phosphate dibasic (Na2 HPO4 ) were acquired from Sigma-Aldrich (St. Louis, MO, USA). Distilled water was used in all aqueous solutions. Table 1. Chemical composition of Ti–V. Element

wt %

Al V Cr C Fe Si Ti

5.84 4.12 0.12 0.35 0.13 0.02 Balance

Coatings 2018, 8, x459 FOR PEER REVIEW

of 22 22 33 of

2.2. Preparation 2.2. Preparation 2.2.1. Preparation of the Substrates 2.2.1. Preparation of the Substrates Before anodization, substrates were polished using SiC emery paper (600, 800, 1000, 1500, and Before anodization, substrates were polished using SiC emery paper (600, 800, 1000, 1500, 2000 grit), and then wet polished in a diamond slurry. The polished samples were then sonicated in and 2000 grit), and then wet polished in a diamond slurry. The polished samples were then sonicated acetone for 30 min at 28 °C,◦ washed three times using distilled water, and dried at 60 °C◦for 1 h. in acetone for 30 min at 28 C, washed three times using distilled water, and dried at 60 C for 1 h. 2.2.2. 2.2.2. Fabrication Fabrication of of Self-Organized Self-Organized TiO TiO22 Nanotubular Nanotubular Arrays Arrays To conductelectrochemical electrochemical anodization, a two-electrode electrochemical cell wasA used. A To conduct anodization, a two-electrode electrochemical cell was used. graphite graphite rod (D = 7 mm) was connected to the cathode and the substrate was attached to the anode rod (D = 7 mm) was connected to the cathode and the substrate was attached to the anode electrode electrode (see 1). The distance the electrodes was cm. Acurrent direct current (DC) (see Figure 1).Figure The distance betweenbetween the electrodes was fixed at fixed 2 cm.atA2direct (DC) power power source (Toyotech TDP-2001B 1CH 200V/1A, Toyotech, Santa Clara, CA, USA) was used to source (Toyotech TDP-2001B 1CH 200V/1A, Toyotech, Santa Clara, CA, USA) was used to supply supply a constant anodization voltage of 10 V for 2 h under magnetic stirring (Daihan MSH-20D, a constant anodization voltage of 10 V for 2 h under magnetic stirring (Daihan MSH-20D, Daihan Daihan Scientific Co., Ltd., Gyeongju, Korea) at 150 rpm. Scientific Co., Ltd., Gyeongju, Korea) at 150 rpm. The electrolyte used for anodization was a mixture of deionized water, 0.2 M0.2 H3M POH 4, and 0.4 M The electrolyte used for anodization was a mixture of deionized water, 3 PO4 , and NH 4F. Following anodization, all specimens were rinsed with deionized water to remove residual 0.4 M NH4 F. Following anodization, all specimens were rinsed with deionized water to remove materials. residual materials.

Figure 1. Schematic view of the anodization process. Figure 1. Schematic view of the anodization process.

2.2.3. TiO2 Nanotube Crystallization 2.2.3. TiO2 Nanotube Crystallization Generally, the as-anodized nanotubes are amorphous. A tube furnace (GTF-1230, KRE SEONG, Generally, the as-anodized nanotubes are amorphous. A tube furnace (GTF-1230,was KRE SEONG, Seoul, Korea) was used for heat-treatment and a normal furnace (air atmosphere) utilized to Seoul, Korea) was used for heat-treatment and a normal furnace (air atmosphere) was utilized to achieve the crystalline phases of TiO2 . To form the crystalline phase, annealing of the anodized samples achieve the crystalline phases . To1 h. form the crystalline phase, annealing of cooling the anodized ◦ C2for was performed at 200, 400, and of 600TiO To avoid thermal cracking, heating and to the samples was performed at 200, 400, and 600 °C for 1 h. To avoid thermal cracking, heating and cooling ◦ set temperatures were performed at a ramping rate of 10 C/min. to the set temperatures were performed at a ramping rate of 10 °C/min. 2.3. TiO2 Nanotube Array Characterization 2.3. TiO2 Nanotube Array Characterization 2.3.1. Phase Analysis and Microstructural Characterization 2.3.1. Phase Analysis and Microstructural Characterization The size and morphology of the coated surfaces and cross sections were characterized using a field emission scanning electron microscope (FESEM, Nova Nano SEMsections 450, Thermo Scientific,using Tokyo, The size and morphology of the coated surfaces and cross wereFisher characterized a Japan). The coating thickness and inner diameter wereNova measured from the SEM micrographs. field emission scanning electron microscope (FESEM, Nanodirectly SEM 450, Thermo Fisher Scientific, Measuring wasThe carried out in several and different in order to measured make suredirectly that thefrom measurement Tokyo, Japan). coating thickness innerareas diameter were the SEM results corresponded to the actual coating thickness, wall thickness, and inner The phase micrographs. Measuring was carried out in several different areas in order todiameter. make sure that the measurement results corresponded to the actual coating thickness, wall thickness, and inner diameter. The phase composition and purity of the samples were studied by XRD analysis using an X-ray diffraction system (9 kW, Rigaku, Tokyo, Japan) over a 2θ range from 10° to 80°.

Coatings 2018, 8, 459

4 of 22

composition and purity of the samples were studied by XRD analysis using an X-ray diffraction system (9 kW, Rigaku, Tokyo, Japan) over a 2θ range from 10◦ to 80◦ . 2.3.2. Surface Roughness (AFM) To assess the roughness of the anodized samples, atomic force microscopy (AFM) images were taken in tapping mode using a Nanoscope IIIA scanning probe (Park System, Suwon, Korea). 2.3.3. Wear Test In order to measure the friction and wear properties of the anodized specimen, a ball-on-disc reciprocating wear tester apparatus (dry) was employed. Stainless steel balls with diameters of 5 mm were utilized. The surface of all samples was cleaned with acetone and deionized water (DI) several times prior to wear tests. During the test, a normal load of 3.76 N was applied to the disc, with a 5 mm displacement amplitude, a speed of 60 rpm, and a test period of 300 s. The wear tests were evaluated through qualitative analysis supported by optical microscope observation (U-LBD-2 Olympus, Tokyo, Japan). To evaluate the wear loss, a high-precision (0.0001 mg) weight balance (METTER TOLEDO, Greifensee, Switzerland) was utilized. 2.3.4. Evaluation of Nanohardness Nanoindentation experiments (MTS XP, MTS Systems Corporation, Eden Prairie, MN, USA) were performed and the continuous loading and unloading displacement curve recorded to get the material hardness and modulus of elasticity [22]. When the penetration depth reached 1000 nm, the loading was stopped and was held constant for 10 s. To ensure the reliability of the data, several indentations were performed for each specimen to obtain an average. 2.3.5. Corrosion Test The set-up for the electrochemical workstation comprised a standard three-electrode microcell and a computer-controlled potentiostat (PARSTAT 3000A, PalmSens, Berwyn, PA, USA) with research corrosion software (Versa Studio 2). The reference electrode used was a saturated calomel electrode (SCE), the counter-electrode was a platinum wire, and the specimen acted as the working electrode. Measurements of the open-circuit potential (OCP) were obtained following the immersion of the specimen in a CaCl2 solution. Prior to testing, the potentiodynamic polarization behavior of the specimen was recorded following immersion in a solution for 30 min in order to achieve stability. Electrochemical impedance spectroscopy (EIS) was conducted at the OCP with an alternative current (AC) amplitude of 10 mV over the frequency range 105 –10−2 Hz. The range of the scan was started from −250 mV with respect to the OCP with a sweep rate of 0.5 mV−1 ·s−1 to a final current density of 0.1 mA·cm−2 . All electrochemical tests were conducted in synthetic serum with the chemical composition shown in Table 2. Table 2. Chemical composition of the synthetic serum. Chemical Composition

Concentration (g/dm3 )

CaCl2 Na2 HPO4 NaH2 PO4

1 22.98 13.78

Coatings 2018, 8, 459

5 of 22

3. Results and Discussion 3.1. TiO2 Nanotube Array Formation Mechanism The formation of TiO2 nanotubes is a chemical, physical, and electrochemical process. Generally, it is believed that in electrolytes containing fluoride ions, the formation of TiO2 nanotubes occurs because of competing for electric field-assisted processes [23]: 2H2 O → O2 + 4e− + 4H+

(1)

Ti + O2 → TiO2

(2)

TiO2 + 6F− + 4H+ → (TiF6 )2− + 2H2 O

(3)

The anodic oxidation process is divided into three stages:

•

•

•

For titanium metal oxide under the action of an electric field, the anode current density decreases exponentially until it reaches a steady state. Due to the creation of a compact oxide film, the current drops rapidly [24]; Due to the creation of the oxide film, the electric field strength at both ends of the film increases gradually. Due to the corrosion reaction of F− with the oxide film under the influence of an electric field, the oxide film forms dissolved pits, resulting in an increase in current. The rate of decreasing current slows but continues; As the corrosion rate and oxidation rate reach equilibrium, ion mobility becomes a major factor in current formation. The hole bottom electric field is much larger than that of the hole wall, and fluoride ions are more concentrated in the bottom of the hole, further deepening the hole. With the current reduction, oxidation rate, and nozzle dissolution rate, the tube bottom corrosion rate reaches dynamic equilibrium. The length of the nanotube array no longer increases.

3.2. Phase Evolution and Structural Features (XRD Analysis) Different phases have an influence on the performance of titanium dioxide. Nowadays, the rutile and anatase phases have attracted great attention, particularly in electronics and medical applications. The anatase phase is more suitable for the osteal cells nucleation and growth because of higher bioactive properties. The hydroxyapatite phase is well complemented by the crystal lattice of the anatase phase, however, the rutile phase is more efficient to enhance mechanical properties. Scientific research has shown that TiO2 with rutile phase increases the hardness of the TiO2 bulks. Hence, the anodized specimens were annealed at versatile temperatures to attain rutile phase titanium dioxide [24]. Figure 2a–c shows the XRD patterns of the uncoated sample and anodized sample for 2 h at 10 V in an electrolyte containing 0.2 M H3 PO4 and 0.4 M NH4 F before and samples anodized and annealed for 1 h at 400 and 600 ◦ C. From Figure 2a, the anodized samples without heat treatment did not show TiO2 peaks; the XRD pattern mainly shows the base of the titanium peak, indicating that the non-annealed TiO2 -coated samples were air amorphous. At the annealing temperature of 600 ◦ C in the air atmosphere, the TiO2 nanotubes showed anatase and rutile peaks. The XRD characteristic peaks of the TiO2 anatase and rutile configurations appear in the (101) plane at 2θ = 25.334◦ and in the (110) plane at 2θ = 27.507◦ , respectively. Analysis of the XRD pattern showed that the samples annealed at 600 ◦ C in an air atmosphere comprised a tetragonal anatase phase, and so the annealing in air atmosphere influenced the crystalline phase of TiO2 . However, it was found that the sample annealed in the air atmosphere had stronger diffraction peak of anatase phase; the corresponding diffraction peaks of samples annealed in Ar showed anatase phase and no rutile phase was detected at 600 ◦ C. This shows that different atmospheres had an impact on the sample crystallization. This phenomenon occurs due to introducing oxygen into the lattice of TiO2 nanotube arrays when annealed in air atmosphere [25,26]. With annealing at 600 ◦ C, a small amount of rutile-phase diffraction peaks appeared, indicating

Coatings 2018, 8, 459

6 of 22

that with increasing temperature, some of the anatase structure was transformed into a rutile-phase structure. However, there was still a small residual Ti peak at the conversion amount, and thus the annealing temperature of 600 ◦ C was still not enough to transform a large amount of TiO2 nanotubes into the rutile phase. In addition, there is a disparity in phase structure between the samples annealed under air and annealed Coatings 2018, 8, x those FOR PEER REVIEWunder Ar gas. 6 of 22

Figure 2. 2. (a) (a) X-ray X-ray diffraction (XRD) XRD patterns of Figure (XRD) patterns patternsof of(1) (1)substrate substrateand and(2) (2)anodized anodizedsample; sample; XRD patterns ◦ ◦ ◦ thethe anodized samples C, (2) (2) 400 400 °CCand and(3) (3)600 600°C.C. of anodized sampleswith withannealing annealing(b) (b)ininair airand and(c) (c)in inAr Argas gas at at (1) (1) 200 °C,

3.3. Microstructural Characterization 3.3. Microstructural Characterization Figure 3a depicts the top-view FESEM micrograph of the samples anodized for 2 h at 10 V Figure 3a depicts the top-view FESEM micrograph of the samples anodized for 2 h at 10 V in an in an electrolyte containing 0.4 M NH4 F and 0.2 M H3 PO4 without annealing. Figure 3b shows electrolyte containing 0.4 M NH4F and 0.2 M H3PO4 without annealing. Figure 3b shows a highera higher-magnification FESEM top-view image. An SEM micrograph of the top-view with higher magnification FESEM top-view image. An SEM micrograph of the top-view with higher magnification is shown in the inset in Figure 3b. It can be observed that the average inner diameter of magnification is shown in the inset in Figure 3b. It can be observed that the average inner diameter the nanotubes was 36.4 nm and the tube wall thickness was 7.8 nm. Figure 3c,d displays cross-sectional of the nanotubes was 36.4 nm and the tube wall thickness was 7.8 nm. Figure 3c,d displays crossviews of the sample after anodization under different magnifications. A nanotubular structure sectional views of the sample after anodization under different magnifications. A nanotubular was formed after anodization. This feature is a particularly critical parameter contributing to good structure was formed after anodization. This feature is a particularly critical parameter contributing mechanical properties [27]. From the FESEM images shown in Figure 3a,d, the surface was rough and to good mechanical properties [27]. From the FESEM images shown in Figure 3a,d, the surface was mainly comprised of lamellar solid matter. Localized dissolution of the oxide layer resulted in the rough and mainly comprised of lamellar solid matter. Localized dissolution of the oxide layer creation of irregular pits that subsequently transformed into pores. With increasing anodization time, resulted in the creation of irregular pits that subsequently transformed into pores. With increasing the compact TiO layer collapsed, after which a porous titanium dioxide layer gradually formed to anodization time,2 the compact TiO2 layer collapsed, after which a porous titanium dioxide layer achieve TiO2 nanotube arrays [28]. gradually formed to achieve TiO2 nanotube arrays [28].

Coatings 2018, 8, 459 Coatings 2018, 8, x FOR PEER REVIEW

7 of 22 7 of 22

36.4 nm

7.8 nm

Figure Lowandand (b,d)(b,d) high-magnification field emission scanningscanning electron microscope (FESEM) Figure3.3.(a,c) (a,c) Lowhigh-magnification field emission electron microscope images of the anodized substrate for 2 h obtained from 0.4 M NH F and 0.2 M H PO electrolyte at 4 4 0.4 M NH4F3 and 4 0.2 M H3PO (FESEM) images of the anodized substrate for 2 h obtained from 10 V before thermal annealing: (a,b) top-views and (c,d) cross-sectional views. electrolyte at 10 V before thermal annealing: (a,b) top-views and (c,d) cross-sectional views.

Figures Figures44and and55display displayFESEM FESEMimages imagesofofthe thesamples samplesanodized anodizedfor for22hhatat10 10VVininthe the0.4 0.4M MNH NH4 4FF ◦ C for 1 h in normal atmosphere and 0.2 M H PO electrolyte after annealing at 200, 400, and 600 and 0.2 M H33PO44 electrolyte after annealing at 200, 400, and 600 °C for 1 h in normal atmosphere and and Ar atmosphere, respectively. Annealing differentatmospheres atmospheresdid didnot not result result in in any Ar atmosphere, respectively. Annealing in in different any significant significant change in nanotube size. The average tubes’ size is around 35 nm and evenly distributed on the surface change in nanotube size. The average tubes’ size is around 35 nm and evenly distributed on the ◦ C, of the substrate. As shown in Figure 4c,d and Figure 5c,d, when the calcination temperature was 400 surface of the substrate. As shown in Figures 4c,d and 5c,d, when the calcination temperature was the morphology of the sample approximately the samethe as that anodized sample. 400apparent °C, the apparent morphology of thewas sample was approximately sameofasthe that of the anodized This is due to the small anatase crystallites grains growing along the curvature of the nanotube sample. This is due to the small anatase crystallites grains growing along the curvature length of the ◦ C, and anatase rather than through the wall thickness. Thus, the nanotube structure was stable at 400 nanotube length rather than through the wall thickness. Thus, the nanotube structure was stable at formation had no significant on the morphology. air atmosphere, were still 400 °C, and anatase formationinfluence had no significant influence onIn theanmorphology. In anthere air atmosphere, ◦ nanotubes when the annealing temperature wastemperature increased towas 600 increased C. However, in the there wereeven still nanotubes even when the annealing to 600 °C. anodized However, ◦ C in the Ar tube furnace, some of the surfaces were covered by isolated sample annealed at 600 in the anodized sample annealed at 600 °C in the Ar tube furnace, some of the surfaces were covered island aggregates 5e,f),(Figure indicating that the ordered were destroyed to some extent. by isolated island(Figure aggregates 5e,f), indicating thatnanotubes the ordered nanotubes were destroyed to This may have been responsible for the anatase to rutile and titanium to rutile phase transitions [29]. some extent. This may have been responsible for the anatase to rutile and titanium to rutile phase This indicates thatThis the indicates changes inthat microstructural characteristics withcharacteristics annealing temperature in Ar transitions [29]. the changes in microstructural with annealing atmosphere were probably associated with the excessive diffusion of Ti ions along the nanotube walls, temperature in Ar atmosphere were probably associated with the excessive diffusion of Ti ions along which resulted walls, in oxidation thus oxide wall thickening From SEM pictures, there were no the nanotube which and resulted in oxidation and thus[30]. oxide wallthe thickening [30]. From the SEM ◦ C in the air morphological differences in the porous structure after thermal treatment for 1 h at 600 pictures, there were no morphological differences in the porous structure after thermal treatment for atmosphere. the annealing treatment wasannealing carried attreatment 600 ◦ C in was Ar and the sample was 1 h at 600 °CHowever, in the air when atmosphere. However, when the carried at 600 °C in held at this temperature for an extended period of time, clear and significant morphological changes Ar and the sample was held at this temperature for an extended period of time, clear and significant ◦ C Ar atmosphere showed a spheroidization phenomenon were observed. The nanotubes a 600The morphological changes were under observed. nanotubes under a 600 °C Ar atmosphere showed a (Figure 5e). The findings suggest that the annealing temperature significant effect on the spheroidization phenomenon (Figure 5e). The findings suggest thathad theno annealing temperature had surface morphology of the TiO nanotube array film (air atmosphere), whereas annealing in the Ar 2 no significant effect on the surface morphology of the TiO2 nanotube array film (air atmosphere), tube furnace changed of the nanotubes at 600 ◦ C. of the nanotubes at 600 °C. whereas annealing in the themorphology Ar tube furnace changed the morphology

Coatings 2018, 8, 459 Coatings 2018, 2018, 8, 8, xx FOR FOR PEER PEER REVIEW REVIEW Coatings

8 of 22 of 22 21 88 of

500 nm nm 500

μm 55 μm (a) (a)

(b) (b)

500 nm nm 500

μm 55 μm (c) (c)

(d) (d)

μm 55 μm

500 nm nm 500

(e) (e)

(f) (f)

◦ C, Figure 4. 4. FESEM images of the the samples afterafter annealing for 11 h h at at 1(a,b) (a,b) 200 °C, 200 (c,d)◦400 400 °C, and and (e,f) 4. FESEM FESEM images of the samples annealing for h at200 (a,b) C, (c,d) 400(e,f) Figure images of samples after annealing for °C, (c,d) °C, ◦ 600 °C °C in600 an air air atmosphere. and (e,f) C atmosphere. in an air atmosphere. 600 in an

500 nm

5 μm (a)

(b) Figure 5. Cont.

Coatings 2018, 8, 459

9 of 22

Coatings 2018, 8, x FOR PEER REVIEW

9 of 21

5 μm (c)

500 nm (d)

1 μm

5 μm (e)

(f)

Figure 5. FESEM images of the samples annealed for 1 h at C, (c,d) 400 ◦°C, C, and (e,f) 600 ◦°C C at (a,b) (a,b) 200 200 ◦°C, in Ar gas.

For EDS analysis analysis was was conducted conducted in in order For further further investigation, investigation, EDS order to to characterize characterize the the chemical chemical composition of the samples. Figure 6a,b is the SEM top-view, EDS spectra, and elemental composition composition of the samples. Figure 6a,b is the SEM top-view, EDS spectra, and elemental composition of the substrate. substrate. The The oxygen oxygen atom atom content content of of the the anodized anodized samples samples increased increased as as compared compared to the pure pure of the to the substrate Figure 6e–h 6e–h shows shows SEM SEM images, images, EDS EDS spectra, spectra, and and elemental elemental compositions compositions substrate (see (see Figure Figure 6c,d). 6c,d). Figure ◦ C in different atmospheres. The samples annealed in different of anodized samples annealed at 600 of anodized samples annealed at 600 °C in different atmospheres. The samples annealed in different atmospheres andand O elements, without other impurity elements,elements, indicatingindicating that annealing atmospheres contained containedTi Ti O elements, without other impurity that in differentinatmospheres did not introduce impurity elements intoelements the samples. atomic ratios annealing different atmospheres did notother introduce other impurity into The the samples. The ◦ C in normal atmosphere and Ar atmosphere of Ti and O for anodized sample and annealed at 600 atomic ratios of Ti and O for anodized sample and annealed at 600 °C in normal atmosphere and Ar were 7.65, 1.68, 3.0, respectively. In addition, are easilyare generated in TiO2 , atmosphere wereand 7.65, 1.68, and 3.0, respectively. In oxygen addition,vacancies oxygen vacancies easily generated and thus oxygen vacancies are ubiquitous in various TiO2 TiO systems. These defects alsoalso existed in in TiO 2, and thus oxygen vacancies are ubiquitous in various 2 systems. These defects existed the TiO nanotube array annealed in a normal atmosphere, affecting the atomic ratio of Ti and O. 2 2 nanotube array annealed in a normal atmosphere, affecting the atomic ratio of Ti and O. in the TiO The in Ar Ar is is lower ratio of The atomic atomic ratio ratio of of the the anodized anodized sample sample annealed annealed in lower than than the the atomic atomic ratio of the the one one annealed in air atmosphere due to absence oxygen during annealing in Ar gas. annealed in air atmosphere due to absence oxygen during annealing in Ar gas.

Coatings 2018, Coatings 2018, 8, 8, 459 x FOR PEER REVIEW

(a)

(c)

10 of of 21 22 10

(b)

(d)

(e)

(f)

(g)

(h)

Figure 6. Energy-dispersive X-ray spectroscopy (EDS) elemental analysis of TiO2 nanotube thin Figure 6. Energy-dispersive X-ray spectroscopy (EDS) elemental of atmosphere TiO2 nanotube thin film: film: (a,b) substrate; (c,d) anodized sample annealed at 600 ◦ C analysis (e,f) in air and (g,h) in (a,b) substrate; (c,d) anodized sample annealed at 600 °C (e,f) in air atmosphere and (g,h) in Ar Ar atmosphere. atmosphere.

In this study, contact mode AFM was employed to observe the microscopic topography by using the interaction forces between and employed molecules,toasobserve shown the in Figure 7. AFM images of by samples In this study, contact modeatoms AFM was microscopic topography using were collected over anbetween area of 20atoms µm ×and 20 µm. As shown in Figure the anodized sampleofdisplayed the interaction forces molecules, as shown in 7a, Figure 7. AFM images samples a rough surfaceover with (RMS) 156.53 nm. According 7b, the sample anodized sample were collected anroughness area of 20 μm × 20ofμm. As shown in Figure to 7a,Figure the anodized displayed at 600 ◦with C in atmosphere showedofa 156.53 roughness of 125.99 nm. The of the anodized aannealed rough surface roughness (RMS) nm.(RMS) According to Figure 7b,surface the anodized sample ◦ sample annealed at 600 C in Ar gasshowed resultedainroughness the highest surface of 239.43 annealed at 600 °C in atmosphere (RMS) of roughness 125.99 nm.(RMS) The surface of nm. the It should be notedannealed that the RMS values here are ainrelative comparison the (RMS) analyzed anodized sample at 600 °C inprovided Ar gas resulted the highest surfacebetween roughness of samples and not absolute 239.43 nm. It are should be notedvalues. that the RMS values provided here are a relative comparison between the analyzed samples and are not absolute values.


11 of 22 11 of 22


11 of 21

(a)

(b)

(c)

Figure 7. Atomic Atomic force force microscopy microscopy (AFM) (AFM) images images of surfaces for (a) as-anodized substrate and TiO22 Figure ◦ C under (b) normal atmosphere and (c) Ar atmosphere. nanotubular arrays annealed at 600 nanotubular arrays °C under (b) normal atmosphere (a)

(b)

(c)

Anodized(2h) + 600 ℃ in Ar

℃ in Ar

℃ in Ar

Anodized(2h) + 400 Anodized(2h) + 600 ℃ in Ar

Anodized(2h) + 200

℃ in Ar Anodized(2h) + 200 ℃ in Ar Anodized(2h) + 400

Anodized(2h) +600

Anodized(2h)℃+600 in atmosphere ℃ in atmosphere

Anodized(2h) + 400 + 400 Anodized(2h) ℃ in atmosphere ℃ in atmosphere

℃ in atmosphere

Anodized(2h) + 200 Anodized(2h) + 200 ℃ in atmosphere

Anodized(2h)

Anodized(2h)

Pure Ti

0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

Pure Ti

0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

Coefficient of Friction (μk)

Coefficient of Friction (μk)

3.4. Friction and Wear Behaviors 3.4. Friction andFigure Wear Behaviors 7. Atomic force microscopy (AFM) images of surfaces for (a) as-anodized substrate and TiO2 Implantsnanotubular in service areannealed affected by°Chumoral erosion, external fretting wear, and other arrays at 600 under (b) normal atmosphere and (c)forces, Ar atmosphere. Implants in service are affected by humoral erosion, external forces, fretting wear, and other effects. The mechanical properties of thin film-coated substrates, such as hardness, elastic modulus, effects. The3.4.mechanical properties Friction and Wear Behaviors of thin film-coated substrates, such as hardness, elastic modulus, the adhesion strength between the thin film and the implant materials, the frictional properties of the the adhesion strength between theaffected thin film and theerosion, implant materials, frictional properties of the Implants in service are humoral external forces, the fretting wear, and other film surface, etc., relate to the service lifebyand service quality of the implant. Therefore, it is of great film surface, etc.,The relate to theproperties service life and service substrates, quality ofsuch theasimplant. it is of great effects. mechanical of thin film-coated hardness, Therefore, elastic modulus, significance to study the nanohardness, elastic modulus, wear resistance, and friction coefficient of theto adhesion between the thinelastic film andmodulus, the implantwear materials, the frictional properties the significance study strength the nanohardness, resistance, and frictionofcoefficient of samples prior to the application of service the titanium alloy quality material in the human body.it is of great film surface, etc., relate to the life and service of the implant. Therefore, samples prior to the application of the titanium alloy material in the human body. significance to study elastic modulus, wear to resistance, andthe friction coefficientproperties of The reciprocating wearthe testnanohardness, (ball-on-disc) was conducted evaluate tribological of The reciprocating test (ball-on-disc) was conducted to evaluate the tribological properties samples prior towear the application of the titanium alloy material in the human body. the TiO2 -coated substrates. Figure 8 shows the friction coefficient of uncoated and TiO2 -coated samples of the TiO 2-coated substrates. Figure 8 shows the friction coefficient oftribological uncoatedproperties and TiO2-coated The reciprocating wear test (ball-on-disc) was conducted to evaluate the prior to and following heat treatment at different temperatures and different atmospheres under of the TiO 2-coated substrates. Figure 8 shows the friction coefficient of uncoated and TiO2-coated samples prior to and following heat treatment at different temperatures and different atmospheres a normal load of 3.76 It can be seen heat treatment had an important influence on the friction samples prior N. to and following heatthat treatment at different temperatures and different atmospheres under a normal load of 3.76 N. It can be seen that heat treatment had an important influence on the a normal load of 3.76 N. It can beThe seen average that heat treatment had anofimportant the substrate, coefficientunder of the TiO2 nanotube coating. coefficients frictioninfluence for the on bare friction coefficient of the TiO nanotube coating. The average coefficients of friction for the bare friction coefficient the2 TiO 2 nanotube coating. The average coefficients of friction bare the anodized sample, andofthe samples annealed at 200, 400, and 600 ◦ C in air andfor Arthe atmosphere are substrate, substrate, the anodized sample, andand thethe samples and°C600 °Cand inAr air and Ar the anodized sample, samples annealed annealed at at 200,200, 400, 400, and 600 in air around 0.76, 0.72, 0.44, 0.42, 0.31, 0.30, and 0.27, atmosphere are around 0.76, 0.29, 0.72, 0.42, 0.31, 0.29,respectively. 0.30, 0.27, respectively. atmosphere are around 0.76, 0.72, 0.44,0.44, 0.42, 0.31, 0.29, 0.30,and and 0.27, respectively.

8. The friction coefficients TiO2 coatings coatings before andand afterafter heat treatment at different Figure 8. Figure The friction coefficients of ofTiO before heat treatment at different 2 temperatures and different atmospheres. temperatures and different atmospheres.

The average coefficients of friction of the annealed coatings in Ar are lower than those of the samples annealed the aircoefficients atmosphere. friction the samples annealed at Figure 8. The in friction of The TiO2 average coatingscoefficients before and of after heat of treatment at different temperatures and different atmospheres.

Coatings 2018, 8, x FOR PEER REVIEW Coatings 2018, 8, 459

12 of 21

12 of 22

The average coefficients of friction of the annealed coatings in Ar are lower than those of the samples annealed in the air atmosphere. The average coefficients of friction of the samples annealed 600 ◦ C were significantly lower than those of other samples. This behavior can be associated with the at 600 °C were significantly lower than those of other samples. This behavior can be associated with formation and crystallization of TiO the changes in the phase structure from anatase to rutile. 2 and the formation and crystallization of TiO 2 and the changes in the phase structure from anatase to rutile. These differences ininthe natureof ofthe theAr Arenvironment, environment, which leads These differences thebehavior behaviorare aredue dueto to the the reductive reductive nature which leads to to a partial reduction and vacancy formation. a partial reduction and vacancy formation. Figure 9 shows theanodized anodizedsubstrate, substrate, and TiO Figure 9 showsoptical opticalmicrographs micrographs of of the the bare bare substrate, substrate, the and thethe TiO 2 2 nanotube-coated substrates (top-view)after afterthe thewear weartest. test. nanotube-coated substratesannealed annealedunder under different different conditions conditions (top-view) Figure 9a,b opticalmicrographs micrographsofofwear wear track Figure 9a,bshows showsthe thelowlow-and andhigh-magnification high-magnification optical track onon thethe substrates tested coefficientofoffriction frictionofofapproximately approximately substrates testedagainst againstaasteel steelball, ball, with with aa corresponding corresponding coefficient 0.7641.Figure Figure9c–o 9c–oshows showslowlow-and and high-magnification high-magnification optical samples 0.7641. opticalmicrographs micrographsofofanodized anodized samples annealed under differentconditions conditionsafter afterwear. wear. Most Most of the exhibited annealed under different the debris debrisproduced producedbybythe thecoating coating exhibited largely aggregated particles chipping flakes. Additionally, wear shows scratches a largely aggregated particles or or chipping flakes. Additionally, thethe wear spotspot shows scratches as aas result debris abrasion. the friction the continues film continues to undergo plastic deformation of result debrisofabrasion. DuringDuring the friction cycle,cycle, the film to undergo plastic deformation and and breaks. The thin film off comes off the surface and act debris as an abrasive. As the temperature breaks. The thin film comes the surface and debris as anactabrasive. As the temperature increased increased from 200 to 600 °C, because of theadhesive increased adhesive of the the substrate film to theand substrate ◦ C, from 200 to 600 because of the increased strength of strength the film to stronger and stronger structure, the films appeared to have better wear resistance. The weight of the coated structure, the films appeared to have better wear resistance. The weight of the coated samples was samples was measured before and after the wear test, and a slight decrease in the weight of all the measured before and after the wear test, and a slight decrease in the weight of all the samples was samples was found. Experiments showed that the existence of a TiO2 thin film layer can significantly found. Experiments showed that the existence of a TiO2 thin film layer can significantly reduce reduce the friction coefficient. Additionally, it can be established that the wear resistance of the Ti–V the friction coefficient. Additionally, it can be established that the wear resistance of the Ti–V alloy alloy improved noticeably due to the high hardness and low coefficient of friction of the highly improved noticeably due to the high hardness and low coefficient of friction of the highly crystalline crystalline structure of the TiO2 nanotubes, which provides a good protective effect for the substrate. structure of the TiO2 nanotubes, which provides a good protective effect for the substrate. The wear The wear loss result of different samples is tabulated in Table 3. As can be seen, the anodized sample loss result ofatdifferent samples tabulated in sample Table 3.has As can be seen,and thethe anodized annealed 600 °C in the air,isand the bare the lowest highestsample values annealed in this ◦ C in the air, and the bare sample has the lowest and the highest values in this investigation. at 600 investigation.

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

(i)

(j) Figure 9. Cont.


13 of 21

Coatings 2018, 8, 459

13 of 22


13 of 21

(k)

(l)

(k)

(l)

(m)

(n)

(m)

(n)

(o)

(p)

(o) optical micrographs of worn scar on (p) Figure 9. Low- and high-magnification (a,b) substrate, (c,d) anodized substrate annealed in air atmosphere at (e,f) 200 °C, (g,h) 400 °C, and (i,j) 600 °C and Figure 9. Lowand high-magnification optical micrographs of worn scar on (a,b) (c,d) Figure 9. and high-magnification optical micrographs of worn scar onsubstrate, (a,b) substrate, ◦ ◦ annealed in Ar gas at (k,l) 200 °C, (m,n) 400 °C, and (o,p) 600 °C. anodized substrate annealed in air atmosphere at (e,f) 200 °C, (g,h) 400 °C, and (i,j) 600 °C and (c,d) anodized substrate annealed in air atmosphere at (e,f) 200 C, (g,h) 400 C, and (i,j) 600 ◦ C ◦ C, (o,p) annealed in in ArAr gasgas at (k,l) 200200 °C, ◦(m,n) 400 °C, 600 °C. and annealed at (k,l) C, (m,n) 400and and (o,p) 600 ◦ C. Table 3. Wear loss of different samples. Table 3. Wear loss of different samples. Wear Loss (g) Table 3.Sample Wear loss of different samples. Sample Wear 0.0007 Loss (g) Bare sample Wear Loss (g) 0.0007 Anodized sample 0.0005 Bare sample 0.0007 Anodized sample 0.0005 Anodized + 200 °C in Ar 0.0004 Anodized sample 0.0005 Anodized + 200 °C in Ar 0.0004 ◦ Anodized + 400 °C in Ar 0.0004 Anodized + 200 C in Ar 0.0004 Anodized 400 in Ar 0.0004 ◦C Anodized + 400++ in °C Ar 0.0004 Anodized 600 °C in Ar 0.0002 ◦ Anodized + 600 °C in Ar 0.0002 Anodized + 600 C in Ar 0.0002 Anodized +◦ 200 °C in air 0.0004 Anodized + 200+ C in °C air in air 0.0004 Anodized 200 0.0004 Anodized +◦ C400 °C in air 0.0001 Anodized + 400 in air 0.0001 Anodized +◦ 400 °C in air 0.0001 Anodized °C in air 0.0001 Anodized + 600 +C600 in air 0.0001 Anodized + 600 °C in air 0.0001 Sample Bare sample

3.5. Nanoindentation 3.5. Nanoindentation 3.5. Nanoindentation Nanoindentation experiments were carried carriedout outby byrecording recordingthe thecontinuous continuous loading and Nanoindentation out by recording the continuous loading and Nanoindentationexperiments experiments were were carried loading and unloading displacement curves (Figures 10–12), thus describing the interaction between the tip and unloading displacement describingthe theinteraction interactionbetween between and unloading displacementcurves curves(Figures (Figures 10–12), 10–12), thus thus describing thethe tiptip and sample during the indentation process toget get the material hardness and modulus of elasticity. In the sample during the indentation process to to get thethe material hardness and modulus of of elasticity. InIn thethe past sample during the indentation process material hardness and modulus elasticity. past few years, several studies of the mechanical properties of materials have focused on the past fewseveral years, studies several ofstudies of the mechanical of have materials have on the few years, the mechanical propertiesproperties of materials focused on focused the measurement of modulus (E)and and indentation hardness (H) using nanoindentation [31–34]. measurement ofYoung’s Young’s modulus (E) indentation (H) using nanoindentation datadata [31–34]. ofmeasurement Young’s modulus (E) and indentation hardness (H)hardness using nanoindentation data [31–34].

(a) (a)

(b)

(b)

Figure 10. Cont.


(c)

14 of 22 14 of 21

(d)

Characteristic load load vs. vs. nanoindentation nanoindentationdepth depthfor for(a) (a) substrate substrateand and (c) (c) anodized anodized sample. sample. Figure 10. Characteristic 15 of 22 Hardness and modulus vs. nanoindentation depth for (b) substrate and (d) anodized sample.


(a)

(a)

(c)

(c)

(e)

(b)

(b)

(d)

(d)

(f)

Figure 11. 11. Characteristic loadload vs. nanoindentation depth depth for anodized samples samples annealed annealed in atmosphere Figure Characteristic vs. nanoindentation for anodized in ◦ ◦ ◦ (f) atmosphere at 400 (a) 200 °C, (c) and (e) 600 °C. Hardness vs. nanoindentation depth at (a) 200 C, (c) C (e) and (e)400 600°CC. Hardness and modulusand vs.modulus nanoindentation depth for anodized ◦ C,°C, for anodized samples air(d) at (b) (d)(f) 400 °C,◦ C. and (f) 600 °C. samples annealed in airannealed at (b) 200in◦ C, 400200 and 600 Figure 11. Characteristic load vs. nanoindentation depth for anodized samples annealed in atmosphere at (a) 200 °C, (c) 400 °C and (e) 600 °C. Hardness and modulus vs. nanoindentation depth for anodized samples annealed in air at (b) 200 °C, (d) 400 °C, and (f) 600 °C.


15 of 22 16 of 22

(a)

(b)

(c)

(d)

(e)

(f)

12. Characteristic Characteristicload loadvs. vs.nanoindentation nanoindentation depth anodized samples annealed inatAr at Figure 12. depth forfor anodized samples annealed in Ar ◦ C, (c) 400 ◦ C and (e) 600 ◦ C. Hardness and modulus vs. nanoindentation depth for anodized (a) 200 °C, °C °C. Hardness nanoindentation depth ◦ C, ◦ C, and (f) 600 ◦ C. samples annealed in Ar at (b) 200 °C, (d) 400 °C, and (f) 600 °C.

The deformation deformation mechanisms mechanisms are are correlated correlated to to the the measured measured modulus modulus as as aa function function of of depth. depth. The The deformation deformation process process of of the the TiO nanotube array array under under Berkovich Berkovich indenter indenter can can be be described described as as The TiO22 nanotube follows; at a very low indentation depth, the TiO nanotubes bend elastically, to a certain penetration follows; at a very low indentation depth, the TiO22 nanotubes bend elastically, to a certain penetration depth which which is is characterized characterized by by aa linear linear increase increase in in modulus modulus with with increasing increasing depth. depth. In In this this region, region, depth the increasing modulus is mainly because of amplified densification of the TiO nanotubes. Due to 2 the increasing modulus is mainly because of amplified densification of the TiO2 nanotubes. Due to the brittle nature of TiO nanotubes, they elastically bend to a very small strain of around 5%. As the the brittle nature of TiO22 nanotubes, they elastically bend to a very small strain of around 5%. As the indentation depth depth enhances, enhances, fracture fracture occurs occurs (“pop-in” (“pop-in” in in the the load-displacement load-displacement curves). curves). A A parabolic parabolic indentation increase in the indentation modulus is observed. In this area, the indentation depth is comparable to increase in the indentation modulus is observed. In this area, the indentation depth is comparable to the thickness of the TiO coating. Therefore, surges in elastic moduli are considered as the result of the thickness of the TiO22 coating. Therefore, surges in elastic moduli are considered as the result of an increasing increasing contribution contribution from from the the substrate. substrate. With With increasing increasing indentation indentation depth, depth, aa fracture fracture in in TiO an TiO22 nanotubes interact with adjacent nanotubes, causing them to bend and fracture. The minor fragments nanotubes interact with adjacent nanotubes, causing them to bend and fracture. The minor fragments from fractured fractured TiO TiO22 nanotubes nanotubes become become compacted compacted slowly, slowly, causing TiO22 thin thin from causing densification. densification. When When the the TiO film underneath the indentation tip becomes gradually dense, the elastic modulus increases. Lastly, film underneath the indentation tip becomes gradually dense, the elastic modulus increases. Lastly,

the coating becomes almost fully dense. This area is characterized by a plateau in indentation

Coatings 2018, 8, 459

16 of 22

the coating becomes almost fully dense. This area is characterized by a plateau in indentation modulus and named the composite modulus, as it is a combination of the elastic modulus of the Ti substrate and the dense coating [33]. Table 4 shows the elastic modulus and hardness mean values (10 indentations per sample) of different samples at different annealing temperatures and different atmospheres. As can be seen, with increasing the annealing temperature from 200 to 600 ◦ C, the hardness and modulus increase in both air and Ar atmosphere. It seems that annealing at 600 ◦ C in Ar and air result in close elastic modulus for both samples, 156.64 and 160.87 GPa. Nevertheless, there is a significant difference in hardness of anodized samples annealed at 600 ◦ C in air compared to its counterpart annealed in Ar. This major difference could be due to the contribution of O2 for better oxidization of TiO2 coated substrate in an air atmosphere. Table 4. Hardness and elastic modulus of different samples—Nanoindentation test. Sample

Hardness (GPa)

Elastic Modulus (GPa)

Bare substrate Anodized sample Anodized + 200 ◦ C in air Anodized + 400 ◦ C in air Anodized + 600 ◦ C in air Anodized + 200 ◦ C in Ar Anodized + 400 ◦ C in Ar Anodized + 600 ◦ C in Ar

3.38 3.66 3.71 4.11 6.15 3.28 3.57 4.36

115.51 135.55 140.34 147.46 160.87 140.45 148.35 156.64

Figures 10–12 show the typical load-displacement curves, nanohardness, and modulus of elasticity of the substrate, the anodized sample, and the anodized samples annealed under different conditions. The load was applied at various penetration depths between 0 and 1000 nm. Figure 10a,c, Figure 11a,c,e and Figure 12a,c,e show the indentation load versus penetration depth on the surfaces of the substrates. The loads on the surface increased to the maximum at the 1000 nm penetration depth, held for a while, and then unloaded. In general, heat treatment under different annealing conditions (temperature and atmosphere) resulted in changes in material properties such as strength and hardness. Figure 10b,d, Figure 11b,d,f, and Figure 12b,d,f display the hardness and modulus of elasticity achieved by indentation at different depths. According to the results, the hardness and modulus values of the uncoated specimen were 3.38 and 115.514 GPa, respectively. During anodization, an amorphous TiO2 phase was formed, which increased the hardness of the entire sample slightly. It can be seen that the hardness and modulus values of the oxidized sample increased to 3.664 and 135.547 GPa, respectively (Figure 10d), because of the formation of the TiO2 thin film. Variations aside, it is clear from Figures 10–12 that the dominant trend in the hardness of these materials is an almost linear steady increase in hardness and modulus with increasing annealing temperature. The results show that hardness and Young’s modulus both initially increased with increasing depth of penetration after annealing. Both these parameters dropped slightly behind the depth of 1000 nm. There are two key processes that were caused by the densification of the nanotube layer and the further densification of the dense surface, including the two main deformation processes occurring at the tip of the indenter during indentation. The area under the indenter is dense, whereas the other side of the indenter is subjected to shear stress, resulting in densification and wear. On the one side, because of the contribution of the substrate to the overall strength of the delaminated material, the modulus of elasticity increases during penetration of the top of the indenter into the coating surface. Densification of the area around the indentation has a slight effect on the strength of the material and results in wear between the indenter surface and the densified coating surface [34]. As the density of TiO2 around the indenter increases, the modulus increases. After thermal annealing at 600 ◦ C in an air atmosphere, crystallization of TiO2 (from amorphous to crystalline) took place, with the result that the surface hardness and modulus increased to 6.15 and 160.87 GPa, respectively, values that were


17 of 22 18 of 22

atmosphere is the most advantageous of TiO 2 nanotubes coated on Ti– generally higher than those for samplescondition annealedfor in the Ar. annealing This indicates that annealing at 600 ◦ Ca in V alloy. a normal atmosphere is the most advantageous condition for the annealing of TiO2 nanotubes coated Therefore, on a Ti–V alloy. it can be established that the high hardness and low coefficient of friction of the crystalline structure of established the TiO2 nanotubes improved wear the Ti–V Therefore, it can be that the high hardnessthe and low resistance coefficient of of friction of alloy the substantially. crystalline structure of the TiO2 nanotubes improved the wear resistance of the Ti–V alloy substantially. 3.6.Corrosion CorrosionResistance Resistance 3.6. Thecorrosion corrosionbehaviors behaviorsofofthe thesubstrate substrateand andanodized anodizedsamples samplesannealed annealedunder underdifferent different The conditions were evaluated by potentiodynamic polarization tests (PDS) in CaCl 2 solution. The results conditions were evaluated by potentiodynamic polarization tests (PDS) in CaCl2 solution. The results of corresponding corrosion potential (E corr ) and corrosion current density (I corr ) are shown in Table of corresponding corrosion potential (Ecorr ) and corrosion current density (Icorr ) are shown in Table 5.5. Table Electrochemical parameters the potentiodynamic polarization curve. Table 5. 5. Electrochemical parameters ofof the potentiodynamic polarization curve.

Sample Sample

Ecorr (V) Ecorr (V)

Substrate

2.32 × 10−1

Anodized + 600 °C in Ar

−5.19 × 10−2

βc (V) βc (V)

βa (V) βa (V)

1.72 × 10−1 8.138.13 × 10−1 × 10−1 1 −1 7.017.01 × 10×−10 1.041.04 2 −2 6.916.91 × 10×−10 1.091.09 8.08 × 10−1 −1 8.08 × 10 6.4 × 10−1 ×−10 1 −1 1.41 6.4 × 10 −2 1.76 × 10 1.41 × 10−1

Substrate 2.32 × 10−1 1.72 × 10−1 −2 1 Anodized 2.55 × 10−17.53 × 7.53 Anodized 2.55 × 10− 10−2 × 10 ◦ − 1 − 2 −1 −2 Anodized °C in air 7.4 × 10 Anodized + 200 +C200 in air 2.82 × 10 2.82 × 10 7.4 × 10 1 Anodized + 400 ◦+C400 in air 2.33 × 10− 10−2 × 10−2 Anodized °C in air 2.33 × 10−18.28 × 8.28 1 Anodized + 600 ◦+C600 in air 2.64 × 10− 10−2 × 10−2 Anodized °C in air 2.64 × 10−11.18 × 1.18 ◦ − 1 Anodized + 200 C in Ar 2.18 × 10 4.58 × 10−2 Anodized + 200 °C in Ar 2.18 × 10−1 4.58 × 10−2 − 1 Anodized + 400 ◦ C in Ar 2.17 × 10 4.16 × 10−2 −1 −2 Anodized °C in − Ar5.19 × 102.17 −2 × 10 1.76 × 4.16 Anodized + 600 ◦+C400 in Ar 10−2 × 10

2) ) Icorr(A/cm (A/cm Icorr 2

1.04 × 10−7

1.04 × 10−7 −8−8 2.33 2.33 × ×1010 − 3.53 3.53 × ×10109−9 −9−9 6.35 × ×1010 6.35 − 1.87 × ×10108−8 1.87 1.86 × 10−8−8 1.86 × 10 4.54 × 10−8 4.54 −7−8 4.33 × ×1010

4.33 × 10−7

Ecorrrepresents representsthe thecorrosion corrosiontendency tendencyofofthe thesubstrate substrateand andIcorr Icorrrepresents representsthe thecorrosion corrosionrate. rate. Ecorr According to the data, the E corr for the sample was 232 mV, whereas the E corr for the anodized sample According to the data, Ecorr for the sample was 232 mV, whereas the Ecorr for the anodized 7 A −2 and −8 A cm−2 was 255 mV. In mV. addition, the Icorr of theIcorr substrate 1.04 × was 10−7 A cm× reached sample was 255 In addition, the of the was substrate 1.04 10−it cm−2 2.33 and ×it10 reached − 8 − 2 after anodization. the Additionally, reduction of Ithe corr reduction means that rate This 2.33 × 10 A cm Additionally, after anodization. of the Icorrcorrosion means that thedecreased. corrosion rate shows that the prepared coating can provide corrosion protection of a Ti alloy substrate in a CaCl decreased. This shows that the prepared coating can provide corrosion protection of a Ti alloy substrate 2 reach the in Figure 13, polarization curves of the insolution. a CaCl2 We solution. We same reachconclusion the same conclusion in which Figureillustrates 13, whichthe illustrates the polarization uncoated sample and sample the anodized sample. curves of the uncoated and the anodized sample.

Figure 13.13.The behaviors ofof the uncoated sample and anodized sample. Figure Thecorrosion corrosion behaviors the uncoated sample and anodized sample.

The Thepolarization polarizationplot plotclearly clearlyshows showsthat thatasascompared comparedtotothe theuncoated uncoatedsample, sample,the theanodized anodized specimens showed very small I values. This shows that a titanium dioxide layer on the corr specimens showed very small Icorr values. This shows that a titanium dioxide layer on thesubstrate substrate can effectively improve the substrate corrosion resistance. Thus, the specimens with nanotubes showed can effectively improve the substrate corrosion resistance. Thus, the specimens with nanotubes higher corrosion compared compared to those with smooth surfaces (Table 3). (Table 3). showed higher resistance corrosion resistance to those with smooth surfaces Figures 14 and 15 show typical potentiodynamic polarization plots acquired Figures 14 and 15 show typical potentiodynamic polarization plots acquiredfor forthe theanodized anodized sample and samples annealed at different temperatures in the air atmosphere and Ar gas, respectively. sample and samples annealed at different temperatures in the air atmosphere and Ar gas,

respectively. After annealing, the corrosion current density (Icorr) of the annealed samples was


18 of 22 19 of 22


19 of 22

considerably lower than that of the anodized sample, demonstrating an increase in corrosion After annealing, the corrosion current density (Icorr ) of the annealed samples was considerably lower resistance. Thislower increase may to changes that occur in the interface (substrate/coating) considerably than thatbe ofrelated the anodized sample, an increase in corrosion than that of the anodized sample, demonstrating an increase indemonstrating corrosion resistance. This increase may region. For the anodized sample, the nanotubes can act as effective channels for the electrolyte to resistance. This increase may be related to changes that occur in the interface (substrate/coating) be related to changes that occur in the interface (substrate/coating) region. For the anodized sample, reach the This sample, behavior be attributed the crystallization of the theelectrolyte nanotubes. Forinterface. the act anodized thecan nanotubes can act to as effective theregion. nanotubes can as effective channels for the electrolyte to reach the channels interface. for This behavior can to Amorphous-structured nanotubes have higher corrosion resistance than high crystalline nanotubes. the to interface. This behavior be attributed to the crystallization of the nanotubes. be reach attributed the crystallization of thecan nanotubes. Amorphous-structured nanotubes have higher From Table 3, it can be observed that TiO 2 coated substrate annealed at 200 and 400 °C in the air Amorphous-structured nanotubes have higher corrosion resistance than high crystalline nanotubes. corrosion resistance than high crystalline nanotubes. From Table 3, it can be observed that TiO2 coated atmosphere had corrosion protection than those annealed in Ar gas. From Table 3, itbetter can observed that coated substrate annealed at 200 and 400 than °C inthose the air substrate annealed at 200beand 400 ◦ C in theTiO air2atmosphere had better corrosion protection atmosphere had better corrosion protection than those annealed in Ar gas. annealed in Ar gas.

Figure 14. The corrosion behaviors of the anodized and annealed samples in the atmosphere. Figure 14. The corrosion behaviors of the anodized and annealed samples in the atmosphere. Figure 14. The corrosion behaviors of the anodized and annealed samples in the atmosphere.

Figure 15.15. The corrosion behaviors of of thethe annealed samples in in ArAr gas. Figure The corrosion behaviors annealed samples gas.

Figure 15. The corrosion behaviors of the annealed samplessample, in Ar gas. Figures 1616 and 17 display FESEM images ofof thethe substrate, anodized anodized sample Figures and 17 display FESEM images substrate, anodized sample, anodized sample ◦ annealed at 200 C in the air atmosphere and Ar gas after the corrosion test. Corrosion is carried annealed at 200 °C in the air atmosphere and Ar gas after the corrosion test. Corrosion is carried out Figures 16 and 17 display FESEM images of the substrate, anodized sample, anodized sample out in the form of pitting corrosion. Figure 16a,b shows pitting corrosion of the substrate with deep in the form of pitting corrosion. Figure 16a,b shows pitting corrosion of the substrate with deep holes annealed at 200 °C in the air atmosphere and Ar gas after the corrosion test. Corrosion is carried out holes having a large diameter. Figure 16c,ddisplays displaysthat thatcorrosion corrosion of of the sample is reduced having a large diameter. Figure 16c,d theofanodized anodized sample reduced in the form of pitting corrosion. Figure 16a,b shows pitting corrosion the substrate withisdeep holes compared to the substrate’s corrosion. After anodization, the specimens with nanotubes exhibited compared to the substrate's corrosion. After anodization, the specimens with nanotubes exhibited having a large diameter. Figure 16c,d displays that corrosion of the anodized sample is reduced higher corrosion resistance than the As in Figure the pitting corrosion corrosion is higher corrosion than thebare baresubstrate. substrate. Asshown shown inspecimens Figure 17a,b, 17a,b, thenanotubes pitting compared to theresistance substrate's corrosion. After anodization, the with exhibited is almost almost invisible, invisible, and and the nanotube structure is clearly visible. However, the pitting corrosion of the nanotube structure is clearly visible. However, the pitting corrosion higher corrosion resistance than the bare substrate. As shown in Figure 17a,b, the pitting corrosionofis sample annealed in the air seems to be lower compared to sample annealed in Ar. These can be sample the the air seems to bestructure lower compared sample However, annealed in Ar.pitting These corrosion can be seen almost annealed invisible,inand nanotube is clearlytovisible. the of seen (nanotubes distributed over the structure) clearly from Figure 17c,d. Further annealing in Ar (nanotubes distributed the structure) clearly from to Figure 17c,d. Further in Ar sample annealed in theover air seems to be lower compared sample annealed in annealing Ar. These can be gas, seen gas, results morphologicalchanges changesofofthe the nanotube structure. These differences ininthe results in in morphological structure. differences thebehavior behavior (nanotubes distributed over the structure)nanotube clearly from Figure These 17c,d. Further annealing in Ar gas, (mechanical stability) are due to the reductive nature of the Ar environment. Such changes are not (mechanical stability) are due to the reductive nature of the Ar environment. Such changes are not results in morphological changes of the nanotube structure. These differences in the behavior witnessed when the annealing procedure is performed in the air atmosphere. This indicates that the witnessed when the annealing is performed in the indicates (mechanical stability) are due procedure to the reductive nature of the air Ar atmosphere. environment.This Such changesthat are the not changes inin microstructural characteristics with annealing temperature were perhaps related toto the changes microstructural characteristics with annealing temperature were perhaps related witnessed when the annealing procedure is performed in the air atmosphere. This indicates thatthe the excessive diffusion of Ti ionscharacteristics along the nanotube walls, causing oxidation, but in argon gas due to changes in microstructural with annealing temperature were perhaps related to the the lack of oxygen, oxidation was suppressed [30]. excessive diffusion of Ti ions along the nanotube walls, causing oxidation, but in argon gas due to

the lack of oxygen, oxidation was suppressed [30].

Coatings 2018, 8, 459

19 of 22

Coatings 8, 22 excessive diffusion of TiREVIEW ions along the nanotube walls, causing oxidation, but in argon gas due20 toof the Coatings 2018, 2018, 8, xx FOR FOR PEER PEER REVIEW 20 of 22 lack of oxygen, oxidation was suppressed [30].

5 μm 5 μm

(a) (a)

1 μm 1 μm

(b) (b)

5 μm 5 μm

(c) (c)

1 μm 1 μm

(d) (d)

Figure 16. high-magnification FESEM (a,b) substrate substrate and (c,d) anodized TiO Figure 16. Low- and and high-magnification FESEM images images of of (a,b) 16. Lowsubstrate and and (c,d) (c,d) anodized anodized TiO TiO222 nanotube after the corrosion test. nanotube after the corrosion corrosion test. test.

55 μm μm

(a) (a)

11 μm μm

(b) (b)

55 μm μm

(c) (c)

11 μm μm

(d) (d)

Figure 17. Low- and high-magnification FESEM images of the annealed samples at 200 ◦°C C in (a,b) air Figure 17. Low- and high-magnification FESEM images of the annealed samples at 200 °C in (a,b) air atmosphere and (c,d) Ar gas after the corrosion test. atmosphere and (c,d) Ar gas after the corrosion corrosion test. test.

4. Conclusions 4. 4. Conclusions Conclusions In summary, self-organized TiO2 nanotubes were fabricated by the anodization of a Ti were fabricated by of aa Ti In summary, self-organized TiO nanotubes were by the the anodization anodization Ti alloy. alloy. The The summary, self-organized TiO22 nanotubes alloy.InThe effect of different annealing treatments at fabricated different atmospheres on theofmorphological effect of different annealing treatments at different atmospheres on the morphological characteristics, effect of different annealing treatments at different atmospheres on the morphological characteristics, tribological tribological behavior, behavior, corrosion corrosion resistance, resistance, and and mechanical mechanical properties properties of of aa TiO TiO22 nanotube-coated nanotube-coated Ti Ti alloy were investigated. Annealing in different atmospheres led to great alloy were investigated. Annealing in different atmospheres led to great differences differences in in the the microstructure, microstructure, mechanical mechanical properties, properties, and and corrosion corrosion resistance resistance of of the the nanotubes. nanotubes. From From the the XRD XRD

Coatings 2018, 8, 459

20 of 22

characteristics, tribological behavior, corrosion resistance, and mechanical properties of a TiO2 nanotube-coated Ti alloy were investigated. Annealing in different atmospheres led to great differences in the microstructure, mechanical properties, and corrosion resistance of the nanotubes. From the XRD pattern analysis, oxygen vacancies were introduced into the crystal lattice of the samples when annealed in air. The XRD data demonstrated that no characteristic peaks of TiO2 were detected after anodization. After annealing in air at 600 ◦ C for 1 h, anatase and rutile phases were detected, while only anatase phase was detected when the sample was annealed at 600 ◦ C in Ar gas. From the wear test, it was found that the average coefficient of friction of the samples annealed at 600 ◦ C in Ar gas was lower than that of the other samples, reaching 0.27. The coefficient of friction of anodized sample annealed at 600 ◦ C in the air was 0.31. In addition, after thermal annealing at 600 ◦ C in the air atmosphere, crystallization of TiO2 (from amorphous to highly crystalline) took place, with the result that the surface hardness and modulus increased to 6.15 and 160.87 GPa, respectively, which are higher than those annealed in Ar atmosphere. This suggests that annealing at 600 ◦ C in air atmosphere has a better effect on tribo-mechanical properties of anodized Ti–V alloys compared with the one annealed at 600 ◦ C in Ar. The results of the tribological evaluation showed that the high hardness and low coefficient of friction for the crystalline structure of the TiO2 nanotubes improved the wear resistance of the Ti–V alloy considerably. Moreover, the Icorr of the anodized samples after annealing at 200 ◦ C in air atmosphere provided the lowest value among the tested samples of 3.53 × 10−9 , which was the best compared to the other samples. Author Contributions: Conceptualization, B.H.; Methodology, B.H.; Software, B.H.; Validation, B.H. and E.Z.N.; Formal Analysis, B.H.; Investigation, B.H., E.Z.N., and F.M.; Resources, E.Z.N., F.J., S.B., and F.M.; Data Curation, B.H.; Writing–Original Draft Preparation, B.H., and E.Z.N.; Writing–Review & Editing, E.Z.N., F.J., S.B., and F.M.; Visualization, E.Z.N.; Supervision, E.Z.N.; Project Administration, E.Z.N. and F.M.; Funding Acquisition, F.J., S.B., and F.M. Funding: This research was funded by Hanyang University’s financial support through the Young Faculty Forum Fund (201600000001555). Conflicts of Interest: The authors declare no conflict of interest.

References 1. 2. 3. 4. 5. 6. 7. 8.

9.

Geetha, M.; Singh, A.K.; Asokamani, R.; Gogia, A.K. Ti-based biomaterials, the ultimate choice for orthopedic implants—A review. Prog. Mater. Sci. 2009, 54, 397–425. [CrossRef] Narayanan, R.; Seshadri, S.K. Phosphoric acid anodization of Ti–6Al–4V—Structural and corrosion aspects. Corros. Sci. 2007, 49, 542–558. [CrossRef] Hedzelek, W.; Sikorska, B.; Domka, L. Evaluation of selected mechanical and chemical methods of modifications of titanium. Physicochem. Probl. Miner. Process. 2005, 39, 149–154. Variola, F.; Yi, J.-H.; Richert, L.; Wuest, J.D.; Rosei, F.; Nanci, A. Tailoring the surface properties of Ti6Al4V by controlled chemical oxidation. Biomaterials 2008, 29, 1285–1298. [CrossRef] Jonášová, L.; Müller, F.A.; Helebrant, A.; Strnad, J.; Greil, P. Biomimetic apatite formation on chemically treated titanium. Biomaterials 2004, 25, 1187–1194. [CrossRef] Maiyalagan, T.; Viswanathan, B.; Varadaraju, U.V. Fabrication and characterization of uniform TiO2 nanotube arrays by sol-gel template method. Bull. Mater. Sci. 2006, 29, 705–708. Raja, K.S.; Misra, M.; Paramguru, K. Deposition of calcium phosphate coating on nanotubular anodized titanium. Mater. Lett. 2005, 59, 2137–2141. [CrossRef] Yu, X.; Li, Y.; Wlodarski, W.; Kandasamy, S.; Kalantar-zadeh, K. Fabrication of nanostructured TiO2 by anodization: A comparison between electrolytes and substrates. Sensor. Actuator B Chem. 2008, 130, 25–31. [CrossRef] Macak, J.M.; Tsuchiya, H.; Taveira, L.; Ghicov, A.; Schmuki, P. Self-organized nanotubular oxide layers on Ti–6Al–7Nb and Ti–6Al–4V formed by anodization in NH4 F solutions. J. Biomed. Mater. Res. 2005, 75, 928–933. [CrossRef]

Coatings 2018, 8, 459

10.

11.

12. 13. 14.

15. 16. 17. 18. 19.

20. 21. 22. 23. 24. 25.

26.

27.

28. 29. 30. 31.

21 of 22

Kim, H.M.; Kaneko, H.; Masakazu, K.; Kokubo, T.; Nakamura, T. Mechanism of apatite formation on anodically oxidized titanium metal in simulated body fluid. Key Eng. Mater. 2004, 254–265, 741–744. [CrossRef] Zwilling, V.; Darque-Ceretti, E.; Boutry-Forveille, A.; David, D.; Perrin, M.Y.; Aucouturier, M. Structure and physicochemistry of anodic oxide films on titanium and TA6V alloy. Surf. Interface Anal. 1999, 27, 629–637. [CrossRef] Gong, D.; Grimes, C.A.; Varghese, O.K.; Hu, W.; Singh, R.S.; Chen, Z.; Dickey, E.C. Titanium oxide nanotube arrays prepared by anodic oxidation. J. Mater. Res. 2001, 16, 3331–3334. [CrossRef] Macak, J.M.; Tsuchiya, H.; Taveira, L.; Aldabergerova, S.; Schmuki, P. Smooth anodic TiO2 nanotubes. Angew. Chem. Int. Ed. 2005, 44, 7463–7465. [CrossRef] Macak, J.M.; Taveira, L.V.; Tsuchiya, H.; Sirotna, K.; Macak, J.; Schmuki, P. Influence of different fluoride containing electrolytes on the formation of self-organized titania nanotubes by Ti anodization. J. Electroceram. 2006, 16, 29–34. [CrossRef] Ghicov, A.; Macak, J.M.; Tsuchiya, H.; Kunze, J.; Haeublein, V.; Frey, L.; Schmuki, P. Ion implantation and annealing for an efficient N-doping of TiO2 nanotubes. Nano Lett. 2006, 6, 1080–1082. [CrossRef] Yu, W.; Qiu, J.; Xu, L.; Zhang, F. Corrosion behaviors of TiO2 nanotube layers on titanium in Hank’s solution. Biomed. Mater. 2009, 4, 065012. [CrossRef] Kunze, J.; Müller, L.; Macak, J.M.; Greil, P.; Schmuki, P.; Müller, F.A. Time-dependent growth of biomimetic apatite on anodic TiO2 nanotubes. Electrochim. Acta 2008, 53, 6995–7003. [CrossRef] Zhang, W.; Li, G.; Li, Y.; Yu, Z.; Xi, Z. Fabrication of TiO2 nanotube arrays on biologic titanium alloy and properties. Trans. Nonferr. Met. Soc. Chin. 2007, 17, 692–695. Petukhov, D.I.; Eliseeva, A.A.; Kolesnik, I.V.; Napolskii, K.S.; Lukashin, A.V.; Tretyakov, Y.D.; Grigoriev, S.V.; Grigorieva, N.A.; Eckerlebe, H. Formation mechanism and packing options in tubular anodic titania films. Microporous Microporous Mater. 2008, 114, 440–447. [CrossRef] Yang, B.; Uchida, M.; Kim, H.-M.; Zhang, X.; Kokubo, T. Preparation of bioactive titanium metal via anodic oxidation treatment. Biomaterials 2004, 25, 1003–1010. [CrossRef] Boehlert, C.J.; Cowen, C.J.; Quast, J.P.; Akahori, T.; Niinomi, M. Fatigue and wear evaluation of Ti–Al–Nb alloys for biomedical applications. Mater. Sci. Eng. C 2008, 28, 323–330. [CrossRef] Oliver, W.C.; Pharr, G.M. An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. J. Mater. Res. 1992, 7, 1564–1583. [CrossRef] Gupta, S.M.; Tripathi, M. A review of TiO2 nanoparticles. Chin. Sci. Bull. 2011, 56, 1639. [CrossRef] Chang, W.-Y.; Fang, T.-H.; Chiu, Z.-W.; Hsiao, Y.-J.; Ji, L.-W. Nanomechanical properties of array TiO2 nanotubes. Microporous Microporous Mater. 2011, 145, 87–92. [CrossRef] Prida, V.M.; Manova, E.; Vega, V.; Hernandez-Velez, M.; Aranda, P.; Pirota, K.R.; Vázquez, M.; Ruiz-Hitzky, E. Temperature influence on the anodic growth of self-aligned Titanium dioxide nanotube arrays. J. Magn. Magn. Mater. 2007, 316, 110–113. [CrossRef] Naldoni, A.; Allieta, M.; Santangelo, S.; Marelli, M.; Fabbri, F.; Cappelli, S.; Bianchi, C.L.; Psaro, R.; Santo, V.D. Effect of nature and location of defects on bandgap narrowing in black TiO2 nanoparticles. J. Am. Chem. Soc. 2012, 134, 7600–7603. [CrossRef] Sarraf, M.; Zalnezhad, E.; Bushroa, A.R.; Hamouda, A.M.S.; Baradaran, S.; Nasiri-Tabrizi, B.; Rafieerad, A.R. Structural and mechanical characterization of Al/Al2 O3 nanotube thin film on TiV alloy. Appl. Surf. Sci. 2014, 321, 511–519. [CrossRef] Saharudin, K.A.; Sreekantan, S.; Aziz, S.N.; Hazan, R.; Lai, C.W.; Mydin, R.B.; Mat, I. Surface modification and bioactivity of anodic Ti6Al4V alloy. J. Nanosci. Nanotechnol. 2013, 13, 1696–1705. [CrossRef] Tian, T.; Xiao, X.; Liu, R.; She, H.; Hu, X. Study on titania nanotube arrays prepared by titanium anodization in NH4 F/H2 SO4 solution. J. Mater. Sci. 2007, 42, 5539–5543. [CrossRef] Kummer, K.M.; Taylor, E.; Webster, T.J. Biological applications of anodized TiO2 nanostructures: A review from orthopedic to stent applications. Nanosci. Nanotechnol. Lett. 2012, 4, 483–493. [CrossRef] Baradaran, S.; Basirun, W.J.; Zalnezhad, E.; Hamdi, M.; Sarhan, A.A.; Alias, Y. Fabrication and deformation behaviour of multilayer Al2 O3 /Ti/TiO2 nanotube arrays. J. Mech. Behav. Biomed. Mater. 2013, 20, 272–282. [CrossRef]

Coatings 2018, 8, 459

32.

33. 34.

22 of 22

Zalnezhad, E.; Baradaran, S.; Bushroa, A.R.; Sarhan, A.A.D. Mechanical property enhancement of Ti-6Al-4V by multilayer thin solid film Ti/TiO2 nanotubular array coating for biomedical application. Metall. Mater. Trans. A 2014, 45, 785–797. [CrossRef] Yaghoubi, H.; Taghavinia, N.; Alamdari, E.K.; Volinsky, A.A. Nanomechanical properties of TiO2 granular thin films. ACS Appl. Mater. Interfaces 2010, 2, 2629–2636. [CrossRef] Sarraf, M.; Zalnezhad, E.; Bushroa, A.R.; Hamouda, A.M.S.; Rafieerad, A.R.; Nasiri-Tabrizi, B. Effect of microstructural evolution on wettability and tribological behavior of TiO2 nanotubular arrays coated on Ti–6Al–4V. Ceram. Int. 2015, 41, 7952–7962. [CrossRef] © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).