Preparation of a Modified Micro-arc Oxidation Coating Using Al2O3 Particles on Ti6Al4V

A micro-arc oxidation coating on Ti6Al4V alloy was modified by addition of micro-Al2O3 particles to a sodium phosphate solution. The coating structure and phase were characterized by scanning electron microscopy and X-ray diffraction, and the oxidation resistance and thermal shock properties of the coating were investigated. Results showed that a coating denser than the original coating was produced. This new coating was composed of Al2TiO5 and TiO2. The oxidation resistance and thermal shock property of the coating improved with addition of Al2O3 particles to the electrolyte relative to the sample prepared without the particles in the electrolyte. Moving Al2O3 particles were adsorbed on the coating surface and penetrated through it. As a result, the phase structure and properties of the original coating were modified. *Corresponding author: Hong Li, Guangdong Institute of New Materials, National Engineering Laboratory for Modern Materials Surface Engineering Technology, The Key Lab of Guangdong for Modern Surface Engineering Technology, Guangzhou 510650, PR China, Tel: 86-20 37238071; E-mail: leehongaaron@163.com Received October 18, 2017; Accepted November 02, 2017; Published November 12, 2017 Citation: Li H, Zhang J (2017) Preparation of a Modified Micro-arc Oxidation Coating Using Al2O3 Particles on Ti6Al4V. J Material Sci Eng 6: 400. doi: 10.4172/2169-0022.1000400 Copyright: © 2017 Li H, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.


Introduction
Titanium alloys are widely used in the aerospace and biomedical industries for their low density, high relative strength, and good corrosion resistance. However, these alloys exhibit poor tribological behavior and anti-oxidation resistance at high temperatures. Microarc oxidation (MAO) is a new surface modification technique widely investigated in recent years and employed to produce different kinds of coatings on Ti alloys with improved properties [1][2][3][4][5][6][7][8]. Basically, the composition of the MAO coating depends on the substrate (Mg, Al, or Ti alloy) and the electrolyte. For example, TiO 2 , Al 2 O 3 , and SiO 2 coatings are generally prepared in phosphate [9], aluminate [10], and silicate [9] solutions, respectively. Other composite coatings in mixing solutions [11] are used on titanium alloys. The porous structure of the MAO coating is generated from the micro-arc property of the approach [12]. The in situ growth behavior of the MAO coating underlies its good adhesion to the substrate [13]. Favorable thermal resistance of the MAO coating has also been reported [14,15]. The MAO coating prepared in Na 2 SiO 3 -Na 2 CO 3 -NaOH on Ti6Al4V exhibited good anti-oxidation properties at 500°C for 200 h [14]. The coating prepared in sodium phosphate with Co(CH 3 COO) 2 addition on Ti6Al4V exhibited thermal shock resistance at 500°C for 40 cycles [15]. Also, the functional coatings prepared on Ti alloys using MAO technology attracted recent attention. The black coating, first produced in the 1980s by the original anode oxidation technique or the cathode plasma oxidation method [16,17], is obtained in mixed electrolyte [18]. Overall, the coating properties are mostly dependent on the electrolyte employed. In this paper, a MAO coating much denser than usual was prepared on Ti6Al4V alloy by addition of Al 2 O 3 particles to sodium phosphate solution. The coating's effects on anti-oxidation at 700°C and thermal shock at 850°C were studied.

Coating preparation
The substrate material used in this investigation was Ti6Al4V titanium alloy, with a chemical composition (wt.%) of 6.3 Al, 4.2 V, 0.15 O, 0.11 Fe, 0.03 C, 0.02 N, 0.001 H, and Ti balance. Specimens measuring 20 mm × 15 mm × 2 mm were ground using 60 # , 120 # , 400 # , 600 # , 1000 # , and 1500 # grit silicon carbide papers. The specimens were then cleaned using distilled water and acetone and subsequently air dried. For MAO treatment, a pulse power supply was employed, and the Ti6Al4V plate was used as the anode electrode. A graphite plate was used as the cathode in the electrolytic cell. The electrolyte was composed of an aqueous solution containing 0.3 mol/L sodium phosphate solution and 6 g/L Al 2 O 3 particles dispersed by a magnetic stirrer. During MAO treatment, the temperature of the electrolyte was maintained below 45°C. The applied parameters are shown in Table  1. MAO coatings prepared in sodium phosphate solution without Al 2 O 3 particles were used as a basis for comparison. After treatment, the obtained samples were washed with distilled water and dried at room temperature. The samples were then designated as TMPAl (Al 2 O 3 particles within electrolyte) and TMP (without Al 2 O 3 particles in the electrolyte).

Characterization of the coating
The Al 2 O 3 particle sizes were measured by a laser diffraction particle size analyzer (LMS-30). The current was noted and the growth kinetics curves were plotted. The roughness of the samples was measured by a roughness gauge (TR200, Time Group Inc., China). The thicknesses of the coatings were measured by an eddy-current coating thickness gauge at three different points, and measurements were performed thrice

Coating properties
A thermal high-temperature cyclic oxidation test was performed in a kryptol heater furnace at 700°C in air for 100 h. A ceramic crucible was used to hold the sample and pre-heated to a constant weight. The Ti6Al4V substrate and the coating sample, both previously weighed, were oxidized at 700°C for 10 h. All of the specimens were then brought out, cooled naturally in air to room temperature, and then weighed again. Afterward, the samples were returned to the furnace for the next cycle. The test was performed for 10 thermal cycles, and the kinetic curve of each weight variant per unit area versus time was plotted.
A thermal shock test was conducted to evaluate the thermal shock property of the coating and the bond strength between the coating and substrate. The coated sample was placed inside a furnace at 850°C for 10 min. The specimen was then immediately removed, immersed in cool water, and returned to the furnace for the subsequent cycle. The thermal shock test was repeated for 30 cycles. The surface morphology and thermal-control property of the coatings were observed.

Result and Discussion
The particle size distribution of Al 2 O 3

Current-time response
The current-time response of the TMP and TMPAl coatings are shown in Figure 2; three main regions [19] can be identified in both curves. The current-time curves initially declined linearly, gradually dipped, and then stabilized. By comparing the two curves, we noted that the current in the coat-forming process of TMPAl is lower than that of TMP and stabilizes at a very low value. Moving Al 2 O 3 particles were adsorbed onto the coating surface and penetrated through it. The particles were then surrounded by the melting ceramic because of the high temperature (4000 K) caused by micro-arc formation [20]. The Al 2 O 3 particles melted and recrystallized together, thereby increasing in volume. Many micro-arc pores were filled, and a denser coating was formed. A much lower current was also observed. Figure 3a shows the weight gains and thicknesses of the coatings prepared in different electrolytes. The weight of TMPAl increased by about three times that of TMP because of addition of Al 2 O 3 particles to the electrolyte. The absorbed Al 2 O 3 particles are responsible for the weight gain. However, not much difference in thickness was observed between the coatings. The Al 2 O 3 particles absorbed in the coating improved its compactness, thereby leaving the coating thickness unaffected. The weight gain and thickness data indicate that TMPAl is more compact than TMP. Figure 3b displays the hardness and roughness of the coatings. The hardness only slightly changes. As the hardness of a ceramic is usually affected by crystal texture, grain orientation, and grain size, no serious difference was noted between TMP and TMPAl, and both of the coatings exhibited similar crystal textures and grain sizes. However, the roughness of the TMPAl coating decreases from 4.5 µm to 3.5 µm with respect to that of TMP. Thus, coating roughness can be decreased by Al 2 O 3 particle addition to the electrolyte. Lower current of TMP at the stable period causes the sparse and small micro-arc reaction that leads to the gentle structure of the corresponding coating. Hence, the roughness of TMP slightly decreased.

Microcosmic surface morphologies of the coatings
The microscopic surface morphologies of the coatings are depicted in Figure 5. Many micron-sized pores [21,22] generated by the microarc reaction are observed on the surface of TMP, whereas a few pores exist on the TMPAl surface. The micro-arc occurs at applied voltages above the breakdown voltage of the gas layer enshrouding the substrate.  Point B, which appears to correspond to an added Al 2 O 3 particle, appears to be composed of O, Al, Ti, and some P. Compared with that in point A, the content of Ti in point B is apparently low. By contrast, the relative amounts of O, Al and Ti also resembles the composition of Al 2 O 3 and TiO 2 . And, the main composition is Al 2 O 3 , so the point B can be concluded to the Al 2 O 3 particle. Figure 7 reveals the section morphologies of the coatings. The existence of both porous and dense layers considered to be typical of The generation of a micro-arc depends on the applied voltage and the gas layer formed by the anodic oxidation reaction. The moving Al 2 O 3 particles constantly affect the gas layer; hence, these particles do not produce sufficient plasma to create a micro-arc. Pores were fewer and smaller on the surface of the TMPAl coating than on the TMP coating. Meanwhile, the coating density and conductivity affects the process of anodic oxidation. The density of TMPAl was better than that of TMP, judging from the difference in weight gain and thickness (Figure 2). Throughout the experiment, the current of TMPAl was lower than that of TMP. Hence, less micro-arc was generated on the surface of TMPAl leading to fewer pores than that of TMP.

Section morphologies of the coatings
Two key points on the surface of the TMPAl coating were marked    the MAO coating structure [24] was noted in TMP, whereas a uniform layer was observed in TMPAl.
In MAO processes, temperatures generated by the microarc reaction can reach 4000 K [20]. When this event occurs, the surrounding material (mainly a TiO 2 ceramic) is melted and sputtered. As the micro-arc disappears and the electrolyte cools, the melted material begins to solidify to form volcano-like pores. The micro-arc reaction is also produced inside the coating because of the high voltage, thereby producing internal pores. With the increase in reaction time, the quantity and intensity of the micro-arc on the surface decreases. The oxygen penetrates the coating through a discharge channel to reach the surface of the Ti substrate and form titanium dioxide, which constitutes the dense layer.
The moving Al 2 O 3 particle is adsorbed on the sample surface and penetrates into the melting material [25]. The melting points of titanium dioxide and aluminum oxide both approach 2100 K; thus, penetrating Al 2 O 3 fuses and solidifies with the melting TiO 2 . Two theories may explain the dense-layer formation with Al 2 O 3 addition. The first conjecture proposes that the volume increases with the particle addition and aids help in filling the pores. The other hypothesis suggests that the decrease in current with Al 2 O 3 addition to electrolyte under the same parameters reduces the quantity and intensity of the micro-arc. As free pores form in the entire process, less pores form inside the coating, and less micro-arc discharge is generated internally.
The elemental distribution across the TMPAl coating cross section is depicted in Figure 8. Similar to the EDS results in Table 2, a small amount of P was found in the cross section. The elemental composition of the TMPAl coating on the surface to a thickness of 10 μm constitutes Ti, O, and Al in stable values. From 10 μm to nearly 20 μm deeper into the coating, Ti content gradually increases to the Ti content in the Ti6Al4V substrate. By contrast, the Al and O contents gradually decrease to the contents of Al and O in Ti6Al4V, respectively. Figure 9 shows the XRD patterns of TMP, TMPAl, and their oxidized counterparts. The main phase of the coating prepared in sodium phosphate solution without Al 2 O 3 particles correspond to metastable anatase TiO 2 at low temperature, thermodynamically stable rutile TiO 2 at almost all temperatures, and Ti (Figure 9d). These results are similar to that previously reported [26]. During the MAO process, the anode Ti6Al4V is oxidized to form anatase TiO 2 , part of which transforms to rutile TiO 2 under the high temperatures produced by the micro-arc discharge. Furthermore, the discharge channel promotes the migration of Ti in the substrate to the coating. With increasing coating thickness, the previously formed coating becomes calcined and sputtered, allowing the transfer of some of the Ti in the substrate to the coating. After oxidation at 700°C for 100 h, Ti oxidizes to titanium dioxide ( Figure 9c). As the transformation temperature of anatase TiO 2 to rutile TiO 2 is reached at 600°C, much more rutile TiO 2 is found.

Coating phases
The main phase of the coating prepared in sodium phosphate solution with Al 2 O 3 particles constitute Al 2 TiO 5 , Ti, and some anatase TiO 2 (Figure 9b). The reported temperature of the following reaction [27] was 1553 K. In the formation process, the Al 2 O 3 particle is absorbed into the melting TiO 2 to form Al 2 TiO 5 . The presence of Ti is caused by the permeation in high-voltage conditions and sputtering by the micro-arc. Similar to that in TMP, the Ti in the TMPAl coating becomes oxidized into anatase TiO 2 and rutile TiO 2 (Figure 9a). Moreover, Al 2 TiO 5 is partially decomposed to TiO 2 and Al2O 3 as shown in the equation below: TiO 2 + Al2O 3 → Al 2 TiO 5 (1)      oxidation at 700°C for 100 h. After oxidation, the color of Ti6Al4V drastically changes; the brown oxide coating is generated and obtains massive spalling with time. The TMP transforms from light grey to light yellow, whereas the TMPAl changed only slightly. No spalling pieces were found on both TMP and TMPAl after oxidation. Notably, the oxidation kinetics curve of Ti6Al4V follows parabolic kinetics before 30 h because the oxide coating does not spall from the substrate. Then, the curve assumes linear kinetics, as the oxide coating begins to spall gradually. TMP and TMPAl showed good surface morphologies; no piece spalled from the substrate. The weight gains of TMP and TMPAl are 0.78 mg/cm 2 and 0.60 mg/cm 2 , respectively, illustrating that the coating samples of TMP and TMPAl can both provide good oxidation resistance for Ti6Al4V at 700°C for 100 h. The weight gains of TMP and TMPAl mainly caused by the oxidation of Ti were observed in the coatings. The content of Ti in TMP is substantially greater than that in TMPAl as shown by the contrast in XRD patterns (Figure 7b and 7d). Hence, the weight gain of TMP is much higher than that of TMPAl. The good oxidation resistance properties of TMP and TMPAl are attributed to the phase and coating structure. The main phases of TMP and TMPAl comprise TiO 2 , Al 2 O 3 , and Al 2 TiO 5 , having low oxygen diffusion coefficients. The structure of TMP contains both porous and dense layers. Although many pores exist in the porous layer, these pores are insufficient, and thus, no channels are available for oxygen to reach the substrate. By contrast, TMPAl possess fewer pores, and consequently, oxygen is much more difficult to pass through the TMPAl coating.

High-temperature oxidation of samples
According to the Wagner's theory of oxidation, oxidation weight gain and oxidation time can be expressed by the following relationship: ΔW n =k p t (2) where n is the reaction index, k p is the reaction constant (function of temperature), ΔW is the weight gain, and t is the oxidation time. The method of linear regression was adopted to fit the curve and calculate the reaction index and reaction constant. The fitting equation is show in Table 3.
From the value of n in Table 3, continuous oxidation is used to describe the oxidation of the Ti6Al4V substrate as the oxidation film spalls off with time. Linear oxidation occurs on the TMP coating, whereas parabolic oxidation takes place on the TMPAl coating. Hence, the TMPAl coating can provide better anti-oxidation protection for Ti6Al4V than TMP in the long term.

Thermal shock of coatings
The surface macro-photograph of the coating samples is exhibited in Figure 11 after 30 times of thermal shock testing at 850°C. A large area of TMP peeled off, whereas some scattered oxidation points are displayed on the surface of TMPAl. These points indicate that the thermal shock resistance of the MAO coating prepared in sodium phosphate solution can be improved by the addition of appropriate Al 2 O 3 particles in the electrolyte. The thermal shock property is used to describe the anti-thermal shock property of some coatings and the adhesion between the coating and substrate. The coating with high thermal expansion coefficient easily cracks with strong thermal stress. The thermal expansion coefficient disparity between the coating and substrate can cause the peeling from the substrate. The thermal expansion coefficient of TiO 2 and TiAl4V are 9 × 10 −6 and 8.6 × 10 −6 m/K, respectively, whereas that of Al 2 TiO 5 is about 1.5 × 10 −6 m/K [28]. The thermal shock test showed that the coating of TMP spalls more severely than TMPAl, which is attributed to the higher thermal expansion coefficient of TiO 2 than that of AlTiO5. Although a large gap in thermal expansion coefficient exists between Al 2 TiO 5 and Ti6Al4V, the metallurgical bond between the coating and substrate can be protected from spalling. Moreover, during the coating preparation process, the current of TMP is higher than that of TMPAl. Stronger and more numerous micro-arc discharges are also created in the former than in the latter. Greater stress is generated in the process of repeated calcination, which magnifies the thermal stress and causes the cracking and spalling off of the coating.

Conclusion
Addition of Al 2 O 3 particles to a sodium phosphate solution led to the preparation of a slightly porous coating by MAO on Ti6Al4V. The Al 2 O 3 particles added to solution not only decreased the roughness but also improve the compactness of the coating. Al 2 O 3 and Al 2 TiO 5 phases combined in the slightly porous structure to provide good antioxidation effects at 700°C for 100 h. The low coefficient of thermal expansion of Al 2 TiO 5 plays a crucial role in the anti-thermal-shock property of the coating at 850°C for 30 cycles.