High temperature oxidation behaviors of Al/Cr composite coating on Ti2AlNb alloy

The Al/Cr coatings were deposited on Ti2AlNb alloy by magnetron sputtering and double glow plasma alloying technology to improve the oxidation resistance. The morphology of the protective coating was investigated by scanning electron microcopy and the phase composition of the coating was analyzed using x-ray diffraction. The results suggested that the protective coating with multilayer structure was dense and homogeneous. The formation of Cr inner diffusion layer was beneficial to the bonding strength between the coating and substrate, which was attributable to the double glow technology. The oxidation behaviors of Al/Cr coating were investigated by the isothermal oxidation tests for 100 h at 700 °C, 800 °C and 900 °C, respectively. The results indicated that the protective coating showed a lower oxidation rate. At 700 °C, the Al2O3 oxidation products were formed on the surface of coating at high temperature, which were dense and homogeneous against oxygen diffusion. With the increase of oxidation temperature, the external diffusion of Cr element reacted with oxygen to form Cr2O3, which provided the further protection. As for 900 °C, the volatile CrO3 was formed by the reaction of the external diffusion of Cr and Cr2O3, and the oxidation products Al2O3 and Cr2O3 continued to provide protection for Ti2AlNb alloy.


Introduction
Ti 2 AlNb based alloys have attracted widespread attention for application in aircraft engines and automotive components [1] because of the excellent properties of low density, creep resistance and fracture toughness [2][3][4].And Ti 2 AlNb alloys are popular for demanding applications [5] due to the excellent strength-weight ratios of it.However, although Ti 2 AlNb alloys possess excellent mechanical properties, it can not meet the advancedaeroengines requirements due to the poor oxidation resistance in high temperature environment.J J Dai et al [6] reported that operating temperature above 873 K, the conventional titanium alloys were oxidized and brittle oxides (TiO 2 , AlNbO 4 and so on) with porous structure were formed on the alloy surface, which was unfavorable for the application of alloys.The oxygen-containing and nitrogen-containing products are advantageous to the subsurface brittleness [7,8].Thus, searching a method to enhance the oxidation resistance of Ti 2 AlNb alloy, which is significant to the application of Ti 2 AlNb alloy at high temperature.
Currently, many pieces of research were carried out to the oxidation behaviors of Ti 2 AlNb alloy.M M Wang et al [9][10][11] reported that the element of Al can enhance the oxidation resistance of Ni-based superalloy, which attribute to the formation of Al 2 O 3 phase with extremely dense.Y S He et al [12] reported that Nb could improve oxidation resistance because of the controlled of mass transfer of TiO 2 .However, the with the content of Nb increasing, the oxidation resistance was depressed due to the other products formed such as AlNbO 4 and TiNb 2 O 7 phase.B Zhou et al [13] fabricated Cr-coating on Ti-45Al-8.5Nballoy to improve the oxidation resistance, the results suggested that Cr is an effective antioxidant, when the content of Cr is in the range of 8%-10%, the diffusion rate of oxygen is controlled and the oxidation is decreased.
However, the bonding strength between substrate and coating is a challenge, poor bonding strength has a serious influence on the application of Ti 2 AlNb alloy.To obtain the stable application, many efforts are been made though different preparation methods [14].Lots of methods have been used to deposition protective coating on Ti 2 AlNb alloy, including laser melting, plasma spraying, and magnetron sputtering etc [7,15,16].Magnetron sputtering technique is effective method to fabricate various kinds of coatings, which employ ions with high ionization of argon gas to strike target and beneficial elements was deposited on the substrate surface [17].The interface of coating and substrate is obvious due to the lower processing temperature.Furthermore, a new method called double glow technique arouses significant attention.Double glow technique has been used to fabricate Nb, Cr, Cu and Mo etc coatings successfully [18,19].Coating fabricated by double glow technique has excellent bonding strength, which attributes to the formation of diffusion layer [20].
In the present study, the composite coating of Al/Cr layer was fabricated via magnetron sputtering and double glow technique together, which was expected to obtain a strong bonding strength and an excellent oxidation resistance coating on the surface of Ti 2 AlNb alloy.The study aimed to investigate the oxidation behaviors of the protective coating at high temperature environment.The phase evolution and the oxidation resistance mechanism of Al/Cr composite coating was also analyzed.

Materials and experimental procedure 2.1. Raw materials
In this study, Ti-22Al-25Nb alloys with the shape of 15 mm × 15 mm × 4 mm were used as the substate, the chemical composition (wt.%) of Ti 2 AlNb alloy was 10.82 Al, 42.12 Nb, 0.034 O, 0.0054 N and Ti rest.The substrates were polished and cleaned in alcohol by ultrasound equipment before the preparation process.

Preparation of the coating
The Al/Cr composite coating was fabricated via magnetron sputtering and double glow technique.First, Cr layer deposited on surface of Ti 2 AlNb alloy by double glow technique.The target was pure Cr (99.99 wt%) with diameter of 90 mm.The voltage of sources electrode and the cathode was 920 V and substrate 400 V, respectively.The distance of two electrodes was 20 mm, and the treating pressure was 40 Pa.The temperature was 850 °C-900 °C for 3.5 h.Second, the Al layer deposited on Cr-sample by magnetron sputtering.The target was pure Al (99.99 wt%) with diameter of 90 mm, and the deposition power was 180 W. The distance between Cr-sample and target was 25 mm and the pressure was 4-6 Pa.The treating temperature was 200 °C-300 °C for 3 h.Finally, vacuum annealing applicated for 10 h at 600 °C.

Characterization of the coating and oxidation test
The surface and cross-section microstructures were observed by scan electron microscopy (FEI, Quanta450, USA), and the crystal phases were detected by a x-ray diffraction (Rigaku, DMAX-RB12KW, Japan) with copper Kα radiation (λ = 1.5418Å) over the range from 20°to 90°.
To evaluation the oxidation behaviors of the protective coating, the samples with and without coating were detected by isothermal oxidation test in the muffle furnace at 700 °C, 800 °C and 900 °C for 100 h, respectively.The samples were weighted by electronic scales (JingKe, FA1044, China) every 10 h and then put it back to the muffle furnace to continue the remaining oxidation process.

Microstructure of Al/Cr composite coating
Figure 1 shows the characters of Al/Cr composite coating on the surface of substrate.The surface of Cr layer and Al layer is given in figures 1(a) and (b), respectively.The results indicated that the deposition of Cr and Al are dense without cracks or holes.Cr layer suggested polycrystalline structure and the surface was rough, which attributed to the ion bombardment with high energy during the double glow treatment.However, the Al layer suggested a layer growth mechanism.The cross-section morphology and XRD (figures 1(c) and (d)) indicated that the protective coating had a multilayer structure.The outermost layer was Al (about 10 μm), and the Al-Cr alloy layer (about 22 μm) formed under the Al layer due to the mutual diffusion of Al and Cr during the heat treatment.The thickness of Cr deposited layer was about 8 μm.The innermost layer was Cr diffusion layer (about 19 μm), which was mainly attributable to the feature of double glow process, the Cr atoms sputtered from the target to the substrate and easily diffused into the surface of substrate due to the activated surface by glow discharge during the double glow process at high treating temperature.The bonding strength between the coating and substrate was excellent because of the interdiffusion layer of Al/Cr and the Cr diffusion layer, which prevented the coating from breaking off during the application.

Oxidation behaviors of Al/Cr Coating
To reveal the effect of the oxidation temperature on the oxidation behaviors of protective coating, the Ti 2 AlNb alloy with and without Al/Cr coating were subjected the isothermal oxidation for 100 h at 700 °C, 800 °C and 900 °C, respectively.
Figure 2 shows the mass gain curve of the samples after isothermal oxidation tests.In order to ensure the accuracy of experimental data, five samples were tested at each temperature and the average of five weighting results was taken for consideration.The results suggests that the protective coating showed a lower oxidation rate than substrate at the same conditions.Figure 2(a) shows the mass gain of substrate, which indicates that the weight continuously increased with the oxidation duration in a linear manner.However, the mass gain of Al/Cr coating was such that the weight curve was exponentially increasing one, which fitted the Wagner's theory of oxidation [21].Based on Wagner's theory, the mass gain due to the oxidation can be described by the follow equation [22]: Where A represents the oxidation rate, t represents oxidation time, n represents the oxidation index, ∆W represents the mass gain.The fitting results for the protective coating and substrate are listed in table 1.The slopes of substrate suggested that the slow growth with the increase from 0.023 to 0.260 and the constant dramatically increased from 0.457 to 2.251 mg cm −2 , which indicated that the substrate surface was oxidized and formed the oxidation products at the beginning of oxidation process.As for the protective coating, the oxidation index at the range from 1.16 to 1.93 suggested that the oxidation spread was not only attributed to the oxygen diffusion but also the chemical reaction at the surface area.The results were combined with mass gain curves suggested that the maximum weight gain at 700 °C (1.89 mg cm −2 ) was one-quarter of that at 900 °C (8.43 mg/cm 2 ), So the Al/Cr coating provided ranking protectiveness at 700 °C. Figure 3 shows the XRD patterns of Al/Cr coatings after isothermal oxidation treatment for 100 h at 700 °C, 800 °C and 900 °C, respectively.The results suggested that Al 2 O 3 , Cr 2 O 3 and AlCr 2 phases were detected after the oxidation at 700 °C.The Al 2 O 3 phase could reduce the diffusion of oxygen and protected the sample from further oxidation because of the ceramic nature and super density of it.The small amount of oxygen diffused into sample and reacted with Cr to form Cr 2 O 3 phase, which provide a further protection against oxidation.With the oxidation temperature was increased to 800 °C, no new oxidation products were formed, however, the content of Cr 2 O 3 phase was elevated.The oxidation products Cr 2 Ti, Al 5 Ti 2 and Cr 2 Nb were detected in the coating because the external diffusion of Ti and Nb occurred at high oxidation temperature.
The surface morphologies of Al/Cr protective coating subjected oxidation treatment at 700 °C, 800 °C and 900 °C are shown in figure 4. The results suggested that the protective coating oxidated at 700 °C exhibited a mountain-like hillocks and pits structure on the coating and many cracks appeared on the surface of sample,    Al 2 O 3 film without cracks, which can effectively protect the sample against oxidation.Many bubbles appeared on the surface of sample after 900 °C oxidation treatment and some of it were broken, which suggested that the external diffusion of Cr reacted with oxygen to form Cr 2 O 3 under the Al 2 O 3 film, then some of Cr 2 O 3 reacted with oxygen again to form volatile CrO 3 .When the pressure exceeded a certain value, the bubble was broken and CrO 3 was released.The EDS results of area I, II and III were listed in table 2. The results indicated that the main protective mechanism was the dense Al 2 O 3 film, which isolated the inner parts from the high temperature  environment.The external diffusion of Cr occurred during the oxidation process, and the Cr content (2.91 At.% at 700 °C) was increased to 4.72 At.% when the temperature raised to 800 °C.However, the content of Cr decreased to 3.82 At.% at 900 °C due to the volatile CrO 3 released.Thus, the results analyzed with XRD patterns (figure 3) suggested that the continuous dense layer of oxide scales (Al 2 O 3 and Cr 2 O 3 ) were formed to control the oxygen diffusion during the oxidation treatment, which can improve the oxidation resistance of substrate effectively.
The cross-section morphologies and corresponding EDS results of Al/Cr protective coating oxidized at 700 °C, 800 °C and 900 °C are shown in figure 5.The results suggested that the Cr diffusion film can be found around substrate, which can be attributed to the feature of double glow technique.This diffusion film can ensure the strong bonding strength between protective coating and substrate, which can prevent the separation of the protective coating from the substrate at high temperature.At 700 °C, the oxidation layer was formed at the top layer of protective coating, and a Al rich layer was below the oxide layer.The main phase of the oxidation layer was Al 2 O 3 , which was dense to prevent the oxidation.Cr element diffused outwards during the oxidation process, a small amount of Cr was oxidized to Cr 2 O 3 which prevented the further oxidation.The spallation was appeared at several points in the cross-section, which attributed to the different thermal expansion coefficients between oxides and protective coating.The results were analyzed with figure 4 suggested that the oxidation products on the surface of the coating peeling off after oxidation at 800 °C, which attributed to the shrinkage stress of the phase transition and the non-uniform distribution of Al 2 O 3 products.The EDS results showed that there was an obvious Cr peak, which mainly attributed to the consumption of the Al element.The internal Al element diffused outwards, which reacted with the diffused oxygen atoms to Al 2 O 3 and the consumption of Al in the Cr-rich region caused the peak of Cr content to increase.The newly formed dense oxidation product prevented the further oxidation.At 900 °C, the content of Al in Cr rich layer decreased due to the external diffusion of Al.The peak type of Cr was sharper and the peak value was higher than that at 800 °C.The external diffusion behavior of Al element provided a strong guarantee for the oxidation resistance of the coating.
Above all, although the outermost oxide layer would be peeling off, the outwardly diffused Al element would continue to be oxidized to form the oxide film to control the oxidation behaviors of coating during the oxidation process.In addition, the external diffusion of Cr could react with oxygen atoms, which prevented the further oxidation.

Conclusion
In this paper, the Al/Cr coatings were fabricated on Ti2AlNb alloy via magnetron sputtering and double glow plasma alloying technique, which was expected to enhance oxidation resistance of substrate.This resulted in coatings that had a multilayer structure, which consisted of 10 μm outermost Al deposit layer, 22 μm Al/Cr layer, 8 μm Cr deposit layer and 19 μm Cr diffusion layer.
(1) The bonding strength between the protective coating and substrate was doing well due to the formation of Cr diffusion layer, which was the feature double glow plasma alloying treatment.
(2) The Al-Cr coating formed on Ti2AlNb alloy exhibited oxidation resistance.The continuous Al 2 O 3 oxide layer was formed on the top of the coating to against oxygen and the Cr 2 O 3 was for formed under the Al 2 O 3 products to provide the further protection at 700 °C and 800 °C.However, the continuity oxide layer occurred peeling off, the subsequent Al continued to be consumed to provide protection.
(3) The oxidation of Cr present at 900 °C suggested that the external diffusion of Cr reacted with oxygen to form Cr 2 O 3 under the Al 2 O 3 film then some of Cr 2 O 3 reacted with oxygen again to form volatile CrO 3 .The CrO 3 was released when the pressure exceeded a certain value.

Table 1 .
Oxidation models fitted to mass gain date of the substrates and Al/Cr coatings1.93= 2.00 × 10 -2 t 10-100 800 (ΔW) 1.60 = 7.73 × 10 -2 t 10-100 900 (ΔW) 1.16 = 1.25 × 10 -1 t 10-100 which attributed to the phase transition of Al 2 O 3 occurred and the shrinkage stress of the volume change led to the cracks and wrinkling during the oxidation treatment [10], another reason caused the pits structure and cracks on the surface of sample is the non-uniform distribution of Al 2 O 3 products because of the mutual diffusion of Al and Cr elements.With the oxidation temperature was increased, the local fluctuation of the surface was reduced due to the recrystallization of Al 2 O 3 , which could limit the number of oxygen diffusion channels effectively.In the case of sample oxidized at 900 °C, the surface was covered a continuous dense layer of

Figure 3 .
Figure 3. XRD patterns of Al/Cr coatings after isothermal oxidation tests.

Table 2 .
EDS results of Area I, II and III of Al/Cr coatings after oxidation treatment.