High temperature oxidation behaviour of Al 2 O 3 /Al composite coating on γ-TiAl

Specimens

The substrate γ-TiAl (Ti–46·5Al–1Cr–1·5V, wt-%) alloy, which was developed by Central Iron and Steel Research Institute of China, was sliced to 15×15×5 mm by wire electrode cutting, and all sliced specimens were polished to no. 1500 with SiC paper, then cleaned ultrasonically in ethanol and dried finally.

Coating process

An Al₂O₃/Al composite coating was deposited by rf magnetron sputtering in two steps, Al deposition and Al₂O₃ reactive deposition, using the following set-up parameters and then heat treating the coated specimens:

The Al target studied in this work was 99·99% pure aluminium with a diameter of 100 mm.

Scratch test

Adhesion strength of the Al₂O₃/Al composite coatings on γ-TiAl was measured by a scratch tester (WS-2006), equipped with a 200 μm radius Rockwell diamond indenter, which was drawn across the coatings’ surfaces using a linearly increased normal load from 0 to 100 N. The rate of load increase was set at 20 N min⁻¹, and the speed of indenter was 2 mm min⁻¹. The scratch length was 10 mm. An acoustic emission sensor was attached near the diamond indenter to detect the acoustic signals emitted from the coating failure. The minimum load at which coating failure occurs is defined as the critical load and represents an indication of the coating adhesion.

Oxidation test

A vacuum annealing was designed at 650°C for 15 h before the isothermal oxidation test. Three pieces of Al₂O₃/Al coated specimens obtained in this manner as a group underwent tests of isothermal oxidation test at 1000°C for 100 h in a muffle furnace with the substrate γ-TiAl alloy as a comparison. The mass changes due to oxidation processes were counted by analytical balance with the sensitivity of 10⁻⁵ g. The high temperature oxidation test was repeated three times, and the presented test results are averaged.

Evaluation of coating

The phase compositions of the Al₂O₃/Al composite coatings before and after oxidation at different temperatures were identified by XRD (BurkerD8 ADVANCED) using a Cu K_α radiation over a range from 20 to 90°. The morphologies and chemical compositions of coatings and oxide scales were analysed by an SEM (Quanta200, FEI Company) equipped with an energy dispersive spectroscope (EDS) (XMS60S).

Coating characterisation

Figure 1 shows the SEM morphologies and EDS results of Al₂O₃/Al composite coating before the oxidation test. The surface of the as deposited Al₂O₃/Al composite coating is very dense and uniform (Fig. 1a), with image analysis suggesting porosity of <1·4% (the surface porosity of Al₂O₃/Al composite coating was analysed by software Image J and Photoshop with results 1·077 and 1·354% respectively). The EDS analysis (shown in Fig. 1b) indicates that the chemical composition of the surface contained 39·65%Al and 60·35%O (in at-%), which accord with the aluminium oxygen atom ratio of Al₂O₃. The Al₂O₃/Al composite coating grew at a rate about 20–80 nm min⁻¹, up to a thickness of 20 μm, as shown in Fig. 1c. No cracks or other defects were found either in the coating or on the interface, and the interface between the coating and the substrate is clean. In magnetron sputtering, vacant sites that are generated by ion bombardments improve the coating adhesion and lead to the gradient element distribution in the Al₂O₃/Al and Al/γ-TiAl interfaces. ³⁰ It is notable that a diffusion layer was formed because of the interdiffusion between the Al interlayer and the substrate γ-TiAl alloy during the coating deposition. The XRD result of Al₂O₃/Al composite coating shown in Fig. 2 suggests that the phase structure of surface and subsurface is α-Al₂O₃ [Joint Committee on Powder Diffraction Standards (JCPDS) no. 84-1468] and Al (JCPDS no. 04-0787) because of the high penetrability of X-ray. (The theoretical calculated value of X-ray penetrability in the Al₂O₃ film is 106·9 μm. However, this is not a simple question, as there are many factors involved, such as interface in the bilayer structure, crystal imperfections and inhomogeneity of chemical composition and structure. In this study, the thickness of Al₂O₃ layer is ∼5 μm. Therefore, the XRD result should suggest the phase structure of surface and subsurface zone.)

OPEN IN VIEWER

Figure 1

a surface image, b EDS result, c cross-section image and d line scanning result ofAl₂O₃/Al composite coating

OPEN IN VIEWER

Figure 2

X-ray diffraction pattern of Al₂O₃/Al composite coating on γ-TiAl alloy before oxidation test

Microstructure of coating after isothermal oxidation

Figure 3 shows the SEM surface morphology and EDS result of Al₂O₃/Al composite coating after the isothermal oxidation at 1000°C for 100 h. The oxide scale exhibits a cauliflower-like surface appearance. No cracks or spallation can be found in the coating after isothermal oxidation. It is remarkable that a small amount of Ti (4·19 at-%) was detected on the coating surface, indicating the out diffusion of Ti during the oxidation test.

OPEN IN VIEWER

Figure 3

a surface image and b EDS result of Al₂O₃/Al composite coating on γ-TiAl alloy after oxidation test

The cross-section morphologies and EDS line scan results of Al₂O₃/Al composite coatings after 5, 20 and 100 h oxidation test are shown in Figs. 4 and 5 respectively. In these pictures, the evolutions of a layered structure can be observed obviously. The interface between Al and Al₂O₃ layer disappeared after 5 h oxidation, as shown in Fig. 4a. It was found that the Al interlayer transformed to a Ti–Al diffusion zone gradually due to the outward diffusion of Ti during the oxidation test. As the oxidation time increased, the diffusion zone grew quickly, and it reached 30 μm thickness after 100 h oxidation test, as shown in Figs. 4c and 5c. As an active element forming intermetallic with Al, Ti from the substrate had diffused outward and formed Ti–Al intermetallics (see arrows in Fig. 4c). A small amount of Ti, which is in the form of TiAl₂ (JCPDS no. 47-1147) and TiAl₃ (JCPDS no. 49-1446), could be detected in the surface and subsurface zone (Fig. 6) because of the high penetration of X-ray. ³¹ The coating without through wall cracks is still compact, as shown in Fig. 4c, indicating that the Al₂O₃/Al composite coating system exhibited excellent oxidation resistance performance after 100 h air exposure at 1000°C.

OPEN IN VIEWER

Figure 4

Cross-section images of Al₂O₃/Al composite coating on γ-TiAl alloy after a 5 h, b 20 h and c 100 h at 1000°C

OPEN IN VIEWER

Figure 5

Line scanning results of Al₂O₃/Al composite coating on γ-TiAl alloy after a 5 h, b 20 h and c 100 h at 1000°C

OPEN IN VIEWER

Figure 6

X-ray diffraction pattern of Al₂O₃/Al composite coating on γ-TiAl alloy after oxidation test

Oxidation kinetics and mechanisms

Figure 7 shows the isothermal oxidation kinetics and the morphology transformations of the substrate γ-TiAl alloy and Al₂O₃/Al coated specimen at 1000°C. The isothermal oxidation kinetics of the substrate and the coated specimen tested at the same temperature are plotted together for comparison. As the oxidation test results found in the literature, the mass gain of substrate was more than 17 mg cm⁻² after 100 h oxidation, which indicates poor oxidation resistance at 1000°C. One of the reasons that lead to the more mass gain of the substrate γ-TiAl alloy is the new surface oxidation after the oxide scale spallation. The mass gain of Al₂O₃/Al coated specimen is much less than that of substrate γ-TiAl alloy. It is remarkable that the mass gain of Al₂O₃/Al coated specimen had grown slowly after 10 h oxidation exposure. During the 100 h oxidation test, the surface colour of Al₂O₃/Al composite coating had changed from white to grey. However, no spallation and cracks can be observed.

OPEN IN VIEWER

Figure 7

Mass change versus time at oxidation test exposed to air under isothermal oxidation at 1000°C

Figure 8 shows the microstructural evolutions of coating during the oxidation test base on the results of microstructural and compositional analyses using SEM/EDS. Experimental results have revealed that the Al₂O₃/Al composite coating, fabricated by magnetron sputtering, exhibits excellent high temperature oxidation resistance during isothermal oxidation. The mechanisms accounting for this beneficial effect are illustrated by this sketch map and discussed as follows.

OPEN IN VIEWER

Figure 8

Schematic illustration for element supply and diffusion barrier mechanism in Al₂O₃/Al coated γ-TiAl alloy during isothermal oxidation at 1000°C

First, the outward and inward diffusion of the Al element from interlayer leads to a gradient section structure of the coating system. Meanwhile, the Al interlayer provides sufficient Al source for the formation of a continuous Al₂O₃ layer in the coating surface. Increasing the oxidation time, the Al interlayer is exhausted, while the thickness of intermediate diffusion layer grows. Thus, the presence of an Al interlayer assures the beneficial effect of a stable and compact Al₂O₃ layer. To some extent, the employ time of coating depends on its thickness. ³²

Second, the outward diffusion of Ti was detected by EDS and XRD techniques during oxidation test, as shown in Figs. 5 and 6. As an active intermetallic element with Ti, Al suppresses the out diffusion of Ti to the coating surface effectively because of the synthesis of Ti–Al intermetallics, such as TiAl₂ and TiAl₃ (Figs. 4c and 6).

In all, this coating structure is beneficial for the spallation resistance, which results from interdiffusion and the improvement of oxidation resistance of γ-TiAl alloy.

Scratch test

The coating adhesion strength was one of the main quality indicators of the coatings fabricated by magnetron sputtering. The acoustic emission curves are plotted as a function of the normal load in Fig. 9a, and the corresponding scratch morphologies are displayed in Fig. 9b–e. The major differences between two curves are the intensity of acoustic emission peak and the starting position of first acoustic emission signals. Although the critical loads of coatings on γ-TiAl alloy before and after oxidation test are larger than 50 N, above which the adhesion strength between the coating and substrate is generally sufficient for applications,^34–36 it is remarkable that the Al₂O₃/Al composite coated specimen after oxidation test displayed lower intensity of acoustic emission signal and higher critical load in comparison to that before oxidation test. The evidence of plastic deformation can be seen in the scratch track of the Al₂O₃/Al composite coating after oxidation test, which indicates better adhesion strength between the coating and the substrate than that before oxidation test.

OPEN IN VIEWER

Figure 9

a acoustic emission signal peaks versus normal load curves and optical images of scratches on Al₂O₃/Al composite coating b, d before and c, e after oxidation test

An explanation can be proposed to understand the obtained results from the scratch test. A gradient section structure of composite coating contributes to the reduction of the bending stress at the coating/substrate interface, which is usually the main cause of interfacial failure in the composite coated systems. The heat treatment before oxidation test and the isothermal oxidation air exposure lead to the formation of the gradient section structure. Thus, the interdiffusion process that occurred during oxidation test gives a contribution partly to the adhesion of the coating to the substrate.