The effect of yttrium incorporation on the formation and properties of α-Al₂O₃ coatings

3.1 Microstructure and composition of the AlYα coatings

As seen in Figure 1A and B, the surface structures of the coating were uniform and compact without pores or cracks. The interface between the coating and the matrix was clear without significant stratification. The EDX points were used to analyze the different sites of the coating section, as shown in Table 1. It was known from the FeAl phase diagram that the main elements in the coating exhibited a gradient distribution [19]. The quantitative Al diffused onto the surface of the backing material, thus producing a gradient concentration containing Al. The FeAl₃ compound might form on the coating surface. The Fe₂Al₅ compound might form in the interlayer, whereas the FeAl phase formed at the juncture of the coating and matrix. The presence of the transition layer (FeAl phase) allowed for a securer combination between the coating and the matrix and offset a series of phenomena such as exfoliation as a result of thermal expansion coefficient during the subsequent oxidization between the matrix and the coating [20]. In addition, the content of element Y was not high (2.53%, 2.36%, and 1.82%) at the three EDX points, which did not coincide with the proportion of Al and Y elements in the target material. In light of the irregular distribution curve for element Y during line scanning, the low content of Y in the coating may be due to the fact that the sputtering rate of the rare earth Y is lower than that of the metal Al, and the composite target material sputters much fewer Y particles after ionic bombardment. In addition, as Y³⁺ has a larger ionic radius (0.093 nm) than Al³⁺ (0.05 nm), the diffusion of Y in the coating is restricted and the diffusion and migration energy is high [21]. Meanwhile, the entry of Y³⁺ ions into the coating will cause considerable lattice distortion and produce great activation energy. Y³⁺ ions are more likely to spontaneously aggregate at the grain boundaries and defects to reduce the free energy of the system by producing segregation because of the loose arrangement of the atoms at the grain boundaries and the presence of many electron holes [22]. Therefore, the Y atoms are distributed irregularly.

Figure 1:

Morphology and phase of the AlYα coating: (A) surface morphology, (B) section morphology, and (C) GAXRD pattern.

The GAXRD pattern for the AlYα coating in Figure 1C further verified the presence of diffraction peaks of multiple iron-aluminum intermetallic compounds: FeAl phase, FeAl₃ phase, and Fe₂Al₅ phase. A large number of high-energy argon ions bombarded the matrix surface of the target material and stainless steel during the process of double glow plasma diffusion metallizing. The elements (Al, Y, α-Al₂O₃) were sputtered from the target material onto the surface of workpieces. Meanwhile, the argon ions heated the stainless steel matrix to a high temperature by bombardment. The argon ion bombardment caused several defects within the matrix and coating, thus allowing for mutual diffusion of the alloy elements sputtered onto the matrix surface and the elements within the matrix. Meanwhile, the elements of Al and Fe were liable to form iron-aluminum intermetallic compounds as a result of their very strong chemical affinity. With continuous mutual diffusion, a large number of Fe within the matrix migrated onto the surface and left many vacancies. The increasing Al would diffuse to these internal vacancies, thus generating the Fe₂Al₅ and the FeAl phases with a low content of aluminum. Therefore, the intermetallic compound layer grew toward either side of the matrix, enabling the coating thickness to increase. As seen in Figure 1C, in addition to the FeAl phase, a small amount of α-Al₂O₃ was also present in the coating. This was due to the fact that some atomic groups or particle groups consisting of a few atoms might sputter from the source electrode material as a result of the very high energy of the argon ion bombardment. However, there was no diffraction peak containing rare earth Y alloys or Y chemical compounds in Figure 1C.

3.2 Microstructure and composition of the AlYα oxidization coatings

As seen in Figure 2A, the coating surface became more compact and exhibited an island-shaped raised structure during oxidation at an oxygen flow rate of 2 sccm. The projection in Figure 2A was magnified, as indicated in Figure 2B, and the projection mainly comprised a large number of 100–200 nm fine particles. The EDX was used to analyze the raised point 1 and the flat surface point 2, as shown in Table 2. The main elements were Al and O in both regions of the coating. A small amount of element Y was distributed at the raised site of the coating. These projections might be caused by the pinning effect of the Y particles [23]. Li et al. [24] have discovered that the addition of Y can inhibit the growth of nanometer crystal grains to some extent. The surface of the coating prepared was coarse, but the release of element Y can improve the mechanic performance of the coating and increase the binding force of the coating. The coating thickness was approximately 10.92 μm after oxidization (Figure 2C). As seen in Figure 2D, the oxide coating mainly included diffraction peaks of α-Al₂O₃ and Fe₂O₃. FeAl and Fe₂Al₅ phases were still present in the coating. Meanwhile, no Al₂O₃ of metastable phase crystal form was present in the coating. This was possibly due to the fact that the addition of the rare earth element of Y promoted transformation from the metastable phase to α-Al₂O₃ compared with the result of the research where only α-Al₂O₃ seed crystals were added [18].

Figure 2:

Morphology and phase of the plasma oxide coating (A) surface morphology, (B) the enlarged image of the boxed region in (A), (C) section morphology, (D) GAXRD pattern.

3.3 Distribution and induction mechanism of element Y in the coating

To verify the existence form and distribution of element Y, the coatings before and after oxidization were subject to a FETEM test, and the EDX in the FETEM was used to analyze the coating, as shown in Figure 3. As seen in Figure 3A, there was a transition layer at the junction between the coating and the matrix. At the initial stage of sputtering, during the process of double glow plasma diffusion, metalizing, sputtering, heavy ion sputtering, and sedimentation occurred simultaneously at a low temperature, thus producing a transition layer at the junction between the matrix and the coating. The crystalline grains grew in a columnar manner because of a low sputtering temperature. As seen in Figure 3B and D, element Y really existed in the coating before and after oxidization. Meanwhile, the distribution of element Y was not continuous. On the basis of the amplification of the local region in Figure 3D, as shown in Figure 3E, and the EDX analysis, the white bright spots in the figure represented the Y₂O₃ particles. It can be inferred that element Y was distributed at the grain boundary primarily in the form of fine Y₂O₃ particles. Kuntal and Anjan [15] have discovered that the content of Y at the grain boundary (0.26 at%) is higher than that within the crystals 0.00 at%, and element Y is mainly concentrated at the grain boundary of the crystals in the Y element-adulterated Al₂O₃-ZrO₂ ceramic composite material. Previous research has discovered that the O ions diffused through short-circuit diffusion paths during the process of plasma oxidation. Meanwhile, element Y has a higher affinity for oxygen than element Al. The phase transition heat of Y₂O₃, ΔHf is -1905 KJ/mol, and the phase transition heat of Al₂O₃ is -1621 KJ/mol. Hence, Y is more likely to be oxidized into Y₂O₃ at the grain boundary.

Figure 3:

TEM analysis and EDX spectrogram for the coating before and after oxidization: (A) coating before oxidization, (B) EDX analysis for the coating before oxidization, (C) coating after oxidization, (D) EDX analysis for the coating after oxidization, and (E) image for STEM magnified in Zone 1 in panel c.

3.4 The binding force between Al₂O₃ coatings and the substrate

Figure 4 presents the scratch curves for plasma oxide coatings doped with the α-Al₂O₃ seed crystals alone and plasma oxide coatings co-doped with the α-Al₂O₃ seed crystals and the rare earth element Y. As seen in Figure 4, the binding forces of the coatings were more than 60 N, and their binding forces were good. The presence of the transition layer (FeAl layer) avoided spalling as a result of mismatching of the thermal expansion coefficients between the matrix and the coating material, thus raising the bonding strength between the coating and the matrix. Research has found that Y is able to promote the selective oxidation of the Al₂O₃ coating and reduce the critical content of Al of the Al₂O₃ coating formed by the alloy [25]. Meanwhile, the rare earth element Y or its oxide exists at the alloy interface, thus allowing the oxide coating to firmly bind to the alloy surface. The rare earth element is able to eliminate crystal lattice vacancies and impede the formation of vacancies at the alloy/oxide coating interface thus increasing the binding force as a result of its large diameter [26]. It can be seen from Figure 4 that the plasma oxide coating co-doped with multiple elements has a better binding force than the oxide coating doped with only one element.

Figure 4:

Scratch curve for plasma oxide coating: (A) Alα oxide coating and (B) AlYα oxide coating.

3.5 Corrosion property of oxide coatings

Figure 5 presents the polarization curve for the two oxide coatings and 316L stainless steel in the 3.5% NaCl solution. Table 3 presents the corrosion potential and corrosion current. The research group has obtained a corrosion potential of -0.291 V for the 60 wt% α-Al₂O₃ coating by using a single factor (plasma bombardment) [27]. The research group has obtained higher corrosion potentials of -0.206 and -0.130 V for the oxide coating by using double factors (ion bombardment and α-Al₂O₃ seed crystals) and three factors (ion bombardment, α-Al₂O₃ seed crystals, and the rare earth element Y). The improvement in the corrosion resistance of the coatings may be attributable to the following three reasons. First, the content of α-Al₂O₃ increased in the research, which helped increase the corrosion resistance of the matrix [28]. Second, the compact aluminum oxide coating consisting of numerous fine particles could serve as a barrier to impede diffusion of the corrosive media. Finally, the corrosion property of the AlYα oxide coating was better than that of the Alα oxide coating. The improvement in corrosion property may be due to the fact that α-Al₂O₃ and Y₂O₃ could serve as “physical barriers” in a corrosive environment to impede corrosion, or α-Al₂O₃ and Y₂O₃ particles and the stainless steel matrix could form many “corrosion microbatteries”. The oxide particles served as the cathode, and the stainless steel matrix served as the anode, thus improving the corrosion resistance of the matrix. Meanwhile, the addition of the α-Al₂O₃ seed crystals and the rare earth element Y fined the crystalline grains of the coating, thus improving the corrosion property of the oxide coating [29]. The minimum corrosion current of the AlYα oxide coating was 0.009 μA·cm^-2, three orders of magnitude lower than that of the 316L stainless steel matrix. The corrosion property of the coating could reflect the compactness of the coating to a great extent. Relevant parameters were calculated in accordance with the Tafel formula, as shown in Table 3. It could be seen from the computational formula for the porosity that the porosities of the Alα oxide coating and the Alα oxide coating were 0.13% and 0.10%, respectively. It was thus clear that the prepared oxide coatings were very compact.

Figure 5:

Polarization curves for plasma oxide coating and 316L stainless steel in 3.5% NaCl solution.