3.1. Experimental Results
To investigate the phase components of the NiCrAlY BC after oxidation at 900 °C, the YSZ TC was removed. The XRD patterns of the oxidized specimens under different stresses at 900 °C for 100 h are displayed in
Figure 2. Only Al
2O
3 is observed in the specimens without stress and under tensile stress, while NiCr
2O
4 appears in the specimen under compressive stress. Hence, compressive stress changes the oxidation behavior of the NiCrAlY BC.
Figure 3 shows the cross-section morphologies of the oxidized specimens under different stresses at 900 °C for 100 h. In the case of the specimen without stress, a thin TGO layer in the NiCrAlY BC appears (
Figure 3a), which has been obtained in the literature [
25,
26]. The average thickness of the TGO layer is about 0.9 μm. In the case of the specimen under 100 MPa tensile stress, a thicker oxide scale of approximately 1.2 μm is observed (
Figure 3b), indicating that the induced tensile stress greatly promotes the oxidation of the NiCrAlY BC. The thickness of the TGO layer in the NiCrAlY BC increases to 1.5 μm with the introduction of the 100 MPa compressive stress (
Figure 3c). Thus, compared with tensile stress, compressive stress further promotes the oxidation of the NiCrAlY BC. No crack is obtained in the TGO layer under different stresses, which means that the applied stress does not exceed the critical stress for oxide failure by cracking. Moreover, the surface morphology of the oxidized specimens without stress at 900 °C for 100 h is shown in
Figure S1, similar to those of the oxidized specimens under tensile and compressive stresses (not shown). Hence, the effect of the external stress on the surface morphology of the TBC is limited.
Figure 4 and
Table 1 display the EDS line and spot composition diagrams of the specimens after oxidation at 900 °C for 100 h under different external loads. The main component of the TGO layer in the NiCrAlY BC without stress is Al
2O
3 (
Figure 4a and
Table 1), which is consistent with the reported literature [
27,
28]. A similar phenomenon is observed in the specimen under 100 MPa tensile stress (
Figure 4b and
Table 1). Thus, the external tensile stress possesses a limited effect on the composition of the TGO layer in NiCrAlY BC, while the TGO composition changes obviously when the specimen suffers from compressive stress according to the EDS results (
Figure 4c and
Table 1). Unlike the specimens without stress and under 100 MPa tensile stress, a two-layer TGO layer is formed in the oxidized specimen under 100 MPa compressive stress. More Ni and Cr elements are observed on the side close to the YSZ coating (upper TGO layer) with a thickness of about 0.3 μm. Then, the content of Ni and Cr elements decreases rapidly, and more Al
2O
3 appears (lower TGO layer). Since the upper TGO layer is not dense, the oxygen penetrates and reacts with Al to form more Al
2O
3. The change in the oxidation behavior of the NiCrAlY BC might be attributed to the various oxidation energy barriers of the elements under different stresses. Thus, the first-principles simulation was employed to explain the experimental results.
3.2. Ideal Oxidation of Al(111), Ni(111), and Cr(110) under Different Stresses
To more deeply understand the effect of the different stresses on the oxidation behavior of the NiCrAlY BC, simulations of the O
2 molecule adsorption on the surfaces of the NiCrAlY BC under different stresses were carried out. To simplify the simulation, Al(111), Ni(111), and Cr(110) instead of NiCrAlY alloy were used. This simplification is reasonable because the main products are the oxides of Al, Ni, and Cr under different stresses (
Figure 2). After geometry optimization, all low-symmetry structures relax into the high-symmetry ones and some high-symmetry vertical adsorption states are unstable.
For the stress-free Al(111), the most stable structure with an adsorption energy of −12.43 eV is displayed in
Figure 5a. For the Al(111) under tensile stress, the adsorption energy of the most stable structure is −12.39 eV, and its structure is similar to that of the oxidized stress-free Al(111). For the Al(111) under compressive stress, the most stable structure possesses the adsorption energy of −12.17 eV, resembling that of the oxidized stress-free Al(111). The O atom adsorbs at the threefold hollow connecting with three Al atoms in the most favorable adsorption configuration, thus reducing the number of dangling bonds on the surface. The negative adsorption energy implies that strong bonds are created between the adsorbed O atoms and the topmost Al atoms.
Comparing the adsorption energies, the O
2 molecule prefers to adsorb on the stress-free Al(111) surface. Nevertheless, the difference in the adsorption energies of the various most stable structures is only 0.26 eV. Even though this is a small energetic difference, there are significant electronic differences among the three configurations, as shown in
Figure 6.
Figure 6a displays the partial density of states (PDOS) of the O atom in the oxidized Al(111) under different stresses. The O 2p-orbital peak appears at −5.21 eV in the oxidized stress-free Al(111), while it shifts to −4.98 eV in the oxidized Al(111) under tensile stress. The O 2p-orbital peak in the oxidized Al(111) under compressive stress is at the lowest energy of −5.58 eV, indicating that the O atom in the oxidized Al(111) under tensile stress is the least stable. Similarly, the oxidized Al 3s- and 3p-orbital peaks in the oxidized Al(111) under tensile stress possess the highest energies from −10.78 eV to the Fermi level, causing it to possess the lowest stability (
Figure 6b), which is related to its initial state before oxidation. The Al 3s- and 3p-orbital peaks in the stress-free Al(111) surface appear between −11.04 eV and the Fermi level, while they shift to between −10.85 and the Fermi level in the Al(111) under tensile stress (
Figure 6c). The Al 3s- and 3p-orbital peaks in the Al(111) under compressive stress are between −11.20 eV and the Fermi level, which implies that the Al atom in the Al(111) under tensile stress is the most unstable. Hence, the instability of the Al atom in the Al(111) under tensile stress might be the main reason for its easy oxidation.
The interactions between the O
2 molecules and the topmost Al atoms in the Al(111) under different stresses were investigated, and detailed positions for the interactions along the reaction paths with the lowest energy barriers are shown in
Figure 7. All of the reaction paths possess similar transition states where the O
2 molecule dissociates into O species; the O atom bonds to a surface Al atom, but remains unbonded for the remaining two Al atoms. The O
2 molecule adsorption path on the stress-free Al(111) surface is characterized by an energy barrier of 0.52 eV (
Figure 7a) and the O–O interatomic distance in the transition state structure is 1.79 Å (
Figure 7b). The energy barrier for the O
2 molecule adsorption on the surface of the Al(111) under tensile stress is 0.34 eV, 0.03 eV higher than that on the surface of the Al(111) under compressive stress (
Figure 7a). The distance between the two O atoms is 1.76 Å in the transition state structure of the oxidized Al(111) under tensile stress (
Figure 7c), while it is 1.80 Å in the transition state structure of the oxidized Al(111) under compressive stress (
Figure 7d). Thus, O
2 dissociation becomes the easiest on the surface of the Al(111) under tensile stress, agreeing well with the decreasing stability of the Al atom in the Al(111) under tensile stress (
Figure 6). When Al is stretched, the instability of the Al atom increases, causing easier oxidation to occur, which supports the experimental results in which tensile stress results in the easier oxidation of Al (
Figure 3).
The most stable configurations after the O
2 molecule adsorption on the surface of the Ni(111) under different stresses are also observed, as shown in
Figure 5b. The most stable structure after O
2 molecule adsorption on the stress-free Ni(111) surface is similar to that on the Ni(111) under tensile or compressive stress (not shown). Each O atom bonds with the three topmost Ni atoms to decrease the dangling bonds on the surface. The lowest adsorption energy of −6.01 eV is obtained for the oxidized stress-free Ni(111), only 0.23 eV higher than that for the Ni(111) under tensile stress and 0.26 eV lower than that for the Ni(111) under compressive stress. The lowest adsorption energies of the O
2 molecules on the surfaces of the Ni(111) under different stresses are much bigger (approximately 6 eV) than those on the surfaces of the Al(111) under different stresses. Thus, the interaction between the Ni atoms and O
2 molecule is much weaker than that between the Al atoms and O
2 molecule. In addition, the main difference among the three most stable structures of Ni(111) under different stresses is the various state of the oxidized Ni atoms. A charge transfer of 0.38 e from the oxidized Ni1 atom (marked in
Figure 5b) to the O atom is observed after O
2 molecule adsorption on the stress-free Ni(111) surface, leading to the formation of an O atom with a negative charge and a strong Ni1–O bond. This is a 0.39 e charge transfer from the oxidized Ni1 atom to the O atom after O
2 molecule adsorption on the surface of the Ni(111) under tensile stress. The greatest charge transfer (0.45 e) from the oxidized Ni1 atom to the O atom occurs after O
2 molecule adsorption on the surface of the Ni(111) under compressive stress, which means a strong interaction between the O atom and the Ni1 atom. Thus, the oxidized Ni1 atom in the Ni(111) under compressive stress possesses the most stability, which might cause the easiest oxidation of the Ni atom.
To further elucidate the oxidation behaviors of the Ni(111) under different stresses, we plotted the PDOS patterns of the Ni atom in the Ni(111) under different stresses before oxidation, as displayed in
Figure 8. The Ni 3d orbital exerts a significant role in bonding (
Figure 8b) in the Ni(111), different from the Al atom (
Figure 6c), which might be a significant reason for their distinguishable oxidation behaviors (
Figure 3). The Ni 3d orbital peak in the stress-free Ni(111) is located at −0.36 eV and shifts to the higher −0.24 eV in the Ni(111) under compressive stress, as opposed to the higher stabilization observed in Al(111) under compressive stress. The Ni 3d orbital peak in the Ni(111) under tensile stress appears at the highest energy of −0.17 eV, indicating that the Ni atom in the Ni(111) under tensile stress possesses the least stability, followed by the Ni atom in the Ni(111) under compressive stress, different from the Al atom in the Al(111) under compressive stress possessing the highest stability. Otherwise, all O 2s and 2p orbital peaks appear at the same position in the oxidized Ni(111) under different stresses after oxidation (
Figure 8a), meaning similar O electronic structures. Therefore, the Ni 3d orbital with low bound exercises a major effect on the stability of the Ni atom under different stresses, which indicates that the oxidation behavior of Ni might be different from that of Al under different stresses.
A further study of the adsorption of an O
2 molecule on the surface of the Ni(111) under different stresses was carried out. The key positions for the adsorption along the reaction path with the lowest energy barrier are displayed in
Figure 9. All of the reaction paths possess similar transition states where the O
2 molecule dissociates into two O atoms and the O atom bonds to a topmost Ni atom. The transition state of the O
2 molecule adsorption on the stress-free Ni(111) surface has an energy barrier of 1.85 eV (
Figure 9a) and the O–O distance in the transition state structure is 2.00 Å (
Figure 9b). The O
2 molecule adsorption path on the surface of the Ni(111) under tensile stress is characterized by an energy barrier of 1.78 eV, and the distance between the two O atoms in the transition state structure is 1.97 Å (
Figure 9c). The transition state of the O
2 molecule adsorption on the surface of Ni(111) under compressive stress is the one with the lowest energetic barrier, 1.50 eV, and the O–O distance in the transition state structure is 1.95 Å (
Figure 9d). Hence, it is easiest for the O
2 dissociation on the surface of the Ni(111) under compressive stress, which is consistent with the experimental results indicating easier oxidation occurrence for the Ni under compressive stress (
Figure 3). Hence, as Ni suffers from compressive stress, the lowest energy is needed for the dissociation of the O
2 molecule, causing it to be more easily oxidized.
The most stable configurations after the adsorption of the O
2 molecules on the surfaces of the Cr(110) under different stresses are presented in
Figure 10. In the case of the stress-free Cr(110), the most stable adsorption structure with an adsorption energy of −10.38 eV is displayed in
Figure 10a. When the Cr(110) suffers from tensile stress, the adsorption energy of the most stable structure is −10.39 eV, and its structure is similar to that of the oxidized stress-free Cr(110), as shown in
Figure 10b. For the Cr(110) under compressive stress, the most stable structure possesses the adsorption energy of −10.12 eV, and it is distinguished from that of the oxidized stress-free Cr(110) or Cr(110) under tensile stress. In the former, the O atom adsorbs at the fourfold hollow connecting with four Ni atoms due to the shorter distance between the Ni atoms, while in the latter two, it adsorbs at the threefold hollow connecting with three Ni atoms.
To further study the effect of stress on the electronic structure, the PDOS patterns of the Cr atom in the Cr(110) under different stresses are plotted and displayed in
Figure 11b. The Cr 3d orbital is a great contributor to bonding, similar to the Ni atom (
Figure 8b). The Cr 3d orbital peaks in the stress-free Cr(110) are between −4.88 eV and the Fermi level, while they appear between the higher −4.80 eV and Fermi level in the Cr(110) under compressive stress. The Cr 3d orbital peaks in the Cr(110) under tensile stress located between the highest energy of −4.71 eV and the Fermi level, which implies that the Cr atom in the Cr(110) under tensile stress possesses the least stability, followed by the Cr atom in the Cr(110) under compressive stress. The effect of stress on the characteristics of the Cr atom in Cr(110) is similar to that of the Ni atom in Ni(111). Thus, the oxidation behavior of Cr under different stresses might be similar to that of Ni under different stresses.
The adsorption behaviors of the O
2 molecules on the surfaces of the Cr(110) under different stresses were analyzed further, and the PDOS patterns of the O atoms after adsorption are plotted and displayed in
Figure 11a. The O 2p orbital peak appears at −5.74 eV in the oxidized stress-free Cr(110), while it shifts to −6.03 eV in the oxidized Cr(110) under compressive stress, meaning that the O atom in the oxidized Cr(110) under compressive stress is the more stable. Differently, two O 2p orbital peaks shift to the higher energies (−5.55 and −6.00 eV) in the oxidized Cr(110) under tensile stress. The various PDOS patterns might be due to the different stable configurations (
Figure 10).
The interactions between the O
2 molecules and the topmost Cr atoms in the Cr(111) under different stresses were investigated. The detailed positions for the interactions along the reaction paths with the lowest energy barriers are shown in
Figure 12. The transition states in the Cr(110) under different stresses are varied, although the O
2 molecule dissociates into O species in the transition states and the O atom bonds to a topmost Cr atom. The transition state of the O
2 molecule adsorption on the stress-free Cr(110) surface has an energy barrier of 2.92 eV (
Figure 12a) and the O–O distance in the transition state structure is 2.80 Å (
Figure 12b). The barrier decreases to 2.76 eV on the surface of the Cr(110) under tensile stress and the distance between the two O atoms in the transition state structure is 2.71 Å (
Figure 12c), which implies that the dissociation of the O
2 molecule is feasible on the latter surface. The most favorable path for the adsorption of the O
2 molecule occurs on the surface of the Cr(110) under compressive stress with an energy barrier of 2.63 eV and the O–O distance in the transition state structure is 2.27 Å (
Figure 12d). Thus, the O
2 molecule prefers to dissociate and adsorb on the surface of the Cr under compressive stress rather than that without stress and under tensile stress, resulting in it being the easiest to be oxidized.