Microstructure and Corrosion Properties of La2Zr2O7/NiCoAlY Thermal Barrier Coatings Deposited on Inconel 718 Superalloy by Laser Cladding
Abstract
:1. Introduction
2. Materials and Methods
3. Results and Discussion
3.1. Microstructure
- (1)
- Table 3 shows that EDS results at position D are mainly composed of O, Zr, and La. Combined with XRD calibration results in Figure 3, it can be seen that the coating is mainly La2Zr2O7 (JCPDS 01-073-0444) phase. The other small amounts of Al, Ni, Nb, Cr, Ti, Fe, Co, Mo, and Y in position D should come from the laser-heated molten bond coat NiCoCrAlY powder and partially melted Inconel 718 substrate. This is caused by the rapid laser heating and solidification during laser cladding, and the mixed alloy elements have no time to separate.
- (2)
- Table 3 shows that the Nb content of Spot3 and Spot6 exceed 10 at.% and they are probably Laves phases. Since Laves phase is generally A(Fe, Ni, Cr)2B(Nb, Mo, Ti) type, they might be Cr2Nb phase in combination with EDS results of Spot3 and Spot6. This is consistent with the results reported in literature [32,33,34,35].
- (3)
- Table 3 shows that the Nb content of Spot2 and Spot4 is 8.51 at.% and 5.62 at.%, respectively. They are probably δ phase (Ni3Nb). The reason is that the solidification temperature of δ phase is 860–995 °C and Nb concentration requirement is 6–8 at.% [35]. This is consistent with the results reported in literature [32].
- (4)
- Table 3 shows that Fe, Ni and Cr contents in Spot1, Spot5, and Spot7 are dominant, which should be γ phase ((Fe, Ni, and Cr) solid solution).
- (5)
- A large number of literature studies have shown [36,37,38] that the solidification process sequence of Inconel 718 substrate is L → γ → γ + NbC (high temperature) eutectic reaction → γ + laves (low temperature) eutectic reaction. According to EDS results in Table 3, in the laser cladding process, the chemical composition of the laser heating melted preset powder layer, and partially melted Inconel 718 substrate deviated from the chemical composition of Inconel 718 substrate due to mixing, but the degree of deviation should be small, because the chemical composition of the added NiCoCrAlY bond coat powder is basically the composition of Inconel 718 substrate. Therefore, we believe that the solidification process of the laser modified layer is as follows: The high melting point La2Zr2O7 (2300 °C) solidified first, and the remaining alloy liquid followed the solidification sequence of Inconel 718 alloy: L → γ → (γ + NbC) eutectic reaction → (γ + Laves) eutectic reaction. In other words, the solidification of remaining alloy liquid was mainly caused by L → γ reaction at the beginning. As time went on, Nb, Mo, Ti, C, and other elements were enriched between dendrites, leading to the eutectic reaction L → (γ + NbC) and the consumption of a large number of C atoms in the alloy. As the L → γ reaction continued and progressed, further enrichment of the interdendrite liquid solute atoms occurred until the eutectic reaction L → (γ + Laves) occurred and the solidification process was complete. At the same time, due to the characteristics of rapid laser heating and fast solidification, Nb element segregation was serious, and part of the regions with low Nb content generated δ phase instead of Laves phase, because Nb content in δ phase is usually 6–8 at. %, while Nb content in Laves phase usually exceeds 10 at. % [32,33,34,35].
3.2. Corrosion Properties
- (1)
- When there is no oxidation film, the following electrochemical reactions occur in 3.5 wt.% NaCl aqueous solution containing oxygen for Cr, Ni, Nb, Al, Co, Mo, Ti, and Fe elements in Inconel 718 substrate:For Cr element:
- Negative: Cr − 3e− = Cr3+
- Positive: O2 + 2H2O + 4e− = 4OH−
- Total reaction equation of galvanic cell: 4Cr + 3O2 + 6H2O = 4Cr(OH)3.
- For Ni element:
- Negative: Ni − 2e− = Ni2+
- Positive: O2 + 2H2O + 4e− = 4OH−
- Total reaction equation of galvanic cell: 2Ni + O2 + 2H2O = 2Ni(OH)2.
- For Nb element:
- Negative: Nb − 5e− = Nb5+
- Positive: O2 + 2H2O + 4e− = 4OH−
- Total reaction equation of galvanic cell: 4Nb + 5O2 + 10H2O = 4Nb(OH)5.
- For Al element:
- Negative: Al − 3e− = Al3+
- Positive: O2 + 2H2O + 4e− = 4OH−
- Total reaction equation of galvanic cell: 4Al + 3O2 + 6H2O = 4Al(OH)3. Due to the instability of Al(OH)3, it is finally decomposed into water and alumina: 2Al(OH)3 = Al2O3 + 3H2O.
- For Co element:
- Negative: Co − 2e− = Co2+
- Positive: O2 + 2H2O + 4e− = 4OH−
- Total reaction equation of galvanic cell: Co + O2 + 2H2O = Co(OH)4.
- For Mo element:
- Negative: Mo − 6e− = Mo6+Positive: O2 +2H2O + 4e− = 4OH−
- Total reaction equation of galvanic cell: 2Mo + 3O2 + 6H2O = 2Mo(OH)6.
- For Ti element:
- Negative: Ti − 4e− = Ti4+
- Positive: O2 + 2H2O + 4e− = 4OH−
- Total reaction equation of galvanic cell: Ti + O2 + 2H2O = Ti(OH)4.
- For Fe element:
- Negative: Fe − 2e− = Fe2+Positive: O2 + 2H2O + 4e− = 4OH−
- Total reaction equation of galvanic cell: 2Fe + O2 + 2H2O = 2Fe(OH)2. Fe(OH)2 can also continue to react with oxygen in solution: 4Fe(OH)2 + O2 + 2H2O = 4Fe(OH)3.
- (2)
- When there is oxidation film, the following chemical reactions occur in 3.5 wt.% NaCl aqueous solution for Cr, Ni, Nb, Al, Co, Mo, Ti, and Fe elements in Inconel 718 substrate:Generally speaking, all metal elements will form oxide film on their surface at room temperature. Table 6 is the Pilling–Bedworth (P-B) ratio of alloy oxides of Cr, Ni, Nb, Al, Co, Mo, Ti, and Fe. According to the criterion that “the integrity of metal oxide film is a necessary condition for its protection, while the P-B ratio is greater than 1 is a necessary condition for the integrity of metal oxide film”, it can be seen that the oxide film formed by these alloy-elements is complete. However, if the P-B ratio is too large (such as more than 2), the internal stress of the oxide film is large, and the oxide film is easy to break and lose protection or the protection is very poor. Therefore, according to the results in Table 1 and Table 6, the presence of Nb and Mo may make the oxidation film formed on the local surface of Inconel 718 substrate worse.
4. Conclusions
- (1)
- The outermost layer of the coating is La2Zr2O7 layer, and the secondary layer is mainly composed of γ + laves/δ phase eutectic structure.
- (2)
- The corrosion resistance of the coating is superior to that of Inconel 718 substrate in 3.5 wt.% NaCl aqueous solution, and the presence of La2Zr2O7 phase is the main reason for the improvement of the corrosion resistance of the coating.
Author Contributions
Funding
Data Availability Statement
Acknowledgments
Conflicts of Interest
References
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Element | Cr | Ni | Nb | Al | Co | Mo | Ti | C | Fe |
---|---|---|---|---|---|---|---|---|---|
wt.% | 18.8 | 52.7 | 5.3 | 0.5 | 0.02 | 2.9 | 0.9 | 0.03 | Bal. |
Element | Ni | Co | Cr | Al | Y |
---|---|---|---|---|---|
wt.% | 43.0 | 24.0 | 20.0 | 12.0 | 1.0 |
Element | O | Zr | La | Ni | Nb | Cr | Ti | Fe | Al | Co | Mo | Y | Possible Phase |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Area D | 58.2 | 20.4 | 15.6 | 0.1 | 0.0 | 0.4 | 0.8 | 0.1 | 3.5 | 0.0 | 0.0 | 0.9 | La2Zr2O7 |
Spot1 | 11.5 | 4.1 | 0.2 | 32.2 | 0.4 | 18.1 | 0.4 | 30.5 | 1.9 | 0.1 | 0.5 | 0.1 | γ |
Spot2 | 21.1 | 5.2 | 0.2 | 22.8 | 8.5 | 14.8 | 1.8 | 21.2 | 2.8 | 0.1 | 1.0 | 0.5 | δ |
Spot3 | 28.0 | 4.9 | 0.2 | 13.6 | 11.0 | 19.4 | 1.2 | 14.6 | 2.1 | 0.2 | 4.3 | 0.5 | Laves |
Spot4 | 19.9 | 4.6 | 0.2 | 22.4 | 5.6 | 19.8 | 1.0 | 20.2 | 2.6 | 0.2 | 3.0 | 0.5 | δ |
Spot5 | 12.2 | 4.3 | 0.2 | 31.1 | 0.5 | 18.1 | 0.6 | 29.9 | 2.2 | 0.1 | 0.6 | 0.2 | γ |
Spot6 | 28.7 | 5.3 | 0.2 | 13.6 | 10.9 | 18.8 | 0.9 | 14.4 | 2.2 | 0.2 | 4.3 | 0.5 | Laves |
Spot7 | 13.3 | 4.5 | 0.2 | 31.2 | 0.6 | 18.0 | 0.5 | 28.7 | 2.1 | 0.0 | 0.7 | 0.2 | γ |
Specimen | Ecorr (V) | Icorr (A/cm2) |
---|---|---|
Inconel 718 | −1.13 | 1.44 × 10−4 |
Laser clad coating | −0.91 | 1.29 × 10−5 |
Samples | Position | O | Zr | La | Al | Nb | Mo | Ti | Cr | Fe | Co | Ni | Y |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Inconel 718 | Area A | 15.0 | 0.0 | 0.0 | 2.2 | 5.3 | 0.7 | 0.7 | 17.0 | 39.5 | 0.2 | 19.4 | 0 |
Inconel 718 | Area B | 12.5 | 0.0 | 0.0 | 1.4 | 5.4 | 0.5 | 0.9 | 17.8 | 38.8 | 0.4 | 22.3 | 0 |
Inconel 718 | Spot 1 | 10.6 | 0.0 | 0.0 | 1.4 | 5.3 | 0.8 | 0.7 | 17.6 | 42.4 | 0.2 | 21.0 | 0 |
Coating | Area C | 61.5 | 16.0 | 15.1 | 4.6 | 0.2 | 0.1 | 0.9 | 0.5 | 0.1 | 0.1 | 0.2 | 0.7 |
Metal | Cr | Ni | Co | Fe | Fe | Ti | Nb | Al | Mo |
---|---|---|---|---|---|---|---|---|---|
oxide | Cr2O3 | NiO | CoO | FeO | Fe2O3 | TiO2 | Nb2O5 | Al2O3 | MoO3 |
P-B ratio | 2.07 | 1.65 | 1.86 | 1.70 | 2.14 | 1.73 | 2.68 | 1.28 | 3.4 |
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Huang, K.; Li, W.; Pan, K.; Lin, X.; Wang, A. Microstructure and Corrosion Properties of La2Zr2O7/NiCoAlY Thermal Barrier Coatings Deposited on Inconel 718 Superalloy by Laser Cladding. Coatings 2021, 11, 101. https://doi.org/10.3390/coatings11010101
Huang K, Li W, Pan K, Lin X, Wang A. Microstructure and Corrosion Properties of La2Zr2O7/NiCoAlY Thermal Barrier Coatings Deposited on Inconel 718 Superalloy by Laser Cladding. Coatings. 2021; 11(1):101. https://doi.org/10.3390/coatings11010101
Chicago/Turabian StyleHuang, Kaijin, Wei Li, Kai Pan, Xin Lin, and Aihua Wang. 2021. "Microstructure and Corrosion Properties of La2Zr2O7/NiCoAlY Thermal Barrier Coatings Deposited on Inconel 718 Superalloy by Laser Cladding" Coatings 11, no. 1: 101. https://doi.org/10.3390/coatings11010101
APA StyleHuang, K., Li, W., Pan, K., Lin, X., & Wang, A. (2021). Microstructure and Corrosion Properties of La2Zr2O7/NiCoAlY Thermal Barrier Coatings Deposited on Inconel 718 Superalloy by Laser Cladding. Coatings, 11(1), 101. https://doi.org/10.3390/coatings11010101