3.1. Coating Composition and Structure
Figure 2 shows the XRD patterns of a single aluminate coating on the K444 alloy deposited at 1050 °C and three kinds of yttrium-modified aluminate coatings on the K444 alloy deposited at 950 °C, 1000 °C, and 1050 °C. The characteristic peaks of the four coatings are similar, mainly β-NiAl and NiFe phases, indicating that adding yttrium and changing the deposition temperature has little effect on the surface structure of the coating.
Figure 3 shows the cross-sectional microstructure of (a) the single aluminide coating and (b) the yttrium-modified aluminide coating on the K444 alloy deposited at 1050 °C and element line scan maps of Al, Ti, Co, Ni, Cr, and Y. The direction of arrows in the
Figure 3a,b indicates the element line scanning direction. The deposition time of the two groups was 2 h, and the deposition temperature was 1050 °C. It can be seen from the figure that both groups of coatings have two layers, the outer layer and the diffusion layer. The thickness of the single aluminide coating is about 20 μm, and that of the yttrium-modified aluminide coating is about 28 μm. It can be seen from element line scan maps that the Al of the yttrium-modified aluminide coating is more widely distributed, while the external diffusion degree of Ti and Cr is lower than that of single aluminide coating, indicating that the addition of yttrium can promote the internal diffusion of Al atoms, hinder the external diffusion of Ti, Cr, and other matrix elements, and increase the coating thickness.
Figure 4a–c show the cross-sectional microstructure of yttrium-modified aluminide coatings on the K444 alloy deposited at 950 °C, 1000 °C, and 1050 °C. The number 1 in the
Figure 4a–c represents the zone of outer layer and the number 2represents the zone of interdiffusion layer. The coatings of the three groups are all double-layer structures, with an outer layer and a diffusion layer. Among them, the thickness of the yttrium-modified aluminide coating deposited at 1050 °C is the biggest, about 28 μm, the thickness of the coating deposited at 1000 °C is about 15 μm, and the thickness of the coating deposited at 950 °C is the smallest, about 10 μm. The principle of aluminide coating prepared by chemical vapor deposition is mainly the internal diffusion of Al and the external diffusion of Ni [
19,
20], and the outer layer is mainly composed of Al and Ni. The inner layer is an interdiffusion zone, and the bright white area is a Cr-rich phase [
21].
Figure 5 shows the distribution curves of yttrium in three groups of yttrium-modified aluminide coatings, corresponding to zone 1 and 2 of the three groups of coatings in
Figure 4. The yttrium in the three groups was distributed evenly, and the content in the outer layer of the coating was slightly higher than that in the diffusion layer. The content of yttrium in the yttrium-modified aluminide coating deposited at 1050 °C was the highest (0.85 wt.% as a result of line scanning), the content of yttrium in the yttrium-modified aluminide coating deposited at 1000 °C was less (0.75 wt.% as a result of line scanning), and the content of yttrium in the yttrium-modified aluminide coating deposited at 950 °C was the least (0.245 wt.% as a result of line scanning). According to research [
22], the addition of yttrium can accelerate the deposition rate of active aluminum atoms, increase the content of aluminum in the coating, and increase the coating thickness. The higher the deposition temperature, the faster the reaction rate and the more yttrium content in the coating. Under the influence of deposition temperature and yttrium, the diffusion rate of aluminum element is further accelerated, and the coating thickness is further increased.
3.2. Cyclic Oxidation Behavior of Coating
Figure 6 shows the kinetics curves of the single aluminide coating deposited at 1050 °C and the three groups of yttrium-modified aluminide coating deposited at 950 °C, 1000 °C, and 1050 °C of cyclic oxidation for 100 h at 1100 °C. It can be seen that the single aluminide coating and the three groups of yttrium-modified aluminide coating gain weight faster from 0–10 h, and the β-NiAl in the coating quickly forms a dense Al
2O
3 film, and after 10 h, the weight gain is slow, indicating that a complete Al
2O
3 film has been formed on the surface of the coating to protect the matrix from further oxidation. The weight gain of the single aluminide coating was 0.55 mg·cm
−2, the weight gain of the yttrium-modified aluminide coating deposited at 1050 °C was 0.3 mg·cm
−2, and the resistance of the coating to high temperature oxidation was increased by 45%. The weight gain of the three groups of yttrium-modified aluminide coatings is slower than that of the single aluminide coating, indicating that the high temperature oxidation resistance of the yttrium-modified aluminide coating is better than that of the single aluminide coating. Compared with the three groups of yttrium-modified aluminide coatings, the weight gain of the yttrium-modified aluminide coating deposited at 950 °C is the largest, indicating that its oxide film has a greater tendency to crack and flake, and its oxidation resistance at high temperature is poor. The slope of the weight gain curve of the modified coating deposited at 1000 °C and 1050 °C is similar, indicating that the resistance of the two coatings to high temperature oxidation tends to be consistent.
Figure 7 shows the XRD patterns of single aluminide coating deposited at 1050 °C and three groups of yttrium-modified aluminide coating deposited at 950 °C, 1000 °C, and 1050 °C on K444 alloy after cyclic oxidation for 100 h at 1100 °C. The oxidized aluminide coatings are mainly Al
2O
3, TiO
2, and γ-Ni
3Al phases, and the yttrium-modified aluminide coatings are Al
2O
3, TiO
2, γ-Ni
3Al, and β-NiAl phases. This indicates that part of the aluminum in the coating is consumed by oxidation, and the β-NiAl phase is transformed into the more brittle γ-Ni
3Al phase. There is a γ-Ni
3Al phase in the single aluminide coating, but no β-NiAl phase, which shows that the coating damage is more serious.
Figure 8 shows the cross-sectional microstructure of a single aluminide coating and a yttrium-modified aluminide coatings deposited at 950 °C, 1000 °C, and 1050 °C on the K444 alloy, and element line scan maps of Al, Ti, O, Ni, Cr, and Y after cyclic oxidation for 100 h at 1100 °C. The direction of arrows in the
Figure 8a–d indicates the element line scanning direction. As can be seen from the
Figure 7 and
Figure 8, the outer layer of a single aluminide coating is mainly NiO compounds and Cr
2O
3, and the inner layer is TiO
2. The outer layer of the modified coating deposited at 950 °C is mainly Cr
2O
3, and the inner layer is Al
2O
3 and TiO
2. The outer layer of the modified aluminide coating deposited at 1000 °C is Al
2O
3, TiO
2, and the inner layer is Cr
2O
3. The outer layer of the modified aluminide coating deposited at 1050 °C is Al
2O
3, Cr
2O
3, and the inner layer is TiO
2. The outer layer of the single aluminide coating is undulating, the content of Ni is high, the oxide layer is peeling off, and the local failure of the coating is serious. The outer layer of the modified aluminide coating is relatively flat, among which the oxidation product layer of the yttrium-modified coating deposited at 950 °C is thicker, about 14.3 μm, and the oxidation product of the coating deposited at 1000 °C and 1050 °C is thinner than that of the coating deposited at 950 °C, about 2 μm. The yttrium-modified aluminide coating deposited at 1000 °C and 1050 °C has dense surface oxide film, therefore it has good high-temperature oxidation resistance. The high-temperature oxidation resistance of the yttrium-modified aluminide coating is better than that of single aluminide coating, and the high temperature oxidation resistance of the modified aluminide coating deposited at 1000 °C and 1050 °C is similar, but is better than that of modified aluminide coating deposited at 950 °C.
Figure 9 shows the surface morphologies of the single aluminide coating and three groups of yttrium-modified aluminide coatings on the K444 alloy after cyclic oxidation for 100 h at 1100 °C, where (a) is single aluminide coating deposited at 1050 °C. (b), (c) and (d) are three groups of yttrium-modified aluminide coatings on the K444 alloy deposited at 950 °C, 1000 °C, and 1050 °C. Both the single aluminide coating and the yttrium-modified aluminide coating deposited at 950 °C have hole defects, but the single aluminide coating has more defects. The modified coatings deposited at 1000 °C and 1050 °C are denser.
Table 2 shows the EDS surface scan element content table of single aluminide coating and three groups of yttrium-modified aluminide coatings on the K444 alloy, corresponding to
Figure 9a–d. It can be seen that the atomic proportion of Al + O on the surface of the four groups of coatings is more than 80%. The single aluminide coating and the modified aluminide coating deposited at 950 °C have more types of swept elements, including Co and Fe, the content of Co is 1.82 wt.% and 0.86 wt.%, and the content of Fe is 3.18 wt.% and 2.36 wt.%, indicating that the coating has a certain degree of damage. However, no Co was detected on the surface of modified coating deposited at 1000 °C, the matrix element types were less, the content was lower (Fe content was 0.34 wt.%), and the oxidation resistance was excellent. Substrate element types of yttrium-modified coating deposited at 1050 °C are the least, and Co and Fe are not detected, so the oxidation resistance is the best.
The coating surface of single aluminide coating and three groups of yttrium-modified aluminide coatings is the β-NiAl phase, and the metastable cubic crystal Al
2O
3, including γ phase, δ phase and θ phase, is first formed during oxidation. As the oxidation process continues, these metastable Al
2O
3 will gradually transform into stable α-Al
2O
3. Studies have shown that [
23,
24] rare earth elements can promote the nucleation of steady-state α-Al
2O
3 by forming oxidation particles, thus promoting the transition from metastable Al
2O
3 to stable Al
2O
3. In addition, rare earth elements can also reduce the critical Al content of Al
2O
3 film formation on the surface, reduce the critical concentration of Al
2O
3 formation on the surface, and extend the deposition time of the coating. Therefore, more yttrium can better promote the formation of the α-Al
2O
3 film, thereby improving the high-temperature oxidation resistance of the coating.
3.3. Thermal Corrosion Behavior of Coatings
Figure 10 shows the corrosion weight gain kinetics curves of the single aluminide coating and three groups of yttrium-modified aluminide coatings on the K444 alloy after corrosion at 900 °C-Na
2SO
4 + NaCl for 75 h. The single aluminide coating gained weight rapidly in the early stage of corrosion (0–20 h), reaching 0.0265 mg·cm
−2 at 20 h. It remained stable in the middle stage of corrosion (20–40 h), gaining 0.0293 mg·cm
−2 at 30 h. At the later stage of corrosion (40–75 h), the weight gain decreased rapidly, reaching 0.0108 mg·cm
−2 at 75 h. The corrosion weight gain of the three groups of modified coatings showed an approximate change law, that is, the weight gain was faster in the initial stage of corrosion (0–10 h), and the rate tended to be stable as time went on. The yttrium modified aluminide coating deposited at 1050 °C had the least corrosion weight gain (0.0100 mg·cm
−2 at 75 h), and the sample deposited at 950 °C had the most corrosion weight gain (0.0151 mg·cm
−2 at 50 h), and there was a trend of weight loss at 50–75 h. The weight gain decreased from 0.0151 mg·cm
−2 to 0.0133 mg·cm
−2, indicating that the yttrium-modified coating deposited at 950 °C was seriously corroded. The corrosion weight gain of the three groups of yttrium-modified aluminide coatings was smaller than that of the single aluminide coating, indicating that the thermal corrosion resistance of the yttrium-modified aluminide coating is better than that of the single aluminide coating. The corrosion weight gain of the two groups of modified aluminide coatings deposited at 1000 °C and 1050 °C kept a slow growth trend, indicating that the coating has a good protection effect. The thermal corrosion resistance of the modified coating deposited at 1000 °C and 1050 °C is close, and better than that deposited at 950 °C. The modified aluminide coating with more yttrium content has better resistance to high temperature corrosion.
Figure 11 shows the XRD patterns of single aluminide coating and three groups of yttrium-modified aluminide coatings on the K444 alloy after corrosion at 900 °C-Na
2SO
4 + NaCl for 75 h. It was found that the single aluminide coating has Al
2O
3, Cr
2O
3, and γ-Ni
3Al phases, but no β-NiAl phase, which shows that the coating damage is more serious. The structure of the three groups of modified aluminide coatings is similar, with Al
2O
3, TiO
2, Cr
2O
3, β-NiAl and γ-Ni
3Al phases, indicating that the three groups of modified coatings still have a good function of protecting the internal matrix.
Figure 12 shows the cross-sectional microstructure of single aluminide coating and yttrium-modified aluminide coatings deposited at 950 °C, 1000 °C, and 1050 °C and element line scan maps of Al, Ti, O, Ni, Cr, and S on the K444 alloy after corrosion at 900 °C-Na
2SO
4 + NaCl for 75 h. The direction of arrows in the
Figure 12a–d indicates the element line scanning direction.
Figure 12a is single aluminide coating deposited at 1050 °C, (b), (c), and (d) are the three groups of yttrium-modified aluminide coatings on the K444 alloy deposited at 950 °C, 1000 °C, and 1050 °C. As can be seen from this figure, the outer layer of the single aluminide coating and the yttrium-modified coating deposited at 950 °C are mainly NiO compounds, and the inner layer is Al
2O
3, Cr
2O
3, and TiO
2. The content of S in the matrix is relatively high, internal vulcanization occurs, and the coating is seriously damaged.
Figure 13 shows the thickness curves of the corrosion products of the K444 alloy with single aluminide coating and three groups of yttrium-modified aluminide coatings after salt hot corrosion for 75 h. The thickness of the corrosion product of the single aluminide coating is about 29.3 μm, the corrosion product appears in the matrix, and the local damage of the coating is serious. The thickness of the corrosion products of the yttrium-modified aluminide coating deposited at 950 °C is about 26.8 μm, the thickness of the corrosion products of the modified coating deposited at 1000 °C is about 18.5 μm, and the thickness of the corrosion products of the modified coating deposited at 1050 °C is about 9.8 μm, indicating that the thermal corrosion resistance of the three groups of yttrium-modified aluminide coating is better than that of the single aluminide coating. Compared with the three groups of modified coatings, the thermal corrosion resistance of yttrium-modified aluminate coating deposited at 1050 °C is better than that of the modified coating deposited at 1000 °C, and both are better than that of the modified coating deposited at 950 °C.
Yttrium can improve the toughness of the coating, thus effectively slowing down the cracking and falling off of the oxide film. Moreover, the yttrium can inhibit the outward diffusion of Cr, Ti, and other elements, and inhibit the inward diffusion of S, thus improving the thermal corrosion resistance of the coating. The higher the content of yttrium, the better the thermal corrosion resistance of the coating.
Figure 14 shows the surface morphology of the (a) single aluminide coating and yttrium-modified aluminide coatings deposited at (b) 950 °C, (c) 1000 °C, and (d) 1050 °C on the K444 alloy after corrosion at 900 °C-Na
2SO
4 + NaCl for 75 h. The single aluminide coating has corrosion holes, and the yttrium-modified coating deposited at 950 °C has corrosion pits, while the samples deposited at 1000 °C and 1050 °C have dense surface coatings without obvious defects.
Table 3 shows the surface scan element content of single aluminide coating and three groups of yttrium-modified aluminide coating on K444 alloy after corrosion at 900 °C-Na
2SO
4 + NaCl for 75 h that are corresponded to
Figure 14a–d. By synthesizing
Figure 14 and
Table 3, it can be found that S (0.33 wt.%) and a small number of matrix elements (0.23 wt.% W and 0.29 wt.% Ta) were detected on the surface of the single aluminide coating. It may have internal vulcanization, and the local damage of the coating is relatively serious. The S element (0.28 wt.%) was detected on the surface of the yttrium-modified aluminide coating deposited at 950 °C, but no matrix element is detected. The thermal corrosion resistance of the coating is better than that of the single aluminide coating. The surface of the modified coating deposited at 1000 °C is mainly Al
2O
3 and TiO
2, with a small amount of Cr
2O
3, and no S is found, indicating that the coating has a certain degree of damage, but it still has a good protective effect. The surface of the sample deposited at 1050 °C is mainly Al
2O
3, with a small amount of TiO
2 and Cr
2O
3, indicating that the coating is not corroded and has good thermal corrosion resistance.