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Article

Effect of Al Content on the High-Temperature Oxidation Resistance and Structure of CrAl Coatings

by
Jiaojiao Ma
1,
Chuiyi Meng
1,
Hui Wang
2 and
Xiujie He
1,*
1
Sino-French Institute of Nuclear Engineering and Technology, Sun Yat-sen University, Zhuhai 519082, China
2
Interdisciplinary Materials Research Center, Institute for Advanced Study, Chengdu University, Chengdu 610106, China
*
Author to whom correspondence should be addressed.
Coatings 2021, 11(12), 1434; https://doi.org/10.3390/coatings11121434
Submission received: 13 October 2021 / Revised: 10 November 2021 / Accepted: 16 November 2021 / Published: 23 November 2021
(This article belongs to the Special Issue Friction, Wear Properties and Applications of Coatings)
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Versions Notes

Abstract

:
The oxidation behaviors of Cr, Cr93.4Al6.6, Cr58.1Al41.9, and Cr34.5Al65.5 coatings, deposited by using multi-arc ion plating technology, at high temperature were studied. The weight gain, oxide thickness, morphology, and phase composition of the coatings before and after oxidation were analyzed in detail. The results show that there is an Al content window available for tuning the oxidation behaviors of the CrAl-based coatings. The Cr93.4Al6.6 coating is considered to be most protective and can effectively improve the high-temperature oxidation resistance of the substrate; whereas, too high an Al content has a harmful effect on the antioxidant properties of the coatings. The oxidation mechanism of Cr and CrAl coatings were also discussed.

1. Introduction

Zr alloy is widely used in fuel cladding due to its superior high-temperature mechanical properties [1,2], high-temperature corrosion resistance, and low thermal neutron absorption cross-section. However, under severe accident conditions, such as loss of coolant accidents (LOCAs), fuel rods and cooling water cannot circulate heat, and the reactor cannot be cooled in time. The fuel rod will continue to heat up under the action of decay heat. At high temperatures, Zircaloy will oxidize with water/vapor, causing the fuel cladding to fail and producing a large amount of hydrogen [3] that accumulates in the shell mixed with oxygen and air and then explodes. This is how the explosion at the Fukushima nuclear power plant in March 2011 came about. Therefore, the world nuclear industry proposed accident tolerant fuel (ATF), by adding a coating to the surface of the traditional Zircaloy cladding, to improve its resistance to high-temperature water vapor oxidation. In the high-temperature oxidation environment of the reactor, the oxidation resistance of the ATF cladding depends on the formation of attached, non-volatile, and crack-free oxide scale. Nowadays, Cr is considered an ideal material to study as a hard protective coating with high hardness, corrosion resistance, and oxidation resistance [4,5,6,7,8]. There have been extensive studies on the oxidation resistance of Cr coatings under LOCA conditions. The Atomic Energy Commission (CEA) in the framework of the French Nuclear Institute, in partnership with AREVA and Electricite De France (EDF), used magnetron sputtering to deposit a Cr coating on the surface of the Zircaloy-4 (Zr-4) substrate, and then the samples were oxidized under different high-temperature water vapor environments. The results showed that the Cr coating greatly improved the high-temperature steam antioxidant ability of the substrate [9,10,11,12]. Park et al. [5], at the Korea Institute of Atomic Energy, used arc ion plating technology to prepare a Cr coating on the surface of Zr-4. The oxidation test resulted in a water vapor environment at 1200 °C and 2000 s and showed that the high-temperature oxidation resistance of the coated zirconium alloy was significantly stronger than that of the zirconium alloy. The substrate and the Cr-coated zirconium cladding have better ductility. He et al. [13] used multi-arc ion plating technology to prepare Cr coatings on Zr-4 alloy under different air pressures and bias voltages and studied the Cr coatings in the air at different temperatures (the highest temperature was 1060 °C). In the oxidation behavior, the Cr coating forms a dense Cr2O3 layer on the surface to prevent oxygen from diffusing inward and has a protective effect on the Zr alloy matrix at high temperatures. Wei et al. [14] found that, when a 20 µm Cr coating is oxidized in the air at 800 °C, a dense Cr2O3 film is formed on the surface, which has excellent oxidation resistance. When oxidized at 1200 °C, Cr2O3 and O2 generate volatile CrO3, causing cracks in the Cr2O3 oxide film and destroying the oxidation resistance of the film. The main drawback for high-temperature applications of the aforementioned conventional Cr coatings is their limited oxidation resistance and oxidization to volatile CrO3 above around 1000 °C.
As known, α-Al2O3 scale remains protective at oxidation temperatures higher than 1000 °C, due to its stable thermodynamics and low growth kinetics [15]. According to Zhong et al. [16], CrAl coatings have higher oxidation resistance because both Cr and Al can form protective oxides that inhibit oxygen diffusion; whereas during high-temperature oxidation, Al consumption to the intermetallic phase formation is significant. The study on laser coating of a CrAl layer on Zr-4 alloy also showed that the mixing of the CrAl coating with Zr deteriorated the oxidation resistance [17]. Therefore, in order to further improve the oxidation performance of the CrAl coating, the atomic ratio, preparation method, and oxidation mechanism of the CrAl coating still need to be further studied.
In this work, we performed a systematic study of the oxidation properties of four kinds of coatings with different CrAl ratios prepared by multi-arc ion plating. The CrAl coating with 6.6% Al atomic ratio had the best oxidation performance. At the same time, the oxidation mechanism of Cr coating and Cr93.4Al6.6 coating was also compared. This work will provide data support for the development of CrAl coatings with excellent oxidation properties.

2. Experimental Methods

2.1. Coating Deposition

By multi-arc ion plating technology, Cr, Cr93.4Al6.6, Cr58.1Al41.9, and Cr34.5Al65.5 coatings were prepared on a Zr-4 substrate using four kinds of CrAl targets (with atomic ratios of Cr:Al 100:0, 90:10, 50:50, and 30:70, respectively), as depicted in Table 1. According to the Cr-Al phase diagram, at different temperatures, different ratios of CrxAly coatings generated a variety of compounds, as shown in Figure 1 [18]. The target size was ϕ100 mm × 13 mm. The size of the Zr-4 sheet (Western Energy Materials Technologies Co., Ltd., Xi’an, Shanxi, China) was 25 mm × 15 mm × 1.5 mm, and the chemical composition of its main elements is shown in Table 2. Before deposition, 100#, 240#, 360#, 600#, and 800# water sandpaper was used to polish the substrate step by step; then it was ultrasonically cleaned in a mixture of acetone and absolute ethanol for 10 min and dried for later use. The Zr-4 substrates were mounted on a rotating sample fixture inside the deposition system, four arc targets were installed around the deposition chamber to ensure the uniformity of the deposited coating. When the vacuum of the vacuum chamber reached about 0.008 Pa, the deposition process started, and Ar gas was introduced, followed by Ar+ cleaning and target bombardment cleaning. Then the bias voltage was changed to 200 V to deposit a transition layer. Finally, while preparing the CrAl coatings, the bias voltage was set to −75 V [13], the target current to 100 A, the temperature to 360 °C, the gas pressure to 1.3 Pa, and the film deposition time to 4 h.

2.2. Oxidation Experiment

The coating was subjected to a high-temperature oxidation experiment using the oxidation weight gain method. After the muffle furnace was heated to 1160 °C, the specimens were rested in a crucible and placed into the furnace. High-temperature oxidation tests were performed in air at 1160 °C for 1 h/2 h. The heating rate of the furnace was set at 10 °C/min. After the oxidation tests, the sample was taken out for natural cooling, weighed, and recorded with an electronic balance.

2.3. Characterization

Samples for the characterization of the cross-section were cut from the as-prepared coated specimen using the spark-erosion wire cutting method and then carefully ground and polished according to the standard procedure. The surface and cross-sectional morphologies of the samples with the imaging mode of secondary electron were characterized by scanning electron microscopy (SEM, Tescan Mira 3, Brno, Czech Republic). Energy-dispersive X-ray spectroscopy (EDS, EDAX, Mahwah, USA) elemental mapping images and line profiles operated at 20 kV were acquired to characterize the distribution of the main elements. X-ray diffraction (XRD, Ultima IV, Tokyo, Japan) θ–2θ scans using Cu Kα light source were performed to better reveal the phases in the samples.

3. Results and Discussion

3.1. Oxidation Resistance

Firstly, we compared the average corrected weight gain and oxide thickness (see Table 3) of each coating after isothermal oxidation at 1160 °C for 1 h. The latter was the average of a large number of measurements obtained from multiple metallographic sections of each oxidized sample. After being oxidized at 1160 °C for 1 h, it was found that the Cr93.4Al6.6 coating had the least weight gain due to oxidation. Although the Cr coating had the least oxide thickness, the oxidation weight gain of the Cr coating was about 28.8% larger than that of the Cr93.4Al6.6 coating. This result shows that the oxide thickness is not completely positively correlated with oxidative weight gain.
Figure 2 shows the oxidation weight gain curve of the deposited CrAl coatings with different Al contents at 1160 °C. It can be seen from the result that the curve of oxidation at 1160 °C for 1 h was divided into three trends: (1) When the Al content increased to a critical value, which was 6.6% in this study, the oxidative weight gain decreased. (2) As the Al content reached 41.9%, oxidation weight gain increased slowly. (3) As the Al content increased to 65.5%, oxidation weight gain increased sharply.
For the sake of further testing of the oxidation performance of these CrAl coatings with different Al content, a 2 h oxidation experiment was also performed at 1160 °C. The Cr58.1Al41.9 and Cr34.5Al65.5 coatings fell off due to severe oxidation. Therefore, Figure 2 only shows the oxidation weight gain results of the Cr and Cr93.4Al6.6 coatings oxidized at 1160 °C for 2 h. It can be seen from the result that the oxidation weight gain of the Cr93.4Al6.6 coating sample was 52.13% lower than that of the Cr coating sample. The results show that the superiority of the oxidation resistance of the Cr93.4Al6.6 coating became more obvious when the oxidation time was longer.

3.2. Coating Phase

In order to explore the effect of Al content on the oxidation performance of CrAl-based coatings, it is necessary to characterize the phase of the coatings in detail. Figure 3a shows XRD patterns of as-deposited coatings prepared on Zr-4 substrates. Tracks of the Cr-Al and Zr-Al intermetallics were hardly detected in Cr93.4Al6.6 in the XRD pattern, implying that the intermetallic phase did not form during the depositional process at the selected deposition parameters and at the certain Al content (6.62 at.%). Similar to the case for the Cr coating, the phase corresponding to the XRD peak of the Cr93.4Al6.6 coating was still mainly Cr. However, the peak positions corresponding to Cr phase in Cr93.4Al6.6 coating tended to shift to smaller angles than the Cr peak positions in Cr coating. According to the Bragg formula, this shift indicates that the lattice constant of the Cr phase in Cr93.4Al6.6 coating is slightly larger than that in Cr coating, which is due to the fact that (1) Al atoms with a larger radius (1.43Å) enter the crystal lattice of Cr (1.27Å) to form a solid solution structure and (2) different deposition rates result in different residual stresses in the coatings. In addition, after adding 6.6 at.% Al in the Cr coating, the preferred growth orientation of Cr phase changed from (200) to (110). This indicates that the solid solution structure formed by Al doping can significantly reduce the surface energy of the (110) plane during the deposition of the Cr coating. The Al-Cr phases in the other two as-deposited coatings with higher Al content were indexed as follows: Cubic Al3Cr7 with the space group of Im-3 m and shifted Al8Cr5 peak position for Cr58.1Al41.9 coating, cubic Al8Cr5 with the space group of I-43 m for the Cr34.5Al65.5 coating. There are two potential reasons for the shift of Al8Cr5 peaks in Cr58.1Al41.9 coating: the low coating deposition temperature [19] and off-stoichiometry of the coating composition. At the coating-substrate interface, due to the interdiffusion of elements, a transition zone where Zr and Al coexist appeared, especially the existence of Al2Zr3, Al2Zr, and AlZr2 intermetallic compounds, which buried hidden dangers for subsequent oxide layer cracking.
Figure 3b shows the XRD patterns of the Cr and CrAl coating samples after oxidation at 1160 °C for 1 h. According to the XRD spectra, the Al3Cr7 and Al8Cr5 peaks in the Cr58.1Al41.9 and Cr34.5Al65.5 coatings disappeared completely. Al3Cr7 and Al8Cr5 phases transformed into the AlCr2 phase on the one hand and were oxidized to Al2O3 and Cr2O3 on the other hand. Due to the limited protection of Cr58.1Al41.9 and Cr34.5Al65.5 coatings, O diffused into the substrate to oxidize Zr to ZrO2. For the Cr and Cr93.4Al6.6 coatings, the Cr peaks did not completely disappear after the high-temperature oxidation, which proved that the coating itself had not been completely oxidized. Although the α-Al2O3 film has excellent oxidation resistance to prevent O from diffusing inward [20], in the oxidized Cr93.4Al6.6 coating, no signal of Al2O3 was detected. As in the case of the oxidized Cr coating, only the peak of Cr2O3 was detected by XRD. It can be assumed that Al2O3 is amorphous together with crystal Cr2O3 [21], or the amount of Al2O3 is below the detection limit of XRD. In addition, in the XRD spectra of the oxidized Cr and Cr93.4Al6.6 coatings, within the XRD detection accuracy, no Zr oxide was detected. This shows that the Cr and Cr93.4Al6.6 coatings were still protective after being oxidized at 1160 °C for 1 h.

3.3. Coating Composition and Morphology

The composition and thickness of the deposited coatings are shown in Table 1. The atomic percentage of Cr and Al in the CrAl coating was different from the atomic ratio of the target. This is because the ionization rates of Cr and Al are different, and the difference in sputtering effect of the two will affect the atom number ratio of the coating. In addition, the thickness of the A, B, C, and D coatings measured by SEM were 8.08, 17.48, 18.05, and 18.34 μm, respectively. Although the deposition time and conditions were the same, the CrAl coating was thicker than the Cr coating. This is mainly because the melting point of Al (660.3 °C) is lower than that of Cr (1907 °C); therefore, the target material was easily melted during the deposition process after doping with Al atoms.
The initial surface morphology of the CrAl coatings is shown in Figure 4a–d. Similar to the case for the surface of other coatings deposited by multi-arc ion plating [22,23,24,25,26,27], droplet-like particles appeared on the surface of our four coatings. The surface of sample A was flatter and more homogeneous, whereas a greater number of larger droplets appeared on the surface of samples B, C, and D. This is mainly because the melting point of Al is lower than that of Cr, and therefore, it is easy to form large droplets during the deposition process. When the Al content reaches 41.9%, there are aggregated spherical grains on the surface of the coating, and there are pores, resulting in a greater probability of oxygen diffusing into the coating during oxidation process.
After oxidation in air for 1 h, the surface of the Cr coating was covered with oxide crystal grains, with a size of about 1.10 μm, as shown in Figure 4e. When 6.6 at.% Al was introduced into the Cr coating, the surface oxides became dense and the particle size was reduced to 0.61 μm, as shown in Figure 4f. The O ions and the Al ions that diffuse toward the gas meet somewhere in the existing oxide film, and the two react to form Al2O3 oxide crystals [28]. The oxide particle size of the Cr58.1Al41.9 coating and the Cr34.5Al65.5 coating after oxidation was larger and looser than that of the Cr93.4Al6.6 coating. As a result, its resistance to high-temperature oxidation was reduced, which is consistent with the result of oxidative weight gain (see Figure 2). When the Al content continued to rise to 65.5 at.%, cracks appeared on the surface after oxidation. After oxidation in air for 2 h, cracks also began to appear on the surface of the Cr coating. The surface of the Cr93.4Al6.6 coating was more uneven than the surface after oxidation for 2 h, but no cracks appeared on the surface. For the two coatings with higher Al content, namely, the Cr58.1Al41.9 and Cr34.5Al65.5 coatings, the coatings began to fall off severely (not shown here). From the surface morphology after oxidation, it can also be directly seen that the Cr93.4Al6.6 coating should have the best oxidation resistance.
The initial cross-sectional morphology of the CrAl-based coatings is shown in Figure 5a–d. Clearly, the substrate was completely covered by a dense coating. Due to the presence of a transition layer, the coating was well combined with the substrate without obvious gaps. Compared with as-deposited Cr coatings, as-deposited CrAl-based coatings had pores. According to the report of Polcar et al. [19], it is inferred that the pores in the CrAl coatings are caused by the detachment of larger droplets during the initial growth of the coating, which were, consequently, covered by newly deposited coating.
It is worth noting that, compared with the Cr coating after 1 h oxidation in air, the Cr93.4Al6.6 coating contain dispersed, isolated voids and microporosity. Small voids may coarsen into larger isolated pores, possibly as a result of Al selective oxidation in them. EDS analysis shows that the pore area contained 53.05% of Al and 8.54% Cr, which is rich in Al and poor in Cr; while other areas contained 9.30% of Al and 48.32% of Cr, which was rich in Cr and poor in Al. The results indicate that during the oxidation process, Cr and Al elements segregated and oxidized under the action of high temperature. Void formation may also arise during the transformation of γ-A12O3 to α-A12O3, which occurs with a volume contraction of 14.3% [29]. Clearly, with the increase of Al content, the porosity in CrAl coatings increased after oxidation. After 2 h of oxidation, the Cr coating became rather uneven due to the volatilization of CrO3 and the diffusion of Cr to the substrate. With the extension of the oxidation time, the pores in the Cr93.4Al6.6 coating gradually increased. For the Cr58.1Al41.9 and Cr34.5Al65.5 coatings, the enlarged pores aggravated the generation of internal stress, making the coating weak, and causing the oxide layer to locally crack or peel off at high temperature. Next, the substrate will also begin to be oxidized.
The cross-section of the coating was further analyzed by SEM/EDX, and interesting information about the element distribution was obtained. After the Cr93.4Al6.6 coating is oxidized in high-temperature air, Al will diffuse to the surface and form (Al,Cr)2O3 together with Cr and O. In the pores of the coating, the amount of O is higher than that of adjacent locations, and the content of Al is obviously higher than that of Cr, which indicates that Al in the pores will form Al2O3 with the diffused O, as shown in Figure 6. Due to the limited Al content, there was no obvious sign of Al diffusion to the Zr substrate in the Cr93.4Al6.6 coating, and the inter-diffusion mainly occurred between Cr and Zr.
In the Cr58.1Al41.9 coating large mass fraction of Al, the standard Gibbs free energy of Al2O3 (−1424.887 kJ/mol) was obviously more negative than that of Cr2O3 (−926.911 kJ/mol). Therefore, compared to Cr, Al will preferentially diffuse to the surface of the coating and oxidize to form a continuous Al2O3 dense layer, and prevent the diffusion and migration of Cr on the surface of this area, as shown in Figure 7. Like the Cr93.4Al6.6 coating, Al2O3 will still be generated in the pores of the Cr58.1Al41.9 coating, and the Al-rich regions in the pores and Cr-poor regions correspond on a one-to-one basis. However, according to EDS results, the Al content of the coating surface was significantly higher than the Al content of the pores inside the coating. Compared with Cr93.4Al6.6 coating, Cr58.1Al41.9 coating had larger pores. In addition, a small amount of Al began to diffuse into the Zr substrate.
When the Al content continues to increase to 65.5%, the formation of α-Al2O3 and Al diffusion will be accompanied by a large number of cracks and pores, as shown in Figure 8. This makes it easier for O to diffuse through the coating to the substrate, resulting in a substantial increase in oxidative weight gain. Compared with the Cr58.1Al41.9 coating with a lower Al content in Figure 7, the section of the Cr34.5Al65.5 coating with a higher Al content showed more dense “black holes”. EDS analysis shows that the “black holes” were rich in Al, whereas the white area outside the holes was rich in Cr. Further, the Al diffused into the Zr substrate also increased.
According to the above results, it is obvious that adding 6.6% of Al can improve the oxidation performance of the Cr coating, but an excessively high Al content will instead cause a decrease in the oxidation performance. Nevertheless, the influence of coating thickness on oxidation performance is not considered in this work, which needs further study in future work.

3.4. Oxidation Mechanism of Cr and CrAl Coatings

All of the oxidized coatings demonstrated porous structures, as shown in Figure 5e–j. In addition, the higher the Al content, the higher the porosity after oxidation, and the larger the pore size. For Cr coating oxidized for 1 h, these pores mainly existed either at the coating/substrate phase interface or at the Cr2O3/coating interface, as shown in Figure 9a. The voids at the coating/substrate interface are caused by the Kirkendall Effect [30,31]. At the boundary between Cr coating and substrate diffusing into each other at different rates, their intermetallic phase grows, and unfilled vacancies coalesce into large voids. The porosity at the Cr2O3/coating interface is attributed to the outward growth of the Cr2O3 layer and the Cr needed for this growth. Similar to Kirkendall effect, outward Cr diffusion from the coating must be accompanied by a counter flow of vacancies across the Cr2O3/coating interface. When the oxidation time of the Cr coating is extended to 2 h, the protective effect of the Cr coating becomes rather limited. The oxidation process involves outward intergranular diffusion of Zr through the residual metallic (un-oxidized) Cr layer with subsequent formation of a zirconia intergranular network [9], as shown in Figure 9b. With the high inward diffusion flux of oxygen at the Zr/Cr interface, the prior ZrCr2 intermetallic phase layer gradually disappeared and transformed back into metallic Cr grains together with zirconia grains, according to the assumed chemical reaction described below:
ZrCr2 + O2 → ZrO2 + 2Cr,
When 6.6 at.% Al was added to the Cr coating, there was not enough Al to form a new compound with Cr, but it existed in the Cr crystal lattice in the form of a solid solution. When the Cr93.4Al6.6 coating is oxidized at 1160 °C for 1 h, Al will inhibit the diffusion and oxidation of Cr. Therefore, no pores were found at the coating/substrate interface or at the oxide layer/coating interface, as shown in Figure 9c. Nevertheless, there will be Al- and O-rich pores inside the Cr93.4Al6.6 coating. After prolonging the oxidation time to 2 h, Al-rich pores in the Cr93.4Al6.6 coating are more concentrated in the area near the coating surface, which is caused by the further diffusion of Al, as shown in Figure 9d. However, the Cr93.4Al6.6 coating still adhered well to the substrate and prevented O from invading the substrate, showing more excellent oxidation resistance than the Cr coating.
When the Al content continues to increase, after 1 h of oxidation, the oxide layer on the surface of the CrAl coating will be obviously stratified, with a layer of Al2O3 being covered on the outside of the Cr2O3, as shown in Figure 9e. The phase of the outermost Al2O3 is mainly α-Al2O3, which is mainly because the pre-formation of Cr2O3 provides a structural template to stimulate the growth and stabilize the rhombohedral structure of crystalline α-Al2O3 [32,33]. The main reactions in the oxidation process include the following:
4Al + 3O2 → 2Al2O3,
4Cr + 3O2 → 2Cr2O3,
Cr2O3 + 2Al → Al2O3 + 2Cr,
Reaction (4) was considered reasonable in the oxidation process, which was because the standard free energy change of Equation (2) was much smaller than that of Equation (3) [34]. Thus, the formation of Al2O3 was realized mainly via two methods, either based on the corundum-type Cr2O3, which formed as cores in the initial stage of the oxidation process for the growth of α-Al2O3, or through Equation (4), in which Cr in the corundum-type Cr2O3 was replaced by Al, and the original lattice remained. Due to the diffusion of Al and Cr elements, many holes appear in the coating, and Al-Zr and Cr-Zr compounds are formed between the coating and the substrate. When the oxidation time was extended to 2 h, the coating was completely peeled off, as shown in Figure 9f. The ability of the CrAl coatings to protect the substrate was lost and the Zr alloy was severely oxidized.
It is worth mentioning that a steam environment is indeed the expected environment for coated cladding in an accident scenario, and most investigations of core degradation during severe nuclear reactor accidents consider oxidation of metal core components by steam only. Nevertheless, it is still very meaningful for us to study the high-temperature behavior of CrAl-based coatings in air. This is because (i) the reaction mechanism of zirconium with steam or water is similar to that of oxygen, and the air contains 20.9% oxygen; (ii) our work is mainly to investigate the influence of Al element on the oxidation performance of Cr coating in a high-temperature environment; (iii) the coating deteriorates more severely in high-temperature air than in oxygen and steam [35,36]. If the coating has excellent performance in high-temperature air, its performance in the steam environment will be more reassuring.

4. Conclusions

In this study, four CrAl-based coatings with different Al content were prepared by multi-arc ion plating method, and their oxidation performance and mechanism were characterized in detail. The details can be summarized as follows:
(1) The Cr-Al ratio had obvious effects on the morphology, phase, and oxidation resistance of the coatings. When the Al content was 6.6 at.%, the phase in the CrAl coating was still the Cr phase, and Al existed in the Cr lattice in the form of a solid solution. With the increase of Al content, Al3Cr7 and Al8Cr5 phases gradually appeared. High Al content resulted in the formation of a loose structure and was unfavorable to the high-temperature oxidation resistance of the coating. The CrAl coating with an Al content of 6.6 at.% had the best oxidation resistance.
(2) The solid solution strengthening of Al can enhance the oxidation resistance of the Cr coating. The Cr2O3 + Al2O3/CrAl/Zr-Cr composite layer can effectively protect the substrate. However, the excessive introduction of Al can cause the coating to crack or fall off during the oxidation process.

Author Contributions

Conceptualization, X.H.; methodology, J.M.; software, J.M.; validation, H.W.; formal analysis, H.W.; investigation, J.M. and C.M.; resources, J.M.; data curation, J.M.; writing—original draft preparation, J.M.; writing—review and editing, X.H. and H.W.; visualization, C.M.; supervision, X.H.; project administration, X.H.; funding acquisition, X.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Guangzhou Science and Technology Plan Project (no. 202102020989) and the Fundamental Research Funds for the Central Universities, Sun Yat-sen University (no. 2021qntd12).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data sharing is not applicable to this article.

Conflicts of Interest

The authors declare no conflict of interest.

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Coatings 11 01434 g001 550
Figure 1. Cr-Al phase diagram [18].
Figure 1. Cr-Al phase diagram [18].
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Coatings 11 01434 g002 550
Figure 2. Oxidation weight gain of CrAl-based coatings with different Al contents at 1160 °C. The peeling coating was also included for the higher Al coatings.
Figure 2. Oxidation weight gain of CrAl-based coatings with different Al contents at 1160 °C. The peeling coating was also included for the higher Al coatings.
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Coatings 11 01434 g003 550
Figure 3. XRD diffractograms of the CrAl-based coatings (a) before and (b) after oxidation at 1160 °C for 1 h.
Figure 3. XRD diffractograms of the CrAl-based coatings (a) before and (b) after oxidation at 1160 °C for 1 h.
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Coatings 11 01434 g004 550
Figure 4. Surface morphology of CrAl-based coatings deposited on Zr-4 alloy (a–d) before and after oxidation for (e–h) 1 h and (i,j) 2 h. (a,e,i) Cr; (b,f,j) Cr93.4Al6.6; (c,g) Cr58.1Al41.9; (d,h) Cr34.5Al65.5.
Figure 4. Surface morphology of CrAl-based coatings deposited on Zr-4 alloy (a–d) before and after oxidation for (e–h) 1 h and (i,j) 2 h. (a,e,i) Cr; (b,f,j) Cr93.4Al6.6; (c,g) Cr58.1Al41.9; (d,h) Cr34.5Al65.5.
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Coatings 11 01434 g005 550
Figure 5. Cross-sectional morphology of CrAl-based coatings deposited on Zr-4 alloy (a–d) before and after oxidation for (e–h) 1 h and (i,j) 2 h. (a,e,i) Cr; (b,f,j) Cr93.4Al6.6; (c,g) Cr58.1Al41.9; (d,h) Cr34.5Al65.5.
Figure 5. Cross-sectional morphology of CrAl-based coatings deposited on Zr-4 alloy (a–d) before and after oxidation for (e–h) 1 h and (i,j) 2 h. (a,e,i) Cr; (b,f,j) Cr93.4Al6.6; (c,g) Cr58.1Al41.9; (d,h) Cr34.5Al65.5.
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Figure 6. SEM images and EDS patterns of Cr93.4Al6.6-coated Zr-4 after oxidation for 1 h at 1160 °C.
Figure 6. SEM images and EDS patterns of Cr93.4Al6.6-coated Zr-4 after oxidation for 1 h at 1160 °C.
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Coatings 11 01434 g007 550
Figure 7. SEM images and EDS patterns of Cr58.1Al41.9-coated Zr-4 after oxidation for 1 h at 1160 °C.
Figure 7. SEM images and EDS patterns of Cr58.1Al41.9-coated Zr-4 after oxidation for 1 h at 1160 °C.
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Figure 8. SEM images and EDS patterns of Cr34.5Al65.5-coated Zr-4 after oxidation for 1 h at 1160 °C.
Figure 8. SEM images and EDS patterns of Cr34.5Al65.5-coated Zr-4 after oxidation for 1 h at 1160 °C.
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Figure 9. Schematic diagram of the oxidation process of Cr-coated and CrAl-coated Zr-4 at 1160 °C. Figure 9b is adopted from the diagram in Brachet’s work [9].
Figure 9. Schematic diagram of the oxidation process of Cr-coated and CrAl-coated Zr-4 at 1160 °C. Figure 9b is adopted from the diagram in Brachet’s work [9].
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Table
Table 1. Actual composition (at.%) of Cr-Al coatings analyzed by SEM/EDS.
Table 1. Actual composition (at.%) of Cr-Al coatings analyzed by SEM/EDS.
Composition of Cr–Al Coating (at.%) Thickness (μm)
TargetCoating NotationCrAlDeposition Time (h)
CrA100048.08
Cr90Al10B93.46.6417.48
Cr50Al50C58.141.9418.05
Cr30Al70D34.565.5418.34
Table
Table 2. Chemical composition of Zr-4 cladding material used in present investigation (wt.%).
Table 2. Chemical composition of Zr-4 cladding material used in present investigation (wt.%).
ONHfCrFeSnZr
0.160.0080.010.07 ~ 0.130.18 ~ 0.241.2 ~ 1.7Balance
Table
Table 3. Weight gain and oxide thickness after isothermal oxidation for 1 h at 1160 °C.
Table 3. Weight gain and oxide thickness after isothermal oxidation for 1 h at 1160 °C.
CoatingMean Corrected
Weight Gain (mg/cm2)		Oxide Thickness
(μm)
Cr3.165.52
Cr93.4Al6.62.2512.01
Cr58.1Al41.919.018.95
Cr34.5Al65.591.622.81
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Ma, J.; Meng, C.; Wang, H.; He, X. Effect of Al Content on the High-Temperature Oxidation Resistance and Structure of CrAl Coatings. Coatings 2021, 11, 1434. https://doi.org/10.3390/coatings11121434

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Ma J, Meng C, Wang H, He X. Effect of Al Content on the High-Temperature Oxidation Resistance and Structure of CrAl Coatings. Coatings. 2021; 11(12):1434. https://doi.org/10.3390/coatings11121434

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Ma, Jiaojiao, Chuiyi Meng, Hui Wang, and Xiujie He. 2021. "Effect of Al Content on the High-Temperature Oxidation Resistance and Structure of CrAl Coatings" Coatings 11, no. 12: 1434. https://doi.org/10.3390/coatings11121434

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