Microstructure evolution and adhesion properties of thick Cr coatings under different thermal shock temperatures
Introduction
The use of Cr coatings has become the most promising prospect for accident-tolerant fuel coatings (ATFCs) due to their excellent high temperature steam oxidation resistance [[1], [2], [3]], irradiation resistance [[4], [5], [6]] and corrosion resistance [1,7]. Among the selection of Cr coating preparation technologies, the deposition rate of coatings is the primary consideration in manufacturing full-length Cr-coated Zr alloy fuel claddings; additionally, it is crucial to evaluate the adhesion strength of the coating/substrate as well as the thickness uniformity for engineering applications. Based on these results, the reported coating preparation technologies mainly include a special PVD system [8], cold spraying [[9], [10], [11], [12]], 3D laser coating [13,14], arc ion plating [7,[15], [16], [17]], electroplating [6,18] and magnetron sputtering [1,[19], [20], [21], [22]]. Nevertheless, it can be noted that these studies mainly focus on the microstructure, mechanical properties, oxidation behaviour and corrosion resistance, and these have shown encouraging experimental results.
The original concept of ATFC is to effectively improve the accident-tolerant performance of zirconium alloy cladding under accident conditions or to substantially delay the embrittlement failure of the cladding under design basis accident (DBA) and beyond design basis accident (BDBA) conditions, thereby providing enough coping time to deal with subsequent problems. Moreover, the ATFCs have similar or even better performance than current commercial cladding under normal operating conditions [3,23,24]. In fact, a Cr coating can be used as an ATFC to protect Zr-4 alloy claddings. This protection can be attributed to the formation of a dense Cr2O3 layer under both corrosion conditions and high-temperature oxidation conditions, which can prevent the diffusion of oxygen to the substrate and delay the degradation of a nuclear reactor core during various accidents. However, it is necessary to determine whether Cr coatings can still protect Zr alloy substrates when a sudden temperature change occurs [25]. In other words, when a loss of coolant accident (LOCA) occurs, the coatings will face a severe thermal shock process once the coolant is reinjected, and it is very meaningful to investigate the structural integrity of Zr alloy fuel claddings protected by Cr coatings to avoid nuclear leakage accidents during this process. Unfortunately, the microstructure evolution of the oxide layer and degradation of coatings during thermal shock tests seem to have been overlooked, despite these issues being critical for evaluating the effectiveness of coatings. Hence, only by studying the thermal shock resistance of the coating can we further determine the fundamental feasibility of ATFCs.
In this study, based on our previous studies on the high-temperature steam oxidation performance of thick Cr coatings [22], the effect of different thermal shock temperatures on the microstructure evolution and adhesion properties of thick Cr coatings on Zr-4 substrates is further studied, and the differences in the thermal shock behaviour under different temperatures are comparatively discussed. In addition, the adhesion strengths of the coatings after the thermal shock tests are analysed to evaluate their protective effect.
Section snippets
Coating deposition
Thick Cr coatings were deposited on the two main sides of Zr-4 substrates (1 × 1 × 0.25 cm) by radio frequency magnetron sputtering (RFMS, Chengdu Qixing Vacuum Coating Technology Co., Ltd., China), as described previously and briefly reviewed here. Briefly, thick Cr coatings with a thickness of 27 μm were deposited on polished Zr-4 substrates at an RF power of 160 W in a 0.4 Pa Ar atmosphere. The substrate holder was heated to 400 °C, and the bias was −50 V during the deposition process.
Thermal shock tests
Thermal shock tests
Fig. 1 shows the surface appearance of thick Cr coatings before and after thermal shock tests. Fig. 1(a) indicates that the as-deposited thick Cr coating had a typical silvery-white metal surface, and the coating surface was complete and compact without obvious spallation. After the thermal shock tests at 1000 °C, 1100 °C and 1200 °C, the surface colour of the coatings changed significantly from silvery white to dark grey, and no cracks or spallation of the coatings were observed on the oxide
Conclusions
In this work, the effects of different thermal shock temperatures on the microstructure evolution and adhesion properties of thick Cr coatings on Zr-4 substrates were systematically investigated. Thick Cr coatings remained intact at thermal shock temperatures ranging from 1000 °C to 1200 °C, and the corresponding adhesion tests showed that these coatings remained well adhered to the Zr-4 substrates. After the thermal shock at 1300 °C, the main reasons for the degradation of coating properties
Author contribution statement
Qingsong Chen: Conceptualization, Experiment, Investigation, Writing - Original Draft, Writing - Review & Editing, Visualization.
Yang Xiang: Experiment, Investigation.
Zhuo Li: Experiment, Investigation.
Hengji He: Experiment, Investigation.
Yuxin Zhong: Experiment, Investigation.
Changda Zhu: Experiment, Investigation.
Ning Liu: Data Curation, Supervision.
Yuanyou Yang: Data Curation, Supervision.
Jiali Liao: Data Curation, Supervision.
Hong Chang: Resources.
Chunhai Liu: Conceptualization,
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant No. U2067218).
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