Microstructure evolution and adhesion properties of thick Cr coatings under different thermal shock temperatures

Highlights

• The thermal shock performance of thick Cr coating was systematically studied.

• The degradation of coating properties was the severe diffusion and migration of Cr and Zr at near-eutectic temperature.

• The formation of pores in Cr₂O₃ layer may be related to the evaporation of volatile CrO₂(OH)₂.

Abstract

The effect of thermal shock temperatures of 1000 °C, 1100 °C, 1200 °C and 1300 °C on the microstructure evolution and adhesion properties of thick Cr coatings on Zr-4 alloy substrates are systematically studied. The results show that the Cr coating has excellent thermal shock resistance even at 1300 °C and still adheres well to the substrate after the thermal shock tests. The main reasons for the degradation of coating properties are the severe diffusion and migration of Cr and Zr at near-eutectic temperatures, while the formation of pores on the Cr₂O₃ layer may be related to the evaporation of volatile CrO₂(OH)₂ at the grain boundaries of the Cr₂O₃ oxides. In addition, the microstructural evolution of thick Cr coatings is also discussed in detail.

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.