Friction and Wear Properties of Cr-Nx Coatings for Nuclear Fuel Cladding

Friction and wear resistance are important factors that affect the selection of accident-tolerant fuel coating materials. In this study, the wear behavior of a series of Cr-Nx coatings with different N contents was investigated using a reciprocating sliding tester. The coating morphology, change in the coefficient of friction during the friction and wear tests, and wear volume after the friction and wear tests, were characterized and discussed in detail. The results show that the Cr2N coating has better anti-friction and wear resistance behaviors than the Cr and CrN coatings under anhydrous and aqueous conditions. In addition, the water environment promoted the wear of the Zr-4 alloy and Cr coatings and inhibited the wear of the CrN and Cr2N coatings. The mechanisms of friction and wear were also discussed.


Introduction
Nuclear reactor fuel cladding is the first barrier between radioactive fuel rods and the environment and plays an important role in ensuring reactor safety [1]. At present, Zr alloys are widely used as nuclear reactor fuel cladding materials owing to their low neutron capture cross-section, thermal conductivity, and mechanical properties [2]. However, these alloys show a lack of protection ability under certain conditions, particularly during loss-ofcoolant accidents (LOCAs) [3,4]. When a LOCA occurs, the core temperature of the reactor increases sharply due to the loss of coolant, and Zr reacts violently with water steam at high temperatures: leading to the formation of zirconia and hydrogen [5]. Hydrogen gas is highly explosive when mixed with oxygen, which was the main cause of the 2011 Fukushima Daiichi nuclear disaster [6]. After the Fukushima nuclear accident, methods to improve the oxidation resistance of fuel cladding have been considered as extremely important in the fields of nuclear materials and safety. Therefore, a new reactor fuel system, comprising accident-tolerant fuel (ATF), was proposed to improve accident tolerance in light water reactors (LWRs). In addition to replacing UO 2 fuel by using materials such as U 3 Si 2 [7], UN [8], etc., with higher thermal conductivity, there are two main ways to improve the performance of fuel cladding in ATF systems. The first method is to replace the Zr alloy with untraditional fuel-cladding materials, such as SiC [9][10][11] and FeCrAl [4,[12][13][14]). The second one is to deposit a layer of coating on the surface of the Zr alloy to improve its oxidation resistance and mechanical performance under accident conditions [15]. Although the use of new cladding materials can fundamentally eliminate the issue of the high-temperature oxidation of the Zr alloys, the disadvantage of this method is that it is difficult to replace the original cladding in an operating nuclear power plant and it requires extensive research and extra funding [1]. Hence, based on a comprehensive consideration of its compatibility with existing nuclear power plants and economic conditions, coating technology has been widely studied because it can substantially improve the oxidation of the cladding without replacing the Zr alloys [1].
In recent years, many studies have been conducted on different ATF coatings. Meng et al. found that the weight gain of CrN-coated Zr-4 alloy after oxidation at 1160 • C decreased by 97.7% compared with that of the uncoated Zr-4 alloy [16]. Additionally, He et al. found that the Si in a CrSi-based coating improved the oxidation resistance performance of the Zr-4 alloy [17]. Effects of ion irradiation on Cr coatings of various thicknesses on a Zr alloy were also studied in detail [18]. In addition to oxidation and irradiation resistance, friction and wear resistances are also important factors that affect the selection of ATF-coating candidates. The flow of coolant around the fuel rods in the reactor primary circuit can cause the rods to vibrate, known as flow-induced vibration (FIV), which leads to the relative slip displacement between the fuel rod and the grid spring that causes the degradation of the fuel rod, known as grid-to-rod fretting (GTRF) [19][20][21][22]. Under conditions of high neutron irradiation and thermal relaxation, the grid-to-rod gap gradually increases, resulting in the increased vibration of the fuel rod under the primary coolant [23,24]. Notably, GTRF is the most common cause of failure in fuel cladding materials [25]. In studies on the friction and wear properties of Cr-based ATF coatings, Lee et al. found that a CrAl coating with a high-load support capability considerably improved the wear performance of the Zr-4 alloy by forming a load-bearing layer [26]. Our research group has thoroughly studied the influence mechanisms of Si [24] and Al [27] on the friction and wear properties of Cr-based ATF coatings, including the effect of N on the oxidation properties of Cr-based ATF coatings [16]. However, the effect of N on the friction and wear properties of Cr-based ATF coatings has not been studied in depth. As the introduction of N can directly change the crystal form and material properties of the coating (metal or ceramic), it is important to study the effects of N contents on the friction and wear properties of coatings.

Coating Deposition
In this study, a Zr-4 alloy (the chemical composition is listed in Table 1) was used as the substrate material. Before coating deposition, each substrate was cut via wire electrical discharge machining to the dimensions of 25 mm × 15 mm × 2.5 mm. A 2 mm diameter hole was punched near the edge of the Zr-4 sample before hanging the material in the deposition chamber. To eliminate the residual stress inside the material, it underwent an annealing treatment. The surface of the substrate was then ground using 240-, 600-, 800-, 1200-, 1500-, 2000-, 2500-, and 3000-grit SiC papers before the coating was deposited. Furthermore, automatic polishing was performed for 3 h using a vibratory polisher (Buehler, Chicago, IL, USA) to obtain a smooth surface for the friction and wear tests. It was then cleaned for 10 min using an ultrasonic cleaner (Keweixin, Wuxi, Jiangsu, China) with anhydrous ethanol and acetone as the cleaning solutions. Three ATF coatings with different compositions of Cr-N x (i.e., Cr, CrN, and Cr 2 N) were deposited by multi-arc ion plating with specific parameters ( Table 2). The schematic diagram of the multi-arc ion coating machine (Yulang, Linyi, Shandong, China) is shown in Figure 1. Table 1. Chemical composition (wt.%) of the Zr-4 cladding material used in this study [28].

Friction and Wear Experiment
The schematic diagram of the reciprocating sliding tester (model CFT-I) (Zhongkekaihua, Lanzhou, Gansu, China) used in this study is shown in Figure 2. The instrument was used for the friction and wear tests of these samples to determine the coefficient of friction (COF), which is an important index in determining the friction properties of a material. The friction and wear experiments were conducted at the room temperature of 27 • C under unlubricated (air) and lubricated (deionized water) conditions. The reciprocating stroke and reciprocating frequency were fixed at 5 mm and 13 Hz, respectively, while the test times were set at 5 min for the Zr-4 alloy and Cr coating and 20 min for the CrN and Cr 2 N coatings. As zirconia will be formed on the surface of the grid in the nuclear reactor, zirconia balls were used as the friction pairs in this study. The contact forces used were 1 N for the Zr-4 alloy and Cr coating and 10 N for the CrN and Cr 2 N coatings.

Characterization
To characterize the coatings, an X-ray diffractometer (theta-theta type) (Rigaku Ultima IV, Tokyo, Japan) using Cu Kα light source was used to determine the chemical composition of each coating sample. The diffraction angle was varied from 10 to 90 • . Scanning electron microscopy in conjunction with energy dispersive X-ray spectrophotometry (SEM-EDS) (Tescan Mira 3, Brno, Czech Republic) was used to observe the surface and cross-sectional morphologies of each coating and to determine the elemental distribution. A non-contact three-dimensional (3D) laser interferometer (NewView™ 8000, Zygo, Middlefield, CT, USA) was used to observe the 3D morphology of the scratches on the coating and to measure their corresponding wear volume and scratch depth. A Vickers hardness tester (Wilson, Lake Bluff, IL, USA) was used to measure the Vickers hardness of each coating sample.  (311) were also observed in the XRD patterns. Furthermore, in the XRD profile of the Cr 2 N coating, only the Cr 2 N peak was observed, indicating that under the selected deposition conditions, the Cr 2 N coating did not contain any CrN components. The surface morphologies of the Cr-N x coatings are shown in Figure 4. The figure shows the presence of microdroplets and shallow craters on the surface of the coatings. The appearance of these microdroplets is a typical characteristic of multi-arc ion plating. According to the point scanning results, the Cr content on microdroplets is significantly higher for CrN and Cr 2 N coatings, which further indicates that the microdroplets were because of the emission of the cathode Cr target material. The shallow craters were mainly formed owing to the following reasons: solid droplets that peeled off the coating surface, liquid droplets that shrank during the deposition, or solid droplets that bombarded into liquid droplets [29]. A comparison of the number of shallow craters on the surface of the coatings showed that the number of shallow craters increased with an increase in the amount of N introduced in the deposition chamber, which indicates that N can promote the formation of shallow craters. The cross-sectional morphologies of the Cr-N x coatings are shown in Figure 5. The coatings deposited with the selected parameters were dense and smooth. No signs of defects, such as pores inside the layer, were observed. Moreover, the coatings that were not peeled off or cracked were well integrated with the substrate. The average thicknesses of the Cr, CrN, and Cr 2 N coatings were 7.76, 21.05, and 12.16 µm, respectively. For the same deposition time of 6 h, the thickness of the CrN coating was higher than that of the Cr 2 N coating. Hence, it can be concluded that an increase in the amount of nitrogen introduced can accelerate the deposition rate of the Cr-N x coatings on the surface of the Zr-4 alloy. Additionally, we observed a thin Cr transition layer between the Cr-N x coatings and substrate material. The presence of the transition layer can overcome the problem of coating adhesion, considering that CrN and Cr 2 N pertain to ceramic coatings while Zr-4 alloy belongs to the metal category. Therefore, the Cr transition layer formed a reasonable composition gradient distribution, which reduces the residual stress between the interface and improves the bonding strength of the CrN and Cr 2 N coatings. The elemental distribution of the Cr-N x coatings is shown in Figure 6. According to the line scanning results, the Cr and N elements in the CrN and Cr 2 N coatings, as well as the Cr element in the Cr coating, were uniformly distributed, and the distribution range was consistent with the thickness of the coating. The line scanning results further confirmed that the N content in the Cr 2 N coating was lower than that in the CrN coating, as shown in Figure 6a,b.

Wear Behavior
The results of the analysis of samples using a non-contact 3D laser interferometer are shown in Figure 7. For the Zr-4 alloy and Cr coating, the sliding tests were conducted under a load of 1 N and a duration of 5 min, whereas the test was conducted under a load of 10 N and a duration of 20 min for the CrN and Cr 2 N coatings. The scratch morphologies of the Zr-4 alloy and Cr coating were different from those of the CrN and Cr 2 N coatings. The scratches of the Zr-4 alloy and Cr coating demonstrate more pits with uneven depth while the scratch pits of the CrN and Cr 2 N coatings are more uniform and shallower. During the sliding test, the Cr coating was completely worn through and followed by the wear of the Zr-4 substrate. Consequently, two types of abrasive particles, Cr and Zr, were generated during the friction and wear tests of the Cr coating. The wear volume of the Cr coating was larger than that of the Zr-4 alloy under both unlubricated and water-lubricated conditions, as shown in Figure 7a-d. The wear area of the friction pair corresponding to the Cr coating was also larger than that of the Zr-4 alloy, as shown in Figure 8a-d. Additionally, the wears of the Zr-4 alloy and Cr coating were more severe under water-lubricated conditions. Meanwhile, the wear volume of the Cr 2 N coating was considerably smaller than that of the CrN coating. In addition, the wear volumes of the CrN and Cr 2 N coatings in water were considerably smaller than those in air, indicating that water acts as a lubricant in the friction and wear processes, as shown in Figure 7e-h. The wear area of the friction pair corresponding to the CrN and Cr 2 N coatings is proportional to the wear volume of the coatings, as shown in Figure 8e-h. The results show that the introduction of nitrogen during the coating deposition can substantially improve the wear resistance of the Crbased coatings. Moreover, the Cr 2 N coating exhibited a better wear resistance than the CrN coating.

Hardness Analysis
The hardness of the friction pair, Zr-4 alloy, and the Cr-N x coatings (Figure 9) was each determined using a Vickers hardness tester. To avoid the contingency of the experimental results and ensure the reliability of the experimental data, five points on the surface of each sample were selected for the Vickers hardness measurement. As the hardness of the Zr-4 alloy and the Cr coating were lower than that of the friction pair, the wear debris generated in the friction and wear processes can alleviate the degree of damage to the samples. After the friction pair corresponding to the Cr coating was rubbed in air, Cr was evenly distributed on the wear surface, as shown in Figure 10a. In contrast, for the friction and wear tests in water, the water washed away some of the wear debris, and the Cr content on the wear surface was substantially reduced. As a result, the Cr content was concentrated at the edge of the wear surface, as shown in Figure 10b. Therefore, the decrease in Cr and Zr wear debris due to water scouring aggravated the wear of the Cr coating and Zr-4 alloy. As the Cr coating was slightly harder than the Zr-4 alloy, once the coating was worn out, the harder Cr wear debris also aggravated the wear of the Zr-4 alloy, resulting in a slightly larger wear volume of the Zr-4 alloy that was protected by the Cr coating.  For the Cr-N x coatings, the hardness of both the ceramic-like CrN and Cr 2 N coatings were considerably higher than that of the metal-like Cr coating, indicating that an increase in the N content can improve the hardness of the Cr-N x coatings. The addition of N makes the lattice structure of the CrN coating appear as a face-centered cubic structure, substantially improving the hardness of the coating [24]. Unlike the Cr coating and Zr-4 alloy, the hardness of both the CrN and Cr 2 N coatings were higher than that of the friction pair. The debris of the CrN and Cr 2 N coatings that remained in the scratch, aggravated the damage of the coating. The hardness of the CrN coating was higher and the damage to the coating was more severe, resulting in a greater wear volume of CrN than Cr 2 N. As the water washed away the debris, as shown in Figure 10c,d, the friction and wear damage to the coatings were reduced; therefore, the wear volumes of the CrN and Cr 2 N coatings in water were smaller.

Coefficient of Friction
The changes in the coefficients of friction of the Zr-4 alloy and Cr coating, during the sliding tests, when the load was 1 N and the duration was 5 min, are shown in Figure 11a-d. The coefficient of friction of the Zr-4 alloy was stable at 0.357 ± 0.039 when the sliding test was conducted in air, in which the coefficient of friction of the Zr-4 alloy increased rapidly from 0.2 to 0.35 and stabilized. When the Cr coating was used for the friction and wear tests in an anhydrous environment, the initial coefficient of friction was 0.2. The Cr coating was worn through after 1.4 min while the coefficient of friction rose sharply and then stabilized at approximately 0.586 ± 0.135. The stabilized value was larger than that of the Zr-4 alloy due to the residual Cr wear debris in the wear scar. In contrast, when the friction and wear tests of the Cr coating were conducted in an aqueous environment, the initial coefficient of friction was lower (approximately 0.1). The Cr coating was worn through at a faster rate; therefore, the coefficient of friction increased sharply and finally stabilized at approximately 0.416 ± 0.051. The Cr coating was worn through faster in the water, which was consistent with the previous results suggesting that the wear resistance of the Cr coating in water is very low (Figure 7c,d). The lower coefficient of friction in water was due to the washing away of some of the wear debris by water. Under a load of 10 N and a duration of 20 min, the coefficient of friction of the Cr 2 N coating (Figure 11g,h) was slightly smaller than that of the CrN coating (Figure 11e,f), regardless of whether the coating was lubricated or not. At the beginning of the sliding test, the coefficient of friction of the coating surface increased sharply owing to the rough coating surface. In addition, the hard abrasive particles produced by the abrasive wear of the CrN and Cr 2 N coatings aggravated the wear of coating in the initial stage through furrow action. After a certain period, the change in the coefficient of friction reached a relatively dynamic equilibrium, in which the production and discharge of hard abrasive particles were balanced; consequently, the interface between the zirconia friction pairs and coating became smooth. After the friction became stable under unlubricated conditions, the coefficients of friction of the Cr 2 N and CrN coatings fluctuated at approximately 0.462 ± 0.053 and 0.537 ± 0.067, respectively. In contrast, under water-lubricated conditions, the coefficients of friction of the Cr 2 N and CrN coatings fluctuated at approximately 0.465 ± 0.045 and 0.529 ± 0.049, respectively. These values indicate that under the lubricated condition, water promoted the discharge of hard abrasive particles that were produced in the processes of friction and wear and slowed down the fluctuation of the coefficients of friction of the coatings, consequently reducing the wear of the coatings.

Conclusions
In this study, three types of Cr-N x coatings with different N contents were prepared using multi-arc ion plating, and their wear behaviors were investigated in detail. The results are summarized as follows: (1) The N content can control the phase composition of the Cr-based coatings. As the N content increased, the hardness of the Cr-based coatings increased gradually; (2) Under anhydrous and water-lubricated conditions, the Cr 2 N coating showed better friction reduction and anti-wear behaviors than the Cr and CrN coatings; therefore, the Cr 2 N coating is more resistant to GTRF degradation in LWRs; (3) For the Zr-4 alloy and Cr coating, the reduction in wear debris in the wear scar in the water environment promoted the wear of the samples. In contrast, for the CrN and Cr 2 N coatings, the reduction in wear debris in the wear scar in the water environment inhibited the wear of the coatings.