Experimental evaluation of direct current magnetron sputtered and high-power impulse magnetron sputtered Cr coatings on SiC for light water reactor applications

Highlights

• Two types of sputtered Cr coatings were deposited on SiC substrates.

• Direct current magnetron sputtered (DCMS) coating contained intercolumnar channels.

• High-power impulse magnetron sputtered (HiPIMS) coating under compressive stress.

• The HiPIMS coating exhibited a lower oxidation rate during water corrosion testing.

• Intercolumnar channels in the DCMS coating acted as pathways for water permeation.

Abstract

Two types of physical vapor deposition Cr coatings were deposited on chemical vapor deposited SiC coupons using conventional direct current magnetron sputtering (DCMS) and high-power impulse magnetron sputtering (HiPIMS) to mitigate the corrosion of SiC, an advanced fuel and structural material in light water nuclear reactor cores. Coating microstructure was characterized using scanning electron microscopy and scanning transmission election microscopy. X-ray diffraction analysis was conducted for phase identification and qualitative analysis of residual stresses. The mechanical integrity of the coatings was evaluated using scratch testing, micro-indentation, and nano-indentation tests. The microstructure of the DCMS coating consisted of fine columnar grains with nanoscale intercolumnar channels, while the HiPIMS microstructure was denser and free of any columnar defects. The DCMS coating showed a small tensile residual stress, while the HiPIMS coating was under compressive stress. Both types of coatings showed good mechanical integrity in terms of ductility and adhesion to the substrate. Corrosion performance of the coatings was assessed with a 30-day high temperature water autoclave test. Both types of coatings formed a protective Cr₂O₃ surface layer 20-30 nm thick during the autoclave test. In the DCMS coating, the columnar defects provided permeation pathways for water penetration resulting in oxide formation in the intercolumnar regions in the interior of the coating. Overall, both types of Cr coatings offer promise for the mitigation of hydrothermal corrosion of SiC in light water reactor operating environments.

Introduction

A primary concern associated with current zirconium-alloy (Zr-alloy) fuel cladding in light water nuclear reactors (LWRs) is the severe exothermic reaction of Zr with steam in the event of a loss-of-coolant accident, leading to cladding failure and explosions associated with hydrogen production [1,2]. Near-term solutions to improve the accident tolerance of existing Zr-alloy cladding rely on a thin coating with a material that forms a protective oxide scale, such as Cr or FeCrAl, and extends the coping time during severe accident scenarios [3], [4], [5], [6]. Long-term accident-tolerant fuel designs involve the complete replacement of Zr-alloy cladding with a new material. These options include FeCrAl alloys [7,8] or SiC-SiC_f composites [9,10]. The SiC-SiC_f composite design is a promising candidate based on its excellent thermal stability, oxidation resistance, and high-temperature strength, which provide superior safety performance in accident conditions [11]. SiC also has favorable properties for normal operating conditions, such as a low neutron absorption cross section, minimal radiation-induced swelling, and allows for higher fuel burn-ups and higher efficiency for power generation [12].

Two obstacles hindering the implementation of SiC-SiC_f composite fuel cladding in LWRs are cladding hermeticity [13,14] and hydrothermal corrosion at normal reactor operating conditions [14,15]. Hermeticity concerns arise due to the brittle nature of SiC and its propensity to form microcracks under mechanical loads, which can lead to escape of gaseous fission products. While the fiber reinforcement does provide some “pseudo-ductility” to the composite, microcracks have been shown to form at strain levels as low as 0.1% [16], while irradiation-induced volumetric swelling reaches roughly 2% at normal conditions [9,14].

The hydrothermal corrosion behavior of monolithic SiC and SiC-SiC_f composites has been studied extensively in pressurized water reactor (PWR) and boiling water reactor (BWR) water chemistries both with and without irradiation [15,[17], [18], [19], [20], [21], [22]]. In LWR operating conditions, which normally range from 280-340°C with pressures between 7.5-15 MPa, SiC oxidizes to form a SiO₂ surface layer [15]. Eqs. (1)-(4) show possible oxidation pathways for SiC in high temperature water in order of increasing oxygen activity [15]. All reactions result in the formation of SiO₂, which subsequently reacts with water to form soluble Si(OH)₄, as in Eqn. (5) [15,17].

$$\text{Si}{\mathrm{C}}_{\left(\mathrm{s}\right)}+2{\mathrm{H}}_2{\mathrm{O}}_{\left(\text{aq}\right)}\to \text{Si}{\mathrm{O}}_{2\left(\mathrm{s}\right)}+\mathrm{C}{\mathrm{H}}_{4\left(\mathrm{g}\right)}$$

$$\text{Si}{\mathrm{C}}_{\left(\mathrm{s}\right)}+2{\mathrm{H}}_2{\mathrm{O}}_{\left(\text{aq}\right)}\to \text{Si}{\mathrm{O}}_{2\left(\mathrm{s}\right)}+2{\mathrm{H}}_{2\left(\mathrm{g}\right)}+{\mathrm{C}}_{\left(\mathrm{s}\right)}$$

$$\text{Si}{\mathrm{C}}_{\left(\mathrm{s}\right)}+3{\mathrm{H}}_2{\mathrm{O}}_{\left(\text{aq}\right)}\to \text{Si}{\mathrm{O}}_{2\left(\mathrm{s}\right)}+3{\mathrm{H}}_{2\left(\mathrm{g}\right)}+\mathrm{C}{\mathrm{O}}_{\left(\mathrm{g}\right)}$$

$$\text{Si}{\mathrm{C}}_{\left(\mathrm{s}\right)}+4{\mathrm{H}}_2{\mathrm{O}}_{\left(\text{aq}\right)}\to \text{Si}{\mathrm{O}}_{2\left(\mathrm{s}\right)}+4{\mathrm{H}}_{2\left(\mathrm{g}\right)}+\mathrm{C}{\mathrm{O}}_{2\left(\mathrm{g}\right)}$$

$$\text{Si}{\mathrm{O}}_{2\left(\mathrm{s}\right)}+2{\mathrm{H}}_2{\mathrm{O}}_{\left(\text{aq}\right)}\rightleftharpoons \text{Si}{\left(\text{OH}\right)}_{4\left(\text{aq}\right)}$$

These chemical reactions can lead to gradual recession of the SiC cladding tube, contamination of the water coolant, and precipitation and buildup of SiO₂ on relatively cooler surfaces of the system [15]. Therefore, environmental barrier coatings are under development to mitigate the hydrothermal corrosion of SiC by preventing the exposure of the SiC cladding surface to the water coolant. These coatings may also function as a layer to maintain the hermiticity of the cladding [23].

Preliminary work by Ang et al. included the deposition of Cr, Zr, CrN, TiN, ZrN coatings on SiC via electroplating [24], vacuum plasma spray, and cathodic arc physical vapor deposition (PVD) [23]. Electroplated Cr coatings pose challenges in terms of microcracking and coating uniformity, which are significant issues in environmental barrier coating applications, in addition to toxicity concerns related to hexavalent chromium used in electroplating baths. Plasma-sprayed coatings also raise concerns related phase purity, porosity, and adhesion. This study demonstrated that PVD coatings offered the most promise to produce a coating for mitigation of corrosion based on the ease of depositing a high-purity, crack-free coating.

Raiman et al. [25] tested a selection of the cathodic arc PVD coatings in a 400-hour corrosion test under simulated BWR coolant conditions. The results from this study suggest that CrN coatings may be feasible for use in BWRs under hydrogenated water chemistry, but Cr coatings showed the best performance, demonstrating small weight changes in both BWR hydrogenated water and normal water chemistries. Mouche et al. [26] conducted in-depth characterization of these coatings. One key result was higher adhesion strength and ductility in the metallic Cr coatings compared to ceramic coatings. This is an important consideration for coatings to accommodate volumetric swelling of the SiC cladding under neutron irradiation [27]. However, interfacial cracking was observed in the cathodic arc Cr coatings resulting from stress buildup along with hydrogen impurity content in the coating. Additionally, all cathodic arc microstructures contained detrimental macroparticles which are artifacts of this deposition process. For these reasons, it is valuable to investigate different types of PVD processes which may remedy these issues and provide further insight into how coating microstructure affects corrosion behavior.

Cr coatings are being researched extensively as an accident tolerant fuel coating for current Zr-alloy cladding. Promising coating processes under investigation include cold spray deposition [4] and PVD [28], [29], [30]. The Cr coatings provide increased high-temperature oxidation resistance for Zr-alloy cladding in beyond-design-basis accident conditions while also slowing oxidation of the cladding at operating conditions [28,30]. It is reasonable to expect that Cr coatings can mitigate corrosion of SiC cladding at operating conditions based on their slow oxidation kinetics and ability to form an adherent, protective Cr₂O₃ layer on the surface [25].

In the current study, two types of sputter deposition were selected to fabricate Cr coatings: traditional direct current magnetron sputtering (DCMS) and high-power impulse magnetron sputtering (HiPIMS). In the sputter deposition process a gas, commonly argon, is ionized and forms a plasma. The positively charged gas ions collide with a negatively biased target of the desired coating material, ejecting atoms from the target surface which then deposit onto the substrate. The DCMS process is a well-established coating method for microelectronic and industrial tooling applications. The HiPIMS process was developed more recently [31] and has risen in popularity over the last two decades. In HiPIMS deposition, high-power pulses are applied to the target creating very high instantaneous power densities. As a result, a large fraction, upwards of 70%, of the sputtered flux is ionized [32]. In contrast, only a few percent of the sputtered atoms are ionized in the conventional DCMS method. The degree of ionization depends on factors such as the ionization potential of the sputtered atom species and power cycle characteristics [33]. Many studies have analyzed the composition of HiPIMS discharges for chromium [34], [35], [36], [37], [38]. These studies have concluded that there is a significant increase in the proportion of Cr⁺ and Cr²⁺ ions present in the plasma compared to DCMS discharges. In fact, it has been found that Cr⁺ ions are the most prevalent species in the HiPIMS plasma, whereas Ar⁺ ions dominate the DCMS plasma [34]. Ehiasarian et al. have estimated a 30% ionization fraction for chromium compared to the few percent usually obtained in a DCMS discharge [36].

Ion bombardment of the growing coating during the HiPIMS process provides some distinct advantages over traditional DCMS, most notably an improvement in coating density, a less columnar grain structure, an increase in hardness and adhesion strength, and a decrease in surface roughness [33]. Typical sputter deposition processes are capable of depositing high-quality coatings with deposition rates usually on the order of a few nm/s [39]. It has been widely reported that HiPIMS deposition rates are lower than the conventional DCMS process mainly due to the back-attraction of ionized sputtered material [39]. HiPIMS deposition rates can range from between 30-80% of DCMS deposition rates for the same material at the same average power, but these values can vary depending on the system and processing procedures [39]. Target power densities during HiPIMS pulses are typically on the order of a few kW/cm² compared to a few W/cm² during conventional DCMS deposition [32]. HiPIMS duty cycles (the fraction of time the pulse is active) are relatively small, usually no more than 10% [32]. Consequently, the time-averaged power densities between both deposition methods are similar.

This study expands upon the previous work [23,25] through the investigation of sputter deposition, rather than cathodic arc deposition, to produce Cr coatings more suited for LWR cladding application. In this study, a 30-day pure water static autoclave test was conducted to evaluate the corrosion behavior of the two types of sputtered Cr coatings on SiC flat samples fabricated using the chemical vapor deposition (CVD) process. The surface roughness evolution of the coated samples throughout the autoclave test was measured using atomic force microscopy (AFM). Detailed microstructural examination via scanning electron microscopy (SEM) and transmission electron microscopy (TEM) was used to correlate process parameters to corrosion performance. X-ray diffraction (XRD) analysis was used to qualitatively characterize the residual stress level and texture of the coatings. Scratch testing and microindentation testing was conducted for qualitative assessments of mechanical integrity. Finally, the hardness and modulus of the coatings was measured using nanoindentation tests. These evaluation methodologies allow for an understanding of the efficacy of Cr coatings on SiC substrates deposited with DCMS and HiPIMS for LWRs as well as other moisture-bearing environments.