High Temperature Steam Oxidation Behavior Of CrAlSiN Coatings On Zirconium Alloy

Nuclear energy is one of the main forces of current power generation,its safety has always been the focus of attention.The main cause of the Fukushima nuclear power plant accident in 2011 was that the zirconium（Zr）-alloy cladding tube,one of the nuclear fuel components,reacted with high-temperature steam when loss of coolant accidents occurs,releasing a large amount of heat and hydrogen,which caused an explosion.Since then,the ability of cladding materials to withstand accident conditions has been reassessed,and accident tolerant fuel（ATF）cladding materials have begun to receive widespread attention from researchers.Among them,the surface modification of the active Zr-alloy cladding to improve the accident tolerance of the Zr-alloy cladding tube itself is one of the current development directions of ATF materials.The motivation of this study is to prepare Cr-based protective coatings on the Zr-alloy substrate,to improve the high-temperature steam oxidation resistance of the Zr-alloy,and to study the structural evolution of the coating-substrate during the oxidation process.In this work,Cr Al Si N protective coatings with various Cr/Al（0.6～4.5）were prepared on the surface of Zr-alloy by magnetron sputtering technology,namely:R1-0.6,R2-1.3,R3-2.0,R4-3.1,and R5-4.5.The N content in Cr Al Si N coatings was19～28 at.%,The Si content was 4～7 at.%,and the thickness was about 8～12μm.The five coatings all exhibited the non-columnar structure.As the value of Cr/Al increase,the crystallinity of the coating first decreases and then increases.The cross-sectional structure of the five coatings is relatively dense,without obvious column gaps.The surface of the R2,R3 and R4 coatings is flat and dense,there are a few cracks on the surface of the R1 coating,and there are tiny voids on the surface of the R5 coating.In terms of performance,the mechanical properties of coatings,high-temperature steam oxidation resistance,and high-pressure hot water corrosion resistance were tested successively.The effect of Cr/Al on the oxidation behavior of the coatings was studied.Meanwhile,the protection-failure process of Cr Al Si N coating in the oxidation process was explored.（1）The Cr Al Si N coatings exhibit excellent mechanical properties.The coating’s hardness is 7 to 16 GPa,which is 2 to 5 times that of the uncoated Zr-alloy,and the elastic modulus is 139 to 204 GPa,which is 1.1 to 1.8 times that of the uncoated Zr-alloy.As Cr/Al increases,the overall hardness of the coatings shows a downward trend.The adhesion between the R3 coating and Zr-alloy substrate was tested by a scratch system.The coating and Zr-alloy substrate showed good bonding both at the as-deposited and the post-oxidation state.The critical load L_c2 corresponding to the peeling of the as-deposited coating is～23 N;after 6 hours of oxidation at 1200°C high temperature steam,the bonding strength between the R3 coating sample and the substrate increases significantly,the chipping initially appears along the edges at the load of～40 N,followed by a total delamination of the coating at the load of～53 N.（2）Using high-temperature steam oxidation equipment,the five coatings were tested for their oxidation resistance in steam at 1200°C,and their oxidation behavior was analyzed.The R3 coating has the best oxidation resistance,and the protection time for the Zr-alloy substrate exceeds 6 h.At this time,the oxidation depth of the uncoated Zr-alloy is greater than 300μm.the protection of the R1 and R2 coatings for the Zr-alloy substrate is less than 2 h.After 2 h oxidation,the R1 and R2 coating samples showed excessive oxidation and striped oxidation arch,respectively.The R4and R5 coatings showed similar protective effects and could protect the Zr-alloy substrate from oxidation within 4 h.The protection and failure process of R3 coating was studied,and it was found that the coating will form a uniform and dense Cr₂O₃/α-Al₂O₃ double-scale structure during the oxidation process,which effectively prevents the diffusion of oxygen to the Zr-alloy substrate.When the oxidation time exceeds 6 h,the protection fails,and the scale exhibits a loose and porous structure.（3）The corrosion tests were carried out in a static autoclave in static deionized water at 360°C and saturation pressure,corresponding to 18.7 MPa at this temperature.After 12 days of corrosion,the R2 and R3 coatings performed the most stable,the coating phase and microstructure did not change significantly,and the corrosion resistance was better than the uncoated zirconium alloy.Fine particles appear on the surface of the R1-coated sample.Cracks and flaking appear on the R4-coated sample,and there are tiny attachments.Grooves appear on the surface of the R5-coated sample,and crystalline oxides with a size of about 1～3μm are attached.Based on the evaluation in terms of mechanical properties,high-temperature oxidation resistance,and high-pressure corrosion test,Cr Al Si N coating with non-columnar structure and Cr/Al of 2.0 has a better overall performance.The coating can significantly improve the hardness and high-temperature steam oxidation resistance of the Zr-alloy substrate and has a good application prospect in the application of ATF materials for Zr-alloy protective coating systems.