Study On The Preparation And Property Of SiC Coating On Zr-4 Alloy

Zirconium alloys,as being an important structural material in water-cooled nuclear power reactors have been used for fuel cladding,due to their combination of low thermal neutron capture cross section,adequate mechanical properties and good corrosion resistance.The main problem threating the safety of the zirconium alloy cladding is the reaction of zirconium alloy with cooling water.The Part of the hydrogen released by zirconium and cooling water reaction will be absorbed by zirconium alloy,which will decreasing the strength and plasticity,as while as increasing the brittleness.With the further improvement of the fuel consumption of the reactor,fuel cladding tubes face with a harsher environment mainly involving corrosion reaction between zirconium alloy and cooling water,which puts forward higher requirements on the corrosion resistance of zirconium alloys.SiC has excellent corrosion resistance,high thermal conductivity,high hardness and good wear resistance,in addition,it has attractive nuclear properties,such as low thermal neutron cross-section,high resistance to neutron irradiation,low induced radioactivity.Therefore,the SiC coating can be used to improve the corrosion resistance of zirconium alloy,bringing an important practical significance on improving the safety and economy of the reactor.In this thesis,Radio Frequence(RF)magnetron sputtering technique was used to prepare SiC coating on Zr-4 substrate.The phase of SiC coating was analysised by XRD.The morphology was investigated by SEM,AFM.The composition and adhesion of SiC coating were investigated by EDS and Scratch Tester.At the same time the high temperature oxidation corrosion resistance properties,microstructure and hydrogen content of Zr-4 substrate were studied by Integrated Thermal Gravimetric Analyzer,Optical Microscopy and Hydrogen Determinator.The effect of bias voltage(0 V,-50 V,-80 V,-120 V)and working pressure(0.2 Pa,0.8 Pa,1.5 Pa,3 Pa)on deposition rate,roughness,coating growth behavior and adhesion of SiC coating were studied,the optimal process parameters were obtained.The corrosion resistance of SiC coatings prepared by different process parameters in 3.5wt%NaCI solution were studied by electrochemical workstation,at the same time,the corrosion resistance of the SiC coating(prepared by optimal process parameters)and the Zr-4 substrate were studied by the static immersion corrosion method.The oxidation corrosion resistance of Zr-4 alloy and SiC coating at 800 ℃ high temperature steam were investigated by Integrated Thermal Gravimetric Analyzer.The results indict that:when the SiC coating is prepared by RF magnetron sputtering,the microstructure of the Zr-4 substrate will not be affected,neither additional absorption of hydrogen.The SiC coating sample prepared by 200 W,-80 V,3 Pa possesses a high density with small particle size,proper deposition rate(1085nm/h),good adhesion strength.Meanwhile,the surface roughness is 11.9nm,so it is the best preparation parameter in this thesis.The electrochemical test show that the corrosion resistance of Zr-4 substrate is highly improved with sputtered SiC coating.The coating prepared at 3Pa and-80V has the biggest corrosion potential and the least corrosion current density in 3.5wt%NaCl solution.The static immersion test indicates that SiC coating sample has excellent corrosion resistance too.The experiments of high temperature steam oxidation corrosion at 800 ℃ demonstrate that:when the oxidation time is less than 1800 seconds,SiC coating has excellent high temperature protection performance.Once the oxidation time lasted longer than 1800 seconds,the coating peeled off,losing its protective effect on the matrix.