Preparation And Microstructure And Properties Of Metal And Metal-ceramic Composite Coatings On ODS Steel Surfac

China’s long-term for 2035 and the goal of"dual carbon economy"clearly point out that we will accelerate the promotion of the energy revolution,build a clean,low-carbon,safe and efficient modern energy system,improve energy supply and security capabilities,and"accelerate the development of non fossil fuels.".Nuclear energy has the advantages of abundant resources,high energy density,good adaptability to harsh environments,and no dependence on solar radiation,making it an important component of the non fossil energy structure.With the deepening of scientific research,nuclear energy technology has developed rapidly.From the use of nuclear fission energy to the development of nuclear fusion energy,people strongly expect to use nuclear energy to solve energy problems in sustainable development.Oxide dispersion strengthened（ODS）ferritic steel has excellent high-temperature tensile and creep properties,as well as good radiation expansion resistance,resulting in a higher service temperature window.It is considered one of the most promising candidates for the fourth generation advanced fission reactor fuel cladding and the main heat transfer components of future fusion reactors.Advanced fission nuclear energy and future fusion reactors have higher operating temperatures and more complex environments,and ODS is a candidate material.However,when they are exposed to high-temperature supercritical water,carbon oxide,liquid metal,and other cooling media,transmutation media,or neutron breeding media for a long time,they are prone to oxidative corrosion,wear corrosion,and liquid metal corrosion.In the pursuit of higher accident tolerance and higher thermal effi-ciency,such as ultra-high temperature gas-cooled reactor（VHTR）,lead cooling fast reactor（LFR）,etc.,it poses more stringent requirements on the coating materials,including better high-temperature mechanical properties and better corrosion resistance.In response to the above issues,surface coating technology has been proven to be one of the most effective methods for improving the corrosion resistance and wear resistance of cladding materials.Cr has the advantages of high melting temperature,good corrosion resistance,and high hardness.In recent years,the application of surface chromizing technology in various reactor cladding materials has been widely reported.Cr coated fuel cladding materials have excellent high-temperature steam oxidation resistance,corrosion resistance,and radiation resistance.On the other hand,aluminum nitride（AlN）has received widespread attention due to its high electrical resistivity,high thermal conductivity,and good chemical stability.AlN is considered to be a suitable coating material for the test cladding module（TBM）of fusion reactors.In addition,first principles calculation has become a common method for theoretical analysis and experimental results prediction in material science research.However,there are few researches directly relying on first-principles calculations on the scientific problems existing in the surface coating of materials.In this study,Cr coating and Cr-AlN composite coating were prepared on the surfaces of 12CrODS steel and Fe Cr Al-ODS steel by pack-cementation,the effects of air cooling and water cooling on the coating and substrate after pack-cementation were studied.The electronic structure and elastic properties of the second phase in the coating were studied by first principles calculation.By comprehensively utilizing techniques such as SEM,EDS,XRD,and EBSD,the microstructure characterization and phase analysis of Cr coating and Cr-AlN composite coating were carried out,revealing the formation mechanism of metal coating and composite coating.The microhardness,friction and wear resistance,and corrosion resistance of Cr coatings and Cr-AlN composite coatings were studied,and the strengthening and corrosion mechanisms of the coatings were elucidated.The main conclusions are as follows:（1）The XRD and EDS results indicate that different cooling methods do not alter the phase composition of the coating.Due to the faster cooling rate,there is more Cr on the surface of the water-cooled sample,resulting in a higher Cr/Fe value in the outermost layer of the W1 sample.Compared to the original substrate,the substrate grains of A1and W1 samples after pack-cementation are significantly coarsened.（2）The first principle calculation results show that the Cr and C atoms in Cr₂C crystals are bound by ionic bonds,while the Fe and Y atoms in Fe₂Y crystals are mainly bound by polar covalent bonds.Compared with Fe₂Y,Cr₂C crystals exhibit more significant elastic anisotropy.Secondly,the elastic constants and modulus of Cr₂C crystals significantly increase with increasing pressure,while the elastic modulus of Fe₂Y crystals slowly increases with increasing pressure.Under a certain pressure,Cr₂C will change from brittleness to toughness,while Fe₂Y always appears as a ductile phase within a large pressure range,indicating that the presence of Fe₂Y phase has a good effect on improving the impact toughness of the coating.In addition,the results of the surface work function show that the average work function of Cr₂C is greater than that of Fe₂Y,which indicates that when the Cr coating works in corrosive media,the Fe₂Y phase will be corroded before the Cr₂C phase.（3）Under the same ratio of embedded powder,the significant difference from aluminum free ODS steel is that the surface of Fe Cr Al ODS steel will generate a Cr-AlN composite coating,consisting of a Cr rich layer（outer layer）and an AlN rich layer（inner layer）.The Cr rich layer is mainly composed of Cr-Fe solid solution,in which a small amount of{Cr,Fe}₂N_1-x,Cr₂C and Fe₂Y phases are dispersed inside,while the AlN rich layer is composed of Fe-Cr solid solution and AlN particles dispersed on it,and the size and density of AlN particles show a gradient distribution from outside to inside.（4）The introduction of Cr coating and Cr-AlN composite coating on aluminum free ODS steel and Fe Cr Al ODS steel will significantly improve their surface hardness.For Cr coatings,the maximum hardness of the coating is about 870HV,while for Cr-AlN composite coatings,the maximum hardness of the coating is about 870HV.The introduction of Cr-AlN composite coating significantly improves the friction and wear resistance of ODS steel.However,due to the high content of Al inside Fe Cr Al ODS steel,the introduction of Cr AlN composite coating will reduce its corrosion resistance at 3.5wt%Na Cl solution.