Study On The Microstructure And Properties Of NiCr Composite Thermal Barrier Coating For Continuous Casting Mould

With the improvement of the production level of the metallurgical industry,the surface quality of the steel billets and slabs drawn from the mold in the continuous casting production process have received more and more attention.The oscillation marks and other surface defects need to be polished,which increases the energy consumption.Therefore,the development of surface defect-free casting technology has become especially important for energy saving and improving metal yield.Applying thermal barrier coating to the surface of the copper plate at the meniscus of the mold is one of the important development directions.At present,the commonly used thermal barrier coating process is plasma sprayed composite thermal barrier coating.However,there is no latest report on the composition of the transition layer material and its influence on the structure and performance of the composite thermal barrier coating.Therefore,NiCr-based composite thermal barrier coatings are selected as the research objects in this study.And three composite coatings,i.e.,NiCr/YSZ,NiCrAl/YSZ and NiCrAlY/YSZ are prepared respectively to study the influence of the composition of the transition layer material on the structure and performance of the composite thermal barrier coating.In this paper,tensile test,hardness test,porosity measurement and thermal shock resistance test are used to study the comprehensive performance of the three composite thermal barrier coatings mentioned above.And the effects of the addition of aluminum and yttrium elements to the transition layer on the structure and performance of NiCr composite thermal barrier coating were analyzed.The goal of the study is to provide experimental data and theoretical support for the design of high-performance composite thermal barrier coatings.The main results and conclusions obtained are as follows.The results of microstructure analysis of NiCr/YSZ and NiCrAl/YSZ composite thermal barrier coatings show that due to the addition of Al to the transition layer,the stacking phenomenon of the NiCrAl transition layer is significantly reduced.Y-shaped and hook-shaped joints are found at the layer boundaries.The internal defects such as pores,holes,unmelted particles and isolated oxides are also significantly reduced.It is also found that after thermal shock resistance testing,Ni-Al-based alloy phases are formed in the NiCrAl/YSZ coating,which hinders continuous growth of thermally-growth oxide(TGO).It can be found NiCrAl/YSZ coating has advantages over NiCr/YSZ coating.The superiority manifest in lower microhardness and porosity,stronger bonding strength,and a significant improvement in thermal shock resistance.Though the microstructure analysis of NiCrAl/YSZ and NiCrAlY/YSZ composite thermal barrier coatings,the results show that due to the addition of yttrium element to the transition layer,the formation of spinel-type oxides,pores,and isolated oxides are reduced.Due to the increases of surface tension of the sprayed molten droplets,the stacking phenomenon of the transition layer increases.As a result,the bonding strength of the coatings decreases.In the NiCrAlY transition layer,the Al2Ni6Y3 phase is formed during the thermal spraying process.After multiple thermal cycles,the Al2Ni6Y3 phase is oxidized to form Y3Al5O12 phase,which hinders the growth of TGO layer and the formation of harmful oxides such as NiCr2O4.There,the creep resistance and thermal shock resistance of the coating improve accordingly.Compared with those of NiCrAl/YSZ coating,the microhardness of NiCrAlY/YSZ coating is increased by 51.78%,the tensile performance is reduced by 6.25%,the porosity is reduced by 1.59%,and the thermal shock lifetime is greatly improved.