| Heat engines like gas turbine and diesel engine are widely used in types of warships like aircraft carriers,frigates,destroyers,and flight power devices,electric power production,and other fields.During the service,the hot end parts of the engine have to face high temperature oxidation,erosion,corrosion,and other complex and poor conditions.The thermal barrier coatings(TBCs)system with 6～8 wt.%Y2O3 stabilized ZrO2(6～8YSZ)as the top ceramic material can provide effective protection.However,if 6-8YSZ continues to work at a high temperature above 1200℃ for a long time,problems such as sintering,phase transformation and corrosion will occur,which will lead to a decrease in the service life and a failure to meet the development needs.Referring to the in situ strengthening concept,this paper mainly studied the preparation methods of Gd2O3,Yb2O3 co-doped YSZ composite coatings(Gd2O3-Yb2O3-YSZ composite coatings,8YSZ/Gd2O3-Yb2O3YSZ double-ceramic-layer coatings),and ways to improve the overall thermal insulation effect and service life.The finite element simulation and optimization study of the structure of doubleceramic-layer TBCs were carried out by using the simple model constructed.It was found that the thicker the top Gd2O3-Yb2O3-YSZ coatings were,the better the heat insulation effect was.Considering only the heat insulation effect,the thicker the Gd2O3-Yb2O3-YSZ coatings in this experiment were,the better.However,with the increase of the thickness of Gd2O3-Yb2O3-YSZ coatings,the Von Mises equivalent stress and the S11 stress in the process of thermal shock cycle tended to increase,while the strain degree was relatively small,which could be ignored.With the consideration of the difference of fracture toughness of ceramic coatings,the thickness of Gd2O3-Yb2O3-YSZ coatings and 8YSZ coatings was respectively selected as 125 μm and 125 μm.With this thickness,the coatings could not only maintain good heat insulation performance,but also maintain effective service life.The orthogonal experiment and the result analysis of Gd2O3-Yb2O3-YSZ coatings on the top showed that the bonding strength of the coatings decreased and the porosity increased slightly with the increase of spraying distance.The bonding strength and porosity of the coatings increased with the increase of spraying power.With the increase of powder feeding rate,the bonding strength of the coatings increased first and then decreased,but tended to increase overall,and the porosity decreased.With the increase of the moving speed of the spray gun,the bonding strength and the porosity of the coatings increased.Based on the above analysis results,125 μm Gd2O3-Yb2O3-YSZ coatings were prepared on 125 μm 8YSZ coatings with the main parameters of spraying distance 70 mm,argon flow rate 58 L/min,hydrogen flow rate 16 L/min,current 600 A,power 53 kW,powder feeding rate 46.8%and moving speed 1000 mm/s.The thermal insulation performance,high temperature oxidation resistance,thermal shock resistance and high temperature corrosion resistance of 8YSZ TBCs and 8YSZ/Gd2O3-Yb2O3-YSZ composite coatings were tested.It was concluded that the comprehensive performance of 8YSZ/Gd2O3-Yb2O3-YSZ coatings was superior.For Gd2O3-Yb2O3-YSZ coatings on the top,the co-doping of Gd2O3 and Yb2O3 improved the sintering resistance and chemical stability of YSZ coatings,reduced the thermal conductivity of coatings,inhibited the phase transition of ZrO2,and released excessive thermal stress.The 8YSZ/Gd2O3-Yb2O3-YSZ coatings had some advantages,such as high thermal insulation and high corrosion resistance,etc.However,due to the insufficient bonding strength between the top Gd2O3Yb2O3-YSZ coatings and the bottom 8YSZ coatings,the top Gd2O3-Yb2O3-YSZ coatings fell-off prematurely.The properties of 8YSZ/Gd2O3-Yb2O3-YSZ coatings were weakened.The 8YSZ/Gd2O3-Yb2O3-YSZ composite coatings designed in this study could improve the service life of the coatings to a certain extent.This was mainly due to the buffer effect of 8YSZ intermediate layer and the thermal corrosion protection effect of Gd2O3-Yb2O3-YSZ top layer,which reduced the thermal stress concentration of the double-ceramic-layer structure and prevented the infiltration of corrosion molten salt.The research results of this paper can provide guidance for the research and application of new TBCs which are reinforced by multi-layer gradient and in-situ doping. |
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