Study On Ceramic Coating By Plasma Spraying With Laser Micromelt Sealing Hole

After plasma spraying,the volume shrinkage and residual tensile stress of ZrO₂coating produce crack network on its surface.The interconnected pores and cracks in plasma sprayed ZrO₂coatings not only affect the mechanical properties,but also reduce the high temperature oxidation resistance and corrosion resistance of ZrO₂coatings.Two mechanisms have been proposed for oxygen transfer through plasma sprayed zirconia coatings:diffusion of ions from the crystal structure of ZrO₂,and penetration of gas through pores and microcracks,the latter of which plays a major role.During the oxidation process of ZrO₂coating,with the increase of the oxide layer thickness on the surface of the bonding layer,the interface stress between the bonding layer and the ceramic layer is generated.Sometimes,this stress exceeds the strain tolerance of ZrO₂,so coating delamination may occur at the junction of the ceramic layer and the adhesive layer.It is of great significance to add a dense surface layer on the surface of ceramic layer to reduce the porosity of ZrO₂coating and improve the high temperature oxidation resistance of ZrO₂coating.In this paper,ZrO₂coating was prepared on GH4169 alloy by plasma spraying technology.Laser remelting of ZrO₂coating was carried out with different laser power,and the surface porosity of ZrO₂coating was obtained by using Image J software to process the surface morphology of ZrO₂coating after laser remelting.The optimal process parameters of laser remelting were determined.ZrO₂/SiO₂/Y₂O₃coating,ZrO₂/SiO₂/Y₂O₃coating,ZrO₂/Al₂O₃coating,ZrO₂/Al₂O₃coating,ZrO₂/Al₂O₃/SiO₂/Al₂O₃coating(laser remelting),ZrO₂/Al₂O₃/SiO₂/Al₂O₃coating(no laser remelting)were prepared by sol-gel sealing treatment of nano-sized yttrium oxide sol,alkaline nano-aluminum sol and neutral silicon sol.The samples were oxidized at high temperature in a box-type resistance furnace,and the oxidation resistance of different coatings was determined by thermogravimetric method.Image J was used to process the micromorphology of the prepared coating samples to determine the influence of the sealing process on the surface porosity of the ZrO₂coating.JB-4C precision roughness tester was used to detect the roughness of the prepared coating,and the influence of hole sealing process on the surface roughness of the ZrO₂coating was determined.The macroscopic morphology of the coating before high temperature oxidation was analyzed by 3D depth of field microscope.X-ray diffractometer was used to determine the surface phase of the coating before high temperature oxidation.The microstructure and elemental changes of the coating before and after high temperature oxidation were determined by SEM and EDS.The nanometer yttrium oxide sol,basic nanometer aluminum sol and neutral silicon sol were treated at 120?,450?and 1000?to determine the phase changes of the sol after different temperature treatment.Based on the oxidation kinetics curve of the prepared coating,the surface morphology and phase composition of the coating before and after high temperature oxidation,the conclusions are drawn,which provide experimental basis and theoretical reference for the improvement of the properties of the ZrO₂coating by sol-gel sealing and laser remelting.The main conclusions are as follows:(1)The phase of nano yttrium oxide sol,alkaline nano aluminum sol and neutral silicon sol changed after high temperature treatment.When the neutral silica sol is heated at 120?and 450?,it is dominated by amorphous silica.When it is treated at 1000?,it is dominated by?-quartzite and?-squamous quartz.After heating at 120?and 450?,the alkaline nano-aluminum sol is mainly non-crystalline state,and is treated at 1000?,and becomes mainly?-Al₂O₃mixed with a small amount of?-Al₂O₃.Nano yttrium oxide sol after 450?,1000?high temperature treatment into the body centered cubic Y₂O₃.(2)Laser remelting of ZrO₂coating was carried out with different power laser,and the optimal process parameters of laser remelting were preliminarilydetermined as follows:laser power 35W,defocus 200mm,scanning speed 4.67cm/s,lap rate 50%,sample preheating temperature100?.(3)The porosity of untreated ZrO₂coating is 15.1%,the porosity of ZrO₂/Al₂O₃coating(laserremeltingtreatment)and ZrO₂/Al₂O₃/SiO₂/Al₂O₃coating(without laser remelting treatment)are2.38%,2.81%and 5.58%,respectively.Laser remelting and sol-gel sealing treatment significantly reduce the porosity of ZrO₂coating.(4)Si element enrichment exists in pores and cracks of ZrO₂/Al₂O₃/SiO₂/Al₂O₃coating(laser remelting treatment)before and after high temperature oxidation,which indicates that the coating structure remains intact after a long time of high temperature oxidation.(5)At 1000?,the life of ZrO₂/SiO₂coating is 32.7%of that of ZrO₂coating.The life of ZrO₂/Y₂O₃coating is similar to that of ZrO₂coating.The oxidation rate of ZrO₂coating is 0.102mg·cm^-2h^-1,the oxidation rate of ZrO₂/Al₂O₃coating is 79.4%of that of ZrO₂coating,the high temperature oxidation rate of ZrO₂/Al₂O₃coating(laser remelting)is3.8%of that of ZrO₂coating and the high temperature oxidation rate of ZrO₂/Al₂O₃coating(without laser remelting)is 6.3%of that of ZrO₂coating.Al₂O₃/SiO₂/Al₂O₃composite coating can significantly improve the high temperature oxidation resistance of ZrO₂coating,The high temperature oxidation resistance of ZrO₂/Al₂O₃/SiO₂/Al₂O₃coating increased by 39.6%after laser micromelting treatment.