Microstructure And Properties Of CPED Thermal Protective Coating On Al-Si Alloy Surface

In this paper,thermal protective coating was prepared on the surface of Al-Si alloy by cathodic plasma electrolytic deposition（CPED）technique under the addition of ZrO₂ nanorods,carbon nanotubes and chloroplatinic acid.The effects of ZrO₂ nanorods,carbon nanotubes and chloroplatinic acid on the microstructure,phase,fracture toughness,flexural strength,thermal insulation properties and thermal shock properties of CPED thermal protective coating were studied.The evolution of microstructure and properties was investigated and explored the formation mechanism of coating.The effects of prefabricated barrier films on the formation process,microstructure,phase,thermal insulation properties and the formation mechanism of CPED thermal protective coating were studied.The surface,cross-section morphologies,and the phase composition of the ceramic layers were observed by scanning electron microscope（SEM）and X-ray diffraction（XRD）.The thickness,fracture toughness,heat insulation properties and thermal shock resistance were measured by eddy current thickness gauge,electronic universal testing machine,self-made heat insulation test device and thermal shock equipment,respectively.Using the high speed video camera to obtain the plasma spark discharge time and the number of sparks calculate the plasma discharge channel heat source density,and then the temperature field distribution in and around the discharge channel is simulated by ANSYS software.Simulated calculation by ANSYS software found that the temperature in the plasma discharge channel produced by CPED is obviously higher than that in the plasma discharge process produced by PEO during the preparation of the coating process.The temperatures in the plasma discharge channel were higher than 5500 ℃.CPED plasma discharge produces higher energy than PEO plasma discharge,and the reaction is more intense.Through the study of the thickness of the prefabricated coating,it is found that there were more discharge channels,more uniform coating and better insulation performance when the thickness of the prefabricated coating is 9 micron.The prefabricated coating when the thickness of is 9 micron can promote the plasma discharge and accelerate the formation of high temperature stable phase Zr3Y4O12.Compared to the traditional electrolyte,the addition of ZrO₂ nanorods,carbon nanotubes and chloroplatinic acid can not only reduce the arc voltage,promote the micro plasma discharge of the cathode plasma,but also increase the number of discharge channels on the surface of the ceramic layer.The results by XRD pattern show that the main phase of the coating is α-Al2O3,SiO₂,t-ZrO₂ and Zr3Y4O12.With the increasing of chloroplatinic acid of the concentration,the diffraction peak of platinum enhanced and the content of platinum particles increased.With the thermal growth of ZrO₂ nanorods in the electrolyte,the thermal insulation performance of the ceramic coatings prepared by CPED was better;With the increase of the concentration of carbon nanotubes in the electrolyte,the thermal insulation temperature of the ceramic layer prepared by CPED increased first and then decreased;With the increase of the concentration of chloroplatinic acid,the heat insulation temperature of the coating was firstly increased,and the heat insulation temperature remained at the concentration of 0.16g/L.500 times thermal shock test results show that the ceramic layer has a good thermal shock performance.Compared with ordinary zirconium nitrate-yttrium nitrate system,the introduction of ZrO₂ nanorods,carbon nanotubes,chloroplatinic acid can improve the fracture toughness of ceramic coating.With the increase of the hydrothermal time of the zirconia nanorods,the fracture toughness of the coatings increased first and then decreased,and the fracture toughness of the CPED ceramic coatings prepared by adding the zirconia nanorods obtained by hydrothermal 24 h was the best.With the increase of carbon nanotubes and chloroplatinic acid,the fracture toughness of CPED ceramic coating was better.ZrO₂ nanorods,carbon nanotubes mainly through the bridge,crack deflection（or bifurcation）and pull out the mechanism toughen the ceramic coating,while the platinum particles are absorbed by the plastic deformation of the crack propagation energy,passivation crack tip and increasing the radius of the crack tip radius to prevent the continued expansion of the crack,but also through the bridge,crack deflection toughened ceramic coating.