The Effect Of Gradient Structure And High Temperature Treatment On The Characteristics Of Ceramic Composite Coatings

Al₂O₃-TiO₂ ceramic coating has high hardness,high temperature resistance,good corrosion resistance and wear resistance properties,of which Al₂O₃-40 wt%TiO₂composite coating has a high bonding strength with steel,aluminum and other matrix materials,and the coating has a good density and toughness.However,as for the traditional coating structure,the low bonding strength between the coating and the substrate,the mismatch of thermal expansion coefficient and other problems result in the failure of the coating which is caused by the stress concentration at the interface during the service.Currently,the commonly used solution is to increase the bonding strength and performance of the coating by adding a bond layer to the coating,but the improvement effect is limited.In this paper,the Al₂O₃-40 wt%TiO₂/NiCoCrAl gradient coating was prepared on the surface of 45^#steel by plasma spraying technology.Through the changing characteristics of the volume fraction of components in the gradient structure in the thickness direction,the bonding strength in the interface zone was improved and the residual stress and thermal stress in the material were reduced.On this basis,the coating was treated at different temperatures to improve the interface state and optimize the coating structure,so as to improve the bonding strength of the coating and improve the overall performance of the coating.Scanning electron microscope?SEM?,X-ray energy spectrum analysis?EDS?,electron probe microanalysis?EPMA?and X-ray diffractometer?XRD?were used to observe and analyze the stacking morphology,phase composition and coating defects of the coating.The mechanical properties of the coating were tested by means of Vickers microhardness tester,three-point bending tester and thermal shock tester.The main conclusions of the study are as follows:?1?Compared with the traditional multilayer coating,the Al₂O₃-40wt%TiO₂/NiCoCrAl gradient coating has a gradient structure of composition.In the gradient region,the ceramic splats and NiCoCrAl splats show the morphology of a mixed stack,with a few holes,and the Al and Ti enrichment areas can be observed in the ceramic region.?2?In the ceramic coating and the bond coating,the ceramic particles and NiCoCrAl particles are stacked in splats to form lamellar coating structure,and the stacking of some semi-molten ceramic particles results in pore defects in the ceramic layer.In the gradient region,ceramics and NiCoCrAl splats are mixed and stacked to form a stacked topography covering each other.The NiCoCrAl splats filled the holes formed by the superimposition of semi-fused ceramic particles through plastic deformation,which enhanced the density of the region.?3?The failure process of Al₂O₃-40 wt%TiO₂/NiCoCrAl gradient coating three-point bending test is divided into two stages.The crack inside the coating had two rapid propagation processes in the test,corresponding to two peaks on the load-displacement curve.The gradient change of coating composition affects the crack propagation path,and the gradient coating has better fracture toughness.?4?In the thermal shock experiment,the generation and accumulation of thermal stress lead to the non-directional crack propagation path in the gradient region,along with the increase of lamellar gap in some regions,the local shedding and the propagation mode of crack propagation along the lamellar boundary.There are a certain number of cracks and oxides of Fe at the interface between the matrix and the bonding layer,and the oxidation reaction of the coating also leads to the formation of a small amount of oxides in the shedding area.?5?After 800? high-temperature treatment,the needle-like phase and ingrowth Cr-rich phase were found inside of the coating.Among them,the formation of needle-like phase enhances the bonding strength between layers.The microhardness of the coating first increased and then decreased from the outside to the inside,and the strengthening of the needle-like phase significantly increased the maximum microhardness of the coating,which was higher than the maximum microhardness of the original coating without high-temperature treatment.?6?After 900? high-temperature treatment,the internal pores of the coating increased.The ceramic region forms a nonhomogeneous distributed strip of Al₂O₃.The Al₂O_`layer is formed at the boundary of the NiCoCrAl region,Cr-rich phase is formed inside of the region.After 1000? high-temperature treatment,dense pore defects and shedding areas appear inside the coating,and some Cr-rich areas appear in the NiCoCrAl region.Compared with 800? high temperature treatment coating samples,the hardness value of coating after 900? and 1000? high-temperature treatment is lower.