The Effect Of Heat Treatment On The Microstructure And Mechanical Properties Of Plasma Sprayed Al₂O₃-TiO₂-MgO Coating

Due to Al₂O₃-40%TiO₂ ceramic coating has outstanding properties such as wear,corrosion and heat resistance which produced by thermal spraying techniques.It's widely application in surface reinforcement of mechanical parts.During the thermal spraying processes,the ceramic coating always content some defects such as voids,induces cracks,unmelt particles and residual stress.However,ceramic coatings are very sensitive to residual stress and cracks which can reduce the coating lifetime critically.With the development of industrial technology,the requirements for the service environment of mechanical components are more and more complicated.Therefore,it is very urgent to improve the microstructure of ceramic coating and improve the mechanical properties of the coating.Heat treatment can effectively change the instability of material structure,eliminate residual stress and obtain ideal performance.MgO as a ceramic material additive can block the growth of?-A1₂O₃grain and improve the hardness of materials.The composite coatings were prepared on steel by atmospheric plasma spraying?APS?.The effect of MgO addition on the microstructure and mechanical properties of the ceramic coatings and the optimum content of MgO in the coating will be discussed in this thesis.The coatings were analyzed using scanning electron microscopy with electron probe x-ray microanalysis and x-ray diffraction.The mechanical properties of the coating were tested by a Vickers hardness test and three point bending test on the as-sprayed coatings and after heat treatment.The main conclusions are as follows:?1?Increase the MgO addition content,the pores,porosity,unmelted particles and microcracks in coatings increased significantly,and the hardness of the coating decreased seriously..When the coating containing 5 wt.%MgO the defects in the coating are the least and the hardness of the coating is the highest.?2?After heat treatment,the diffusion layer formed and the thickness of this layer is60⁷0?m at 1000°C,the interface between the bond coating and the substrate disappeared,the heat treatment promoted an interfacial diffusion between bond coat and substrates.?3?Heat treatment induced a morphological transformation from lamellae with clear interface to alternant-layer structure with mutual pinning at temperature between800°C and 1000°C.·?4?Temperature and time during heat treatment could influence the phase evolution within the coating.During the lower temperature heat treatment,the Al₂TiO₅formation and decomposition could suppress the new phase Mg₂Al₆Ti₇O₂₅ formation which can improve the coating hardness compared to 1000°C.?5?Interfacial diffusion between bond coat and substrates also made contribution to the mechanical properties of the ceramic coatings after heat treatment at 1000?for24 h in Ar,significantly increasing their fracture toughness.