Microstructure And Bonding Mechanism Of Plasma Sprayed Metal/Al₂O₃-TiO₂ Coating Interface

With the continuous development of thermal spraying technology,thermal spraying ceramic coating is widely used in aerospace,military and even ordinary civil machinery of iron-based parts surface wear and corrosion protection and other fields.Due to thermal expansion coefficient of the ceramic,mechanical properties,such as lattice structure varied with the metal/alloy bond-coats,as more and more demanding industrial service condition,put forward higher requirements on the performance of the coating,bond-coats and ceramic top-coats interface determines the service life of the whole material,which greatly limits the further development of thermal spraying ceramic coating use.The failure of AT40 wear-resistant ceramic coating on the surface of iron-based parts is mainly caused by the crack propagation at the interface between the ceramic top-coats and the bond-coats.From the current failure mode of wear-resisting ceramic coating,the interface mechanical property gradient between the bond-coats and the ceramic top-coats is one of the most important reasons for the failure of wear-resisting ceramic coating.However,there is a great relationship between the interface mechanical property gradient and the choice of bond-coat materials and the design of interface structure.At present,the failure mechanism of different interface structures composed of different bonding layer materials is not clear,and the optimization of bond-coat materials and interfaces of wear-resistant ceramic coatings lacks detailed theoretical support and systematic research on interface mechanical properties.In this work,Al₂O₃-40wt%Ti O₂(AT40)ceramic top-coats was optimized by analyzing the working conditions of iron-based wear parts.The Cu-AT40 coupling interface and the in-situ oxide nailing interface of Fe Cr Al-AT40 were prepared by heat treatment process design on the basis of the traditional double-layer structure interface between metal(Cu)and alloy(Fe Cr Al)bond-coat and AT40 ceramic top-coat.The amorphous-AT40 continuous gradient transition interface and Cu-Ti₃Al C₂nanocomposite continuous gradient transition interface were studied by interface structure design and bond-coat material optimization.The microstructure and phase composition of different interface structures were systematically analyzed,the bonding mechanism of different interface structures was revealed,and the mechanical properties of different interface were tested and analyzed at multiple scales.By comparing the interface mechanical properties of different interface structures of different bond-coat materials,the bond-coat material and interface structure of AT40wear-resistant ceramic top-coat on the surface of iron-based parts were optimized based on the service conditions of iron-based parts.The microstructure,morphology and phase of the interface between ceramic top-coats and bond-coats were studied by scanning electron microscope and transmission electron microscope.Then the multi-scale mechanical properties and failure of the interface were analyzed by using nano-indentation test technology,micro-indentation test technology,three-point bending test technology and adhesive-tensile method.The bond-coats material and interface structure of AT40 wear-resistant ceramic coating on the surface of iron-based parts are optimized to reveal the bonding mechanism and failure mechanism of different interface structures.More than 19,000tests of microstructure characterization and mechanical properties were carried out on approximately 15 groups of approximately 700 samples of wear-resistant ceramic coating system on the substrate surface of Q235.According to the test results,the bonding mechanism,multi-scale mechanical properties and failure mechanism of different interfaces are systematically analyzed,and the following main research conclusions are obtained:Double-layer Structure Interface:The traditional double-layer structure interface is mainly connected by mechanical chimera.Cu-AT40 interface was heat treated under Ar atmosphere protection at 900 ^oC/12h to generate a coupling interface layer less than 10?m which was composed of Cu Al O₂,Cu Al₂O₄,Al₂O₃,Ti O₂ and Cu.The coupling interface realizes the metallurgical bonding between the bond-coats and the ceramic top-coats,and improves the bonding strength of the interface by more than37.5%.The traditional double-layer structure interface of Fe Cr Al-AT40 was heat treated at 900 ^oC/12h in Ar+1 vol%O₂ atmosphere.A large number of strips of Al₂O₃ are generated in the interface pores and connected with the ceramic top-coats to form the in-situ oxide nailing interface,which alleviates the interface failure caused by the difference in bonding modes between the ceramic and alloy atoms.The formation of Al₂O₃ near the interface of the bond-coats blocks the subsequent entry of O in the heat treatment process,forming the Al₂O₃ gradient distribution from the bond-coats to the substrate,thus forming the elastic modulus gradient bond-coat.The elastic modulus gradient-in situ oxide nailing interface improves the mechanical property gradient on both sides of the interface and improves the interface bonding strength by more than 20%.Continuous Gradient Transition Interface:AT40-amorphous(Fe₅₆Cr₂₃Mo₁₃B₈)continuous gradient transition interface was successfully prepared on the iron substrate surface by technology of two simultaneous powder servo for atmospheric plasma spraying.In this interface,the ceramic phase from the amorphous bond-coats to the ceramic top-coats presents the gradient microstructure with the amorphous phase gradually decreasing and the ceramic phase gradually increasing.The gradient change of the composition is realized through the spraying process design,forming the fuzzy interface and the gradient transition of hardness and fracture toughness.The interface further reduces the mechanical property gradient on both sides of the interface and improves the fracture toughness and crack propagation resistance of the interface.Nanocomposite Coating:(1)The amorphous bond-coat was partially crystallized by heat induction,and a large number of nanoparticles of alloy and solid solution were formed.The nanocomposite bond-coat was prepared by introducing nanoparticles into the bond-coat.The bond-coats has high hardness and Young's modulus,which can effectively hinder the crack propagation and improve the interface performance of the coating.(2)Based on the characteristic that Ti₃Al C₂ is easy to decompose and oxidize at high temperature,the nanocomposite bond-coats was successfully prepared by atmospheric plasma spraying.A large number of nanocrystalline ceramic phases appeared in the coating and Cu formed a spatial network structure around the nanocrystalline ceramic particles.Through the 3PB test,the fracture toughness of the nanocomposite bond-coats was as high as 9.4 MPa·m^1/2,and the energy release rate was only 15.1 Nm^-1,showing good fracture toughness and crack resistance.Under the protection of Ar atmosphere of 900 ^oC/12h,the heat treatment produced more nano-ceramic particles and the gradient interpenetrating structure of reticulated Cu,which tightly connected the ceramic top-coats with the substrate.Nanoparticle and solid solution effectively hinder the crack propagation and improve the coating performance.Nanocomposite Continuous Gradient Transition Interface:The nanocomposite continuous gradient transition bond-coats was successfully prepared by Cu/Ti₃Al C₂mixed powder plasma single route powder servo spray technology,with a large number of Al₂O₃ and Ti O₂ nanoparticles of about 30 nm-200 nm in size.The interfacial bond strength of Cu/Ti₃Al C₂-AT40 nanocomposite continuous gradient transition coatings was more than 80%higher than that of traditional double-layer coatings.After heat treatment at 900 ^oC/12h,the interfacial bonding strength was increased by 151.99%compared with that of traditional double-layer coating.The order of the interface hardness and elastic modulus gradient of the four coating systems from small to large is:Cu/Ti₃Al C₂-AT40 nanocomposite continuous gradient transition coating,amorphous-AT40 continuous gradient transition adhesive coating,Fe Cr Al-AT40 double-layer in-situ oxide napping interface coating,Cu-AT40 double-layer coupling interface coating.The bonding strength of interfacial was as follows:Cu/Ti₃Al C₂-AT40 nanocomposite continuous gradient transition coating,Cu-AT40 double-layer coupling interface coating,and Fe Cr Al-AT40 double-layer in-situ oxide nailing interface coating.Based on the comprehensive analysis of service environment,interface mechanical property gradient and interface bonding strength between the bond-coats and AT40 wear-resistant ceramic top-coats,Cu/Ti₃Al C₂-AT40 nano-composite continuous gradient transition bond-coat is the preferred bond-coats for the AT40 wear-resistant ceramic top-coats on the surface of the iron-based parts.