Preparation,Microstructures And High Temperature Oxidation Mechanism Of Low Expansion Nanocrystalline Cermet Coatings

With the development of aero-engine towards high flow ratio,high thrust weight ratio and high inlet temperature,it is usually necessary to apply thermal barrier coating system(TBCs)on the turbine blade surface to meet the increasing temperature and pressure in the combustion chamber.A typical TBCs consists of a superalloy substrate,a metallic bond coat,a thermally grown oxide and a ceramic insulation top coat.In general,the coefficients of thermal expansion(CTE)of metallic bond coat represented by MCrAlY or β-NiPtAl is quite different from that of α-Al2O3 film formed the surface.Therefore,upon cooling,the thermal stress formed and accumulated in the oxide film will destroy the integrity of TGO,arousing cracking and spallation.In this regard,the cermet coatings are designed and studied using the arc ion plating equipment to solve this problem for the paper.The purpose is to add ceramic phase particles with lower CTE value into metallic coatings during depositing,narrow the gap of CTE value between coating and TGO film,reduce the stress in TGO film and improve the oxidation resistance of coating.In this paper,the cermet coatings were prepared on Ni-base superalloy K417 by arc ion plating using a NiCrAlY target,whose evolution process and degradation mode are explored.At the same time,for the sake of providing theoretical and practical basis for further using cermet coating as a new bond coating in the TBCs,the methods of improving high-temperature oxidation resistance were studied from the two aspects on composition modification and structure optimization.The main results are as follows:1.Effects of oxygen incorporation in low expansion Ni+CrAlYN nanocomposite coatings on the structure and properties were studied.Ni+CrAlYNO cermet coatings were obtained by doping a certain amount of oxygen into Ni+CrAlYN cermet coatings.With the increase of oxygen content,the grain size of nanocomposite coating is refined.During the oxidation,the internal AlN particles would be used as the Al source through 4AlN+3O2→2A1203+2N2(gas)to maintain the growth of oxide film.After oxidation at 1000℃ for 500 h,Ni+CrAlYN cermet coating had almost degraded into a metal coating,and inclusion of substrate element,e.g.Ti,Mo and Co were found in the TGO film.However,oxide particles decorated at the grain boundaries in the Ni+CrAlYNO coating had the ability of inhibiting the cermet coatings degradation,in which there were still abundant AlN particles and TGO film was relatively pure.Therefore,the oxidation resistance of coating was improved via incorporating oxygen.In addition,when the content of doping oxygen was 20 sccm,the microhardness of coating had reached the value of 9.5 GPa.In short,the microhardness and wear resistance of the Ni+CrAlYNO were superior to the NiCrAlY and the Ni+CrAlYN coatings.2.The duplex coating comprised an outer layer of low-expansion Ni+CrAlYNO cermet and an inner NiCrAlY layer was designed and prepared,and the oxidation mechanism was studied.After annealed treatment at 1000℃ for 6 h,the outer cermet layer consisted of γ/γ’ matrix with dispersive particles of AlN,Al2O3,and Y2O3,while the inner NiCrAlY layer consisted of γ’-Ni3Al and α-Cr.After isothermal oxidation for 100 h at 1000℃,the oxidation rate of duplex coating was lower than the monolayer NiCrAlY coating and the monolayer Ni+CrAlYNO cermet coating,and only pure and single α-Al2O3 formed on the surface of duplex coating.The reasons for improving the oxidation resistance of the duplex coating were attributed to the uphill of Al from the inner NiCrAlY layer,driven by reactions of 2AlN+3O→Al2O3+2N and N+Al→AlN.And the sustainable Al content at TGO/coating interface was maintained.3.The effect of oxygen content on high-temperature oxidation behavior of Ni+CrAlYNO/NiCrAlY duplex coating was investigated.Based on the oxygen content of flowing into cermet layer during depositing,the duplex coatings were referred to as N+10O,N+20O and N+30O coatings,respectively.After isothermal oxidation or cycle oxidation at 1000℃,the oxidation resistance of N+30O coating was better than that of other two coatings.This was mainly due to oxygen content on Y distribution of outer layer and coatings degradation.When the oxygen content of the duplex coating was at a suitable level,Y would be dragged inside the coating by oxygen in the form of nano-sized YAG particles,so the inclusion of Al/Y oxide in the TGO film would be reduced and the integrity of the TGO would be maintained.4.The oxidation behavior of the Ni+CrAlYNO/NiCrAlY duplex coating with diffusion barrier was studied.Ni+CrAlYNO/NiCrAlY duplex coating with diffusion resistance,referred to as DB+Duplex,was prepared by one-step method,that is,using a NiCrAlY target and adjusting the difference of gas flowing into the vacuum chamber.After annealed treatment at 1000℃,thin α-Al2O3 films with diffusion resistance were formed on the both sides of NiCrAlYO diffusion barrier closed to the interface of between coating and substrate.After oxidation at 1000℃ for 500 h,the oxidation mass gain of the DB+Duplex coating was reduced by 44%than that of Ni+CrAlYNO/NiCrAlY coating.In addition,the diffusion barrier layer obviously inhibited the interdiffusion behavior between coating and substrate,avoided the formation of TCP phases and improved the oxidation resistance of coatings.Compared with the traditional method of depositing diffusion barrier,this method was easy to operate and promoted the cleanliness of the both sides of diffusion barrier layer closed to the interface of between coating and substrate.