| Thermal barrier coatings(TBCs)are widely used in heat engines because of their low thermal conductivity and good high temperature stability,in protecting high temperature alloys and improving their operating temperature.It plays an active role in improving work efficiency,reducing the loss rate of use,and reducing the wear rate of turbine blades of aerospace engines.However,due to its working environment and its harsh structure,its shape and microstructure are very complicated,which makes it difficult to accurately predict the failure of the coating,which is extremely harmful to the safety of use.Therefore,the research on the failure mechanism of thermal barrier coatings contributes to the healthy development of aviation industry,and also contributes to the further optimization and improvement of thermal barrier coating materials and structures.In this paper,a sprayed adhesive layer was prepared on a nickel-based alloy by atmospheric plasma spraying(APS)system.The growth process and composition evolution of the thermally grown oxide(TGO)were studied by constant temperature oxidation.Two kinds of thermal barrier coating samples of 30 μm and 200 μm thick ceramic layers were prepared.The effect of oxygen diffusion on the growth of TGO in the coating was studied.For the 200 μm thick ceramic layer samples,the heating frequency was studied under the same constant temperature oxidation period.Different effects on the failure of thermal barrier coatings;the microstructures in the coatings were analyzed by means of optical microscopy and adhesion tester,and the relationship between stress changes and coating failures was discussed.Research indicates:(1)After the high temperature oxidation of the sprayed adhesive layer,the main component of TGO is Al2O3 and grows in needle shape,and the nucleation rate is much larger than the growth rate;with the high temperature oxidation,the growth gradually becomes uniform from needle to particle size.The bulk transition indicates the formation of a dense oxide whose main component is Cr,Co oxide.(2)The oxidation process of the thermal barrier coating is divided into three stages.The first stage is the initial stage of oxidation.The interface between the ceramic layer and the bonding layer does not form a continuous oxide film,which cannot prevent the entry of oxygen,and selectively oxidizes with Al,and the weight gain is obvious.TGO grows fastest.The second stage is the middle stage of oxidation,the oxidation weight gain is stable,the TGO growth is slow and delamination occurs,and the ceramic layer and the bonding layer form a dense oxide film,and the oxidation rate is affected by the diffusion rate.In the middle and late stages of oxidation,the selective oxidation ends,the concentration of Al does not reach the oxidation requirement,and the less active Ni,Cr,Co have diffused and started to oxidize.After the black TGO is formed by Al,the other elements are oxidized together with Al to form a gray color.TGO layer.The third stage is the stationary period,and the oxidation weight gain is stagnant.When the gray layer in TGO rapidly grows to a certain thickness,the overall growth is slow,and the transverse longitudinal cracks in the ceramic layer begin to connect and penetrate,and the coating partially peels off.Although the overall oxidation weight gain trend is about the same,the TGO growth rate and total increment of the 30 μm ceramic layer sample are significantly higher than the 200 μm ceramic layer sample.(3)The effect of the thermal cycle frequency on the failure of the thermal barrier coating found that the more the number of thermal cycles experienced,the shorter the high temperature oxidation time experienced when the coating sample failed.Compared with the 5 min thermal cycle sample,the residual stress value of the thermal cycle sample is small for several hours to several tens of hours,and the residual stress is the cause of crack initiation and expansion.Under the high temperature oxidation for a long time,the thermal mismatch caused by the high frequency of thermal cycling plays a certain role in promoting the initiation and propagation of cracks.The initiation and expansion of cracks are beneficial to the transportation of oxygen,which accelerates the growth of TGO.In the thermal cycle,due to the different materials of the thermal barrier coating,the temperature change causes the thermal mismatch between the layers,which promotes the formation,growth and connectivity of microcracks,accelerates the growth of TGO,and ultimately leads to accelerated coating failure.(4)The residual stress analysis of the thermal barrier coating shows that the coating stress does not increase with the increase of high temperature oxidation time,but decreases first and then rises.At 50 h,a stress peak appears and finally falls.The adhesion test of each layer of the coating found that the tendency of adhesion and stress was similar,indicating that the residual stress had a certain influence on the adhesion of each layer of the thermal barrier coating. |
