Research On The Controlling Of The Compound Composition At The Interface Of Thermal Barrier Coatings Deposited By Thermal Spraying Process And Its Failure Mechanism Of High Temperature

Thermal barrier coatings are widely used to protect gas turbine of aero-engineand land-based gas turbine from high temperature oxidation and corrosion, which canenhance the engine lifetime and its efficiency. This paper reviewed the currentresearch status of thermal barrier coatings with regard to classic yttria stabilizedzirconia and new-type pyrochlore materials, prevailing deposition processes(including Plasma Spraying, High Velocity Oxygen Fuel, Cold Gas DynamicSpraying, Electron Beam-Physical Vapor Deposition), the interfacial degradation andits failure mechanisms of thermal barrier coatings systems as well as the currentmethods to prolong the lifetime of TBCs. The bond coat（CoNiCrAlY, NiCrAlY andCoCrAlY）and top ceramic coat (8wt.%YSZ) were prepared by means of HighVelocity Oxygen Fuel and Air Plasma Spraying, respectively. To study the correlationbetween the interfacial morphology of the bond coat and different compositions ofsubstrate alloys and the phase transformations in the TGO (θ-Al2O3â†'α-Al2O3etc.)and the residual stress in the TGO and the growth of spinel and the TGO growth rate,as well as their effects on the oxidation of thermal barrier coatings, especially thephase transformations in the TGO and the mechanical properties of the interfaceTGO/TBC that subjected to vacuum preheating treatment after thermal exposure. Themain results of studies obtained are as follows:â‘´The characteristic of thermally grown oxides of TBCs depositing onto the castiron-based substrate is significantly different. The change trend law of the thickness ofupper CS layer and lower Al2O3oxide layer at the interface of the bond coat and thetopcoat were evaluated by the area rule. Meanwhile, the morphologies of thermallygrown oxides at the interface of bond coat and substrate were classed into twoparts layer and bulk，and the main phase is alumina.⑵The paper suggests that different chemical compositions can influence the phasetransformations in TGO after isothermal oxidation. Compared with the castiron-based alloy, the TGO growth rate of the nickel-based substrate was significantlyslower as the thermal exposure proceeded. It should be noted that the Cr3+and Al3+with smaller radii may influence the mechanisms of phase transformation in the TGO,and these smaller ions can promote the θ-Al2O3â†'α-Al2O3phase transformation.Furthermore, the TBCs system with iron-based substrate are prone to have muchshorter stage of θ-Al2O3â†'α-Al2O3phase transformation.⑶It is disclosed that the morphology of bond coat and the phases in TGO have great effect on the residual stress of TBCs. The apex of the convex of the undulated TGOusually has the largest compressive residual stress. The spinel oxide was inclined toform at the location of the apex of the convex of the undulated TGO with a lowervalue of b/a geometric parameter, but only a small magnitude of spinel oxide at thelocation of the flank of the convex. However, the spinel cap· morphology usuallyforms at the location of the apex and the flank of convex of the undulated TGO with ahigher value of b/a geometric parameter. Compared with the nickel-based substrate,the more presence of spinel cap· at the location of the convex of the undulated TGOwith a cast iron-based substrate can be found at the convex of the undulated TGO, andits earlier end of the θ-Al2O3â†'α-Al2O3phase transformation may result in the lowervalue of compressive residual stress in overall.â‘·It can be disclosed that the vacuum preheating treatment can significantlysuppressed a great quantity of the chrome oxide, Cobalt oxide and spinel oxides in theTGO of the high temperature corrosion CoCrAlY-based coating. And the TGO mainlycomprised of the Al2O3grain (grain size:0.4-1μm) with lower Y content dotted witha small magnitude of Al2O3with higher Y content. A layer of micron scale thermallygrown oxides mainly consisting of continuous Al2O3with approximately1μm grainsize was formed at the surface of the coating, only a small quantity of spinel wasfound in the TGO after the isothermal oxidation.⑸A well-designed compound vacuum preheating treatment was successfullyoptimized experimentally, and the processes of the vacuum preheating treatment(1050℃,4h)&ageing (1000℃or1100℃) for10h were recommended to apply toTBCs system prior to the isothermal oxidation service (Heating-up vacuumdegree:7×10-3Pa, insulated vacuum degree:3×10-3Pa).⑹It is proposed that the control of the θ-Al2O3â†'α-Al2O3phase transformation at the0-17h stage of the1050℃isothermal oxidation by means of the newly-compoundvacuum preheating treatment process can have significant effect on the prolonging thelifetime of TBCs. Moreover, the formation of (Al, Cr)2O3oxide sub-layer result fromthe textured and larger grain of α-Al2O3can show a longer service lifetime of TBCs.⑺The modeling of mechanical evolution of TGO or TGO/TBC interface wasestablished. The steady-state energy release rate (Gss) first reaches the max value.The imperfection or microcrack cannot unstably propagate to macro scale becausethey absorb a great deal of energy after the appropriate vacuum preheating treatment,and the lifespan of TBCs can be enhanced. â‘»The categories of cracks in thermal barrier coatings includes surface crack,horizontal propagation crack at the TGO region, horizontal initiation crack at the TGOregion and net microcrack. And the combination of different kinds of microcrack canhave significant effect on the thermal shock behavior of thermal barrier coatings. TheTGO area can be strengthened by Vac.(1050,4h)+1000,10h and Vac.(1050,4h)+1100,10h processes.