Micromechanism Of CMAS Corroding YSZ And YSZ(Er) Thermal Barrier Coatings: First Principles Calculation

YSZ?Y₂O₃ partially Stabilized ZrO₂?thermal barrier coatings?TBCs?is the main surface treatment material to enhance the high temperature performance of aircraft engine blades and the thrust-weight ratio of engines.However,volcanic clouds and regions with high concentrations of sand,dust,and gravel mainly containing CaO,MgO,Al₂O₃,and SiO₂,which is named calcium magnesium aluminosilicate?CMAS?,can accessibly corrode the YSZ in high temperature environment to lose efficacy.Then by first-principles calculation,several thermodynamic parameters such as cohesion energy,formation heat,Griffith rupture work and Gibbs free energy,et al.,is calculated to explore the structural stability of YSZ in CMAS corrosion process.The interface strength of CMAS/YSZ,the mechanism of crack initiation and propagation are estimated by parameters as thermal expansion coefficient,residual stress and diffusion coefficient.At last,the micro-mechanism of corrosion resistance of YSZ and Er-doped YSZ to CMAS is further revealed from analysis on Mulliken population and diffusion activation energy.The following conclusions are drawn:?1?When CMAS melt adsorbs on the surface of YSZ,atomic diffusion?Ca?Y?and?Si?Y?occur at the interface of CMAS/YSZ.And the diffusion of?Ca?Y?and?Si?Y?does not need to overcome any activation energies when Y is located in the surface layer of YSZ?111?.This diffusion process can strengthen structure stability of CMAS but weaken that of YSZ,enhance the bonding strength of CMAS/YSZ interface,as a result in making CMAS peel off much more difficultly.In addition,loss of Y atoms in YSZ results in the instability of t-ZrO₂ phase to transform into m-ZrO₂ phase.This phase transformation accompanies by 4%volume expanding,resulting in thermal stress to ultimately induce the exfoliation of YSZ thermal barrier coatings.?2?The corrosion product CaAl₂Si₂O₈ with high formation ability(H_formation=-7.832eV/atom)and structural stability(E_cohesive=-3.447 eV/atom)will be formed at the CMAS/YSZ interface.Because the thermal expansion coefficient of CaAl₂Si₂O₈ phase has great difference from that of YSZ,it is easy to produce internal stress to induce cracks nucleating at their weak atomic bonds of Zr-O(Q_Zr-O=0.050)and Ca-O(Q_Ca-O=0.050)during thermal cycle.The large residual stresses??_r=122.89MPa?and small interface adhesive strength(W=-0.442 Jm^-2)are attributed to the cracks growing along the CaAl₂Si₂O₈/Al₂O₃ interface.And the small interface adhesive strength(W=-1.320 Jm^-22 and W=-1.354 Jm^-2)and large residual stresses make the cracks further expand along the interface of CaAl₂Si₂O₈/MCrAlY and MgAl₂O₄/Al₂O₃ easily,leading to the failure of YSZ thermal barrier coating.?3?In order to improve the corrosion resistance of YSZ TBCs to CMAS,Er rare earth element are doped into YSZ coatings.It is found that the YSZ?Er?greatly reduces the ability of CMAS corrosivity due to its smaller Griffith rupture work(W=-1.665 Jm^-2)of CMAS/YSZ?Er?than that(W=-2.253 Jm^-2)of CMAS/YSZ.Compared with CMAS/YSZ interface,Er-doped can greatly reduce the diffusion ability of other atoms,especially Y atoms at CMAS/YSZ?Er?interface in high temperature environment.The diffusion of Y atom can increase the resistance of t-ZrO₂ to m-ZrO₂ phase transformation.Furthermore,the Er dopant in YSZ can decrease the diffusion activation energy??E₁?Zr?Si?=-7.705 eV?in CMAS/YSZ?Er?relative to that??E₀?Zr?Si?=1.088 eV?of CMAS/YSZ,which promotes the generation of ZrSiO₄ due to its powerful structural stability and facile formation(E_cohesive=-8.541 eV and H_formation=-3.558 eV).Once the reaction product ZrSiO₄ adsorbs on the surface of the coating or in the pore,the smaller Griffith rupture works of CMAS/ZrSiO₄(W=-0.740 Jm^-2)and CMAS/3Al₂O₃·2SiO₂(W=-1.299 Jm^-2),which are even weaker than that of CMAS/YSZ?Er?(W=-1.665 Jm^-2),will reduce the ability of the stripping YSZ?Er?TBCs and hinder the continuous infiltration of CMAS.Then the corrosion resistance of YSZ?Er?to CMAS is enhanced absolutely.