Study On The Surface Activity And Wetting Mechanism With CMAS Of YSZ Nanomaterials Based On First-principles Caculation

Yttria-stabilized Zirconia?YSZ?is widely used in aeroengine thermal barrier coatings?TBCs?.However,the environmental sediment sediments?mainly CaO,MgO,Al₂O₃,SiO₂,CMAS?will seriously corrode the TBCss to decrease its service life.Therefore,how to improve the corrosion resistance of TBCs is one of the key technologies to improve their service life.In this paper,the wettability of molten CMAS on the surface of YSZ bulk surface and microfacet is systematically studied by using density functional theory?DFT?calculation method.Several parameters,such as the work of adhesion,surface tension and viscosity,are used to identify the relationship between the wettability of CMAS on YSZ surface.Then the worst wettability of YSZ could be got to improve the corrosion resistance of YSZ.The results show that:?1?From the analysis on the surface activity of YSZ nano structure,tt is found that the surface energies of YSZ bulk surface and microsurface follow the trend:E_surface[?010×010?]>E_surface[?111×010?]>E_surface[?101×010?]>E_surface[?111×101?]>E_surface[?101×101?]>E_surface?111?bulk surface>E_surface?101?bulk surface>E_surface?010?bulk surface,wherein the maximum surface energy of the microfacet model E_surface[?010×010?](6.7667 Jm^-2)is five times greater than that(1.3964 Jm^-2)of the YSZ?111?bulk surfaces.The surface activity of the YSZ microfacet is therefore much more vigorous than that of the bulk surface.Electronic structure analysis shows that the highest chemical activity of the YSZ microfacets is derived from their large Fermi energy(E_F-[?010×010?]?-1.8403 eV?,pseudo energy gap,and the maximum bond length?d-?O-Zr??0.03?,and the charge?Q_A-?O-Zr??0.26?.At the same time,it is found that the surface energies of the YSZ microfacets and bulk surface exhibit a quantization phenomenon as Y=N×B,wherein B is the average of the surface energies of the two Miller indices,and N is a positive integer,Y is the surface energies of the microfaces.?2?The wetting process of CMAS melt on YSZ surface is studied by molecular dynamics?MD?simulations.The results of the contact angle analysis show that the wetting trend of CMAS on YSZ follow as YSZ?110?>YSZ?001?>YSZ?111?>YSZ?101?>YSZ?010?.The trend of the adhesion work of CMAS on YSZ surfaces varies according to the order W_adYSZ?010?<W_adYSZ?101?<W_adYSZ?111?<W_adYSZ?001?<W_adYSZ?110?.This indicates that the chemical reaction is more vigorous on CMAS/YSZ?110?than on the CMAS/YSZ?010?interface.Some other properties,such as surface tension,viscosity of the CMAS melt,and work of separation of CMAS/YSZ further confirm this conclusion.CMAS/YSZ?110?has a much lower surface tension(0.27 Jm^-2)than(0.37 Jm^-2)CMAS/YSZ?010?.However,the viscosities of CMAS melt on YSZ?110???=4.91*s Pa?and YSZ?010???=4.97*s Pa?are nearly similar with each other.Depth study reveals that the CMAS melt shows its highest wettability on the YSZ?110?surface.This interaction originates from the highly energetic Mg,Si,and O ions,especially the Mg ions of CMAS penetrating the YSZ surface layer.The intrinsic driving force for such a strong movement of CMAS ions is stimulated by the oxygen ions on the YSZ?110?surface layer,which provide a higher electronegativity compared to other crystal surfaces.The distribution of oxygen ions on the YSZ surface layer is the key factor affecting the wettability of CMAS on YSZ TBCs.?3?The wetting process of CMAS melt on YSZ microfacet is studied by MD simulations.The results of the contact angle show that the wetting trend of CMAS on YSZ follow as YSZ[?101×101?]>YSZ[?010×101?]>YSZ[?010×010?].The trend of the adhesion of CMAS melt on YSZ surfaces varies according to the order YSZ[?101×101?]<YSZ[?010×101?]<YSZ[?010×010?].This indicates that the chemical reaction is more vigorous for CMAS/YSZ[?010×010?]than for CMAS/YSZ[?101×101?]interface.However,all of the wetting angle of CMAS on YSZ microfacet is less than 38o±2o,which means all of the YSZ microfacest show good wettability to CMAS.Deep study reveals that the difference of wettability of CMAS melt on YSZ microfacet and on YSZ bulk surface.It is found that Y element in CMAS/YSZ interface precipitates from YSZ and segregates into CMAS melt at high temperature,especially to Ca and Si ions in CMAS segregating into YSZ.As a result,the high active sites in CMAS/YSZ interface melt completely.Then the YSZ microfacets shows better wettability than that of YSZ bulk surface...