| Phase transformation and sintering (>1200℃) of the traditional 6～8wt.%Y2O3-ZrO2 (YSZ) thermal barrier coatings (TBCs) limit its application at higher temperature. So it is very important to investigate and prepare novel TBCs materials which have phase stability, sintering-resistance and low thermal conductivity above 1200℃in order to meet the requirements of the latest aircraft engines with higher thrust-to-weight ratio.(La2O3, Y2O3)-ZrO2 (LaYSZ)、(Yb2O3, Y2O3)-ZrO2 (YbYSZ) and (Sc2O3, Y2O3)-ZrO2 (ScYSZ) rare earth oxides co-doped zirconia ceramic powders were prepared by chemical co-precipitation calcination method. Phase stability and thermo-physical properties of the powders were studied in order to estimate their possibilities to be as TBCs materials. The characterizations of crystallography, effect of stabilizer contents on phase stability as well as on phase transformation mechanisms are represented. ScYSZ thermal barrier coatings were prepared by air plasma spraying (APS) technique. Phase stability, phase transformation kinetics characterization and effect of stabilizer characterizations (valence, atomic weight and ionic radius) on thermal conductivity are investigated. Evolution of coating structures as well as failure mechanisms under high temperature oxidation, corrosion and thermal shock environment were also investigated respectively, which may provide theoretical knowledge for development and application of novel thermal barrier coatings.Phase stability and thermo-physical properties of LaYSZ, YbYSZ and ScYSZ ceramic powders were studied in detail.1.0LaYSZ has better phase stability at 1400℃in comparison with YSZ, and the addition of La2O3 can suppress sintering effectively.3.5YbYSZ and (5.3～7.1) ScYSZ both have better phase stabilities, higher sintering-resistance and lower thermal conductivity. ScYSZ has larger region of t’phase, higher sintering resistance and lower thermal conductivity, so among these three systems, ScYSZ is the promising TBCs candidate.The studies on crystal characterization of ceramic powders reveal that stabilizer contents and heat treated temperature are important factors in determining stability of t’phase. Quantitative relationship between tetragonalities (c/(?) a) and stabilizer contents is established. Tetragonalities of these three systems all decline with stabilizer contents increasing. t’ and c phases are easily stabilized in high-stabilizer region, while t and m phases are formed in low-stabilizer region. Phase transformation kinetics of ScYSZ powder indicated that phase transformation from t’to t and c is relative to temperature and time, which was diffusion controlled. The velocity of phase transformation depends on diffusive velocity of the doped atoms. The diffusion of doped cations which leads to the non-uniform distribution of stabilizers produces phase transformation of t’during high temperature aging.Avrami equation is adopted to establish relationship between the amounts of phase transformation and time. The incubation time of phase transformation decreases with temperature rising. For the same composition, the incubation time of phase transformation of coating is obviously longer than that of powder, which indicates that microstructures, stress, defects and cell distortion have important effects on phase transformation. No phase transformations occur in both 6.4Sc0.5YSZ and 7.1Sc0.53YSZ coatings under the heat treatments of either 1400℃/550h or 1500℃/300h, which prove that both the coatings have excellent phase stability.The effect of do-pant characterization on thermal conductivity is analyzed according to mechanisms of phonon thermal conduction and chemical theory of defects. For the zirconia-based materials doped by cations with lower valence, Oxygen vacancy and substitutive defects both strengthen phonon scattering and decrease phonon mean free path, as a result, thermal conductivity is reduced. The rather larger differences of atom characterization between Sc and Zr contribute to the lower thermal conductivity of ScYSZ. Additionally, increasing Sc2O3 contents can produce more oxygen vacancies and also can reduce thermal conductivity effectively. Phase composition, grain boundaries, quantity and shape of cracks and pores between layers are important factors influencing thermal conductivity. Higer phase stability and sintering-resistance play an important role on lowering thermal conductivity of ScYSZ coatings.The failure mechanisms of ScYSZ(TC)/NiCoCrAlTaY (BC) TBCs under high temperature oxidation, corrosion and thermal cycling conditions are investigated. The results of oxidation experiment indicated that thermally grown oxides (TGO) layer is formed at TC/BC interface during oxidation. The stresses caused by TGO growth and thermal mismatch between TGO and TC and/or BC are main reasons leading to the oxidation failure. Stresses produced by TGO growth mainly includes the growing stress of oxides including (Cr,Al)2O3+(Co,Ni)(Cr,Al)2O4 +NiO(CSN). As continuous TGO layer is formed between TC/BC interface, thermal stress produced by thermal mismatch between TGO and TC and/or BC begins acting. The simulating results of stress distribution in TC/TGO/BC coatings obtained by ANSYS software indicated that large compressed radial stress is produced by the mismatch between each layer in the coatings, which is destructive to TBCs. The stress of TC/TGO interface is the largest and that decreases form the sample center to edge. With the oxidation time increasing, the failure pattern of the coating is characterized by TC turning up flaking. The failure place is TC/TGO interface. The results of high temperature corrosion and thermal shock experiments indicated that ScYSZ coating has better properties of V, S molten salts corrosion-resistance and thermal shock resistance. However, the oxidation of bond coat is accelerated under the effect of corrosives. Thick TGO layer is formed under long-term thermal cycling, so the oxidation of bond coatings is the main reason leading to thermal shock failure. So the denser and thicker coatings are needed under the condition that the thermal shock property can not be influenced.ScYSZ coatings have stable phase structures at 1400℃and 1500℃, additionally, ScYSZ TBCs also have higher sintering-resistance, thermal shock resistance, molten salt resistance and lower thermal conductivity, which may be promising materials to be used as insulating coat at ultra-high temperatures. |
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