ZrO2-Y2O3 92/8
By means of the ZrO2-Y2O3 phase diagram for 8% Y2O3-containing plasma spray coatings Miller(1981) had discussed which phase rations are present in the spray coating.The powders and in particular the plasma spray coatings thus produced are not the thermodynamic equilibrium.With 8% Y2O3 predominantly tetragonal and cubic ZrO2 are obtained-besides certain contents of monocline ZrO2,depending on the powder type used.These Y2O3-rich tetragonal phases in the coating disproportionate during longer high-temperature use into high Y2O3-containing cubic ZrO2 and less Y2O3-containing teragonal ZrO2, the latter is converted into the monocline phase when cooling down and can lead to strains in the coating and perhaps to their failure.
At this point two coating approaches are considered.On the one hand to spray coatings so porously that these conversion-induced strains and thermal stresses are being compensated in the coating-porous or hollow powders are especially suitable for this purpose.But possibly higher-stabilized powders with higher Y2O3 contents are suited in order to overcome this problem on dense sprayed coatings,even if Levinde(1981)found 8%Y2O3,as otimum,but it seems that he did not specify and consider the powder type in detail and consequence.In a recent work Stecura(1986)reported an optimum in thermocycle resistivity of the coating system as a function of the NiCrAlY bond coat and the ZrO2-Y2O3 to coat quality.This optimum was found to be determined by a certain ceramic coating porosity using a sintered ZrO2-Y2O3 powder type of 6 to 8% Y2O3 in combination with certain favourable NiCrAlY compositions of the bond coatings.What finally is most successful that's still left to empiric tests.With the differ of different powders both approaches shall be given the possibility to do well-aimed developments.
The variety of the powder types is greatest in ZrO2-Y2O3 92/8 being at present the most important material for advanced thermal barrier coatings.Almost all possible powder manufacturing processes are realized even if not all gained importance.Figure1 shows these various powder types in the SEM-picture as spheriacl and irregular shaped powders.The other important powder characteristics become clearer when compairing the microstructure in fig2.There are four porous powder types having spherical and irregular particle shapes,and also two powder typeswith dense grain,on the one hand with spherical,on the other hand with irregular particle shape.Additional production-specific difference in properties concerning phase-contents and the comparison of morphological powder properties are shown in table .
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