Fig 25 Thermal Strain In Ttbc Coatings Cylinder Head Equal Thickness Layers

Figure 25 shows the prediction of what happens to the thermal strain when we spray these bond-coat/ceramic mixtures in equal thicknesses using the same conductance goal. Because of the difference of thermal expansion of the mixtures, we greatly decrease the strain near the bond coat interface. Each coating layer produces a strain gradient based upon the expansion of the layer. The coating in this case ends up haying a mean strain of about 400 MPa in tension throughout the coating thickness; however, note that because of the mixing of the metallic bond coat into the ceramic (in order to obtain the same conductance goal) we have to increase the total thickness of the layer from 1.25mm to 2.10mm.

To go one step further, rather than produce layers of equal thickness of the various mixes of bond coat and ceramic, we produce layers of equal conductance which is shown in Figure 26. The model now predicts where the mean strain throughout the coating thickness in those five layers is almost zero -- which would be an ideal situation. In doing so, of course, the layer thicknesses are no longer equal because we are really trying to match equal conductance through the layers. But because more metallic bond coat is used in the layers we would actually have to grade out to almost a 3mm thick coating.

This data allows a designer to come up with the optimum thickness and optimum conditions, in other words, to minimize the stresses by allowing a nominal increase in coating thickness. In actual practice the ideal process would be to continuously grade the coating based upon the optimized design.

400 vv/m2 eC

400 vv/m2 eC

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