Fig 23 To Obtain Data For Design

The thicknesses of these mixed layers will depend upon the analysis as shown in the next figure. Here the analysts have modeled the strains developed, at temperature, in a thermal barrier coating on a cast substrate. Figure 24 represents what actually would happen with a ceramic coated (non-graded) TBC on a cast iron cylinder head with a target of getting the conductance of 400 watts per meter squared degree C.

'FIG. 24. THERMAL STRAIN IN TTBC COATINGS CYLINDER HEAD

What this figure shows is that the highest tensile strain, reaching nearly 1600 microinches, occurs near the bond coat in the ceramic coating and then decreases thru the coating thickness toward the top surface. Obviously the temperature increases from the bond coat out to the top surface due to the thermal barrier effect and in this case the top surface is at about 650 C. The top surface of the ceramic coating based on no grading is actually in compression so that the strains have gone from 1600 in tension just above the bond coat to nearly 400 in compression at the top surface of the ceramic. This is based upon the 1.25mm thick coating which, with the porosity used in this particular coating, would reach the thermal conductance goal. Our purpose in the grading is to greatly decrease the 1600 microinch strain which occurs near the bond coat/ ceramic interface and indeed this is the area where we often see the failure of such a plasma sprayed coating initiate.

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