Hi Mani, i am not sure of your intention whether this is to assess the plasma torches or to look at the role of processing conditions. Invariably, when you do sand blasting on thinner substrates with large aspect ratio, it is sometimes preferred to have it done on both sides to avoid the stress building up? Otherwise, such curvature will be seen. Usually, plasma spraying tends to induce tensile residual stresses and the typical compressive stress (built during sand blasting) would have got nullified. Without knowing the state of curvature after deposition, it can't be speculated or correlated with the spallation event. The curvatures that form during spraying will provide lots of information in terms of deposition stress, cooling stresses and evolving stress, etc. If you have generated additional specimens, have a look at those and observe the curvature..

I suppose you must have generated with 3/7MB and 9MB torches? With respect to spallation, the possible event could be attributed to higher stresses or bond coat oxidation or stresses due to higher TGO kinetics..What was the porosity levels before and after the thermal cycling? Did you observe any top coat densification? What was your exposure temperature and i also believe that such temperatures may not promote phase transformations. 

Why do need deareted conditions? Do you mean, spraying was done in shroud atmosphere?

Hi siva, thank you for your detailed reply. Let me clarify the following:

We use 9MB torch

Density of the top coat was similar for both the specimens made with 3MB and 9MB

Exposure temperature was not measured. But back wall temperature was 1250 deg.C

Derated condition: I mean the full power capacity of the torch was not used.

Flat specimen after grit blasting was studied recently and the same phenomenon was observed.

Kindly suggest

Hi Mani, I suppose you must have performed the tests exposing to a flame or burner and measure the back surface temperature. What is the typical duration of heating-steady state-cooling cycles? How long is your expected life from the TBCs?

Since your back wall temperature itself is so high, then the exposure temperature should be certainly higher and under such circumstances, i am not sure whether we get consistent performance with CaO stabilized ZrO2 (whose service temperatures are around 900 deg C)

If you can measure the porosity levels in both specimens post-spallation, that should give some idea about the possible densification. Also, look at the TGO thickness and some say, a threshold of 5.5 microns is the onset of delamination...Does your spalled specimens such high thickness levels?