TBC (7-8% YSZ - NiCrAlY) on the titanium substrate by APS isused in a application where the service temperature of the system would be 1500 C. From the literature survey, it is clearly explained that there will be a phase change at 1200C of Zirconia, which leads to coating failure. Can anyone suggest the alternate coating technique or changes in the coating material which can extend the life of substrate? Also, anyone explain the failure modes possible of the coating?
Note: The substrate is subjected to elevated temperature at highly oxidizing environment.
Maximum operating temperature of the best alpha titanium alloys is about 700°C. At these temperatures, titanium alloys not only oxidize quickly, but also massive lose the strength. Only with titanium aluminides the maximum operating temperature rises to about 800°C, but that is still relatively little in comparison with nickel superalloys (maximum operating temperature about 900°C).
In addition, there is no working oxidation protection layer for titanium alloys and no bond coat for TBCs. It is therefore not possible to operate a TBC system on titanium alloy above 600°C.
I do not know where you have the number 1500°C, maybe from a since fiction book, but it has nothing to do with reality. For normal TBC systems with MCrAlY bondcoat and YSZ topcoat and a substrate out nickel superalloy, the maximum temperature at the top cot surface is approx. 1250°C (substrate must not be over 900°C hot).
Thanks Mr. Verlotski,
Thank you for the suggestion. It is clear that Titanium substrates can not with stand temperatures above 700 C. I was doubtful, that if TBC is possible over Titanium then its life can be extended for a longer period.
As you have suggested that YSZ coating by APS requires preheating of the substrate to higher temperature, it is impossible to coat APS over Titanium as the service temperature of Titanium is 700 C. Even though, I reduce preheat temperature of the Titanium substrate (in-order to prevent the micro-structural damage of Titanium at higher temperature) for APS coating, it is risky to get good adhesion between Titanium and bond coat.
I am searching for a material system which can withstand 1500 C (as my system operating temperature is 1500 C) and Titanium can not solve my purpose.
As you have suggested, Nickel based super alloys service temperatures are 950 C, whereas with TBC can withstand 1250 C. Nickel based alloy system also can not solve my requirement.
As ceramics are brittle in nature, I can not choose Ceramics as construction material. Also, I can not use active cooling system to operate it. Only passive cooling is possible.
Can you suggest me any system that I can choose for my application?
Thanks for your suggestion.
Saagar.
Dear Saagar,
Range of metals that can be used in air at 1500°C is very short: platinum, rhodium and alloys of these two. Even other platinum metals (palladium, osmium, iridium and ruthenium) can not be used at 1500°C in air (palladium melts at 1555°C and other three oxidize).
For working in vacuum or inert gas you would still have molybdenum, tungsten, tantalum and niobium, but with that range of metals are already exhausted.
In other words, for the structures that work in air at 1500°C, you have to use ceramics, other options just do not exist.
Regards
Vadim
Dear Vadim,
Thanks for your suggestion. I will alter my design to give extended active cooling over the substrate to prevent it.
Thank you.
I would look at high temperature turbine blades, which are internally cooled and work with (practically) air at temperatures that suit your needs.
Best luck for your work.
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