Thermal Spray TBCs porosity

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Thermal spray coatings are inherently porous in nature. The porosity levels in certain thermal sprayed coatings such as CoNiCrAlY coatings performed by the low pressure plasma sprayed process may be much lower than its corresponding plasma sprayed cousin, but nevertheless porosity is a given. Ok, some hvof fans could join in the lpps group, but only in the tail end.

Nevertheless, porosity in thermal sprayed coatings is not all that evil when it comes to thermal barrier oatings. On the contrary, a certain level of porosity IS required of all thermal barrier coatings for them to function properly and that is one reason why thermal sprayed coatings processes are preferred in the application of these materials such as yttria stabilized zirconia.

The ability of a thermal barrier coating to withstand thermal cycling is very much dependent upon this inherent porosity. Plasma sprayed TBCs generally are required to have about fifteen percent porosity levels in their microstructure. This is what gives them their thermal cycling strain tolerance. But there is a significant culprit that can mask the actual porosity levels in these TBCs.

Those that are familiar with such coatings will know that it is pullout as observed in metallographic testing. Such pullout is many times wrongly diagnosed as porosity, but is caused by improper polishing techniques. Conversely, improper polishing techniques can smear over the coating and give you a lower porosity observation. Thus the importance of metallurgical polishing techniques cannot be overstated.

Metallurgical polishing that either overstates or underestimates the actual porosity in TBCs is very dangerous. Even with automatic polishers, it is the establishment of a proper polishing procedure that is imperative, especially in the case of TBCs. Computerized image analysis that is touted by some companies is great in establishing real numbers, but then again the sample preparation technique is not a variable that these gizmos can fix.

The porosity level of such TBCs is more important in certain service cycle components than in others. For example, a TBC coated component that is in a high temperature furnace that ramps up to a high temperature and stays there for very long periods of time does not cycle much and so a lower porosity may not affect its life that much. Same is the case with a jet engine component that flies across the atlantic between two far away continents as opposed to a commuter plane engine that starts and stops every hour or so and cycles to high and low loads often.

Unfortunately, the thermal spray coater that is the applicator of the coating is not privy to this information and so it is in his or her best interests to meet the necessary specifications every single time and not take a gamble with the end use of the component.