What is the best way to prepare sample for TEM analysis of wornout surface without damaging the wear features in thermally sprayed ceramic as well as cermet coatings?
If you need cross section - FIB. If you need surface topography - also an old, almost forgotten replica method, but you'll need to dissolve your base material. Even better for topography - use SEM instead of TEM.
The most interesting view that allows observing tribological layers and strain gradients is cross section view parallel to the sliding direction. The foils may be obtained by coating the worn surface using, for example , epoxy resin, curing it and then EDM cutting out a platelet for mechanical grinding and ion thinning. The final result looks like an elongated hole whose edges show the microstructures below the worn surface.