NP3 is Teflon flakes impregnating a sacrificial nickel plating and if I remember right close CTE to non anodized (Bare) aluminum of a common type. Forget the specifics now. It exposes more Teflon as it wears reducing Friction CE. DLC and other nano vapor depositions are pretty hard. Cerakote has proven sensitive to some solvents IME and can flake depending on surface prep.
I’ve use NP3+ in other industries and really liked it. Bummed to see Robar is out. Always wanted to have trigger internals and bolts plated.
If memory serves Birdsong T is Teflon encapsulated in a “coating” of some kind. Not sure if it’s ceramic or another sprayed finish but don’t think it was a PVD or plating process.
Nitride if I remember correctly is a salt or something like a halide reacting with carbon creating a hardened surface with great corrosion resistance. Nitride Carborizing May be the process. Bluing is also a salt/rusting process to form a protective surface rust layer.
My details may be foggy as it’s been years since I have looked at metal treatments. I’m sure others on here can jump in with better details. There were a few good articles comparing properties of many metal finishes floating around.
Really depends on the properties that matter. Protective layer, low sheen, low CEF, corrosion resistance in harsh environments, etc. and then the chemicals the coating will interact with. I have had NP3+ subjected to heavy temp cycles in abusive environments basically coated with caustic acids perform well. I’ve had ammonia based cleaners not harm cerakote albeit I was careful. I’ve had SU-100(denatured Kerosene) eat right through cerakote. I’ve seen Halide solutions and other Acetones fail to desolve optical coatings that IPA(Alcohol) cleaned off in seconds. I was never the Material Scientist in these trials but did work closely with the Designers and they drove home the point that a fairly innocuous solvent can have a strong reaction breaking chemical bonds if chosen correctly.
