Academia threw this one up for me at random. I couldn't find where it had been discussed in detail before (Duncan will put me right if it has)
https://www.academia.edu/35308142/Crowns_hats_turbans_and_helmets_The_headgear_in_Iranian_history_volume_I_Pre_Islamic_Period_Edited_by_Katarzyna_Maksymiuk_and_Gholamreza_Karamian_Siedlce_Tehran_2017
There is a whole book here, with lots of pictures, including Seleukids, parthians, sassanians and gandarans. We have hugely more helmets from this region than I was aware of, though I'm sure the more specialised of us will be aware of most, if not all, of them.
I don't think it's been discussed as a whole, though we may have cited individual essays.