Related to Can a shield be disarmed.
Can a piece of armour be disarmed?
For example, can an attacker knock their foe's helm off? Remove their gauntlets? Disarm them of a cloak?
Why would an attacker want to do this? Maybe the item is magical and the attacker wants to deny its use. Maybe the item is just valuable and the attacker wants to steal it.
If so, what rules would cover this? If not, what rules would you suggest?
There are no existing rules that explicitly allow this. The only methods of disarming that exist at present are the Battlemaster Fighter's Disarming Attack, and the variant Disarm rule in the DMG. Disarming Attack only lets you force a creature to drop an item that it's holding, not one that it's wearing:
When you hit a creature with a weapon attack, you can expend one superiority die to attempt to disarm the target, forcing it to drop one item of your choice that it's holding.
The variant Disarm rule likewise only lets you knock items from a target's grasp, not from their body (page 271 of the DMG):
A creature can use a weapon attack to knock a weapon or another item from a target's grasp.
If a player wanted to do this, it would presumably be either a Strength or Dexterity ability check, albeit a very difficult one.
There are not currently any options for doing so.
The ability to disarm opponents come either from the disarm variant rule or the battlemaster’s disarming attack. These both specify that they work only on items that are “grasped” or “held,” respectively, so unless someone is holding onto some piece of their armor, they can’t be disarmed of it.
But if you caught someone with hat in hand, then you could.
Armor is worn, not armed. Consider that armor takes time to put on due to it's straps, belts, laces, or other various parts, and is considered a lengthy procedure taking up to 10 minutes (PHB pg. 146). Contrast that with a weapon or shield which takes an action (6 seconds).
The difference is very plain. Armor takes care and consideration to be worn, whereas a shield or weapon is simply held in your hand. This also means you couldn't disarm a greatsword sheathed on somebody's back either.
The bottom line is that disarm only applies to what you're currently holding.
I think you might be able to appropriate the rules for grappling. Sure, WotC simplified the rules for 5e, but that doesn't mean you can't pull DM fiat; as others have stated, there aren't any rules specifically regarding this kind of situation to provide precedent.
If you can grab hold of your opponent and immobilize them, you or a companion might be able to tear off some armor or clothing; maybe not just any part, but a boot or a gauntlet or a hat/helmet could work. The necessary rolls could vary depending on what is being removed and how (fighters and rogues might try different methods, after all).
While the rules do state that armor takes about 10 minutes to fully equip/remove (others have already provided the references), that's for a full set of equipment; a single piece of armor or equipment such as a glove/boot/hat/helmet being removed in a haphazard way, if you aren't being careful not to harm the wearer or the item itself, might be removable from a pinned individual in a few rounds (probably more if they're struggling and roll well on their opposed checks).
For that matter, most hats aren't firmly attached to whoever is wearing them, so one of those could just fall off during the struggle if that's what you're after. (That may not apply to magic items depending on your setting, of course.)
