If the noncreature artifact had any other supertypes, types, or subtypes, it will keep those. In most cases, the artifact creature won’t have any creature types. (2013-10-17)
Once Sarkhan’s first loyalty ability has resolved, each planeswalker you control (including Sarkhan) is no longer a planeswalker for the rest of the turn. They don’t lose any loyalty counters or abilities, and you can still activate their loyalty abilities if you haven’t done so yet this turn. They don’t lose loyalty if they’re dealt damage while they’re not planeswalkers. (2019-05-03)
Quote from peteroupc » Note that "becom[ing] an artifact creature" is treated differently by the rules than, say, "becom[ing] a land creature" or "becom[ing] an enchantment creature" or, as in the case of Sarkhan's first loyalty ability, "becom[ing] a ... Dragon creature" (compare C.R. 205.1b with C.R. 205.1a).