Darksteel Axe

• WOTC Official Reminder Textofficial rules
  Indestructible — Effects that say "destroy" don't destroy this permanent. A creature with indestructible can't be destroyed by damage.
• WOTC Official Reminder Textofficial rules
  Equip — Equip [cost] ([cost]: Attach to target creature you control. Equip only as a sorcery.)

• WWizards of the Coast2013-07-01
  Official Ruling
  Although Darksteel Axe has indestructible, it can still be put into the graveyard for other reasons. The most likely reason is if it's sacrificed.
• WWizards of the Coast2013-07-01
  Official Ruling
  Darksteel Axe itself has indestructible, not the creature it's equipping.

Rulings via Scryfall