Some writings on ottoman military, including "The Enemy at the Gate: Habsburgs, Ottomans, and the Battle for Europe" mentions ottoman sipahis slashing western pikemen's spear tips with their sabres. From what I can recall, similar account also appears in Charles Boutell's book about history of arms and armour. Are there any primary sources supporting such claims?

Can't remember any, and just like similar accounts--the Roman cavalry riding across the Macedonian front and breaking the pikes at Pydna comes to mind--I prefer to put this in the "highly dubious" bin. It might have been done by desperate individuals or small groups but I strongly doubt its overall effectiveness as a deliberate tactical device.