As a matter of general principle, the second noun in a combination like 'key card' describes the nature of the object. Thus, a key card is a card that works/look/functions, etc. as a key, whereas a 'card key' would be a key that works/look/functions, etc. as a card. In your case, I have seen and heard of cards that function as keys (e.g. in hotels) but I have no idea what a 'card key' would be.