Can I use my iPhone charger with my iPad?
Yes, you can use your iPhone charger with the iPad.

This should not damage the iPad or the charger.

However, since the iPad is provided with a lower current the charging takes longer. Both the iPhone and iPad charge with 5V, but the iPad charger provides 2A while the iPhone charger only provides 1A.

This is why it takes longer to charge. You should completely turn off the iPad in order to speed up the charging process.