Bricks are cool, but brick chimneys have an impossible task. Bricks absorb water when it rains, then get rid of it as they heat up from sunshine or a hot fire. They are constantly subjected to freeze-thaw cycles, and because water expands when it freezes, you end up with "spalling" which is what your bricks have done. Follow Jay J's advice and see a brick mason. There are no "quick cheap fixes" for this one.