I once had a problem looking for cracks in a graphite coating on quartz fibers. This problem is a good example of the general case of defining visible light microscopy inspection criteria. Since specimen lighting may influence critically what one can see. The answer to the question depends a bit on the size of the crack and the opacity and reflectivity of the oxide coated surface. First insure that the microscope has a lighting set up that allows one to dependably see the cracks before worrying about a camera. Most of the digital cameras that can be used with a microscope would probably be adequate to recording the crack, once the viewing situation is set up.