Brake Discs

The brake disc or commonly called the brake rotor, is the heart of a car's braking system. It is the component by which the brake caliper clamps its hold unto to stop the the wheels' rotation. If the brake disc is damaged, in any way, the car will ultimately meet a tragic end.
Because of its importance to the car and its passengers, car designers and engineers have made the brake disc, even in its simplicity, a dependable innovation. Some are simply made of solid cast iron, others are hollowed out and have added fins that joins the discs two contact surfaces. Because of its hollow part, it is "ventilated" to dissipate the heat generated. This usually used on front discs. High Performance brake discs have holes which commonly known as cross drilling and was done to redistribute gas on the disc surface. The holes let the gas to pass through it when the brake pads clamp down on the disc. Other brakes are slotted to aid in removing dust and dirt but slotted discs are seldom used on road cars for they easily wear out the pads.
Because of the stress that brake discs can experience it can be damaged in one of these three ways, they can warp, scar or even crack. Warping is caused by the heat generated from friction. The heat softens the metal which causes it to be reshaped. Another cause of warping is caused when the overheated disc is immediately cooled. This happens when a clamped disc is immediately released and is immediately cooled down. Improper torquing of lug nuts can also cause warping. Cracking occurs only in drilled discs. Holes near the edge of the disc develop cracks because of the disc's uneven rate of expansion when heated. scarring can also be contributed to overheating. 
When brake discs are damaged, it is better to replace them than having them to be repaired for such measures can cause graver problems.