Armor-plating

Though the spark is the life of the Transformer, they are still made up of circuits and processors that are vulnerable. And years of warfare have left those circuits and processors even more vulnerable. To protect them, armor-plating is placed over their structures to preserve their vital functions. Though the armor-plating is not invulnerable, and can succumb with enough weapons fire, the armor is still more resilient than most things on Earth.

Fiction

Live Action Movies

When Scorponok's tail was severed in battle, it was taken by the military for research. It was discovered that the exterior of the severed tail was composed of a kind of "self-regenerating molecular armor".