Solid particle erosion is progressive loss of material from a solid surface by repeated impacts from fluid-borne solid substances-particles. The particles can range from oxygen molecules, as in outer space, to sand particles in ambient air (Figure 9.1) to roadway stones a centimeter in diameter that produce chips on automobile body parts. The potential for an impacting particle to cause damage to a solid surface depends on its impact energy, specifically its mass (m) and velocity (v). The classic physics relationship for momentum of an object in motion applies: ½ mv2. However, the velocity exponent may be 2.2 or higher.