Tribologically impact is when a solid or liquid object strikes a surface. This could be a single high-energy event, such as a hammer or even a projectile. For the former, in automotive engine this includes the opening and closing of the exhaust and inlet valves against the valve sets, particularly for race engines where the accelerations involved (to get more air in) lead more violent impact events. Projectiles impacts can be from ballistics and understanding how materials perform under these high-strain events help with the development of personnel and vehicle armour.
Impact can also be more of a fatigue-type process where multiple particles or droplets strike a surface gradually leading to damage and this is commonly referred to as erosion. Erosion can occur on leading edges of various blades from wind turbine to aero-engine fan blades and energy-generating gas turbine blades through water droplet erosion. Water droplet and particle erosion is a particular problem in the high velocity flows encountered in the oil and gas industry.