This paper discusses the decrease in current generated by the impact between particles anti a metal plate with time. Final currents decrease to about one-tenth of the initial ones. In order to explain this fact, we have investigated the material transfer from particles; to the metal surface and work hardening of the metal plate. EDS analysis has revealed that the amount of transferred chemical elements increases with the passage of time, and then tends towards a constant value. It is also found that the surface of the metal plate is hardened through impact of particles, and therefore the yield pressure increases with time. The current decrease can he explained by a proposed model which takes into account the change of yield pressure and the material transfer with time.