The Critical Impact Velocity in Particles Impact Erosion

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Solid particles impact erosion of metallic materials proceed through two kinds of damage processes. One is the removal of material due to repeated plastic deformation, and the other is cutting. These processes occcur simultaneously and the ratio of their contribution to the total damage depends not only on the impact angle but also on the impact velocity. As the impact velocity goes down, a solid particle tends not to skid over the target material surface, which subsequently decreases the damage through cutting. At a certain lower velocity, the particle does not skid at all, which results in no cutting damage but plastic deformation damage only. This velocity was defined as the ""critical impact velocity". In this study the methodology to determine the critical velocity through a measurement of the coefficient of friction established, that is the dynamic friction coefficient during skidding and the static friction coefficient during rolling without skidding. In order to measure the coe … More fficient of friction at the moment of particle impact, a rotating target apparatus was developed. The critical impact velocity thus determined depended on the hardness of both material and particle, as well as on the shape and size of the solid particles. Furthermore, the critical impact velocity was theoretically derived through analyzing the behavior of the target material surface impacted by a spherical solid particle : at the moment of oblique impact, an indentation is formed ; at the same time, the material is strained tangentially to the surface in the direction of particle movement. As the extent of strain goes over the elastic limit, the impacted solid particle does skid over the surface, which brings about wear to the surface by cutting. The threshold strain was derived as a function of the impact velocity, the rotating velocities as well as the duration of particle surface contact. Those parameters which characterized the impact behavior were derived from the coefficient of friction and the rebounding coefficient, all of which were obtained from the mechanical properties of the target material and particle, and other factors concerning the particle. Consequently, the theoretical value of the critical impact velocity was given solely as a function of the mechanical properties of the target and the particle. A good correlation was found between the critical velocity determined by experiment and that predicted by the theoretical equation. Less