Erosion debris particles produced by particle impact erosion of pure Ni and a stainless steel have been examined in the scanning electron microscope for the purpose of determining whether micro-machining is an operative mechanism of erosion by alumina particles. Macroscopic machining chips generally exhibit well-defined lamellae on the side of the chip away from the tool face, and such lamellae are also observed in micromachining chips produced by abrasion or scratch testing. The aspect ratio of such chips is generally large. In the present work, the aspect ratios and shapes of erosion debris particles formed at angles of incidence below the peak erosion angle (αc) were generally consistent with the dimensions of the impact craters formed on the eroded surface and with the hypothesis that they were formed by micromachining. However, most of the debris particles did not exhibit characteristic lamellae. This may be explained by the fact that the surface from which they are formed is very rough even on a scale similar to the size of the debris particles. This is not true in abrasion: Micromachining chips formed from such a surface would be expected to have surfaces which would obscure the existence of lamellae. However, some chips would be expected to come from the few relatively smooth areas of the surface, and these should show lamellae. Examples of such chips were, indeed, found, and micrographs of these chips are nearly indistinguishable from micrographs of micromachining chips formed by abrasion or scratch tests. It is concluded that micromachining is an operative mechanism of erosion which is of greatest importance at low angles of incidence. Debris particles formed at higher angles of incidence are generally more platelike.Presented at the 39th Annual Meeting, in Chicago, Illinois, May 7–10, 1984