Conditions leading to the embedding of angular and spherical particles during the solid particle erosion of polymers

Embedding of erodent particles is observed during abrasive jet micromachining (AJM) of polymers. Embedded particles may shield the surface from subsequent impacts resulting in a reduced erosion rate. They are also a potential concern in the performance of AJM fabricated devices such as micro-fluidic chips. In this work, the conditions leading to particle embedding in polymers were determined for both angular and spherical particles. Using experimental data and a rigid-plastic model of particle impact, it was shown that angular particles embed only if the impact conditions are such that particle-target interface remains contiguous throughout the impact. A further requirement for angular particles to embed is that the static coefficient of friction between the particle and the target material must be sufficiently high to prevent the particle from being ejected on rebound. These criteria are consistent with the intuitive observation that embedding increases with elongated particles, and when the major axis is parallel to the incident velocity. The size distributions of the particles that were predicted to embed by the model were similar to those in the abrasive stream indicating that particle embedding did not depend on particle size for the investigated particle/target systems. The embedding of spherical particles was then analyzed using a rigid-plastic impact model in comparison with published experimental data. It was shown that a spherical particle will embed if it penetrates the substrate to a depth greater than its radius, so that displaced material can surround it to prevent rebound. Embedding of spherical particles at 90° is unlikely because the rebound forces will be larger than the frictional forces that would retain the particle.