A re-examination of the ‘particle size effect’ in slurry erosion

The nature and significance of what has been called the ‘particle size effect’ on the rate of slurry erosion and the problem of its experimental evaluation are discussed. Experiments involving the change of erodent particle size in typical laboratory test equipment, while revealing a decrease in erosion rate with decreasing erodent particle size, also produce significant changes in the slurry flow conditions and particle motion which, unless evaluated quantitatively, will mask the nature of the particle size effect. Some model for the rate of material removal must be assumed to allow comparison between the conditions examined. We assume this to be the rate of dissipation of erodent particle kinetic energy at impact. Wear results have been obtained using a slurry pot erosion tester (for which the flow conditions about, and impact conditions on, the 6 mm diameter cylindrical specimens have been modelled and experimentally confirmed), using 0.94 wt.% suspensions of SiC (varying in mean particle size from 14 to 780 μm) in diesel oil for aluminium and Pyrex glass. For aluminium no damage threshold was observed and the experimental erosion rate varied as dp2 over the whole particle size range. A contribution to this of dp was found to be ascribable to fluid flow changes in the experimental apparatus and the remaining contribution of dp between 100 and 780 μm tentatively assigned to the decrease in surface energy of debris particles with increasing particle size. Below 100 μm there is a transition from wear by particle impact to wear by wet abrasion by particles moving across the surface. Quantitative analysis of erosion rates on the basis of particle impact energies was not possible in this range. For Pyrex glass, erosion wear was measured only between 390 and 780 μm and was found to vary as dp4. This dependence was made up of dp for fluid flow changes, dp for debris particle size and dp2 for the influence of an energy threshold for damage of between 2 and 8 μJ for each particle impact. After taking flow conditions and squeeze film retardation at impact into account, it is concluded that there is no fundamental change in the interaction between impacting erodent particles and the wearing surface with increasing particle size between 100 and 780 μm.