CFD-aided prediction of the shape of abrasive slurry jet micro-machined channels in sintered ceramics

Abstract

The extreme hardness of sintered ceramics makes it difficult to machine them economically. Abrasive slurry-jet micro-machining (ASJM), in which a target is eroded by the impingement of a micro-jet of water containing fine abrasive particles, is a low-cost alternative for micro-machining of sintered ceramic materials without tool wear and thermal damage, and without the use of patterned masks. Existing profile prediction models could not account for changes in the flow field observed in the ASJM of sintered ceramics as channel depth increased. These changes in the flow of abrasive particles fundamentally altered the channel profiles so that the specific erosion rate (mass of material removed per mass of erodent) of the channel centerline decreased with increasing depth and, when machined at 90° incidence, the profiles changed shape. Computational fluid dynamic (CFD) modeling was used to derive a generalized relation between channel geometry and erosive flow (the nonlinearity function), which was used in an existing numerical-empirical model to predict the depths, widths, and shapes of ASJM micro-channels in sintered ceramics; i.e. aluminum nitride (AlN), alumina (Al2O3), and zirconium tin titanate (Zn–Sn–TiO2). The specific erosion rate-particle impact angle and specific erosion rate-particle impact velocity relations, measured for 1wt%, 10μm-diameter alumina slurry jet, were used in a CFD model of a first-pass channel to obtain the erosive pattern (erosive efficacy distribution) of the slurry jet within a shallow ceramic channel. This shallow, first-pass erosion pattern was then generalized and used with the nonlinearity function to predict the shapes of deeper channels. The predicted depths in each of the three ceramics at any point on the cross-section were within 6% of those of measured channels up to a depth/width aspect ratio of about 0.5 for nozzle angles of both 90° and 45° in both the forward or backward channel-machining configurations.