Flow Analysis of Physical Vapour Deposition Process in OpenFOAM

Abstract

Tata Steel employs physical vapour deposition (PVD) as a novel galvanization technique in their line production to prevent corrosion. In this process, zinc is evaporated in a vapour distribution box (VDB) and directed via nozzles into a vacuum where it is deposited on a steel substrate. In order to reduce stray deposition, achieve high and uniform mass flow rate from the nozzles and avoid condensation, understanding of the flow phenomena in the VDB and nozzles are necessary. Seeing that the PVD process is carried out at high temperatures in vacuum which makes it infeasible to experimentally characterise the flow for optimization purposes, thus, this research utilizes computational fluid dynamics (CFD).

In this study, the flow behaviour of the zinc vapour in the VDB and nozzles are investigated by solving the fluid governing equations numerically using finite volume method (FVM) in openFOAM. This analysis is done using the pressure-based sonicFoam solver to cope with the subsonic flow in the VDB and supersonic flow in the nozzles. The mass flow rate was compared to the ideal isentropic expression and experimental results. Regions in the set-up with high probability of condensation was investigated using a four coefficient Antoine equation.

A uniform pressure build-up in the VDB enabled uniform deposition. Around the corner of the inlet channel, a stable recirculation pattern affected the nozzle flow, however, this effect reduced as the outlet pressure decreased. The simulated total mass flow rate was greater than the experimental data by a factor of four possibly as a result of numerical errors and experimental stray depositions. And the simulated nozzle mass flow rate was less than the analytical isentropic expression by a factor of five due to the nozzle viscous boundary effect, and VDB wall heating. The mass flow rate from the nozzles decreased with increase in distance from the inlet channel. However, the nozzles far away are less likely to form zinc droplets as compared to those close to the inlet stream. For scale-up under the same conditions, it is expected that non-uniformity would be pronounced and the effect of the small eddies would become more significant. Thus, the simulation of the PVD process gives a qualitative description of the flow and a first approximation for the mass flow rate. To further improve the mass flow prediction, the sensitivity of the mass flow rate depending on the melt temperature and inlet velocity boundary condition should be studied in future works.