A comparison of continuum, DSMC and free molecular modeling techniques for physical vapor deposition

Advanced Physical Vapor Deposition (PVD) techniques are available that produce thin-film coatings with adaptive nano-structure and nano-chemistry. However, such components are manufactured through trial-and-error methods or in repeated small increments due to a lack of adequate knowledge of the underlying physics. Successful computational modeling of PVD technologies would allow coatings to be designed before fabrication, substantially improving manufacturing potential and efficiency. Previous PVD modeling efforts have utilized three different physical models depending on the expected manufacturing pressure: continuum mechanics for high pressure flows, Direct Simulation Monte Carlo (DSMC) modeling for intermediate pressure flows or free-molecular (FM) dynamics for low pressure flows. However, preliminary calculations of the evaporation process have shown that a multi-physics fluidic solver that includes all three models may be required to accurately simulate PVD coating processes. This is due to the high vacuum and intermolecular forces present in vapor metals which cause a dense continuum region to form immediately after evaporation and expands to a rarefied region before depositing on the target surface. This paper seeks to understand the effect flow regime selection has on the predicted deposition profile of PVD processes. The model is based on experiments performed at the Electron-Beam PVD (EB-PVD) laboratory at the Applied Research Lab at Penn State. CFD, DSMC and FM models are separately used to simulate a coating process and the deposition profiles are compared. The mass deposition rates and overall flow fields of each model are compared to determine if the underlying physics significantly alter the predicted coating profile. Conclusions are drawn on the appropriate selection of fluid physics for future PVD simulations.