Numerical Modelling of Laser Absorption and Heat Transport in Metallic-Overcoated APT Specimens

Laser-assisted atom probe tomography (APT) of materials with poor thermal and electrical conductivity generally suffers from data artefacts and low experimental yield. Depositing a thin metallic overcoating on specimens has become a common mitigation strategy, yet the physical basis for its effectiveness remains poorly understood. In this work, a coupled three-dimensional finite element model is applied to metallic-overcoated APT specimens to resolve how overcoating may influence laser absorption and heat dissipation. Using an experimentally derived steady-state geometry, the optical and thermal response of silicon (Si) specimens overcoated with four metals is simulated and compared with an uncoated baseline. The overcoating is shown to redistribute laser absorption and produce a more symmetric apex temperature distribution. However, because Si has a particularly high thermal diffusivity over the temperature range spanned during analysis (50 to 300 K), the metallic overcoating impedes rather than enhances heat dissipation, reducing the apex cooling rate. Experimental mass spectra are in general agreement, with most overcoated specimens showing more extended thermal tails than the uncoated baseline. For a specimen with poor thermal conductivity, such as calcite, the opposite behaviour emerges and simulations show that a metallic overcoating substantially improves apex cooling.