Evaluation of DC-MS and HiPIMS TiB₂ and TaN coatings as diffusion barriers against molten aluminum: An insight into the wetting mechanism

TaN and TiB₂ ceramic coatings on steel were tested as barriers to protect steel against molten aluminum by sessile drop experiments. Coatings tested were deposited by PVD using conventional DC-MS (Direct Current Magnetron Sputtering) and HiPIMS (High-Power Impulse Magnetron Sputtering) techniques. The evolution of contact angle and contact radius of the molten aluminum drop was recorded at 800 °C for H13 tool steel bare and coated samples. The influence of the thickness and microstructure of the coatings was investigated by XRD, SEM/EDS to elucidate the mechanism of the wetting process. Spreading rate of aluminum on coated samples was between 10 and 50 times slower than on bare steel samples. Coated samples showed reactive wettability with a reaction-controlled spreading stage observed as a constant spreading rate after an initial transient stage. The spreading mechanism of the molten metal on coated samples is a two-step mechanism: (i) molten metal infiltration through the coatings and (ii) fracture of the coating after the formation of Fe-Al intermetallic compounds that leads to an increased volume of the metallic substrate. The quantification of both steps proves that the coating's thickness plays a key role in the reactive wettability kinetics. Results show that HiPIMS deposited TaN coatings could be a promising candidate to protect steels against molten aluminum attack.